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

Proof of de broglie equation?

  1. Mar 10, 2009 #1
    I saw a proof in a book for the de broglie equation which I thought was wrong.

    if l = lambda, so wavelength and then all other notation is standard they said:




    but for a photon, c=v


    but mv=P


    and v=fl


    and finally:


    Anyway, that's what they said. But I thought since a photon doesn't have a mass "mv" couldnt be its momentum. Or does it not matter?
  2. jcsd
  3. Mar 10, 2009 #2
    It's correct because it uses relativistic mass, but still it doesn't prove anything about non-photons. I think de Broglie had a slightly stronger proof: relativity implies that if a particular wave packet represents a particular particle (photon or otherwise) then it will always have the same relation between wavelength and momentum, but even that doesn't require the same value of h to be shared for all particle types. Perhaps there is no theoretical a priori proof?
    Last edited: Mar 10, 2009
  4. Mar 10, 2009 #3
    the mometum of a photon is P=E/c where E=hf the energy of a photon
    and i thought E=mc^2 cannot be used for a photon beacuse that equation is for
    particles at rest and light is never at rest.
  5. Mar 18, 2009 #4
    no, for a photon v=C, so E=mCC=hn, thus mass of a photon=hn/CC (m=hn/CC) for an electron at rest E=mCC but for an electron in motion E=mCC+ mvv/2 or E=m(CC+(vv/2)) ie. the mass energy plus the kinetic energy. now take special relativity and E=hn=hn+h(nsub1-nsub2); C is just the constant of proportionality for conservation of mass/energy, think of it this way: CC=E/m or C= plus or minus the square root of E/m
  6. Mar 18, 2009 #5
    E=mCC=hn or CC=E/m.
    C is the constant for conservation of mass/energy and follows special relativity.
    G is the constant for conservation of space/time and follows general relativity.
    If one takes the mass of a photon as m=hn/CC then we have the the observed accelleration of a photon in a gravitational field and Albert's eclipse test.
    Albert's photo-electric effect is just this: hn=mCC+mvv/2, if hn is greater than the fermi energy an electron is moved up.
    The converse is Sammy's electro-photic effect: mCC+mvv/2=E=hn=h(nsub1+nsub2); if an electron of sufficient total energy impinges on metal then a photon is emmited; also called a light bulb.
    A special case is when an anti-electron impinges on anti-metal an anti-photon is emmited; this would be an anti-flashlight or a black-hole.
    Doppler is just special relativity:E=hn=h(n1-n2)=m(CC-vv/2) where the shift is the frequency change based on the velocity of the observer relative to the light source.
    In Albert's eclipse test the curvature of light in a gravitational field is shown but they did not check for the frequency shift that would have been evident.
    Plank's constant is the constant of proportionality between mass/energy and space/time.
    Thus we have the relationship between gravity and electromagnetic events.
    Next time I will address the atomic scale and do away with the misconceptions of strong and weak forces.
  7. Mar 18, 2009 #6
    As I have learned from my own topic m=E/c² is valid, IF m is the relativistic mass (and not rest mass, since that is zero for a photon).


    So photons do not have restmass, but they do have relativistic mass. Then this mass can be plugged into p=mv=mc to calculate the momentum.
    Last edited by a moderator: Apr 24, 2017
  8. Mar 18, 2009 #7
    no, no, no! E=mCC is the energy of a particle and is equal to the mass times C squared. True this is the mass energy of a particle at rest, so for a particle in motion just add 1/2 mass times velocity squared: E total= mCC+mvv/2, but for a photon all the energy is mass in motion at the speed of light, ie. v is defined as C for a photon.
    The mass of a photon is found from E=mCC=hn, thus m=hn/CC ( I use CC for C^2, it is easier to read and to type on a computer, n is nu.)
  9. Mar 18, 2009 #8
    C is the constant of mass/energy; G is the constant of space/time; h is the conversion factor between the two. Plank wants quanta: how about charge? Is it 1/3 of an electron charge? Then we have quarks with 1 or 2 quanta (+ or -), electron with 3(-), proton with 3(+) and neutron with 0. So 0, 1, 2, or 3 quanta. Any thoughts on this?
  10. Mar 18, 2009 #9
    No. Total energy (for a free particle) is given by:

    E^2 = (mc^2)^2 + (pc)^2
    This is true for all particles (massless or not).
    The E = mc^2 is just a special case for a massive particle at rest.

    Your equation instead tries to mix relativistic (mc^2) and non-relativistic quantities ( 1/2 mv^2 ). Please do not mix these together like that.
  11. Mar 18, 2009 #10
    The mass/energy of say an electron at rest is E=mCC, now give it a shove and add the kinetic energy: E total= mCC+mvv/2 ; this is just special relativity in terms of mass; in terms of frequency it is just doppler effect. C and G are just constants, h is the convertion factor. This stuff is so simple for me, yet few can understand it!
  12. Mar 19, 2009 #11
    So you are saying [itex]E=mc^2 + \frac{1}{2}mv^2[/itex] and because for photons c = v: [itex]E=mc^2 + \frac{1}{2}mc^2=\frac{3}{2}mc^2[/itex]. And then you say the mass of a photon can be found from [itex]E=h\nu=mc^2[/itex] (instead of (3/2)mc²)
    That seems rather inconsistent to me, as Justin Levy pointed out as well. You aren't taking into account that [itex]E_k_i_n=\frac{1}{2}m_0v^2[/itex] deals with [itex]m_0[/itex] rather than [itex]m_r_e_l[/itex].

    [itex]mc^2 = m_0c^2 + \frac{1}{2}m_0v^2 [/itex] is only a good approximation at low speeds.
    Last edited: Mar 19, 2009
  13. Mar 19, 2009 #12
    no, I said that e=mCC is the mass/energy, e=mvv/2 is the kinetic energy; sum of energy is E=mCC+mvv/2; also e=hn is the mass/energy of a photon, E=hn+h(n1+n2) is the mass/energy for a moving light source relative to the observer, ie. special relativity, doppler. Think of it this way: if the light source is moving towards the observer it is as if the photons have added kinetic energy, but C is constant so the energy change is expressed as an increase in frequency; if the light source is moving away from the observer then it is as if there is a decrease in kinetic energy, but again C is constant so there is a decrease in frequency. This is just doppler ( red-shift, blue-shift); this is just linear/special relativity.
    For general relativity just use mass of a photon,m=hn/CC, in the gravity formula. The observed curvature of light path is an accelleration, but again C is constant along the path of light so the observer would see an increase in frequency of the photons. In the Eclipse test the curvature was observed, but I do not think they looked for the frequency shift nor could they have documented it with black and white film.
    In summary, E=mCC=hn is just the conservation of mass/energy: CC=E/m=hn/m.

    My first law of thermodynamics: There is one system, and this is it baby! a) there is conservation of mass/energy; b) there is conservation of space/time; c) there is conservation of angular momentum. Thus there is no need for the cat in a hat and no uncertainty.
    My second law: there are two frames of reference a) this one: home of electric monopoles and magnetic dipoles b) the one that photons have: the orthogonal universe is the home of the magnetic monopoles (magnetons), gravitons, and time particles (chronotons); there may be a third frame of reference but I have not done the math yet. I can get enough dimensions to satisfy the Oiler equation. Thus no parallel universes only an orthogonal one (maybe two orthogonal universes).

    You may wonder who I am; I proposed spherical carbon (bucky balls) 30 years ago and was laughed at; I did general relativity in rotational coordinates to simplefy the math but my professors could not get a handle on it. I am known as Sammy k-space because I can think in multi-dimensions. If you want to see what I see then try revolutions/sec instead of cycles/sec and photons become 3-D!!!
  14. Mar 19, 2009 #13
    Cragar, Albert's photo-electric effect is just the opposite of Sammy's electro-photic effect; ph>metal>e- vs e->metal>ph; for a black-hole then it is: e+>anti-metal>anti-ph, the photons are moving backwards in time and thus appear to fall back into the singularity(anti-light source). Quite simple!
  15. Mar 23, 2009 #14
    Here is proof of the de Broglie: Gh/CC is the constant of space/time, units are meters cubed per second, so we can warp space/time but with this constraint. This is the most significant equation since E=mCC and maybe ever. The space/time constant is proof of relativity, conservation of space/time, and m=hn/CC. Ask me how if you dare.
  16. Mar 23, 2009 #15
    Yes, space/time is invariant!
  17. Mar 23, 2009 #16
    no, v is the velocity of the observer relative to the source: I did not say 3/2, I said mCC+mvv/2=E relative for observer moving towards the source and mCC-mvv/2=E relative for observer moving away from the source. It is a vector sum.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook