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Trying to generally understand Relativity.

  1. Sep 30, 2009 #1
    I am have had quite the issue trying to grasp it, and I know deep down it should not be this difficult. I am very visual oriented. Seems to me that Einstein did it this way to, by visualizing? So what I seem to understand is:

    I don't get the relationships between stationary (more so non accelerating) observer and an accelerated observer? Both will see themselves as normal but in relation to each others level of acceleration they will see the other as being altered in some way?

    Is there any simple mathematical equations that will express it correctly.

    Say like 2 observers - 1 is non acceleration the other is (same direction). what do both of the see from their respects, and what do they see of each other in ierms of time and space. How is that an accelerated object differ? Is only appearance or does spacetime actually distort?

    I think I had a better handle on before I started learning it, mostly from library material. Sometimes when I model it in my head what they show it works out exactly how I conclude than other times it totally wrong...and it's really stressing me out hehe.

    Any help would be appreciated, maybe someone was in the same boat once.
  2. jcsd
  3. Oct 1, 2009 #2


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    You may be confusing acceleration ( changing velocity) with relative velocity. Think about two observers who have a constant relative velocity. Each one will see the other approaching ( or departing) at a constant speed. Special relativity gives rules about how each will perceive the other's clocks and rulers.
  4. Oct 1, 2009 #3


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    You seem to expect that in the case of one observer non-accelerating, and another observer accelerating, only the relation between the two should count. In other words, you seem expect a relational theory.

    Here's the thing: relativistic physics is not a relational theory. This cannot be emphasized enough: relativistic physics is not a relational theory.
    This may sound paradoxical: we have the principle of relativity of inertial motion, and at the same time we have that relativistic physics is not a relational theory. How can that be reconciled?

    Let me paint an analogy:
    Imagine a perfectly motionless sea, with two ships floating alongside. The sea is featureless; all locations are indistinguishable from each other. When ships set in motion they can measure their velocity relative to the water. Using dead reckoning they can return to a location they departed from some time earlier, for example to rendez-vous with another ship.

    Now, let's say we cannot measure velocity relative to space, but that we can measure acceleration relative to space. Acceleration is (mathematically) a derivative of velocity. In that sense velocity and acceleration are connected, but they are on different levels.

    Dead reckoning at sea uses velocity with respect to the background. When it's acceleration with respect to the background that you can measure you can still have a form of dead reckoning, but on a higher level. Let's say a spaceship has intervals of acceleration, with phases of coasting along in between. If you have exact logs of direction and duration of acceleration, and duration of phases of coasting along, you can plot your trajectory. And if you want to you can plot a course back to some rendez-vous point.

    Relativistic physics asserts that no such thing as velocity with respect to some backgrounds structure exists, but at the same time relativistic physics hinges on the fact that we do measure acceleration with respect to the background structure. We assume that that combination is possible, and we call the environment that makes that combination possible Minkowski spacetime.

    The Minkowski metric expresses the relation between space and time in Minkowski spacetime.

  5. Oct 1, 2009 #4
    I think I got that part. There is no universal acceptable frame or reference. If (me) as an observer is watching (you) fly by (me) in space there is no way for me to tell whether or not I was moving at a specific velocity or if you were.

    what I grasp or think I grasp is the faster you go towards the speed of light the more time slows down, and that length contract in the direction of motion.

    I just can't see how if me in non-accelerating frame of reference will see someone one else who is accelerating have their length seem as contracted and their time slowed down, and from their perspective the same.

    It's hard for me to grasp something if the way it's described is "this is just how it is" there must be something physical I can model in my head so I can grab the concept.

    I borrow this book (Relativity Visualized) from the library here is an example:

    3 ship same distance appart, the middle one is the flag ship, traveling at the same speed you are. They send a light signal to burst the engine and start accelerating at the same time. Now the flag ship send another signal but this time you see the signal arrive at the rear ship, while the crew see is at the same time. Why? I can just accept that this true...but with no idea as to why I can't see it. I see it has to do in that example that the speed of light doesn't change and that both observe (ship or you) see themselves as stationary, but because one is accelerating somehow both views of each other has been skewed, and I am not getting why.

    I apologize I know it must be frustrating but I really need to understand this.
  6. Oct 1, 2009 #5


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    Both of you have an objective way to decide which person is accelerating and which isn't--the one that isn't accelerating will feel weightless (if we are talking about flat SR spacetime where there's no gravity), while the one that is will feel G-forces. And it's just part of the rules of relativity that the time dilation and length contraction equations are only supposed to hold in the frames of inertial (non-accelerating) observers, these equations were never meant to apply to non-inertial frames.
  7. Oct 2, 2009 #6


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    This is a very nice book. In a later chapter you will get an nice geometrical model for your head. Here an interactive version of it:
    Because the light signal propagates a the same speed in every direction for every observer: for you and for the crew of the flag ship moving (acceleration is not relevant here) relative to you. From that the relativity of simultaneity follows.
  8. Oct 2, 2009 #7
    I think I get it, I believe I am assuming that would be the normal consequence of it so I am not seeing how it's different. So when is acceleration relevant? or does that matter at all?

    Thank you all for all the help!
  9. Oct 2, 2009 #8


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    Keep in mind that the length contraction is mutual.

    Let A and B be two spaceships, that are equal in size when they are co-moving. If A and B have a velocity relative to each other then A is length contracted as compared to B, and B is length contracted as compared to A.
    That is, in the frame of reference that is co-moving with A the other spaceship, B, is reckoned to be contracted. Conversely, in the frame of reference that is co-moving with B its the other spaceship that is reckoned to be contracted.

    You express an expectation (or a demand): 'there must be something physical I can model in my head'. What would it take to satisfy your desire? What counts as 'a physical model' for you?

  10. Oct 2, 2009 #9


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    I don't have the book here, but from my head he derives length contraction there:

    Initially the three space-ships are moving inertially in line equally spaced, without acceleration. Then the (middle) flagship sends a signal to accelerate. The inertial movment already causes different simultaneities here:
    - the flagship observes the other two ships simultaneously firing their rockets
    - we observe the last ship firing the rockets before the leading ship
    As a result the formation shortens in our frame during acceleration, and stays shorter after the acceleration.
  11. Oct 3, 2009 #10
    I see I think correct me if I am wrong.

    Two objects with the same velocity will see each other exactly the same.
    Two objects with different velocity will see each other different? (in respects to space and time, either slower or contracted)

    I assume the change in spacetime regardless is proportional to the change in velocities?


    The best example I can give you is that: I am trying to picture two rockets traveling at different velocities. I picture myself in the first ship and I am trying to visualize how space and time is changed relative from my perspective. A book will simply state that it does. Which kinda leaves me to guess how it happens. When either I or the other ship accelerates. Something physical must happen to spacetime, and that what I am trying to figure out. If I can see that then I understand completey. I kinda have to see the basics of everything involved I guess than I can advance. I can't build up, it just confuses me. Maybe I need to find a new method to learn.

    It's just hard to derive from words. I have to constantly change it so that it's correct. It maybe very simple, but I tend to over complicate things. Like as I am typing in my mind I am not even seeing words I am see visuals, It's really hard to explain. It's just how I've always been. It does make me good at problem solving though, I've always been able to solve practical problems a lot faster. Maybe most people do what I do, and I just haven't developed my literature or something, but most people I tell me they don't think in the same manner.


    Wow I think I get it totally. Is that really how spacetime is? Can I do experiments to see this? in the book it states that I can take a stop watch run with it, and then leave the stop watch stationary at some point and when I make the same trip the time will be different? I am sure it would be minute but still, I want to try it if so.
    Last edited: Oct 3, 2009
  12. Oct 3, 2009 #11


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    Spacetime is not "really real", it is a mathematical model that describes how reality behaves. You can have three real apples, or three real bananas. But the number three is not something real, it is just an human idea. In your book, in the chapter about the "myth" there is a nice geometrical model for people like you who like to visualize things.
    If the effects where so big, relativity would have been developed much earlier. No, you cannot measure time dilation with stop watches by running.
    Last edited: Oct 3, 2009
  13. Oct 3, 2009 #12
    Ok cool thanks guy, I got it.

    I guess photons are then "timeless" correct?
  14. Oct 3, 2009 #13


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    They don't experience proper-time according to any observer. Or in simple terms: they don't age.
  15. Oct 3, 2009 #14


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    I should point out that perspective is not relevant for relativistic physics. Certainly not for understanding relativistic physics.

    Here I am using the expression 'from my/your perspective' to mean the recorded observations from a single location in space, without the corrections for transmission delays. Signals from sources at different distances have different transmission delays, and that muddles the picture. A good overall picture is one that is corrected for transmission delays.

    The substance of relativistic physics is about the effects that come into focus after transmission delays have been corrected for. That is why for visualization spacetime diagrams are used. Worldlines are plotted in the diagram; it is as if there is a fine grid of observers, all logging events, and the overall diagram represent the assembled information, jundistorted by transmission delay.

    I understand your urge to address that question, of course, but I think you are too ambitious there.

    Let me make a comparison.
    Newton proposed that the planets revolve around the Sun due to a universal gravitational attraction. Some of his contemporaries insisted that this supposed gravitational attraction should first be explained. But explaining gravitation was way, way beyond the powers of 17th century science (arguably it is to this day beyond our powers). It would have been very wrong if Newton would have decided against publishing because he couldn't explain why gravitational attraction exists. Newton addressed the problems he could solve (making giant strides), and stopped short of what was beyond understanding anyway.

    Returning to relativistic physics:
    We can describe the relativistic effects quantatively, but we have no means of figuring out how they happen. It's not a matter of being difficult; the means just aren't there. If we would stubbornly insist that we should figure out first what is happening underneath, then we would just bog ourselves down.

    Last edited: Oct 3, 2009
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