“All our sweetest hours fly the fastest.” -Virgil
If you’ve been around the block once or twice, you know that the speed of light in a vacuum — 299,792,458 meters-per-second — is the absolute maximum speed that any form of energy in the Universe can travel at. In shorthand, this speed is known as c to physicists.
But you or I, no matter how hard we try, will never attain that speed. There’s a simple reason for this: we have mass. And for an object with mass, you can accelerate it all you want, but it would take an infinite amount of energy to reach c, and I’m sorry, folks, there’s only a finite amount of energy in the Universe.
But that doesn’t mean we settle for 90% of c, or 99%, or 99.9999%. We always strive for that extra fraction of speed, that extra bit of energy, that extra push ever-closer to the unattainable limit. You may be most familiar with our latest attempts to do this at CERN, where we’ve recently discovered the Higgs Boson.
By smashing two protons into one another, one moving at 299,792,447 meters per second (just 11 m/s shy of the speed of light) in one direction and the other moving at the same speed in the opposite direction, we can produce incredibly energetic particles, bounded only by the energy available via Einstein’s E=mc2. After the LHC’s upgrade is complete, that speed will increase to 299,792,455 m/s, which will make these the fastest protons ever created on Earth.
But they’re hardly the fastest particles we’ve ever made.
After all, a proton is a relatively heavy particle, some 1,836 times heavier than it’s orbiting friend, the electron! Even though we’ve created protons that are at higher energies than electrons, it only takes one-1,836th the energy (or 0.054%) to get the electron up to the same speed. Which means that LEP — the Large Electron-Positron Collider (and the LHC’s predecessor) — where they got electrons up to 104.5 GeV of energy (compared to the 6,500 GeV expected for the LHC after the upgrade), still holds the record for particle accelerator record speed.
What is that speed? 299,792,457.9964 meters per second, or a whopping 99.9999999988% the speed of light, just 3.6 millimeters-per-second slower than light in a vacuum!
But that’s just here on Earth, with our lame superconducting electromagnet accelerators, powered by puny chemical energy sources. Compared to what comes out of the Universe, our terrestrial sources don’t stand a chance.
Outer space is filled with collapsed stars, supernovae and supermassive black holes — including ones at the center of active galaxies, above — where magnetic fields billions of times the likes of anything ever to appear on Earth are routine. From all directions in space, cosmic rays — high energy particles, mostly protons — fly through the Universe at energies dwarfing anything we’ve ever created or even experienced here on Earth.
Yes, there are fewer particles as we go to higher and higher energies, but the highest energies are no longer measured in terms of GeVs (Giga-electronVolts, or 109 eV), TeVs (Tera-electronVolts, or 1012 eVs) or even PeVs (Peta-electronVolts, or 1015 eVs). Instead, these energies can spike all the way up into the 1019 eV-range!
Now, this number is really, really interesting! Because above about 4-or-5 × 1019 eV, the Universe won’t let you stay at that energy! The problem, believe it or not, is that no matter how high the energy is of the particle you made, it has to pass through the radiation bath that’s left over from the Big Bang in order to reach you.
This radiation is incredibly cold, at an average temperature of some 2.725 Kelvin, or less than three degrees above absolute zero. If we sought to calculate the root-mean-squared energy of each photon in there, it’s on the order of just 0.00023 electron-Volts, a tiny number. Every time a high-energy charged particle has a chance to interact with a photon, it has the same possibility that all interacting particles have: if it’s energetically allowed, by E=mc2, then there’s a chance it can make a new particle!
And that particle doesn’t get that energy for free; it has to come from the system that created it! The lightest particle you can create from a collision like this is a neutral pion, which you need 135 MeV of energy to make. There’s a threshold for this that’s relatively easy to compute (previously done here), and what it tells you is that as long as you’re above a certain energy threshold — known as the GZK cutoff, named for Greisen-Zatsepin-and-Kuzmin — you’re going to emit those pions until you’re below that energy threshold!
For a long time — until the last few years — we thought we’d observed particles that exceeded this threshold, which meant that either they were being generated within our galaxy (somehow), which is the only location that would allow them to survive the trip to Earth, there was something wrong with our understanding of relativity (fat chance), or, as most people thought likely, there was a problem measuring these unprecedented high energies.
Lo and behold, now the two most modern observatories/experiments looking for these — Pierre Auger Observatory and the High-Resolution Fly’s Eye Experiment — both see the GZK cutoff clearly, and no cosmic rays above about 5 × 1019 eV. As far as a proton traveling with that energy goes, do you know what that means for speed? It tells us that a proton traveling at the GZK limit has a speed of:
Or, to put that in perspective, if you raced a proton of this energy and a photon to the nearest star-and-back, the photon would arrive first… with the proton just 22 microns behind, arriving 700 femtoseconds later.
And every charged particle in the cosmos — every cosmic ray, every proton, every atomic nucleus — is limited by this speed! Not just the speed of light, but a little bit lower, thanks to the leftover glow from the Big Bang! So when you dream of traveling through the Universe, it’s not quite arbitrarily close to the speed of light; the radiation from the Big Bang — so low-energy in the microwave — will fry you if you move too quickly. And that’s the cosmic speed limit for you, me, and everything else made of matter!