Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Mass Equivalence of Light

  1. Jul 18, 2013 #1
    I have done reading on the momentum versus mass of light. However there is one issue which I'm still wondering about:

    When we use a magnesium lamp which is enclosed by a glas-bulb and ignite it, it will emit light during the chemical reaction. We know that two atoms have a higher mass when they are separate than when they form a molecule. Now we can conclude that the magnesium lamp has a lower mass after than before by Δm. The only thing that left the lamp was light.(?) Is it right to say that the mass difference of the lamp equals the energy of the photons that were emitted by n*hf=Δm*c^2? Is it at that size?
    From what I read this is probably nonsense(?) So how is the mass difference accounted for? Thank you in advance.
  2. jcsd
  3. Jul 18, 2013 #2


    User Avatar
    Science Advisor

    We should never use mass in that context, but energy. The rest mass of the lamp becomes lower (due to equivalence of rest mass m and rest energy E0; this is the meaning of E0 = mc2)

    But photons do not "carry this mass" b/c their rest mass is zero. They carry energy hf.

    For the whole system "lamp + radiation" the total energy E is conseverd. One could use M = E/c2 to define the invariant mass M (or rest mass) of the system, but as you see this new mass M is not additive (it is not the sum of the rest mass of the lamp which shrinks due to emission and the rest mass of the photon which is zero)

    Instead one should use the general relation

    ##E^2 = (mc^2)^2 + (pc)^2##

    between rest mass m, 3-momentum p and total energy E.
    Last edited: Jul 18, 2013
  4. Jul 18, 2013 #3
    I'm merely familiar with this topic to the level of Sears and Zemanskys book Physics. So I expected an answer similar to yours. May I ask you again:

    Where did the mass difference of the lamp end up if not accounted for by the energy of the light emmitted? Gravitational field changes would be much too small to account for.(?)
    I'm aware about light not having any rest mass - following from relativity. That's why I used the term "equivalence". And that very carefully.

    Shooting two photons onto each other we create a particle pair with "mass" - which can anihilate again to light.
    I thought if light is the propagation of energy through space, it is the propagation of mass through space to be able to do so, or add mass onto things if they absorb light ec..

    I have been using this very basic idea of mass/energy equivalence for any sort of wave to transform an equation to do with mass into a corresponding one to see what happens to the wavelength of things in my study. And it worked. I thought it could have only worked if this energy/mass equivalence was basically right. So just very basically I was thinking that the amount of energy that a certain amount of photons possess is capable of creating matter with a mass of the equivalence of that energy or adding on to matter by the same size? This is not contradicted by the rest mass principle, is it?
  5. Jul 18, 2013 #4


    Staff: Mentor

    A single photon has no invariant mass, but a system of multiple photons can have mass. The mass of the system of the bulb and photons is constant.
  6. Jul 18, 2013 #5
    No matter how you call it, the article from which you cited ends with the following concluding remark (and surely that's what the OP meant):

    "If the theory corresponds to the facts, radiation conveys inertia between the emitting and absorbing bodies."
    - http://www.fourmilab.ch/etexts/einstein/E_mc2/www/

    By the same amount you mean? Indeed. That's the conclusion of that paper (it's worth reading the full conclusion). And there is no "rest mass principle" - only some confusion due to the coexistence of differing definitions.
    Last edited: Jul 18, 2013
  7. Jul 19, 2013 #6
    Thank you so much Harrylin - I'm working on something with my heart and my soul at the moment and your response and that paper was very important to me - what a wonderful surprise today - Thank you.
  8. Jul 19, 2013 #7
    So my stuff worked with reason after all - I just must say how happy I am and how great this Physics Forum is because with out it I probably would still be wondering about this issue tomorrow. Thank you.
  9. Jul 19, 2013 #8


    User Avatar
    Science Advisor

    I don't know why but this comment was just really warm feeling :)
  10. Jul 19, 2013 #9


    User Avatar
    Staff Emeritus
    Science Advisor

    The main source of confusion, I find, is that people who are interested in "mass" are usually interested because they expect it to cause gravity.

    Unfortunately, mass causes gravity only in Newtonian physics. In GR, mass, energy, momentum, and pressure all contribute to gravity (via the stress energy tensor).
  11. Jul 21, 2013 #10
    Thx Newton, that's just what I needed today after being repeatedly told by L.M.Krauss that my hypothesis is nonsense because I'm using SR and Newtonian Approximations...

    I have had the idea that the fact that the universe expands gives everything a higher mass by Δm=E/c^2, and that this Δ-mass is invisble to us because we are affected the same - yet gravitates space..... It's some pages of calculations and OH BIG SURPRISE - I couldn't believe it- my number is that the invisible proportion today is 83.44% (no puzzling - first ever hit - and been very critically testing it since 6 month). Do you know a similar number? Maybe from DM? My hypothesis explains why the cosmic microwave imprint... falling apple....

    I'm using the GR radius of the universe (4.4x10^26 m).... I double checked all my single steps with you guys and other Physicists. Yet, according to Krauss, I'm getting this COINCIDING number because I'm doing it all wrong? I'm using Newtonian approximation (universe as Sphere) to calculate the energy. So now I'm asking others how big the enrgy is in [kgm2/s2] according to GR...

    Maybe you can understand how I feel :/ because there is also another big coincidence, my hypothesis also gets the right number for DE (saying it is the PE of the universe and where it locates), I feel coincidenced out...
  12. Jul 21, 2013 #11


    Staff: Mentor

    We cannot discusss personal hypotheses here. It needs to be published in the mainstream peer reviewed scientific literature first.
  13. Jul 22, 2013 #12
    Sorry Dale. Would you know anywhere where this is possible?
  14. Jul 22, 2013 #13


    Staff: Mentor

    Yes, the peer reviewed scientific literature and scientific conferences. That is what they are for, and it is the only way to get a valuable new idea recognized by the scientific community.
  15. Jul 22, 2013 #14
    Is it possible for an independent out of academia person to publish anything in the peer reviewed scientific literature?
  16. Jul 22, 2013 #15
    Sure. Just send your article to a journal and it will get peer reviewed. If it passes peer review, then it will be published.
  17. Jul 23, 2013 #16


    Staff: Mentor

    Certainly. With most peer reviewed journals the process is double-blind. The reviewers don't know who the author is and vice versa. So the authors credentials are irrelevant, all that matters is the quality of the manuscript.

    Another way is to get an academic coauthor; again, their credentials don't matter but they can help improve the quality. Personally, I am in industry, not academia, and I am a coauthor on about a half-dozen manuscripts per year with my academic collaborators.
    Last edited: Jul 23, 2013
  18. Jul 24, 2013 #17
    Home with little children at the moment :/ - otherwise academia but biology - friend of mine lectures Physicist and I hope will help. Would it be better to publish when I'm back at work, even if it was as a Biology Academic staff or Postdoc ec.? I have published as a Biologist before. Or would I publish as a private person then anyway?
  19. Jul 25, 2013 #18


    Staff: Mentor

    You should publish as yourself. There is nothing wrong with being a biologist.

    I would recommend co-authoring with your physicist friend. He or she should be able to help you navigate any differences between the biology literature and the physics literature. Plus, if you cannot convince a single physicist friend then it is unlikely that you will convince the whole physics community at large. Plus, it always helps to have more people reading and reviewing the manuscript prior to submission.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook