Meteors have the power to transform our
planet, to wipe out huge swaths of life
on Earth in one impact.
But, not all of that dramatic. We are
hit by more than 100 tons of small,
sand-sized particles every day, and they
are essentially unnoticeable.
About once a year, a car-sized meteor
comes crashing through our atmosphere,
but the resultant impressive fiery
streak burns up long before hitting the
ground.
It's only on the scale of millions of
years that we are in danger of being hit
by meteors a kilometer or two wide, big
enough to do some serious damage.
But, we've not seen one of those since
the dinosaurs, lucky for us.
But, size isn't everything. In fact, the
biggest impacts may not even be from the
largest objects.
If one particularly interesting theory
is true, some very rare meteors may be
made of something a little more exotic
than rock.
Externally, there would be not much to
differentiate these rarer meteors. In
the vacuum of space, they would appear
exactly the same as any other space
rock.
But, while regular meteors might create
a fiery streak in the sky, these
outliers would have impacts that exceed
the largest of nuclear bombs.
And those are just the small ones.
If a kilometer-sized meteor of this
variety hit the Earth, similar in size
to the normal meteor that killed the
dinosaurs, we might not have a planet
anymore.
That is the power of a meteor made of
antimatter.
Could antimatter meteors really exist?
What clues would help us identify them
from regular matter meteors? And how big
would they have to be to become a major
problem.
I'm Alex McCollum and you're watching
Astro. Join me today as we test the
scientific theory behind antimatter
meteors and explore the odds of such
objects lurking in our solar system.
Antimatter is a funny substance. The
fact that it exists means that we
shouldn't be here. It is almost
identical to regular matter with a few
key differences.
One is that it has an inverted charge.
Another is that when antimatter meets
regular matter, the two annihilate each
other completely converting almost
entirely into energy.
Now, in the early universe, it wasn't
just matter that was created. In theory,
an equal amount of antimatter came
[music] into being, too.
All the matter should have bumped into
the antimatter and everything would have
canceled each other out. This would have
left no universe for us as there would
be nothing to make the universe out of.
It is one of the mysteries of science
that this did not happen and for some
reason, a slightly larger amount of
matter coalesced into existence than
antimatter. Perhaps as small a
discrepancy as a billion and one matter
particles to every billion antimatter
ones.
Scientists are still trying to figure
out why this might have occurred, but
the mystery remains unsolved for now.
>> [music]
>> Perhaps there is some rule at play that
we've not yet identified.
Whatever caused it, this imbalance is
the reason the universe we see around us
is almost entirely made of regular
matter and the only place we reliably
see antimatter is when they make tiny
amounts of it in experiments at CERN and
in other particle accelerators.
Some hospitals even have small particle
accelerators to create positrons for PET
or positron emission tomography scans.
However, just because we don't see it,
doesn't mean that tiny pockets of
antimatter
>> [music]
>> couldn't exist.
When the raw primordial soup quark-gluon
plasma of the Big Bang began to form
into particles, while overall there
would have been more matter than
antimatter, in local areas there were
fluctuations. [music]
So, it's logical that clumps of matter
could have emerged and dominated as
gravity pulled them together.
After all, if you flip a coin enough
times, you'll inevitably end up with
runs where you flip nothing but heads or
nothing but tails.
If these clumps were large enough in
scale, they wouldn't annihilate away in
a burgeoning solar system. Instead, they
would be the solar system, and it would
be pockets of matter that would
eventually annihilate out, leaving
behind stars and protoplanetary disks
made entirely of antimatter.
This seems highly theoretical, but could
such solar systems [music] actually
exist?
For the most part, an antistar would be
visually identical to a regular one as
antimatter behaves in all the ways you
might expect matter to. It obeys the
same laws of gravity and mostly looks
the same. The only telltale giveaway
would be when regular matter interacted
with it, such as when the interstellar
winds met the edge of the fledgling
solar system and resulting annihilations
would emit gamma radiation. And
curiously, there are some systems that
seem to do this.
14 such gamma-ray emitting star
candidates were found in the Milky Way
thanks to the Fermi Large Area Telescope
in 2021.
If these are antistars
and their profiles do match what we
would expect out of anti-stars, so this
is not ruled out, and [music] they're
not other objects that emit gamma rays
like pulsars or black holes, then
systems made of antimatter with planets
and asteroids do exist. The ratio of
anti-stars to stars would be about 1 in
400,000.
So, the stage is set. If anti-star
systems could exist, it's not illogical
to think that antimatter meteors do,
too.
And that leads us to the question, what
are the chances of them coming to Earth?
The idea of antimatter meteors striking
our planet is not a new one.
Even back in 1940, only a decade or so
after the discovery of antimatter as a
concept, Russian-American physicist
Vladimir Rojansky began to speculate
about their existence.
All it would take was for our solar
system to have passed one of these
antimatter solar systems at some point
in the past. The sun's gravity would
then knock a few outlying antimatter
meteors out of their precarious orbits
and into ours.
Antimatter meteors may not have been
able to form in our solar system, their
particles would have been annihilated by
interaction with matter far too early
for that. But in space, just an empty
vacuum somewhat devoid of matter, there
would be nothing for them to annihilate
with.
>> [music]
>> So, there's a chance they could be among
us.
And if they were, would we even notice?
Soon after Rojansky, American astronomer
Lincoln LaPaz began to wonder whether
any of the craters on Earth could be
attributed to antimatter meteors. So,
perhaps this is the time to consider
what an antimatter meteor could do if it
came in contact with the Earth, so we
know what to look out for.
Thankfully, it's not necessarily a
simple journey.
The first issue such [music] an
antimatter meteor would encounter before
reaching earth would be our atmosphere.
To be fair, this is a problem for
regular meteors, too. When one of those
trades vacuum for air, the speed at
which it's traveling causes it to
generate incredible friction,
causing some or all of the meteor to
burn up on the way down, depending on
its starting size, leading to the fiery
streak I mentioned at the start.
However, antimatter meteors would have
it much worse.
Each particle of atmosphere that the
antimatter meteor encountered on the way
down would annihilate a similar amount
of the meteor.
So, to understand what happens next, we
need to turn to Einstein's famous
equation E = mc squared.
The meteor's mass, multiplied by the
speed of light squared, tells us how
much energy would be released by the
meteor on the way down. Technically,
twice that, as for each particle of the
meteor that's annihilating, a particle
of atmosphere is doing so, too.
This is potentially a huge amount of
energy being released.
Let's do a bit of a thought experiment
to work out how much.
While the dividing line between the
atmosphere and space is either 100 km
up, if you're talking to Europeans, or
80 km up, if you're talking to
Americans, as the two haven't reached a
consensus on that point yet, over 90% of
the mass of Earth's atmosphere is
actually clumped below 16 km, clinging
to the planet as closely as possible
thanks to the pull of gravity.
So, by working out how much air exists
between this line and the Earth's
surface, we can get a rough idea of how
much antimatter you'd need to have
before an antimatter meteor hits the
ground.
This is a little tricky to calculate,
but thanks to a chart released by the
International Standardization
Organization, I was able to add up the
sum total of the atmosphere at different
elevations between the surface and 20 km
high, and found that in a 1 m by 1 m
corridor between space and the surface,
you'd have roughly 10,132
kg worth of atmosphere.
A meteor would need to be roughly this
large to even hit the surface, assuming
it traveled the most direct route
straight down.
Now, antimatter is similar in mass to
matter, so we can assume that they would
have comparable compositions, too.
Most meteors are made of chondrites, so
assuming an average antimatter meteor
made of the antimatter version of the
same stuff, we can put the density of
our antimatter meteor at around 3,400 kg
per cubic meter.
Of course, there's then a balancing act
you need to do. The bigger the meteor,
the larger the surface area, and the
more air it doesn't counters before it
reaches the surface.
However, as volume scales up in cubes,
while surface area is only squared,
there is a sweet spot you can hit where
the amount of air being annihilated by
the meteor would theoretically perfectly
match the amount of mass the meteor
possesses. And then we know that any
mass of meteor above that would make it
through the atmosphere to hit the
ground. Using very rough math, I found
that a 3 by 3 by 3 m meteor, or 27 cubic
meters, worked best for this
calculation, which had a mass of 91,800
kg, or 3,400 kg per cubic meter * 27
cubic meters.
This would only encounter 91,191
kg worth of air on the way down, meaning
608 kg worth of meteor would actually
hit the surface.
That's approximately the mass of a large
grand piano. How much damage could 600
kg of antimatter actually do?
It's time for our equation, E = mc².
Using this calculation, we learn that
600 kg of remaining meteor releases 5.4
* 10 ^ 19 J of energy, or 54 quintillion
J.
Then, double that as the ground being
annihilated releases that amount too.
So, 108 quintillion J.
For a point of reference, a 1 megaton
nuke gives off 4.18 * 10 ^ 15 J of
energy.
Even if we scaled up to the largest nuke
ever detonated, the Tsar Bomba, which
had a yield of 50 megatons, we're still
only looking at 2.09 * 10 ^ 17 J. A full
5,000 times weaker than our antimatter
grand piano. And the Tsar Bomba blast
was so great, towns within 55 km, like
70, were leveled. Wooden buildings 160
km away were reportedly damaged. The
light from the blast was seen 1,000 km
away.
Windows in Norway and Finland were
shattered by the explosion.
If just 600 kg of our antimatter meteor
impacted in the middle of a state like
Texas,
the whole state would be destroyed. The
center vaporized, the rest devastated.
This is just the energy released from
our antimatter meteor annihilating on
the ground, not even going into things
like kinetic energy.
But even then, it's actually not just
the 600 kg that hits the ground we need
to worry about.
When you consider the rest of that
91,200
kg of mass that was annihilated in the
atmosphere, the blast radius becomes a
lot bigger.
While some of that energy will travel
upwards into space, minimizing the
damage below,
suddenly, you're not just worrying about
Texas. You're worrying about the
entirety of the USA.
All that's just from an antimatter
meteor that's comparable in size to a
car.
We really do not want to get hit by an
antimatter meteor.
So, what actually are the risks here?
Let's say that an antimatter meteor had
got swept up in the sun's gravity well
millions to billions of years ago.
Could it now be one of the 40,155
near-Earth asteroids that NASA tracks?
The good news is no.
For one simple reason.
Space isn't actually empty.
While we talk of space being a vacuum,
even in space, there are trace amounts
of dust floating in the void. As such,
an antimatter asteroid traveling even
through the interstellar medium would
not likely have a survival rate of
longer than around 300 years.
As we last clipped another star 70,000
years ago, Scholz's star, in case you're
interested, it would have to have been
an exceptionally lucky antimatter meteor
to not only dodge all the other
asteroids in that time, but also to have
not encountered enough dust since then
that it would have disintegrated into
gamma radiation.
For this same reason, an interstellar
antimatter comet coming through our
solar system
>> [music]
>> would be unlikely.
Small ones would burn up before reaching
us, and large masses would be
noticeable. They would emit a steady
stream of gamma rays as they traveled,
making them detectable to our
telescopes.
No such sparkling asteroids have ever
been detected.
So, all in all, while it would be
devastating to be hit by an antimatter
meteor, it is unlikely that one would
survive long enough to reach our planet,
assuming they and antimatter stars exist
in the first place.
We are probably quite safe.
Besides, if antimatter meteors existed,
we might have seen some evidence of them
before. Meteor impacts with unusual
destructive capacity, but leaving no
traces of the meteor that caused it.
We've not seen anything like that.
Have we?
In June 1908, a fireball lit up the sky
in a remote part of Siberia. The meteor
exploded before hitting the ground, its
detonation causing massive forest fires
and sending trees crashing to the ground
like bowling pins in an area of
destruction kilometers wide.
Witnesses more than 30 km away reported
seeing a flash brighter than the sun
followed by a roar of thunder. Due to
its remoteness, scientific teams did not
arrive at the site until 1927,
but even then, the destruction caused by
the blast was easy to see.
Strangely, for an object that caused
[music] such destruction, almost no
trace of the meteor was ever found
beyond a few microparticles.
But the Tunguska event couldn't have
been caused by an antimatter meteor,
could it?
Don't worry, we know that it probably
wasn't an antimatter meteor,
but there was a lot of debate about the
topic in the past, and we couldn't
resist being a little spooky.
Have a look around you.
Everything you see, from the skin of
your hands to the screen you're watching
this video on, is a different
combination of the same three building
blocks of matter:
protons, neutrons, and electrons.
Now, let's look a little farther. Say at
Mars or the Andromeda galaxy, or even
halfway across the observable universe,
and still there is matter made of
protons, neutrons, and electrons as far
as the eye can see.
At first, this might not sound all that
surprising.
But for once, the mystery here isn't
that we've seen something we can't
explain, but rather that we haven't seen
something we were expecting: [music]
a universe just as full of antimatter.
I'm Alex McColgan, and you're watching
Astrium. Join me today as we explore the
world of antimatter and learn about its
interactions with other particles and
even with gravity.
By the end of this video, you'll
probably agree that antimatter is a bit
weird, but you'll also see why some
physicists are frustrated that it isn't
weird enough.
Let's get one thing out of the way
first. Although it might sound like
something straight out of science
fiction, antimatter is very real.
It forms a critical part of the standard
model of particle physics, and particles
of antimatter have been observed in
experiments going back nearly a century.
The very first detection of antimatter
dates back to a 1932 experiment
conducted by Carl D. Anderson at Caltech
using a cloud chamber immersed in a
magnetic field.
When charged particles from outer space,
broadly called cosmic rays, intercept
the Earth's orbit and fly through this
chamber, the magnetic field curves their
paths according to the charge and mass
of each particle.
And the clouds show a visible imprint of
their resulting trajectories.
Anderson was hoping this experiment
would help determine just what kinds of
particles were streaming into the Earth
from the cosmos, and he may have found
just a little bit more than he bargained
for.
What Anderson saw was that these cosmic
rays included both positively and
negatively charged particles.
The masses of the negatively charged
particles lined up exactly with the
known mass of an electron,
but some of the positively charged
particles were far too light to be
protons.
Instead, they appeared to have the mass
of an electron despite having the
opposite charge.
And so, these never-before-seen
particles
came to be known as antielectrons,
or later, positrons for short.
In 1936, Anderson would win the Nobel
Prize in physics for this discovery.
Meanwhile, a British physicist, who was
also destined to win a Nobel,
had been developing a description of
electrons that would fit nicely within
the framework of quantum field theory.
His name was Paul Dirac.
By 1928, Dirac had realized that in
order to describe electrons as quantum
fields in a way that was physically
consistent with special relativity,
they had to be part of a larger
mathematical structure, later known as a
Dirac spinner, that inevitably gave rise
to both positively and negatively
charged versions of the same particle.
In this way, Dirac had predicted the
existence of positrons before Anderson
had even built the cloud chamber that
would detect them 4 years later.
What's even more incredible is that
electrons aren't the only fundamental
particle to come in a two-for-one Dirac
spinner package.
Other particles of matter, like the
quarks that make up protons and
neutrons, each have their own antiquark
counterparts.
These antiquarks can come together to
form antiprotons and antineutrons,
which can then bond with positrons to
form antiatoms [music]
and antimolecules.
You could make a whole planet out of
antimatter, and from the outside, it
would look quite similar to an ordinary
planet made of ordinary matter.
But, [music] if antimatter were too
similar to matter, if the only
difference were the sign of its charge,
then it would be impossible to explain
why our universe contains so much of one
and so little of the other.
This cosmic mystery, known as the
baryonic asymmetry of the universe, sent
physicists on a decades-long quest to
try and find as many differences as they
could between matter and antimatter.
That quest lives on today, spearheaded
by particle colliders at CERN that are
capable of producing, trapping, and
studying both positrons and antiprotons.
But, before we talk about these
experiments, let's try to summarize what
we already know about the properties of
antimatter.
When studying antiparticles in
isolation, experiments have confirmed
with ever-greater precision that their
intrinsic properties, namely their
masses, are exactly the same as for
ordinary particles.
And when studying how antiparticles are
affected by electromagnetic forces,
experiments have again found that they
behave the the
exact way as ordinary particles, except
with the opposite electric charge, just
as Anderson had observed in his cloud
chamber. But, electromagnetism is just
one of the four fundamental forces of
nature, alongside gravity and the weak
and strong nuclear forces.
And as physicists began to better
understand the weak force in the 1950s
and '60s, they realized that particles
and antiparticles are actually affected
by it quite differently.
The first surprise was that ordinary
particles could only feel the weak force
if they were left-handed.
And antiparticles could only feel it if
they were right-handed.
The concept of handedness, or chirality,
is subtle and difficult to conceptualize
for particles with mass.
But, a loose analogy can be drawn with a
particle's helicity, which describes
whether a particle is spin up or spin
down along its direction of motion. In
this analogy, a spin up particle is
called right-handed, while a spin down
particle is called left-handed.
The second, and even crazier, surprise
was that right-handed antiparticles
experienced a different strength of the
weak force as compared to left-handed
ordinary particles.
In practice, this means that the quantum
probabilities for radioactive decay in
ordinary nuclei are somewhat different
from the probabilities of the analogous
decay processes in antinuclei.
This fundamental asymmetry between
particles and antiparticles was first
observed in a 1963 experiment run by
James Cronin and Val Fitch of Princeton
University, who would be awarded yet
another Nobel Prize for their discovery.
When this asymmetry was discovered,
there was some hope that it would
explain the baryonic asymmetry of the
universe.
Perhaps these differences in the weak
force were responsible for the abundance
of matter and utter lack of antimatter
around us.
But the math didn't quite work out.
There simply wasn't enough of a
difference between the strength of the
weak force acting on particles versus
antiparticles.
That was when physicists began to turn
their attention to the strong nuclear
force.
Theoretical models predicted that, just
like in the weak interaction, there
should be some differences in how
left-handed particles and right-handed
particles feel the strong force.
But antimatter just keeps surprising us.
Every experiment to date suggests that
the strong force treats particles and
antiparticles just the same.
This brings us to the last of the four
fundamental forces and the subject of
today's ongoing experiments at CERN,
gravity.
To be honest, suggesting that gravity
might treat matter and antimatter
differently is kind of a long shot.
Think back to the popular legend of
Galileo tossing stones of different
sizes and materials from the Tower of
Pisa. They all fell at the same rate
because the gravitational acceleration
on Earth is 9.8 m/s²
regardless of which object is falling.
Of course, the experiment works even
better in a vacuum chamber where air
resistance is taken out of the equation.
Newton expanded on this idea and showed
in the 17th century that your
gravitational acceleration anywhere in
space depends only on the mass of the
object pulling you and your distance
from it, but not on any of your personal
properties, not even your own mass.
This famous result, known as the
equivalence principle, is the foundation
of Einstein's theory of general
relativity, our most accurate and
successful model of gravity to date.
With that in mind, physics is still an
experimental science at its core, and we
can't know for sure whether matter and
antimatter obey the same laws of gravity
unless we check for ourselves.
The physicists at CERN set out to do
just that, motivated not only by the
baryonic asymmetry of the universe, but
also by a few speculative papers
suggesting that the cosmological
properties of dark matter and dark
energy could be more easily explained if
antimatter were to have a negative
gravitational charge,
or to put it simply, if antimatter were
to fall up rather than down.
There are several ongoing experiments at
CERN testing the gravitational
properties of antimatter, including
AEgIS,
GBAR, and ALPHA.
Today, we will focus specifically on a
key experiment coming out of the ALPHA
group that was published in the journal
Nature this past September.
After decades of assumptions, this
experiment has brought us real-world
data on the gravitational acceleration
of antimatter on Earth's surface.
But, before we show you the results,
let's take a moment to appreciate just
how intricately this experiment was
designed in order to isolate and measure
the effects of gravity.
The first step in the experiment is to
secure a beam of several million
positrons per second emitted from a
radioactive isotope of sodium.
Most of these positrons end up colliding
with ordinary matter in the experiment,
causing miniature explosions in which
positrons and electrons annihilate each
other and release a small burst of
energy in the form of light.
But, a small fraction of the positrons
survive as they are guided through the
experimental apparatus where they are
cooled by low-pressure gases and trapped
by electric and magnetic fields.
But, observing the effects of gravity on
these positrons would be nearly
impossible. Their masses are so small
that the tiny force of gravity felt by
each particle is overshadowed by even
the smallest fluctuations in the
surrounding electromagnetic fields.
That's why this collection of positrons
is merged with a separate container of
antiprotons where they bond and form
neutral antihydrogen atoms that are much
less responsive to stray electromagnetic
fields.
And where did the antiprotons come from?
Suffice it to say that they were
produced by firing ordinary protons into
a block of metal really, really fast.
Yes, physics is awesome like that.
Once the antihydrogen atoms are created,
they behave like tiny, weak magnets that
can remain trapped by complicated
arrangements of external magnetic
fields.
Now, this magnetic interaction is weak
enough that it no longer overwhelms the
gravitational effects that we are trying
to measure.
The chamber containing these
antihydrogen atoms is nearly a vacuum.
There are just about 200,000 atoms of
ordinary gas per cubic centimeter
compared to a typical atmospheric
density of 20 quintillion atoms per
cubic centimeter. Under these
conditions, the trapped antihydrogen
atoms almost never collide or annihilate
with atoms of ordinary matter.
Instead, they can more or less just
float around the chamber for minutes or
longer.
But, as the magnetic fields used to
vertically trap the antihydrogen atoms
are weakened,
this random floating eventually allows
the antihydrogen atoms to escape through
either the top or the bottom of the
chamber, where they can collide with a
wall of apparatus, annihilate with some
ordinary atoms, and release a small
burst of light.
In the ALPHA experiment, this happens
over the course of about 20 seconds.
The theory behind the experiment is that
if gravity really pulls antimatter
downwards, more of the antihydrogen
atoms escape through the bottom than the
top.
The stronger the gravitational force,
the more atoms escape through the
bottom.
The simulations the ALPHA team ran
showed that under normal gravitational
attraction, about 85% of the
antihydrogen atoms should escape through
the bottom.
Whereas, only 20% of them would escape
through the bottom if gravity pulled
antimatter upwards.
If there were no gravitational force at
all, the simulation showed a more even
distribution of 55% escape through the
bottom, probably only differing from 50%
due to asymmetries in the experimental
apparatus itself.
What did the actual experiment find?
Well, roughly 75%
of antihydrogen atoms escaped through
the bottom of the chamber, showing a
clear preference for downward pulling
gravity.
As any thorough scientist would, the
ALPHA team repeated this experiment to
collect a variety of data points that
could tell a more complete story.
They redid the procedure under various
levels of magnetic field bias, which
applied external upward or downward
magnetic forces on the antihydrogen
atoms.
On this graph, a bias of -1g
means that enough magnetic force is
applied to counteract normal gravity,
while a bias of +1g means there's an
extra g of magnetic force is applied to
push the antihydrogen atoms downward and
so on.
The team made predictions through
simulations for each bias and for
various possible gravitational
interactions, which produced the orange,
green, and purple curves shown here.
As you can see, the experimental data
points shown in blue best match the
orange curve, which represents the
normal simulation where gravity pulls
antimatter downwards.
But because the data falls just a bit
below this curve, the best-fit
gravitational acceleration was only
0.75g,
3/4 of the strength of gravity acting on
ordinary matter.
Does this mean that gravity affects
matter and antimatter particles
differently after all?
Not necessarily.
Let's have a look at the error bars.
They indicate that there are two major
forces of uncertainty in the results,
including an uncertainty in the applied
bias, possible errors in alignment, and
other systematic and statistical
uncertainties.
When accounting for these uncertainties,
the best-fit gravitational acceleration
is actually reported as 0.75g
plus or minus 0.13g
plus or minus 1.6g.
This means that a full 1g of
gravitational acceleration is still
fairly consistent with the collected
data.
Future experiments will be able to
determine more precisely how strongly
gravity acts on antimatter,
but we can already rule out speculative
theories that rely on antimatter falling
up instead of down.
In the end, despite how weird and
backwards the world of antimatter is, it
seems that only the weak force actually
applies differently to particles and
antiparticles.
But, explaining the baryonic asymmetry
of the universe would require much more
drastic differences between the two.
So, scientists aren't looking for them.
Could there be new forces and particles
that interact even more weirdly with
antimatter?
Or would you be willing to accept that
having so much more matter than
antimatter around us is a mere
coincidence?
In any case, let us know if you've
learned something new about antimatter
from watching this video, and whether
this is a topic you'd like to hear more
on.
800 light-years away, there's an unseen
antimatter factory churning out
high-energy positrons,
tiny particles of antimatter that are
streaming through the cosmos and
colliding with our planet.
For much of history, we didn't know this
strange source existed. Most of the
positrons bombarding us went completely
undetected, instead getting absorbed in
our planet's atmosphere.
It wasn't until we started looking from
beyond the bounds of our planet that we
noticed them.
In 2011, NASA's Alpha Magnetic
Spectrometer, a state-of-the-art
particle detector some 200 mi up aboard
the International Space Station, was
switched on.
What did it find? You guessed it,
positrons.
The presence of these subatomic
particles was to be expected, but not in
the numbers they were finding.
Such was the sheer volume of positrons
being detected that the usual sources
like natural radioactive decay and
cosmic rays no longer offered a
sufficient explanation.
So, where were they coming from?
We've only recently been able to trace
the culprit of this cosmic antimatter
shower, and it all comes down to another
high-energy discovery.
A strange gamma-ray haze named Geminga,
first identified in the 1970s.
What is this mysterious source of gamma
radiation? And what does it have to do
with the unusual abundance of
high-energy positrons hitting our
planet?
I'm Alex McConaughey, and you're
watching Astrium. Join me today as we
tune into the enigmatic frequency of
Geminga, whose gamma radiation has lit
up the world of astrophysics for
decades.
Up in our night sky, nestled in the
Gemini constellation in the northern
celestial hemisphere, there is something
peculiar going on.
In 1972, NASA's Small Astronomy
Satellite 2, or SAS 2, identified an
unknown source of gamma radiation.
But with the technology available at the
time, the best it could do was trace its
origin to this wider region of our Milky
Way.
So, the radiation's ultimate source
remained hidden among the stars for
decades.
Nevertheless, it was given a name,
Geminga, as coined in 1976 by Italian
physicist Giovanni Bignami, who would
dedicate his career to studying it.
It's a play on words, a combination of
Gemini, the region where it's located,
and gamma, the type of radiation it
emits.
Geminga is also a pun in Beniamino's
Milanese dialect, meaning "it's not
there". A fitting name for a gamma-ray
haze with unknown origin.
It wasn't until 1983
when Beniamino and his team finally had
their big break.
They managed to identify a weak X-ray
signal from Geminga using the Einstein
X-ray satellite.
This meant, although its exact position
remained unknown, they could narrow down
their search area and were getting
closer [music] to uncovering Geminga's
hiding place. But it didn't answer the
big question, "What is it?" as
astronomers could still only offer vague
guesses about the true nature of the
source.
That was until 1991
when they had another lucky break. Two
separate missions identified radiation
coming from Geminga and they weren't
constant signals, but pulses.
The first of these discoveries was made
with a German-built X-ray telescope
known as ROSAT, short for Röntgen
Satellite, named after the German word
for X-rays.
ROSAT was the first to identify pulses
in the X-ray signal coming from Geminga
and soon after they were also confirmed
in the gamma wavelengths by the
Energetic Gamma Ray Experiment Telescope
or EGRET, a telescope aboard NASA's
Compton Gamma Ray Observatory Satellite.
Not only did these complementary
observations demonstrate that the X-rays
and the gamma rays were both coming from
Geminga, but for the first time they
revealed what Geminga was.
With a period of 0.237
seconds, flashing as it spins around its
axis a little more than 4 hertz or four
times per second, Geminga behaved like a
pulsar.
A pulsar is a type of neutron star that
spins rapidly, emitting beams of
radiation that sweep across space like a
cosmic lighthouse.
From across the galaxy, most pulsars
appear to flash in radio waves anywhere
from a few times a minute to as fast as
700 times per second.
And at this point in the early '90s,
they were incredibly rare.
You see, before ROSAT and EGRET, only
two other high-energy gamma-ray pulsars
had ever been identified, the Crab and
Vela pulsars.
And Crab and Vela were different to
Geminga in a couple of key ways.
First, in addition to gamma rays, both
of these pulsars also produced radio
waves and were therefore visible using
radio telescopes.
So, if Geminga was a pulsar, then it
would be the first discovery of one that
was apparently radio silent, only
emitting enough radiation to be seen in
the gamma and x-ray wavelengths.
And second, Crab and Vela were
surrounded by their respective nebulae,
remnants from when they were created
from supernova explosions.
But Geminga's nebula was conspicuous by
its absence.
So, why is Geminga this powerful source
of gamma rays so good at hiding from our
radio telescopes? Where is its nebula?
Or could it be a different type of
object altogether?
Well, the answer to the first question
is, in part, because we hadn't been
listening properly due to the
limitations of the available technology
and due to our understanding of the
radio emissions of such stellar
remnants.
You see, while radio pulsars can emit
radio waves across a wide bandwidth,
from as low as 17 MHz to above 87 GHz,
around half the radio spectrum, not all
of these frequencies travel well through
space.
Even though we've known since the 1970s
that radio pulsars often peak between
100 to 200 MHz, where they are
intrinsically brightest, things like the
interstellar medium, background sky
temperature, and effects from the
ionosphere mean that lower frequencies
are dampened as they make their way
across space, resulting in very weak
signals that are much more difficult to
detect.
Because those radio signals are so weak,
most radio telescopes hadn't been
looking for them, instead confining
themselves to searching for signals
between 430
and 1,600 MHz.
This would have been fine had Geminga
behaved as expected for one of its kind.
Since it did not, it took until 1997 for
scientists to realize what was
happening.
Three independent observations from the
Pushchino Radio Astronomy Observatory
were able to identify extremely weak
pulses from Geminga using a sensitive
transit [music] antenna.
The faint radio pulses came in around
100 MHz, which explains why previous
radio searches for Geminga had come up
silent.
Turns out Geminga wasn't truly hiding
and had been sending us signals. We just
weren't listening correctly.
That same year, a team led by the late
astronomer Janusz Gil theorized that
another reason Geminga had appeared to
be radio silent may be its magnetic
field. Models showed that radio waves
may be absorbed or refracted within the
pulsar's magnetosphere, leaving only
weak pulses around 100 MHz to be
detectable.
This would effectively leave it quiet at
the higher radio bands than most
telescopes used.
Confirming Geminga was a pulsar, and
specifically a gamma-ray pulsar, was a
big deal.
In fact, 99% of its output is in the
gamma range, making it one of the
brightest gamma-ray sources in our
entire galaxy.
It is, as it turns out, all that's left
after a star several times more massive
than our sun exploded about 350,000
years ago.
But, its relative radio silence and its
apparent lack of a nebula weren't the
only unusual things about this pulsar.
When Italian astrophysicists, including
Bignami, Geminga's name-giver, compared
a series of observations from the
European Southern Observatory's 3.6-m
telescope and New Technology Telescope
with observations from the
Canada-France-Hawaii
Telescope, they found [music]
that Geminga was moving.
Not only that, it was traveling at an
unusually high speed of around 0.2 arc
seconds [music] per year.
As a reminder, an arc second is a very
small unit of angular measurement. Used
when we need more precise measurements
than a degree would allow.
Within each arc degree, there are 60 arc
minutes, and within each arc minute are
60 arc seconds. These arc seconds are a
common unit used in astronomy to talk
about the movement of objects across the
sky from our perspective on Earth.
If you were to draw a circle around the
orbit of the moon around the Earth,
there would be 360°
around that circular path. So, at any
given time of day or night, assuming
nothing is blocking your view of the
horizon, you can see about 180° of the
sky. And from the horizon to the zenith,
the top of [music] the sky, it is 90°.
If you hold out your little finger at
arm's length and close one eye, the tip
of your little finger covers about 1° of
the sky, roughly.
Next time you're outside on a clear
night, try this and see if your little
finger can cover the moon. It should,
[music] because the moon takes up only
about half of a degree, or about 31 arc
minutes in the night sky.
Geminga traveling 0.2 arc seconds across
our sky each year may not sound like a
lot, but from our perspective on Earth,
the typical star only moves a few
thousandths of an arc second per year.
Yet, despite being 800 light-years from
us, Geminga will travel 30 arc minutes,
the equivalent to the apparent diameter
of the moon, across our sky in just over
10,000 years.
In other words, this stellar corpse is
racing through the galaxy at nearly 210
km per second.
Heading towards the border between the
constellations Gemini and Lynx, and at
its current rate of motion, Geminga will
remain in Gemini for another half
million years. But it may need a new
name after that.
However, this mysterious pulsar gets
stranger still.
It is its vast speed that helps produce
another feature that scientists were
about to discover.
As it hurtles through space, Geminga
leaves behind two ghostly X-ray tails
that streak 3 trillion kilometers across
the sky.
As I discussed earlier, despite the fact
that this fast-moving pulsar is nearly
radio silent, it certainly isn't quiet
in the gamma-ray and x-ray wavelengths.
In 1999, ESA's X-ray Multi Mirror
Mission, or [music] XMM-Newton,
was launched to peer deeper into this
x-ray universe, and 4 years later, a
team led by Patrizia Caraveo uncovered
these comet-like x-ray trails.
Their shape and brightness are partly
explained by the shock wave created by
Geminga's motion through space and its
rotation as a pulsar,
but they are also revealing of another
attribute, Geminga's colossal mass.
Measuring only about 20 to 30 kilometers
across, Geminga is extremely dense,
containing about as much mass as a one
and a half of our suns.
To put that in perspective, if you had a
teaspoon of neutron star material, it
would weigh about 4 billion tons, as
much as 10,000 Empire State Buildings.
As this dense, high-mass object races
forward through the low-density
interstellar medium, just 0.06 to 0.15
atoms per cubic centimeter,
it compresses the interstellar medium
and its own embedded magnetic field by a
factor of four.
Meanwhile, the incessant spinning of the
neutron star creates an environment
where electrons and their antimatter
counterparts, called positrons, can be
accelerated to extreme energies,
powerful enough to emit high-energy
gamma-rays.
While most of these electrons are seen
in the gamma radiation that escapes from
the pulsar, some get trapped and spiral
within this enhanced magnetic field.
In these images from a computer model,
the tails can be seen streaking along
the edges of Geminga's three-dimensional
shock wave, like the wake created by a
boat going through water.
Only this boat is more massive than our
sun, and the wake is made up of
extremely high-energy X-rays.
The final piece of the Geminga puzzle
wasn't discovered until 2005.
It's nebula.
Taking the form of a shell of neutral
hydrogen gas with a radius of 0.4
parsecs wide, it turned out to be what
we call a pulsar wind nebula.
This type of nebula is created from the
wind plasma that emanates from a
pulsar's magnetic poles.
The plasma, made of charged particles
that can be accelerated to to near light
speed, surrounds the pulsar, creating a
nebula of high-energy particles that
give off strong X-ray emissions.
With the confirmation that Geminga did
have a nebula, its identity as a pulsar
could finally be confirmed.
But Chandra went even further.
In addition to imaging Geminga, Chandra
also looked at a second pulsar called
B0355
+ 54.
And by comparing the two, astronomers
uncovered another possible explanation
for the absence of radio pulses from
Geminga.
On the surface, these pulsars seem quite
similar. They are both about half a
million years old, and they spin about
four to five times per second.
However, as you know, Geminga is seen
primarily in gamma-ray pulses with no
bright radio emissions.
By contrast, the other pulsar, which
I'll refer to as pulsar B, is not seen
in gamma rays.
And instead, is one of the brightest
known radio pulsars.
How could these two pulsars be so
similar, yet so vastly different in how
we see them?
The answer may be as simple as how each
of these pulsars are oriented relative
to our observation from Earth.
Astronomers believe that these images of
Geminga and pulsar B have revealed their
spin axes and uncovered a reason for why
radio and gamma ray pulses may be
present or absent on different pulsars.
Like our own magnetic field around
Earth, both of these pulsars have
magnetic poles close to their spin
poles.
These poles are where the beams of
pulsing radio emissions come from.
You could try to model this if you
skewer a little foam ball right down the
middle.
The foam ball is a pulsar and the radio
beams coming from the poles are
represented by the wooden skewer coming
out both ends. If you spin the ball
around the skewer like a spin axis, you
create an equator around the middle.
To illustrate the gamma ray source along
the spin equator, called the torus, you
could cut a hole in a paper plate and
squeeze it over the foam ball.
Now you've got yourself a disk of gamma
rays beaming out from the equator in
every direction.
With Geminga, the edge of the paper
plate is pointing towards us, meaning
the gamma rays are heading to Earth.
But for pulsar B, its relative position
to us is at a different angle, as if
looking at the flat surface of the
plate.
The gamma rays are moving perpendicular
to our line of sight, therefore missing
Earth.
Let's look again at the two Chandra
images of Geminga on the left and pulsar
B on the right, along with artist
illustrations of what astronomers
believe the pulsar wind nebulas look
like for each of these.
In the image of pulsar B, the long
trailing blue tails represent the radio
jets emanating from its poles, and the
skewers coming out of both ends.
Only instead of being straight like a
wooden skewer, pulsar B is moving so
fast through space that these jets
appear bent backwards, trailing behind
as the pulsar moves through space.
Now, look at the image of Geminga.
Here, the long twin tails on either side
of the image are the radio jets,
trailing behind as it, too, rushes
through space.
But this time, instead of pointing
almost directly toward and away from our
vantage point on Earth, these jets
appear to be coming off to the sides,
not aimed at Earth.
So, when astronomers look at Geminga,
they see powerful gamma-ray emissions
from the spin equator, but the radio
jets point to the sides and remain
unseen.
And when they look at pulsar B, the
opposite happens. The radio jets are
pointed almost straight toward our
planet, while the gamma-ray source at
the equator is missing Earth.
Sometimes, the most simple explanation
is the correct one.
And that finally brings us back to the
decades-long mystery of an unusual
abundance of antimatter bombarding our
planet.
For more than a decade, a particle
detector called the Alpha Magnetic
Spectrometer, or AMS-02, has been
attached [music] to the International
Space Station, collecting information on
antimatter, dark matter, and cosmic ray
sources.
As a reminder, cosmic rays are energetic
particles, [music]
fragments of atoms that travel through
space at nearly the speed of light.
These can be made by the sun, by
supernova explosions, or other cosmic
means.
And in 2013,
the first results of the AMS-02
experiment were announced. The detector
had recorded more than 400,000
positrons, the largest sample of cosmic
ray positron data ever collected, and
[music] increasing the world's total
cosmic ray positron data by a
hundredfold.
For years, many astronomers and
physicists hoped that this excess
antimatter may be the byproduct of a
dark matter annihilation, offering
possible clues about this mysterious
substance. After all, dark matter could
make up around 27% of the cosmos, and
yet we still don't know what it is.
Like regular matter, dark matter holds
mass and takes up space, but it doesn't
seem to absorb, reflect, or interact
with light, at least not in a way we can
detect.
Some theorize that dark matter may be
made of yet unidentified types of
particles.
Whatever it is, scientists had high
hopes that the overabundance of
antimatter being detected aboard the ISS
may hold clues about dark matter's true
nature.
Unfortunately, they've been left
disappointed. The more scientists dig
into the data, the clearer it's
becoming. The most likely source of
these positrons may actually be pulsars.
Astrophysicists had long suspected this,
but until 2017,
there simply wasn't proof.
It was the High-Altitude Water Cherenkov
Gamma Ray Observatory that finally added
evidence to this hypothesis.
A small halo of gamma radiation was
identified surrounding Geminga with
trillions of times more energy than
[music] is visible to our eyes, from 5
to 40 trillion electron volts.
The sort of radiation usually produced
by positrons. [music]
This was the first real observational
evidence pointing to a pulsar as a
potential source.
Pulsars naturally surround themselves
with a haze of both electrons and their
positron counterparts as a result of the
star's intense magnetic field.
This intense magnetic field pulls
particles from the pulsar's surface and
accelerates them to near the speed of
light.
Scientists think that these accelerated
positrons and electrons are then
colliding with starlight, boosting the
light to higher energies, which then
radiates as the gamma-ray halo observed.
But, based on the size of the halo that
the Hawk team saw,
Geminga's positrons would rarely have
the energy required to reach our planet.
And so, they believed the excess
positrons must have a more exotic
source.
That was until stunning new information
was uncovered a few years later thanks
to a team led by astrophysicist Matea Di
Mauro.
Using a decade of gamma-ray data from
Geminga acquired from Fermi's Large Area
Telescope, which is able to observe
[music] lower energy light than the Hawk
Gamma-Ray Observatory, Di Mauro's team
was able to subtract out all other
gamma-ray sources to reveal a
spectacular glow coming from Geminga,
much, much bigger than what the
scientists had ever seen before.
The vast, oblong halo of glowing
gamma-rays at an energy of 10 billion
electron volts spanned 20 degrees of the
sky, similar to the area the Big Dipper
constellation occupies.
And that's not all. The glow of gamma
radiation is even bigger at lower
energies.
If we could see it all with the naked
eye, Geminga's gamma-ray glow would
dominate our sky, covering an area 40
times bigger than the full moon.
With this new information,
astrophysicists found that the size of
Geminga's halo meant that this one
pulsar alone could be responsible for as
much as 20% of the excess positrons
detected near Earth.
From there, it's no stretch to imagine
that other pulsars are the most likely
culprit for the remaining antimatter
abundance we found.
This explanation may not have solved the
mystery of dark matter, but it is
certainly a magnificent revelation.
It was Jocelyn Bell Burnell who
discovered the first pulsar in 1967.
Back when people thought that those
regular signals could be the work of
extraterrestrial life.
In the nearly 60 years that have passed
since, we have found thousands of
pulsars, and our understanding of these
neutron stars has grown with every one.
And since Geminga was identified as only
the third known gamma-ray pulsar in
1991, we've now spotted over 300 thanks
to NASA's Fermi mission.
But given Geminga's track record of
defying expectations and furthering
science,
I like to think that this particular
pulsar has more secrets still to reveal.
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