Two beams of light affecting each other

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Avian
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Two beams of light affecting each other

Postby Avian » Sat Aug 31, 2013 6:58 pm UTC

Is it possible for two intersecting beams of light to affect each other? I'm thinking of the following experiment: A beam of light is emitted from a source A (e.g. a laser) to a detector. At one point in time, source B is switched on and its beam intersects the first beam somewhere on its path between source A and the detector (but doesn't directly hit the detector). Is there any way the detector would be able to detect this change? The experiment is done in vacuum (to prevent any non-linear effects from the medium) and with wavelengths that don't allow for electron-positron pair production.

I know from classical electromagnetic field theory that there is no way for the second beam to change what the detector sees. However I've heard some conflicting views whether quantum mechanics allows for such photon-photon interactions. I only have rudimentary understanding of quantum mechanics and I was wondering if someone can clear this up.

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thoughtfully
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Re: Two beams of light affecting each other

Postby thoughtfully » Sat Aug 31, 2013 7:29 pm UTC

I would guess there's going to be some chance of virtual electrons and the like interacting. A rather small chance, I'd imagine.
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PM 2Ring
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Re: Two beams of light affecting each other

Postby PM 2Ring » Sat Aug 31, 2013 7:38 pm UTC

Yes, low energy photon-photon scattering is possible, but it has a low probability and as yet there are no confirmed direct observations of this phenomenon.

From http://physics.aps.org/synopsis-for/10. ... 111.080405
The whole process appears as two photons ricocheting off each other, but it has only been observed indirectly by its effect on the magnetic moments of the electron and muon.


According to http://www.ncbi.nlm.nih.gov/pubmed/16606179 a low energy photon-photon experiment was proposed in 2006, but I gather that the results were not positive.


I don't think the arrangement you propose would be able to detect 1 photon in zillions being deflected. But maybe if your light source A was very dim and you waited a very long time, you might get an observable deflection. :)

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Tass
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Re: Two beams of light affecting each other

Postby Tass » Sun Sep 01, 2013 7:19 am UTC

If they are not parallel, then there is also a gravitational effect, albeit very small.

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Re: Two beams of light affecting each other

Postby Avian » Sun Sep 01, 2013 12:52 pm UTC

I don't think the arrangement you propose would be able to detect 1 photon in zillions being deflected. But maybe if your light source A was very dim and you waited a very long time, you might get an observable deflection. :)


Thanks for the explanation. I was more interested in theoretical possibility than practical observation, but the articles you linked were exactly what I was looking for and gave me some keywords for further search.

Using them I have also just found this masters thesis that has a nice overview and models the effect as a classical non-linear medium, which I found a bit more accessible: http://arxiv.org/abs/hep-ph/0512033

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Re: Two beams of light affecting each other

Postby PM 2Ring » Sun Sep 01, 2013 1:17 pm UTC

My pleasure.

I see that there's a third paper on this topic by E. Lundstrom (et al) on arXiv, but I can't see any actual results of photon-photon scattering experiments, and Lundstrom appears to have moved on to other topics. Maybe they're waiting for the XFEL X-ray laser at DESY to come on-line...

Please let us know if you do find anything more tangible on this interesting topic!

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Re: Two beams of light affecting each other

Postby flownt » Sun Sep 01, 2013 4:27 pm UTC

Tass wrote:If they are not parallel, then there is also a gravitational effect, albeit very small.


Why are there no gravitational effects if they are paralel?

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Re: Two beams of light affecting each other

Postby Tass » Mon Sep 02, 2013 5:01 am UTC

flownt wrote:
Tass wrote:If they are not parallel, then there is also a gravitational effect, albeit very small.


Why are there no gravitational effects if they are paralel?


Basically because gravity "cannot catch up". This only goes if they are going in the same direction, mind, if they are anti-parallel they will attract.

That there cannot be any attraction between parallel beams can be seen from the fact that you can change their energy by a Lorentz transformation. If they attract both by amount x and amount 2x, then x must be zero.

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Re: Two beams of light affecting each other

Postby Qaanol » Mon Sep 02, 2013 2:49 pm UTC

Tass wrote:Basically because gravity "cannot catch up". This only goes if they are going in the same direction, mind, if they are anti-parallel they will attract.

This explanation never quite sat right with me.

Picture two parallel laser beams 1 light-second apart, firing continuously. The laser on the left starts firing, photons stream out, and their energy density starts warping space-time. The photons shoot forward at c, and their initial warping of space-time expands spherically from the laser.

After 1 second, the gravity sphere from the first photons out of the left-hand laser reaches the right-hand laser, warping space-time at that laser. Intuitively it seems like photons launched from the right-hand laser at that time should feel the effects of that warped space-time.

Tass wrote:That there cannot be any attraction between parallel beams can be seen from the fact that you can change their energy by a Lorentz transformation. If they attract both by amount x and amount 2x, then x must be zero.

Or infinity. But yeah, this makes sense, though it does not explain where the intuition that photons should have a “gravitational wake” goes wrong.
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davidstarlingm
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Re: Two beams of light affecting each other

Postby davidstarlingm » Tue Sep 03, 2013 3:24 pm UTC

Qaanol wrote:
Tass wrote:Basically because gravity "cannot catch up". This only goes if they are going in the same direction, mind, if they are anti-parallel they will attract.

This explanation never quite sat right with me.

Picture two parallel laser beams 1 light-second apart, firing continuously. The laser on the left starts firing, photons stream out, and their energy density starts warping space-time. The photons shoot forward at c, and their initial warping of space-time expands spherically from the laser.

After 1 second, the gravity sphere from the first photons out of the left-hand laser reaches the right-hand laser, warping space-time at that laser. Intuitively it seems like photons launched from the right-hand laser at that time should feel the effects of that warped space-time.

Tass wrote:That there cannot be any attraction between parallel beams can be seen from the fact that you can change their energy by a Lorentz transformation. If they attract both by amount x and amount 2x, then x must be zero.

Or infinity. But yeah, this makes sense, though it does not explain where the intuition that photons should have a “gravitational wake” goes wrong.

If photons had a non-zero rest mass, this would work. Because it would mean they weren't traveling at the speed of light, which would allow for two-way "communication" of the gravitational force.

If the left-hand photons could produce a gravitational effect on the photons emitted slightly later from the right-hand laser, then momentum would not be conserved; the photons in the right-hand laser would have an altered momentum due to the altered gravitational field, but there could be no equal and opposite force on the left-hand photons. In order for the gravitational force to take effect, both objects/particles/whatever have to be able to operate on each other.

It may be counter-intuitive, but remember: length contraction. A particle traveling at the speed of light (in this case, a photon) interacts with the entire universe in a light-like way rather than a timelike way. The entire universe is compressed into a plane perpendicular to its direction of travel. If two photons are parallel and traveling in the same direction, then their normal planes are also parallel, meaning they never intersect and can thus never act on each other. From the perspective of a photon, no other photons can exist unless their normal planes intersect.


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