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Are there two types of mass?

  1. Jun 12, 2012 #1

    in my studies (of popular physics texts written for the lay-man, and wikipedia,) I have a question:

    it seems that mass for many elementary particles is the immediate consequence of interaction with the higgs field, via the higgs mechanism

    but there are other particles that can be considered as having mass, such as photons, per einsteins Mass-Energy Equivalence Principle. That is, because photons can be considered as having energy, then they can simultaneously be considered as having mass, as well.

    and so, I ask, does this mean that there are two types of mass?

    I dont know what to think :O( can anyone clear this up for me?
  2. jcsd
  3. Jun 13, 2012 #2
    The mass m and the energy E are two parameters related by

    In any given Galilean frame (for which the particle may be moving),

    E = γ mc2
    where γ = 1/ √(1-v2/c2))

    m = (what would E appear to be relatively to the particle itself) / c2.

    The law of conservation of energy is that in any fixed referential, the sum of all energies is constant.
    Thus a collision of two photons, that have nonzero energy but zero mass, will produce something with nonzero mass (from to the sum of energies of both photons that are equal to each other in the involved frame) which can then annihilate again giving back 2 photons as started.
    Thus mass is not conserved.
  4. Jun 13, 2012 #3
    I see, so are you saying that the photons do not have mass, but rather that their energy values can create mass, (ie as in collisions.)

    hmm, ok... So, if photons do not have mass, then why do they have gravitational fields?? I thought that only things with mass have gravity... but I know that light has gravity, too...

    so why does light have gravity if it does not have mass??
  5. Jun 13, 2012 #4
    That what Newton said. Mass and gravity.
    Then came Einstein said that it is gravity and space.
    The light just follow the line, which by gravity effect make the line crooked or bend.
  6. Jun 13, 2012 #5


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    Gold Member

    This does not address your question regarding "relativistic mass" of photons, but there are two kinds of mass: inertial mass, which is the mass required to produce a certain Newtonian acceleration given a certain force, and gravitational mass, which is the mass required to produce a certain gravitational acceleration with a given gravitational mass. As far as we know, inertial mass = gravitational mass. This is called the equivalence principle.
  7. Jun 13, 2012 #6


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    General relativity uses the (stress-)energy-momentum tensor as source of gravity. Photons have energy. Nothing wrong there.
    You can relate the energy to something which can be called gravitational mass. But it is fine to work with the energy only.
  8. Jun 13, 2012 #7
    I think you (the OP) are referring to the rest mass of a particle and the relativistic mass of a particle in relative motion. In modern physics mass is usually the former i.e. it is the amount of stuff present in the rest frame of the particle. In this context things like photons have no mass. Relativistic mass is used in older texts but has fallen out of favour because it can be confusing, relativistic mass is kinetic energy. It is still used in particle physics where the mass-energy is given in electron volts.

    Photons although they have no mass gravitate as all energy does because it is the stress-energy tensor that creates the curvature of spacetime not the mass.
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