[MUSIC PLAYING]
Long before the God particle, there is the Oh-My-God
particle, a cosmic ray vastly more energetic than had ever
been seen or was even thought possible.
These ultra-high energy cosmic rays still perplex scientists.
Where are these extra galactic death rays coming from?
On October 15th, 1991, a single atomic nucleus travelling
at 99.99999999999999999999951% of the speed of light crashed
through our atmosphere and streaked across the Utah sky.
The nucleus quickly disintegrated
into a shower of subatomic particles and lights.
That light was seen by the Fly's Eye Observatory, a collection
of oversized tin cans that was an early experiment
by the University of Utah to spot the highest energy
cosmic rays in the universe.
Scientists analyzing the Fly's Eye data
calculated that the cosmic ray responsible for this particular
flash must have had a kinetic energy of 300 exaelectron
volts.
That's 48 joules, an amount of energy
we associate with macroscopic, not subatomic objects.
That single atomic nucleus carried as much kinetic energy
as a good sized stone thrown at your head at 50 miles an hour.
The particle was dubbed the Oh-My-God particle.
Nothing like it had ever been seen before.
In fact, cosmic rays of that energy
was supposed to be impossible.
Let's talk about cosmic rays for a second.
Radioactivity was discovered by Mary Curie and Henri Bacquerel
at the end of the 1800s.
High energy particles, electrons,
and small atomic nuclei, as well as gamma rays,
are ejected when heavier radioactive elements decay.
We're bathed in a very low level of this radiation
due to naturally occurring radioactive elements
in the Earth.
Soon after its discovery, this ambient radioactive flux
was found to weaken with height above the ground,
because the radiation loses energy to air molecules.
But weirdly, above a certain height,
this radiation starts to increase again.
Theodore Wulf first noticed this in 1909
when he took a detector to the top of the Eiffel Tower.
But the real proof came a few years later in 1912
when Victor Hess took some of Wulf's detectors
on a hot air balloon ride.
Radiation levels increased with height.
And that meant there had to be a source
of these high-energy particles somewhere above.
It turns out they were coming from space.
Cosmic rays had been discovered.
In the years following Hess's balloon ride,
and even following the Fly's Eye detection
of the Oh-My-God particle, we've come a long way
in the art of catching cosmic rays.
They are elusive little critters.
You can't focus them into a camera like you can with light.
Also, as Hess discovered, most don't make it to the ground
anyway.
For lower energy cosmic rays, one approach
is to look for their Cherenkov radiation.
All cosmic rays are traveling at pretty close
to the speed of light, but that's
the speed of light in a vacuum.
Light travels slower in a medium like air.
So when cosmic rays enter the atmosphere,
they're actually traveling faster than the new, lower
speed of light.
This results in a burst of gamma rays, Cherenkov radiation,
that is actually detectable from the ground.
Higher energy cosmic rays tend to obliterate themselves
several kilometers above the ground in massive collisions
with nuclei of air molecules.
The result is a cascade of subatomic particles, the debris
of the collision, that can spread itself out
over several kilometers.
These cascades are called air showers,
streams of charged particles cause the air
to fluoresce, a glow that can be seen by specialized telescopes.
Many of the debris particles also reach the ground
and can be detected there.
By analyzing the energies and trajectories of the debris,
the collision that produced them and the nature
of the original cosmic ray can be reconstructed.
Several facilities around the world
are devoted to catching cosmic rays.
For example, the Pierre Auger Observatory in Argentina
monitors a region around 3,000 square kilometers
for high-energy cosmic rays.
It includes 1,660 giant tanks of water designed
to see Cherenkov radiation when air shower particles pass
through them, as well as telescopes
to spot fluorescence in the air above,
just like the old Fly's Eye.
The Fly's Eye itself has evolved into the Telescope Array
Project.
It still uses upgraded fluorescence telescopes
and has added scintillation detectors on the ground.
These are simple slabs of acrylic
between metal plates designed to stop air shower particles
and detect the light produced as they smack
into nuclei within the slab.
By now, we have a pretty good census
of the types of cosmic rays that tend to hit the Earth.
Most of them are single protons, the nuclei of hydrogen atoms.
And a fair number are helium nuclei.
But about 1% of cosmic rays are heavier nuclei,
as heavier as iron.
We also see gamma rays and even anti-matter particles.
They come in at all energies, from a sickly billion electron
volts at the low end to the crazy 10 to the power of 20
electron volts or higher, like the Oh-My-God particle.
The higher the energy, the rarer they are.
At the lowest energies, the cosmos
flings one particle every second per square meter
of the Earth's surface.
At energies up near that of the OMG particle,
they are incredibly rare.
Only a handful have been spotted since the first,
giving a rare estimate of 1 per square kilometer
every couple of centuries.
So where do these things come from anyway?
To accelerate a particle to the energies of cosmic rays,
you need a particle accelerator.
We build artificial ones on Earth using giant rings
and powerful magnetic fields.
It turns out that the universe is full of natural particle
accelerators.
For lower energy cosmic rays, it's
believed that many, and perhaps most, come from supernova
explosions within our galaxy.
When a star explodes, the expanding shock wave
carries a strong magnetic field.
It can trap particles and accelerate them
until they're energetic enough to escape the shock.
The higher the energy of the cosmic ray, though,
the more likely it is to have originated
from outside our galaxy.
The exact sources of these so-called extra galactic
cosmic rays are more mysterious.
But they may come from magnetic acceleration in quasars,
or perhaps they're blasted out in gamma ray bursts.
The most ridiculous cosmic rays, like the Oh-My-God particle,
shouldn't exist at all.
See, the universe is basically opaque to particles
with such high energies.
Empty space isn't really empty, it's
full of low-energy microwave photons leftover
from the heat glow of the very earliest of times.
This is the cosmic microwave background radiation.
We talk about it in this episode.
Cosmic rays with energies over 5 times 10
to the power of 19 electron volts, about 8 joules,
can't travel far before smacking into these photons
and giving up some of their energy.
This is the so-called Greisen-Zatsenpin-Kuzmin,
or GZK, limit.
For years, it was thought that no cosmic ray could exceed it,
except that the OMG particle was six times more energetic.
Only a very small number of these extreme energy
cosmic rays have been seen since the OMG particle.
But those are perplexing.
See, they must have come from nearby,
from close enough to our galaxy to not be wiped out by the CMB.
In Now I'm talking close by on cosmic scales, so within 1
to 200 million light years.
But at that distance, sources like quasars and gamma ray
bursts should be very obvious.
Yet they seem to come from nowhere in particular.
We do see more than the average number coming
from the direction of the Ursa Major cluster,
but there's no obvious source there.
It's still a mystery exactly what
produces these extreme cosmic rays and how close to us
the sources are.
For cosmic ray astrophysicists, there's
a giant invisible particle accelerating
elephant in the room.
Part of the challenge in understanding cosmic rays
is that our atmosphere and magnetic field
shield the surface of the earth so well.
But we should count that as a blessing.
Apollo astronauts traveling outside Earth's magnetosphere
reported strange flashes of light,
which may be due to Cherenkov radiation
from cosmic rays passing through their eye's vitreous humor,
or from the particles hitting their optic nerves.
Even ISS astronauts are subjected
to a significant radiation risk.
Along with solar outbursts, cosmic rays
are one of the most serious obstacles
to manned interplanetary travel.
As we figure out the origins of these particles,
cosmic ray astronomy is becoming an increasingly powerful tool
for investigating our amazing universe.
But even now, these particles are extremely useful.
The highest energy cosmic rays, like the Oh-My-God particle,
generate collisions far more energetic
than our largest particle accelerator, the Large Hadron
Collider.
Studying cosmic rays may crack open
the mysteries of both the largest and the smallest scales
of space time.
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Last week, we talked about the prospects of building
telescopes on the moon.
Let's see what you had to say.
Acousticpsychosis wonders why we're
so obsessed with going to Mars when the moon makes
such a perfect nearer station.
Well, I absolutely agree with this.
The moon is an incredible gift and we should make
good and peaceful use of it.
Ragdala asks, who gives permission for moon landings.
In the case of the Moon Express program,
the approval was from the US government.
But you're right, no one owns the moon.
So in principle, no one can regulate landing on it.
However, a country can regulate the activities
of companies based in that country,
even if those activities don't happen within national borders.
However, there are real legally binding regulations.
The United Nations developed the Treaty on Principles
Governing the Activities of States in the Exploration
and Use of Outer Space, including
the Moon and Other Celestial Bodies,
or, the Outer Space Treaty, which
the United States has ratified.
It restricts the use of space for peaceful purposes,
but otherwise grants free access to nation states.
That doesn't mean a nation state can't impose legal restrictions
on its own citizens.
David Connolly emailed to remind us
that the Constellation Program and its Ares rockets
was discontinued in 2009 in favor of the Space Launch
System.
Yet, and I even commiserated with rocket scientist friends
when that happened.
Don't you hate it when knowing something
doesn't translate to remembering you know it
when it really matters?
Oh well, a few people wondered if I
was sick during the comment responses of the last episode
and wished me a speedy recovery.
Well, thanks guys.
I am feeling a lot better.
I shot the episode you just watched that same day, so
apologies for the voice.
And Alexander Sedov and others wondered
why I was wearing cowboy underwear in that episode.
Dude, I was sick.
You're lucky I got out of bed at all.
Having any type of underwear on was a major victory.