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Thread: Does a substance emitting light lose mass?

  1. #1 Does a substance emitting light lose mass? 
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    Given that a photon has mass and is emitted from a source under excitation (heat, electricity, nuclear fission), does the substance emitting the light lose mass in the form of light packets?


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  3. #2  
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    Yes, in principle. But it's a teeny tiny amount, too small to measure. The reason is that the 'effective mass' of a photon is its energy divided by the velocity of light squared. The energy of a photon of yellowish light is about 2 eV (electron Volts). One ev corresponds to 1.6 10-19ergs. That number divided by c2 is 1.8 10-36 kilograms. Even though there are lots and lots of photons in a laser beam it still adds up to an unmeasurable amount. Even if you radiate a kilowatt for a day, you lose about 108 Joules of energy. That's equivalent to 10-9 kg of mass. Not a lot.


    Last edited by dmwyant; May 6th, 2012 at 01:01 PM.
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  4. #3  
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    A photon has no mass, but since it does have energy, technically an emitting object does lose mass. (E=mc^2)
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  5. #4  
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    Err, try the first post in this thread
    Last edited by Guitarist; May 6th, 2012 at 01:12 PM.
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  6. #5  
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    But..

    The mass lost, what does that equal of particles, superstrings?
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    Quote Originally Posted by HexHammer View Post
    But..

    The mass lost, what does that equal of particles, superstrings?
    Can you rephrase this question in a way that makes sense?
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  8. #7  
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    When you talk about electron flow, it is like adding electrons into a sea of electrons and they flow to complete a circuit. Could it be that light is the same; that light is created by adding energy (in whatever form) to the substance causes the light particle to leave but the net mass stays the same because it is replaced?
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  9. #8  
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    Quote Originally Posted by jetstove View Post
    When you talk about electron flow, it is like adding electrons into a sea of electrons and they flow to complete a circuit. Could it be that light is the same; that light is created by adding energy (in whatever form) to the substance causes the light particle to leave but the net mass stays the same because it is replaced?
    No. When the photon of light leaves the mass, the mass does not stay the same. It becomes lesser by the amount of m=E/C^2, where E is the energy of the photon, in accordance with Einstein's famous equation.
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  10. #9  
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    Quote Originally Posted by Harold14370 View Post
    No. When the photon of light leaves the mass, the mass does not stay the same. It becomes lesser by the amount of m=E/C^2, where E is the energy of the photon, in accordance with Einstein's famous equation.
    Although ... wouldn't this depend, to some extent, on why the mass is emitting photons? If it is an object that is cooling down or undergoing radioactive decay then the loss of energy will cause a corresponding (tiny) loss of mass. But if it is something like an electric light, then the energy will come from the electricity - and so no net mass loss. (But I'm not quite sure if that is what jetstove was getting at.)
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    Yes, I may have misinterpreted the question. Going back to the original question, in the examples given (heat, electricity, nuclear fission) if you are adding heat, by electricity or some external source, so as to keep the temperature constant, then the mass lost due to photon emission would be replaced by mass added due to the heat input. Nuclear fission, though, will result in a net loss of mass from the original bomb material or reactor core.
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  12. #11  
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    It's possible for the emitting object to lose momentum/kinetic energy instead of mass, as in the case of an electron emitting a photon when it gets accelerated, and then slowing down a little bit as a result.
    Some clocks are only right twice a day, but they are still right when they are right.
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  13. #12  
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    Quote Originally Posted by kojax View Post
    It's possible for the emitting object to lose momentum/kinetic energy instead of mass, as in the case of an electron emitting a photon when it gets accelerated, and then slowing down a little bit as a result.
    But remember that mass and energy are equivalent so if it loses energy it will lose mass...
    Without wishing to overstate my case, everything in the observable universe definitely has its origins in Northamptonshire -- Alan Moore
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    Certain posts have been moved to to a new thread named "Electrons and energy" in New Hypotheses and Ideas.
    "Men are apt to mistake the strength of their feelings for the strength of their argument.
    The heated mind resents the chill touch & relentless scrutiny of logic"-W.E. Gladstone


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  15. #14  
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    Quote Originally Posted by Harold14370 View Post
    Quote Originally Posted by HexHammer View Post
    But..

    The mass lost, what does that equal of particles, superstrings?
    Can you rephrase this question in a way that makes sense?
    I does make sense, just get off your high horse.

    Electron Capture (also called inverse beta decay) there you can theorize a new particle due to the changed mass i perspectivly Protons and Neutrons. The same goes for the substance emitting light, as it will lose weight/mass we can thereby theorize new particles. So the question is, has this been done or is it new grounds?
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  16. #15  
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    Quote Originally Posted by HexHammer View Post
    I does make sense, just get off your high horse.
    Hexhammer, I am sure what you wrote made sense to you. It made no sense to Harold and it made no sense to me. The primary responsibility for clear communication rests with the writer, not the reader. I think you may have mentioned in another thread that English is not your native language. If that is the case then you would benefit from paying attention when a native English speaker says he does not understand you. If I am mistaken and you are a native English speaker then you should certainly know better!

    Your next sentence, intended to clarify, left me even more confused. What does it mean. The grammar is totally screwed up. Absence of commas to indicate where clauses begin and end isn't helping.
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  17. #16  
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    Quote Originally Posted by HexHammer View Post
    The same goes for the substance emitting light, as it will lose weight/mass we can thereby theorize new particles. So the question is, has this been done or is it new grounds?
    I'm not sure why you think this requires any "new particles"; if something loses energy, it also loses mass (e=mc2).

    Apart from that, I was going to write almost exactly what John wrote. What you said wasn't clear (to others - which is the important bit).
    Without wishing to overstate my case, everything in the observable universe definitely has its origins in Northamptonshire -- Alan Moore
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