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Thread: Does light have mass?

  1. #1
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    Scientist say that even light can't escape the gravity pull of a black, then light must have mass in order to be affected by the gravity pull of the black hole.
    So do light have mass?

  2. #2
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    Yep!
    Light has two parts: a wave and a very very small piece of mass known as a photon. Photons are emitted by electrons when they drop from a high energy state to a low energy state.
    Random fact: EVERYTHING in the universe has both a wave and particle form. So technically you are a wave, but due to the size of a person the wave part of you is negligible.

  3. #3
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    No, light do not have mass, but the equivalence principle says that everything, heavy masses as well as massless objects are affected by gravity equally. Gravity is described as the curvature of the spacetime, which all objects has to pass through.

  4. #4
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    Hmm we have a contradiction. Personally I think that light does have mass even if it is extremely extremely little. Otherwise it makes no sense that it would be sucked into black holes.

  5. #5
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    Otherwise it makes no sense that it would be sucked into black holes.
    Yes actually it does. The equivalence principle.

    Anyway, some links that might explain it:

    http://math.ucr.edu/home/baez/physics/Rela...light_mass.html

    http://www.discover.com/ask/main52.html

    http://van.hep.uiuc.edu/van/qa/section/Lig...t/949533197.htm

  6. #6
    Faulkner Guest
    Do positrons emit photons too? If so, light could be massless, but still be affected by matter, because it exists beyond the frontiers of mere matter & mass etc...Or are there such things as anti-photons?

  7. #7
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    Originally posted by Faulkner@Oct 9 2003, 10:50 AM
    Do positrons emit photons too? If so, light could be massless, but still be affected by matter, because it exists beyond the frontiers of mere matter & mass etc...Or are there such things as anti-photons?
    Not only could light be massless, it is always massless. As for positrons, yes that can happen but the photon has no charge and so there would be no difference between a photon coming from an electron and one from a positron.

  8. #8
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    Originally posted by Faulkner@Oct 9 2003, 10:50 AM
    Or are there such things as anti-photons?
    The dark? Kidding!

    What about when light waves combine to make a dark spot? I remember some high-school physics about that.

    As well, since there is constant background radiation, are photons (of some wavelength) present almost anywhere?

    Oh, as well, light can act like it has momentum. That's the concept behind a solar sail.

  9. #9
    GreekJimbo Guest
    Well, I think I can answer our question without even answering your question. Check this out. . . .

    All object travel in a stright line in a universe. Doesnt matter is they are weightless or someone's overweight grandmother falling out of a plane. Every object moves in the direction of least resistance (in the cae of the overweight grandmother, that's straight down).

    So, we have light . . . it travels in a "straight" path most of the time (by our observations). But whenever sapce-time warps, so does the path of light. Forget about the idea of gravity "pulling" light. Gravity changes the shape of the road that the light is travelling on. Light is just moving in the path of least resistance (and even if it seems to curve to us. . . it is still travelling straight acording to the light particle. In the case of the blackhole, the light is moving on a "road" that infinitally loops back on itself. So, the light is still travelling straight, but the road is bent. Poor light.

    Peace!

  10. #10
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    Let me clairify one thing- Photons have no antiparticle.

  11. #11
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    The Mass of the Photon

    Here is an interesting take on the old Photon debate. Apparently it has no rest mass, but it acquires a mass when moving.....


    http://www.physlink.com/Education/AskExperts/ae180.cfm

  12. #12
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    Can’t light waves cause a pressure on the objects the light hits, without the light being a “particle” or having “mass”. Can’t the force of the rapidly moving electric and magnetic fields of the light cause the pressure without the fields having “mass”?

  13. #13
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    Quote Originally Posted by rahuldandekar
    Let me clairify one thing- Photons have no antiparticle.
    Photons are actually their own antiparticle.

  14. #14
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    Quote Originally Posted by Sam5
    Can’t light waves cause a pressure on the objects the light hits, without the light being a “particle” or having “mass”. Can’t the force of the rapidly moving electric and magnetic fields of the light cause the pressure without the fields having “mass”?
    Yes, basically. Photon scattering or absorption does exert a pressure, because the photons has momentum, given by E/c

  15. #15
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    Thanks Swansont, for pointing out that photons ARE their own anti-particles.

    If photons have mass, how can they go from not existing(an electron in an elevated orbit) to instantly travelling through space at C? All objects with mass must accelerate, and cannot travel at c. On ther other hand, if light is massless, it cannot accelerate and must travel at c. Therefore, we can safely state the photon has no REST MASS.

    It is possible to try and measure the mass of the photon, but all attempts have failed. This allows us to say that if the photon does have mass, it is smaller than anything we have tried to measure.

  16. #16
    I have nothing to add to this but i realy enjoyed reading all the answers, very interesting.

    esp. alanprice and GreekJimbo and Swansont

  17. #17
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    Photons do not have mass. This is one of the fundamental principles of Quantum ElectroDynamics. If photons have mass, a lot of stuff in this start to fall apart, and we don't see that happening, and we're talking any mass. Now, I wish I could go into this more, but that's all the info I've been able to glean from chatting with one of the faculty here (a solid state physicist).

    Now gravity isn't an interaction between masses (think of the standard force of gravity equation as a Newtonian approximation). Mass bends spacetime in such a way as to alter the paths of objects around it. Without mass, spacetime is flat, and the objects go in a straight line (they always go in a straight line). However, with mass, spacetime is bent, and those straightline paths, which are "straight" but restricted to following spacetime, are bent and curved.

    This is analogous to walking a straight line on earth. However, we are restricted to the surface, and so that straight line will eventually curve around on itself. Light, and matter, is restricted to the "surface" of spacetime. So if spacetime bends, so do their trajetories.

  18. #18
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    But what if a photon heads towards a blackhole, swings around it and then heads back in the direction that it came from? Will it not have imparted some momentum to that black hole? There fore there must be some kind of interaction between mass'?

  19. #19
    Quote Originally Posted by TwAgIssmuDe
    Scientist say that even light can't escape the gravity pull of a black hole, then light must have mass in order to be affected by the gravity pull of the black hole.
    So do light have mass?
    According to Albert Einstein, "mass" expands exponentially as it approaches light speed and such exponential expansion prohibits any mass from attaining the speed of light because of the (also) expanding resistance (like the force against an auto's windscreen grows greater as speed increases).

    Light is warped around a black hole as it follows the curvature (warp and compression) of space in the vicinity of a black hole. Theoretically, a black hole is mass that has, in fact, expanded exponentially and is traveling at a speed approaching the speed of light.

    Light, it seems, follows the path of least resistance and its natural "physic" is to travel at the speed of light when un-emcumbered by matter. It is my opinion that a black hole contains no light, nor does it stop light but, rather, repels light because of the dense compression of space surrounding it so it seems that no light is eminating from a black hole.

    I have written an essay addressing this issue:
    http://www.templeofsolomon.org/Quant...tg/thought.htm

    It is also my belief that matter does not occupy space but, rather, displaces it. It is this belief that has caused me to formulate my own definition of "gravity" which is a result of the compression (from within) and stretching (from without) of space by matter.

    Einstein and Hawking seem to disagree about what happens inside of a black hole. Einstein (interpreted) claims that light cannot be "stopped" and Hawking claims that time stops inside of a black hole. If time stops then it is impossible to have "velocity", therefore light , if any, must stop. It is at this junction that Einstein and Hawking part company.

  20. #20
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    Quote Originally Posted by John_Charles_Webb

    snip....
    Actually, this particular forum (Questions and Answers) is for mainstream answers. That silly essay and its misunderstandings of General Relativity really belong in the ATM forum.

  21. #21
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    Quote Originally Posted by swansont
    Yes, basically. Photon scattering or absorption does exert a pressure, because the photons has momentum, given by E/c
    Do you have a link to a white paper or essay on this? Light for me is by far the most interesting thing in the universe, and this bit got me thinking about the subject.

  22. #22
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    Try here. Click on each of the boxes below the top chart. Hope this helps.

  23. #23
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    In addition to Tensor's link (love the hyperphysics site) try this and that

  24. #24
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    Do photons have energy?

    Quote Originally Posted by Ricimer
    Photons do not have mass.
    If photons do not have mass, then do they also not have energy?

    If a photon has energy but not mass then I guess I have a fundamental misunderstanding of the equation E=mc^2, which seems to be saying that energy and mass are equivalent, except for a conversion factor of c^2.

    Can someone help clear up my confusion, here?

    Thanks!

  25. #25
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    E = mc^2 is actually the formula for converting between rest energy and mass.

    For a moving object, the equation to use is E = E0 + pc, where E0 is the rest energy. For photons, there is no rest energy, so you get E = pc, i.e momentum times the speed of light.

    Edit: I might as well add that the photon's energy according to quantum mechanics is E = hf, i.e Planck's constant times the frequency. Combine the two equations, and you get all sorts of interesting factoids such as the momentum of a photon with a certain frequency and so forth. Interesting stuff.

  26. #26
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    Actually it's E^2 = p^2c^2 + m^2c^4

    m is rest mass. You get E = pc because the rest mass is zero.

  27. #27
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    I knew there was something I'd forgotten ... serves me right for trying to do this without doing some fact-checking beforehand.

  28. #28
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    The only thing you did wrong was to say that E = E0 + pc.
    Those 2 vectors(E0, pc) do not add up the same way as E^2 = p^2c^2 + m^2c^4.

  29. #29
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    Question, and I may have completely and totally misinterpreted what was said here:

    Photons are trapped in solar cells to get energy, to my understanding, so my question is, were you to slow a photon to lesser than the speed of light, utilizing E=mc^2, you're converting energy to mass, so what if black holes, rather than bending or repelling light, actually slow photons to sub-light speed and thus your photon becomes particulate matter and is then affected directly by the black hole's gravity (IE. light does not escape a black hole). Your converted photon would only have to weigh a marginal fraction of anything in order to be affected, and by being marginally affected in any way shape or form, it's slowed down even moreso and thus gripped by your black hole.

    Warning, my logic hurts even my own brain, I apologize if I'm a blithering idiot.

  30. #30
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    Not quite.

    A photon's energy is measured by it's color. The shorter the wavelength, the stronger the photon. Photons ALWAYS travel at C(in vacuo). No faster, no slower, but C.

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