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Basic Q about Relativity

  1. Jan 31, 2008 #1
    Okay, I don't know really if this is the right forum to ask this in, so I apologize if it ain't. I will also add that I am 16 years old and from Sweden, so pardon my English.

    The question I have is the following:
    I read a short part of a chapter in our physics book in school which was about Einstein's theory of relativity, and there were an example in the book about time and relativity, I didn't really get all of it but at least I understood a bit of it. It was about a train travelling between two lights with a constant speed. If you used a sensor at the exact time you where in the exact middle of these two lights, and measured the time it took for each of the lights to reach the sensor, the light which you were travelling towards would reach the sensor first, since the train had a speed in that direction, while on the other hand the light which you were travelling from would be delayed. On the other hand, if you would just stand still in the exact middle of these two lights, theoretically the sensor would display that both of the lights reached the sensor at the exact same time.

    So, what if our whole universe was moving in a direction with a very high speed. This would mean that the stars we measured which was in the direction from earth which the universe was moving would be further away than we have measured. Because if it takes the light from a star 9.000 years to reach earth, if we have a velocity towards that star, that would mean the light would reach us in a speed of the velocity of light + the velocity of universe towards that star, which would result in a shorter time than if only the light was moving.

    I am sorry for explaining so bad, but I hope you got about what I meant. I hope this ain't the theory of relativity (what I just explained with universe and everything), because when I think about it that would make pretty much sense. I didn't read the whole chapter so I don't know. What do we relate to when we do all of our measurements? What is the "original" position which ain't got no speed or anything, and how can we be sure about that? If there were something that was unclear please ask me what I meant with it.

  2. jcsd
  3. Jan 31, 2008 #2
    Don't stop reading your book just yet! The main point of (special) relativity is that the speed of light is the same for all observers. Even if you're moving towards a star at half the speed of light, you still record light from that star as reaching you at the speed of light. Strange but true, and this fact leads to even stranger things such as the slowing of time for moving observers...
  4. Jan 31, 2008 #3
    To clarify - the observer starting at a point equa-distant between two light sources will according SR, measure the local light speed of both signals as c - but the signal from the source which you are approaching will be received first - this has to do with the coordinate frame for the experiment - the measurement of the effective speed of light for an object moving wrt to a coordinate frame attached to the light source will not, in general, be equal to c.
  5. Jan 31, 2008 #4
    I think you are confusing the OP. Are you talking about closing speed?
  6. Jan 31, 2008 #5
    Your assesment is correct, you are talking about something called "closing speed" that can be as large as twice the speed of light. Since the receptor is moving towards the star (with a speed v<c) and the light is coming towards the receptor at speed c, the distance between the receptor (Earth?) and the star is covered at c+v.
    Now, with the risk of confusing you, the speed of the light is unchanged (c) and it will be measured by an observer on Earth as "c". It is just that the distance between Earth and the star is covered by the ray of light at the "closing speed" of c+v.
    Last edited: Jan 31, 2008
  7. Feb 1, 2008 #6
    When you finish the chapter or the book you will realise that there is no "original" position that we relate all our measurements to. There is no way to determine if you are "really" stationary or moving relative to some imaginary absolute reference frame.

    The same applies to the universe as whole. When you asked the question "So, what if our whole universe was moving in a direction with a very high speed." you have to ask yourself "moving relative to what?" Since any observer can not absolutely determine their own motion they can assume they are stationary and assume everything else is moving relative to them. The observer on the train can assume he (and the train) are stationary and that the observer at the side of the track (and the track) are moving. When the observer on the ground sees two flashes arriving at the same time coming from points that are the same distance from him he assumes the twoo flashes occured simultaneously. If he saw one arrive earlier he would assume that the flashes were not emmited at the same time. It is the same for the observer on the train. When the observer on the train sees the flash from the front of the train arrive before the flash from the back of the train he assumes the flash at the front of the train happened before the flash at the back. What appears to have happened simultaneously to the observer beside the track is not simultaneous as far as the the observer on the train is conscerned. This is called the relativity of simultaneity. If the universe was moving in 'some direction with very high speed' it would not appear any different to us (because we are moving along with the universe) than if the universe was stationary. There are no measurements that we can perform to find out the "absolute motion" of ourselves or of the universe.
  8. Feb 1, 2008 #7
    Aren't you two suggesting two different things? As I read it 1effect is suggesting that the light will approach the receptor at the "closing speed" which was v+c, and cadnr on the other hand is suggesting that the light will reach the receptor at the speed of light (just c), no matter if it is moving against the source or not.

    Correct me if I'm wrong.
  9. Feb 1, 2008 #8
    If I understand correctly (it's been a few years since I studied relativity in any detail), the moving observer receives one signal first because they have moved to a new location in space closer to the source of that signal (and further from the source of the other one). Thus, one signal reaches you before the other because it has less ground to cover to reach you, even though the speed of light is the same for both.
  10. Feb 1, 2008 #9
    They are not contradicting each other. They are both right but talking about subtlely different things. An observer that measures the speed of the receptor as v relative to himself would calculate a hypothetical "closing speed" of v+c. The observer with the receptor would measure the speed of light as c no matter if he (and the receptor) are moving towards or away from the source (as cadnr stated). They are talking about measurements or calculations by different observers. Any observer measures the speed of light as c relative to themselves.
    Last edited: Feb 1, 2008
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