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)
