First, the concept of time travel is one of those fun parts of physics. Whether true or not, it is entertaining to play the ‘what if’ game. If nothing else, the concept makes or forces one to think about the nature of reality.
Secondly, Einstein and others have postulated that time travel is a theoretical reality and I’m not in their sort of league that I can dispute the theories. I’ll leave that to others who know the field inside and out.
But thirdly, and most importantly, you can never actually be in the future or the past, only in the future or the past compared to where and when you are now. In other words, no matter how you slice and dice things, you exist in the where-ever and in the whenever in that where-ever’s or whenever’s NOW or in other words in the present. You cannot literally be in any future or in any past since you only experience the NOW which is the present. If you should somehow travel back one hour, you would still experience things as belonging to NOW. If you sleep for one hour then wake up, you are in the future relative to when you went to sleep, but you still find yourself in the NOW.
Is time travel possible?
The answer is both yes and no. Yes, we can travel into the future at one second per second, we do that anyway whether we like it or not. Yes we can travel into the future at a slightly quicker rate by going to sleep or otherwise having our sense of consciousness, our awareness of rate of change (which is what time really is or measures) incapacitated. You get drunk and pass out and the next thing you know you are 12 hours into the future. Yes we can travel into the future as outlined by Einstein’s twin ‘paradox’ where one twin travels at a very high rate of speed outward bound, stops and returns to home base, while the stay at home twin, well, stays home. Upon their reunion the travelling twin finds their stay at home twin to be far older, so the travelling twin has travelled into the future more rapidly than would otherwise have been the case. Yes, you can travel back in time, in theory, according to the apparent theoretical properties that wormholes or black holes can have. No, you can’t travel to the past because of all of those nasty paradoxes. I like the variation on the grandfather paradox whereby you travel back just one hour into the past and shoot yourself dead. That’s a novel way of committing suicide! The other paradox I like is when you go back in time to have Shakespeare autograph your copy of “Hamlet”. Shakespeare isn’t home but the maid promises to have him autograph your book when he returns. Alas, your timing is slightly off and Shakespeare hasn’t yet written “Hamlet”, so when he receives your copy from his maid to autograph, he reads it, and after you return to Shakespeare’s home and receive back your now autographed copy and return home to your own time, Shakespeare now writes “Hamlet”. The paradox is, where did “Hamlet” come from since Shakespeare only wrote it after he had already seen your copy. No, you can’t travel back to the past because if that were possible there would be hoards of time-travelling tourists who went back in time to witness some important historical event or other. No hoards of photo-snapping tourists have ever been documented being present at Custer’s Last Stand, the Battle of the Alamo, the sinking of RMS Titanic, or any one of thousands of similar historical events. Yes, you can travel back in time but only into a parallel universe. If you shoot yourself but it is another you in another universe, no paradox arises. You travel back in time to have Shakespeare autograph your copy of “Hamlet” but in that parallel universe Shakespeare can now write “Hamlet” based on your copy and no paradox results. However, the one point I find interesting is that if you end up in the future, or in the past, are you really in the future or the past? No, the only time you can exist in is the present, your right here and NOW time. It might be a different time from what you previously knew, but still wherever and whenever you exist, you only exist in the NOW. Is time travel possible?
It could already be the case that time travel has been documented at the quantum level although that could be open to interpretation. Before I get to the specifics, I just need to point out that with respect to the laws, principles and relationships of physics, time is invariant. Operations in physics remain invariant in time whether time is moving as we normally perceive it (past to future) or back to front (future to past). For example, gravity would operate as per its normal grab-ity self in a world where time flowed backwards. There’s many an operation one could film that when the film were run backwards, one wouldn’t be any the wiser. Tree branches blowing in the wind comes to mind, or the coming together, collision, and rebounding or separation of two billiard balls. Okay, having established that when it comes to physics, physics doesn’t care which direction time is flowing, there will be no violations in those laws, principles and relationships of physics future to past, we now come to the delayed double slit experiment.
In the normal double slit experiment
You have an electron gun that fires one electron particle at a time, such that one electron completes its journey before the next one is fired, at two side-by-side slits. If one or the other slit is open, the one-at-a-time electrons pass through the open slit to a detector screen behind the slits. The detector screen gets hit in nearly the same spot every time after each and every electron particle passes through the single open slit. That is straight forward. If both slits are open, the electron shape-shifts into a wave (how I don’t know), passes through both slits (as only a wave can), morphs back into a particle and hits the detector screen. The difference is that after enough electrons have been fired, and have passed or waved through the double slits, the hits on the detector screen are not in just one or two spots but all-over-the-map, albeit all-over-the-map in a classic wave interference pattern. Okay, that’s the classic experiment.