The Highest Energy Cosmic Ray Ever Seen | Unveiled

The Highest-Energy Cosmic Ray Ever Seen On October 15th, 1991, an observatory in Utah
detected something remarkable: a particle so powerful it was quickly nicknamed the “Oh-My-God
particle”. But what exactly was it? And why haven’t we witnessed anything like
it before or since? This is Unveiled, and today we’re uncovering
the extraordinary story of the highest-energy cosmic ray the world has ever seen. Are you a fiend for facts? Are you constantly curious? Then why not subscribe to Unveiled for more
clips like this one? And ring the bell for more fascinating content! Cosmic rays were properly discovered in 1912,
by Austrian-American physicist Victor Hess. They are high-energy, subatomic particles
that are blasted through space. And they’re also incredibly common; with
Earth constantly being bombarded by them, though most are low-energy and not particularly
noteworthy. It’s thought that cosmic rays, in general,
originate from very bright points in the universe, like distant stars, supernovas, quasars and
blazars. Our own sun could potentially throw out cosmic
rays, but it’s thought that the higher-energy rays (from our perspective) usually come from
much further away. For a cosmic ray to be considered ultra-high-energy,
it needs to score higher than one “EeV”, or one “exa-electronvolt”; which has the
same energy as a quintillion standard electronvolts… So, anything measuring more than one EeV is
packing a truly staggering amount of energy! Seeing as we’re getting hit by cosmic rays
all the time, though, what is it that makes the one sighted in Utah in 1991 so special? Well, to this day the Oh-My-God particle remains
the highest-energy cosmic ray ever seen, and though there have since been some similar
cosmic rays detected, none have truly come close to matching this one. The “OMG” particle had around 320 exa-electronvolts
of energy, and was travelling at 99.99 (followed by 19 more 9s, 5, 1) percent the speed of
light. This particular ray was so baffling that the
scientists who observed it reportedly held back their discovery for another year before
announcing it to the world… because they feared that it had simply been a crazy anomaly. We’d really never seen anything like this
before! Even by today’s standards, the OMG Particle
was around ten million times faster than any of the particles we’ve since accelerated
inside the world’s largest machine, the Large Hadron Collider. Specifically, the Oh-My-God shattered something
called the “Greisen-Zatsepin-Kuzmin Limit”; which was named after physicists who came
up with it, and is more commonly called the “GZK Limit”. Formulated in 1966, the GZK limit said that
no cosmic ray could ever have energy higher than 60 exa-electronvolts. This was because, despite how it appears to
us, space isn’t “just space”… It’s full of particles, including (and most
importantly, in this case), the photons that make up the cosmic microwave background radiation,
or CMB. The GZK Limit posits that if a cosmic ray
had a higher energy than 60 exa-electronvolts, the photons in the CMB would effectively stop
it in its tracks – with cosmic ray particles crashing into CMB particles. But the OMG Particle broke the GZK Limit more
than five times over, and since its discovery there have been a few hundred other extreme-energy
rays found to have exceeded 60 EeV. One explanation for violations of the GZK
Limit has to do with the particles themselves. It’s suggested that the GZK Limit still
applies and cannot be broken when the particle in question is a proton, but that the particles
in these “trans-GZK events” (the extreme-energy cosmic rays) are made of heavier elements. Unfortunately, since it came and went so quickly,
we have no way to know now whether the OMG Particle was a proton or something else. And it’s the same story for all those other
extreme-energy cosmic rays detected afterwards, as well. For the OMG particle, in particular, we also
have very little idea about exactly where it came from. While it’s true that these particles are
generally supercharged by incredibly bright objects like quasars or supernovas, when we
look into the sky to try and find which quasar or supernova could have launched the OMG Particle
specifically, scientists are still drawing a blank. One part of the problem is that the trajectory
of cosmic rays can be altered by something like the Milky Way’s own magnetic field… Observatories have detected that some other
cosmic rays appear to come from the vague direction of the constellation Ursa Major,
but there’s no conclusive path to a source beyond that. Of course, the other main problem when trying
to understand cosmic rays of any kind is time. We don’t really know about them until they
enter Earth’s atmosphere, and then it all happens so quickly! We can only see them at all because of Cherenkov
Radiation, which is the radiation produced when an object exceeds the speed of light
relative to the medium it’s in – water, perhaps. But, even then, they’re only very briefly
visible. With the OMG Particle, because of the huge
amount of energy it harboured, it was almost impossible to draw any meaningful conclusions
other than; “it happened.” We are making some progress in studying cosmic
rays, however. The Telescope Array Project in Utah – the
same state the OMG Particle was discovered in – started up in 2008, with 507 surface
detectors. One key finding of the Array so far has been
that about 27% of all the detected ultra-high-energy cosmic rays we experience come from just 6%
of the sky. This apparent “hotspot” once again appears
to lie in the direction of Ursa Major, so we are (and were) at least searching in roughly
the right place! But there are still many more puzzles to solve…
especially as the Pierre Auger Observatory in Argentina has reportedly found evidence
of another “warm spot”, in the direction of Centaurus A – the fifth brightest galaxy
in Earth’s sky. Scientists and analysts hope that one day
we’ll be able to locate exactly what’s throwing out these cosmic rays like there’s
no tomorrow… and precisely where it is (or they are). And perhaps then we’ll better understand
what we once saw thrown towards our galaxy at more than 99.99% the speed of light, back
in 1991. For some, it’s one of the greatest mysteries
of space… But, who knows, given the endless possibilities
that the universe holds, maybe we’ll one day observe something to beat even the seemingly
inimitable “Oh-My-God Particle”. Maybe. What do you think? Is there anything we missed? Let us know in the comments, check out these
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