I am wondering if there is only one object in the universe than does it make any sense to talk about its velocity. If empty space can be thought of as a reference to measure its velocity than it might.
I read somewhere that in empty space someone cannot tell if she is moving unless she has some acceleration. Now if it does not make sense to talk about velocity how can it make sense to talk about acceleration?
Answer
The Equivalence Principle of General Relativity holds that acceleration and gravity can be described identically. With an accelerometer, you can tell whether or not you are accelerating in empty space, regardless of whether another object is available to act as a reference point. Under acceleration, your weight will change just as though you were approaching or standing in a gravitational field.
Velocity, however, has meaning only in relation to relative movement between two or more objects. No one has yet discovered an edge to empty space which can be used as a reference point for measuring the velocity of an object. Therefore, a reference point is necessary to measure velocity, and the velocity will be attributable to both objects.
Acceleration is the rate of change of velocity. You do not need to know the magnitude of velocity, only its rate of change in order to measure acceleration.
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