Earth is showered with cosmic rays. They are protons, electrons and atomic nuclei traveling at nearly the speed of light, and strike our atmosphere to create the most power particle collisions ever observed. As a particle approaches the speed of light, it’s energy increases exponentially, so it might seem that there is no upper limit to just how much energy cosmic rays can have. But it turns out there is a limit, at least in theory.
The limit is imposed by the cosmic microwave background (CMB). This thermal remnant of the big bang fills the Universe with a sea of microwave photons, which is why we observe the CMB from all directions in space. But because of relativity, a cosmic ray moving at nearly the speed of light will observe this radiation greatly blue shifted. Instead of a sea of faint microwaves, these cosmic rays observe CMB photons as high energy gamma rays. Occasionally the cosmic ray will collide with a photon, producing particles such as pions and taking some of the energy from the cosmic ray. This will continue until the cosmic ray isn’t powerful enough to produce pion collisions. As a result, over the vast expanse of intergalactic space any really high energy cosmic ray will be lowered to this cutoff energy.
High energy protons collide with CMB photons, producing pions while losing energy. Credit: Wolfgang Bietenholz
This cutoff is known as the GZK limit, after Kenneth Greisen,Vadim Kuzmin, and Georgiy Zatsepin, who calculated the limit to be about 8 joules of energy (a proton traveling at 99.999998% of light speed), and that any cosmic ray traveling at least 160 million light years will have dropped below this limit. While that’s a huge amount of energy, there have been observations of cosmic rays with even higher energy. The highest energy cosmic ray had an energy of about 50 joules. So how is this possible?
The short answer is that we aren’t sure. High energy cosmic rays are more powerful than any particle accelerator we have, so these kinds of particles can’t be recreated in the lab. One possibility is that our measurement of high energy cosmic rays is somehow wrong. We don’t observe cosmic rays directly, but instead observe the shower of particles they create when striking our atmosphere. From this we infer its energy and composition. While that’s certainly a possibility, the observations we have seem pretty robust.
Another solution is that these cosmic rays are produced locally (in a cosmic sense). Most cosmic rays have traveled billions of light years before reaching us, but if a cosmic ray was produced less than 160 million light years away it could have more energy than the GZK limit. The problem with this idea is that there is no known source of high energy cosmic rays within 160 million light years, so this answer simply replaces the GZK paradox with the mystery of their origin. Another possibility is that the highest energy cosmic rays might be heavier nuclei. About 90% of cosmic rays are protons, and another 9% are alpha particles (helium nuclei), with the rest mostly electrons. It’s possible that a few cosmic rays are nuclei of heavier elements such as carbon, nitrogen, or even iron. Such heavy nuclei might be able to sustain their energy over greater cosmic distances, thus overcoming the GZK limit.
But one other option is perhaps the most tantalizing. Since these cosmic rays have more energy than anything we can create in the lab, they are a test of really high energy particle physics. It’s possible that the GZK limit is simply invalid. It’s based upon our current understanding of the standard model, and if the standard model is wrong so could the GZK limit. The answer to the GZK paradox might be new physics we don’t yet understand.
The energy of the most powerful cosmic rays might just be too big to fail.
Next time: The event horizon of a black hole marks a one way trip to oblivion. It also seems to defy some of the most foundational ideas of physics. We look at the hottest paradox in physics tomorrow.