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Back in grade 12 I was told by my physics teacher that one would have no way of determining the velocity of an inertial frame of reference without comparing it to another (e.g. if a person was in a train travelling at constant velocity without any windows, he could not know how fast or in which direction the train is going).

I was wondering if this statement is true because I kind of figured out a way by taking advantage of basic special relativity, which I won't describe here as it would take a lot of writing and time (though my teacher said he could not find any problems though).

Anyway I looked everywhere and could not find whether it would be possible to determine the velocity of a frame of reference without comparing it to another. so if anyone could tell me the truth about the statement I would be very happy.

Thank you.

edit: another question that's more broad, is the term "at rest" absolute, or is it relative? like is it correct say "I am at rest" with no extra information or you can only say "I am at rest with respect to ..."

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  • $\begingroup$ so in case that wasn't clear enough, imagine you are in a room with no windows, and you have to determine if the room was moving or is at rest, how would you determine it $\endgroup$
    – Spongebob Squareroot-pants
    Oct 2, 2015 at 0:21
  • $\begingroup$ This statement is one of the basic principles relativity is built on so you cannot use relativity to disprove it. $\endgroup$
    – Winther
    Oct 2, 2015 at 0:34

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This is not possible. It violates the law that there is no preferred frame. If you were in the moving train, you'd see only the relative velocity between you and other objects. Your rulers, and clocks would all be completely normal to you, so there is no way to solve for your absolute speed. You could find other peoples relative speed by comparing measurements and using the stand Lorentz transforms to solve for their speed relative to you, but that is it.

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  • $\begingroup$ but another question, is there such thing as "absolute velocities" or are velocities only "relative" to the observer from different frames $\endgroup$
    – Spongebob Squareroot-pants
    Oct 2, 2015 at 0:39
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    $\begingroup$ If an inertial frame could be determined to have an absolute velocity, then the frame which has zero absolute velocity would be called the preferred frame. That is what "preferred frame" means. Now reread the second sentence of the answer. $\endgroup$
    – WhatRoughBeast
    Oct 2, 2015 at 0:50

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