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Thread: photon mass

  1. #1 photon mass 
    Moderator Moderator AlexP's Avatar
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    Is a photon travelling at c massless? I know it has to be massless to be travelling at c, but it seems to me that for as fast as it's moving, it must have a lot of KE, which would impart to it mass... Unless it can't have KE because it has no mass to begin with. I'm obviously a bit unclear on the matter, so clarification would be great.


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  3. #2  
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    Well, I guess it boils down to light having the properties of both a wave and particle. It's still a mystery to me and I can't help but wonder if light has mass too. It is effected by gravity as any other object. Seems weight is an indicator of mass, right? Some say it is not gravity, but spacetime or some such that effects the space light is traveling through.

    I dunno. I got the lawyer style passive beat down when discussing the matter in another topic. Everyone seems to think the space which the light passes through is bent by mass and so the light's path is bent too. I don't affirm this yet simply because light also has momentum and quantity which leaves room to wonder.


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  4. #3  
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    Chemboy: Maybe look at a feeble explanation here for the relationship between light energy, mass and kinetic energy.
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  5. #4  
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    Light is composed of electrons right? And as i remember electrons have mass.

    Look at lasers, they fire a form of light and as can be seen in various tests, a powerful enough laser melts and blasts the steel out of the way, allowing it to cut it. If light didn't have any mass it would just melt it and the material would flow down; even if it is the slightest amount, the energy in a laser is so much that it shows this fact.

    F=MA

    F=MC

    F=(0)C

    F=0

    No force, no laser cut.
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    Quote Originally Posted by Cold Fusion
    Light is composed of electrons right? And as i remember electrons have mass.

    Look at lasers, they fire a form of light and as can be seen in various tests, a powerful enough laser melts and blasts the steel out of the way, allowing it to cut it. If light didn't have any mass it would just melt it and the material would flow down; even if it is the slightest amount, the energy in a laser is so much that it shows this fact.

    F=MA

    F=MC

    F=(0)C

    F=0

    No force, no laser cut.
    What about the fact that a laser cut is done by heat energy, not the light itself?

    Light is a strange thing, how can it be massless if it exists, but at the same time, how can it travel through space if it contains particles?

    It is interesting that a laser cutter 'blasts' steel away, but I don't think that is actually the case.

    Also, I seem to remember that the mass of an electron is negligible, and the effect of this mass (on steel or similar) would also be negligible.

    I may be completely wrong, but I thought I'd put my thoughts across in order to gain a better understanding...
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  7. #6  
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    Quote Originally Posted by Cold Fusion
    Light is composed of electrons right?
    Are you sure? As far as I know, electrons have charge, photons don't. How can they be one and the same?

    True they are both entities of the quantized EM field, but that's surely no reason to equate them?
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    Yes I would agree with that, but it doesn't widen my understanding of this
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  9. #8  
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    Quote Originally Posted by ColdFusion
    Light is composed of electrons right?
    No, light is composed of photons. You may be confusing this with the fact that photons are released when electron transition in an atom occurs...

    Quote Originally Posted by ColdFusion
    Light is a strange thing, how can it be massless if it exists
    Mass is not a given. Just because something exists doesn't mean it has to have mass.

    Quote Originally Posted by ColdFusion
    but at the same time, how can it travel through space if it contains particles?
    What's the problem with something travelling through space if something contains particles? Our bodies contain particles and we can travel through space.

    Quote Originally Posted by Coldfusion
    Also, I seem to remember that the mass of an electron is negligible
    The mass of an electron is not negligible. In considering atomic masses in chemistry, it's negligible purely because it's so small (edit: in comparison to the rest of the atom (nucleus)) (apprx. 1/1836AMU), but if you're considering anything outside of atomic masses, such as individual electrons, they most definitely have a significant mass. If they were massless they could travel at c, which isn't the case.
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  10. #9  
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    but at the same time, how can it travel through space if it contains particles?
    It's physical waves like sound that cannot travel through space because they have no medium to travel through. a photon travels in it's own medium. (Electromagnetic radiation)

    As to light having mass, there have been many debates on this forum on that matter. Personally, if you look at Einstein's equation on the increase of mass you will see that since light travels at light speed, if it had mass, the mass (and energy required to move it forwards) would be infinite. Since light does move, I would guess that it had no mass.

    Mabey though, the light does not actually travel through our three dimensions . If Light did have mass, than to do what it does it might just shift from dimension to dimension, not ever traveling. We all exist in the singularity! It would not need speed to travel! How would you describe the effects of material objects on light? I don't know. I was just theorizing what I think would happen if light had mass.
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  11. #10 Re: photon mass 
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    Quote Originally Posted by Chemboy
    Is a photon travelling at c massless? I know it has to be massless to be travelling at c, but it seems to me that for as fast as it's moving, it must have a lot of KE, which would impart to it mass... Unless it can't have KE because it has no mass to begin with. I'm obviously a bit unclear on the matter, so clarification would be great.
    No one has mentioned KE, The alternative that Chemboy mentioned in the first place. This, I don't know. I would guess that light had no KE. What exactly is the exact nature of this? Does something need KE to move?

    Kinetic Energy is defined as the work needed to accelerate a body of a given mass from rest to its current velocity. Light wants to travel at light speed. When you slow it down in a near absolute zero gelatinous solution, when it comes out the other end, it just continues to travel at light speed.

    The fact that we can slow down light does say something about this though.
    What effect would that have on your hypothesis?
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    Thank you guys, all is coming clearer

    I am an A-level student so I'm not learning things at this level, but I'm just interested/fascinated by these things.

    If light had a mass, it's photons surely could NOT travel at c, correct?

    As you say, we don't know about other dimensions, so we can only speculate, at least until we have technology to prove otherwise lol.

    Anyway, hopefully you'll be able to help with the things I should be concentrating on as and when I need it lol
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  13. #12  
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    Quote Originally Posted by ColdFusion
    If light had a mass, it's photons surely could NOT travel at c, correct?
    If photons had mass they could not accelerate to c. Photons do travel at c, but I still wonder if they have some sort of mass due to their large amount of KE, that's what I was trying to figure out in this thread.

    Something to point out... Isn't what's normally given as 0 the photon's rest mass? What's the significance of that?
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  14. #13  
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    Photons have no "rest mass", but they do have mass because of their energy. Anywith with energy will also have some mass.
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  15. #14  
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    Quote Originally Posted by Shaderwolf
    but at the same time, how can it travel through space if it contains particles?
    It's physical waves like sound that cannot travel through space because they have no medium to travel through. a photon travels in it's own medium. (Electromagnetic radiation)

    As to light having mass, there have been many debates on this forum on that matter. Personally, if you look at Einstein's equation on the increase of mass you will see that since light travels at light speed, if it had mass, the mass (and energy required to move it forwards) would be infinite. Since light does move, I would guess that it had no mass.

    Mabey though, the light does not actually travel through our three dimensions . If Light did have mass, than to do what it does it might just shift from dimension to dimension, not ever traveling. We all exist in the singularity! It would not need speed to travel! How would you describe the effects of material objects on light? I don't know. I was just theorizing what I think would happen if light had mass.
    Well, the trick is that, by that definition, even light isn't really traveling at C. It's so close to C that you could never measure the difference, and that's part of why you can increase its energy by adding acceleration to it. You're pushing it closer and closer to C.

    If it ever actually reaches C, it will have infinite mass, but that would require infinity acceleration.

    In other words, the closer you get to C itself, you need more and more energy to increase your speed less and less. It's kind of like a hyperbolic curve, if you've ever studied those in High School algebra. You never actually reach your goal, just get closer and closer.
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  16. #15  
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    Quote Originally Posted by kojax
    Well, the trick is that, by that definition, even light isn't really traveling at C. It's so close to C that you could never measure the difference, and that's part of why you can increase its energy by adding acceleration to it. You're pushing it closer and closer to C.

    If it ever actually reaches C, it will have infinite mass, but that would require infinity acceleration.

    In other words, the closer you get to C itself, you need more and more energy to increase your speed less and less. It's kind of like a hyperbolic curve, if you've ever studied those in High School algebra. You never actually reach your goal, just get closer and closer.
    This is wrong. Light travels at exactly C in a vacuum. You need to look up the difference between relativistic mass and rest mass. Light has no rest mass, but it has relitivistic mass because of its energy.
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  17. #16  
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    Light does have mass. Energy is mass.
    Is that mass ever shown in physical form?
    DOES GRAVITY HAVE AN EFFECT ON PURE ENERGY BECAUSE IT IS ANOTER FORM OF MASS???????
    I think I'll post that.
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    I don't think photons have any form of mass.. rest or relativistic.
    For text book answers:
    E^2 = (m^2)(c^4) +( p^2)(c^2)
    m=0

    E=pc=hf
    p=hf/c

    Photon has momentum but not mass, hope it helps.
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  19. #18  
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    Light has energy though.
    Isn't energy mass?
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  20. #19  
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    Quote Originally Posted by Wildreamz
    hope it helps.
    No it doesn't.
    For text book answers: E<sup>2</sup> = m<sup>2</sup>c<sup>4</sup> + p<sup>2</sup>c<sup>2</sup>
    OK but then you assume the result you think you are showing:
    m=0
    E=pc=hf
    p=hf/c
    Assuming that p = momentum, f = frequency, what is h? Planck constant? Where does that come in? Explain what it is! What's it got to do with the discussion anyway?

    You are not explaining yourself very well. How have you shown that the photon has momentum but not mass?
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  21. #20  
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    The energy that u are refering to, E=mc^2, is the rest energy of a matter.
    Photon's rest mass(or invariant mass) = 0, since it is an EM wave, it is not even matter to begin with.

    Therefore, using E=mc^2 is invalid in this case. However, if u are able to convert all the energy in 1 photon into matter. Yes, that will be the supposed mass of this matter created.

    The equation i quote is the correct relativistic expression for the energy of a particle of mass m with momentum p.

    It is generally taught in textbooks that, photon has no rest mass but has momentum. And yes , h is planck's constant used to calculate this momentum. Remember, energy of light is E=hf.

    Of course, although the rest mass of photon is considered as 0 in mainstream physics. You might hypothesis that it does contain some mass, but of course, this will screw up the whole theory of relativity and how the speed of light is invariant regardless of the speed of observer and source.

    However, the theory of relativity is backed by much experimental edvidence.. so i doubt ur hypothesis will be taken seriously unless u can find new experimental edvidence to back it.

    My 2cents worth.
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  23. #22  
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    I was not saying that the photon itself contained physical mass. Question: how does the energy of light differ from the energy released from an atomic bomb? Doesn't an atomic bomb release photons?
    I was merely stating that, since that energy is another form of mass, maybe gravity still has the exact same effect on pure energy as it does on mass.
    I do not believe that a photon has physical mass. It would be impossible for it to reach light speed.
    Maybe the fact that energy and mass are two sides of the same coin though can explain why black holes love to swallow light. Are there more similarities in the two than just the fact that you can create one from another?
    The way that they react would explain why a particle can exist without mass, and also why the particle is effected by gravity.
    Does this still defy relativity? if it does, i have no reason not to disregard the idea. I did not think of this for any practical reason. It was just an Idea. What is wrong with this idea? I am not challenging you, I merely want to learn.
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  24. #23  
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    Quote Originally Posted by Shaderwolf
    was not saying that the proton itself contained physical mass.
    Make sure you say "photon," we don't want people to become (more) confused.

    (Now to everyone in general)

    E=mc<sup>2</sup> does not say that energy has mass or that energy is mass. It says that energy is equivalent to mass. Just take note of that.

    Now, the next matter... I want to establish something here. When a photon comes into existence, due to say, electron transition, does it start at speed = 0 and accelerate to c, or is its speed instantly c?
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  25. #24  
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    I'd say instantly C. The bose-einstein condensate situation is atypical of how a photon behaves.
    Disclaimer: I do not declare myself to be an expert on ANY subject. If I state something as fact that is obviously wrong, please don't hesitate to correct me. I welcome such corrections in an attempt to be as truthful and accurate as possible.

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  26. #25  
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    One word...momenergy
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  27. #26  
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    You might as well have said "daberettaboot". I don't know what that is. Sounds interesting though.
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  28. #27  
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    look at this

    I just found this. what do you think?
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  29. #28  
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    Quote Originally Posted by Shaderwolf
    look at this

    I just found this. what do you think?
    That was good... Can't honestly say I got a ton out of it, but still good. This (from the same site): http://math.ucr.edu/home/baez/physic...hotonMass.html was good too.
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