Not to put too fine a point on it, but the present by definition does not exist in a fixed state. It is in a constant state of flux, chewing up the future to spit out the past.
Yup. This explains it well.
Worth a watch (short):
I missed the edit but also remember that, inside the event horizon of a black hole, space and time switch places. Granted we cannot see in there and it cannot affect the outside universe but physics lets things get that extreme.
A friend contends that the continued existence of Facebook is proof that time travel will never be possible.
I always figured that we know time travel is not possible otherwise we’d see a few million or billion people appear in the 1900’s trying to kill Hitler.
Robert Charles Wilson’s novel “Last Year” postulated a time travel tech that established a bridge between the present era and the past, but the past was that of an alternate reality created at the moment the bridge was established. The other side of the bridge came out in nineteenth century Ohio and one crosser decided to travel to Germany to kill Hitler’s father. https://www.amazon.com/Last-Year-Robert-Charles-Wilson/dp/0765332639
Yeah, about that…: Wikihistory - Reactor
By the time (heh) we invent time travel, Hitler will be distant history, and there will likely be more recent and more vicious figures to thwart.
Also, most theoretically possible time travel devices can’t take you back before the device was created as far as I know.
All this discussion of paradoxes and violations of common sense feels too far divorced from actual physics. Could we start with something like a solution of Schrödinger’s equation where a particle travels backwards through time and interferes with itself, or explain why such a system is not possible?
If you have a universe that includes time travel, then the many-worlds concept can help explain the inconsistencies. Here’s the scenario. You live in a universe with no time travel, but you know how to do it. You then successfully create a wormhole loop that allows travel back in time, and you start to interact with the past. You are now busily creating new many-worlds which didn’t exist before, and which are all deterministic, even if they are affected by events that happened later in the timeline.
But the history of these many-worlds is different to the one you started in, before you built the wormhole; that timeline did not include a wormhole because you hadn’t built it yet. How can this be? There was a different timeline before you built the wormhole, with no wormhole it it, and you know it exists because you have cellphone footage of it.
In a many-worlds universe, this original universe with no wormholes is easy to explain. Because of some random quantum event, like a radioactive element decaying at the wrong time, there exists a whole sheaf of universes where you failed to create the wormhole in the first place. You were in one of those universes when you successfully created the wormhole. So by attempting to create a wormhole back into the past, you created a whole new set of universes with backwards time travel, but in some of those universes you failed, thereby allowing the existence of the original timeline with no wormhole.
Backwards timetravel in a universe with many-worlds is a nightmare - I think it is possible you may never visit exactly the same timeline twice.
Novikov consistency is a lot simpler, but also a lot more boring. Not as boring perhaps as the universe with no time travel at all, which seems to be the one we are in. For now.
‘Many worlds’ is merely a name and/or interpretation of a linear combination of states in basic quantum mechanics. So I still say we cannot do without a quantitatively worked out example here.
By the way, it is probably best to discount “sci-fi” interpretations— for instance, consider the recent article where a simulated quantum system was evolved backwards in time, then disrupted, then run forwards again. Surprise, there was no “butterfly effect”.
Yes, I read that. But the model they were using may have been incomplete, as so many are.
With a precision of how many centimeters? How would you like to apparate? On your sofa, in your sofa (with a spring though your brain), halfway through the floor or stuck on a wall? And what speed do you wish to have at that moment, and in what direction? Because that varies on the surface of the Earth depending on your latitude: being smacked at your nearest wall or by your nearest wall at 10 mph is an inconvenience, but at 300 mph it becomes a hassle. And the Earth moves much faster than that any given moment relative to the Sun, but in the opposite direction six months later. The rounding errors keep on adding up and I still do not know wich is the right frame of reference to calculate the coordinates of the apparition.
If you need to take all that into account, then you probably need to also take into account the Sun’s orbit around the galaxy, as well as the galaxy’s proper motion.
My theory is boring. I think some sort of time travel might be possible but not in the way we generally think or write about it. I think most “timelines” are REALLY short and that would be a serious limitation on time travel.
I could see a technology for fast and lightweight commuting equipment that achieved speedy answers to complicated problems by sending the answers back in time. But that technology is never going to work in a way that gives you the answer before you ask the question. The information is traveling backwards on a timeline that started when you asked the questions and that’s the limit to how far back it can travel.
Boring, but it avoids the paradoxes.
It seems misleading to think of time travel as “apparition.” The classical picture is that you make a seemingly normal trip, except space-time was distorted in some weird way so that you end up at the same time and place where you started.
There was a short, short sci-fi story involving a very stubborn boy named Stevie who swore “he would stand here and not move!” The story then listed all the various velocities involved and ended with “Needless to say, little Stevie got what he deserved.”
Indeed. But which one is the correct one to take into account? You must make a choice, and only one (perhaps?) is correct.
Misleading? Not for me: I picture time travel as beaming (of Star Trek fame) in time. Because if it is not like that, how does it work? Do you take the time travel machine with you? Or does it precede you, and then you apparate inside it? Or do you move through time, like if walking? But then the space in which your goal is moves in most sensible frames of reference faster than you (and your machine) can move with chemical rockets, how are you going to keep up?