As far as universal limits go, the speed of light gets all the glory. But did you know there is a different speed limit for particles? It's called the GZK limit, and some people think it has already been exceeded. Which has some pretty weird implications for the laws of the universe.

We know that the ultimate speed limit of the universe, the limit that nothing can exceed, is the speed of light. Nothing gets around faster than a photon. But the universe is not exclusively made up of photons. It is also made up of physical particles. Of course, as Einstein proved, these particles can be equivalent to photons. The equation E = mc2 showed us that mass can change into energy and energy into mass the same way that water can be poured from one cup to another — although with considerably more difficulty. While energy is tough to get a hold on, mass is notoriously sluggish. No matter how little the quantity of mass we try to accelerate, we can never quite get it up to the speed of light.


Cosmic Rays and Cosmic Microwaves

But if there's anything that really should come close to the speed of light it's cosmic rays. A few people are surprised to find that cosmic rays don't move at the speed of light, since the word "ray" implies photons. The term "cosmic ray" is a hold-over from when scientists thought these high-energy entities were light. They're actually particles, generally protons and atomic nuclei that have been thrown our way by distant supernovae. There are few things with more power than a supernova, but cosmic rays still don't achieve the speed of light. Indeed, the don't even supersede an energy level that's slower than light. This energy level is known as the Greisen-Zatsepin-Kuzmin limit, or GZK limit. The GZK limit exists not because of the cosmic events that accelerate the particles, or deficiencies in the particles themselves, but because of the cosmic microwave background through which the particles move.

The cosmic microwave background is the leftover light of the Big Bang. When it started moving through the universe, it was as hot as the surface of a star. That was 14 billion years ago. Not it's just a few degrees above absolute zero, but it's still there. In fact, it's still everywhere. There is nowhere in the universe anyone can go what won't contain these photons.


When a Particle Meets a Photon

When two particles slam together at a certain energy level, there are consequences. The cosmic microwave background photons are not high energy, but that doesn't matter if the matter that's slamming into them is high energy. Get above a certain energy threshold for a collision and it forms a new particle. This particle, a neutral pion, will go sailing off, taking some of the energy from the existing particle with it.


Loss of energy means loss of speed. The new particle takes away some of the old particle's will to hurtle through the universe. If the old particle were somehow to pick up speed, it would only keep that speed until it hit another photon again. So the GZK limit is not theoretical, but practical. If a particle goes any large distance in the universe, certainly if it moves between solar systems, it will get knocked down to below the GZK limit.


Can Anything Exceed the Limit?

And so, when astronomers saw cosmic rays that exceeded the GZK limit, they were puzzled. There were a few possible explanations. The least exciting one was equipment error, of course. Then there was the intriguing possibility that something nearby was emitting those impossible cosmic rays. The particles need to collide with something to bring their energy down. Over short distances, a few particles might escape collision. The question was, what nearby source were those rays coming from? Nothing astronomers saw nearby could have emitted them. So to dark energy and dark matter, scientists added the possibility of dark supernovae, or other invisible, incredibly energetic events.


Scientists also theorized that something called Deformed Special Relativity might be responsible for the GZK-exceeding cosmic rays. This version of special relativity claims that, not only is the speed of light the maximum speed for the universe, the Planck energy and Planck length are the maximum energy and minimum length for the universe. The collision of the particles with the cosmic background radiation might be so energetic that this new kind of relativity came into play.


Unfortunately, just this once the universe decided to be boring. It seems that, as equipment has gotten better, the sightings of cosmic rays exceeding the GZK limit have stopped. Hopefully, the universe will soon find another way to mess with our heads.

Top Image: NASA/JPL

Pinwheel Galaxy Image: NASA, ESA, and the Hubble Heritage Team (STScI/AURA)

Supernova Image: NASA/ESA/JHU/R.Sankrit & W.Blair

[Via Explanation of the GZK Limit, Astrophysics Update, The Cosmic Speed Limit, Cosmos.]


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