Colliding Photons: Matter-Energy Problems & Wavefunction

[misnoma]

whilst we are successful in colliding matter to identify yet further fundamental particles, and being aware there is a limit on speed due to mass end energy problems, and given that matter and mass are interchangeable, might it be possible to collide a single photon with another (this must rely on the particle nature of light). How could this be done? Ultimately every fundamental particle has a wavefunction and asscoiated energy. So if we remove the problems of mass can collision of photons show any new discoveries? Oh, and please do not go on about c being infinite and independant of direction etc, the question simply targets collision of photons.

 

[jtbell]

Photon-photon interactions have been studied at particle colliders. High-energy electron and positron colliders also produce lots of high-energy photons which can be used for this.

http://www.hep.ucl.ac.uk/opal/gammagamma/gg-tutorial.html

There are also proposed experiments using lasers, for example

http://2physics.blogspot.com/2006/03/photon-photon-scattering.html

(I found these with a Google search for "photon-photon scattering".)

 

[disregardthat]

What would a single photon scatter?

 

[Peter McKenna]

How about creatng a virtual vacuum using two plates at distance < wavelength of visible light, using a full spectrum light source, a single slit filter just large enough for a single photon to pass through at a time, and using Casimir effect, measure the electrical potential difference between the two plates. If a photon photon collision occurs, an electron positron pair will result causing an electrical potential difference between the two plates, the potential difference will cause an oscillating pulse, indicating the energy of the photons.

 

[Bob_for_short]

Photon-photon scattering makes sense and its cross section was calculated long ago (A. I. Akhiezer?). This cross section is very small so one needs many-photon beams (laser beams) to collide in order to observe such rare events.

Bob.

 

[Peter McKenna]

How could you identify a single photon photon collision?

 

[Bob_for_short]

How could you identify a single photon photon collision?

Just as for any projectile-target scattering. The simplest way is to detect the photon deviation from its initial direction. No target, no deviation.

Another effect is the polarization change. You know that a static magnetic field affects the photon polarization. The same effect will be produced in scattering of a high-frequency, short-wave polarized photons from a long-wave electromagnetic radio-wave with quasi-static magnetic field.

Bob.

 

[Peter McKenna]

Isn't that simply vector analysis? A Feynman diagram can be used for reflectivity in the same way and is a vector respresentation of the energy exchange. But can the exchange be isolated, that is, can two discrete photons be isolated and the resulting collison observed?

 

[Peter McKenna]

Can a photon be considered to be the composition of an electron or a positron combined with a hadron or a lepton? Does a photon include the possibility of both a matter and antimatter waveform?

 

[Bob_for_short]

Isn't that simply vector analysis? A Feynman diagram can be used for reflectivity in the same way and is a vector respresentation of the energy exchange. But can the exchange be isolated, that is, can two discrete photons be isolated and the resulting collison observed?

I do not know, we have to compare the cross sections. If (one photon)-(one photon) cross section prevails and multi-photon one is much smaller, then yes.

In case of Faraday effect mentioned above (one photon)-(multi photon) effect prevails.

Bob.

 

[Peter McKenna]

What would a single photon scatter?

Do you mean why?

Heisenberg's uncertainty principle?

 

[Peter McKenna]

"What would a single photon scatter?" An electron, minus the energy needed to release it. If insufficient energy is present, once the uncertainty plus photon has sufficient energy an electron would release. By my understanding, sooner or later a single photon can cause scatter. Its not a mtter of if but when.

 

[jnorman]

as usual, out of my depth here, but i do not think you can have a photon-photon collision. in between the time a photon is emitted and the time it is detected/absorbed, a photon only exists as a probability - it is not a real entity at some specific place and time, it permeates the entire volume of the universe. perhaps i am misunderstanding what you guys are talking about...

 

[Bob_for_short]

as usual, out of my depth here, but i do not think you can have a photon-photon collision. in between the time a photon is emitted and the time it is detected/absorbed, a photon only exists as a probability - it is not a real entity at some specific place and time, it permeates the entire volume of the universe. perhaps i am misunderstanding what you guys are talking about...

Put some dust in the volume and you will see the photon beam premises.

Bob.

 

[jnorman]

thanks bob - i didnt go too far in the literature, but this is from wikipedia:
"Two photons cannot ever collide.In fact light is quantized only when interaction with matter."

i understand that photon beams (system) can affect each other gravitationally, but i thought the OP was aksing about individual photon collisions.

 

[jtbell]

Indeed, two photons cannot interact directly (first-order Feynman diagram), but they can nevertheless interact indirectly via Delbrück scattering (a higher-order Feynman diagram).

 

[Bob_for_short]

thanks bob - i didnt go too far in the literature, but this is from wikipedia:
"Two photons cannot ever collide.In fact light is quantized only when interaction with matter."

i understand that photon beams (system) can affect each other gravitationally, but i thought the OP was aksing about individual photon collisions.

The gravitational photon beam interaction is negligible in comparison with the direct photon-photon interaction. The direct photon-photon scattering makes sense (exists) since the corresponding wave equations are non linear. It is a non-linear effect, if you like. The wikipedia statement is not correct.

The Delbruk scattering is also a non linear effect - deviation of a photon in an external stationary strong electric field (near a nucleus) which itself is not a photon.

Bob.

 

[Peter McKenna]

Is Delbruck scattering where a photon collision results in an electron-positron annhilation which produces another photon?

 

[Peter McKenna]

I looked it up and I was thinking of Pair Production but it describes a similar condition, photon collision in a nucleas field, energy > hv, electron-positron annhilation, and a photon is released (gamma I believe). If I remember correctly, one of the particles is moving in opposite time continuum to maintain symmetry. Must be different, the Feynman diagram doesn't reflect this.

 

[Peter McKenna]

Um, doesn't the two-slit experiment embody direct photon-photon interactions?

Of course the Feynman diagram utilizes other virtual particles, but two photons can cancel each other. No?

Note: This is in response to the guy who said two photons cannot directly interact.

 

[Cthugha]

Um, doesn't the two-slit experiment embody direct photon-photon interactions?

No, double-slit interference happens at the single photon level. There are two fields, which interfere, but just one photon. Two photon interference is possible, too, but it is very rare. It can be seen for example in delayed choice quantum eraser experiments. However this only requires a well defined phase of the two-photon state, but no direct photon-photon interactions.

Of course the Feynman diagram utilizes other virtual particles, but two photons can cancel each other. No?

If they could cancel each other without creating some other product, conservation of energy would be severely violated.

 

[gregarob]

as usual, out of my depth here, but i do not think you can have a photon-photon collision. in between the time a photon is emitted and the time it is detected/absorbed, a photon only exists as a probability - it is not a real entity at some specific place and time, it permeates the entire volume of the universe. perhaps i am misunderstanding what you guys are talking about...

If existence only as a wavefunction per se precluded photon interaction, then so too would the interaction of electrons, protons, quarks... all of these are only wavefunctions in spacetime permeating the entirety of the universe. However, these wavefunctions can still interact with each other, and those interactions can cause collapse of the wavefunction to a "point". One dramatic example is matter-antimatter annihilation, in which a particle and its antimatter equivalent interact in a way which transforms them into entirely new particles.

Um, doesn't the two-slit experiment embody direct photon-photon interactions?

Not quite. The photons interact with matter, not other photons passing through the slit. In fact, one can pass a single photon at a time and still wind up with an interference pattern.

 

[Peter McKenna]

The same experiment could be performed using electrons and a diffraction grating. The probability waves will tend to concentrate at the central "source" of the electrons.

When the event (particle interference) is observed (at least the diffraction pattern), doesn't the probability wave collapse and the "event" then actually occurs? Why doesn't photon interference actually occur in this case? Also why can't that version of "reality" actually exist, as well as the other possible realities (eg Hugh Everett; the many worlds theory)? Where does the case of a single photon causing the diffraction pattern sit with the Many Worlds theory?

Doesn't the probability wave field view follow the Copenhagen Interpretation?

 

[Peter McKenna]

No, double-slit interference happens at the single photon level. There are two fields, which interfere, but just one photon. Two photon interference is possible, too, but it is very rare. It can be seen for example in delayed choice quantum eraser experiments. However this only requires a well defined phase of the two-photon state, but no direct photon-photon interactions.



If they could cancel each other without creating some other product, conservation of energy would be severely violated.

I'm not debating that, I seem to remember an electron positron pair production, or two electrons, one in reverse time, to satisfy symmetry, no?

 

[Peter McKenna]

If existence only as a wavefunction per se precluded photon interaction, then so too would the interaction of electrons, protons, quarks... all of these are only wavefunctions in spacetime permeating the entirety of the universe. However, these wavefunctions can still interact with each other, and those interactions can cause collapse of the wavefunction to a "point". One dramatic example is matter-antimatter annihilation, in which a particle and its antimatter equivalent interact in a way which transforms them into entirely new particles.



Not quite. The photons interact with matter, not other photons passing through the slit. In fact, one can pass a single photon at a time and still wind up with an interference pattern.

I saw the term particle "entanglement" to describe photon interactions.

In Everett's "Many Worlds" theory is "entanglement" the proper term? Or does this term only apply within the framework of the Copenhagen Interpetation, and the wave function view of particle interaction probabilities?