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Newbie vs Einstein

  1. Aug 11, 2008 #1
    A couple questions.

    Assume that you can travel arbitraily close to the speed of light and that you have figured out how to communicate via Quantum entanglement (Like Ender's Game).

    I speed away from earth at almost the speed of light. At the point when I stop accelerating, is it not valid to say that I am standing still and the earth is moving away from me? If so why does time slow for me, instead of the earth? Why can this not be used to determine absolute speed instead of being limited to relative speed? What does this do to my quantum entangled communications? At what point does my mass cause me to become a black hole?

    The second related question is, how do we know that time actually slows? If all of the atoms in my body slow (because their vibrations/motion would also have a speed limit) it would seem to me like time has slowed but it is in fact more like suspended animation. It seems to me that without knowing what time is there is no way to know. If it is in fact just suspended animation then it isn't time travel but just a speed limit. I don't think this is just semantics. My question really asks if time is slowed at all or just our perception of time. Am I missing something?
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  3. Aug 11, 2008 #2


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    From your point of view clocks on Earth tick slower. From the point of view of an observer on Earth your clocks tick slower. It's a symetrical phenomenum.

    Absolute with respect to what? That's the problem. Any inertial observer is free to consider themselves at rest.

    At no point do you become a black hole. There's some good explanations in the FAQs.

    Again, there are good explanations in the FAQs but the fact that muons created by cosmic rays in the upper atmospere reach the Earths surface, even though thay shouldn't last long enough, is pretty compelling proof.

    Time really does slow down when moving clocks are observed from the rest frame of an observer. It's not an issue of perception.
  4. Aug 11, 2008 #3
    I'm still not really getting it. I understand that point about it being from the perspective of the observer, but if I come back to earth and stop, time has slowed for me not earth. I guess I don't get the relative part.
  5. Aug 12, 2008 #4


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    You are right, it is not relative, it is absolute. For you to come back to earth you must change directions, ie. accelerate. In special relativity, motions with constant velocity are relative, but accelerations are absolute.
  6. Aug 12, 2008 #5
    But say you can accelerate almost instantly, so that virtually all the travel time is at a constant velocity. Then the acceleration is an insignificant part of the trip. Another approach is to return at a normal velocity so that time dilation is insignificant. The time effects from the first part still caused you to age less than earth. Is it, just that it appears relative while you are moving, but the absolute effects are apparent when you stop?

    Another, way of asking the question is, if while I am traveling I should be able to tell time by the position of the planets/stars (I know that no light would catch me, but pretend). If it is relative there movement should slow down from my perspective (since they are moving away from me at relativistic speeds). But a stationary observer would see them move normal. If I stop moving where are the planets? If Time slows for me, they should have moved faster from my perspective, which is what the stationary observer would see.

    Still confused. I am not questioning 100 years of relativity , but I cannot, make sense of these apparent contradictions. I accept that it is true, but I am having trouble wrapping my mind around it.
  7. Aug 12, 2008 #6


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    That's not the point. It doesn't matter how long the acceleration takes, it's the fact that you accelerated that breaks the symetry.
  8. Aug 12, 2008 #7
    It feels like we are going in circles here. If I leave the earth at the speed of light then come back time has slowed for me not earth. I'm not sure what acceleration has to do with anything. While I am traveling time appears to slow for the earth from my perspective and for me from their perspective. In reality I am the only one affected. In fact when I stop it will appear from my perspective to have sped up for them when it in fact hasn't. This is what I am trying to understand.

    The reason I mentioned instant acceleration, is that the difference in time between me and earth is determined by my velocity and how long I traveled not by how fast I got up to speed.

    In my original question I mentioned using Quantum entanglement for communications. Which would allow you to communicate with the stationary observer. Would I not be able to tell that Time has slowed for me not him?
  9. Aug 12, 2008 #8


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    It seems to me that your question is a version of the twin paradox. You can search the S&GR forum for amny threads on this topic, which may help you understand.

    I don't see this as a related question. It is more a question of Quantum Physics that relativity.
  10. Aug 12, 2008 #9

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    There is still an asymmetry. You travel way form Earth in one inertial reference, and you travel back to Earth in another inertial reference frame, i.e., you switch inertial reference frames. The Earth stays in one (approximate) inertial reference frame.
  11. Aug 12, 2008 #10
    In all of the resolutions I've read, from the ship's point of view, earth's clock does run slow compared to the ship's clock, except during the acceleration (turnaround). If the turnaround is instantaneous, earth's clock simply "jumps" ahead. Otherwise earth's clock will run faster than the ship's clock during the acceleration, from the ship's point of view. There are many different opinions on this, but as far as I know, the consensus is that the earth clock will "jump" or run faster than the ship's clock during the turnaround according to the ship observer.

    Some say that this isn't due to acceleration, but due to the ship switching reference frames. Well, if we consider the ship to change velocity in arbitrarily small increments, and earth's clock to "jump" ahead a little for each increment, that's equivalent to saying that the ship will consider earth's clock to run faster during the acceleration.

    According to Einstein's clock paradox resolution, the ship's clock run's slower than earth's during the acceleration due to the equivalence principle (the ship observer can consider himself at rest in a gravity field with the earth in freefall) causing the ship's clock to run slower than earth's during the "turnaround".

  12. Aug 12, 2008 #11


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    Not exactly. The problem is that there's no natural way to extend the accelerating coordinate system to points that aren't close on the rocket's world line so that it assigns a time coordinate to events on Earth as well. (There is a choice that can be considered natural when the acceleration is constant, but not when the acceleration is arbitrary).

    You are of course right that the ship can't switch reference frames without accelerating, so it's not wrong to attribute the "jump" of the Earth clock to acceleration in this case. However, there is a version of the twin paradox where there's no acceleration. Instead of two twins, you consider three clocks with (straight) world lines that form a triangle in Minkowski space. (You can probably figure out the rest). In this case, we can't claim that acceleration resolved the paradox.

    Also interesting in this context is the version of the twin paradox suggested by Kev in this thread. See DrGreg's spacetime diagram in #4. In this case, both twins have accelerated in exactly the same way, but SR still predicts that they won't be the same age when they meet. I like this version of the problem because it shows why it doesn't really make sense to think of acceleration as the cause of a second kind of time dilation.

    This can only be considered a resolution if you already understand gravitational time dilation very well, and I don't see how anyone can understand that without first having a very thorough understanding of time dilation in SR. For example it's instructive to think about the relative ticking rates of two clocks at the front and back of an accelerating (Born) rigid spaceship, and then use the equivalence principle to carry the result over to GR, where the situation is supposed to be equivalent to two clocks attached to the ceiling and floor of a room in a gravitational field. What you want to do is pretty much the opposite.

    I don't see why we would want to resort to a resolution that demands that you understand GR and trust the equivalence principle when there are much simpler resolutions.
  13. Aug 12, 2008 #12
    Thanks to all the people who responded. I think, I at least understand why I didn't understand. It does sort of seem like a consequence of math, rather than reality :eek: But given all the other strange things we have found out about the universe maybe not. I am a programmer, not a physicist. In my world a paradox is usually a mistake that ends badly :) This is something that my logical brain will probably never be able to accept as reality.
    Last edited: Aug 12, 2008
  14. Aug 12, 2008 #13
    I have a related question. We are accelerating around the sun by the forces of gravity. Does acceleration due to gravity also cause time dilation? I know that gravity itself causes time dilation. Is the time dilation that we call a factor of g actually a factor of acceleration due to G?
  15. Aug 12, 2008 #14


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    I always think it's easiest to think about the role of acceleration in terms of spacetime geometry. If both of us travel between two points on a 2D plane, and one of us goes in a path which has constant slope in a cartesian coordinate system on that plane (meaning the path is a straight line) while another goes on a path which has a changing slope in just one small region whereas the sections of the path both before and after have (different) constant slopes (so the path resembles a bent straw with two straight segments joined by a small curve), then this is enough to guarantee that the distance along the second path will be significantly larger than the distance along the first. It isn't as if all the extra distance suddenly accumulates along the short bent section of the second path, it's because of the geometry of the situation--in a 2D plane a straight line is always the shortest distance between two points, any other path between those points will be longer.

    Something fairly similar is true for paths through 4D spacetime in relativity, except with the notion of "proper time" along a path through spacetime (the time as measured by a clock that has that path as its worldline) replacing the notion of distance along a path through 2D space. I elaborated on this in another post while I'll copy here:
    I also talked a little more about this analogy to 2D geometry in post #9 from this thread.
  16. Aug 12, 2008 #15
    I think where you are confused is that really there are two distinct time dilation effects. One is the symmetrical "clock running slow" from each observers point of view, and the other is the time accumulated on a clock due to different paths through spacetime. The latter is much like the odometer on your car. We both go to the same place but on different paths and our odometers do not read the same. The clock in relativity plays the part of an odometer. In order for two people to meet again one has to change frames (whether by acceleration or something else) thus traveling a different distance through spacetime.
  17. Aug 12, 2008 #16


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    Newton's theory describes gravity as a force, with a corresponding acceleration, but there's no time dilation in Newton's theory. In Einstein's theory, gravity isn't a force. Objects in free fall (such as a planet in orbit around the sun) are unaffected by forces.

    Gravity does however cause a kind of time dilation. For example, if you put a clock on the floor and one in the ceiling of the room you're in, the one on the floor will be slower. This is because the motion of the clock in the ceiling deviates less from free fall than the motion of the clock on the floor. Any deviation will slow down the clock. This is exactly the same thing that happens if you put a clock on the floor and ceiling of an accelerating rocket that's far away from massive objects (and therefore unaffected by gravity).
  18. Aug 12, 2008 #17
    Is my problem that I am thinking in 3d plus time instead of treating it like 4 spatial dimensions? I really have no problem with the time dilation aspect. It has been demonstrated and isn't really in dispute. The basic problem is in the frame of reference aspect. If I speed away from earth, it may be valid to talk about frames of reference with respect to perception but, my motion doesn't actually affect the earth. I still think that there is a "normal" time (earths) and a "dilated" time (mine). There seems to me that their should be an explanation, that doesn't imply that I have an effect on the earth. Their really is only one person moving, with respect to the starting position.
  19. Aug 12, 2008 #18


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    Suppose instead of Earth, you depart from a space station, and move away inertially. Then instead of firing your rockets to turn around and return to the station, the station briefly fires its own rockets in your direction, and then coasts inertially until it catches up with you. In this case it'll be the station's clock that shows less elapsed time, not yours. Of course, you can analyze the whole thing from the point of view of an inertial observer who was originally at rest relative to the station before it fired its rockets, and continues to move inertially afterwards--in this frame your clock was ticking slower than the station's clock before the station accelerated, but then after the station accelerated it was moving even faster than you in this frame so its clock was ticking even slower, and it works out that the total time on the station's clock is predicted to be less in this frame. On the other hand, you could analyze the whole thing from the perspective of an inertial observer who saw your ship as being at rest after leaving the station, and in this frame the station starts out moving away from you, then fires its rockets to move back towards you (just like the traveling twin in the twin paradox), so its clock is ticking slower than yours during both phases in this frame. But if you do the math in this frame, you still find that the amount that the station's clock is predicted to be behind yours when you meet is exactly the same as the prediction made in the first frame (where your clock was ticking slower than the station's prior to the acceleration, and the station's clock was ticking slower than yours after the acceleration). So there isn't any "true" answer to whose clock is running slower at any given moment (like a moment before the station accelerates, where the two frames disagree about whose clock is ticking slower), but there is an answer all frames agree on about how much time has passed on each clock when they reunite, and the answer is always that the one who accelerated (the station in this example, you in the previous example) has elapsed less time.
  20. Aug 12, 2008 #19


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    Yes, there really is only one person moving, if by "moving" you don't mean moving with constant velocity, but you do mean accelerating/decelerating/changing direction.

    Somewhat unrelated but perhaps helpful is the discussion in d'Inverno's Introducing Einstein's Relativity that it is not a trivial matter as to what an constitutes an ideal clock (ie. one that is unaffected by acceleration). He also says that it is not certain that time dilation applies to human aging, although it is likely, since we are made of things that constitute ideal clocks.

    Another discussion I found helpful is in Woodhouse's book on special relativity - notes similar to his book may be found at his website (http://people.maths.ox.ac.uk/~nwoodh/sr/index.html [Broken]). Sections 2.3, 7.2, 8.0 were helpful to me. He states when discussing the proper time of an accelerated path, that only an ideal clock will measure the proper time, and gives the example of a pendulum as a non-ideal clock. He also says in his book, but not the notes, that it is important not to be dazzled by time dilation - proper time and coordinate time correspond to two different physical operations, and it is not surprising that different operations give different results.
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  21. Aug 13, 2008 #20


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    To avoid diverting this thread off-topic, I've responded to this in a new thread.
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