Stand under the night sky on a clear,
cold evening, and the universe will
seduce you.
Stars [music] pin prick the darkness in
familiar constellations. Jupiter's moons
march in perfect order. [music] Galaxies
spiral like frozen fireworks.
It is beauty on the [music] grandest
possible scale.
But beauty in nature is more than just
what the eye can see.
In the subatomic world, the same
patterns repeat. Symmetries,
proportions, harmonies. Beauty is not
simply decoration, but structure. A
language written deep into the laws of
reality. Indeed, again and again, our
study of physics has confirmed this.
Newton found a single equation to
explain both the fall of an apple and
the orbit of the moon. Einstein's
equations bent space and time
themselves.
And yet, the history of science is also
littered with failed, beautiful
theories. The beauty alone is not
enough. A successful theory must survive
its trial in nature's courtroom. And
now, at the center of that courtroom
stands string theory.
String theory is arguably the most
audacious and seductive vision
physicists have ever produced. The
remarkable [music] suggestion that all
particles and forces in the universe are
nothing more than the vibrations of tiny
strings. Gravity, electromagnetism,
quantum mechanics, even time itself.
[music] All could be woven together by a
single thread. A final theory, the
ultimate unification, a glorious hymn to
symmetry and elegance.
But there's a catch. For unlike Newton's
gravity or Einstein's curvature, string
theory is still on trial. For almost 60
years, it has resisted every attempt at
empirical confirmation. Each time
experiment has cornered it, the theory
has adapted, reformed, and survived.
And so string theory finds itself in the
dock.
On one side, the defense invoking beauty
as their alibi. On the other, the
prosecution demanding hard evidence.
Every triumph will be cross-examined,
every failure entered into the record.
Witnesses will testify. Some foly,
[music] others against it. And finally,
the jury will weigh the evidence in
pursuit of a verdict.
Is string theory a glimpse of the
ultimate truth, or is it a beautiful
illusion destined to be remembered as
physics most seductive false promise?
The courtroom is now in session.
link in the description and get that appointment off your to-do list.
We have to remember that what we observe
is not nature itself, but nature exposed
to our method of questioning.
Flames leap from the stovetop. Knives
flash. Pans hiss [music] and simmer.
Somewhere behind a wall, a chef is
cooking. But you can see none of this.
All you have is a hatch in the wall.
From one side, you can see a tray of
ingredients. Lamb, garlic, turmeric.
Later, a finished plate is pushed back
out. [music] But you never glimpse
inside the kitchen. You never hear the
chef. You simply see what went in and
what [music] came out.
So you start paying attention. When you
provide chili, coconut, and lime, the
same dish always returns. Swap the lime
for tamarind, [music] and something
entirely different emerges. Over time,
the patterns become clear. The recipes
may stay hidden, but the correlations
don't. Eventually, you stop asking how
the food is made. You just learn the
rules.
Input, output.
This was the situation facing particle
physicists in the late 1960s.
Only their ingredients were colliding
particles and the resulting dishes were
sprays of stranger, more exotic
particles. But the recipe, the thing
that told them how one became the other,
remained completely [music]
inaccessible.
And nowhere was this more vividly
embodied than in California, a place
where two very different revolutions
were unfolding.
The air is thick with incense and
rebellion. Long-head teenagers dance
barefoot in Golden Gate Park, preaching
peace and rejecting the establishment.
Jefferson Airplane, [music] Janice
Joplin, and Jimmyi Hendris blare from
nearby speakers as a 100,000 hippies
descend on San Francisco to turn on,
tune in, and drop out.
It is 1967, the summer of love, and
couldn't be more different from the
sterile scene unfolding high upon a
hillside just across the bay. For there
physicists clad in starched shirts,
ties, and elbow patches hover over dusty
chalkboards and computer printouts.
And yet, arguably, the hippies and the
physicists have more in common than it
first appears. For in their own ways,
both are seeking a higher truth. One
through free love and LSD, the other
through fastmoving protons and liquid
hydrogen.
By the late 1960s, the Beverron at the
Lawrence Berkeley National Laboratory in
California ranked among the world's most
advanced particle accelerators. The
Beverron was a proton synretron. [music]
Within this type of accelerator,
physicists speed protons up to nearly
the speed of light before aiming them at
a tank of [music] liquid hydrogen held
just below boiling point. The protons
ripped through the hydrogen like machine
gun fire. New particles ricochet into
existence. children of the carefully
instigated collisions.
The physicists have cleverly set all
this up inside [music] a magnetic field
which cajolles the charged particles
into curved paths the exact shape of
which betrays the mass and charge of
their creators with millions of
photographic plates capturing this
intricate dance as particles pirouet
like ice skaters. This way and [music]
that.
Thanks to [music] experiments like
these, the number of new subatomic
particles exploded as the 1960s drew to
a close. But when physicists tried
[music] to use their existing quantum
rule book to predict the trajectories of
these new particles, they failed
miserably. Whatever was happening at the
point of collision, they were missing
something fundamental. However hard they
tried, [music] they couldn't glimpse
within the particle kitchen. So instead,
[music] they were reduced to focusing on
results, not process.
Eventually, they turned to a pragmatic
workaround called scattering matrix
[music] or smatrix theory. This method
sidestepped the messy details of what
actually happened during the collision.
And like the chef's hatch in the kitchen
wall, focused only on matching inputs
with outputs. For example, the more
energy physicists fed into their
collisions, the bigger the portion sizes
[music] that came back. But still, the
recipe itself stayed firmly locked
behind the wall. That was until 1968
when a young physicist working in
relative obscurity found a key to
unlocking the subatomic secrets.
Gabrielle Venetsiano.
Venetsiano was born in [music] Florence
in 1942. A shy but technically gifted
student, Venetsiano had moved to Israel
to complete his PhD before taking up a
research position at CERN near the Swiss
city of Geneva. Although widely
respected, CERN had yet to become the
particle physics powerhouse that it is
today. And it would be at CERN in a
small office that Venetiano stumbled
across a formula originally discovered
by Swiss mathematician Lenhard Oiler
known as the Oiler beta equation.
Venetiano plucked it from the annals of
18th century mathematics and applied it
to 20th century particle physics. It was
as if he'd stumbled across the chef's
secret menu. the guide that for the
first time revealed exactly how the
portion sizes scaled with the
ingredients. What input [music]
matched what output. Fetsiano published
his paper and then headed off on a
4-week vacation and in his own words did
not think much about it. [music] But by
the time he returned, his formula had
already become the talk of the town.
And it would be this formula that would
turn out to be the founding discovery of
string theory.
To the defense, this was beauty in its
rurest form. A centuries old equation
traced perfectly over brand new data.
Venetiano's equation would become the
first exhibit in what would grow into
one of the longest running trials in
theoretical physics. Although at the
time, no one realized it. Indeed, as
Vanetsiano later admitted, "I was just
looking for a model. I had no idea it
would lead to strings." And so, armed
with Venetsiano's equation, physicists
could finally match the inputs to the
outputs. It was a persuasive piece of
evidence, but circumstantial. There was
still no closer to describing what
exactly was actually occurring at the
point of collision. They could still not
see inside the kitchen, understand why
or how all these new particles were
created. The answer to this, however,
came sooner than expected. For in
September 1969, mere weeks after the
hippie movement reached its cultural
crescendo at Woodstock, a radical
reinterpretation of Venetiano's idea
would come in part from physicist
Leonard Suskin, a witness for the
defense of string theory we will meet
many, many times.
Suskin was born in the New York burough
of the Bronx in 1940, a cultural world
away from hippie, California. His father
was a local plumber [music]
working the rotten tenementss in and
around Harlem. And after attending
college to study heating engineering,
Suskin Jr. fell in love with an unlikely
muse, theoretical physics. Eventually,
he plucked up the courage to tell his
father that he wanted to be a physicist.
Get out of here. You ain't going to work
in no drugstore, came the reply. Not a
pharmacist, a physicist like Einstein,
[music] Suskin Jr. had clarified.
At the end of the 1960s, Suskin hit upon
a new interpretation of Venetsiano's
work. In Chicago, [music] the same truth
was dawning on Japanese physicist
Yuichiri Nambu and Danish physicist
Holgo Nielsen also independently had the
same light bulb moment. The trio each
realized that the root cause of
physicists problems [music] with
colliding protons was that they'd been
treating them as pointlike particles.
Infinite [music] decimally small specs
with no dimensions, much as we still
think of electrons today. Namboo,
Neielson, and Suskin suspected a deeper
truth was hiding behind Venitziano's
equation. Protons weren't pointlike,
they proposed. They were strings,
[music] one-dimensional ropes that could
vibrate in different ways. Rejection was
swift. Reviewers dismissed it, and the
[music] dismissal cut deep. Boom. I felt
like I'd gotten hit over the head with a
trash can. I was very, [music] very
deeply upset. Suskin would later recall,
"According to the new hydronic string
theory they had proposed, the mysterious
escalating melody of masses inside the
Bevetron wasn't a collection of
completely different particles, but
[music] instead came from the same
string vibrating at different
frequencies.
Was this the chef's secret recipe?
[music] Was nature cooking with
strings?"
For a brief moment, physicists, just
like the hippies, had elevated
themselves [music] to a state of deep
enlightenment.
The trouble is they were wrong.
This unsettling realization came on the
back of an experiment taking place less
than an hour's drive south of Golden
Gate Park at a completely different type
of accelerator to the Beverron, the
Stanford linear accelerator center.
Here, physicists weren't firing protons.
They were using them for target
practice. The accelerator center was
effectively a high-speed gun shooting
electrons down a 2-m long range. The
hope was that by relentlessly bombarding
protons, physicists could finally see
what they were made of. The Beverron was
like a butterfly net, hoping [music] to
catch whichever strange subatomic
creatures fluttered past. The Stanford
Linear Accelerator Center was more like
a surgical scalpel slicing open the
proton to see what was inside. If
protons were large clouds of charge, as
had been the assumption, then the
incoming electrons shouldn't lose a lot
of energy upon collision with their tiny
targets and would be scattered off at
shallow angles. Except that's not what
physicists saw at the accelerator
center. Instead, electrons lost
significant energy and pinged off at
surprisingly large angles. It was as if
the electrons were hitting hard
substructures within the proton itself.
The idea had already been floating
around in the physics ether for half a
decade. In 1964, physicists Murray
Galman and George Zvig had proposed that
some particles were in turn made of
smaller particles, quarks. Some models
worked better with them, but until the
experiments at the Stanford Linear
Accelerator Center, it wasn't clear
whether quarks were real or mere
mathematical slight of hand. But it
wouldn't take long for physicists to tie
the loose ends together. Within a few
years, the proton structure had been
neatly bundled into a theory called
quantum chromodnamics or [music] QCD.
This theory perfectly explained how
protons are made of three quarks bound
together by the exchange of particles
called gluons. And so the electrons of
the Stanford Linear Accelerator Center
were pinging off the quarks inside the
protons.
Quarks and quantum chromodnamics were a
hammer blow for the infant idea of
string theory. In courtroom terms, it
was as if the prosecution had landed a
devastating cross-examination. The case
for string theory had barely been
opened, but now it seemed to slam firmly
shut.
Most physicists quietly left the field
behind and moved [music] on to other
projects. Only a few hardy souls
remained, leaving string theory lifeless
but not completely dead. But those who
stayed behind started asking a different
question. What if string theory wasn't
just a failed detour in particle
physics, but the first clue in a far
deeper mystery? Not just a theory of the
subatomic,
but a theory of everything.
String theory is a part of 21st century
physics that fell by accident into the
20th century.
[music] A pipe rests gently on a
scattered pile of dogeared papers. Books
jut out precariously from the [music]
edge of the table. A single piece of
chalk lies expectantly on a ledge
beneath a blackboard filled with
frantically written equations.
After Albert Einstein [music] died in
April 1955, this was the state of his
office, and it was a scene we were
[music] never supposed to see. When Life
magazine photographer Ralph Morse
[music] heard of the physicist's death,
he raced the 90 mi from his home to
Princeton. And after bribing his way in
with a bottle of scotch, Morse was able
to photograph Einstein's desk [music]
within hours of his death.
But it's what's on the blackboard that's
perhaps [music] the most telling. This
was Einstein's final attempt to solve a
problem that had become a decadesl long
obsession.
Einstein was searching for a unified
[music] field theory, a single framework
that could explain both gravity and the
electromagnetic force. [music] And yet,
despite 30 years of trying, his holy
grail remained agonizingly [music]
out of reach. Someone else would have to
step forward, reopen the case, and find
what even the great man could not.
By the time string theory appeared at
the end of the 1960s, physicists were
attempting to do just that. To tie
gravity to not only the electromagnetic
[music]
force, but to the strong and weak
forces, too. One beautiful, elegant, and
coherent [music] theory of everything.
The trouble was, gravity spoke a very
different language to the [music] other
three fundamental forces. It is best
described by Einstein's own general
theory of relativity and introduces the
smooth continuous fabric of spacetime.
[music]
A fabric that is warped by objects with
mass to provide the illusion of an
invisible gravitational pull. In quantum
physics, on the other hand, the language
of the other three fundamental forces,
electromagnetism and the strong and weak
force, this smoothness [music] is
jarred. In the quantum world, everything
comes in blocks of fixed pre-ordained
sizes. The universe it describes is
[music] grainy, the very opposite of
that described by Einstein in his
theory. These two worlds seemed
unresolvably at odds.
>> [music]
>> Remarkably, however, string theory,
despite being bruised and battered by
its run-in with quarks and quantum
chromodnamics, offered a ray of hope, a
faint glimmer of a theory of everything,
a potential way to explain all four
fundamental forces [music] using the
same language, the language of vibrating
strings.
Originally, physicists had only
considered open strings, [music] those
where the ends were free to dangle like
an ordinary piece of rope. A photon, for
example, the particle that carries the
electromagnetic force, is represented by
a single vibrating string. The ends of
the string represent charged particles.
At one end is the electron and at the
other end is its antimatter partner, a
particle called a posetron. The photon
string is literally the link between
them, explaining the attraction between
their opposing electric charges. When a
particle meets its antimatter
equivalent, they annihilate one another
with their combined masses turning into
a flurry of energy. String theory,
therefore, elegantly paints annihilation
as the simple act of two string ends
meeting. So far, so straightforward. But
it was only when string theorists
started exploring the properties of
closed strings. Those were the ends
joined together to form a continuous
loop that their jaws began to drop. And
to understand [music] the importance of
closed strings to a theory of
everything, we need to talk about a
strange property of subatomic particles
called spin.
About a century ago, physicists realized
that orbiting subatomic particles like
electrons behave like they were
generating two magnetic fields. Magnetic
fields are created when electric charges
move. So, it followed that if the
negatively charged electron [music] has
two magnetic fields, then it must be
moving in two different ways. An obvious
movement was the electron's orbital
motion around the nucleus of an atom.
But physicists wondered whether
electrons could also be spinning [music]
on the spot. If true, electrons would be
a bit like the Earth, which both spins
on its axis and orbits around the sun.
However, physicists quickly realized
their mistake. In order to generate that
[music] magnetic field, electrons would
have to be spinning faster than the
speed of light. And of course, that
would be impossible. So while physicists
no longer think of subatomic particles
as actually spinning, they still use
[music] the word spin when it comes to
describing the source of that second
magnetic field, which turned out to
simply be a property of the particle,
not something caused by actual spinning.
Different particles have different spin
values and that's what distinguishes the
two families of particles from one
another. Firmians and Bzons. Firmians
such as electrons, quarks, and neutrinos
all have spins equal to a half. By
contrast, bzons always have a spin
that's equal to a whole number. The Higs
Bzon, for example, has a spin of zero.
Other Bzons are responsible for carrying
the electromagnetic, weak, and strong
forces, and all of them have spins equal
to one. Yet, according to string theory,
the vibration of a closed string will
always represent a particle with a spin
equal to two. And though it may not seem
like it, this subtle realization was a
landmark moment both in the storied
history of string theory and in the
entire history of physics itself.
For the defense, this was exhibit A.
Long before string theory, physicists
had speculated about a spin 2 particle.
Now it appeared to fall naturally out of
closed strings. The jury surely had to
take notice. As so to hammer home their
case, the defense called Soviet
physicist Dmitri Blockv to the [music]
stand. Blockins was a big hitter in
Soviet society. Indeed, he once wrote a
book on the implications of quantum
mechanics that aligned physics with
Marxist Leninist philosophy. It won him
the Stalin Prize in 1952 and four years
later he was made a hero of socialist
labor. When Dincan died in 1979, his
obituary was signed by Soviet leader
Leonard Brenv personally who lamented
that Soviet science has suffered a heavy
loss. And it was in 1934 working
alongside Fodor Galperin that Blockinf
coined the word graviton [music] to
describe a hypothetical particle that
would carry the gravitational force.
The other three forces, electromagnetic,
weak, and [music] strong, can all be
neatly explained by the exchange of
particles. These particles have been
detected through experiments [music]
time and time again, like the photon for
the electromagnetic force. However, no
one has ever found a graviton. This is
because the idea of a graviton has
deeper problems. The other three forces
and their Bzons can all be [music]
explained by a set of rules known as a
quantum field theory. A type of theory
that physicists can use clever [music]
tricks to keep stable. With these other
three forces, physicists can
mathematically [music] patch up any
holes that appear in their theories
using a trick called reormalization.
Not so with gravity. Instead, patching
one hole seems to create several more.
Trying to patch [music] those holes
creates more still. In other words, a
quantum field theory of gravity, one
that contains gravitons, quickly breaks
down at very high energies.
Of course, this hasn't stopped
physicists [music] from working out just
what properties a graviton should have.
First of all, it has to be massless,
just like the photon. The [music]
distance over which a force operates,
its range, is linked to the mass of the
Bzons that carry it. The heavier the
Bzon, the shorter the range. [music]
Gravity, just like the electromagnetic
force, has an infinite range. And so,
gravitons must have a mass of zero.
[music] Gravity also affects all matter
in the universe, regardless of other
properties such as electrical charge.
And it turns out the only way gravitons
would be able to do this is if they are
spin 2 particles. Indeed, this
relationship is so sacrosanked that in
fact any massless particle with a spin
equal to two must behave like a
graviton. And according to string
theory, a closed string creates a spin 2
particle. On paper, that's a graviton.
[music]
And it is here that we arrive at the
single biggest [music] attraction of
string theory. The thing that has seen
schools of scientists rally behind it.
The reason billions of dollars have been
poured into the field in the decades
[music] since. A key pillar of its
courtroom defense. And the very idea
that saw it resurrected [music] from the
dead without even trying, merely by
considering what happens if a string
[music] forms a closed loop. Suddenly,
one of the most sought-after particles
in all of physics [music] pops out of
the theory. It was as if the witness had
suddenly confessed before anyone had
even posed the question and in [music]
the same breath laid out the rest of the
evidence. A single theory that would be
able to predict the existence and
properties of the Bzons that carry
[music] all four fundamental forces,
even gravity. And so unsurprisingly,
this version of string theory came to be
known as Bzonic string theory.
This was physics at its most beautiful.
And the defense argued that this
inherent beauty strengthened the case
for Bzonic string theory [music]
even further. By treating the graviton
as a closed string rather than a
pointlike particle, the holes in its
quantum field theory suddenly could be
patched just like the other forces.
[music] There's no runaway chaos that
obliterates the theory at high energies.
And to understand this, [music] let's
imagine two masses inching ever closer
to one another.
Physicists describe gravity using an
inverse square law. This means that
every time you have the distance between
the masses, their gravitational
attraction quadruples.
As the two masses edge closer and closer
together, the attraction between them
skyrockets. And as the gap shrinks to
zero, the strength of gravity becomes
infinite and the theory breaks down. But
the gap can only shrink to zero if the
particles are treated as pointlike
[music] with no width, depth, or height.
Bzonic string theory replaces this idea
with particles as one-dimensional
strings. And so if gravity works through
the exchange of gravitons, and gravitons
are closed strings that have a
dimension, the two masses can no longer
get infinitely close to one another and
still be exchanging gravitons.
With no runaway rampage towards
infinity, physicists can patch the holes
in the theory just as they did with the
other three forces.
This was a prime example of beauty
offering more than mere decoration.
Instead, it offered salvation, the
elegant geometry of strings vanquishing
infinities in a way that pointlike
particles simply couldn't. The defense
had seized the momentum with a simple,
intoxicating idea. The apparent
differences between gravity [music] and
its fellow forces could be explained
away as the difference between a closed
string and an open one. And at the heart
of the matter, the forces were united by
a single notion. Everything is vibrating
strings.
If bonic [music] string theory were
correct, it seemed to offer something
that even Einstein couldn't muster. A
possible path to a theory of everything.
The end of physics as we knew it,
[music] and the beginning of something
far greater.
And yet, if it were that
straightforward, physicists wouldn't
still be arguing over string theory to
this day. There would be no need to put
string theory on trial. [music] No
courtroom, no lawyers, and plenty of
Nobel prizes.
Every trial begins with opening
statements designed to dazzle. The
defense had made its case with elegance
and flare, but now came the
cross-examination,
and the prosecution's evidence was
damning.
In physics, as in art, symmetry is a key
to beauty.
[music]
A large crowd gathers around the tiny
computer screen, eager to watch the
drama unfold. Suddenly, the message
arrives. It's clear, timestamped,
[music] and intact. But 3 minutes later,
the physicist sends it.
Something [music] has clearly gone
wrong. But not with the equipment, with
reality.
The message arrived before it was sent.
Its information traveling backwards in
time. The usually ironclad rules of
cause and effect upended entirely.
This unnerving paradox is the kind of
thing that happens when you introduce
[music] hypothetical particles called
tachons. particles that can travel
faster than the speed of light. Tachons
were a fundamental flaw in Bzonic string
theory which bizarrely seemed to [music]
predict their existence. For all its
elegance and beauty, Bonic string theory
seemed to summon the spectre [music] of
super lumininal travel. The prosecution
pounced. This was not beauty, but a
grotesque gargoyle. Surely the correct
theory of everything couldn't allow
this. And yet Tachons were just [music]
the beginning. For faster than light
particles turned out to be the least of
string theory's problems. They were
merely the first piece of evidence
introduced by the prosecution in a long
and seemingly watertight rebuttal.
[music]
The speed of light as the ultimate
cosmic limit is a cornerstone of
Einstein's special relativity. and
special relativity rests on an
assumption so important that breaking it
would be tantamount to physics perjury.
Lorent invariance.
Lorent invariance is the idea that the
laws of physics don't play favorites.
Whether you're drifting in space,
sprinting down a corridor, or hurtling
past the Earth at 99.9% the speed of
light, the rules should remain
unchanged.
No observer can have a privileged view
of reality. The speed of light is the
same for all observers. And so strip
away Len's invariance and reality starts
to waver. Particles might behave
differently depending on how fast you're
moving. The whole of physics begins to
crumble. In other words, Lorent's
invariance seems non-negotiable.
And yet, when string theorists tried to
apply quantum physics to their elegant
strings, the algebra buckled. Loren
invariance seemed to collapse. And that
wasn't all. The prosecution pressed on.
They reminded the jury that in quantum
theory, particles can't take on just any
value, only certain allowed ones. Yet,
when physicists tried to quantize
bosonic strings, the math spat out
phantom states with negative
probabilities,
ghosts.
Probabilities [music]
usually live on a scale from 0 to one.
Zero means impossible. One means
certain. But bzonic string theory seem
to introduce probabilities below zero.
And a negative probability isn't more
impossible. It's nonsensical. Like
betting on a horse with odds of minus
one, it just doesn't work. And so a
theory that allows such absurdities is a
theory in contempt of court. The
prosecution had landed yet another
hammer blow. But they weren't finished.
They were [music] going three for three.
The string theory also failed to
reproduce a phenomenon already proven
beyond reasonable doubt.
Parity violation. The prosecution now
called its star witness. Shen Shan Wu.
Wu was born in the Chinese province
[music] of Djang Su in 1912, mere weeks
after the sinking of the Titanic.
She'd stay in and listen to the radio
instead of playing outside with other
children. Her father would read her
passages from scientific journals
instead of children's stories. In 1936,
she left China for the United States. It
was a jarring transition. She looked
into attending the University of
Michigan, but women weren't even allowed
to use the front entrance. [music] The
University of California, Berkeley,
promised a more liberal life. Yet, Woo
remained culturally a drift and far from
home, often finding comfort in meals
from a nearby Chinese restaurant.
But scientifically, Woo flourished. She
was recruited into the Manhattan Project
and helped develop the atomic bomb. She
later became known as the first lady of
physics and in some circles the Chinese
Mari.
But her most famous work came in 1956
when she performed a clever intricate
experiment to show that the weak force,
the one responsible for radioactive
decay, violates par.
[music]
To understand par, imagine a clock
reflected in a mirror. The numbers may
be reversed, but the hands still go
round in the usual direction from 12 to
3 to 6 to 9.
The reflected clock behaves just like a
clock in the real world. And so
physicists would say that the two clocks
exhibit parity.
However, not all the fundamental forces
behave this way. And it was Woo that
proved it. Woo trapped spinning cobalt
nuclei in a magnetic field and waited
for some of the neutrons to decay into
protons, emitting electrons in the
process. The first time she ran the
experiment, most of the electrons were
emitted downwards. Look at this
reflected in a mirror, and you'd see the
nuclei spin direction reversed, just as
the numbers on the clock were, but like
the hand on the clock, the direction of
the emitted electrons would stay the
same. And so, Woo tested this reflection
by flipping the orientation of the
magnetic field, which sent the nuclei
spinning in the opposite direction, just
as in the mirror image. Except now most
of the electrons were spat out upwards.
The mirror image did not match what we
see in the real world and so the weak
force did not obey par. Woo found that
there was a left and right to
fundamental physics.
By the early 1970s, Wu's result wasn't
in question. Parity violation had long
been written into the rules of physics,
proven time and time again. And yet once
again, ozonic string theory, for all its
elegance and potential as a theory of
everything, simply couldn't account for
it. It was yet another gaping hole in
the defense's testimony.
Here, the contradiction at the heart of
string theory was laid bare. Sometimes
it wasn't beautiful enough, spawning
hideous tachons. Other times it was too
beautiful, unable to accommodate the
messy asymmetries of the real world like
parity violation. Beauty, it seemed,
could cut both ways.
Tachons, impossible ghosts, [music] no
Lorentin variance or parity violation.
Any one of these would be enough to
consign a theory to the dustpin of
history. And yet, string theory physics
perennial survivor squirmed and wriggled
its way out of all of them.
Cornered and [music] battered, the
defense turned to an unlikely savior.
English physicist [music]
Claude Love Lace.
Love Lace was a paid up member of the
eccentric physicist club. Known to be a
bit of a loner, he would often avoid
major conferences yet still sent in
important scientific papers. In later
life, Love Lace filled his home with
parakeetses [music] and thousands of
classical music CDs. The unusual
combination helped him think. Somehow,
Love Lace had managed to become a
physics professor without ever obtaining
a PhD. He studied one in London, but
dropped out before completing it.
That doesn't seem to have held him back.
For in 1971, Love Lace [music] made a
fundamental but often overlooked
contribution to string theory. He argued
that the universe has [music] hidden
dimensions, secret locked rooms. The
defense urged the jury to imagine,
evidence [music] concealed from view,
but essential to their case.
The world around us seems to be
four-dimensional. There are the three
spatial dimensions, left, right, up,
down, front, back. Then the one
dimension of time. In other words, we
always need four numbers to accurately
describe an object's location. Three for
where it is and another one for when it
is. However, Love Lace wondered what
would happen if vibrating quantum
strings didn't have to be confined to
our fourdimensional world. And slowly
but surely, he ramped up the number of
dimensions.
First five, [music]
then six, 7, 8, 9, eventually had a
total of 26 dimensions. 25 space and one
time. String theory seemed to stabilize.
Others, including Holbeck Neielson, ran
with the idea, proving that it both
eliminated the probabilistic ghosts and
maintained Loren variance. Born in
Copenhagen in 1941, Neielson was another
of physics eccentrics. the unckempt
hair, the everpresent bow tie, and
indeed back in the earliest days of
string theory, it had been Neielen who
alongside Nambo and Suskin [music] had
realized that Venitziano's work implied
strings. Unfiltered and unpredictable,
perhaps Lovely and Neielson's maverick
nature was just what was needed to open
up the extra dimensions.
String theory was suddenly on slightly
stronger footing. The defense lawyers
were regaining ground. But this newfound
stability involved a massive trade-off,
one that the prosecution will seize upon
to undermine the case for string theory
time and time again. In order to explain
things, we have a lot of evidence for
normal probabilities and the rent
invariance. String theory would have to
invent 22 additional dimensions that we
have no evidence for. And even then,
this sprawling mega-dimensional reality
can't solve all of string theory's
problems.
Timetraveling tachons were still center
stage, and the lack of parity violation
remained a gaping hole in the theory,
but there were answers to be found
there, too. The defense calls for its
own star witness, Yuri Gund.
Gund was born in 1922 in KKE which today
is Ukraine's second largest city. Even
from an early age, he showed precocious
talent. He started playing chess with
his father aged just [music] four and
soon became the better player,
underscoring the impact he would have
both on physics and the people around
him. Fellow physicist and Nobel Peace
Prize winner Andre Sakarov once called
Gund a Mozart of physics. Picture a
chessboard like the one Dolphins spent
so much time playing on as a boy. A
chessboard has 64 squares divided
equally between black and [music] white.
In other words, for every white square,
there is a corresponding black square.
In this way, a chessboard has an
underlying symmetry. In 1971,
Gund became one of the first to propose
that the subatomic world exhibits a
similar kind of symmetry,
super symmetry.
Just as every black square on a
chessboard has a corresponding white
square, so suggested Gund, every Bzon
has a corresponding firm and vice versa.
To name the super symmetric partner of a
firm, you simply add an s before it. So
charmingly, the partner of a quark is a
squawk. An electron is paired with a
selectron and neutrinos are matched to
neutrinos. Indeed, laser string theory
pioneer John Schwarz once quipped that
he wasn't responsible for this slang.
However, the super symmetric partners of
Bzons are named slightly differently
simply by adding an eno to the end. So,
the famous Higs Bzon has a partner that
sounds a little bit like a magician, the
stupendous Higzeno.
Though in practice, physicists often
bend the rules. Hence names like gluino
for the firmionic partner of the gluon.
Although to this day no super symmetric
particles have yet been found. An
absence that the prosecution will pounce
upon later in the case.
And so at the start of the 1970s several
other physicists were also dabbling with
similar ideas. Eventually, they combined
super symmetry with Love Lac's
26dimensional version of Bzonic string
theory and super string theory was born.
Initial results [music] were promising.
This new theory could now account for
firmians as well as Bzons. The inclusion
of firmians could see off the par issue,
providing a way for it to be violated.
It also did away with those pesky
tachons. No faster than light travel, no
effect preceding cause. What's more, the
number of spatial dimensions came
crashing down from Love Lacy's unruly 25
to a slightly more manageable nine. And
so, having been on the ropes for much of
the prosecution's cross-examination, the
defense team had rallied. For each of
the major issues with the original
Bzonic string theory, super string
theory now provided an alibi. What had
looked like a lost cause had yet again
surged back to life. Beauty had become
the compass once more, pointing towards
a hidden country where string theories
problems seem to dissolve into the mist.
But that wasn't the end, for the
prosecution were forced to pivot, honing
in on a new line of attack and not an
unreasonable one.
If these extra dimensions exist, why
can't we see them? Six extra dimensions
that we had no evidence for were hardly
more credible to the jury than 22. And
so, for [music] String Theory's case to
remain stable, string theorists
desperately needed an answer to a very
basic question. If these extra
dimensions really did exist, [music]
where were they?
Why, you may ask, do the string
theorists insist that space is
nine-dimensional?
Simply because string theory doesn't
make sense in any other kind of space.
Theodore Kuza stands on the edge of the
water, ready to prove a point. Now in
his 30s, [music] the polygot theoretical
physicist never learned to swim as a
child. But who needs practice when you
have books and theories, Kuza has
recently swatted up on how to swim in
the pages of a manual. And upon entering
the water, he duly swims at [music] his
first attempt, a triumph for the
theoretical over the practical.
Kuza's mind was certainly [music] a
fertile one. He could speak at least 15
languages and like literature and
philosophy almost as much as mathematics
and physics. And like many physicists
before and since, [music] Kuza became
transfixed with the idea of unification.
Just as medieval kings would try to
unite their respective kingdoms through
key and strategic marriages, physicists
[music] try to do the same with
theories. Why have two different
theories when a single combined theory
could suffice? Kuza's big idea was to
seek a matrimonial bond between
Einstein's general theory of relativity
[music] and the rules of
electromagnetism, a love match between
gravity and [music] light. The obstacle
to such wedded bliss is that the
mathematics of each of the theories is
fundamentally incompatible with the
other. As we've seen before, the
equations explode in a fit of
infinities. And yet, in a 1921 paper,
Kuza was able to get them to gel under
one condition, that the universe [music]
had five dimensions, one more than the
four we are used to. Kuza was clearly
excited by the benefit of adding an
extra dimension of [music] space,
writing in his paper that it could not
amount to the mere alluring play of a
capricious accident. [music] And indeed,
when he wrote to Einstein, the great man
was more than encouraging. The idea of
achieving this through a [music]
five-dimensional world has never
occurred to me and is probably new. I
like your idea immensely.
But of course, as with any attempt to
explain the world through extra
dimensions, the fact that we don't
experience more than four dimensions in
our everyday lives was the proverbial
elephant in the room. If Kuza's extra
dimension really did exist, where
exactly was it? A potential answer would
come from Swedish physicist Oscar Klene.
The son of Stockholm's first rabbi,
Klene became enchanted with science at
an early age. Years later, he would
recount the time when he borrowed his
mother's opera [music] glasses and used
them to stare up at the stars. Working
his way through school and university,
Klene ended up studying chemistry
[music] before pivoting to physics. And
independently to Kuza, he came up with
the idea of a five-dimensional universe.
However, this [music] time offering up
an explanation for why we don't see this
mysterious fifth dimension. It is Klene
argued curled up incredibly small. How
small? On the order of the plank length,
the smallest meaningful distance in the
universe.
When we think of truly small things in
physics, one of the first objects that
comes to mind is the atom. Indeed, a
typical atom is just a tenth of a
nanometer across. You'd need to line up
150 million atoms side by side in order
to match the width of one of your
fingernails.
Small, but not small enough for the
plank length is 16 trillion trillion
times smaller still. In fact, the plank
length is to the width of an atom, but
the width of an atom is to the size of
the entire solar system. In the language
of physicists, the extra dimension in
Kuza Klein theory is compactified.
In other words, it is so vanishingly
[music]
small that we can't see it. And to see
how this works in practice, imagine an
ant crawling along the branch of a tree.
From some distance away, the tree looks
flat and two-dimensional, as if painted
on some faroff canvas. But the ant knows
the truth. There's a third dimension,
and safe in this knowledge, it can
happily crawl around the circumference
of the branch. In the same way our
universe would look four-dimensional
from up here on the scale of plants,
people and planets, but there could be
[music] compactified curled up
dimensions languishing in the hidden
depths of reality.
Despite receiving a warm reception from
several towering figures at the time,
Kuzu and Klein's insights were largely
conceded to a mere footnote in the
history of early 20th century physics
until string theory once more summoned
the spectre of hidden dimensions in the
1970s.
By the start of the 1980s, superstring
theory predicted the existence of no
fewer than six extra [music]
spatial dimensions. The prosecution was
circling. With the Tachion problem
subdued and super symmetry now on life
support, [music] string theory still had
to answer for the six extra spatial
dimensions that stubbornly refused to
show themselves. The court was losing
patience. If the defense had no credible
explanation for where these hidden
dimensions were hiding, the whole case
might collapse.
Enter the expert [music] witnesses for
the defense. Auheno Kalabi and Shing
Tang Yao.
Just like Yuri Gund, both Kalabi and Yao
had had their political troubles. Kalabi
was born into a Jewish family in Italy
in 1923.
Family was forced to flee when Kalabi
was just 15. Kalabi eventually settled
in the United States.
Yao, however, had ended up stateless for
more than a decade. He'd grown up in
Hong Kong in the 1950s, but the British
consulate removed his status in 1978
after he was granted US residency.
However, he didn't gain US citizenship
until 1990. [music] And so, when Yao won
the prestigious Fields Medal in 1982,
the mathematical equivalent of a Nobel
Prize, he didn't hold a passport of any
country. [music]
Yao's big breakthrough in the late 1970s
was to build on the early work of Kalabi
to develop what are now known as Kalabi
Yao manifolds.
To understand the Kabi Yao manifold,
imagine you have a 9-dimensional piece
of origami paper that represents space.
Simple enough, sort of, except you now
have to fold it up so that it only looks
three-dimensional, matching the three
spatial dimensions we experience
daytoday. But if you fold it
half-hazardly, your new
three-dimensional universe will have no
super symmetry, a key part of string
theory. This means that the extra
dimensions aren't some mere hidden
inconvenience.
They play a pivotal role in the shaping
of our reality. They are the silent
invisible conductors of the great
superstring orchestra. It turns out that
only one incredibly ornate and intricate
folding process works. One with all the
creases perfectly aligned so that super
symmetry survives. And that final
origami shape is the calabi yao
manifold.
Even then, however, only some super
symmetry survives the folding process.
Not all, just enough that it can still
explain the particles we see in the
subatomic world and the interactions
between them.
And so the folds of Calabi and Yao were
undeniably beautiful, but the
prosecution was suspicious. [music]
Was this physics or mere mathematical
stage craft? And so it took a while for
the abstract mathematical concept of
calabao manifolds to cross over into
physics and reach string theorists.
[music] But by 1985 they had built it
into their ideas. Once more they'd taken
a defibrillator to a dying theory. Super
string theory rose Lazaruslike once
again. The realization that calabao
manifolds could curl up the six extra
dimensions while still preserving super
[music] symmetry was a huge part of what
is known today as the first super string
revolution. It granted string theory a
[music] stay of execution, a lastminute
reprieve, but not a pardon. Indeed, the
[music] prosecution justifiably argued
that such intricate and unobservable
constructs amounted to little more than
theoretical hearsay. Calabia manifolds
weren't evidence. They were just an
elegant alibi engineered to [music] hide
a lack of evidence. String theory was
still very much on trial and the stakes
[music] were rising.
And so the courtroom buzzed as two more
key witnesses prepared to testify.
Michael Green and John Schwarz.
[music]
Imagine that the universe is a
smartphone and that a theory of
everything like string theory represents
the phone's operating system. It powers
the phone's key processes ensuring they
all run smoothly.
In the early 1980s, the string theory
operating system kept producing
anomalies in the universe. Deep bugs in
the code that would make the apps crash
and the clock skip. Clearly, any theory
of everything worth its [music] salt had
to be more stable. much more stable. And
so in 1984, Green and Schwarz invented
the equivalent of a software patch known
as the Green Schwarz mechanism. It
deployed a sort of diagnostic tool into
the code that could identify and fix
bugs in real time. And at a stroke,
Superstring theory was stable.
In a sense, beauty had been called to
the stand again, but not the soaring
elegance of symmetry this time, but the
precise economy of a patch that saved
the theory from collapse.
To the defense, it showed resilience and
adaptability under pressure. But to the
prosecution, it was but a bandage over a
deeper wound, a clever fix masking
fragility.
But of course, the case for and against
string theory is never straightforward.
It's arguably the most complex trial
physics has ever faced, which is exactly
what makes arriving at a verdict such a
gargantuan and intimidating task. And
indeed, under cross-examination, the
prosecution found a hole in Green and
Schwarz's testimony.
In order for the bugs to be fixed and
for the resulting strings to still carry
charges and interact with one another,
only certain symmetries were allowed.
Even more careful and specific folds in
the intricate origami of the universe.
And there was an even bigger problem.
Suddenly, there wasn't just one coherent
superstring theory that worked. There
were three. Which one, if any,
represented the true operating system of
the universe? String theorists were
about to sink deeper into the quagmire.
At the beginning of the 1970s, after
Quarks had put the original string
theory to the sword, there had been a
massive exodus of researchers from the
field. The number of people working on
string theory had dwindled to a handful,
toiling away on the fringes of physics.
But the combination of the green schwarz
mechanism and the calabao manifolds
ushered in the first superstring
revolution, convincing many that string
theory was viable once again. And so a
tsunami of new physicists joined the
fry. And among them were four physicists
from Princeton University. David Gross,
Jeffrey Harvey, Emil Martin, and Ryan
Rome. They quickly became known as the
Princeton String Quartet.
Together they worked on the idea of
heterotic strings. A heterotic string is
a chimera. It is a hybrid of a super
string and the original bzonic string.
The Princeton string quartet realized
that there were two different heterotic
superstring theories that could also run
like the universe's operating system
without bugs, still yielding the
necessary super symmetry and summoning
all the particles and forces to boot.
Add those two theories to the three that
Green and Schwarz had discovered. And
there was no longer a single super
string theory, but a total of five
possible super string theories. Any one
of them could be the long sought after
theory of everything or none of them.
And so let's summarize the case so far.
String theory [music] started out
simple. Particles were vibrating
strings, not pointlike. But abominations
like tachons entered the fray, and
suddenly the universe needed to have 22
extra dimensions of space. First, super
symmetry was required to vanquish the
tachons. But this [music] proceeded to
predict the existence of a whole range
of particles that still no one has ever
seen. Although super symmetry did manage
to reduce the number of extra spatial
dimensions from 22 to 6, but these still
needed to rather conveniently be curled
up so small that they remain out of
sight. And the only way to [music]
actually curl them up and preserve super
symmetry was by following the elaborate
folds prescribed by Kalabi and Yao. Even
then, only a watered down version of
super symmetry remained. [music] a
diluted superstring theory that was
unstable and less patched by the green
Schwarz mechanism and that created five
possible superstring theories instead of
[music] one.
The courtroom was drowning in
contradictions.
Five different versions of string
theory, each [music] promising to be the
one true narrative.
In a real trial, five mutually exclusive
alibis are rarely [music] a sign of
innocence. And that wasn't all. The
prosecution was about to call in a heavy
hitter. Richard Fineman took the stand
and with surgical precision tore through
the defense's increasingly tangled
testimony.
String theorists don't make predictions,
they make excuses. I do feel strongly
that this is nonsense. I can't help it.
Perhaps I could entertain future
historians by saying I think all of this
super string stuff is crazy and is in
the wrong direction. I don't like that
for anything that disagrees with
experiment, they cook up an explanation,
a fixup to say, well, it still might be
true. It has to be excused most of the
time. Doesn't look right.
Here, the prosecution landed its
sharpest [music] counter punch yet.
Beauty, they argued, hadn't become a
compass, but a mask, a way of evading
reality whenever reality failed to
comply. Every time string theory was
challenged, instead of collapsing, it
shapeshifted. And so, if string
theorists were going to make headway and
convince the restless jury, they clearly
were going to have [music] to choose
which string theory was the right one.
Or maybe they wouldn't have to.
It was very clear that if I didn't spend
my life concentrating on string theory,
I would simply be missing my life's
calling.
Is this on?
It was with these innocuous words that
Ed Wittton stepped to the front of the
lecture theater at the University of
Southern California in 1995.
Wearing a black suit, a red tie, and
thick rimmed spectacles, his voice was
high and squeaky, and he was about to
change physics forever.
>> Indeed, physics has often been reshaped
by nomads who crossed scientific
borders.
Einstein, the patent cler who remade
space and time. Claude Love Lace, the
mathematician who solved string theory's
problems through extra dimensions when
no one else could. But nobody has
embodied this pattern more completely
than Ed Witten.
Whitten initially studied history, then
tried economics, then philosophy. Only
then did he finally turn to physics. And
yet within a decade of doing so, he was
being hailed as the greatest living
theoretical physicist. [music]
A man whose command of mathematics was
so complete that colleagues compared him
not to other scientists, but to Mozart.
It's hard to put into words the esteem
with which Whitten is now held by large
parts of the physics community.
By 1990, people were already calling him
the smartest living physicist. And that
year he became the first physicist ever
to receive the prestigious mathematics
prize, the Fields Medal. So when Witten
stepped up to speak at the Future
Perspectives in string theory conference
in March 1995,
expectations were already skyhigh. Heads
turned, notebooks opened, everyone
wanted to hear what Witten had to say.
Within physics, the big revelations
don't arrive in woodpaneled courtrooms
[music] or in front of gavl wielding
judges. They happen at blackboards,
[music] in lecture theaters, and on the
conference floor. And so on that day at
the University of Southern California,
the stage became the deacto courtroom of
theoretical [music] physics. And the
jury was packed. But Witten hadn't come
to this makeshift court empty-handed.
The key to his new defense, something
called dualities.
Physics is full of dualities. Perhaps
the most famous is wave particle
duality. Sometimes light behaves like a
wave, but other times it behaves like a
particle. It all depends on exactly how
you look at it. In the years after the
first superstring revolution, physicists
began to look for dualities between the
five possible superstring theories in an
effort to make sense of them. And in
doing so, interesting relationships
began to emerge.
In 1987, Balacandrron Satia Palan
published an obscure, initially
overlooked paper in which he explored
the idea of T duality. In this case, T
stands for target space, the regions
that strings are able to move in.
To understand this, imagine that one of
the extra hidden dimensions invoked by
superstring theories is curled up in a
circle or a loop. A string like this can
do one of two things. It can either
travel in a circle along the loop or it
can wrap and twist itself around the
loop. Satia Palan's big insight was to
realize something remarkable. A string
traveling along a small circle is
exactly the same as a string wound
around a large circle. Similarly, a
string traveling along a large circle is
the same as a string wound around a
small circle. And this meant that two of
Green and Schwarz's superstring theories
were identical. The apparent differences
simply stemming from the fact that we're
looking at them through differentiz
lenses. Like light behaving as a wave
when traveling and a particle when
interacting, at their heart they are one
and the same. One heads, the other
tails, but it's still the same coin. And
as well as this, the two heterotic
string theories were also found to be
identical in this way. T- duality means
that really there were only three
superstring theories, not five. We
shouldn't double count those that are
mathematically identical. And Tuality
was just the beginning.
Crystal clearar waves lap [music]
against miles and miles of soft white
sand.
Nestled in the Caribbean Sea, 12 mi off
the coast of Venezuela, Margarita Island
is a little slice of paradise. And in
the summer of 1974,
14-year-old Anamaria Font moved to the
island with her family. [music] It was
on Margarita Island that Anamaria would
complete her final year of high school.
And despite the remote location, Font
fondly recalls how the chemistry and
physics lectures included regular lab
sessions. She soon became enthralled.
After leaving Margarita Island behind,
Font moved to the Venezuelan capital,
Caracus, [music]
to study physics. Next on her meandering
itinerary was Austin, Texas, where she
would complete her PhD in 1987 amid the
heady days of the first superstring
revolution.
And 3 years later came a sizable
breakthrough. In 1990, a new type of
duality entered the lexicorn of string
theorists. [music] Thanks to the work of
font and her colleagues Lewis Ibanz,
Dieter Lust and Fernando Cavdo,
something called s duality was applied
to string theory for the first time or
to give it its full name, strong weak
duality.
To understand suality, let's return to
the shores of Margarita Island.
Water is a miraculous material. It can
be liquid as in the Caribbean Sea, but
it can also become ice and steam. These
three states of water look and behave
incredibly differently, but they all
contain the same underlying water
molecules. The only difference is
temperature.
When it comes to suality, temperature is
replaced with interaction strength. How
likely strings are to break apart or
join up with other strings. In liquid
water, the Hto molecules are relatively
free to move and interact weakly.
Whereas in ice, they are rigidly bound
and interact strongly. These two
differing amounts of particle
interactions are responsible for the
contrasting behavior of water and ice.
But it is still just one substance in
two different states.
And so through their work on suuality,
Font and her colleagues were able to
show that one of Green and Schwarz's
string theories was due to one of the
heterotic string theories of the
Princeton string quartet. Not two
separate theories at all, but a single
theory glimpsed through two different
lenses, as if ice and water had been
misfiled as separate substances.
The discovery of both T and sualities
clearly showed that the five superstring
theories were not to be considered in
isolation.
There were deep, intimate and profound
connections between them and this was
more than enough to turn Edwitten's head
and he ferociously set about exploring
further links between the five
superstring theories.
When Witten stepped up to the overhead
projector at the string theory
conference in March 1995,
he was ready to reveal to the audience a
series of staggering truths.
The universe does not have 10 dimensions
after all. He said it has 11. There were
not five different super string
theories. There was just one.
The audience looked on wideeyed and
wide-mouthed.
However, it would be wrong to give
Whitton all of the credit. Indeed, his
talk was not the first hint of a
possible 11th dimension. Back in 1978,
Eugene Kmer, Bernard Julia, and Joel
Sherk had shown that 11 was the maximum
number of dimensions possible if you
wanted to preserve both super symmetry
and a single type of graviton. This
11dimensional theory was known as super
gravity. The trouble was, physicists had
no evidence it was true. Just weeks
before Whitten's [music] talk, British
physicists Christopher Hull and Paul
Townsend proposed something else
extraordinary. That the five known
string theories and 11dimensional
supergravity were not separate at all,
but different parts of one deeper
theory. Their paper, Unity of
Superstring Dualities lit the match.
Witten set it ablaze.
One thing that is certain is that
Whitten was the one to name this brave
new world. He called it M theory and
physicists once again had a single
theory of everything. Whiten is now
regarded as a landmark moment in string
theory and quickly ushered in the second
string theory revolution. Just over a
decade after the first, the defense team
had wrenched back the upper hand. To
understand the basic idea of M theory,
take a look at three shadows on the
inside of a box. The shadows are all
different shapes, so it would be natural
to assume that they are being cast by
three separate objects. However, they
aren't. With one 3D object, three
different 2D shadows can appear
depending on the direction of the torch
beam. And in the same way, in Witten's
mysterious new world, there is only one
11dimensional theory. The five
10-dimensional superstring theories that
physicists have been struggling to
choose between were the equivalent of
the shadows. Everything was actually
one.
There has been much confusion over what
the M in M theory is meant to stand for.
Whitten himself has said that it could
stand for magic or mystery, but much
later did concede. I thought my
colleagues would understand that it
really stood for membrane.
Membranes or brains for short play an
indispensable role in M theory. If a
string is a one-dimensional object, one
specific type of brain is a
two-dimensional sheet. And Whitten
realized that if you wrap one of these
2D brains around [music] the 11th
dimension, then it would look like a 1D
string. Just as compactifying six
dimensions of a 10-dimensional
superstring theory looks like our
everyday 4D world, compactifying the
11th dimension of M theory looks like
the five 10-dimensional superstring
theories.
And so M theory relegates strings from
their privileged position as the most
fundamental building blocks of the
universe. Everything is atoms. Atoms are
subatomic particles. Subatomic particles
are strings. And strings are just one
kind of brain. And yet there is more
than one type of brain in M theory. And
understanding what they are and the
differences between them is a vital step
in grasping what followed. Now that
Witten had opened the floodgates and
kickstarted the second superstring
revolution.
Imagine reality is a grand and beautiful
library. The open strings that make up
ordinary particles are its books.
However, brains are what the library
itself is built from. Just as in a real
library, books cannot simply float in
midair. They must rest on shelves. These
shelves are called dirishle brains or
dbrains for short. Without Drains, open
strings would have nowhere to end and
many of the familiar subatomic particles
and forces we know would vanish. As well
as this, there are the more structural
Mbrains, the equivalent of the library
walls. M theory contains two fundamental
types. The two-dimensional M2 brain and
the five-dimensional M5 brain. Wrap an
M5 brain around the 11th dimension in
just the right way. And from the
perspective [music] of a 10-dimensional
theory, it can appear as a Dbrain. At
first, this may not seem so strange, but
it's actually like folding part of a
vast wall in the library so that it
becomes a row of shelves. From one
perspective, it's still a wall. But from
another, it now supports books. Dbrains
emerging from the geometry of Mbrains.
Simple.
And mere months after Whitten's talk, a
further crucial breakthrough would
emerge. As the second superstring
revolution reached fever pitch, Joseph
Pchinsky was ready to testify with the
most surprising evidence yet.
I could feel my heart pounding. It was
one of those moments when you realize
that something deep and true has been
found, that the world is not as you
thought it was. Writing in his memoirs,
Pulchinsky recalls how it felt to make
the breakthrough of his life, a life
tragically cut short. Those memoirs were
written while he was battling brain
cancer, a battle that he would
ultimately lose in 2018 at the age of
just 63. If there's one thing I hope I'm
remembered for, it's the realization
that DBrains are physical objects.
Before Pchinsky, Dbrains were treated
like the Earth's [music] equator or
rotation axis. Useful but imaginary.
Lines on a map don't bend [music] the
planet. But Bolchinsky showed that
Dbrains are real. We wouldn't be able to
see them directly. They live in the
extra dimensions string theory demands.
Yet, they would be physical objects.
They have mass. They carry charge. and
their presence could warp the very
fabric of the universe. What began as
notional shelves for open strings turned
out to be massive structures that can
bend the space within the library
itself.
And so what had been dismissed as
mathematical window dressing was now
material fact. It was as if the defense
had finally produced a key piece of
forensic evidence. Fingerprints at the
scene.
Pchinsky almost didn't publish this key
insight, thinking it might be too
speculative, a hesitation that could
have delayed the entire second
Superstring Revolution. Indeed, it's the
mark of a humble and cautious man.
Pulchinsk's obituary in the New York
Times focused just as much on his
kindness and thoughtfulness as on his
brilliance.
With the second superstring revolution
now firmly underway, physicists had once
more shocked the theory back to life.
What had looked like an intractable
problem, five superstring theories, not
one, had been seemingly [music]
resolved. Once more, there was a single
theory to work with, one that not only
promised to unite quantum physics with
gravity, but also offered a framework
for understanding [music] subatomic
particles and their interactions. All
that was needed was yet another hidden
dimension
for the defense. This was a rare moment
of solid ground. The phantom scaffolding
of [music] the theory seemed suddenly
tangible. The shelves were real. They
carried charge. They bent space. [music]
Beauty, at least for a moment, looked as
if it might be anchored to reality.
However, the prosecution was unmoved.
Not a single experiment had glimpsed
[music] these hidden structures, and so
for now, the jury could only mark it as
another point scored for beauty and
elegance, not for evidence. The defense
had reduced structure, but the
prosecution wanted sightings.
Yet, Pchinsk's discovery did have a
sting in its tail. What if, in some
strange and beautiful way, the
bookshelves knew everything about the
rooms they stood in? What if the laws
etched on these shelves somehow encoded
the architecture of the library itself?
Soon, this would become more than
metaphor. It would become a blueprint
for an entirely different kind of
universe. One where space, time, and
gravity itself might be a projection.
It is more important to have beauty in
one's equations than to have them fit
experiment.
In 1997, a young Argentine physicist
named Juan Maldis published a paper that
shook the courtroom to its foundations
and would go on to become the most cited
in string theories history. Yet Malda
himself never anticipated the scale of
the impact. At the time, I thought maybe
a few people would be interested.
Indeed, to this day, he remains almost
bashful about [music] it. He never
raised his voice and rarely used more
words than strictly necessary, said one
[music] colleague. And so, as others
projected cosmic significance onto his
equations, Maldisena quietly returned to
work, content to let the mathematics
speak for itself.
Maldisena's big idea went by the rather
arcane name of the ads CFT
correspondence. [music]
And this obtuse title belied a dizzying
new possibility born out of the
exploration of strings and brains that
the entire universe could be a hologram.
To begin to understand the intricate
physics behind Malda's ads CFT
correspondence, let's return to the
grand library of the universe.
Pchinsky had shown that Dbrains weren't
just a mathematical framework. They
could be physical objects, the
libraryies bookshelves. Open strings,
like books, could end on them. But more
than that, these shelves had mass and
charge and could warp the architecture
of the library itself. Dbrains [music]
came in various sizes. A D0 brain is a
point-like shelf. A D1 brain is a
one-dimensional string-like one. And a
D3 brain spans three spatial dimensions,
the same dimensionality as the space we
inhabit. Malda Sena asked, "What happens
when you stack many of these D3 brains
[music] together? in effect filling a
section of the library with this
familiar shelving. And what he uncovered
was extraordinary. The sheer mass and
energy of the stacked D3 brains warps
the [music] library to such an extent
that space curves inwards. This inward
bend is the hallmark of anti-deitter
space, the ads in Maldisena's theory.
And there was more. The open strings
moving on the D3 brains gave rise to a
quantum field theory with remarkable
symmetry. A conformal field theory, the
CFT [music] part. And while this type of
theory is not quite our quantum physics,
it shares much of the same mathematical
structure. Not siblings exactly, but
close cousins built from similar
foundations.
And it was here that lay the radical
heart of Maldisa's proposal. This
quantum field theory doesn't live inside
the warped library. It lives on its
boundary.
Imagine a 2D screen on the library's
outer wall showing everything happening
inside. You never need to step into the
hall. Every event is faithfully recorded
on that screen. But this screen is no
mere effect simile. It doesn't just
reflect the interior. It is the interior
written in a different language.
Every physical process occurring within
the library, every book being borrowed,
every rearrangement of its shelves has a
precise counterpart on the screen. Those
events exist there too, fully and
exactly, just expressed through a
different quantum set of rules. This is
Malda's ADS CFT correspondence.
This wasn't a cross-examination or a
rebuttal. It was something else
entirely. Maldisena's holographic
universe didn't just answer questions.
It redefined what the questions should
be. Beauty had returned in a most
unfamiliar guise, forcing the jury to
reconsider the very nature of the case.
A universe with gravity contorting under
the influence of mass and energy somehow
encoded in a lower dimensional quantum
theory that doesn't even know how to
include gravity. The curvature of space,
the relentless pulling of stars on
planets, even the very fabric of
spaceime itself, all of it could be
translated line by line into the cold
language of quantum fields on the
boundary.
And so yes, in this sense, ADS CFT is a
kind of hologram. Think of the holograms
often found on the back of a bank card.
All the information is encoded in just
two dimensions. Yet it gives rise to an
equivalent 3D image when viewed from the
right perspective. Similarly, in this
theory, a fivedimensional world with
gravity could emerge entirely from a
four-dimensional boundary with no
gravity at all.
Concepts this radical [music]
come along once in a generation at best.
If true, Maldosena's idea would be as
paradigm shifting as discovering the
Earth orbits the Sun or that time
[music] dilates with speed. It forces
physicists to question assumptions they
once considered immutable, that gravity
is real [music] and space has substance.
But unfortunately it turned out we
didn't live in this kind of universe.
Antida space has negative curvature
where the angles in a triangle add up to
less than 180° and the best astronomical
observations available at the time
suggested that space is in fact flat
with no curvature at all. But despite
this, however, ads CFT may be pointing
towards something deeper. It suggests
that gravity and spaceime might not be
fundamental, but emergent, woven from
more basic quantum interactions on a
lower dimensional boundary. And so, even
though our universe didn't appear to be
built from anti-deit space, Malda's
duality hinted at a broader principle
that the fabric of space itself could be
a kind of hologram. And nowhere is this
holographic principle more sharply
tested than with black holes, the most
extreme objects in the universe, where
general relativity and quantum mechanics
directly contradict each other. And
resolving that contradiction is
precisely what string theory was built
to do.
In particular, black holes raise a
question that neither theory can answer
alone. What happens to information that
falls in? This is the infamous black
hole information paradox.
And to explain this, the court now calls
Jacob Beckenstein to the stand.
Born in Mexico City in 1947, Beckinstein
saw the spiritual side of science. I
look at the world as a product of God.
He set very specific laws and we delight
in discovering them through scientific
work. He once told the Israeli newspaper
Harets. But if there's any place in the
universe that stands as the antithesis
of heaven, it's probably a black hole.
Inescapable chasms in the very fabric of
spacetime. [music]
They relentlessly heave and hoard.
Matter is drawn inexorably inwards
towards an event horizon, torn limb from
limb before vanishing over the edge of
the gravitational waterfall, gone
forever.
Or maybe not.
From 1972, Ekinstein founded the field
of black hole thermodynamics,
an achievement that would eerily
foreshadow Maldisena's work a quarter of
a century later.
For at its heart, thermodynamics is the
study of a property known as [music]
entropy. A measure of how many ways a
system can be rearranged internally
without changing its outward appearance.
More possibilities means higher entropy.
And so to understand this, imagine
hurling a book over the event horizon of
a black hole.
According to general relativity, black
holes can be completely described by
just three properties. Their mass, their
charge, and their spin. Of course, your
book adds a tiny bit of mass, but all
the fine grained details about what was
on each page. The carefully crafted
characters and narrative arcs appear
[music] to vanish. And therein lies a
problem. For the second law of
thermodynamics explicitly forbids the
destruction of entropy. You must always
increase or remain constant.
Beckenstein's breakthrough was to
propose that the black hole responds to
your book by increasing the area of its
event horizon and that this area
functions as a kind of entropy. And in
doing [music] so, he preserved the
second law and hinted at a deeper truth,
that the black hole's boundary somehow
stores information about everything
inside.
That sounds a lot like ADS CFT
correspondence. [music]
But a deeper problem remained. In 1974,
Steven Hawking showed the black holes
aren't entirely black. They emit
radiation. Indeed, over immense time
scales, [music] it was revealed that a
black hole can evaporate completely.
This therefore created a new paradox.
Hawking radiation was featureless,
carrying no imprint of what fell in.
When the black hole was gone for good,
so was all the information it once
contained. This violated not just
thermodynamics, but the core of quantum
mechanics, which insists [music] that
information cannot be destroyed. And
this was the paradox, a conflict not
between specific theories, but between
the foundational rules of physics
themselves. And it would [music] take
the machinery of string theory and the
radical duality of ads CFT to begin to
resolve it.
And here perhaps the courtroom reaches
its tensest moment. What began as a case
about strings and particles has become a
trial over the very nature of space,
time, and information itself.
Let's return once more to our grand ads
CFT library. From inside the library,
when a black hole forms and eventually
evaporates, it seems as though
information about what it swallowed has
been lost. A book thrown into the
gravitational abyss appears to vanish
without a trace. But the screen on the
boundary wall, the boundary quantum
theory keeps a perfect record. In the
language of the screen, nothing is ever
truly lost. The ads CFT correspondence
tells us that these two descriptions,
the interior view with gravity and the
screen's quantum field theory without
gravity, are mathematically equivalent.
So if the screen evolves in a way that
preserves [music] information, then the
entire library must as well. What looked
like destruction from a gravitational
perspective is from the boundaries point
[music] of view just a particularly
complicated translation.
And so the black hole information
paradox becomes not a failure of physics
but a sign that space gravity and
information are woven together more
tightly than we'd ever imagined.
But of course, though ADS CFT may seem
to provide a compelling resolution to
the black hole information paradox, it
does so in a universe we don't inhabit.
After all, we don't live in anti-deit
space. And so again, the theory's
insights are deep and possibly
universal, but we still don't know how
to apply it to the real universe.
And so with Maldis's duality, the
courtroom reeled. Beauty had taken the
stand in its purest form. An entire
universe projected onto a
two-dimensional screen. For the defense,
it was a master stroke. For the
prosecution, just more speculation in
silk robes, the emperor's new clothes.
Once more, the jury was suspended
between wonder and doubt. Dazzled by
beauty, but starved of proof.
Indeed, there are those that loathe this
approach to physics, not least the fact
that the theory is currently impossible
to test or falsify. Some argued that
physicists were so desperate to
reconcile quantum theory with gravity
that they'd lost sight of the most
fundamental aspect of a good scientific
theory, that it should make testable
predictions.
Physicist Lee Smolen once remarked,
"It's not really a theory. It's a
conjecture about a theory that we would
love to believe in.
The prosecution pounced to them. Malda
Sena's entire [music] testimony rire of
speculation. No crime scene, no
fingerprints, no verifiable timeline,
just elegant conjectures stacked at top
untestable assumptions. Impressively
told circumstantial evidence at best.
theoretical physics, they reminded the
jury, [music] should not be judged by
aesthetic appeal, but by experimental
accountability. Until ADS CFT made
contact with measurable reality, [music]
until it predicted something nature
could confirm or deny, it remained in
the prosecution's eyes an exquisite,
[music]
outlandish guess.
Not for the first time, the defense team
was scrambling. Then, just as they began
to regroup, the universe threw them
another potentially catastrophic [music]
curveball.
You may find that your most elegant
theory, the one you like the best, is
simply wrong.
It is the opening days of 1998. Bill
Clinton is president. Elton Jong's
reworked Candle in [music] the Wind, a
tribute to Princess Diana, is finishing
its 14-week run at the top of the US
charts. Titanic is [music] still packing
cinemas. And against this cultural
backdrop, the American Astronomical
Society is holding its annual meeting in
Washington DC. Journalists and
scientists [music] pack the room to hear
from astronomer Saul Pearl Matter. On
the screen, a decade's worth of data
reduced to dots and lines. What he is
about to reveal would later win him a
share of the 2011 Nobel Prize in
Physics.
Yet, in the courtroom of theoretical
physics, this was a hostile witness, one
summoned from a completely different
field with no loyalty to string theory.
And the evidence PL Mutter presented
would leave the defense scrambling and
hand the prosecution an explosive new
line of attack.
The vast majority of astronomers and
cosmologists accept that the universe
began in an event known as the Big Bang
nearly 14 billion years ago.
It started out unimaginably small, far
tinier than an atom, and has been
[music] expanding ever since, ballooning
into the cosmos we see today. But by
now, many eons later, [music] that
expansion should be slowing down. The
gravitational pull of all the stuff in
the universe, stars, [music] gas,
galaxies, dark matter ought to be
applying the brakes. And in the mid
1990s, as the second superstring
revolution royiled through theoretical
[music] physics, two teams of
astronomers had been busy trying to
confirm that deceleration. It was
supposed to just be a routine check. But
when Palm Matter announced his team's
results at that press conference in
early 1998, they stunned the
astronomical community. For string
theorists, it was a bombshell in open
court, a line of evidence they hadn't
anticipated, and [music] one they had no
ready answer for.
Both teams had been using exploding
stars called type 1A supernovi to track
the expansion history of the universe.
These cosmic detonations occur in binary
systems where a dead dense white dwarf
orbits alongside an ordinary star. Over
time, the white dwarf's gravity siphons
material from its companion, slowly
growing heavier. But its appetite has a
hard limit. Once the white dwarf nears a
critical mass, about 1.4 four times that
of the sun known as the Chantra Shakar
limit. It becomes unstable. A wave of
thermonuclear reactions rips through it
and the star is blown apart. But what
makes type 1A supernovi so useful is
that they all explode with roughly the
same amount of fuel. That means they all
shine with roughly the same intrinsic
brightness. Astronomers call them
standard [music] candles. And by
comparing how bright they appear to how
bright they should be, they can
determine their distance. The dimst are
the furthest away.
Except the two rival teams, Pelmutter's
supernova cosmology project and the high
supernova search team both found
something deeply unexpected.
The distant supernovi were too dim.
not just farther than expected, but as
if space had expanded more than it
should have since the light began its
journey towards us. And the only
explanation was shocking. The expansion
of the universe wasn't slowing down. It
was speeding up. Even today, astronomers
have no definitive answer as to why this
is happening. The accelerating expansion
is usually put down to something called
dark energy, a shadowy entity whose
potency only grows as the universe ages.
What dark energy actually is, however,
remains, well, dark. But what we do know
is that dark energy implies something
about a term in Einstein's equations,
the cosmological [music]
constant, usually denoted by the Greek
letter lambda. Einstein originally
introduced it as a mathematical fix to
hold the universe still. And when Edwin
Hubble discovered the universe was
actually expanding, [music] Einstein
realized his error. But lambda wasn't a
mistake after all. Today, it's clear
that the cosmological constant, the
amount of energy that occupies all of
empty space or something acting like
that is real [music] and it's driving
the acceleration. And more than just
setting space's expansion rate, lambda
also affects its geometry. If this
energy is negative, the universe has
negative curvature. And if it's
positive, as the supernovi data seem to
suggest, the universe has positive
curvature.
And this was and is a big problem for
string theory.
We've seen negative curvature before.
When Pchrinsky stacked D3 brains in our
holographic library, the mass and charge
of the brains warped the space, curving
the library walls inwards. Indeed,
that's antida ads geometry with negative
curvature. But now a fundamental
cosmological measurement was telling us
that the real universe curves the other
way. The walls don't cave in, they bulge
outwards. In geometric terms, we didn't
live in an anti-deitter [music]
space with negative curvature, but in a
desitter-like universe where the
curvature is positive and the expansion
accelerates indefinitely. And that's not
merely a mismatch. It strikes at the
heart of the most mathematically mature
part of M theory. ADSC for all its
evidence turned out to be built on a
geometry that clearly clashes with
astronomical observations.
Speaking in 2001, Ed Witten said, "I
don't know of any clear way to get to
sitter space from string theory or M
theory."
This was a damning admission from String
Theory's biggest star witness. The
defense [music] was once again on the
ropes. With no explanation for dark
energy, the prosecution smelt blood. And
though what followed wasn't a knockout
blow, it might be the defense's last
chance to stay on [music] its feet.
Desperate, the defense team called
Renata Kosh to the stand.
Kosh was born in 1943 in the culturally
rich city of Chinzi, now part of Ukraine
amid the upheaval of World War II. She
came of age during the Kruev Thor, a
period of relative openness and reform
following Stalin's death. Against this
backdrop, Kosh moved to Moscow to study
physics and by 1968, she'd earned her
PhD from the capital's prestigious Lebed
Physical Institute. She would also find
love there. Seminars on cosmological
phase transitions are typically known as
hot beds of romance. [music] But it was
during one such seminar that she caught
the eye of fellow physicist Andre Linda.
They began talking about physics. And
two years later they met again, this
time at Lake Loga near St. Petersburg.
When I was leaving Loga, I kept Renata's
hand in mine just a second longer than I
was supposed to. [music] Linda recalls.
A few months later, we were married.
The couple recently celebrated their
golden wedding anniversary.
Colossian Linda quickly became a physics
power couple. As the Soviet Union began
to collapse, [music] they left together,
first for CERN and then in 1990 to
accept joint professorships at Stanford
University, home to one of the world's
leading centers for cosmology and
theoretical physics. And they were still
at Stanford in 2003 when they helped
develop an idea that might rescue string
theory from the existential quagmire
brought on by the discovery of dark
energy.
Working alongside Shamid Katru and
Sandep Travetti, they devised what
became known as the KKLT mechanism, a
moniker formed from the initials of
their surnames.
Back in the 1980s, amid the first super
string revolution, physicists had made
headway in curling up the extra
dimensions of string [music] theory. The
work of Calabi and Yao offered a way to
fold the origami of the universe into
intricate compact shapes known as Calabi
Yao manifolds. But even by the early
21st century, this approach faced a
serious problem. The folds weren't
stable. The size and shape of these
extra dimensions could fluctuate wildly,
leading to universes that were either
unstable or unrecognizable. And so, like
the Green Schwarz mechanism before it,
the KKLT mechanism was a patch, a fix
for a deeper flaw. First, it introduced
fluxes, fields that thread through the
folds of the extra dimensions, acting
like invisible tape [music] to hold the
shape in place and prevent it from
warping.
But that alone wasn't enough. These
solutions still tended to produce
universes with negative curvature,
anti-deitter [music] space. And so to
fix this, KKLT introduced antibbrains,
objects with the opposite energy and
charge to regular dbrains. Adding them
tips the balance, finally producing a
universe with positive curvature, just
like the ditter space implied by the
supernova results.
But it turned out that there are a lot
of ways to add fluxes and antibbrains to
calabi yao manifolds. Each one produces
a different version of string theory
resulting in a different version of the
universe with its own laws of physics,
particles, and forces.
And this vast expanse of possibilities
is known as the string theory landscape.
But this landscape doesn't contain just
a handful of different string theories
or even a thousand. Not even a million
billion or trillion different ways, but
at least 100 quincexagent centilion.
That's a one followed by 500 zeros.
It's hard to do justice to just how big
a number that is compared to this 10 to
500 possible universes. The number of
galaxies, stars, or even seconds since
the big bang are mere rounding errors.
Jacob Beckinstein once calculated the
limit of how much information can be
stored in a given region of space. And
so if the entire observable universe
were a cosmic hard drive, it could hold
around 10 the^ of 122 bits of
information. So adding the KKLT
mechanism suddenly gave us more possible
versions of string theory than there is
information available in the entire
universe to list them. No matter how
smart we are, no matter the size of the
computers we build, we can't ever hope
to catalog, simulate, or analyze them
all. Even in principle, the
computational and physical limits of the
universe make that utterly impossible.
And so if string theory had been accused
of untestability before, it was now, in
the eyes of its [music] critics,
irreparably so. To the prosecution, the
defense had exchanged uniqueness for a
wilderness of alibis. One such critic,
[music] Lee Smolan, didn't mince his
words. If an attempt to construct a
unique theory of nature leads to 10 the^
of 500 theories, that approach has been
reduced to absurdity.
The defense argued that this is simply
what frontier physics looks like. Messy,
abstract, and occasionally lost in its
own beauty. The prosecution, however,
saw something very different. A theory
not grounded in evidence, but
permanently insulated from it. And so if
String Theory's defense team were to
stop the judge and jury throwing out the
case for good, they had some key blanks
to fill in. If the universe is
accelerating, not just coasting, but
racing away from itself, [music] then
we're dealing with something far
stranger than anyone had planned for,
instead of a small tweak to the cosmic
script. [music] It was a complete
rewrite of the opening scene.
The trial was no longer just about
strings or particles. With so many
possibilities now on the table, the
courtroom had become something else
entirely. A grand inquest into not just
how the universe exists, but why this
particular version of it emerged from
the vastness of possibility. And
whatever the answer was, it wouldn't be
local. It wouldn't [music] be small. And
it certainly wouldn't leave the early
universe [music] untouched.
The great tragedy of science. The
slaying of a beautiful hypothesis by an
ugly fact.
The sun rises slowly over a picture of
serenity. A glassy pond with not a
ripple to be seen. A cool early morning
mist clings to the calm waters as dragon
flies dance between the reeds. The
silence only broken by the occasional
chirp of a cricket or croak of a frog.
Though it may not seem like it, this
[music] picturesque pond shares a lot in
common with the modern universe. It's
seemingly calm, balanced, and most
importantly, self-contained.
Indeed, according to the theory of the
Big Bang, this cosmic pond grew from a
tiny puddle about 14 billion years ago.
But like any seemingly still [music]
pond, you'd expect to find disturbances
beneath the surface, eddies, whirlpools,
and currents. Yet, instead of this, the
more physicists studied the universe,
[music] the more unsettlingly still it
became. The deeper they looked, the more
uncanny the calm. And so in a sense, it
wasn't what they saw that was alarming.
It was what they didn't see.
To begin with, there was the flatness
problem.
Just like the surface of the serene
pond, [music] space today appears to be
very close to what astronomers call
flat. In other words, it has almost no
curvature at all. Any tiny deviation
from flatness early on should have
become exaggerated as space expanded,
warping the surface into something
visibly bent today. And yet instead,
[music]
it's astonishingly smooth. At first
glance, there are two possible
explanations for this. Perhaps [music]
space was perfectly flat to begin with
and so simply stayed that way as it
grew, a serene puddle growing into a
serene pond. But the odds of that
happening by chance are astronomically
low. The second option is that the pond
ballooned so enormously that any initial
ripples were ironed out, flattened
[music] by sheer size.
But the standard Big Bang model doesn't
allow for nearly enough expansion to do
the job.
And then there's the horizon problem.
Even though the pond is now vast, too
wide for a ripple to have crossed from
one bank to the other since it formed,
the entire [music] surface looks
perfectly coordinated. No currents, no
temperature differences. Every part of
the pond seems to have agreed on the
same conditions. And that shouldn't be
possible. For one side of a system to
know about the other, to equalize,
synchronize, or share information,
something has to travel between them. A
ripple, [music] a current, a message in
a bottle, something. But in our
expanding pond, there simply hasn't been
time for that. The banks are too far
apart. There hasn't been enough time for
[music] ripples or information to reach
from one side to the other for
everything to even out. The speed of
light [music] simply isn't fast enough
to cover the distance in the time the
universe has existed. And yet its
opposite sides still appear identical.
And finally, there's the monopole
problem. predicted particles that like
mythical leviathans lurking in the deep
should be swimming through the cosmos in
droves.
Magnetic poles without a partner, a
south without a north, a north without a
south. Theories that try to unify the
three non-gravitational forces suggest
that at high energies, the early
universe should have produced huge
numbers of them. And yet, we've never
seen one. That's a serious problem for
any theory aiming to describe physics at
the highest energies, string theory
included. And so each [music] of these
three problems alone was troubling. The
three of them together was damning. The
Big Bang was our best model of the early
universe. But it was leaking at the
seams. If no solution could be found, it
might have to be abandoned altogether.
And if even the big bang couldn't be
trusted, how could grander frameworks
like super string theory, which sought
to unify all of physics, hope to
succeed?
Luckily for Super String and the Big
Bang Theory, one man had a single idea
that could patch every leak at once. The
court calls Alan Guth to the stand.
Born in 1947 in New Brunswick, New
Jersey, Guth skipped his senior year of
high school and enrolled at MIT at just
17, earning a bachelor's, masters, and a
PhD all in quick succession. He began
his career in particle physics, studying
quarks before shifting his focus to
cosmology in the mid 1970s. drawn by
puzzles like magnetic monopoles and the
flatness of space. Yet Guth struggled
[music] and went from temporary post to
temporary post until late one night in
December 1979 [music] he experienced
what he described at the time as a
spectacular realization.
Guth figured out that a period of rapid
[music] exponential expansion could
solve not one but all three big bang
problems. [music] And by 1981 he had
formally proposed what we now know as
cosmic inflation and his career
understandably took off.
Inflation is like an underground pipe
[music] beneath the pond. It provides a
sort of pressure surge that for the
briefest moment blasts huge quantities
of water in from below. The puddle
doesn't just grow, it erupts. Then the
pipe shuts off, leaving the pond to
expand at a slower, steadier pace.
According to the theory, this initial
inflationary phase saw the universe's
size skyrocket from considerably smaller
than an atom to about the size of a
grapefruit in just [music] a trillionth
of a trillionth of a trillionth of a
second, doubling in size roughly 80
times. That may not sound like much, but
by the end of inflation, the universe
was roughly 10 ^ of 78 times bigger than
it had been at the start. To put that
number into context, imagine taking an
ordinary shipping container and then
expanding it by the same amount.
Suddenly, it would be larger than the
entire observable universe.
But how did this relatively
straightforward addition solve all the
problems of the Big Bang?
Well, the flatness problem hinged on the
fact that there hadn't been enough
expansion since [music] the Big Bang to
smooth out any initial wrinkles, but
inflation adds in the extra required
expansion. Similarly, the horizon
problem argued that there hasn't been
enough time for ripples to travel
between every part of the pond. Add in
the extra expansion from inflation, and
suddenly the puddle can start much
smaller, much more interconnected, and
still be capable of becoming the
enormous pond we see today. And finally,
what about the missing monopoles? Well,
inflation solves that, [music] too. If
monopoles were formed in the early
universe, they would have been diluted
by inflation's vast expansion of space,
[music]
stretched so thinly across the cosmic
pond that we'd expect to find hardly any
at all.
At first, inflation looked like a
miracle cure. It fixed the flatness and
horizon problems and even swept the
monopoles under the cosmic rug. But
Goop's original idea, although highly
elegant in theory, didn't quite work in
practice. His version of inflation ended
too violently, leaving behind a universe
riddled with bubbles.
And so, as it happened many times before
in physics, [music] Guth's original
model was beautiful, but when pressed,
it cracked under cross-examination. The
defense would need a smoother, [music]
more controlled version of the same
idea. And that refinement would one day
dramatically intersect with the [music]
world of string theory. And it came from
a familiar face, Andre Linda.
Guth's inflationary surge was too
abrupt, like the underground pipe had
burst. Linda added the equivalent of a
pressure valve.
The water still rushed in, expanding the
pond dramatically, but now in a
smoother, more controlled fashion. The
surface could stay calm, coherent.
And that wasn't the end of it. For in
Linda's version, the valve didn't simply
shut off all at once. One region of the
pond might stop swelling and begin
settling, but others could keep
expanding. Some of those regions would
even spawn new valves of their own,
launching entirely new ponds. And this
process would never end. Inflation
became eternal. Our serene pond just won
amongst countless others, each with its
own currents and contours shaped by the
exact circumstances of its [music]
birth. In other words, a vast and
expansive landscape of pawns with each
one having its own particles,
fundamental forces, and laws of physics,
a multiverse.
If this idea sounds familiar, it should.
It's an eerie parallel of the string
theory landscape. And so, here [music]
was a completely separate theory, one
born from an entirely different
perspective, arriving at the same
unsettling conclusion. a vast multitude
of universes, each with its own unique
physics fingerprint. Fixing the Big Bang
theory suddenly brought it into close
alignment with superstring theory. The
defense's case began to glisten.
But [music] just as the two theories
seemed to be converging into something
grander, deeper questions began to
bubble up. Could these lush ponds, so
elegantly imagined, be nothing more than
mirages? [music]
To answer this question, the court
summons Cameron Vafa.
Vafa was born in 1960 in Thran, the
capital of Iran. During a period of
profound political [music] turbulence,
the sha was in the midst of a sweeping
modernization campaign known as the
white revolution. When vafer was four,
Ayatollini, a fierce critic of the sha's
reforms, was exiled. That same year, the
Iranian prime minister was assassinated
in the street. And so, with all of this
happening around him, [music] the young
Vafer turned his eyes a little higher to
the moon. Whenever I would look at the
moon, it was bothering me as to why it
wouldn't fall to the ground. He would
later [music] recall that fact stuck
with me and also why other people were
not bothered by it and treated [music]
it as if it were irrelevant.
These musings would kickstart a lifelong
love affair with the biggest questions
[music] about our universe.
This ensuing quest for answers took Vafa
first to MIT, [music] then to Princeton,
where he completed his PhD in 1985 under
the watchful eye of Ed Wittton. And by
2008, [music] he'd made such important
contributions to string theory that he
was awarded the prestigious Draq medal
alongside Juan Malena [music] and Joseph
Pulchchinsky. And arguably Vafer's most
provocative insight had come three years
before [music] in 2005.
A sharp, unsettling idea that threatened
to throw both string theory and
inflationary cosmology into disarray.
Welcome to the swamp land.
Vafa looked out across the seemingly
boundless landscape of inflationary
ponds, each one a possible universe with
its own shape and physics, and asked a
simple question. Were any of them
actually compatible with string theory?
And in case after case, the answer came
back a resounding no.
No matter how you tweaked the geometry
or rearranged the ingredients, these
pond-like universes refused to form, or
worse, they emerged only by violating
principles string theory seems to uphold
everywhere else, like how forces should
behave or how light particles can be. In
other words, not every mathematically
consistent pond is actually physically
viable. Some of them may look placid and
still, but step in and you'll find
yourself sinking deep into the mud. What
seemed like a serene [music] pond turns
out to be a slurried, stinking mess.
Together, these boggy outcrops make up
Vafer's swamp land. A murky, unstable
region of the inflationary landscape
that cannot be derived from string
theory, no matter how hard you try.
Swampland [music] had serious teeth.
Fafa's conjectures aren't proven, but
they function like legal precedent,
[music]
setting the boundaries any defense must
reckon with. And so what once seemed
like safe theoretical ground was
suddenly treacherous. Vafer wasn't just
surveying the landscape. He was issuing
injunctions.
entire regions were now off limits.
VAF and others now began deriving
specific mathematical tripwires, [music]
constraints that any viable universe
must avoid. Many existing theories
stumbled right into them. One of the
biggest casualties was Colossian Linda's
KKLT mechanism. Most known KKLT type
solutions sit squarely in Fafa
swampland. This is a massive problem for
string theorists because they need the
KKLT mechanism to stabilize the theory
[music] and make it compatible with the
accelerating expansion of the universe.
And indeed, Vafer's trip wires also
entangle the theory of inflation itself.
In particular, they hit slow roll
inflation hard, the gentle controlled
expansion described by Linda. According
to the swampland criteria, the
conditions needed for such a smooth and
steady growth may simply not be allowed.
The process unfolds too slowly or
stretches too far or relies on a kind of
delicate balance [music] that string
theory seems unwilling to permit. For
critics, this was damning. String theory
couldn't comfortably accommodate the
best known explanations for either dark
energy or inflation.
>> [music]
>> If the swampland conjectures are right,
then two of the most widely accepted
pillars of modern cosmology are mutually
incompatible with string theory, not
flawed approximations, [music]
fundamentally incompatible.
Although inflation does at least have
one advantage over KKLT,
it might be testable.
One of the most promising tests comes in
the form of primordial gravitational
waves. Inflation should have left its
mark either through subtle distortions
in the light left over from the big bang
or as faint waves still traveling
through the universe today. And several
experiments are hunting for them.
Groundbased observatories like BICE and
the Simon's Observatory scan the sky for
signatures in the leftover light. Soon a
space-based mission will join the chase.
Lisa, the laser interpherometer space
antenna scheduled for launch in the
2030s, LISA will be the first
gravitational wave observatory in space.
Exquisitely sensitive to low-frequency
waves that groundbased detectors can't
catch. If Lisa does see the right kind
of background signal, it could offer
direct evidence that inflation really
[music] did shake the universe into
being. But of course, even this wouldn't
resolve the deeper question. For proving
inflation shaped our pond is one thing,
but proving [music] it filled a
multiverse of ponds is quite another.
Eternal inflation may be compelling, but
it remains [music] difficult, maybe
impossible to falsify. Indeed, critics
have long warned that it risks [music]
slipping from science into metaphysics.
Paul Steinhart, one of the original
architects of inflation, has since
become one of its most vocal detractors.
>> [music]
>> He now argues that inflation has become
a self-justifying idea, impossible to
rule out and ever more baroque in its
[music] defense. It's the same criticism
we've seen thrown at string theory time
and time again.
And so [music] we reach an impass.
Inflation may be right, but it's
slippery. KKLT may be beautiful, [music]
but it's inadmissible.
The case had grown enormous, a legal
skyscraper of claims and counter claims.
But at its foundation, sat one quiet,
unproven assumption,
super symmetry.
Indeed, the entire [music] defense
rested on it. After all, there could be
no super string theory without super
symmetry. And yet now after decades the
court [music] was finally ready to test
it.
Super symmetry is the distinctive
prediction of string theory that is most
likely to be discovered experimentally.
The spruing ice sheets of Antarctica are
a white, bleak wasteland. There was
nothing for miles save for snowstorms
and blizzards. Yet, as we converge on
the geographic south pole, human-made
castles rise above the perafrost. For
here, on the high plateau of Antarctica,
some 2,835
m above sea level, is the Ammonson Scott
South Pole Station. And among the dozens
of buildings that dominate [music] the
otherwise desolate landscape is Ice
Cube, a state-of-the-art neutrino
observatory. And yet the truly
impressive [music] part of Ice Cube is
found underground.
In 2005, the very same year Cameron
Vafer went public with swampland,
engineers began using drilling towers
and warm water hoses to board deep holes
into the Antarctic ice. [music]
Into each they lowered long strings with
detectors hanging on the ends. The harsh
conditions and inhospitable winters
where the sun doesn't rise for months on
end meant ice cube took five long years
to complete. Thousands of sensors
[music] were spread over a cubic
kilometer of ice, making it the largest
neutrino telescope in the world. And
indeed, Ice Cub's bread and butter is
high energy neutrinos produced [music]
by calamitous events in the deep faroff
universe.
And yet, it also has its subterranean
eyes set a little closer to home. For
Ice Cube is on the lookout for nutrinos
being held up from the core of the
Earth. [music] nutrinos produced by dark
matter.
If dark energy [music] had been an
agonizing thorn in string theory's side,
dark matter might yet prove a powerful
ally. As we've seen already, dark energy
is the mysterious entity that appears to
be accelerating the universe's [music]
expansion. Dark matter, however, in a
sense plays the opposite role. It is the
invisible glue that binds cosmic
structures together. Astronomers first
noticed dark matter's effects as far
back as the 1930s.
Galaxies within clusters seem to be
moving too fast. And one way to explain
why these clusters didn't simply fly
apart is if there was hidden material
inside, exerting an extra gravitational
pull. Indeed, individual galaxies told a
similar story. Stars on their edges
orbited faster than expected. Perhaps
astronomers mused. They too were held in
place by some invisible [music]
adhesive.
At first though, scientists were stumped
as to what dark matter could be made of.
The culprit seemed to have two
properties. It must have mass, otherwise
it wouldn't exert [music] gravity, and
it must all interact with light,
otherwise we'd see it. Unfortunately, no
suspect in the standard model lineup
matched that particular fit. And so they
wondered, could dark matter be lurking
beyond the [music] standard model?
Astronomers interest in dark matter
accelerated in the [music] 1970s just as
string theorists were beginning to
juggle with super symmetry. And it
turned out that the lightest super
symmetric particle, the neutral lenino,
would behave just like dark matter. It
is a form of weakly interacting massive
particle or wimp for short. And from
that moment to this, the search for
wimps gathered pace.
And so it seemed that super symmetry
didn't just streamline string theory, it
also seemed to predict one of the
[music] most fundamental aspects of our
universe. Finding wimps. therefore would
be a ringing endorsement of super
symmetry and bolster the case for string
theory to boot. And indeed, despite all
its promise with dark matter, super
symmetry also [music]
had an older alibi. After all, it was
super symmetry that first let string
theory smuggled gravity into quantum
mechanics. Without it, string theory's
cleanest prediction, the graviton,
vanishes. And without the graviton, the
whole defense begins to unravel at a
precipitous pace. And so string
theorists pinned their hopes on the
detectors hidden beneath the ice.
If the galaxies indeed flooded with
wimps, the solar system should be
plunging through them as it orbits
around the Milky Way. Dark matter
particles should continually sink into
the core of the Earth where they would
collide with one another and produce
neutrinos. Neutrinos that Ice Cube has
been looking for. Simple enough, except
for the fact it hasn't seen any. [music]
Indeed, a study released in early 2025
trolled through a decade of ice cube
data and found absolutely no signatures
of dark matter coming from the center of
the Earth. Perhaps scientists thought
another team would have more luck, this
time with an experiment not tucked away
under the Antarctic ice, but hidden a
mile underneath South Dakota in an
abandoned gold mine. [music] Except once
again, the search here with the Lux
Zeppelin experiment also came up short.
But perhaps the heady maelstrom of the
galactic center where [music] stars and
dust are constantly swirling around a
gargantuan super massive black hole
offers more possibility for wimps.
[music] Indeed, another experiment has
been scouring this region for clues. The
alphaagnetic spectrometer bolted to the
side of the International Space Station.
AMSO2 is a hulking 2 billion cosmic ray
detector. Since 2011, it's been
measuring high energy particles that
might be produced [music] by WIMP
annihilations near the galactic center,
and initial results looked promising.
[music] AMSO2 found more posetrons than
expected, raising hopes that dark matter
might be the culprit. But again, the
signal proved ambiguous. Pulsars,
rapidly spinning neutron stars, [music]
could produce the same excess. This
means that more than a decade on,
physicists are still debating what, if
anything, AMSO2 has seen.
And so the result is [music]
frustratingly familiar. Hints,
suggestions, and shadows, but never
certainty. The evidence for super
symmetry through dark matter searches
remains agonizingly elusive.
But there is another way.
The most [music] direct tests of super
symmetry have taken place at the largest
energies achievable on Earth. At the
large Hadron Collider here, beneath
[music] the French Swiss border, protons
are smashed together at near light
speeds in the hope that something
entirely new might emerge.
If super symmetric particles [music]
exist, some should be within the Large
Hadron Collider's reach.
>> [music]
>> The LHC first turned on to much fanfare
towards the end of 2008 and within 4
years it made headlines around the world
when physicists [music] discovered the
Higs Bzon, the much lorded God particle.
The Higs Bzon and the associated
Higsfield is what gives other particles
mass. And yet the discovery of the Higs
threw up a confounding mystery. It was a
lot lighter than physicists had
expected. With mass and energy
interchangeable, the energy of the Higs
field was surprisingly low. And this
meant that super symmetry became a
[music] white knight riding in to save
the day. This is because if ordinary
particles have sparticle partners, they
would partly cancel each other out
leading to a low energy Higs field. Add
in the ability to explain dark matter
and the unification of forces at high
energies and super symmetry seem [music]
to hold the upper hand. And yet that
cancellation only works if sparticles
are also relatively light. The heavier
they are, the harder it is to use them
to square away the lowly mass of the
Higs.
And this was the problem.
The LHC is smashing particles together
with such ferocity that it should be
producing sparticles [music] at the
lighter end of what is possible. And yet
once again, like Ice Cube and Lux, the
silence was deafening. [music] The jury
heard the hush as loudly as any
testimony. As the prosecution pressed
home the advantage, the defense
scrambled for explanations. [music]
Maybe lights particles do exist, but
somehow the LHC is missing them. It's
unlikely, but possible. [music]
More likely is that if sparticles do
exist, they're too heavy for the LHC to
produce at current energies. [music] But
that then makes it nearly impossible for
super symmetry to explain the
featherweight mass of the Higs.
To the jury, it sounded less like
confidence and more like backtracking.
[music] For decades, super symmetry had
been the defense's star witness. Beauty
incarnate. the promise that string
theory could finally make [music]
contact with reality. Now that witness
looked evasive, its testimony collapsing
under cross-examination.
And even that still wasn't the end. The
higher the mass of the sparticles
becomes, the harder it is to use super
symmetry to explain [music] the full
triumpvirate of dark matter, force
unification, and Higs mass. Physicists
[music] are forced to fine-tune the
theory with surgical precision,
undermining much of super symmetry's
original far-reaching appeal. And so,
yet [music] again, when confronted with
a deluge of experimental evidence to the
contrary, the defense team seems to
squirm and wrigle its way [music] out of
it. In an essay aimed at his peers,
physicist male Schiffman has urged them
[music] to stop developing continued
baro-like aesthetically unappealing
modifications to super symmetry. It's a
sharp prosecution argument with familiar
sting. Super symmetry remains beautiful,
but is increasingly edging ever further
away from testable ground.
Once more, the defense team was forced
to pivot. Their original alibi that
super symmetry elegantly explained the
Higs mass was quietly shelved. Now, a
theory once operating on the fringes was
called to the stand. An idea that came
at least in part from Nema Akani
Hamemed.
Akani Hamemed was born in Houston,
Texas, 10 days before Apollo 16 touched
down on the lunar surface in 1972.
His Iranian father, Jafar, worked on the
program for NASA, analyzing the moon's
physical properties. After the sha of
Iran was overthrown in 1979, the family
returned to the country, but it didn't
last long. Ayatollah Kamini quickly
began closing universities and Jafar was
an outspoken critic. He was soon
blacklisted. He'd be hanged if caught.
Faced with little alternative, Jafar
gave up his $50,000 life savings to
[music] people smugglers who were
charged with ushering Jafar, Nema, and
the rest of the family out of Iran on
horseback.
Except it didn't go to plan. 10-year-old
Nema soon developed a fever and
struggled to walk. Jafar ran for help.
Nomads leapt to the family's aid,
carrying the ailing Nema over the border
on one of their horses.
However, Nema's mother still feared he
would die. And so, desperate, she
pointed up to [music] the Milky Way
overhead and promised her son a
telescope if they made it out. She would
later go on to say that she thinks her
promise [music] saved his life.
And that boy in the mountains would grow
up to become one of the most daring
[music] minds in modern physics. A
theorist unafraid to question orthodoxy
even when it means sacrificing beauty.
Colleagues have called a carnihmed a
messiah. And indeed according to
physicist Raman Sundrum, Nenah's
persuasiveness is hypnotic. A lot of
people follow where he leads. [music]
In 2004, Akani Hamemed working alongside
his former adviser Savast Demopoulos
proposed a radical twist. [music]
What if super symmetry wasn't beautiful
and elegant at all? What if nature has
carved it in two? This was [music] split
super symmetry where the symmetry is
still real but its effects are lopsided.
The particles that help unify the forces
and [music] account for dark matter
could still be light. But the particles
that protect the Higs from quantum
instability might be unimaginably heavy.
So heavy [music] that the cancellation
trick no longer works.
In a sense, split super symmetry was a
plea bargain. The defense dropped one
charge that super symmetry could explain
the Higs mass in favor of doubling down
on dark matter and force unification.
Together they still aligned with the
deep structural architecture of string
theory. Super symmetry even in this
fractured form was still playing by the
rules.
But once again there was [music] yet
another catch and this was a costly one.
By lifting the mass of the super
partners to extremely high energies far
beyond the reach of the LHC or any
foreseeable collider, split super
symmetry pays for its survival with
testability. The heavier the sparticles
become, the further into the fog of
inaccessibility they recede.
For the prosecution, this was the same
old story. A theory that once promised
to be falsifiable at LHC energy scales
was now hiding its most distinctive
particles on energy scales we may never
probe. Experimental accountability was
once again being quietly reclassified as
optional. And yet having said this, the
theory is not entirely beyond reach.
Split super symmetry still predicts
relatively light Genos, the super
partners of the force carrier Bzons.
They could remain within the grasp of
future accelerators or maybe even
cosmological measurements. Split super
symmetry also retains viable dark matter
candidates and its predictions for force
unification could in principle be
cross-cheed against future data on
effects such as proton decay.
And so while most sparticles may be lost
to us, the theory's fingerprints haven't
vanished entirely.
And so [music]
to the jury, the question becomes one of
burden of proof. Does the theory's
ability to evolve in light of new data
count in its favor? Or is this yet
another case of shifting the goalposts,
a defense based on beauty once more
slipping into evasion? The defense was
out of conventional [music] witnesses.
What remained was the last resort.
Another radical reimagining of the case
itself. Beauty would return to the stand
one final time, but in a form neither
jury nor [music] prosecution could have
anticipated.
Since we can never know anything for
sure, it's simply not worth searching
for certainty. But it is well worth
searching for truth.
Thunder cracks as the swirling sea toys
with a shipwrecked sailor.
Exhausted, he closes his eyes for what
seems like the final time. Yet the storm
clears and the ocean calms. The gentle
waves then deliver the sailor to the
safety of a sandy beach. As the sailor
wakes, he takes in his new surroundings.
A forest of trees buckling under the
weight of plentiful fruit. A pool of
fresh water. A sturdy shelter long
abandoned.
The castaway is astonished. Of all the
places he could have washed up, why
here? He sits on the warm sand,
marveling at his luck. Yet, some would
say he wasn't really lucky at all.
Indeed, the question of why here can
only be asked on an island where
survival was possible. After all, the
dead could not reflect on their fate.
Within physics, this logic is known as
the anthropic [music] principle. And in
the early 2000s, the defense would come
to rely on it to justify their case. And
one man above all others took up this
cause. a man who had been part of the
string theory trial since the very
beginning. One of the trio of people who
came up with the idea in the first
place.
The defense recalls Leonard Suskin to
the stand.
Suskin [music] may have started out as
the Bronx plumber's son, but he would
become string theory's street fighter,
[music] arguably its most combative
advocate in the face of all the hand
grenades thrown in by the prosecution.
And the anthropic principle would be his
[music] counterpunch. The controversial
principle comes in two forms, weak and
strong. The weak anthropic principle
simply states that the universe must
allow observers like us. Otherwise, we
obviously wouldn't be here. Take for
example the fact that the Earth is at
just the right distance from the sun to
have liquid water. [music] According to
the weak anthropic principle, that's no
coincidence. Indeed, were it not so, we
wouldn't be here to notice it. If the
Earth had formed at a different distance
from the Sun, no one would be here to
ask the question. Like the sailor, we
marvel at having been washed ashore on a
habitable island. [music] But of course,
we could never have emerged on any of
the countless baron rocks where survival
is impossible. Similarly, if dark energy
were more potent and the universe had
expanded faster, [music]
galaxies could never have formed and
stars like the sun wouldn't have
ignited. [music] It's no surprise then
that we observe an expansion rate
compatible with life. However, [music]
the strong anthropic principle goes
further, arguing that the universe is
fine-tuned for life. In this case, the
right conditions for observers to emerge
aren't just allowed. [music] Nature is
compelled to produce them. The emergence
of life not some serendipitous fluke,
[music] but inevitable.
And so when it came to string theory's
failings, Suskin used the [music] strong
anthropic principle to flip the script.
To the prosecution, the vast landscape
of 10 ^ 500 possible universes had been
[music] a fatal flaw. The theory that
was supposed to explain everything, in
fact, explaining nothing. But to
Suskind, the seemingly endless string
theory landscape explained exactly why
we are here.
>> [music]
>> Physicists had long noted that our
existence is balanced on a beautiful but
precarious knife edge. An orchestra of
particles, each [music] with precisely
tuned masses and charges. Gravity is the
conductor keeping time with just the
right amount of pull. Dark energy is the
counter melody introducing just the
right level of push. And so how, despite
seemingly impossible odds, is the
universe so perfectly fine-tuned for
life?
Because, said Suskin, it isn't the only
one.
If the string theory landscape
represents [music] an unfathomably large
number of different universes with the
constants of physics varying in each,
then in which of those myriad universes
are we going to find ourselves? It's as
though the landscape were a sprawling
archipelago. Most of its islands are
barren, storm lashed rocks where no
sailor could survive. But there are a
few rare oases, places with the cosmic
equivalent of fresh water, fruit, and
shelter. And just as the curious
castaway could only wash up on one of
those hospitable shores, so too can we
only find ourselves in one of the
universes where life is possible.
By allying string theory with the strong
anthropic principle, Suskin was [music]
turning the very feature the prosecution
were calling an unmititigated disaster
into a cornerstone of the defense. And
he laid out his case in a [music] 2003
paper entitled The Anthropic Landscape
of String Theory. It was a move that
enchanted some members of the jury while
enraging the prosecution.
Indeed, the defense was now claiming
that string theory was the very reason
the trial itself could even be convened.
No string theory, no trial. By this
logic, given that we find ourselves in a
courtroom, [music]
the jury has to find in string theory's
favor almost by default.
Of course, the prosecution was quick to
object to what they considered
scandalous circular reasoning. To argue
that a theory must be true, otherwise we
would not be here to argue about it
wasn't evidence. It was yet more slight
of hand. Beauty somehow masquerading as
inevitability. [music]
And so to counter what they saw as an
absurdity, the prosecution turned to a
big hitter of their own. A man who would
repeatedly and publicly clash with
Suskin over his use of the anthropic
principle [music] to justify string
theory.
The court calls Lee Smolan to the stand.
Like Suskin, [music]
Smolan is a New Yorker. His own path to
physics top table was hardly
conventional. He dropped out of high
school due to boredom before talking his
way into college. From the outset,
Smolin was an outsider willing to
challenge the orthodox approach. And by
the early 2000s, that challenge took aim
squarely at Suskind. The two men clashed
bitterly over string theory's growing
reliance on the anthropic principle. At
first it was private emails, then
literary agent John Brockman intervened,
staging a kind of intellectual knife
fight on his website, Edge. Each man
would write one final statement, unseen
by the other until both were published.
Smolan did not hold back.
The anthropic principle, he wrote, was
so vague that one can get any conclusion
one wants. It cannot help us do science.
A theory that could never be falsified,
he argued, had no place in the courtroom
of physics. It was little more than
philosophical hearsay. To Smolan,
invoking the sailor's good fortune was
no explanation at all. It was survival
bias dressed up as science.
and Suskin was equally cutting. Let's
not put the cart before the horse, he
warned. Science is the horse that pulls
the cart of philosophy. What critics
dismissed as unfalsifiable, he reframed,
as simply not yet testable. Then he
twisted the knife.
What people usually mean, he added, is
that they themselves [music] lack the
imagination to see how to test the idea.
Next came the alibi. Quarks, Suskin
reminded the jury, were once derided as
unobservable, too. No one could isolate
a single quark, so critics called the
idea [music] metaphysics. And yet,
quarks went on to become the bedrock of
modern particle physics. Indeed,
ironically, it was quarks that had
killed Suskin's original hydronic string
theory decades earlier. Now, he summoned
them back as corroborating evidence.
The clash between Suskin and Smolan was
more than a personal battle. It was the
entire trial in miniature.
Is string theory a bold but unfinished
science or an elaborate ruse hiding
beneath a veil of beauty and
inevitability.
And so after decades of claims and
counter claims, [music] the long-unning
trial had at last reached its final
stage. It is time for the closing
arguments.
For the defense, the allure of string
theory is simple. It is the only
framework that has ever naturally united
gravity with quantum theory without
breaking the mathematics. Other
contenders for a theory of everything
exist like Smolan's own loop quantum
gravity, but none have been able to
explain such a wide range of phenomena.
It's not like there hasn't been progress
along the way. Super symmetry, [music]
the green Schwarz mechanism, the various
dualities, Dbrains, the holographic
principle. These aren't just beautiful
mathematical concoctions. They've become
important tools in other fields. From
condensed matter and quantum information
[music] to black holes and nuclear
physics, string theorists freely admit
that many of their predictions are
beyond the reach of current experiments.
But one day that could change. The
absence of evidence is not evidence of
absence. Black holes, quarks, and even
parity violation were once dismissed as
absurd and fanciful. [music] Now all
three are mainstays of modern physics.
And so, is it really so hard to imagine
that strings vibrating across [music]
multiple dimensions could one day join
that list?
Science often demands patience. Like the
cathedrals of old, a good theory can
take more than a human lifetime to
build. Those who lay the first stones
know they will never see the spire
completed. But that doesn't diminish the
importance of their contributions. And
so in a sense, modern string theory in
the guise of multi-dimensional M theory
is a blueprint and a scaffold, not a
finished building.
At its core, string theory has always
been about beauty. the enchanting
compass that has so often pointed
physicists towards undiscovered truths
like fingerprints nature has left at the
scene. The defense concludes with an air
of defiance. If string theory is guilty
of anything, it is only of being ahead
of its time.
But of course, the prosecution disagrees
in the strongest possible terms. String
theorists, they argue, have been blinded
by beauty. For nearly 60 years, the
theory has labored on without a single
shred of experimental evidence to
support it. Searches for super symmetry,
for example, the most crucial aspect
that saves string theory from falling
apart into a slag heap of tachons and no
parity violation, continue to fail. And
yet, every time experiment refutes one
aspect of string theory, it's seemingly
put back together with an even more
elaborate sticking plaster. For a theory
that's supposed to be elegant and
beautiful, it has metastasized into a
monstrous chimera. Beauty has become
only camouflage, conceiving
contradictions behind a veil of
aesthetic appeal. A multitude of extra
dimensions, calabia manifolds, wrapped
up [music] embraanes, all of them
necessary to save string theories
blushes. Not one of them ever glimpsed
in the real world. and ads CFT, which
predicts a curvature of the universe
that's the opposite of what we [music]
actually observe. But the most egregious
overstep, the prosecution [music]
insists, is the string theory landscape.
The use of the anthropic principle to
invert the case in string theory's favor
is the final abdication of science's
duty to predict and have those
predictions falsified through
experiment. It is no longer physics as
poetry, but physics as religion. A
compass that points in 10 to the^ of 500
different directions is no compass at
all.
And so
the gavl falls one final time as the
jury retires to consider its verdict.
After days of deliberation, they finally
return to the courtroom. The foreman
stands. The jury is hung. Unable to
reach a unanimous verdict. Some are
dazzled by String Theory's elegance.
Others, however, think it claims too
much and explains too little. The former
notes that beauty has been both savior
and sabotur. It rescued string [music]
theory when evidence failed, but also
lured it into existential culde-sacs
that make it hard to support.
The judge has little choice but to
declare a mistrial.
It is clear that the case cannot be
resolved with the evidence currently
before the court and so the file is left
open pending the arrival of testimony.
Only the universe can provide.
>> [music]
[music]