# Determining the velocity of an inertial frame of reference

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 ..."

• 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 – Spongebob Squareroot-pants Oct 2 '15 at 0:21
• This statement is one of the basic principles relativity is built on so you cannot use relativity to disprove it. – Winther Oct 2 '15 at 0:34