Time Travel and conservation of Energy! [closed]

As of physical laws we know today there is no way either to create energy or mass not to destroy it and it behold true for specific space time continuum (i am trying to not use term universe here) . So of someone travels from space time continuum to other wouldn’t the initial continuum be at loss of some energy or mass while the other will gain a bit? So does time travelling or even inter-universe travelling violates energy conservation?

2 Answers

This is somewhat speculative physics, but physicists studying General Relativity have considered wormholes as theoretical solutions to the equations. These would potentially allow time travel (see Kip Thorne’s book ‘Black Holes and Time Warps’ for a layman’s guide – Thorne is a leading expert in General Relativity research) and possibly inter-branch travel in a Many Worlds multiverse (see David Deutch’s book ‘The Fabric of Reality’). Deutch theorised that the solution to time-travel paradoxes was that any time-travel would arrive in a different branch, and thus allow you to do what you liked without any inconsistency. (The branches are causally connected, via their shared past.)

Technically, they do not violate energy conservation, although they would, as you suppose, allow large amounts of energy to pop up in the past. The reason is that with wormholes, you are connecting one place to the other with a tunnel of space through the wormhole. So long as you account for the energy coming through the wormhole, everything works fine.

Slightly more technically, energy conservation laws can be expressed in two ways: as a differential equation that requires energy to be conserved ‘locally’ at every point, and integral equations that require energy to be conserved globally inside regions. If you pick a region of spacetime, and add up the flow of energy across its boundary, it must add to zero. If you have a wormhole, you have to draw your boundary taking the wormhole into account, possibly with separate pieces of boundary at either end.

Although most physicists would regard wormholes as extremely speculative and unlikely to be possible – in particular, requiring a type of ‘exotic’ matter with a negative energy density widely considered to be unphysical – the maths doesn’t actually rule them out so far as anyone has been able to prove. They’re studied seriously for what they can tell us about the limitations and possibilities of General Relativity. (Not knowing of any reason ruling them out shows at the least a gap in our understanding we’d like to fill in.) And also, it has to be said, because physicists are often inspired by scifi and it’s a fun idea!

You might get more detail asking about wormholes and conservation of energy at the Physics SE. But Kip Thorne’s book in particular goes into the whole question in much greater detail and length than you’ll ever get from an SE question/answer.

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