# The Naked Scientists Forum

### Author Topic: What happens if photons collide?  (Read 2036 times)

#### thedoc

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##### What happens if photons collide?
« on: 11/12/2014 11:30:06 »
Bal Mukund  asked the Naked Scientists:

Could someone explain to me, how would it look when two photons collide? How will they behave?

What do you think?
« Last Edit: 11/12/2014 11:30:06 by _system »

#### PmbPhy

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##### Re: What happens if photons collide?
« Reply #1 on: 11/12/2014 15:38:00 »
Quote from: Bal Mukund

Could someone explain to me, how would it look when two photons collide? How will they behave?
Since photons have no charge they don't couple to each other. At best, if the energy of the two photons are large enough so they are gamma rays they interact through a higher order process to create matter. See:
http://en.wikipedia.org/wiki/Two-photon_physics
http://www.extremetech.com/extreme/182701-scientists-work-out-how-create-matter-from-light-finally-proving-einsteins-emc2

#### JohnDuffield

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##### Re: What happens if photons collide?
« Reply #2 on: 11/12/2014 23:03:26 »
Bal Mukund  asked the Naked Scientists:

Could someone explain to me, how would it look when two photons collide? How will they behave?
Pretty much like two water waves. They will ride over each other and keep going. But as per Pmb's reply, there is such a thing as two-photon physics which is all about creating matter from light. See the Breit-Wheeler process. The thing to note with the water waves is that when one wave rides over another, it changes direction. It was going straight, then it goes up over the other wave, then it goes down the other side, then it's going straight again. So ordinarily the waves "don't couple to one another". But IMHO creating matter is something like changing the direction of a wave so much that it ends up going through itself, then it keeps on changing its own direction and ends up stuck in a closed path. This is the "higher order process". After all, pair production is a definite, as is  the wave nature of matter, and people talk about "spinors" and Dirac's belt. It can't be rocket science, it has to be something simple like this, but I can't explain why this sort of thing isn't clearly explained in the textbooks.
« Last Edit: 11/12/2014 23:06:27 by JohnDuffield »

#### evan_au

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##### Re: What happens if photons collide?
« Reply #3 on: 13/12/2014 10:17:54 »
In a somewhat related topic, scientists have recently shown that humans can see light in the infra-red region of the spectrum, if two infra-red photons collide with the same retinal cell at the same time.

Although one isolated infra-red photon by itself does not have enough energy to trigger the sensation of light, two such photons colliding with the same cell can produce a response from the cone cell.

So this is a case of two photons interfering as they strike a photosensitive pigment.

This is reported here (amongst others):
http://www.sci-news.com/biology/science-humans-can-see-infrared-light-02313.html
http://www.pnas.org/content/early/2014/11/25/1410162111.short

#### yor_on

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##### Re: What happens if photons collide?
« Reply #4 on: 13/12/2014 12:49:14 »
It's a interesting idea. Do they 'collide'?

#### The Naked Scientists Forum

##### Re: What happens if photons collide?
« Reply #4 on: 13/12/2014 12:49:14 »