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Locality, Time, Speed, Distance, and one confused person.

  1. Mar 14, 2008 #1
    Ok. There are some concepts here that I am having trouble reconciling.

    I would first like to "stipulate" that for time to have any meaning, there has to be change. I'm not sure if there is some Brownian motion at 0 Kelvin, but for the sake of argument, lets say it is not. So, if I freeze an entire frame of reference to 0 Kelvin, all movement stops, and you can say that time has stopped in that frame. Once the temperature is increased, things return to normal and that frame has traveled forward in time in respect to its surroundings.

    This seems to suggest that as movement slows down, so does time.

    Now consider the grandfather paradox:

    You stay on earth, I travel to Alpha Centuri and back pretty damn fast (from my point of view, at least). So, I get back and you are old while I have aged just a little.

    This seems to suggest that as movement increases, time slows.

    Those two things seem contradictory, unless you give up the idea of time being a measure of change. But then how do you explain time in a zero movement system?

    Another thing: If I "chase" a beam of light and measure it's speed, I get c, just as I get c if I stand still. If I flip this around and start to meet a beam of light that comes in my direction, it approaches me at C, just as it approaches me at C if I stand still and watch it come.

    So, I am floating in orbit around the earth, while you are in a spaceship ready to take off at high speed towards the sun. Unbeknownst to us, the sun just instantly collapsed into a a black hole for some reason, and the last photon just left it. You start your travel towards the sun and I stay still. According to the constant speed of light, the photon should reach us both at the same time, even if you went to meet it! You measure the incomming photon at speed c, I measure it at C, and since we started at the same place it should reach us at the same time. But by the time it reaches "us", you are far ahead of me. This seems very weird and implies that the light is two places at once, places that are far away from each other in the direction of the lights motion.


    How do you get to point A and B simultaneously if you can only travel along the dotted line? I realize that the word "simultaneous" is probably the perpetrator here, somehow. But I am so ******* lost on how to conceptualize what is happening here. Can this be explained without starting with tensors and how time is an axis and that I am thinking in Euclidean geometry?

    Another question while I am at it: Does SR or GR have any effect on quantum physics? Do the ideas still hold?

    I feel like a douche for posting what is probably the zillionth moron trying to understand these things before he has the tools to do so. But I cant help it.

  2. jcsd
  3. Mar 14, 2008 #2


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    How is this suggested?

    I'm not sure if this is your own idea or that it is your misconception from relativity, but 'time doesn't slow down with movement". Time parametrizes movement, and according to relativity you can construct a manifold called 'space-time'. An observer observes an event on this space-time, and the coordinates are given by one time-component and three spatial components. With a given metric you can find the relation between those components for a given event. The fact that you can build a Lorentz-invariant interval from these coordinates results in the fact that different observers observe different coordinates, and this is the reason for time-dilatation.
  4. Mar 14, 2008 #3
    I think it has some measure of truth. How do you measure time if nothing moves or oscillates? you define second as some huge number of oscillations of cesium. But if it doesn't oscillate, there is no time - it effectively freezed as well.

    But I don't know if temperature decreases, each cesium atom oscillates slowly.
  5. Mar 14, 2008 #4
    But a system with no movement has no events, so what is time in this system? You still have a 3D coordinate at zero movement, since it does not make everything collapse to a single point, but I don't see how you still have time?

    To quote myself from a bit further down:

    Those two things seem contradictory, unless you give up the idea of time being a measure of change. But then how do you explain time in a zero movement system?

  6. Mar 14, 2008 #5


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    Why does any of that need to be true? A book sitting on a table does not move in a perceptable way, but that doesn't mean time doesn't still flow. You are confusing measurement issues with the actual dimension. A stopped clock is unable to measure time, but that doesn't mean that time isn't still flowing.
    Fine. Time is not a measurement of change. Measuring time requires there to be events to count, but that doesn't mean that if there aren't events to count that time isn't still flowing. See the difference?

    Time for a car sitting still: 1, 2, 3, ,4, 5.....
    Distance for a car sitting still: 0, 0, 0, 0, 0...

    This doesn't mean that a parked car does not experience time, it just means that a parked car makes a very poor clock.
    These two concepts you have connected are not connected. The fact that you measure a car's distance change to be zero while time continues to flow doesn't have anything to do with time dilation in Relativity.
    You're missing two things there: First, for the times to be different, the measured distances also have to be different. Second, if one is moving toward the light source, it is moving toward the light source. Even in old Galilean relativity, the distances have to be different.
    Last edited: Mar 14, 2008
  7. Mar 14, 2008 #6
    I'm afraid I don't :/ In order to calculate movement and distance between objects according to relativity, do you not need x,y,z and t? And if the system in which this calculation is done is completely still, can you not set t to be 0 and get the correct values?

    Have I just wandered off into crackpot territory?

  8. Mar 14, 2008 #7
    I am sorry, I am just not getting this.

    A car moves towards me at 10 Unit/s. I am standing still. The distance between us is 100 Units. It reaches me after 10s. Now I reset, and I start walking towards the car at 5 Unit/s. Normally, if a car moves towards me at 10 Unit/s, and I move at 5 Unit/s, I would measure the car to be approaching at 15 Unit/s, is this not correct? But since i still measure the car to be coming towards me at 10 Unit/s, it has to be moving at 5 Unit/s, no? And it reaches me after 10s, if I stand still and if I move towards it.

    Last edited: Mar 14, 2008
  9. Mar 14, 2008 #8

    Doc Al

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    That's 5 units/sec with respect to the ground.
    Not really. But for low speeds, it's close enough. But at high speeds it's wildly inaccurate.

    But someone at rest on the ground will measure you and the car to approach each other at 15 unit/s. But that's not the relative speed that you would measure.
  10. Mar 14, 2008 #9
    AHA! And there in lies the whole trouble I have, I hope. The entire POINT of RELATIVITY is that there is no OBJECTIVITY. So all measurements are relative to your point of view.

    I feel stupid now. Thank you for clarifying.


    Edited with an additional question: When was this inconsistency in observations discovered? After the fact that Einstein proposed so or is this something that has been known for a long time?
    Last edited: Mar 14, 2008
  11. Mar 14, 2008 #10


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    Hi kenewbie! :smile:

    If an object is at 0 Kelvin, that does not mean it has no movement, it only means that its various bits have no movement relative to each other.

    The object as a whole can still be moving.

    For example, imagine two jars containing something at 0 Kelvin, one here and one on Mars.

    There's no frame in which they both have no movement!

    We will always regard the contents of the container on Mars as moving, even though it is at 0 Kelvin, and has no Brownian motion. :smile:
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