That’s right, as I understand it. The least unlikely way to travel through time is to use a temporarily displaced wormhole, so that you can travel through distorted space-time to another location. Wormholes are probably quite massive, maybe as massive as a black hole of similar radius. So you don’t have to worry about rematerialising in the middle of a wall or anything.
All of them.
Either there is some sort of “rest frame” that comes with sitting in a gravity well that will glue you to the surface as you pass through time, or you have to take into account all motion. It’s epicycles all the way down.
Depends on what type of time travel you are using. Hard to say how it would work, other than it wouldn’t.
I think we are in agreement. If the goal is to travel in or through time and stay alive, it would not work. For several reasons, many of them practical, not fundamental.
But it is a nice thought for day dreaming on a hot evening in the middle of a pandemic.
What I had in mind were “closed timeline curves”, which are features of certain solutions to the equations of general relativity. The idea is then, assuming the existence of such a curve passing through where you are now, there is nothing stopping you from taking your rocket ship and following the path back to your own past. Which seems like it ought to be problematic on some grounds… but at no point did you “apparate”; you follow a continuous time-like “world line”.
I wish for you your rocket finds a nice landing pad at the end of this closed timeline curve. But I like this sentence in your link: “Others note that if every closed timelike curve in a given space-time passes through an event horizon, a property which can be called chronological censorship, then that space-time with event horizons excised would still be causally well behaved and an observer might not be able to detect the causal violation.” I read that to mean: will not work, but if it does, nobody will (be able to) notice.
It makes more sense to think of it like the portals in Stargate SG1. You go in one end and come out of the exit that is located in some other time and place. If the exit is in your living room then it moves along with your living room, avoiding the whole problem. The time machine would be something you travel through, not something you take with you, so you can’t go back to before it was built.
Which works fine if
- that living room exists at the time you wish to travel to
- the furniture hasn’t been moved
- someone doesn’t happen to be sitting or standing in the futurely empty space you wish to travel to.
Given the realities of constantly moving matter, I suspect a working time machine would be indistinguishable from a suicide machine (like Star Trek’s transporters).
1, If you’re travelling into the past then you know the living room was there. If you’re going into the future then you just have to risk that the exit could have been moved, or the living room demolished around it, or whatever. Regardless, you come out of the exit, in a defined place, because if the exit has been destroyed then you can’t go through the portal at all.
2 & 3, AIUI, in reality the portal would not be like a door you walk through, but more like a globe you’d travel through in a spaceship. So yeah, if there was something sitting in front of the exit when you came out, you could collide with it. If matter can go through then presumably light can, but I’m not sure exactly what you’d be able to see through the portal before entering and while travelling through it. If you see a warped view of the other side then you should be able to avoid collisions.
Going into The future? That sounds about as possible as reading a book that has yet to be written.
It’s pretty trivial if you can travel at close to the speed of light, just by time passing slower for you than for everyone else. But that method doesn’t allow you to travel back to the past again.
I guess if you prefer the many worlds interpretation you are travelling to a future, but then we’re all doing that all the time anyway.
There is a world of difference between reading a book as it is written, and reading it before it is written.
Maybe when you travel in time to the past all of the matter and energy that make up you go back to their original places and states that it was in at that past time in space time. However if you go back ten million years all of the particles and energy that exist in you now would be scattered all over the planet and even some of the energy that exists in you now would be in the sun. If you went back just ten years, because of the way consciousness and memory work, you would not even be aware that you went back in time. Maybe we travel back in time all the time. We just don’t know it, And no paradoxes at all. I think I will make some coffee and ponder something else the rest of the day.
The multiple worlds theory is good for answering the question “If time travel is possible why haven’t we been visited by time travelers from the future?” The answer is that we exist in the original timeline before time travel was discovered/invented.
The multiple worlds theory also gives one possible mechanism for explaining the ‘Chronological Protection Conjecture’, which postulates that the universe hates time travel, and stops it from happening whenever it can. In a multiple worlds scenario, those timelines which include reverse causality are chaotic, and events repeat themselves over and over again with slightly different outcomes until you get to a timeline where the time machine breaks down, thereby creating a stable timeline with no causal loops.
We may not be in a universe where time travel has never been invented, but one in which it has been uninvented for the sake of cosmic sanity.
There is such a conjecture, due to Stephen Hawking (and there are some arguments that a putative time machine would explode), but if there is a good definitive proof, I would love a link to it. It definitely does not consist of waving one’s hands while mentioning “many-worlds theory”. The latter is merely an interpretation of simple quantum mechanics of the type taught in first-year physics classes. It says nothing about the laws of quantum gravity necessary to resolve such conjectures.
Yeeah. The formation of a CTC also creates a Cauchy Horizon which prevents causality infringements, yada yada.
But I was actually quoting Niven’s Law, a sci-fi concept that some may not have heard of.
NIVEN’S LAW: IF THE UNIVERSE OF DISCOURSE PERMITS THE POSSIBILITY OF TIME TRAVEL AND OF CHANGING THE PAST, THEN NO TIME MACHINE WILL BE INVENTED IN THAT UNIVERSE.
(…in other words, the universe will keep changing until it reaches a stable state, and that state is one where no time machines are invented)
See
http://www.projectrho.com/public_html/rocket/timetravel.php#id--Time_Travel_Theory
On that page Winchell Chung has listed practically every possible permutation of time travel theory, including the ones that are clearly nonsense (i.e. most of them); and if you want to check if your own ideas have already been thought of somewhere, this is a good place to start.
That guy reads a lot of pulp fiction 
Nowhere on that page do I see a sketch of any theory of quantum gravity, though. Just saying…
Sci-Am has a fascinating and disconcerting piece on QM which reveals that there actually are physicists who are “fans of retrocausality”.
Granted, they are talking in terms of experiments and observations in that brief and tiny place where the cat is both alive and dead, but it does sound a bit like a foot in the door, as it were, with respect to the idea of time travel.
I think that’s equivalent to nothing happening at all