If people around you aren’t going anywhere, if their dreams are no bigger than hanging out on the corner, or if they’re dragging you down, get rid of them. Negative people can sap your energy so fast, and they can take your dreams from you, too.
–Earvin “Magic” Johnson
As far as science goes, we all have our own dreams. For me, it’s to understand the largest scales in the Universe: the most massive structures, the highest energies, and the earliest times of existence.
Particle physicists are also after understanding the Universe at its highest energies, and that’s one of the primary goals of the Large Hadron Collider (LHC). After all, the LHC will reach the highest collision energies ever. How? By running two particles of between 5 and 7 TeV (where one TeV is 1012 electron volts, or 1012 eV) into one another, for a total available energy of up to 14 TeV, the most we’ve ever created in the lab. By smacking these two particles into one another, this available energy gets converted into new particles and anti-particles, which we can then look for in detectors surrounding the collision.
And yet I just wrote last week how this is completely safe. How do we know, given that we’ve never produced energies this large in the lab before?
We have bigger accelerators in space than we have on Earth. In particular, the super-massive black holes at the centers of ultra-massive galaxies can produce the highest energies in the Universe, dwarfing the LHC’s energies by factors of many millions. They create particles — known as cosmic rays — of extraordinarily high energies.
But you can’t create something of an arbitrarily high energy and have it reach Earth! Why not?
Because there’s stuff in the way! If you fired a bullet into a tank of water, the water would slow the bullet down.
Well, when you fire a high-energy particle through the Universe, it doesn’t run into water. It runs into the leftover glow from the big bang, known as the cosmic microwave background.
The amount of energy in a typical photon from the microwave background is miniscule: only around 0.00023 eV.
If you have a particle of high enough energy running into the cosmic microwave background, it will emit a new particle and slow down, similar to the way a bullet emits energy (mostly heat, sound, and pressure waves) and slows down in water.
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. There are three energies we need to think about.
1.) The energy of the photon from the microwave background, ECMB,
2.) The energy of the new particle you’re trying to make, Enew, and
3.) The energy of the the high-energy cosmic ray, Eray.
There’s a relatively simple formula relating these three energies:
So that’s the highest energy for a particle that will ever reach Earth. When they do hit Earth, instead of running into photons from the microwave background, they run into an atom, meaning they run into either electrons or protons. If we want to know how much energy is available to create new particles, we use the same formula as above, with one exception. Where we hadECMB before, for the energy of a photon from the microwave background, we now have Eproton, the rest-mass energy of a proton, which is about 938 MeV.
So, we do the math. How much energy do we get to make new particles with? About 274 TeV. About 20 times as great as the LHC will ever be able to create. All coming from these: the highest-energy cosmic rays.
These very high-energy cosmic rays come in somewhat infrequently; only a few thousand of them with energies that high hit the Earth each year. But, if we ask how many of them of that incredible energy have hit Earth over our 4.5 billion year history, we get a number that’s more than 1015!
So we have more than 1015 events that have had energies far exceeding the LHC’s most optimistic predictions, and we’re still here and still ok. And that’s why, until we beat those energies, I am confident that a particle accelerator will never threaten the safety of the Earth.