# Is time dilation an illusion? Variation on the twins paradox [duplicate]

Consider the twins paradox with a slight variation:

Twins A and B are in separate space ships both capable of going at the speed of light instantly (i.e. without any acceleration). Both ships are stationary relative to each other in intergalactic space facing in opposite directions. They synchronize their clocks.

Then Twin A sees ship B zooms off to the "right" at the speed of light, and ship B travels a round trip of 8 years. Twin A sees ship B recede away from him at the speed of light (in fact, ship A just disappears).

When ship B returns, Twin B's clock will show he has been gone 8 years. But from Twin B's perspective, it's ship A that zooms off to the "left" at the speed of light, does an 8 light year round trip, and according to Twin A's clock, he has also been gone 8 years.

So they both would agree that one has been away from the other 8 years, and both have aged the same amount of time.

It would seem clocks do not actually run slower as they move closer to the speed of light. So is time dilatation just an illusion?

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## marked as duplicate by Brandon Enright, Alfred Centauri, Valter Moretti, John Rennie, Waffle's Crazy PeanutMar 6 '14 at 9:33

possible duplicate of Is time dilation an illusion? – Nick Stauner Mar 6 '14 at 0:42
Don't start a question by assuming the impossible. Just don't. In any case saying "they don't accelerate" and then claiming that they do change speeds shows a basic misunderstanding. And as always, if you draw the world-lines diagram you can sort this out. – dmckee Mar 6 '14 at 0:56
This is not--in fact--a variation at all. It is the same old problem with the same old answer, only Peter has tried to avoid the usual answer by misrepresenting what acceleration is (i.e he has introduced an infinite acceleration and claim that it is not acceleration). – dmckee Mar 6 '14 at 1:00
whether there is acceleration or not does not influence the Twins Paradox. There are two frames of reference Spaceship A and Spaceship B. They are moving Relative to each other. whether A is moving or B is moving is equivalent. – Peter Mar 6 '14 at 10:14
Peter, there are three inertial frames in your setup. Frame A, Frame B (on the outbound leg) and frame B' (on the return leg). The fact that the ship B does not remain at rest in a single frame for the whole experiment is the origin of the difference. – dmckee Mar 6 '14 at 15:12