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Q about light.

  1. Jul 6, 2009 #1
    welcome all

    I'd like to know if there is a way to calculate how manny photons are in this universe. We can calculate the mass of the universe so is there a way to claculate the number of photons? I also would like to clear some things i dont understand. I read that if people build a ship that will be able to reach the speed of light then the moment it reaches the speed of light it will have infinite mass, energy, gravity, force and magnetism. Does a photon have all these?

    All answers are welcome.
    Thank you!
  2. jcsd
  3. Jul 6, 2009 #2


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    A photon has no rest mass so it ALWAYS travels at the speed of light (in vacuum), in materials, light travels slower than c.

    Yes, one can, we can evaulate the photon energy/number density fairly easy to a certain accuracy.
  4. Jul 7, 2009 #3
    A photon's energy is just proportional to its frequency f. Since it is massless, it has no kinetic energy, or gravity for that matter. It will always travel at the speed of light.
  5. Jul 7, 2009 #4


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    so why do we have a kinetic term in the QED Lagrangian for the electromagnetic field, and why is light affected by gravity? Gravity is proportional to the energy-momentum tensor.
  6. Jul 7, 2009 #5
    We don't know the mass of the universe with much accuracy, nor do we know how much matter, dark matter, or dark energy there is with much accuracy. But that isn't necessary because you can measure photon spectrum (its a black-body spectrum at about 2.7 degrees K) and flux directly and its fairly even from all directions, and we have a fair* estimate of the volume of the universe (although the concept of universe volume in this context is a little complicated because we need to compensate for the fact that we see the past at great distances because of the finite speed of light). So you could calculate how many photons there are from that direct measurement.

    *Its actually the surface area of a hypersphere (probably), and this assumes we have a fair estimate of its shape. Maybe we don't.

    A photon has no rest mass. That's why it can be "accelerated" to c without infinities rearing their ugly heads. -- at least that's the simplistic way to look at it. its debatable whether a photon's existence in transit has much meaning.
    Last edited: Jul 7, 2009
  7. Jul 7, 2009 #6


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    No one else quite addressed this misconception.

    No ship made of matter will ever be able to reach the speed of light. Matter cannot travel at c.

    As it approaches (though never reaches) c, its mass will approach infinity. (Its length will also approach zero.)
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