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Stupid Question

  1. Aug 25, 2008 #1
    SR says that as an object reaches the speed of light its mass approaches infinity. How can light reach the speed of light without having an infinite mass?
  2. jcsd
  3. Aug 25, 2008 #2


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    Simple, it doesn't reach it :)
    Light doesn't accelerate from a velocity below the speed of light. From the moment a photon gets created until the moment it is destroyed, it travels at the speed of light (in the medium it is in, of course).

    Something which travels slower than c must have mass and cannot reach the speed of light. Something which travels at the speed of light must be massless and cannot reach speeds below c.
  4. Aug 25, 2008 #3

    I guess I don't understand how something can be created / destroyed without acceleration / deceleration. Are you saying that photons are moving at C the instant they are created, and instantly decelerate to C=0 when destroyed?
  5. Aug 25, 2008 #4

    Why are photons affected by gravity of black holes if they have no mass? F=GMm/r2... So, if photons are massless, then they shouldn't be affected but on the contrary, they are sucked in.
  6. Aug 25, 2008 #5
    Re: @CompuChip

    No mass... nothing to suck in.

    That's a good point.
  7. Aug 25, 2008 #6


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    Yes, they are moving at c from the moment they're created, and they don't decelerate when destroyed, they just get absorbed by some other particle.
  8. Aug 25, 2008 #7


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    Re: @CompuChip

    Even in Newtonian gravity the rate at which an object accelerates in a gravitational field is totally independent of its own mass (remember F = ma, so ma=GMm/r^2 which implies a=GM/r^2). In general relativity gravity is not really a force but curvature of spacetime, and photons always follow geodesics which are the closest things to "straight lines" in curved spacetime, but which may look like curved paths from a purely spatial point of view.
  9. Aug 25, 2008 #8
    Has this been measured, or is this an assumption based on SR?
  10. Aug 25, 2008 #9

    c- the speed of all EM radiations is the speed of all particles having zero rest mass and yes, photon travels with c from the moment it is created to the moment it is annihilated. There is no acceleration or deceleration. One of the many nonsensical [to me] things of relativity.
  11. Aug 25, 2008 #10

    No No No.... I didn't mean that. I said if photons have zero mass, then they cann't be affected by gravitation. Actually photons have zero REST mass. But in motion, they too have mass.
  12. Aug 25, 2008 #11
    Re: @Dave9600

    Sorry. = )

    How can photons have a rest mass if they are moving at the speed of light the instant they are created?
  13. Aug 25, 2008 #12
    "I said if photons have zero mass, then they cann't be affected by gravitation."

    This is not correct. Gravity is not viewed as a force, but as the curvature of space-time. Photons will follow geodesics (the shortest path on a curved space) in spacetime.
    Since gravity is the warping of spacetime, it will affect the paths of photons.

    Gravity affecting photons is not related to the photons' momentum.

    "How can photons have a rest mass if they are moving at the speed of light the instant they are created? "

    Photons have 0 rest mass. Rest mass is a misnomer, I think. I prefer the term invariant mass. If something moves at the speed of light, it has 0 invariant mass.
    Last edited: Aug 25, 2008
  14. Aug 25, 2008 #13
    The shortest path to what?
  15. Aug 25, 2008 #14


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    The shortest path between two nearby points- in the same sense that a straight line is, in Euclidean geometry, the shortest path between two points.
  16. Aug 25, 2008 #15
    But what is the other point?
  17. Aug 25, 2008 #16
    I know very little about relativity. Please suggest some book.
  18. Aug 25, 2008 #17
  19. Aug 25, 2008 #18
    For where the points are on the geodesic, think of it as trying to go the shortest distance on the curve as possible, your starting point is wherever you want on the photons path.

    When you warp the space a photon is traveling in, you create a new shortest path for it.
  20. Aug 25, 2008 #19
    But the other point... what is the photon travelling towards and why?
  21. Aug 25, 2008 #20


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    Isn't about a third of all threads in the relativity forum about the mass of the photon? Another third seems to be about the twin paradox. Isn't it time that someone makes a couple of sticky threads about these two questions?
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