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

Speed C is Dependant Upon Source?

  1. Jul 24, 2004 #1
    1. The speed of light is constant.

    2. Light may reflect off objects.

    3. The speed of light is not determined by it's source.

    If the speed of light is constant and this speed is unaffected by it's source, then the expression, the shape light is, is it's natural shape and it's speed simultaneously. Light's inertia is it's shape, it's physical expression and vice versa. Light speeds itself, objects don't speed the light.

    But, light can be reflected. For something to change directions, acceleration must occur. For acceleration to occur, a change in velocity must occur. For velocity to occur, a change of speed and/or direction must occur. Can it be proven that only direction occurs when light is reflected? What rules out a change of speed assuredly during reflection? Is a mirror that reflects light a new source compared to the source where light last emerged?

    If the source does not determine the speed of light, then why does the speed of light always acclerate away from a source at the speed of light consistently? How does the light know that the source is there to speed away from at the constant speed?

    If two mirrors are traveling at different speeds compared to each other and light is reflected from one to another, shouldn't now light change speed dependent upon source or what happens?

    Since light is constant, it must have an exact quantity between the mirrors at any given state, and this quantity of light definitely changes with the distance the mirrors are from each other. When the mirror's distance is compressed, where does this extra light go? Does it begin to reflect faster?

    -Still trying to understand relativity.
  2. jcsd
  3. Jul 24, 2004 #2

    Tom Mattson

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    So far, so good.

    To be blunt, this makes no sense. What is the shape of light, and why do you think that this shape is identical to the inertia of light?

    All you have to do is measure the speed of the incident beam and of the reflected beam. In every case, you'll find that they are the same. Actually, this has nothing whatsoever to do with relativity. If you bounce a tennis ball off a rigid wall, and the collision is elastic, then the speed of the tennis ball will be the same before and after the collision.

    Yes it is, because the photons are absorbed and re-emitted by the mirror.

    You've just answered your own question: The very fact that the light always moves (not accelerates) away from the source at the speed of light consistently is precisely why we say that the source does not determine the speed of light. That's what it means for the speed of light to be independent of the speed of the source.

    It doesn't know anything.

    No, it shouldn't. You said it yourself, the speed of light is independent of the speed of the source.

    What "exact quantity" are you talking about?
  4. Jul 24, 2004 #3


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    When a photon is reflected from a surface, it is not actually stopped and turned around. Instead, it is absorbed, and a new photon is emitted in the opposite direction.

    - Warren
  5. Jul 24, 2004 #4


    User Avatar
    Science Advisor
    Homework Helper

    Ah, you beat me to it. :cry:

    Sounds like someone needs to brush up on their relativity.
  6. Jul 24, 2004 #5


    User Avatar

    Staff: Mentor

    To put a finer point on what warren said:
    Light doesn't accelerate in the way you think it does. It doesn't change speed. It doesn't slow down to a stop, then reverse its course. It is absorbed and re-emitted, and never travels at any speed other than C.
  7. Jul 24, 2004 #6


    User Avatar
    Science Advisor
    Gold Member
    2015 Award

    No acceleration. Photon's are forbidden to travel at any speed other than c.
  8. Jul 24, 2004 #7
    Note: A photon can accelerate in a gravitational field.

  9. Jul 24, 2004 #8
    Note: That is why light changes frequency. Doppler affect.
  10. Jul 24, 2004 #9


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    The direction light is traveling can and will change in a gravitational field, thus an acceleration, but the magnitude of the velocity will remain a constant c.
  11. Jul 25, 2004 #10
    light momentum

    Perhaps someone can answer this related question. In the original 'train with mirror' relativity demonstration the traveller supposes his light to travel perpendicular to the train whilst the observer sees it travel a triangular path,from which the time relations are derived.
    The hidden assumtion is that the DIRECTION of light has changed with the source ( not it's speed).
    There then arises a question -- if light is absorbed by a mirror and then re-emitted it is assumed that it's momentum did not change ( hence the angle of reflection and reception are equal )
    But in the above scenario the momentum did change -- so the question is does a moving mirror have equal angles or not ?
  12. Jul 25, 2004 #11

    Is the above true?

    When light strikes a mirror, almost all of it is reflected, a small amount is absorbed but it would only heat the mirror and not emit photons.

  13. Jul 25, 2004 #12
    What does your quote have to do with the reflection of light on mirrors? All your quote is doing is taking chroot's explanation a little further by saying a very small amount is not reemitted and heats up the mirror.

    chroot, how exactly is a photon absorbed and a new photon emitted in the opposite direction?
  14. Jul 25, 2004 #13


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    That's not entirely accurate; all of it gets absorbed, and then almost all the energy is reemitted.

    You have to be careful to distinguish between the macroscopic and microscopic terms.
  15. Jul 25, 2004 #14

    Tom Mattson

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Right, some momentum is imparted to the mirror itself.

    Yes, the angles are equal.
  16. Jul 25, 2004 #15
    When the photon is absorbed it exerts a force equivalant to all of it's energy. Then according to Newtons laws for every action (force on the mirror) there is an equal and opposite reaction, this being the emission of another photon in the opposite direction. Then when the mirror uses its own momentum to create a new photon in the opposite direction. I don't want to confuse the mircoscopic with the macroscopic but this is the same principle that causes a ball will bounce off a wall instead of just stopping once it hits the wall.
    Last edited: Jul 25, 2004
  17. Jul 25, 2004 #16
    So, photons are created by momentum? How is that possible?
  18. Jul 25, 2004 #17
    Well not exactly. You see since it absorbed the first photon it needs to exert an equal and opposite force back on to the photon. But since the photon is no longer there (it was absorbed) it simply emits a photon in the opposite direction. But photons do have momentum if thats what you mean.
  19. Jul 25, 2004 #18
    No, what I meant is what you previously said.

    How is a photon created in that process? If the photon is not created, where does it come from?
  20. Jul 25, 2004 #19
    stupid makes a good point. u say that the photon gets totally absorbed then a new photon is made from the mirror? that doesn't make sense... or are u saying that the mirror absorbs the photon then emits it?
  21. Jul 25, 2004 #20
    and if the mirror absorbs the photon and emits it then doesn't that mean the photon had to be slowed down, then accelerated in the opposite direction?
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?