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Thread: energy in massless particles?

  1. #1 energy in massless particles? 
    Forum Bachelors Degree 15uliane's Avatar
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    Excuse my ignorance, but if photons do not have mass (I assume that is correct) how can they have any energy? If E=mc^2 then M=E/c^2 but if m=0 then E also has to =0. How can a photon have a mass of 0 but still have energy? Same goes for any other elemental particle with no mass but with energy. If I am wrong on any of my statements please correct me.


    Also, pardon the overuse of the only equation most laymen like me know, it's the only one easy enough for me to manipulate and think about with my algebra 1 education.


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  3. #2 Re: energy in massless particles? 
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    Quote Originally Posted by 15uliane
    Excuse my ignorance, but if photons do not have mass (I assume that is correct) how can they have any energy? If E=mc^2 then M=E/c^2 but if m=0 then E also has to =0. How can a photon have a mass of 0 but still have energy? Same goes for any other elemental particle with no mass but with energy. If I am wrong on any of my statements please correct me.


    Also, pardon the overuse of the only equation most laymen like me know, it's the only one easy enough for me to manipulate and think about with my algebra 1 education.
    A photons rest mass is zero, it still has kinetic energy, enough to go c.


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  4. #3  
    Forum Bachelors Degree 15uliane's Avatar
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    so when its moving it does have mass. correct?
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  5. #4  
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    I was some of the mud that got to sit up and look around.
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  6. #5  
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    Quote Originally Posted by 15uliane
    so when its moving it does have mass. correct?
    A photon ALWAYS moves, and it moves at c.

    It has energy . It has momentum . That energy has a mass equivalent and that corresponds to the momentum and speed

    It cannot be accelerated, so I have no idea what "inertia" or "mass" would mean in that sense.
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  7. #6  
    Forum Bachelors Degree 15uliane's Avatar
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    so the mass equivalent is not actually the real mass of the photon because it cannot be accelerated, so there is no definition of mass? Wouldn't that make the mass equation void in this situation. If you know I'm not going to understand the answer just say so and I'll learn about it when I get there.
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  8. #7  
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    Quote Originally Posted by 15uliane
    so the mass equivalent is not actually the real mass of the photon because it cannot be accelerated, so there is no definition of mass? Wouldn't that make the mass equation void in this situation. If you know I'm not going to understand the answer just say so and I'll learn about it when I get there.
    http://www.thescienceforum.com/What-...ass-31248t.php
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  9. #8  
    Forum Bachelors Degree 15uliane's Avatar
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    thank you. recap (to see if I'm getting this)- rest mass of photon is 0, and when it is moving it has a mass equivalent that describes the mass of the photon, although if is not really clear what the actual mass is because since the photon cannot be accelerated it doesn't and will not have any inertia, therefore there cannot be any inertial mass, and since inertial mass always corresponds to gravitational mass, there can't be any gravitational mass?
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  10. #9 Photons inside superconductors 
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    I found the following line from the wiki page very interesting. Would anyone be able to provide any further information on this?


    http://en.wikipedia.org/wiki/Photon#...on_photon_mass
    Photons inside superconductors do develop a nonzero effective rest mass; as a result, electromagnetic forces become short-range inside superconductors.
    Many thanks.

    Update: I have found this interesting review article: Photon and Graviton Mass Limits
    http://arxiv.org/PS_cache/arxiv/pdf/...809.1003v5.pdf
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  11. #10  
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    Quote Originally Posted by 15uliane
    thank you. recap (to see if I'm getting this)- rest mass of photon is 0, and when it is moving it has a mass equivalent that describes the mass of the photon, although if is not really clear what the actual mass is because since the photon cannot be accelerated it doesn't and will not have any inertia, therefore there cannot be any inertial mass, and since inertial mass always corresponds to gravitational mass, there can't be any gravitational mass?
    Gravitation is explained by general relativity, and mass/energy is what is important there. Gravitational vs inertial mass is really a Newtonian issue.

    Electromagnetic energy is included in the stress-energy tensor of general relativity.
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    photons do have mass, just real small!! they can even move objects!

    i forget what the 1940's device was though, sry!

    something like a piece of tinfoil left side paint black, with a pin down the middle( as spindle) then on the side that photons are reflected is a push force(newtons) causing a motion !!! low power thou, this all happens in a vacuum
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  13. #12  
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    Quote Originally Posted by doggy
    photons do have mass, just real small!! they can even move objects!

    i forget what the 1940's device was though, sry!

    something like a piece of tinfoil left side paint black, with a pin down the middle( as spindle) then on the side that photons are reflected is a push force(newtons) causing a motion !!! low power thou, this all happens in a vacuum
    That effect does not depend on photon mass. It is purported to be due to photon momentum, but is more likely a thermal effect in an imperfect vacuum.
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    Can you elaborate a little on your statement that its likely due to a thermal effect in an imperfect vacuum.Its an interesting and different take on the effect.
    Due to momentum is what I was tought and what 'makes sense' to me since momentum is an energy due to motion .just like a photon's 'mass' is a mass due to motion.
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    Quote Originally Posted by MigL
    Can you elaborate a little on your statement that its likely due to a thermal effect in an imperfect vacuum.Its an interesting and different take on the effect.
    Due to momentum is what I was tought and what 'makes sense' to me since momentum is an energy due to motion .just like a photon's 'mass' is a mass due to motion.
    The black side of the blades is hotter than the white side. Air on that side heats up and the slightly increased momentum of the molecules pushes a bit harder on the black side than the white side.

    The toys actually turn in wrong direction for photon momentum to be the motive power. The reflective side should have greater impulse than the black side.

    http://www.lhup.edu/~dsimanek/museum/themes/fake-pm.htm
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  16. #15 Compton Scattering 
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    Photons do have "inertia" so to speak. When they collide with electrons they cause them to scatter. The Energy and momentum of a photon are both related to it's frequency. Consider this, in modern physics we learn that all particles behave as waves on the very smallest scales
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    Quote Originally Posted by bbrand719 View Post
    Photons do have "inertia" so to speak. When they collide with electrons they cause them to scatter. The Energy and momentum of a photon are both related to it's frequency. Consider this, in modern physics we learn that all particles behave as waves on the very smallest scales
    DrRocket doesn't posts much in this forum anymore. I would suggest you do a search on DrRocket and find some of the other forums he's more active in if your interested in what he has to say.
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  18. #17  
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    photons may not have rest mass but they have momentum
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  19. #18  
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    Quote Originally Posted by ostkef View Post
    photons may not have rest mass but they have momentum
    There must be something to that. After all they are talking about using solar sails on space vessels and that means light must create enough pressure to move a space craft and even accelerate it, if the sails can be made big enough.
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  20. #19  
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    Quote Originally Posted by bbrand719 View Post
    Photons do have "inertia" so to speak. When they collide with electrons they cause them to scatter. The Energy and momentum of a photon are both related to it's frequency. Consider this, in modern physics we learn that all particles behave as waves on the very smallest scales
    In this example, photons do not "collide with electrons and scatter them." They interact with the photon, causing a change in energy state- after which a photon is emitted and the electron drops to a lower energy state. This is due to conservation not to collisions.
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  21. #20  
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    Quote Originally Posted by PhDemon View Post

    Of course photons have momentum, do some basic research, it's not exactly secret knowledge.
    Dayum, you're more cranky than genius duck.
    Last edited by ostkef; April 16th, 2013 at 11:09 AM.
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  22. #21  
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    I think , energy which has a momentum but it has not any mass due to speed of light. It is not possible to talk about mass for anything reach to speed of light. The point that energy has a momentum then it must have mass ? Mass and energy is different kind. Also to understand the situation there is a statment ' mass converted to energy'. Energy has no mass.
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    Quote Originally Posted by PhysicsEngineer View Post
    I think , energy which has a momentum
    No.

    The point that energy has a momentum then it must have mass ?
    Energy doesn't have momentum.

    For someone who chose the user name PhysicsEngineer you don't seem to know much about physics.
    I'm just keeping my fingers crossed that you're also not involved in engineering.
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  24. #23  
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    Quote Originally Posted by PhDemon View Post
    Quote Originally Posted by ostkef View Post
    Dayum, you're more cranky than genius duck.
    Nah, see post #24

    But that's for someone who's wrong, I was right. Lol.
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  25. #24  
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    Quote Originally Posted by Dywyddyr View Post
    Quote Originally Posted by PhysicsEngineer View Post
    I think , energy which has a momentum
    No.

    The point that energy has a momentum then it must have mass ?
    Energy doesn't have momentum.

    For someone who chose the user name PhysicsEngineer you don't seem to know much about physics.
    I'm just keeping my fingers crossed that you're also not involved in engineering.
    I think it's safe to say that anything that has momentum has energy.

    But for energy to have momentum, the phrasing is a little weird.


    Anyway, give those who aren't fluent in English a break.
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  26. #25  
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    Quote Originally Posted by ostkef View Post
    Anyway, give those who aren't fluent in English a break.
    Oh I do.
    I can usually tell when English isn't someone's first language (although, given some posters I've encountered, it can be hard to decide - either way).
    But I'm not going by the phraseology so much as the "overall gist" of what he's written.
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  27. #26  
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    A photon has a momentum p=h*k. Single photon always have a momentum. But ı wrote energy as a photon ofcourse energy is not vectorel quantity therefore momentum is vectorel. It does not matter my words are so clear. In addition when photon is not moving theoretically it has no mass which makes it has no memonetum but other way, it is moving with c it must have some momentum.
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  28. #27  
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    Quote Originally Posted by PhysicsEngineer View Post
    A photon has a momentum p=h*k. Single photon always have a momentum. But ı wrote energy as a photon ofcourse energy is not vectorel quantity therefore momentum is vectorel. It does not matter my words are so clear. In addition when photon is not moving theoretically it has no mass which makes it has no memonetum but other way, it is moving with c it must have some momentum.
    Get it right man. p= ħ*k

    And no, photons are moving at c, hence they must have zero rest mass to have avoid having infinite momentum.
    "For the rest of my life I will reflect on what light is." - Albert E.
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  29. #28  
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    Quote Originally Posted by ostkef View Post

    And no, photons are moving at c, hence they must have zero rest mass to have avoid having infinite momentum.
    First off, the above isn't even a coherent sentence. Second off, can you prove that non-zero rest mass would result into infinite momentum? Try a little math, let's see it.
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  30. #29  
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    Quote Originally Posted by xyzt View Post
    Quote Originally Posted by ostkef View Post

    And no, photons are moving at c, hence they must have zero rest mass to have avoid having infinite momentum.
    First off, the above isn't even a coherent sentence. Second off, can you prove that non-zero rest mass would result into infinite momentum? Try a little math, let's see it.
    At c, gamma is infinite, hence non-zero rest mass multiplied by gamma will give infinite momentum.

    Whereas zero times gamma can give an undefined number. I'm not sure on how to show it rigorously though.
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    Quote Originally Posted by ostkef View Post
    Quote Originally Posted by xyzt View Post
    Quote Originally Posted by ostkef View Post

    And no, photons are moving at c, hence they must have zero rest mass to have avoid having infinite momentum.
    First off, the above isn't even a coherent sentence. Second off, can you prove that non-zero rest mass would result into infinite momentum? Try a little math, let's see it.
    At c, gamma is infinite, hence non-zero rest mass multiplied by gamma will give infinite momentum.
    But the photon momentum has nothing to do with any .

    Whereas zero times gamma can give an undefined number. I'm not sure on how to show it rigorously though.
    This is where knowing math comes in handy.
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  32. #31  
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    Quote Originally Posted by xyzt View Post
    This is where knowing math comes in handy.
    Math is handy, no doubt.


    Quote Originally Posted by xyzt View Post
    But the photon momentum has nothing to do with any .
    Not even an infinite one?
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  33. #32  
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    Quote Originally Posted by ostkef View Post
    Quote Originally Posted by xyzt View Post
    This is where knowing math comes in handy.
    Math is handy, no doubt.


    Quote Originally Posted by xyzt View Post
    But the photon momentum has nothing to do with any .
    Not even an infinite one?
    Nope, not even.
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  34. #33  
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    Quote Originally Posted by xyzt View Post
    Quote Originally Posted by ostkef View Post
    Quote Originally Posted by xyzt View Post
    This is where knowing math comes in handy.
    Math is handy, no doubt.


    Quote Originally Posted by xyzt View Post
    But the photon momentum has nothing to do with any .
    Not even an infinite one?
    Nope, not even.
    Then how would you show it?
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  35. #34  
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    Quote Originally Posted by ostkef View Post
    Quote Originally Posted by xyzt View Post
    Quote Originally Posted by ostkef View Post
    Quote Originally Posted by xyzt View Post
    This is where knowing math comes in handy.
    Math is handy, no doubt.


    Quote Originally Posted by xyzt View Post
    But the photon momentum has nothing to do with any .
    Not even an infinite one?
    Nope, not even.
    Then how would you show it?
    Learn the math , the photon momentum is defined totally different from the momentum of massive particles, is valid only for massive particles. Instead of spraying the forum with posts, study.
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  36. #35  
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    'Twas just a couple extraneous posts.

    Don't need to get into a tizzy, just relax, be cool.
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    Quote Originally Posted by ostkef View Post
    'Twas just a couple extraneous posts.

    Don't need to get into a tizzy, just relax, be cool.
    It was quite a few posts and they are content-free. So, learn to talk less and to listen more.
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  38. #37  
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    Quote Originally Posted by xyzt View Post
    So, learn to talk less and to listen more.
    Yep, definitely. And you need to learn how to cool it.


    In all seriousness, don't work your heart to death.
    Last edited by ostkef; April 16th, 2013 at 11:12 PM.
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  39. #38  
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    Quote Originally Posted by xyzt View Post
    Learn the math , the photon momentum is defined totally different from the momentum of massive particles, is valid only for massive particles.
    To be fair, I think that is what ostkef was trying to say:
    Quote Originally Posted by ostkef View Post
    At c, gamma is infinite, hence non-zero rest mass multiplied by gamma will give infinite momentum.
    In other words, IF a photon were to have (*) non-zero rest mass and travel at c, then it would have infinite momentum. Therefore it (or any particle) either has zero mass or travels at less than c.

    As a hand-wavy argument it is not too bad.

    (*) Note the subjunctive to indicate a counter-factual.
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  40. #39  
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    E^2 = M^2*C^4 + P^2*C^2

    E=mc^2 is rest-mass. Photons are never at rest, hence a photon E = P*C
    In the information age ignorance is a choice.
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