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

What is mass?

  1. Jun 28, 2010 #1
    Can someone tell me what mass is other than particles with mass react to a gravitational field?
    Electrons and photons are quite similar, both are thought at the moment to be elementary particles, both exhibit wave/particle duality. Yet the electron has mass whereas the photon has none. It can't be that particles with mass are made up of "stuff" as photons seem just as substantial as electrons. What does everyone else think?
  2. jcsd
  3. Jun 28, 2010 #2
    Mass is defined as, "A resistance to a change in motion." Don't confuse mass with weight, which is the consequence of matter generating a force (called, "gravitational mass.")

    Gravity isn't a field in the conventional sense of the term. In other words, if you were in an elevator or other similar enclosure in free-fall under gravity (you're in an enclosure that had just been pushed out of an airplane, for example), you would experience no gravitational force whatsoever. This is true for any sources of gravity, black holes included. If you were in the aforementioned enclosure in free-fall above a black hole, you'd feel no gravitational force acting upon you whatsoever. In fact, as you and the enclosure you're in barrels toward the black hole, you would not in any way, shape or form know you're in a gravitational field--the force of gravity would not exist for you.

    Photons "react to the gravitational field" just as other forms of substance do. In fact--relative to a distant observer--gravity can bend a laser beam. Visibly, if the gravitational source is massive enough...
  4. Jun 28, 2010 #3
    I believe that mass is a measurement of inertia, which is an object's tendency to, as the above person wrote "resist changes in motion."
  5. Jun 29, 2010 #4
    Hi, yes I know how mass 'reacts' to motion and to gravity fields. I wanted a more succint answer of what it actually is, perhaps there isn't one. I've studied how the W and Z bosons receive mass via the Higg's Mechanism but that doesn't help explain what it is. The closest answer I can come up with is that mass is just another quantum number like spin, or EM charge and allows a particle to experience gravitational fields.
  6. Jun 29, 2010 #5
    I think you have a reasonable question, that I would ask as "what enables the freedom of some particles to have velocities to be other than c?
  7. Jun 29, 2010 #6


    User Avatar
    Gold Member

    Last edited by a moderator: Apr 25, 2017
  8. Jul 28, 2010 #7
    I wonder if the point of the original interrogative has been missed. I am new to this forum and a born again physics students. I lack knowledge and experience in physics; therefore my questions will be embarrassingly simple.

    I am having difficulty conceptualizing zero mass and I sense it may be due to a misunderstanding or incomplete definition of mass.

    For mass to exist it seems to me it needs to be measurable and in fact mass is measured in kilograms. How then can a photon and other elementary particles have zero mass? I see photons referred to as particles and point particles, which is counter intuitive in my mind, as how can a particle have zero mass.

    I have read somewhere in the forums where the zero mass of a photon represents a point of energy in a force field. This I can conceptualize yet implies a misunderstanding of the definition of mass on my part, as it is used in zero mass particles.

    I would appreciate any input or thread references that will help me understand this phenomenon.
  9. Jul 29, 2010 #8
    One book I was reading said that mass was latent energy.
  10. Jul 30, 2010 #9


    User Avatar
    Science Advisor

    Mass is defined as the energy a particle has in the inertial frame where it is at rest. Photons clearly have energy, but they aren't at rest in any inertial frame whence one speaks of massless particles. Maybe it would be more precise to say that mass isn't defined at all even as a concept in the case of photons. But particle physicists argue the following way: the energy spectrum of an electron begins at a minimum value, 511 keV, which is different from zero, hence it has mass. The spectrum of photons starts at 0 whence they are massless.
  11. Aug 2, 2010 #10
    I liked DRDU's explanation on this topic. My personal understanding is that energy is fundamental quantity and mass etc. are macro level manifestation of energy. For instance, take a solid object of mass 1Kg. Actually, the object comprises of trillions and trillions of molecules/atoms and these tiny objects are oscillating and vibrating very fast. This oscillation has tremendous energy (in a very tiny scale) which gives the object its solidity and so called mass. (This is similar to a very fast rotating fan blades which will have the appearance of a solid disk.) If you heat up the object, the vibrations will be faster and there will be more energy. Hence, in order to reduce the vibrations, we need to cool the object down. Ideally, if we cool the object down to absolute zero, the vibrations of molecules/atoms will completely stop and I think the object will not have any 'mass' left.

    As regards to rest mass, we can not bring any particle to complete rest and measure its mass like a billiard ball or any other large-scale object.

    I believe that mass of photons can be perceived as expressed in the following equations. Suppose the photon (of a certain frequency v of light) has energy E and mass m.

    Then E = hv (Planck's equation) and E = mc2 (Einstein's equation)

    Hence, we can say that the mass m is = hv/c2
  12. Aug 2, 2010 #11
    Not only particles with mass react to a gravitational field? Everything couples to the gravitational field, since everything in the universe has energy!

    For stuff in the universe that does not move at the speed of light, the mass of these massive particles determines their resistance to move in a rest frame.

    But why mass? I speculate, that quantum field theory needs mass to make sense!
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook