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Why is it that if I have two torches, each shining rays in perpendicular directions at perpendicular screens, such that the beams cross, then the images on the respective screens are independent of whether the alternate torch is switched on? In other words, why does light seem to pass through itself without interacting?

Apologies if this question is basic, and apologies if there is a similar question somewhere on the site but I haven't managed to find an answer.

Thanks, A.

Aerinmund Fagelson
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    Because photons very rarely interact with each other (although they sometimes do). – lemon May 18 '15 at 12:47
  • what is required to see this? does it require quantum mechanics or maxwell's equations? – Aerinmund Fagelson May 18 '15 at 13:24
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    #lemon I'm afraid that is just rephrasing my question. my question is really, can anyone explain why (or which model explains why) photons very rarely interact with each other. modelling light as a wave, it seems non-trivial that it doesn't interact with itself. – Aerinmund Fagelson May 18 '15 at 13:35
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    Maxwell's equations are linear, i.e. two light beams will superpose exactly without interaction. So the linearity of Maxwell's equations is another way of stating your phenomenon. The interactions between light beams are in the realm of quantum electrodynamics, where there is a small, but nonzero, amplitude for e.g. box diagrams and other Feynman diagram loops which describe photon-photon interaction. – Selene Routley May 18 '15 at 13:36
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    Look at the proposals for gamma-gamma colliders for the experimental regime necessary to see deviations from the "linearity" of Maxwell's equations. We are technologically borderline able to explore true/pure gamma-gamma-collisions directly and I would expect several such machines to become operational over the next two decades. – CuriousOne May 18 '15 at 14:07

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Beams of light as we observe them are described classically by Maxwell's equations which have been validated over and over again. Optics is a very well studied subject. In this classical framework light is an electromagnetic wave that moves in vacuum with a constant velocity c in all frames and does not need a medium to propagate, in contrast to sound waves or water waves. Electromagnetic waves have been studied to interfere with each other in the two slit experiment. The energy is distributed in an interference pattern.

In the above framework it makes no sense to have two beams interact in the sense of scattering off each other.

In the quantum mechanical frame, the underlying frame not only of light but of all nature, the electromagnetic wave is composed out of zillions of photons, and photons are elementary particles . There exists photon photon scattering but the probability of this to happen is very low. Thus two light beams have no measurable interactions when crossing.

anna v
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  • Would it be worth mentioning nonlinear optics in this context? Or is that too far removed from the question? – Floris May 18 '15 at 14:16
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    @Floris I am not in a position to discuss non linear optics. If you think it is relevant to beam of light versus beam of light interaction maybe you should add an answer? – anna v May 18 '15 at 16:40
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In other words, why does light seem to pass through itself without interacting?

Because it consists of waves. Anna mentioned photons, but see Wikipedia where you can read about E=hf or E=hc/λ, wherein f is frequency and λ is wavelength. Photons have a wave nature. Imagine minor ocean waves, intersecting. They ride over each other and keep on going. They did interact, but not in any permanent way. If however they were both high-energy waves they might ride up each other and turn into "rollers". The Breit-Wheeler process is something like this:

"However, in 1997, researchers at the Stanford Linear Accelerator Centre were able to conduct the so-called multi-photon Breit–Wheeler process using electrons to first create high-energy photons, which then underwent multiple collisions to produce electrons and positrons, all within same chamber."

See this report from Imperial, which relates to what CuriousOne was saying.

John Duffield
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