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Why would I think I'm not moving?

  1. Mar 21, 2014 #1
    As I understand it, SRT requires me, if I'm inertial, to insist that I'm not moving. But why would I do that?

    In Einstein's example designed to illustrate the relativity of simultaneity he says something to the effect that the man on the train will assume he's not moving. Why should he? He bought his ticket, he felt himself accelerate, and he KNOWS that he is moving with respect to the earth. Why in the world would he assume that he is motionless and that the earth is moving with respect to him?

    Does anyone do this in real life? Does anyone actually ask (as the old joke goes) the conductor if Chicago stops here?
    Last edited: Mar 21, 2014
  2. jcsd
  3. Mar 21, 2014 #2
    A related question:

    A related question: If two observers are moving with respect to each other, and each one assumes that he is motionless, doesn't at least one of them have to be wrong?
  4. Mar 21, 2014 #3


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    No, special relativity is about first realizing that it is perfectly consistent to assume there is no absolute motion, and then (in the modern interpretation) realizing that this assumption matches experiment, explicitly for example the Michelson Morely experiment, while other attempts to explain away the null result of the Michelson Morely experiment have issues.

    Einstein starts out with the notion that the mapping between frames of reference must satisfy the axioms of an infinite group, which basically boils down to the notion that any such mapping must have an inverse. (The other group axioms are closure, associativity, and the existence of an identity transformation).

    Einstein considers the possible set of such mappings, and by focusing on the speed of light being indistinguishable between moving and nonmoving fames concludes that using the Lorentz transform to transform between moving and nonmoving observers is necessary to achieve consistency. The structure of this transform involves abandoning some classical ideas, however, such as the existence of absolute time.
  5. Mar 21, 2014 #4
    So are both right?

    I thought it had been well-established that, prediction-wise, SRT is indistinguishable from theories which posit absolute simultaneity (e.g. Sexl and Mansur's studies), so I'm not sure what your assertion that it "matches experiment" is saying. Other theories, which posit absolute simultaniey (and hence absolute motion) also "match experiment," don't they?

    Put another way, isn't it also "perfectly consistent" to assume that there IS absolute motion?

    I have looked at a number of discussions about the twin paradox. Time and again I see prominent, mainstream physicists readily concede that it is logically absurd to claim that "each clock is slower than the other." Isn't is just as absurd to claim that each of two observers is "motionless" when there is relative motion between them?
    Last edited: Mar 21, 2014
  6. Mar 21, 2014 #5
    Your questions are not even about SRT. You can ask them about plain old Galilean relativity.

    Yes, your passenger might know he is moving with respect to the Earth... but he also might not. What if the windows were shaded and the waggon was noise-proof and his acceleration was slow?
    And even if he knew how he is moving with respect to the Earth, why should he care about the Earth? The important part of relativity is that inside of the waggon physics is unchanged and independent of how it moves with respect to the Earth, or how the Earth moves with respect to it, as he might view the situation.

    And why do you center your measurements on the Earth at all, and not on the Sun, or the Galaxy center, or on some hypothetical aether wind that might turn out to be moving at 2 c in regards to the Earth? Sorry, I meant the Earth was moving in regards to it...

    No inertial reference frame is "wrong", because physics works equally no matter which one you use. They are equivalent in formulation of the laws and in predictions, so nominating any with a special status is just adding redundant aspects to your model.
  7. Mar 21, 2014 #6


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    On the contrary, It tells you that you can use any inertial frame to do calculations based on the same laws of physics. That's Galilean relativity, which was introduced centuries before SRT:

    You can assume a lot of undetectable things and still match all experiments. Hence we use Occam's razor:
  8. Mar 21, 2014 #7
    Well, I was asking a question about Einstein's assertions in his explanation of SR. If that isn't a topic that is "about SRT," then I wouldn't know what is about SRT.

    Albert, as I understood him, was simply comparing a stationary observer on the surface of the earth with one in a moving train. He wasn't talking about the sun, so neither was I.
    Last edited: Mar 21, 2014
  9. Mar 21, 2014 #8
    I agree with that, sure, but that's not the point or the question. My understanding is that for SRT to "work out," (isotropic light speed, etc.), each observer MUST assume he is not moving. Am I wrong?

    If both the guy on the embankment, and the guy on the train, agreed that it was the train that was moving, then they would also agree that the clock on the train, not the one on the earth, was truly the one moving slower. Kinda like with the GPS.

    They would further agree that the two lightning bolts struck simultaneously, and there would be no "relativity of simultaneity" about it.
  10. Mar 21, 2014 #9


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    No, he CAN assume he is not moving. But he can pick some other inertial frame to do calculations.

    If both agree to use the same frame, they will both agree on frame dependent things like simultaneity. That doesn't make these things frame independent.
  11. Mar 21, 2014 #10

    I suspect that you really meant that "mathematics (not physics) works equally no matter which one you use."

    But math and physics are completely different "sciences," aren't they? In the physical world, trains do not run at a uniform speed of, say, 80 mph, without somebody constantly shoveling coal into a furnace--back in Al's day, anyway. It seems to me that a guy on a train would be spitting in the face of physics if he actually claimed that, as between the two, the earth, and not the train, was moving.

    A guy buys a train ticket from LA to NY. He does it precisely because he expects to move from LA to NY, not because he thinks some kind of magic associated with the train will bring NY to him, while he remains motionless.
  12. Mar 21, 2014 #11


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    Yes. You are wrong. Each observer CAN assume that he is not moving. Not must.

    Introductory presentations of S.R. tend to talk about inertial frames of reference and observers who are at rest in those frames interchangeably. In effect, observers in these presentations DO assume that they are not moving.

    S.R. can be derived by exploring the requirement that the laws of physics and the speed of light do not depend on this free choice.
  13. Mar 21, 2014 #12
    My question goes beyond what one, and only one, observer assumes. The question is about what each of TWO observers must both assume, in order for SRT to hold.
  14. Mar 21, 2014 #13
    You are taking my post the wrong way. It does not try to critique you for being off-topic. I was just pointing out that your questions apply [and can be answered] in an even broader sense, and not just the specific one you were focused on.
    [Do you have the same issues against Galilean relativity?]

    Again, I don't get why you react so touchy...
    My point here was to remind you that there are multiple candidates for "absolute" frame, and to ask you: how would you pick it?
  15. Mar 21, 2014 #14
    Oops, quoted A.T. in that last post when I intended to respond to jbriggs, who said:

    Not sure if A.T. was making the same claim as jbriggs did.
  16. Mar 21, 2014 #15
    Well, georgir, I'm new here, so I don't know who's who and who does what. But my post was moved from the relativity forum, and I guess I assumed it was you who did it, given your post.
  17. Mar 21, 2014 #16
    Well, that's a different question. My question was merely addressing a guy on a moving train, on earth, claiming that he was motionless. In that case, the "absolute" frame is obviously the surface of the earth.

    The surface of the earth is not moving with respect to the train. The train is moving with respect to the surface.
  18. Mar 21, 2014 #17


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    Both objects move relative to the other.
  19. Mar 21, 2014 #18
    The same issue simply does not arise with Galileo. He readily admits that, so long as he can see the shore, the sailor will know that he is moving with respect to the land, not vice versa. Einstein wants to deny this. It is common for relativists to claim that you "can't know" if you're moving. To me, that claim is contrary to all experience and common sense.
  20. Mar 21, 2014 #19
    That always true when one object is moving and one isn't. On the other hand, it's never the case when each of two objects is truly (rather than just supposedly) motionless. So that statement is missing the point.
  21. Mar 21, 2014 #20
    But both are the same thing... you seem to like one more, and decide to give it special meaning, based upon some criteria that I guess involves the relative sizes of the two bodies, but it really does not matter.

    One can claim that picking the mutual center of gravity in this situation is "more right". Or one might go for the cosmic microwave background instead... and that might just as well coincide with picking the train, and not the Earth like you do.

    What does your choice affect? What happens if you picked "wrong"? What does "wrong" even mean... No matter what you pick, you'll get the same physics. That's relativity for you.
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