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How can a photon move something( light moves), if it doesn't have a ma

  1. Apr 2, 2013 #1
    ....a mass? not matter how fast it hits an object, it doesn't have mass. F=ma.
    (explain without any equations or without too many equations.)
    Last edited: Apr 2, 2013
  2. jcsd
  3. Apr 2, 2013 #2
    It's because a photon has momentum. F=ma, but F=dp/dt, where p is momentum. Also, E=mc^2 +pc. Since photons have no "rest" mass, m, the equation for a photon is simply E=pc. So that's why photons go "bump in the night," as you mentioned (hehehe).

    It's kind a sleight of hand, this whole rest mass, no mass, momentum deal with the photons, I admit, but "us physicists" are working on a more "user friendly" explanation as we speak. Stay tuned. In the meantime, just focus on E=pc. That's not too "equationy," is it?
  4. Apr 2, 2013 #3


    Staff: Mentor

    It has no mass but it does have energy. Energy can be converted from one form to another - the energy of motion being one form.

  5. Apr 2, 2013 #4
    The basic physical properties of photons are described here:


    One simple way you can tell light has energy: you can feel the warmth from the sun, or the infrared warmth from a fire. That's heat energy. It turns out the higher the frequency the more
    energetic is light, so different color light carries different energies.

    Also note that light is electromagnetic energy...the basis for energy transfer in electrical devices like motors and generators and transformers. Coils concentrate electromagnetic energy so we can get work from it.

    massless 'particles' like photons are kind of strange, meaning not entirely intuitive. If you read the early part of the above link you'll find it took an 'Einstein' to figure out that light exhibits both wave and particle characteristics....THAT is common to all particles. For more on that check
    out deBroglie wavelength.
    Last edited: Apr 2, 2013
  6. Apr 2, 2013 #5
    Hard to believe the idea that a massless thing can interact with the other physical things.
  7. Apr 2, 2013 #6
    aha it means that, there are particles and things that photos are made of but it ONLY DOES NOT interact with the higgs field, right?

    the reason for that might be?
  8. Apr 2, 2013 #7
    The photon has mass. The photon doesn't have rest mass.

    The relativistic mass determines the momentum of the photon. The momentum of a photon as measured in any inertial frame is the product of the speed of light times the relativistic mass of the photon as measured in that inertial frame.

    The following analysis refers to special relativity (SR). An analysis in general relativity (GR) is a little more complicated.

    The photon has a nonzero relativistic mass. As measured in an inertial frame, the relativistic mass of a photon is more than zero. The value of the relativistic mass varies with the inertial frame. However, the photon always has a positive relativistic mass.

    The acceleration of a photon is zero in any inertial frame because the photon has to always move at the same velocity. Therefore, "a=0" for a photon. The expression, "F=ma", applies to a photon in a trivial way. Because F=ma, F=0 for a photon. According to SR, one can't apply a force to a photon.

    A photon that hits an electric charge can impart momentum by disappearing. This would be like a exploding bullet that enters a target and explodes. The photon doesn't change speed. The photon vanishes. Reflection and scattering can be explained by the object emitting new photons. One can think of an exploding bullet hit a person, after which the person takes out his gun and fires new bullets.

    Note that Feynmann diagrams never show a photon changing speed. Feynmann diagrams show photons being emitted by charges or absorbed by charges.

    So applying the Newtonian formula for force, "F=ma", is misleading. Photons can never accelerate, so "a=0". Furthermore, "F=0". Always.

    The rest mass of a particle has only an indirect relationship with the momentum. The rest mass is the hypothetical value the mass of the particle would have in the inertial frame where that particle is stationary. A direct measurement of the rest mass could only be made in the inertial frame where the photon is stationary. Any other determination of rest mass has to be made indirectly.

    The photon can not be at rest. Therefore, it can not have a nonzero rest mass. There is no inertial frame where the photon is stationary. According to special relativity, the speed of light is exactly the same in every inertial frame. There is no inertial frame where the photon is stationary. Therefore, there is no way to directly measure the rest mass of a photon.

    The rest mass can be indirectly determined by measuring the momentum of the particle in several different inertial frames. One can not determine the rest mass of a photon in any one inertial frame. However, the rest mass has no bearing on the momentum of a particle measured in just one inertial frame.
    Last edited: Apr 2, 2013
  9. Apr 2, 2013 #8


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    Darwin123: your explanation adds a lot of confusing contradiction here. "Relativistic mass" in SR is simply defined as

    m = gamma m_0

    But yet, for a photon, the rest mass m_0 is zero. So already this "relativistic mass" being non-zero is inconsistent.

    A photon has momentum. That should be sufficient to show why it can interact. The next logical step is to show why something with zero mass can have a momentum. This is gives us the opportunity to introduce a more general idea of what a "momentum" is.

    Please note that there are plenty of supporting arguments on why the concept of "relativistic mass" should not be used, both in the teaching of SR, and also when we deal with general question such as this. I had already outlined this (with references) in another thread.


  10. Apr 2, 2013 #9

    With QM theres no "F=ma". waves interact with others and can change momentum p and energy E (that are related with wavelenght or frequency). photon has momentum and energy it has (wavelenght or frequency) as other particles.

    mass is just a parameter that defines a dispersion relation. for photons is 0.

    If you think with electromagnetic waves you have a electric and magnetic field that "kicks" a charged particle with mass m.
    Last edited: Apr 2, 2013
  11. Apr 2, 2013 #10
    My main defense is that Einstein used the concept of relativistic mass. However, I acknowledge that the idea has a few problems with it. I find the relativistic mass concept useful as a heuristic model.

    Physicists often get around the concept of relativistic mass. I know that this concept runs into problems in the limit of the speed of light. However, I think the problem is more mathematical than physical. The ambiguity has to do with the mathematical definition of limit rather than the physical definition of mass.

    Note that relativistic mass works fine with particles that are moving just below the speed of light. I have seen explanations of cyclotrons that use the concept of "relativistic mass". Engineers working with particle accelerators seem to use the idea of relativistic mass with respect to charge particles. So dismissing the idea as nonsense seems a little premature. Maybe the concept should be introduced with appropriate caveats.

    Another reason that I like the relativistic mass idea is that it leads naturally (for me) to the idea that photons never change. Photons can only be created or destroyed near an electric charge or electric current. So one can "semi-intuitively" get to the idea of Feynmann diagrams. I am not pushing the idea of solving all relativistic problems with "relativistic mass". However, I was trying to answer the OP in a very rough but useful way.

    When one tries to analyze the photon in terms of mass, one look at fractions where the limits as velocity goes to the speed of light of nominator and denominator are both zero. Of course, zero divided by zero is undeterminable. However, there are "intuitive" ways to handle such a limit. If one thinks of a photon with a rest mass about 10^-40 times the rest mass of an electron, then one can sort of see why it acts the way it does.

    I will stop answering this way if one of the moderators tells me that "relativistic mass" is against forum rules. However, I see no rule saying "never use relativistic mass". If engineers use it, then the idea should be considered main stream. Apparently, I have difficulty knowing distinguishing "main stream" from "nonsense".

    I was told that I should always present at least some links if someone disagrees with me. Here are a few.

    Read the following link on mass in special relativity. Note that “some authors” present relativistic mass as a fundamental concept. "It has been argued" otherwise. I am arguing that relativistic mass still has uses as a phenomenological concept. In any case, I don’t know how SR was presented to the OP. Maybe he already has been exposed to “relativistic mass”.

    The term relativistic mass is also sometimes used. This is the sum total quantity of energy in a body or system (divided by c2). As seen from the center of momentum frame, the relativistic mass is also the invariant mass, as discussed above (just as the relativistic energy of a single particle is the same as its rest energy, when seen from its rest frame). For other frames, the relativistic mass (of a body or system of bodies) includes a contribution from the "net" kinetic energy of the body (the kinetic energy of the center of mass of the body), and is larger the faster the body moves. Thus, unlike the invariant mass, the relativistic mass depends on the observer's frame of reference. However, for given single frames of reference and for isolated systems, the relativistic mass is also a conserved quantity.
    Although some authors present relativistic mass as a fundamental concept of the theory, it has been argued that this is wrong as the fundamentals of the theory relate to space-time. There is disagreement over whether the concept is pedagogically useful.[1][2][3] The notion of mass as a property of an object from Newtonian mechanics does not bear a precise relationship to the concept in relativity.[4]

    I am a big H.A. Lorentz fan. I really like the way Lorentz approached electromagnetic theory. I realize that the approach has limitations. However, it is useful.

    Read the following link on Lorentz.

    The idea of relativistic mass actually dates back to Lorentz's work. His 1904 paper Electromagnetic Phenomena in a System Moving With Any Velocity Less Than That of Light introduced the "longitudinal" and "transverse" electromagnetic masses of the electron. With these he could write the equations of motion for an electron in an electromagnetic field in the newtonian form, provided the electron's mass increased with its speed. Between 1905 and 1909, the relativistic theory of force, momentum, and energy was developed by Planck, Lewis, and Tolman. A single mass dependence could be used for any acceleration—thus enabling mass to be now defined independently of direction—if F = d(mv)/dt (where m is relativistic mass) were to replace F = ma. It seems to have been Lewis who introduced the appropriate speed dependence of mass in 1908, but the term "relativistic mass" appeared later. (Gilbert Lewis was a chemist whose other claim to fame in physics was naming the photon in 1926.) Relativistic mass came into common usage in the relativity text books of the early 1920s written by Pauli, Eddington, and Born.

    What does the OP think about "relativistic mass"? Is it confusing or clarifying?
    Last edited: Apr 2, 2013
  12. Apr 2, 2013 #11


    Staff: Mentor

    Photons, as far as we can tell today, are indivisible quantum excitations of the EM field.

    The exact mechanism of how they can transfer their energy to other particles in the form of motional energy is part of QED - but regardless of exactly how its done the fact photons have no rest mass and can cause objects to move in no way violates any law - in fact it confirms energy conservation.

    Last edited: Apr 2, 2013
  13. Apr 2, 2013 #12
    In theory photons can also acquire mass through the Higgs mechanism working through gauge bosons in case of spontaneous symmetry breaking.
  14. Apr 3, 2013 #13


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    Darwin123: I definitely suggest you read the Lev Okun paper, because he has addressed almost every point that you make, even the historical perspective on why there are still "legacy" usage of the relativistic mass concept. Also, read Heicht's paper on why Einstein stopped using that concept after he realized how misleading the term is.

    I work with particle accelerators. I have to deal with relativistic "mass" concept often. But it really, truly, isn't fundamental, because when we have to measure, say, the energy of the relativistic particle via bending magnets, the concept that actually has any meaning is the relativistic momentum! It is from there that we can derive a whole slew of parameters, including, if one wishes, the "relativistic mass".

    We need to keep in mind that, in general, quoting a relativistic mass requires that one also cites "... at what speed?" Yet, in high energy physics, where many of these fundamental particles are created and are often relativistic, you never see a mass being mentioned accompanied by any speed. Therefore, simply from common practice alone, one can see that there is some well-defined value that does not need to be accompanied by another parameter. We create less confusion if we confine what we mean by "mass" to be that well-defined concept, rather than introduce something else that, as you can see, already have disagreements. No one will argue with you if you stick with the covariant mass.

    BTW, citing Wikipedia to me is not the wisest thing in the world to do. :)

    https://www.physicsforums.com/blog.php?b=4257 [Broken]

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