Does photons cause gravitation?

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Jonatan
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#1
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I have asked many people wwho are rather proficient in physics about this but I keep getting contradicting answers.

According to relativity theory, energy and mass are equivalent. Now, light particles (photons) certainly have energy, so should they not have a relativistic mass (I am aware they have no rest mass)? Does a photon curve the spacetime around it causing gravitation?
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jpowell
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Relativistic mass is a bad term to use, it is very outdated. The term mass unambiguously refers to rest mass these days. So photons have no 'mass'. But we do know that they contain energy and according to GR energy causes a gravitational field. So in answer to your original question, yes.
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Jonatan
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(Original post by AntiMagicMan)
Relativistic mass is a bad term to use, it is very outdated. The term mass unambiguously refers to rest mass these days. So photons have no 'mass'. But we do know that they contain energy and according to GR energy causes a gravitational field. So in answer to your original question, yes.
But why on earth does two laser beams cross undeflected then? Should not the gravity acting between two photons which pass close to each other bend their path?
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jpowell
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Hehe, you need an absolutely massive amount of gravity to bend light . The gravitational effect of a laser beam would be negligible.
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shiny
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This is all very intellectual
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Jonatan
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(Original post by AntiMagicMan)
Hehe, you need an absolutely massive amount of gravity to bend light . The gravitational effect of a laser beam would be negligible.
But the distance gets very short as well. Since photons do not experience any mutual repulsion, could they not theoretically pass straight through each other making the radi between them neglichable? Then I would suspect the gravitational force between them to become quite extreme. Of course, Einsteins general theory of relativity doesnt work very well on small distances to begin with ( Quantum fluctuations etc).
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rts
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(Original post by Jonatan)
But the distance gets very short as well. Since photons do not experience any mutual repulsion, could they not theoretically pass straight through each other making the radi between them neglichable? Then I would suspect the gravitational force between them to become quite extreme. Of course, Einsteins general theory of relativity doesnt work very well on small distances to begin with ( Quantum fluctuations etc).
i think the principle of superposition would apply if they "passed through each other". not that that is any help.
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jpowell
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Photons interactions are definitely in the domain of quantum mechanics, and so it is silly to try and use GR to explain their behavior.
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Jonatan
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(Original post by AntiMagicMan)
Photons interactions are definitely in the domain of quantum mechanics, and so it is silly to try and use GR to explain their behavior.
Yes, ive heard about the chaos that happens down at the planch length when you try to do so...
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elpaw
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Deja vu
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Jonatan
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(Original post by elpaw)
Deja vu
LOL. I have asked it here as well... Hmmm, well at least my memory qualifies for a physics career. I still do not have a clue what to beleive. All I know for certain is that it sooner or later ends up with teh answer "It depends on how you define..."
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jpowell
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I don't think it matters how you define mass. Energy bends space time. Photons have energy. Photons bend space time. But then a photon is going to bend space time far less than say an electron, and we don't consider the gravitational attraction between electrons.

The reason that you don't see a strong interaction between photons even as the distance gets infinitely small, is because as we all know from quantum mechanics infinitely small is not well defined.
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Nylex
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(Original post by AntiMagicMan)
as we all know from quantum mechanics infinitely small is not well defined.
We do? Explain that.. I really didn't understand quantum mechanics .
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Jonatan
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(Original post by Nylex)
We do? Explain that.. I really didn't understand quantum mechanics .
Basicly in order to credit the existance of a small distance you must be able to measure it. It happens that the larger the energy of a photon, the shorter its wavelength , and consequentually the shorter a distance you can measure. An infinitely smalle distance is therefore only meaningful if you have a photon (or other particle) of infinite energy (actually it is the momentum that matters, but infinite momentum implies infinite energy). Currently the shortest distances that have any relevant effect on our experiments are probed using particle accelerators. For the shortest distances one has to pump enough energy into a single proton to give it a kinetic energy equivalent to the sum of the kinetic energy of all particles in a mosquito in full flight. Seing that a mosquito contains several billion particles you can quickly imagine what vasts amounts of energy must be pumped into a single particle. Quantum Mechanics and Relativity theory contradict each other at very short distances. One problem is that in order to probe these extremely short distances using present technology one would need a particle accelerator with a diameter close to that of the solar system. Hardly something that will be affordable in the close future...
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Jonatan
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(Original post by AntiMagicMan)
I don't think it matters how you define mass. Energy bends space time. Photons have energy. Photons bend space time. But then a photon is going to bend space time far less than say an electron, and we don't consider the gravitational attraction between electrons.

The reason that you don't see a strong interaction between photons even as the distance gets infinitely small, is because as we all know from quantum mechanics infinitely small is not well defined.
So if I understand you correctly, you would say that theoretically gravitation does cause two laser beams to deflect slightly as they cross, only that this deflection is so neglectable that we can disregard it?
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jpowell
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Yes... I guess I am.
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Jonatan
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(Original post by AntiMagicMan)
Yes... I guess I am.
Which brings up another interesting question. Is gravitation quantisized? Einstein would probably turn in his grave if you answered yes to that question, and quite honestly it would not surprise me if the answer was no. After all, you get all sorts of weird surprises when you try to mix GR with QM.
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jpowell
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Personally I think it is. General relativity is a good theory but it doesn't quite make the cut. I just can't accept we live in a universe with 4 forces, 3 of which unify and the last one stubbornly refuses to.
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Jonatan
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(Original post by AntiMagicMan)
Personally I think it is. General relativity is a good theory but it doesn't quite make the cut. I just can't accept we live in a universe with 4 forces, 3 of which unify and the last one stubbornly refuses to.
Well, Im quite a stubborn looner myself, maybe thats why I like gravity
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jpowell
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String theory isn't much help either, though physicists like to talk about it, no one can get the equations to make much sense and even when they have equations that look good, no one knows how to solve them or even use them .
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