Alice is driving a race car around an essentially circular track. Brian, who is

Question

Alice is driving a race car around an essentially circular track.  Brian, who is sitting at a fixed position at the edge of the track, measures the time that Alice takes to complete a lap by starting his watch when Alice passes by his position (call this event E) and stopping it when Alice passes his positin again (call this event F).  This situation is also observed by Cara and Dave, who are passengers in a train that passes very close to Brian.  Cara happens to be passing Brian just as Alice passes Brian for the first time, and Dave happens to pass Brian just as Alice passes Brian the second time.  Assume that the clocks used by Alice, Brian, and Cara are close enough together so that we can consider them all to be "present" at event E, and similarily that those used by Alice, Brian, and Dave are "present" at event F.  Assume that the ground frame is an inertial reference frame.

a) Who measures the proper time between events E and F

b) Who measures the coordinate time between events E and F

c) Who (if anyone) measures the spacetime interval between events E and F?

EXPLAIN YOUR REASONING

Explanation / Answer

A) Brian and alice . They are present at both the events at that point and so measure proper time.

B)coordinate time is measured in single inertial frame.

Brian , ( cara and dave are in the same inertial frame , train . difference in their times also gives coordinate time).

C) Brian . Space time interval is the measurement made by observer at rest so Brian

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