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Constant Speed of Light

  1. Apr 1, 2013 #1
    An observer is moving at 10% the speed of light. A light ray whizzes past him. He measures its speed to be exactly 186000 miles/sec.

    What happens at the physical level that he does not measure the speed of light to be less?

    Has the time of the observer slowed down? Or what?
  2. jcsd
  3. Apr 1, 2013 #2
    The faster you go the more time slows down for you and it stops at the speed of c that's why you can't reach it because you have mass which would require and infinite amount of energy to achieve it and also you would become timeless.

    Now this is because of relativity puts that light is traveling at c no matter what the frame of reference is so you can go 80% of c or 99% of c light would always be light with the speed of c for you, unless you would become light itself which you cannot.
    Now It sounds not that cool , the part about the speed of light always being constant but well It's not me who decided the rules.
  4. Apr 1, 2013 #3


    Staff: Mentor

    Einstein postulated the speed of light is constant for all inertial frames of reference in Special Relativity and derived from that measurement of time and distance is different for each observer.

    The postulate conformed to the results of the michaelson-morley experiment where they could find no difference in light speed in any direction even though we are in apparent motion around the sun. They were looking for the luminiferous aether that light as a wave traveled in.
  5. Apr 1, 2013 #4
    ... relative to ME. He could be at rest as far as millions of people are concerned. He could be on the Earth, and Earth observers think it is I who am moving by at 10% c. Earthers say I am moving. I say they're moving. Motion is relative.
  6. Apr 1, 2013 #5


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    Gold Member

    Moving relative to what ? Your statement does not mean anything unless you specity this.

    There seems to be no 'mechanical' explanation for this.

    [edit]beaten to it by 1977ub by a minute.
  7. Apr 1, 2013 #6
    Mentz sez:

    yes, I would say, 'nobody knows'.

    On the other hand, as has been stated in slightly different ways above, right now YOU are moving at 99% the speed of light in many observers frames,,,,and so am I....yet you and I may both be 'stationary' at computers and see each other as 'stationary'...on the earth's surface....
    what's 'real'??........All those perspectives are.....that's RELATIVITY.

    That light is measured at the same speed by all inertial observers makes no 'common sense'... and neither does quantum uncertainty which rules the microscopic world.....

    as Richard Feynman says "We have to accept nature as she is, absurd."
  8. Apr 1, 2013 #7
    You can't say "time stops at the speed of c" when c is calculated using time. That is circular logic.
  9. Apr 1, 2013 #8
    Every time you shoot your boosters for a period and then turn them off, you experience being "at rest" all over again. How can you ever reach c when you're always at rest? Meanwhile, your time never slows down for you. Motion is relative. Whether you can ever be made to reach c relative to somebody *else* is *their* problem. They find it gets harder and harder to accelerate you and takes more and more energy, and that for them your clock keeps slowing down.
  10. Apr 2, 2013 #9
    What Mentz114 said notwithstanding, look what happens if you take the steps literally:

    c is a constant, 300,000 km/s

    so 10% of c is 30,000 km/s

    you can accelerate to 30,000 km/s

    Since c is always constant, relativity has to make a choice whether to let c "upgrade" its speed back to constant after you accelerate, or invoke changes (observed by others) in your time and length, in order to keep the constant c truly constant for all observers at all speeds...

    Its not quite "we had to destroy the village in order to save it", but it does underscore the depth of rethinking about light, space, and time.
  11. Apr 2, 2013 #10
    I'm pretty sure he means he's moving at .10c relative to some "stationary" observer who is shining the light in the travelers direction.

    And again it seems as though we are overcomplicating this matter once again. What is happening is that, according to the stationary guy shining the flashlight, the traveler is moving at .10c and the light beam is moving at c. Therefore, the stationary guy sees the light travel slower (.9c) relative to the traveler than he does relative to himself (c).

    The traveler, on the other hand, sees the light pass him at c because he is aging at a (slower) rate relative to the stationary guy, and that compensates for this lag the stationary guy witnesses. The rate of that aging calibrates to make it appear to the traveler that the light is passing him at c.
  12. Apr 2, 2013 #11
    The logic is not circular because the accurate way of stating it is that time "has no meaning" for a particle or a photon traveling at c. Anything traveling AT the speed of light simply does not experience the phenomenon of time at all. Nothing is actually starting or stopping here.
  13. Apr 2, 2013 #12
    How can you state it? You have put a wrist clock to a photon? :smile:
    "Does not experience time" is a meaningless statement for anything travelling at the speed of light.

  14. Apr 2, 2013 #13


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    Can you make a clock move at exactly 0.5c long enough to measure a non-zero proper time interval? Or will it actually be something like 0.4999...998c in reality? If that is close enough, than 0.999...998c should be close enough to tell what happens at exactly 1c, even if you will never be able to confirm it experimentally.
  15. Apr 2, 2013 #14
    Not for a hyperbolic function, there is no limit.
  16. Apr 2, 2013 #15


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    Science Advisor

    I was actually talking about the requirement for experimental feasibility suggested by lightarrow, not about functions and limits. But neither hyperbolic functions nor limits are needed here:

    [itex]\Delta \tau = \Delta t \sqrt{1-v^{2}}[/itex]

    For [itex]v=1[/itex] we get [itex]\Delta \tau = 0[/itex]
  17. Apr 2, 2013 #16
    I think my point went straight over your head.
  18. Apr 2, 2013 #17
    I sincerely haven't grasped the first time of your post. Anyway, if you say that you want to measure the proper time interval of a spaceship moving from Planet A and Planet B (fixed distance) then yes, that proper time decrease to zero as the spaceship's average speed approach light speed.
    But who said in this thread that we have two fixed points in space A and B? If the spaceship keeps moving, its proper time keeps flowing...
  19. Apr 3, 2013 #18
    Speed of light is not a constant.
    That's why my glasses works.
    Speed of light depend on the media it propagete through.
    Refractive index indicates how the light speed slowing down.
    There are a lot of publications show that air refractive index depends on temperature and on pressure.
    The main element in the universe is hydrogen.
    Why hydrogen refractive index should not depend on pressure (concentration) and on temperature?
    We all agree that hydrogen concentration is bigger closer to the star and lower in interstellar space.
    So the speed of light should be different for different areas of the universe.
  20. Apr 3, 2013 #19


    Staff: Mentor

    When physicists say the speed of light is constant they mean its speed in a vacuum.

  21. Apr 3, 2013 #20
    So I would presume that in increasing densities of Hydrogen, there is an increasing incidence of absorption / re-emission of the light, and thus a change in the *average* speed...?
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