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Absolute position in spacetime?

  1. May 15, 2012 #1
    I am new to the topic of special relativity. Having read a bit of Feynman and googling for answers I am still a bit lost. While I have an aptitude for logic, it does not extend quickly into physics or maths. So quite likely I am barking up the wrong tree and about to demonstrate my ignorance but here goes...

    Special relativity start by saying there is no absolute velocity. No stationary frame of reference. Only relative positions to other frames of reference.

    I am not about to debate whether the equations of special relativity work - they clearly do. They allow to to translate motion from the point of view of different observers. The maths works. It means you can use the laws of physics in whatever isolated frame you like.

    But just because you don't need any absolute stationary frame of reference for your calculations, how does special relativity rule out its existence? I am not looking for ether. But perhaps there is a point in space where time runs fastest - no velocity or gravity effects involved (or at least negligible). Perhaps there is an absolute spacetime field that we move relative to?

    The way I envision it time travels at the speed of light, so when you start moving close to that speed, time has to slow down for you. It does this because it is no longer passing by you at the speed of light. Could this be true?

    Since light always travels at C (in a vacuum), doesn't that make it possible to find your absolute velocity (relative to the fabric of spacetime) by measuring the time light takes travel a set distance (in your frame) in one direction versus another (say forward versus back)?

    For example in The Michelson-Morley Experiment - if they were able to measure the time taken for light to travel one way from A to B at it's constant velocity, would they have seen a difference in the time taken from B to A, given that earth is not stationary and B may be moving towards or away from starting point A - thereby making the absolute distance travelled by the light change? Obviously since they measured in both directions, this difference would cancel out.

    I can buy the time and length distortion. What I don't get is just because you CAN calculate relatively - how does that rule out that you HAVE an absolute position in our physical universe?

    Then again perhaps there is no such thing as constant velocity, since at every place in the universe there is AN acceleration?

    And just for the hell of it - here's another one...
    If someone is moving at the speed of light relative to me. They will see me as moving at the speed of light in the opposite direction. In reality one of us may be stationary - though special relativity suggests you can't know which one - in this case which one of us has infinite mass? Is the mass increase relative? Or is it absolute? Which of us experiences time stop?

    Is the increase in relativistic mass merely an expression of the reduced time that one is able to exert a force when moving at relativistic speeds? IE is it not simply that time is reduced that makes an object act as if it were massive?
  2. jcsd
  3. May 15, 2012 #2


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    Yes. Everything travels through space-time at c so things traveling close to c in space are moving slowly in time.

    Nothing with mass travels AT c, so you should reword your questions about that to be about things traveling CLOSE TO c.

    There have been numerous threads on this forum about why there is no absolute velocity and no absolute reference frame. Do a forum search for either.
  4. May 15, 2012 #3
    Nope. All laws of physics are perfectly valid in all reference frames and unchanged in all reference frames, that's what SR is based on. If there was an "absolute" reference frame of the universe, there would be no way to verify what it was, which really takes out the whole point of an absolute reference frame.

    Basically. Note that this isn't really it, because you'll observe yourself as perfectly normal and everyone else as moving very quickly through spacetime.

    Nope. All the weird effects of SR come from light in a vacuum (or anything else massless, for that matter) traveling at c in all reference frames. Before SR, it was thought that light did travel in a certain reference frame, known as the Aether. (Well, the medium light propagated through, but effectively the same argument.) And you should know how well that went, from the Michelson Morley Experiment.

    Again, you'd lose all meaning of an absolute reference frame.

    Ummm ... ? What do you mean by an acceleration being at every point of the Universe?

    Well, first, that's impossible, but assuming you mean ridiculously close to c, the mass increase is relative.

    Neither. (Again, near time stop.) From your point of view, you see the other person appear time dilated. From the other person's point of view, you appear time dilated.
  5. May 15, 2012 #4
    A lot of questions like this seem similar to what must have been asked or discussed (or a least thought about) in the early days when considering the idea that clocks and rods change rate and length when moving relative to the observer... If clocks and rods change their measures when in motion, are these devices really suitable for doing measurements of something in motion? Think about that as they must have; in almost any other areas of measurement if the intrinsic properties of the measuring device itself was determined to vary based on the conditions one was inclined to reject it and find a consistent device or another method of making the measurement...

    Look at some of the early writings with this question in mind and see if you can grasp to some degree how the answer becomes the foundation of modern relativity.
  6. May 16, 2012 #5
    Thanks for your answers. I think I am slowly getting it.

    What I meant by an acceleration everywhere is that no matter where you are in the universe you are surrounded by matter and are therefore subject to acceleration by gravity.

    If the increase in mass is only relative, how does that line up with particles in a box being heavier if they are travelling at higher velocity (higher temp)?

    I am still stuck on the speed of light. I get that it travels at c at all times. If you are travelling forward at any velocity and you shine a light infront of you at what speed does the light really travel? Surely it is not c plus your velocity as that would exceed its limit. If I can measure it, it will come out as c relative to me - but is it truely travelling at c OR does it only appear that way due to time/space dilation? And if you shine a light behind you it will appear to travel at c relative to you - but does that mean is it really travelling at c minus your velocity? I just don't get that if you're moving at all, that light could appear to travel at the same speed infront of you as behind. Surely time and space are not dialated in one direction (say forward). How can it travel a metre infront of you in the same time that it travels a metre behind you (given that you're moving)? Since I don't get it, can someone explain what is really happening in this scenario? Thanks! =)
  7. May 16, 2012 #6


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    99% of the places you can be in the universe have negligible gravitational force because there is nothing anywhere nearby.

    It is traveling at c. Relative to you. Relative to me. Relative to the little green man on alpha centaur.

    Not, it is traveling at c. Relative to you, relative to me, and relative to the little green man on alpha centaur.

    Yep, it's a headbanger. Get over it.
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