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

Energy and Gravity

  1. Jul 18, 2008 #1
    Gravity is the mutual attraction between matter right?
    I heard photons are light particles, so would that mean theres an attraction between photons and photons or photons and regular particles. Since matter is a concentrated form of energy and light is energy wouldn't that mean there would be a mutual attraction between energy(in this case light). Would there also be a attraction between energy and matter.

    BTW I'm just some curious 14 year old so don't laugh at me :(
    Forgive me for making a lot of typos.
  2. jcsd
  3. Jul 19, 2008 #2


    User Avatar
    Gold Member

    Yes gravity is the attraction between matter. In Newton's equatyion for gravitational attraction you need mass to create a force. Photons how ever have no mass so no force goes between them. Now to avoid confusion I'll explain about E = mc^2, this confused me alot. E= mc^2 is only for rest mass, photons rest mass is 0, since they are always moving we can't use this equation.
  4. Jul 19, 2008 #3
    Here's some info I found... not sure if it helps your understanding.


    "For most average objects, momentum is truly mass x velocity. When motion
    gets close to the speed of light, we find that the momentum relation p=mv is
    only an approximation. It is only correct when speed (v) is much smaller
    than the speed of light (c). The relation that works for all speeds is E^2
    = p^2c^2 + m^2c^4. It is much less convenient to use, and doesn't help
    figure anything out until you reach speeds of perhaps thirty million meters
    per second. For a particle with no mass, the relation reduces to E=pc.
    This works for a photon. For very small speeds, the system reduces to
    E=mc^2 + (1/2)mv^2, and p=mv. This leads to relations with kinetic energy
    and momentum: much more convenient to work with and just as accurate until
    you reach speeds close to the speed of light.

    As for magnetic field, there is no reason why it should behave like gravity.
    For one thing, the strength of magnetic FORCE depends on the speed of the
    particle being pushed or pulled by the field. Also, unlike gravity,
    magnetic force pushes sideways, perpendicular to the field direction.
    Gravitational force is just gravitational field multiplied by the mass being
    pushed or pulled. Electric force is just electric field multiplied by the
    charge being pushed or pulled. Magnetic force depends on the charge, speed,
    AND direction of the charge being pushed or pulled, as well as the strength
    of the field. It is a very different kind of force.

    As for "carrying" the field, it is known that photons of light transmit both
    electric and magnetic force. In fact, light is waves made of oscillating
    electric and magnetic fields."

    Dr. Ken Mellendorf
    Illinois Central College


    Source: http://www.newton.dep.anl.gov/askasci/phy00/phy00332.htm
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook