Nature does not discriminate

We usually make physical measurements with respect to the most convenient reference frame, placing ourselves at the origin. An observer sitting in a laboratory, in an aeroplane or in a train will consider himself or herself to be at rest, while the rest of the world passes by. We often refer to these reference frames by symbols such as S, S' or S''. In Figure 15.6, the laboratory is

S'' (aircraft) frame

S frame

S frame

Figure 15.6 All inertial frames are equivalent.

called the S frame. There is no mystery here — just a matter of notation!

The first postulate states that the laws of physics apply equally to all unaccelerated frames of reference. It is quite legitimate to construct your own 'frame of reference' placing yourself at the origin at all times. No frame of reference can be defined as being absolutely at rest and therefore somehow superior to other frames.

It is certainly in line with common experience to eat your dinner in an aeroplane, walk up and down in a train, or perhaps even play snooker on an executive jet, without noticing that you are travelling. Provided that there are no bumps or stops and starts (accelerations), everything is the same as in the airport lounge.

Another experience most of us have had, at one time or another, is to look out of the window of a train in a station and think that, at last, our train is leaving. A few seconds later we suddenly realise, as the last carriage of a neighbouring train passes by, that it is the other train that is leaving, and we are still in the station! According to the first postulate, if we 'close the blinds', and have no communication with the world outside, no physical experiment can be done that will detect uniform motion. Whether the blinds are shut or open, we can only say that one of us is moving relative to the other.

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