PDA

View Full Version : Photons into matter?



m74z00219
2009-Jun-11, 04:38 AM
If you imagine a stationary proton and stationary anti-proton that are in contact, we would expect them to annihilate into two or three photons.

Is it possible to go the other way? Can you combine photons to get back protons and anti-protons? If so, why not??


I'm VERY curious about this.

Thanks,
m74

Celestial Mechanic
2009-Jun-11, 04:43 AM
You are far more likely to get an electron-positron pair than anything else. To get a proton-antiproton pair the interaction has to form three quark-antiquark pairs with enough energy to give rise to the observed proton-antiproton pair.

m74z00219
2009-Jun-11, 05:17 AM
Okay, that seems to make sense since in terms of it being less complex, but does it happen? Can we do it in the lab?

WayneFrancis
2009-Jun-11, 05:30 AM
Someone correct me if I'm wrong but the big problem here is that photons won't interact with each other in that way when alone. You need other fundamental particles for this to occur. If the energy density was high enough then yes but you'd need a lot more energy then what this produces.

m74z00219
2009-Jun-11, 07:15 AM
Someone correct me if I'm wrong but the big problem here is that photons won't interact with each other in that way when alone. You need other fundamental particles for this to occur. If the energy density was high enough then yes but you'd need a lot more energy then what this produces.

I don't see why energy would necessarily be an issue. You can imagine two photons equal in energy to a positron and an electron.

WayneFrancis
2009-Jun-11, 07:37 AM
I don't see why energy would necessarily be an issue. You can imagine two photons equal in energy to a positron and an electron.

I can imagine many things but it doesn't mean that 2 photons are going to combine into a fundamental particle if the condition aren't right.

korjik
2009-Jun-11, 10:03 AM
I don't see why energy would necessarily be an issue. You can imagine two photons equal in energy to a positron and an electron.

A photon that makes a positron-electron pair is just over 1 MeV of energy. That is a pretty hard gamma ray. Making a proton-antiproton pair requites about 1800 MeV of energy. That is a fairly improbable energy for a single photon.

antoniseb
2009-Jun-11, 10:37 AM
Back when I was a kid, I was using 140MeV photons to make Pions. This happened inside Gold nuclei. Our goal was to make some Mercury-197.

trinitree88
2009-Jun-11, 11:29 AM
A photon that makes a positron-electron pair is just over 1 MeV of energy. That is a pretty hard gamma ray. Making a proton-antiproton pair requites about 1800 MeV of energy. That is a fairly improbable energy for a single photon.

korjik. Yes improbable. That's 1.80 Gev. 3.5 Gev is believed to have been seen though. see:http://www.eurekalert.org/pub_releases/2005-12/nyu-pfe120905.php.
Typically, the highest energy collisions we ever see are in the cosmic ray showers, and proton/antiproton, though rare are not impossible....and even rarer are the same type of events from single neutrinos or neutrino /antineutrino annihilations. The neutral current branch of the weak interaction can produce any particle/antiparticle pair, and W+ /W- pairs have been seen. When a particle antiparticle pair collide, proton/antiproton pairs make messy jets most often, but electron/ positron colliders make very energetic photons that then can interact with ambient particles to produce massive pairs.
The route is this. An extremely energetic photon can lose energy primarily through three routes....1. photoelectron production (Einstein's Nobel)...high speed electrons are ejected from matter, atoms are transiently ionized.
2. Compton scattering. Energy is transferred to electrons as increased kinetic energy, and the photon redshifts.
3. Pair Production. The mass/energy of the pair , E=mc2..is subtracted from the E=hv of the photon.
Eventually, a high energy photon dissipates it's energy through these three channels, with the annihilation energy showing up as heat. Hence sensitive calorimeters are needed to look for conserved energy in particle detectors . pete

m74z00219
2009-Jun-13, 01:42 AM
A photon that makes a positron-electron pair is just over 1 MeV of energy. That is a pretty hard gamma ray. Making a proton-antiproton pair requites about 1800 MeV of energy. That is a fairly improbable energy for a single photon.

It would have to be at least two photons though, in order to conserve momentum.