# Can photons collide?

Discussion in 'Physics & Math' started by Magical Realist, Jun 24, 2014.

1. ### Magical RealistValued Senior Member

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Isn't the collision of two photons the creation of matter? If so, do we create matter when two flashlights are pointed at each other?

3. ### exchemistValued Senior Member

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No. They pass through each other, just as waves from two motor boats pass through each other on a lake.

5. ### btrRegistered Member

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Two photons with sufficiently high energy (greater than 1.022 MeV in the centre-of-mass frame) can collide to produce electron-positron pairs, or even other, more massive charged particle pairs if the photons' energy is really high. The photons would have frequencies in the gamma ray range, though (in the centre-of-mass-frame), so your flashlights won't do the trick unless they are racing towards each other really quickly. You can call this "creation of matter if you want"; some people would say, equally correctly, that you've just converted one form of matter into another.

Lower energy photons can also collide, incidentally, although the probability for such a process is low (and the lower the photons' centre-of-mass energy is, the lower the probability), and of course you won't produce charged particles.

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

7. ### Magical RealistValued Senior Member

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Thanks. I was just looking at this when I saw your post. It confirms what you say:

http://www.theverge.com/2014/5/18/5724658/photon-collider-could-turn-light-into-matter

8. ### Farsight

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"A photon can, within the bounds of the uncertainty principle, fluctuate into a charged fermion/ anti-fermion pair, to either of which the other photon can couple".

I dislike a tutorial that suggests that a 511keV photon magically morphs into a 511keV electron and a 511keV positron, or that pair production occurs because pair production occurred, spontaneously, like worms from mud. It reminds me that "within the standard model" work is crucial for the well-being of HEP. There is a photon-photon interaction that needs to go into QED.

9. ### btrRegistered Member

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No; I'm aware that some people don't like to think of light as being "matter", but for those that do it's quite valid to view the process as converting one form of matter (photons) into another (charged particles).

Note that the purpose of the tutorial was not to explain where the rules of perturbative field theory come from; rather, its scope was certain experimental data obtained by the OPAL experiment at CERN, and how that data compared with theoretical predictions.

There are already several types of photon-photon interactions predicted by the Standard Model, some of which were verified at the OPAL experiment I mentioned above. There is no experimental data I'm aware of that suggests extra photon-photon interactions are required.

10. ### Farsight

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3,492
All points noted btr.