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Why is the speed of light the same for everyone regardless of observers velocity?

  1. Mar 5, 2012 #1
    I was just reading on inflation and the book was saying that since the universe is expanding, the light we see from stars are now ALOT farther away from us now. This seems to my noob mind to be contradictory to relativity.

    If you on a train going 500mph at a beam of light the beam of light is only coming at you at the speed of light. Not the sum of the speeds. First question. Why is this the case? Seems impossible. lol
    Second question. Does the universe expanding faster than the speed of light throw any wrenches in current theory? Do the two statements correlate with eachother at all? The distance that light travels is the distance it was from us + the inflation distance of the universe . & The speed of light is constant for everyone regardless of velocity.
  2. jcsd
  3. Mar 5, 2012 #2


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    SR is a local phenomenon. Two objects that are local to each other cannot exceed the speed of light with their relative velocity, but SR does not forbid objects increasing in distance bwetween each other as a result of expansion. Locally, the two objects might as well be stationary; it is the space between them that is expanding.
    Because your velocity relative to the source of light results in a time dilation.

    Usually, the way this thought experiment is set up, you (F for Fell) are in your spaceship travelling a .9c and you turn on your headlights.

    You wait ten seconds and measure your beams having travelled ten light seconds ahead of you.

    Me, on Earth (D for Dave), sees F turn on his headlights and D waits ten seconds and measures again. D sees the beam of light having travelled ten light seconds, but sees F following closely behind, having travelled 9 light seconds in that time. F is only 1 light second behind his beams.


    Because, at .9c, F is time dilated. Time is moving slower for him as compared to D. During the 10 seconds D waited before measuring, F has experienced much less time passing. As far as he's concerned barely a second has passed. He will wait much longer before deciding that 10 seconds has passed, and taking his measurement.
  4. Mar 5, 2012 #3

    Perfect. Time dilation. Knew generally what it was, but sadly every book i've read on relativity fails to make the connection in a way that stuck. (at least that I could remember. ha)

    But still..."the two objects might as well be stationary; it is the space between them that is expanding" isn't the space still expanding faster than the speed of light? That is how the universe is larger than the age? Are they pretty sure they got this inflation thing right? Sounds to a laymen sorta like an escape goat.(but so did dark matter for the longest time. Till I learned more) Does anyone important think the predicted size and age of the universe not matching c points to a fundamental flaw in our understanding?

    Not being a grammar Nazi, but you should try firefox. Stock spell check. I'm terrible at spelling, but it makes me look somewhat intelligent. Someone like you who at first glance only makes typing mistakes could really benefit too. Little red reminders.
    Last edited: Mar 5, 2012
  5. Mar 5, 2012 #4


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    Again, special relativity has nothing to say about distant objects moving at > c velocities. It does not violate SR. So there's no reason to go looking for a "better" answer.

    I see one typo (bwetween). I do use Firefox, btw.

    I hope you're not taking issue with the correct spelling of travelling....:wink:
  6. Mar 5, 2012 #5
    Dave's post is the way to think about it.....

    From Lineweaver and Davis:
    Lots of good stuff here:
    http://space.mit.edu/~kcooksey/teaching/AY5/MisconceptionsabouttheBigBang_ScientificAmerican.pdf [Broken]

    Unsure if you mean inflation or expansion.....but yes, pretty sure for both.....best explanations and experimental confirmations so far....But only
    discovered and confirmed recently.....this is NOT 100 year old science.
    Last edited by a moderator: May 5, 2017
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