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Black Hole Question, HELP!

  1. Jul 14, 2010 #1
    The speed of light happens at almost all regions around at every place in the universe. Even around black holes, infact, a black hole is a stuffed region of mass. (JUST QUOTEING THE OBVIOUS).

    I know that the when things approch the speed of light that time dialtation can occur. But what happens if the speed of light was slowing down to your speed? Would the time dialtion happen? The black hole makes light slow down, and the only way that you can slow or speed up time IS when you are almost at the speed of light, right? This is just my reasoning.
    Can anyone have any idea how to solve my quesions? THANKS!
  2. jcsd
  3. Jul 14, 2010 #2
    light does not slow down. if it did, the photon would disappear. you have to remember, photons are massless so if they were to slow down they would have no mass, hence, no energy.

    plus, time can be sped up the closer one gets to a massive object (like a black hole).
    hope that helped
  4. Jul 14, 2010 #3


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    Maybe he means situations when light goes slower than usual? Such as light moving through water (photons move at about 75% speed of light).
  5. Jul 14, 2010 #4


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    Please read an entry in the FAQ thread in the General Physics forum.

  6. Jul 14, 2010 #5
    Time dilation is caused when ever light gets red-shifted or blue shifted i learned that a couple of days ago from alxm, and photons always travel at c , when light slows down in water that is the group velocity of the photons and it hast to do with the photons interacting with the matter and i think there is a FAQ answer to this , When light gets near a black hole it will become gravitationally blue-shifted , we can alter lights frequency and momentum by the Doppler effect or with a gravitational field .
  7. Jul 14, 2010 #6
    Who says time has only to do with the speed of light?
  8. Jul 14, 2010 #7
    because the speed of light is always constant relative to all observers ,
    so the only thing that can change in our equations is length or time ,
    And you can show the speed of light is constant through a solution to max-wells equations
  9. Jul 15, 2010 #8
    I thought that time dilation occurs only proportional to the speed of light in a vacuum. A photon in the universe is traveling at c, but since the universe is not a total vacuum the photon's time does not come to a stop because it is traveling less than the speed of light in a vacuum. So the photon is allowed to spin because it has time. Time has not officially stooped. Wrong or Right?
  10. Jul 15, 2010 #9
    I think to the best of my knowledge photons do not experience time .
  11. Jul 15, 2010 #10
    If photons do not experience time then how are they allowed to spin? So you are saying they travel at approximately 300,000 km per second? They only travel at that velocity in a vacuum right?
  12. Jul 15, 2010 #11


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    Note that the density of particles in space is on the order of 1 per 16 cubic centimeters, so light can travel quite a distance without hitting anything.

    I don't know what you mean by "spin".
  13. Jul 15, 2010 #12
    Like a proton a photon has spin. If the photon did not experience time it could not have spin. Gravity is everywhere. The gravitational force and other factors slow the photon just shy of the velocity at which time stops.
  14. Jul 15, 2010 #13
    Gravity does not slow light down , it can alter its path or red-shift or blue-shift it , but not slow it down , im not sure how to answer your question about spin , So when the light is moving at c it would lose its spin angular momentum but then when it slows down it would regain it according to you . What about gravitons (if they exist ) do they move at c and can we alter their path .
  15. Jul 16, 2010 #14
    That's a good point. You are right on that. How can a photon have spin if time does not exist for the photon? The speed at which time stops is the speed of light in a vacuum. In the universe light is traveling through a vacuum. Therefore it is allowed to spin because time has not stopped because it is just shy of the speed in a vacuum. No?
  16. Jul 16, 2010 #15
    Are you sure? Light streches or compresses itself when moveing. Red shift and blue shif indecates motion and we get time dialtion when something is going faster until it reaches c. Which only happens at high speeds. Very high speeds, but your right, just maby that, we could find how much time will change it self by useing red shift/ blue shift to helf find how much somethings is moveing.
  17. Jul 16, 2010 #16


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    There are several severe misunderstanding here.

    1. Photons having spin, etc. do NOT require any such criteria as "having time", whatever that means. This is because ALL of the properties of light are measured and obtained NOT in a photon's frame! We measure its speed, momentum, spin, etc in OUR frame. And by what we know of SR, since c is the same in all inertial frame, this is also a valid set of parameters in other frames.

    2. We do not do a Lorentz transformation to a photon frame. We have no physics to describe what happens there. So asking if a photon would have a "spin" in that frame is meaningless. The fact that we DO have a spin conservation with the photon means that we can measure it in our frame!

    3. Please remember our https://www.physicsforums.com/showthread.php?t=414380" that you have agreed to. If you are unsure, do NOT make unsupported speculation. You may think that you are basing it on something you thought you understand, but there are several errors already here in this thread being offered as "answers".

    Last edited by a moderator: Apr 25, 2017
  18. Jul 16, 2010 #17
    More than a several "misunderstandings" are posted above. Too many to address. [ But that's how we all learn...with many errors along the way....especially physicsts!!! never politicians.]

    It sounds like you think, for example, if you were to accelerate from your normal speed here on earth, for example, to say .99c and remain at that velocity you'd experience significant time dilation. Not so, your local time as you experience it would be UNCHANGED; you would be unaware of any change in the passage of time.

    But from other references, that is other observers, say someone who remained on earth, your time WOULD appear to proceed substantially more slowly. However, you should note that THEIR time would also appear to have slowed substantially from your vantage point...in fact their would time would appear to have slowed just as much as yours. So each sees the other's time as having slowed, not their own!!!! It's RELATIVE.

    There is no absolute measure of the passage of time just as there is no absolute measure of your velocity; it's all relative, depending on the observer's frame of reference. Two different observers will in general disagree.

    So rule #1 is: The speed of light (in a vacuum) is the only constant.

    While a correct statement, it has nothing to do with the passage of time of a photon. The apparent velocity of light might be affected as photons bounce off particles, are absorbed and reemitted, but each individual photon still moves at an observed speed of "c" between those collisions.

    True if in free space, not true in a prisim, for example, sitting in an experimental lab. Have you ever seen a rainbow? Did the passage of your time vary.....no.

    See here for one explanation:
  19. Jul 17, 2010 #18
    That was implied , i figured everyone could gather that . But thanks for pointing it out ,
    Just curious what do you mean by free space
  20. Jul 21, 2010 #19
    free space is vacuum space, I guess 'free' of everything..empty....
  21. Jul 21, 2010 #20
    the photon does not slow, rather its frequency and wave length change.
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