Richard Wolfson’s “Simply Einstein” is a book I would recommend very highly, but even in this work it is possible to find things that don’t seem to add up – at least to me.
Lets consider three scenarios:
1. (Passengers in an aircraft): Wolfson asks: “Am I moving?” He then answers the question as follows: “By looking out of the window you can justifiably assert that the plane is moving relative to Earth, but that is as far as you’ll get. You’re just as correct in asserting that Earth is moving relative to the plane.” So far, so good; this is exactly what relativity tells us.
2. (The twin paradox, involving a return trip from Earth to a distant star): Wolfson argues that because the occupants of the space ship experience acceleration, whereas those on Earth do not; one can say that the spaceship is moving relative to the Earth, but not the other way round.
Is it just nit-picking to insist that the same can be said of the aircraft in the first scenario?
3. (Spaceship passing Earth in uniform motion): Wolfson explains that because both Earth and spaceship are in uniform motion, each must be able to claim to be stationary, relative to the other, and both must observe the other’s clocks to be “running slow”, relative to their own clocks.
Apart from ignoring the fact that the space craft must have accelerated at some point, relative to something; this does seem to leave one question unanswered. Suppose that when the spacecraft passes Earth, there happen to be two people, one on the ship and one on Earth, who are exactly the same age. When the ship passes a distant star, some time in the future, will there be any difference between their ages?
