[LeeE (OP)]

When an electron/positron pair is created from a pair of gamma photons, how important is the relative phase difference between the two gamma photons?  Does it have any significance at all?  I'm wondering this because it would seem to me that the probability of any two gamma photons meeting exactly in-phase is likely to be tiny, but I get the impression that electron-positron pair creation is relatively common.

[lyner]

I imagine that polarization would also have an effect.
The directions in which the pair leave the scene could be a possible outcome variable, dependent upon phase.
This effect must be due to non linearity; mostly, superposition of em fields applies.

[lightarrow]

When an electron/positron pair is created from a pair of gamma photons, how important is the relative phase difference between the two gamma photons?  Does it have any significance at all?  I'm wondering this because it would seem to me that the probability of any two gamma photons meeting exactly in-phase is likely to be tiny, but I get the impression that electron-positron pair creation is relatively common.

Are you sure that a pair could be created from a couple of interacting photons? Photons usually don't interact.
Usually pairs electron/positron are produced by a gamma photon collision with a nucleus or another massive particle, which provides the mass to make the momentum conservation of the process possible.
http://en.wikipedia.org/wiki/Pair_production
http://www.answers.com/topic/electron-positron-pair-production

[Vern]

I think that it is now established that atomic nuclei need not be present. This was recently confirmed by experiment.

Edit: I guess the electron used as a collision catalyst provides the momentum conservation mechanism.

OUT OF PURE LIGHT, PHYSICISTS CREATE PARTICLES OF MATTER
September 16, 1997
A team of 20 physicists from four institutions has literally made something from nothing, creating particles of matter from ordinary light for the first time. The experiment was carried out at the Stanford Linear Accelerator Center (SLAC) by scientists and students from the University of Rochester, Princeton University, the University of Tennessee, and Stanford. The team reported the work in the Sept. 1 issue of Physical Review Letters.
Scientists have long been able to convert matter to energy; the most spectacular example is a nuclear explosion, where a small amount of matter creates tremendous energy. Now physicists have succeeded in doing the opposite: converting energy in the form of light into matter -- in this experiment, electrons and their anti-matter equivalent, positrons.
Converting energy into matter isn't completely new to physicists. When they smash together particles like protons and anti-protons in high-energy accelerator experiments, the initial particles are destroyed and release a fleeting burst of energy. Sometimes this energy burst contains very short-lived packets of light known as "virtual photons" which go on to form new particles. In this experiment scientists observed for the first time the creation of particles from real photons, packets of light that scientists can observe directly in the laboratory.
Physicists accomplished the feat by dumping an incredible amount of power -- nearly as much as it takes to run the entire nation but lasting only for a tiny fraction of a second -- into an area less than one billionth of a square centimeter, which is far smaller than the period at the end of this sentence. They used high-energy electrons traveling near the speed of light, produced by SLAC's two-mile-long accelerator, and photons from a powerful, "tabletop terawatt" glass laser developed at Rochester's Laboratory for Laser Energetics. The laser unleashed a tiny but powerful sliver of light lasting about one trillionth of a second (one picosecond) -- just half a millimeter long. Packed into this sliver were more than two billion billion photons.
The team synchronized the two beams and sent the electrons head-on into the photons. Occasionally an electron barreled into a photon with immense energy, "like a speeding Mack truck colliding with a ping pong ball," says physicist Adrian Melissinos of the University of Rochester. That knocked the photon backward with such tremendous energy that it collided with several of the densely packed photons behind it and combined with them, creating an electron and a positron. In a series of experiments lasting several months the team studied thousands of collisions, leading to the production of more than 100 positrons.
The energy-to-matter conversion was made possible by the incredibly strong electromagnetic fields that the photon-photon collisions produced. Similar conditions are found only rarely in the universe; neutron stars, for instance, have incredibly strong magnetic fields, and some scientists believe that their surfaces are home to the same kind of light-to-matter interactions the team observed. This experiment marks the first time scientists have been able to create such strong fields using laser beams.
By conducting experiments like this scientists test the principles of quantum electrodynamics (QED) in fields so strong that the vacuum "boils" into pairs of electrons and positrons. The scientists say the work could also have applications in designing new particle accelerators.
Spokesmen for the experiment, funded by the U.S. Department of Energy, are Kirk McDonald, professor of physics at Princeton, and Melissinos, professor of physics at Rochester. Also taking part in the experiment were William Bugg, Steve Berridge, Konstantin Shmakov and Achim Weidemann at Tennessee; David Burke, Clive Field, Glenn Horton-Smith, James Spencer and Dieter Walz at SLAC; Christian Bula and Eric Prebys at Princeton; and seven other physicists from Rochester, including Associate Professor David Meyerhofer; graduate students Thomas Koffas, David Reis, Stephen Boege, and Theofilos Kotseroglou; research associate Charles Bamber; and engineer Wolfram Ragg.
________________________________________
CONTACT: Tom Rickey, (716) 275-7954.

[LeeE (OP)]

Thanks for the thoughts and comments.  Interesting point re the polarisation sophicentaur, and you're right lightarrow; we don't usually think of photons interacting, except in terms of wave interference (but then waves and photons are a bit like mutually exclusive descriptions of the same thing), which leaves the photons unchanged.  The interactions are there in theory though, and I thought it had been experimentally observed, as Vern's post suggests.

[JP]

Aren't phase and number conjugate variables for photons?  In other words, isn't the phase of a single photon not well defined (following from uncertainty relations for conjugate variables).

[Vern]

In other words, isn't the phase of a single photon not well defined (following from uncertainty relations for conjugate variables).

I've seen this used as an argument that single photons can't exist. I suspect though that a single photon can exist as a single quantum of energy.

[JP]

In other words, isn't the phase of a single photon not well defined (following from uncertainty relations for conjugate variables).

I've seen this used as an argument that single photons can't exist. I suspect though that a single photon can exist as a single quantum of energy.

I don't think there's doubt that single photons can exist.  But I'm not sure about trying to argue about  single photons meeting in-phase or out-of-phase.  This probably goes back to the problem if trying to think of photons as little bullets that meet at some point in space.

[Vern]

I suspect that the electric and magnetic properties of photons reach saturation at single points. That point is then the most probable place where reaction will happen. We see this reaction at a single point and it looks like a particle.

The electric and magnetic amplitude of photons can not be a variable because it is not needed in equations. We find Planck's constant where we would expect to see that amplitude.

[LeeE (OP)]

I don't think there's doubt that single photons can exist.  But I'm not sure about trying to argue about  single photons meeting in-phase or out-of-phase.  This probably goes back to the problem if trying to think of photons as little bullets that meet at some point in space.

It think this, and your previous answer, was what I was after; phase doesn't seem to be significant, which I think fits with the improbability of phase co-incidence (although I'm now wondering why the phase of a single photon is not well defined - interesting).

Thanks.

[Vern]

Isn't the term "in phase" used to indicate that the changing field amplitude of the photon's fields are in step? Or changing in the same direction at the same time? A single photon could have a phase relationship with another photon, but would need to curl around until front meets back to be in phase with itself IMHO.

[lyner]

Aren't phase and number conjugate variables for photons?  In other words, isn't the phase of a single photon not well defined (following from uncertainty relations for conjugate variables).

It depends on what the photon actually consists of - what time and space it actually occupies. It will only have a single frequency if it exists for all time. (Fourier) Hence, its 'phase' is not an easy quantity to discuss if you want to talk of it as the dreaded 'little bullet'.