Our planet is subject to a constant bombardment of radiation…from space.
Well, maybe it’s not as scary as it makes it seem. “Radiation” is a catch-all term that astronomers use to refer to forms of light, including visible light, the kind we see, as well as subatomic particles that circulate in space. We don’t usually think of these particles as “rays” – cosmic rays, to be precise – but we still use this nomenclature. because of the inertia of jargon.
Some cosmic rays originate from the sun, others from elsewhere in our Milky Way, and others, called extragalactic cosmic rays, originate across vast distances to other galaxies.
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In fact, it’s a remarkable idea: Earth is regularly hit by particles. other galaxies. It’s a long trek, a journey of tens of millions of light years, sometimes more, that ends when one of these wayward rays is harmlessly absorbed by our atmosphere, high above our heads.
These particles arrive with a wide range of speeds, which gives them a wide range of kinetic energy, the energy of motion. In our macroscopic universe, we use a unit like the joule to measure energy, which is still quite small. (It takes about four joules to raise a cubic centimeter of water 1 degree Celsius.) Particle physicists, however, use a much smaller unit called an electron volt (or eV). It takes 26 million billion to heat the same quantity of water! This is a more appropriate unit for particles, most of the time. But cosmic rays travel so fast – close to the speed of light – that they can have a very high kinetic energy, easily reaching the level of megaelectronvolt (MeV) and gigaelectronvolt (GeV).
You still wouldn’t feel it if one of them hit you. But what is shocking is that some cosmic rays have reached far, far higher energies than that.
In 1991, the Fly’s Eye detector, which monitored the sky for the glow caused by energetic particles hitting our atmosphere, detected a flash so huge it defied belief: the cosmic ray that triggered it had an energy of 320 quintillion eV, or 320 billion GeV. That’s millions of times the kinetic energy of the protons we can spin in our most powerful particle accelerators. It’s so energetic, in fact, that it has a decent macroscopic equivalent: This cosmic ray carried 51 joules of kinetic energy, which is about the same as a slow-moving curveball, but that energy came from a single subatomic particle.
It’s been nicknamed the “Oh-My-God” particle, and it makes the hairs on the back of my neck stand up.
For what? Because protons are almost incomprehensibly large – by analogy, the size of a proton compared to that of an orange is about the same as the size of an orange compared to that of an orange. the diameter of Neptune’s orbit around the sun.
The OMG particle is a great mystery. On the one hand, to have so much energy, she must have traveled incredibly fast relative to the Earth. Assuming it was a proton, it was moving at a speed of 99.99999999999999999999995 percent of the speed of light. If a photon and the OMG particle had been in a race since the formation of the universe, the particle would only be about 600 meters behind.
So what could propel a particle like this to such ridiculously high speeds? The answer might shock you.
This isn’t clickbait: shockwaves, especially in catastrophically high-energy structures such as focused beams of matter and energy bursting from a supermassive black hole. The rapidly moving ionized gas out of such events results in extremely strong magnetic fields. Charged subatomic particles (such as protons, which carry a positive electrical charge) are accelerated as they move through such fields, sometimes at high speeds. But if the gas collides with other gas clouds, the subatomic particles can play ping-pong with each other, gaining energy each time they bounce. (It’s called First order Fermi accelerationa term I like for its Star Trek– like cadence.) They can become so energetic that they are thrown like a stone from a trebuchet.
Even so, bringing the particles up to femtometers-per second, slower than light itself is extraordinary, and the specific processes involved are unclear. There are no known sources capable of this in the Milky Way, so the OMG particle most likely came from another galaxy. The second highest energy cosmic ray ever observednicknamed Amaterasu after the Shinto sun goddess, had an energy of 244 quintillion eV and appears to come from a part of the sky that overlaps with the galaxy PKS 1717+177, known for having extremely powerful jets coming out of its central black hole. Many others have also been associated with other active galaxies..
And there is even more mystery to come. The speed of the OMG particle actually violates a cosmic rule of thumb used by particle astrophysicists. The universe is filled with radiation from the Big Bang, called the cosmic microwave background. This is a low power item, assuming you’re not moving fast relative to it.
But a particle moving at close to the speed of light will see the radiation coming from it considerably amplified in energy because of the Doppler shiftand at these speeds this effect operates at a ridiculously extreme level. A proton hit by high-energy photons would be expected to lose energy, which would slow it down, so that at very high speeds it would be rapidly decelerated. There is an even stricter limit; if the photons it sees are energetic enough, the proton will be converted into two other subatomic particles, a neutron and a pion. Both of these elements quickly decay into even more particles, so ultimately, very high-energy protons (with more than 50 quintillion eV) from distant galaxies should never reach us.
So how did the OMG particle get here?
The answer may simply be that it wasn’t a proton. Cosmic rays are a mixture of different subatomic particles, including helium nuclei (two protons and two neutrons bound together) or even heavier elements. An iron core, frequently responsible for cosmic rays, would not be affected in the same way as a proton and could make the long journey to Earth.
The OMG particle is the highest energy cosmic ray ever detected, but many others have been observed with slightly lower but still surprising energies. Clearly the universe has no problem making them, even if they are rare.
Besides looking awesome, they also tell us something important about the cosmos. There are engines of extreme power, capable of producing particles much more energetic than we could hope for on Earth. Energies like this were common, even ubiquitous, in the early universe, so finding particles like this is like having a window into the split second after the big bang.
The universe teaches us about itself and all we have to do is pay attention to the little things.
