1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

How can a photon exists on its own without a mass?

  1. Sep 25, 2014 #1
    For example, thermal energy exists and has no mass, but is carried by particles which have mass. A photon is described as a particle - how can a photon exist on its own, travel in space and even push other particles with mass if it has no mass itself?

    I am not sure if that thread should be in quantum physics section or not so I post it here
  2. jcsd
  3. Sep 25, 2014 #2


    User Avatar
    Science Advisor
    Gold Member
    2017 Award

    radiated thermal energy is carried by infra-red photons ( more precisely IR Electromagnetic radiation) massless particles <-- and I use that term broadly
  4. Sep 26, 2014 #3


    User Avatar
    Gold Member

    mvbn: Welcome to PF!
    As davenn wrote, electomagnetic radiation is how I would consider thermal energy 'travels'. You may be speaking about kinetic energy when you mention particles that have mass, as in the collisions of molecules in a gas or their vibrations in a solid.
    Photons are best thought of as the minimum amount (quanta) of light that can be transferred. You will read that light/photons exhibit a "wave/particle duality", but the experts will tell you this is an analogy, and it's better to think of photons as neither (they are their own unique, quantum object.)
    Light travels as electromagnetic waves - an excitation of the EM field.

    Light has energy and momentum. The E=mc^2 equation from Einstein that most folks are familiar with explains the relationship between energy and mass. But there is more to the right hand side of the equation that's frequently omitted - a momentum component for massless particles. The equation reduces to the simpler E=mc^2 when the momentum term iz zero. So light does indeed exert a pressure, albeit ever so small, from the momentum it carries.
  5. Sep 26, 2014 #4
    I thought When One vibrating particle(atom) hits another vibrating particle and causes it to vibrate more that is transfer of thermal energy without photons. Am I wrong?.

    @TumblingDice If photons are not particles I can't undestrand why it is teached that they are particles. Even wikipedia says "A photon is an elementary particle, the quantum of light and all other forms of electromagnetic radiation,". It says clearly that it is "elementary particle" and not just have particle properties. It is confusing that I don't know if I should think about it as particle or not.
  6. Sep 26, 2014 #5


    User Avatar
    Science Advisor


    Light has particle properties. Photons are particles.
  7. Sep 26, 2014 #6
    You are not wrong. This is an example of conduction. There are additional ways to transfer heat. One, as A.T. pointed out, is radiation.
  8. Sep 26, 2014 #7


    User Avatar
    Gold Member

    ^ Excerpt from PF FAQ: "Is Light A Wave Or A Particle?"
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook