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Q & A: photons and antiphotons

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Most recent answer: 10/22/2007
Q:
If photons can be their own anti-particles, how do scientists distinguish or create them? Also, why is it that high-energy photons are usually the ones to interact when light-on-light scattering occurs? P.S. Is there any way to couple these photons/antiphotons together for an extended period? (a minute?)
- Devon (age 18)
Waverly High, Lansing M.I.
A:
Well, if a particle is its own antiparticle, it is even easier to produce than if it had an antiparticle partner. An electron has an antiparticle partner, the positron. To make an electron, you must also either make a positron or something else to carry off the anti-electron-ness, like an electron antineutrino. You also have to worry about making the charge of everything add up (electrons have one unit of negative charge -- making one means changing the charge of something else). Electrons are made in the decay of a neutron to an electron, a proton, and an electron antineutrino. Electrons can be pair-produced with positrons but you need at least 2*M_e*c^2 of energy to do this.

Photons are neutral and are their own antiparticles so they can be made one at a time. Accelerating a charged particle (like an electron) is sure to make photons. Hot objects radiate photons all the time.

Photons have energy and momentum (but no mass). You can detect photons in a variety of ways -- with your eyes, with a camera (film or digital), with phototubes or photodiodes. Some of these are sensitive to the energy and direction of the photon. Prisms and diffraction gratings steer photons of different energies (related to wavelength and color) in different directions. Lenses focus photons from different positions and directions to different places on film or a CCD array. Very high-energy photons are detected in calorimeters in high-energy physics experiments. These are made of lead sandwiched between scintillating plastic, typically. An incoming photon will pair-produce an electron and a positron, which will scatter off of the lead nuclei and radiate more photons, which will pair-produce electrons and positrons and so on until the energy is used up. The scintillators will flash when the electrons and positrons pass through, and phototubes detect the visible light thus produced.

The scattering probability of visible photons is so small it hasn't yet been detected. Phtons scatter by exchanging a virtual electron which goes around in a quantum-mechanical loop. If the energy of the reaction is tuned to twice the electron mass (times c**2) there's enough energy to make a real electron go around the loop, and the scattering probability goes up.

I don't understand the last question -- photons do not form bound states, if that's what you mean. They do fall into black holes, however.

Tom

(published on 10/22/2007)

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