For Christmas a little walk ! :) But to travel in the past or in the future we must first well understand what is the time …
And in fact intuitively everyone has a pretty good idea of what it is, for example, we all know that if the time stopped suddenly, everything would become simply immobile at once. What we know perhaps a little less is that we could no longer see this « immobile » world, since light itself would be stationary and would not reach us anymore, but hey, that’s alright, and basically we all have a good « intuition » of what is the time… Because time is not so complicated, time is the movement of everything in the universe, if we « slow down » these movements (we’ll see how further) then time slows. And if the movements are « reverse » then the time go back. Finally, if movements are accelerated time accelerates.
There is another thing that should be noticed, and this is important because this is what will allow us to travel in time, it’s that the speed of these movements are not necessarily the same everywhere in the universe : the movements can stop only in a well defined area of the universe and continue normally elsewhere. Or they can go « faster » in an area, and then the time is passing more rapidly in it than in the rest of the universe.
So time is not « absolute » in the universe, this is not something abstract, that embrace the whole universe, almost « separated » from the universe even, no, the time is very concrete, it is the movements, if we stop them all, then we stops the time !.. Quite simply if I may.
And here it is our « means of transport » in time ! If we keep the « normal » time in a machine and we do go back all the movements of the universe (or only the solar system to remain modest :) ) then we « travel » in the past !
But many fantasies are related to travel in the past, to « meet oneself » for example … This is misunderstanding how the time travel in the past is working because as we have seen if a traveler wants to go back in time in a region of the universe then it must reverse all the movements of this area except his own… Indeed the traveler should not go back, otherwise he would not travel in the past finally, he would return with the entire area (and not even being aware of what happens…). So the « matter » of the traveler, inside the machine continues its normal path in time, but it means that this « matter » of the traveler, is no longer « part » of the universe that goes back outside the machine…
And that’s extremely important to take into consideration because that means that this area go back into a different past from what it was ! a past without the « matter » of the traveler… To well understand an example : imagine that the traveler just before boarding his machine had a discussion in a bar with a friend. At the beginning of the trip all the matter in the area of the universe comes back, so the friend « heads » backwards towards the bar. The traveler does not heads backwards to the bar, he’s in the machine, he’s excluded from the area turning back. When the friend arrived at the bar he do not encounter the traveler, the « matter of the traveler, » as it is in the machine… So he talks to alone ?
No, he’s not, he « will » not speak « anymore » we can say, since he has no more interlocutor… Even if the universe evolves in reverse direction the rules of the interactions of matter continues to be be respected, they are just in reverse : for example, if an object had bounced on another, if we remove one of the two object and backwards this area, the object that remains does not rebound alone in this « new past », he continues his course, in the reverse direction of course, but this time without bouncing, so it goes in a different past from what it was, because the matter in presence itself is different.
More generally, excluding a part of the material from one area of the universe that comes backward is a butterfly effect « in reverse » in that area, the « reversed » events will change more and more compared to what they were as the « initial difference » will spread, and gradually all the events « will » be different from what they had been.
So what about killing a « parent » !? it makes us disappear suddenly ? Let study this case : a traveler travel back in time, in fact we saw, we should say instead « all the movements of a region of the universe return back except those of a traveler. » The parent goes back, but in a world WITHOUT the traveler who is in the machine. As we saw, gradually interactions that the traveler made do not occur in reverse as they had been made by him and this leads to a different past. To such an extent that even the mother does not gives birth to the child !! Indeed the matter of the traveler did not come back, and thus he did not become back a baby, and her mother is not « going » to be « pregnant » of the traveler because the baby did not « come » back… She will do something else :) … All the « parents » of the traveler are gone back, but in a past that ended up to be completely different from what it was when the traveler was there. In short, if the machine stop and the passenger kill any of its parent, well nothing happens, he or she does not disappear .. (he just goes to jail because you can bet that the police of that time will not believe his history !! :) )
OK, enough with theory, now to practice ! :)
So, to travel in time in a region of the universe, we must influence the movements in the area, slow down, speed up, or even reverse… But how ?
One of the central « parameter » of movements, so of time, is inertia. Inertia is the resistance to the speed change of an object, the acceleration or slowdown. Usually we are not very aware of inertia because it is proportional to the mass and generally we are moving relatively light objects, but for example you can feel it well when you have to push someone on a swing, especially if the person is heavy. Inertia, proportional to the mass, is the « center » of the way movements behave and therefore inertia/mass is in the « center » of how time passe.
Then, one of the central problem to solve to travel in time, to change the mouvements of one region of the universe, is to change the inertia in this area.
There is two possibilities : either we change the inertia of an entire area and the passenger keep his, or it is the traveler who changes his own inertia in its machine and the entire universe keeps his. We see immediately that the difficulty with the first solution is that you have to influence the inertia of an entire region, large enough (about a solar system let’s say), to increase it or decrease it compared to a traveler who would keep his… In practice it is a little difficult to implement…
On the other hand influencing the inertia of a confined area is much easier to imagine… If we were able to increase the inertia/mass in a machine that would effectively make harder all mouvements changes, for example pushing at the same force the person on the swing would move him more slowly. All movements change in the machine would be more « difficult », the time would be « slowed down ». And since outside of the machine in the world, time always passe normally, well this is it ! we travel in the future !! :) .. The traveler in the machine see everything in the world moving very quickly, and seen from the world, in his machine, he seems almost immobile.
But beware, as for the trip in the past we do not travel in the same future as if we had participated in this world, inevitably, because we are in the machine, so we are away from the world, we have no influence on it, and it evolve without us. But still, it evolves « in the future » relatively to the traveler, this is the most important, and we will arrive 20 years later discovering new technologies, and of course people will have 20 years more than us he he :)
At the opposite if we reduce the inertia in a small area, in a machine, all movements will happen faster, and thus the time passe faster than in the universe.
There it is :) In conclusion, I think that going back in time to the past, even in a small area, is almost certainly, and unfortunately, impossible, and even if it was possible, we saw that it would be under certain conditions, for example in any case it wouldn’t be instantly, it would take « some time », a « backward time », the time to do EVERYTHING « move » in the area back, in any case a « door » that would project us instantly years back could not exist, it would simply misunderstanding the time itself to imagine that way of traveling back in time.
On the over hands traveling in the future is probably possible, by just « simply » increasing the inertia/mass of a traveler in a machine as we saw, time of the universe would passe normally and the traveler would remain « standing still » so he would travel in the future. Applications can be imagined : a new kind of fridge : the inertia fridge !! it stops the time of the food :) that’s the progress ! At the opposite you can also design an inertia prison, in this case we reduce the inertia of the « traveler prisoner » one minute in the world is for him one year.. prison sentences are executed right at the end of the jugement ! in five minutes :) But for the wine, it’s simply a revolution !! :D ;)