Now we have heard time-machine and time travelling so many times but we have never found an implemented machine/way for practical use. But time-machine can never be a reality as per the following points: 1. Speed of light and Gravity. As it is proven that time slows down when we travel nearer the speed of light. So time manipulation is possible by manipulating our speed. Also Gravity affect the space-time hence the object should be of extreme gravity to distort space-time , human does not have that much of mass to attain that extent of gravity. 2. Law of entropy

The thermodynamics law of entropy says that entropy (i.e. nothing but randomness/disorder/forwardness) is always increases. So that implies that a hard boiled egg after boiling can never reverse the denaturing, the egg inside will remain cooked forever and it cannot be reversed.

So, based on what concept or rather "theories" the time machine can be made to work/develop in future?

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    As this is written, it should be answered by physicists, no? There are closely related philosophy questions, but this one, as you have asked it is purely about how such a thing might be constructed in a way consistent with current theories of physics. So at the risk of seeming hypocritical, I am both going to give an answer, and vote to close the question. – user9166 Aug 10 '17 at 16:37
  • There is a more basic problem, the time travel proper where the past can be altered is logically incoherent. So we either get time travel that changes nothing because the past already incorporates the trip (see Novikov's principle), or an absurdity. The "concept" for something like sci-fi scenarios is not "time travel" but parallel worlds, a.k.a. "alternative timelines". SEP discusses potential physical realizations. – Conifold Aug 10 '17 at 21:04

One of the aspects of modern physics that makes time difficult to explain is that all of our current theories predict that things should be reversible, except for the second law of thermodynamics. This is broadly known as CPT-symmetry in physics, and results in a philosophical problem known as Loschmidt's paradox.

The problem is that this law is simply empirical, not a theoretical result of other theories. The detailed mathematics that holds it up, generally known as 'the fluctuation theorem' would work just as well if we consistently saw time going backward, and it does allow for time to reverse in regions of space with extremely high entropy. It just predicts this should be very uncommon.

Boltzmann's original way of looking at this was to predict reversals in time, and in fact to presume that at some point in time entropy would be maximized somewhere in space and time would in fact reverse there and this reversal would spread out to include the whole timeline. This has problems of its own, in particular, it favors the notion that the Universe never began, and we came into being as "Boltzmann Brains".

But ignoring that, since we can move forward in time and laterally in space, if the law of entropy reversed, we should be able to move to that place, at some point in our own timeline after the event of reversal there, and ride that timeline backward, then come back to our own region of space. We would not necessarily end up in a part of space-time from which we could get to our own past. But we would in fact have gone backward in time.

(Of course, in such a chaotic place, we would probably fall apart, and if we suddenly reversed all of our energy-exchange processes, we would probably not be able to function or control anything -- the physics for which all of our matter is evolved would no longer work the same way. So having an object, much less a person, survive this process is likely to be impossible.)

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