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So Much Fun With 'Causality Violation'

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Posted by Michael W. Pearson on December 19, 2004 20:36:32 UTC

Hi Dr. Dick
This question is submitted humbly in good humor:

In your opinion, is "disentanglement" a term for the disentangling of
1) assumptions or 2) a later stage in the explanation of phenomena?
Mike
Causality violation in General Relativity
quoted from http://www.physics.fsu.edu/users/ProsperH/AST3033/relativity.htm
In 2000 Betty sets off to visit a nearby star. Unfortunately, the severe acceleration of Betty's spaceship as she hurtles away from Earth causes a tachyonic shockwave to propagate to another star that is also 8 light years from Earth. Let's suppose that the shockwave arrives at the other star in 2006, Betty's time; that is, coincident with Betty's arrival at the first star. And, alas, the shockwave destroys the second star.
This shockwave propagates into Betty's future, from 2000 to 2006. Nothing odd about that. But the star's destruction is coincident with 1997.2 on Earth. Therefore, from Ann's perspective the shockwave travels from 2000 to 1997.2; that is, backwards in time!
Even stranger, the destruction of the star occurred before Betty set off on her journey! So in Ann's frame of reference, the star's destruction (the effect) occurred prior to the shockwave (the cause). Since the star is 8 light years away, according to Ann, the light heralding the star's demise will not reach her, on Earth, until 1997.2 + 8 = 2005.2. We have engineered a causality violation in Ann's frame of reference: that is, we have arranged for the effect to precede its cause in at least one frame of reference.
Suppose a civilization near the star witnesses its destruction. They send an emergency tachyonic message to Earth that reaches Earth, say, in 1998 before Betty's departure, warning her to forgo her journey and thereby avert the accidental destruction of the star by the shockwave.
Betty acts on this message and abandons her trip; no shockwave occurs and therefore the star is not destroyed. But then since the star is not destroyed no warning message is sent, Betty does not abandon her trip and consequently the star is destroyed inadvertently. We have arrived at a logical contradiction!

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