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Can light collide with light? Answered

If two electromagnetic waves intersect, what happens? Is light 'immaterial' or without density, is light weightless?

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orksecurityBest Answer (author)2010-06-17

Light has mass (relativistic mass only, no rest mass). Gravitational lensing is one example of that; websearch will find you details about that. However, for all practical purposes light has no density, and will not collide with another light beam.

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steveastrouk (author)2010-06-17

"collision" is perhaps a bit emotive, but light can of cource constructively, and destructively interfere with itself, that's how diffraction gratings work for example.

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nepheron (author)steveastrouk2010-06-18

So is this like that whole light-through-slits experiment? When a beam of light it aimed through 2 slits, and it separates into a bunch of bars?

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steveastrouk (author)nepheron2010-06-18

Yes, but along will come Kelsey to point out the single photon version.

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orksecurity (author)steveastrouk2010-06-18

Valid point. Two beams crossing in midair won't noticeably affect either beam farther on, but at the intersection you can get interference effects.

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Jack A Lopez (author)orksecurity2010-06-18

Unless of course the place where the beams intersect is not "midair", but instead a non-linear optical medium, then one beam can affect the future travel (phase, frequency, amplitude, etc) of the other beam.

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Kryptonite (author)2010-06-18

I would say that no, it cannot collide, even though it indeed does have mass (as proved below).

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nickodemus (author)2010-06-17

This looks like a question for Kelsey! :D
My layman's guess, is yes. I don't think light could be weightless, since it can't escape a black hole. I can't wait to see what all the science majors think. :-)

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