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.
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 ..."