It should have been a simple test.
The wormhole stretched from the moon's
orbit to an empty patch of space near
Neptune, where a welcome party was
already being arranged. It had taken
huge effort and resources to reach down
into the quantum foam and pluck a
wormhole from those depths, but general
relativity had predicted their
existence. And now here it was. With
this technology, humanity could finally
achieve the dream of interstellar
travel. And so, after the last
calibrations and tweaks, the ship and
its brave crew entered the wormhole's
throat, passing easily through the
sphere of the entrance. Onlookers could
see the retreating vessel gradually
shrinking away as it appeared to move
effortlessly to its destination, 4 12
billion km away. At the other end, the
reception party could see the ship
approaching, slowly getting larger as
the minutes ticked by. The test probe
had taken roughly 30 minutes to traverse
the distance, and the manned vessel
seemed to be on a similar track.
[Music]
But then,
there were murmurss of concern as the
ship appeared to slow down, then stall.
It looked frozen in place.
30 minutes passed and the ship did not
emerge.
Days passed, then weeks, then months.
With time, sensitive telescopes at
either end were able to determine that
the ship was still in motion, but almost
imperceptibly slowly.
Debates raged about what to do. Send a
rescue party? Turn off the wormhole
generator? Every option seemed to end in
tragedy with the fragile balance of
exotic forces maintaining the stability
of the wormhole thrown into
disequilibrium, tearing the ship and its
crew to pieces. The only safe option was
to wait.
And so they waited.
[Music]
500 years passed before what was left of
the ship finally emerged from the open
end of the wormhole. It was still
intact, but barely functioning. A broken
husk after years of exposure to the
harsh conditions within. The crew was
long dead, their remains still strapped
into the command chairs.
Finally, the wormhole was shut down.
Clearly, they had miscalculated. The
predictions of general relativity were
evidently not enough. They thought they
had solved every corner of the complex
intersection of gravity and quantum
mechanics, accounted for every possible
interaction.
But something simply hadn't worked.
Humanity looked out at the distant stars
and never felt more alone.
Wormholes are complicated.
They appear in the equations of general
relativity and the idea of them is
almost as old as the theory itself. But
unfortunately for those test pilots
heading to Neptune, relativity is just a
machine that tells us how gravity works
in various scenarios, regardless of
whether or not the universe actually
allows those scenarios.
And so what would it take to build a
wormhole? Which mechanisms allow for
wormholes? and which forbid them? And
would they transport us to the
destinations of our dreams across the
stars? Or to an early death.
[Music]
using the code history of the universe and start your passion project today.
Critics called it confusing, dense, and
absurd. But to Edgar Alan Poe, it was
his greatest work yet.
It was 1848, just one year before his
mysterious death. Having struggled as a
professional writer throughout much of
his life, his recent success from The
Raven had been a welcome financial
reprieve, which he took advantage of to
compose what he viewed as his
masterpiece, what he called the one
great purpose in my literary life.
Its title was Eureka.
The text is expansive, and today little
attention is paid to it, especially
compared to his much more famous poems.
But some statements in its tangled mass
still stand out as oddly precient. For
about 2thirds of the way through the
text, in the middle of a section where
Po discusses the expanses of
interstellar space, he declares almost
out of nowhere that space and duration
are one.
In Po's eyes, space and time were the
same.
Working nearly a century later, Albert
Einstein had no idea that in some sense
he was recapitulating the thoughts of a
long deadad New England poet. But that
is exactly what he ended up doing.
It was 1905, a year which would become
known as Einstein's annis morabilis, a
year of miracles. In that single year,
Einstein published five papers, each one
with huge implications for our reality.
And one of those papers, perhaps his
most consequential, concerned the
relationship between movement and
energy. It's in this paper that Einstein
formulated his special theory of
relativity, which showed that the
measurements of length and time were
relative to the observer. But Einstein's
former teacher, Herman Minkovsky,
thought that he hadn't gone far enough.
And so, shortly after, Minkovsky offered
an intuitive geometric formulation of
Einstein's work, presenting the universe
as a four-dimensional object. Unlike
Po's ideas, however, this new spacetime
did not mean that space and time were
identical. Instead, they were different
facets of a single unified entity. But
Einstein would learn from his teacher
once more and quickly adopted this new
idea, using it to make one more gigantic
leap in human imagination.
In 1915, Einstein unleashed into the
unsuspecting world a general theory of
relativity.
[Music]
In the language of general relativity,
spacetime isn't static. It responds to
the presence of matter and energy. The
unified fabric of spacetime bends and
warps due to matter's influence. And in
return, that bending of spacetime
influences the motion of matter. Indeed,
years later, famed physicist John
Wheeler would sum it all up with the
simple phrase, "Matter tells space how
to curve, and space tells matter how to
move."
And it's happening all around you and
within you right now.
Ripples of gravity, known as
gravitational waves, constantly slosh
over the Earth, created by distant
cataclysmic events. As they pass, they
deform space. If you were to measure it
down to the level of an atomic nucleus,
you could feel yourself being gently
stretched and squeezed by those passing
waves.
Much like Po's writings, the theory of
general relativity itself is beautiful,
haunting, and sometimes complex and
confusing. This is because general
relativity is more than a simple
physical theory of nature. General
relativity
is a machine.
You start with an input, a particular
arrangement of matter. Then you feed it
into the equations of relativity and out
pops an answer, a prediction for how
that matter will behave under the
influence of its own gravity. But
general relativity doesn't tell you
whether that arrangement of matter is
real or not. It's up to you to decide
what is possible and what is impossible.
Indeed, relativity is not alone in
physics when it comes to this. Newton's
law of universal gravitation, which
relativity replaced, was similar. You
tell Newton what kind of material
arrangement you have, and the equations
will tell you how that system behaves.
If you want realistic answers, you have
to provide realistic scenarios. For
example, you could in principle figure
out the gravitational influence of a
solid metal rod 10,000 lightyear long.
It's up to you to decide if lightyear
long solid metal rods exist in the
universe. On that matter, Newton will
remain silent. But to make matters even
more complicated, unlike Newton's laws
that preceded it, general relativity is
not a simple equation that could be
taught in high school. Instead, it's a
set of 10 equations in four dimensions,
all tangled with each other, and all
nonlinear.
The physics community knew that it was
correct and useful, but it was so
difficult to wrangle that most
physicists preferred instead to work on
subatomic and astrophysical problems.
Indeed, the pioneering physicist JJ
Thompson once remarked, "I have to
confess that no one has succeeded in
stating in clear language what the
theory of Einstein's really is."
Though there was some progress soon
after Einstein developed relativity,
Carl Fartshield found a solution
involving the simplest arrangement of
matter possible while carrying in the
trenches of World War I. In this case,
Zartshield took the machine of
relativity and input something
resembling the solar system out popped a
description of the strength of gravity
surrounding the sun. While the solution
was intended to mimic the solar system,
it contained something else. the
possibility of a singularity.
The idea that if the ball of matter were
to be squeezed below a certain limit,
then it would just keep collapsing, its
own gravitational pool so strong that it
becomes a singular point of infinite
density surrounded by a one-way membrane
and event horizon. And so the machine of
relativity had done its job. Now it was
up to physicists to figure out whether
the result was possible or not.
And most scoffed at the idea.
Indeed, Einstein himself rejected it. In
a 1939 paper, he argued that no object
could ever shrink to the degree that
gravity would win out over all the other
forces. Frozen stars, as they were named
at the time, simply could not exist in
the universe.
But J. Robert Oppenheimer disagreed.
Just a few months after Einstein's
publication, Oppenheimer was working
with his student Hartland Snder. He had
just figured out the maximum possible
mass of a neutron star, the densest star
possible. And he wondered what would
happen to those stars if they went over
the mass limit. But Oppenheimer and
Snder came to the exact opposite
conclusion to Einstein. Once you folded
in the rest of our knowledge of physics,
they found that there was really nothing
to stop the infinite gravitational
collapse of a massive enough star. And
from there, the machine of general
relativity told you exactly what would
happen. Their paper stating clearly,
"The star thus tends to close itself off
from any communication with a distant
observer. Only its gravitational field
persists.
[Music]
However, the same day that the duo
published their paper, Germany invaded
Poland. Their work would be forgotten
for decades, and Opanima himself would
get swept up in the Manhattan Project
and the fraud politics that ensued,
never to return to astrophysical
mysteries.
Despite this, as time went on, it would
become indisputable. Black holes were
real and possible. But was that all that
the machine of relativity predicted?
Or was there something more?
As Oenheimer worked away in service of
the United States government, behind the
Iron Curtain, Soviet astronomer Igor
Novakov saw something else produced by
the equations. The black hole had a
twin. Novokov had been born in 1935 in
Moscow and quickly ascended to leading
positions at Moscow universities. Little
is known about his personal life except
that working as a physicist in that
environment was a tricky business. You
had to ensure that your research did not
conflict with Soviet ideology. You would
frequently get pulled aside to work on
some state sponsored priority and you
had little hope of making your work
known to the worldwide community of
scientists. Still, Novakov had enough
freedom to pursue some decidedly
unorthodox lines of research. And his
great revelation in 1964 was born out of
a simple realization about gravity.
[Music]
Imagine if we took a video of you
tossing a ball up in the air and
catching it. But we will be clever about
the framing so we don't see your hand,
just the ball going up and down. If we
were to play back that video in reverse,
would you notice?
Both the forward and backward play
through would look identical, just a
ball going up and down. The mathematics
of relativity are similar. They don't
know if processes are going forwards or
backwards in time. Once again, it's up
to you to decide which is which.
Gravity, including general relativity,
doesn't care about time.
[Music]
And a black hole is a particular
solution to general relativity.
[Music]
In 1964, Novokov realized that the
equations of black holes could be run in
reverse. Instead of trapping particles
forever, they could eject particles
forever. Instead of a black hole, it
could be a white hole.
Novel Corford proposed this idea to
explain mysterious bright sources
appearing in astronomical surveys. But
as he was in the Soviet Union and behind
the Iron Curtain, his work was largely
ignored by Western scientists.
And white holes would be very strange
beasts indeed, perhaps even stranger
than black holes.
They would still have singularities at
their centers, but they would be
surrounded by the opposite of an event
horizon, an anti-horizon.
They would still have mass, and so you
would still be gravitationally attracted
to one, but you could never enter. The
anti-horizon forbids passage into the
white hole, just like a black hole's
event horizon forbids travel outwards.
However, if you found yourself inside of
a white hole when it first formed, you
would not stay there for long. You would
find yourself propelled outwards faster
than the speed of light, like a flooding
river that you can't resist until you
exited the white hole. The white hole
would continue spewing matter and
radiation until it was completely empty,
finally consisting of nothing more than
a bare singularity and its anti-
horizon. And if this is not strange
enough, white holes can only have been
formed in the past and are now exploding
outwards. the exact time reversal mirror
of a black hole.
[Music]
In time, physicists will prefer a
different explanation for those distant
bright sources, namely the energies
emitted by matter as it fell into black
holes. But as late as 2014, Novokov
believed that some of them may still be
white holes formed through some exotic
mechanism in the early universe and
still shining today.
While most physicists disregarded the
possibility of a white hole, a few were
intrigued. Roger Penrose, who would go
on to win a Nobel Prize for his work on
black holes, became aware of Novacov's
work and realized that it might just be
the ingredient needed to explain yet
another strange solution in general
relativity. A solution involving a
tunnel from one universe to the next.
But it would be Penrose's friend and
colleague, John Wheeler, who would give
these tunnels their modern name,
wormholes.
He would find that white holes and black
holes could be connected together. And
in fact, the solution had already been
found decades earlier by a relatively
unknown assistant of Einstein's.
And with that solution in place, Weaver
realized that a little branch of
relativity could blossom into a way to
jump light years into the stars. He just
needed to see if the machine of general
relativity was spitting out genius or
madness.
[Music]
Roughly a dozen people from countries
across Europe and North America, men and
women. They would peer deeply into the
inner workings of nature. They would
devise ingenious mathematical proofs.
They would conjure results from
groundbreaking experiments. They were
all, in a word, geniuses.
But they all had the same inescapable
problem. They lived in the shadow of
Albert Einstein.
like being the second best swimmer in a
pool next to Michael Phelps, there was
no hope for Einstein's collaborators in
claiming top prize as the smartest
person in the room. But still, Einstein
himself did not disparage them or hold
them in disregard. Instead, he
frequently asked them for help. Einstein
called one friend, Michelle Besso, the
best sounding board in Europe. And one
of these working relationships became
personal as well. his first wife
marriage.
[Music]
And then there was Nathan Rosen.
He would meet Einstein for the first
time in 1934 at the Institute for
Advanced Studies, which had been founded
on the grounds of Princeton a few years
prior. Rosen was smart, capable, and
hardworking. He'd even proposed a
largely correct model for the atomic
nucleus before its discovery. But
despite his intellect and other work,
it's unlikely his name would be repeated
so often today if it weren't for his
collaborations with Einstein.
Rosen had already spent a few months at
Princeton before he finally built up the
courage to meet with the legendary
physicist. When he did, timidly knocking
on the genius's door, Einstein was
engaged in a deep conversation with
another collaborator, the mathematician
Walter Meer. But immediately, Einstein
put the young Rosen at ease. As Rosen
himself related years later, I told him
about my work on the solutions of the
equations of his unified field theories
and he seemed interested and asked many
questions. After a while, he began
telling me about the work he was doing
at the time and before I realized it,
the three of us were engaged in a lively
discussion of his current problems. What
followed was a three-year partnership.
Officially, Rosen was Einstein's clark,
but their relationship was closer to a
friendship. Rosen was especially
interested in Einstein's doomed efforts
to unify gravity with electromagnetism,
something most other physicists scoffed
at as foolish and misguided, which
probably helped their bond.
And it was during their time together
that Rosen would propose a remarkable
new outcome from the machine of
relativity.
[Music]
At that time, Einstein had been
particularly frustrated by the
appearance of singularities in his
theories. As well as this, he was
equally frustrated by the growing trend
of viewing subatomic particles as having
no size, no spatial extent. He'd sought
a way to eliminate both problems. And
Rosen came up with a way to do it. Rosen
and Einstein imagined following the path
of a line as it fell into a black hole
and crossed the event horizon. Normally,
we would consider this the end of the
story. The line crosses the event
horizon, is sealed off from the rest of
the universe forever, and meets its
ultimate fate at the singularity in the
center. But Rosen had a radical
suggestion. What if the line could keep
going? What geometric construction would
allow them to avoid the singularity
altogether? It was an intriguing
question, and Rosen had just the machine
to answer it. General relativity. Rosen
found that if you followed this path,
you would end up back in the universe,
but not where you started. The path
would form a bridge between two
different regions of the universe. There
could be multiple paths, even shortcuts
within spaceime.
To complete their theory, Rosen and
Einstein added electromagnetic fields to
the calculations, hoping that this
solution that eliminated singularities
could also explain the appearance and
properties of subatomic particles. In
other words, to describe particles as
these bridges in spaceime.
Unfortunately, the attempt did not work.
And so, these Einstein Rosen bridges, as
they came to be known, fell into
relative obscurity.
And so it would be a further two decades
before American physicist John Archerald
Wheeler would resurrect this obscure
idea of Nathan Rosen's and give the
bridges a new name.
Wormholes.
Wheeler had had an appetite for normying
questions since his youth. His mother
recounted that one day when he was 4
years old and splashing around in a
bathtub, he went suddenly still, looked
up at her, and asked, "What happens when
you get to the end of things?"
And indeed, he never let go of that
childhood curiosity and some childhood
mannerisms, including a particular joy
in racing down flights of stairs as
quickly as he could. And it was this
boundless energy that allowed him to
investigate questions that nobody else
dared to explore, including the
mysterious bridges of Rosen and
Einstein. Crew it was Wheeler that
realized they might provide a mechanism
for interstellar travel. That the
bridges were no mere mathematical
construct, but a tunnel in spaceime that
could be shortened. In principle, you
could travel to distant stars with no
more effort than a leisurely stroll or a
race down a flight of stairs.
But almost immediately after this
revelation, he then realized that if you
attempted to travel through a wormhole,
you would be instantly obliterated.
Rosen and Einstein had imagined
traveling into a black hole without
suffering the ultimate fate of the
singularity and instead emerging
somewhere else safe and sound. In this
sense, the other end of their bridge was
a white hole, the mirror image of a
black hole. Wheeler, who was one of the
few to become aware of Novokov's work in
the Soviet Union, realized that there
was nothing in the mathematics of
general relativity to prevent the
existence of such an eternal black hole.
A tunnel in spaceime with a black hole
at one end and a white hole at the
other. Problem solved? Not quite, for
these wormholes suffered from two fatal
flaws. The first is that general
relativity is not the only theory of
physics. There is more to the universe
than pure gravity. The second is that
just because a wormhole can be created,
it doesn't automatically mean that it
will stick around.
For decades, physicists had struggled
with the concept of black holes and the
vexing problems that they brought up.
But eventually, evidence would win the
day, and now all physicists accept their
reality.
However, this is not the case for white
holes.
[Music]
A white hole is a black hole run in
reverse, which is possible in general
relativity only because general
relativity does not care about the flow
of time. But we do, and so do other
theories of physics. The cast iron laws
of thermodynamics, for example, do care
about the flow of time and the rise of
entropy. You can collapse material to
form a black hole. The entropy increases
and all is well in the universe. But you
cannot expand material to form a white
hole. The entropy would have to be
lowered to make this happen and that is
strictly forbidden. There is no
mechanism to create a white hole out of
the materials and physics of the known
universe. When a star collapses, the
rush of energies and forces prohibits
one from forming. In a sense, pinching
off that portion of the Einstein Rosen
Bridge, leaving only the black hole
behind.
Perhaps though the early universe chalk
full of strange and unknown physics was
able to create white holes from some
exotic process and leave them behind as
remnants that inhabit the modern-day
universe. Could that be the solution?
Unfortunately, no. Even if you were able
to create a white hole, either through
some arcane mechanism in the big bang or
in the laboratory, it could not last for
long. The white hole still has positive
mass. It still has a singularity at its
center. If you were to encounter it, you
would still be drawn towards its massive
gravity, but you couldn't ever enter.
Its anti-horizon is the ultimate
exclusive club. It does not admit entry
to anybody, to anything. And so, you
would become trapped, pulled
tantalizingly closer to the edge of the
anti- horizon, but never able to pass
through. The energies of this system of
you approaching infinite decimally
closer to the anti-horizon go up and up
and up and eventually they go out of
control. To put it simply,
they explode.
And so there is no known mechanism that
can create a white hole. And once
formed, there is no known mechanism that
can keep it stable. For these reasons,
understandably, physicists do not
believe that white holes exist in
nature. Unlike their twins, the black
holes, they are destined to be nothing
more than flights of mathematical fancy
embedded in Einstein's relativity.
But dreamers like Weether and Novacov
aren't constrained by a single theory.
After all, we've updated our
understanding of gravity in the past,
and we expect, or at least hope, to
update the general theory of relativity
in the future. So, while our current
knowledge of physics forbids the
existence of white holes, maybe a more
nuanced approach could allow them, maybe
wormholes could be possible.
But unfortunately, even if you could
achieve some feat of magical technology
to create a white hole and keep it open,
turning it into the open end of an
Einstein Rosen bridge, we have found
that you should never ever enter it. The
problem is that the description of
spaceime offered by Einstein and Rosen,
what we now call an eternal black hole,
is static. A simple description of
geometry frozen in time forever. But as
John Wheeler at the age of four asked
his mother, "What happens when you go to
the end of things? What would happen if
you created this wormhole and actually
travel through it?"
[Music]
The interiors of wormholes are not
pleasant places. They are not serene
tunnels through gentle spacetime. They
would be regions of intense, voracious
gravity.
Gravity can do more than just pull
straight inwards. It can also tear.
Indeed, the moon attempts to gently tear
the Earth apart with every rotation,
which we recognize as the tides. An
object falling into a black hole is
stretched and thinned before being
atomized, a process physicists call
spaghettification.
And the gravitational forces inside a
wormhole are also so strong, so intense,
so tortured that if one were to form, it
would be compelled to stretch along its
length and compress across its middle
like a rubber band stretched too far.
Wormholes would simply rip themselves
apart in a flurry of gravitational
energies.
In fact, Wheeler found that this
destructive process happens faster than
the speed of light. Nothing, not even
light, can travel through the throat of
a wormhole and safely emerge from the
other end before the tunnel destroys
itself. Briefly, you might glimpse a
view of your destination. Light from
either end of the wormhole can fall in.
You could see the universe at both your
origin and your end point. If someone
entered from the opposite end, you could
meet inside the wormhole tunnel. You
could share stories and experiences from
across the entire universe.
And then the wormhole would collapse and
you would be left inside the black hole.
It singularity your only possible
destination.
You would die.
And so after Wheeler's pronouncement of
the finality of wormholes, most
physicists accepted that they were
literal dead ends. It was clear once you
crossed the event horizon of a black
hole, whether as a single entity or as
the entrance to a wormhole, you could
not escape. No clever trick of Einstein
and Rosen's geometry could save you.
But predictably, that wasn't quite the
end.
For decades later, a third generation of
physicists would take up the challenge
posed by Einstein, Rosen, and Wheeler.
And in fact, it would be one of
Wheeler's own students who would find a
way to create a stable, traversible
wormhole. He just needed a little help
from science fiction.
[Music]
If you're looking for good physics in
science fiction, movies like Star Wars
are not the best place to find it.
Laser beams that travel slower than real
life bullets. Gigantic moon-sized space
stations that defy their own gravity.
Weapons powerful enough to obliterate
entire planets.
And of course, spaceships that with a
simple flick of a switch can travel
faster than the speed of light.
While Star Wars makes for great
entertainment, it doesn't have the best
grounding in known physics. But
sometimes a writer or director comes
along who wants to hue a little closer
to our modern understanding of how the
universe works. And one example of this
is the film Interstellar directed by
Christopher Nolan.
The world of Interstellar is
recognizable enough with a crop
destroying disease standing in for the
ravages of climate change and NASA a
shadow of its former glory trying to
organize a mission to explore a
mysterious object that has appeared in
the outer solar system. But despite this
attention to real life worldb building,
Nolan wanted his crew of astronauts to
travel vast distances in short amounts
of time. And so he enlisted Kip Thorne,
a physicist, expert in general
relativity and Nobel Prize winner to
help with ideas. Thorne tells the story.
Chris considered traveling through space
faster than the speed of light as
non-negotiable. He used that phrase in
our brainstorms.
But in the end, after in-depth
discussions, he came around.
Faster than light travel was out of the
question for Thorne. But rather than
refusing Nolan entirely, he compromised
and provided a possible solution. The
physicist suggested a wormhole, a
shortcut through space and time that was
stable and traversible. A bit of
fantastical technology certainly, but
one that was not entirely ruled out by
modern physics. And indeed, if anybody,
Thorne should have known, for it was
Thorne himself who realized this when in
the 1980s he was helping out another
famous friend of his, the legendary
science communicator Carl Sean, with his
own fictional story.
In Sean's novel Contact, humanity
receives a set of instructions on how to
build a wormhole, and a team travels
through it. In one exchange of dialogue,
Sean wanted his characters to muse on
how it might be possible, but he didn't
know what the characters should say. So,
he, like Nolan later, turned to Thorne
for help.
Of course, by that time, wormholes were
already known to be catastrophically
unstable. So Thorne and his
collaborators came up with a way to
stabilize them.
The idea was never intended as a serious
venture into theoretical physics.
Indeed, his 1988 paper on the subject,
co-authored with Michael Morris, was
titled Wormholes in Spacetime and Their
Use for Interstellar Travel and
subtitled A to Tool for Teaching General
Relativity.
For Thorne, this was business as usual.
He once said in an interview, "Almost
everything that I've done was something
I never planned to do. It was clear that
the wormholes of Rosen, Einstein, and
Wheeler simply didn't work. White holes
aren't stable, assuming they can be made
at all, and the bridges they form with
black holes are only temporary."
And so, to make a wormhole useful in
Thorn's jargon traversible, you have to
solve two problems at once.
[Music]
One, you have to put the entrance to the
wormhole outside the black holes event
horizon. That's the only way to avoid
doom at the singularity.
And secondly, you have to stabilize the
passage itself, exerting some sort of
pressure on the walls so that they don't
just collapse in an instant. And Thorne
found one simple trick of the universe
that could solve both problems within
the equations of general relativity.
Normally most physicists use general
relativity in one direction. Given an
arrangement of matter and energy, they
use the equations to calculate what the
resulting curvature in spaceime is. From
there, you can calculate the
gravitational forces and predict the
future behavior of the system. But
Thorne worked the equations in reverse.
He started with a specific structure in
spacetime, the wormhole bridge between
two regions of the universe and asked
the equations what configuration of
matter and energy is required to make
that tunnel stable.
Ever dutiful, ignorant of what is and is
not possible in the real universe, the
machine of relativity obliged and
produced an answer.
Negative mass.
Not antimatter which has positive mass
but opposite charge. negative mass. A
football that weighs -500 g. A cut of
meat from the grosser that weighs -2 kg.
[Music]
Negative mass doesn't pull and attract.
It pushes and repels.
If you fashion negative mass into a
shell, spaceime around it responds,
pushing itself apart. This pressure can
inflate the entrance of a wormhole to
sit beyond the event horizon, allowing
you to enter the tunnel without being
swallowed by the black hole at its
heart. That same pressure of the
negative mass keeps the tunnel open,
resisting its natural inclination to
tear itself apart and allowing objects,
light, particles, spaceships to travel
through.
There it was. The key to a working
wormhole laid bare in the cold
mathematics of relativity.
[Music]
There was one problem though. Where
could we find negative mass?
Despite decades of searches and
experiments, physicists and astronomers
have found no sign of matter with
negative mass, dubbed exotic matter. And
indeed, even if it did exist, it would
pose severe problems for our most
fundamental and cherished conceptions of
momentum and energy. For example, exotic
matter might behave in the opposite
manner in response to forces. If you
kicked a football with negative mass, it
would go flying backwards, not forwards.
If you dropped it, it would go flying up
into the sky. Exotic matter would break
our understanding of the conservation of
momentum, the foundational principle
behind almost every single theory of
modern physics. Of course, it's
perfectly possible for momentum
conservation to be wrong or at least
inadequate. But after centuries of
investigation, it has shown no signs of
weakness, no cracks in its ability to
explain how the universe behaves.
And so exotic matter seems to be a dead
end.
But Einstein did show us that matter and
energy are merely two sides of the same
coin. So if exotic matter can't be made
and fashioned into a wormhole, perhaps
exotic energy could do the trick.
and exotic energy found its beginnings
in an old-fashioned shootout.
It was just after World War II and the
streets of Copenhagen were standing in
for the dusty thorough affairs of the
Old West.
Suitwearing cowboys wielded mock weapons
and entered into simulated duels with
each other. One would have the choice to
draw their weapon whenever they chose.
The other had to wait for the opponents
before they could make a move. One such
participant was Hrik Casemir, a young
graduate student who was more interested
in particle physics than fake gunfights.
And watching over all of this odd
experiment, stopwatch and notebook in
hand, was none other than Neil's Boore,
the godfather of quantum physics.
[Music]
Casemir was in Copenhagen to work on
quantum mechanics, but Bore had diverted
from his main interests to briefly
indulge a hypothesis he had created
known today as the gunslinger effect.
He believed that automatic responses
were faster than intentional ones and
had wanted to test the idea.
Reluctantly or not, Casemir had to take
a break from his studies and pretend to
be a gunfighter. And so perhaps that
image of fellow graduate students
staring at each other across a distance,
pretend guns ready at the hip, stuck
with him as he returned to hard physics,
and imagined the tension and forces
between two metal plates held a small
distance apart.
Studying the effects of quantum
mechanics under bore, his calculations
had shown a remarkable property. With
nothing between two metal plates at all,
they would spontaneously move towards
each other as if willed together by an
invisible new force. But this would turn
out to be no new force of nature. It was
instead an artifact of quantum
mechanics.
Quantum mechanics does not envision a
universe full of particles. Instead, it
sees fields that occupy all of space and
time. They are everywhere. These fields
are full of vibrations, constantly
fizzling in and out of existence.
Sometimes these vibrations take on more
substance and travel. This is what we
call a particle. But most simply
disappear before they can be noticed by
the wider cosmos. And there are in fact
infinitely many vibrations occurring all
throughout the expanse of the universe.
However, between the plates, those
vibrations are restricted. Through
careful calculations, Casem was able to
show that the difference in vibrations
outside and inside of the plates led to
a gentle push, a force that would impel
the plates to come together. And since
the region inside the plates had less
energy than the average energy of the
vacuum, it could be said that the
Casemir effect, which was named in his
honor, is an expression of negative
energy.
And a sufficient concentration of
negative energy can hold open a
wormhole, stabilize it, and allow it to
be traveled through.
[Music]
Such a wormhole would be a thing of
wonder completely unlike anything else
observed in the universe. Its entrance
would be a sphere not unlike the event
horizon of a black hole. Meaning you
could enter the wormhole from any
direction.
But it would be a blaze with light. Not
the light of the surrounding stars, but
the light from a distant portion of the
universe that is filtered through the
opposite end. You could wave hello to a
friend in the Andromeda galaxy. You
could examine its stellar contents,
and you could travel there yourself.
Movement through this wormhole would be
relatively stressfree, assuming that the
tunnel was properly stabilized with an
adequate arrangement of negative energy.
You would experience no strong tidal
distortions, no incredible warpings of
gravity. Although your view of the
outside universe would be highly
distorted due to the unique geometry of
the wormhole tunnel, but you would pass
through unscathed, skipping tens,
hundreds, millions of light years
without a hitch.
At least in principle.
For once again, things are not quite as
straightforward as they seem.
We have laws of physics like the general
theory of relativity. But usually the
maths within these laws has much broader
scope than just our physical universe.
For example, some solutions to
relativity are permitted in our universe
like black holes while others like white
holes are not.
And so to help navigate this over the
decades physicists have developed a set
of rules to guide which solutions are
permitted and which are not. These rules
are called energy conditions.
There are various like the strong,
dominant and weak conditions and in
general they tell us how energy can
behave. For example, energy cannot
travel faster than light just like
matter. Another condition states that
local mass cannot be negative. Exotic
matter is simply not allowed due to its
momentuming implications.
Yet another condition says that local
gravity must always be attractive. There
may be repulsion as we see with the
accelerated expansion of the universe,
but that cannot happen at small scales.
The Casemir effect violates the weak
energy condition, the rule that energy
must always be positive. But of course
this is only true in the hyperlocal
space between the plates. If you average
out over a larger volume including the
plates and surrounding environment total
energy is positive and the energy
condition is restored.
It is an open question in physics if any
of these energy conditions can be
violated at large scales. They seem to
hold true, but again, they are only
guesses based on our experience with
gravity. It is possible that strange
quantum oddities like the Casemir effect
could be amplified by a sufficiently
advanced civilization.
Indeed, quantum theory predicts the
existence of numerous, even infinite
wormholes.
The same quantum fields that constantly
vibrate and fizzle in and out of
existence give rise to an invisible sea
of particles which in turn disturb and
disrupt spacetime at the tiniest scales.
This is called the quantum foam.
A roing sea of broken and unruly
spacetime operating at such small scales
in time and space that they escape
notice. That foam contains countless
submicroscopic wormholes. And so
perhaps, as Thorne mused in his 1988
paper, an advanced civilization could
take one of these ultra tiny wormholes
and pluck them out of the quantum foam,
carefully enlarging them to macroscopic
proportions.
Perhaps they could find a way to violate
the energy conditions at large scales,
allowing a Casemir-like effect to go
from minute distances between plates to
span intergalactic lengths.
And perhaps that civilization could use
that technology to allow a brave crew to
travel to distant stars.
[Music]
But a functioning wormhole, as
tantalizing as it sounds, is more than a
bridge to faraway galaxies.
It is also a tunnel into the past.
[Music]
You receive an invitation to a party.
The location is the University of
Cambridge. If you need more precision,
the coordinates of the party are at 52°
12 minutes 21 seconds north and 0° 7
minutes 4.7 seconds east. And the party
takes place precisely at 12 noon
universal time on the 28th of June in
the year 2009.
There will be champagne and your host is
none other than famed astrophysicist
Steven Hawking. At that precise place,
and more importantly, time, Hawking
arrived at the room he had reserved. He
sipped some of the champagne and waited.
And after several hours of nobody
showing up, he departed. And then, and
only then, after the party had
concluded, did he distribute his
invitations.
It was a party for time travelers.
The lack of guests seemed to be
reasonable experimental evidence to show
that time travel was not possible. After
all, who in the distant future would
turn down an opportunity to meet Steven
Hawking? But Hawking himself was not
totally dissuaded. I'm hoping copies of
the invitation in one form or another
will survive for many thousands of
years. Maybe one day someone living in
the future will find the information and
use a wormhole time machine to come back
to my party, proving that time travel
will one day be possible.
And so, was he correct? Could wormholes
hold the key to time travel?
As far as we can observe, time travel
into the past is impossible. And yet,
once again, the machine of relativity
seems to throw a spanner in the works as
its equations permit it.
Let's assume that our knowledge of
physics advances from its present state,
and we discover a way to violate the
energy conditions and construct a
working, traversible wormhole held open
by exotic matter or negative energy. You
build your device and bounce between the
stars faster than light. Never locally,
of course. You never ever measure
yourself moving faster than light in
accordance with relativity. And yet at
the same time, you are able to arrive at
your destination before the news of your
departure reaches it. You then take one
end of your wormhole and accelerate it
to nearly the speed of light. Indeed, it
doesn't matter where you take it. What
matters is that it's fast. Due to
special relativity, that end of the
wormhole experiences time dilation, its
clock becoming unsynchronized with the
other end.
And so if you travel outside the
wormhole back to the other
non-acilerated end and travel through,
if your journey time outside the
wormhole is less than the difference in
time between the two ends, you can
arrive before you leave.
Wormholes aren't just shortcuts in
space. They are shortcuts through time.
If the universe allows us to build
wormholes, it automatically means that
it allows us to build time machines.
[Music]
But time travel opens up so many
paradoxes that most physicists point to
this reason as sufficient evidence that
wormholes are impossible.
But why? Why can't we travel through
time?
Consider this example. You have a
machine that creates a wormhole. You can
turn the wormhole on. You can also turn
the wormhole off. So, you turn the
wormhole on and travel into the past.
Then, you turn the wormhole off. Or you
detonate a bomb and destroy the entire
wormhole making machine entirely. But if
the wormhole is off or destroyed, how
are you able to travel into the past to
turn it off? This is just one example of
the myriad paradoxes inherent to time
travel. And so anyone who has deeply
studied wormholes has been forced to
confront the difficulties of time travel
and concoct solutions around it.
[Music]
Noikov himself, the father of white
holes, created the self-consistency
principle. He suggested that perhaps
time travel is allowed, but only in
certain limited cases where the past
remains unchanged. For example, you
throw an asteroid towards the entrance
of a wormhole. But at the last minute,
just before entering, another asteroid
comes flying in, knocking yours off
course. It still flies into the
wormhole, but at a trajectory you did
not anticipate. The asteroid travels
through the wormhole and emerges out the
other end in its own past only to
collide with itself just before entering
the wormhole. And so the history of the
universe remains consistent with the
past unchanged. Some unknown mechanism
prevents the past from being altered.
But other physicists rejected the idea
of time travel altogether. Indeed, in a
1992 paper, Steven Hawking himself said,
"It seems that there is a chronology
protection agency which prevents the
appearance of closed timelike curves and
so makes the universe safe for
historians."
Hawkings chronology protection
conjecture states flatly that time
travel is forbidden. Instead of trying
to find a way to allow wormholes and
thus time travel to exist, we should
instead use the information provided
that time travel appears to be forbidden
as a guide to building an improved
theory of quantum gravity.
And yet still, that is not quite the end
of it. For perhaps wormholes are not
tunnels into the past, but bridges to
entirely different universes.
In the many worlds interpretation of
quantum mechanics espoused by Hugh
Everett, every quantum interaction leads
to a branching multiverse.
For example, if we perform an experiment
that has a 50/50 chance between two
outcomes, one universe with one set of
uses measures one result, while another
universe with another set of uses
measures the other result.
This interpretation of quantum mechanics
has many flaws, but is not definitively
ruled out by any experiment, and it
opens up the possibility that wormholes
could allow us to travel to the other
branches of the infinitely expanding and
splitting quantum tree. Following this
logic, perhaps there is a universe out
there in the many worlds where revelers
from the future did arrive at Hawings
party. In their universe, Hawings party
was always a success. in ours always a
failure. Hawkings chronology protection
conjecture holds and time travel is
still allowed.
And so the presence of wormholes in the
equations of general relativity opens up
a whole range of intriguing tantalizing
possibilities. the ability to leapfrog
amongst the stars, the capacity to jump
into our own pasts. Perhaps wormholes
are forbidden by some yet to be known
laws of gravity. Perhaps they are
allowed but contain hidden flaws and
hidden traps. Or perhaps they are just
too slow.
In 2019, a team of researchers at
Harvard University and the Institute for
Advanced Study, the same place where
Rosen met Einstein nearly a century
before, found a way to concoct wormholes
without needing exotic energy. But there
was just one catch. These wormholes took
much longer than expected.
They weren't working with a model of
gravity applicable to our universe.
Instead, they were focused on a special
sort of spacetime that only has two
spatial dimensions and one of time.
Surprisingly, this wormhole requires no
exotic matter to stabilize it. Gravity
itself takes on a repulsive character in
the extreme geometries of the wormhole
tunnel, which is more than enough to
hold it open and allow for passage.
But an analysis of that same geometry
revealed that such a tunnel could not be
used to skip around the universe faster
than light. Indeed, paths flowing into
the wormhole were even slower and longer
than paths that remained in flat space.
Not only was this wormhole a poor way to
traverse the cosmos, it was worse than
not building one at all.
Admittedly, the researchers had used a
modified structure of spaceime to study
this wormhole. A structure that is not
the same as the physical universe, but
it still leads to the intriguing, if
frustrating, idea that wormholes may
very well be possible. But they also may
very well be useless.
Despite these recent discoveries, in
general in our universe, the current
consensus among most physicists is that
ultimately wormholes are not allowed.
But they can still serve as useful
playgrounds to test new ideas in gravity
and quantum mechanics to study physics
in one of its most extreme
configurations imaginable. to use their
strange behaviors as a probe not into
unknown regions of the universe, but
into the corners of knowledge currently
hidden from us. As Steven Hawkings said,
when reflecting on the difficulties of
understanding wormholes, time travel,
and quantum gravity, not only does God
play dice, but he sometimes throws them
where they cannot be seen.
[Music]
watching and we'll see you next time.
[Music]