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1

Imagine a 1d space and a little vector in it that can change its location but not its length. It can't continuously change in a way that turns it around. If that vector was the momentum this says in 1d you can't continuously change you direction without your momentum being zero. A similar thing happens in relativity. A massive object has a location in 4d ...


2

Considering 'space-time', rather than space and time, time is essentially just a 4th axis along which we can move (generally at a rate of one second per second in the positive direction); the other three being the familiar x, y and z Sending something backwards through time (if possible) would not involve destroying energy in the present and creating it ...


15

Conservation of energy/mass is the result of a symmetry called time shift symmetry, and if this symmetry is broken energy/mass will no longer be conserved. It is far from obvious that time shift symmetry would be preserved if closed timelike curves were possible, so you can't use conservation of energy as an argument that time travel is impossible.


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Most time machines seem to require having exotic matter which has negative energy density, so perhaps you can send back some negative energy and some positive energy. Creating mass isn't an issue, mass isn't conserved and really that's because the mass of a system isn't the sum of the masses of the parts. And there are other options. For instance, if you ...


1

Sometimes you get a time machine when you were not trying to get a time machine, such as with a wormhole or a FTL transport. The FTL only makes a time machine if space is finite or if you use multiple transports so since the time travel is accidental and involves how multiple parts connect together it isn't obvious how to break it into a before and an after ...


3

The Novikov consistency principle is just an expression of how it is assumed that the laws of physics still apply : even in situations where spacetime can be twisted into having closed timelike curves, you still have to solve differential equations stemming from some initial conditions. And ideally, we want those to be continuous and smooth. Also, we cannot ...



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