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Velocity of light and it's frame of reference.

  1. Jun 3, 2014 #1
    If light has a maximum speed of c, what frame of reference is that speed determined from?

    For example, if you were to turn on a light bulb moving at a constant rate and measured when the light in every direction got to one light year away from it's source, wouldn't some of it reach that distance more quickly than the light moving in the opposite direction and wouldn't it be appropriately redshifted? If so, couldn't you use that data to determine a universal frame of reference?

    It's an obvious question so I'm sure I'm wrong about something. I'd be obliged if someone could explain it to me. :biggrin:
  2. jcsd
  3. Jun 3, 2014 #2


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    The speed c is the same no matter what frame you use. This is assumption #1 in special relativity. If I'm sitting here with a light bulb, and you run past me very fast, and I turn the light bulb on, we both measure the light from this light bulb to be traveling at c.
  4. Jun 3, 2014 #3
    Okay, but if that's the case, how does light end up in the same place for both of us? If I'm running past you and light is still traveling away from both of us at c, shouldn't it end up in different places?
  5. Jun 3, 2014 #4
    The time is flowing differently for you and the length for you is contracted. This causes the light to move exactly as needed.
  6. Jun 3, 2014 #5
    Okay, but what about the light that's moving in the other direction? If time is moving more slowly for me, doesn't that mean that the light moving in the opposite direction also has less time to move and ends up at a different point in space than for the "stationary" observer?
  7. Jun 3, 2014 #6


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    At some point you'll have to try drawing a space-time diagram or playing with the formulas that relate one observers notion of position in space and time to another's; these are called the "lorentz transforms" and google will find many references.

    But before you do that, you should google for "einstein simultaneity train" to find Einstein's classic thought experiment on the relativity of simultaneity; that explains how light signals traveling in opposite directions can be reconciled.
  8. Jun 3, 2014 #7
    Will do. Cheers!
  9. Jun 3, 2014 #8

    D H

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    As Nugatory mentioned, you are now running into the third counterintuitive feature of relativity theory, that simultaneity is relative. Einstein's train thought experiment addresses this seemingly paradoxical aspect of relativity.

    The first counterintuitive feature is that the speed of light is the same to all observers. Suppose I'm sitting on the hood of a moving car, idly tossing rocks forward. You're watching this while standing still on the ground. If the car is going 30 mph and I'm throwing those rocks at 30 mph relative to me, you will see those rocks as moving at 60 mph because 30 mph + 30 mph = 60 mph. If I shine a flashlight instead of throwing rocks, you would expect those photons to be coming at you at c+30 mph. That's not how it works. This is what makes the constant speed of light so counterintuitive.

    One consequence of relativity theory is that our naive velocity addition rules aren't right. This applies not only to beams of photons but to everything. Going back to the tossing pebbles situation, the velocity of those rocks is not quite 60 mph. It is however so close to 60 mph that the error is immeasurably small.

    The second counterintuitive feature is that time and distance are relative. Again, this doesn't make a bit of sense from the perspective of our everyday world. It's the passage of time and the distance between objects that are the same to all observers in our everyday world. You have to surrender your grip on those everyday concepts to understand relativity theory.

    As already mentioned, the third counterintuitive feature is relativity of simultaneity. The constancy of c, the velocity addition rules, the Lorentz transformations that describe how space and time are related, and the simultaneity of relativity go hand in hand. The constancy of c dictates that all of those other features must follow.

    The two way (round trip) constancy of the speed of light is a physically observed fact. Some physicists from Einstein's time were working on how to explain this fundamental fact away. Einstein's brilliance was in taking this observed fact as being fundamental rather than explaining it away.
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