I think all the models for a time machine that I have seen involve moving in a defined way through spacetime. Eg if you use a wormhole you end up whereever the exit is positioned, so if the wormhole exit was in orbit around the sun, you could predict where you would emerge.
You joke, but it really does help, by many orders of magnitude. I’ve read serious scientific papers on the topic. The most efficient designs for time machines (that is to say, the ones that require the least absolute amount of apparently-impossible substance) are, in fact, bigger on the inside than on the outside.
In the ‘many worlds’ cosmology model every time a random event could happen new timelines are forked out where each possible outcome occurs - but in each of those timelines, only a single outcome did occur. This means that each timeline itself only has a single outcome, and ends up looking like a deterministic timeline where specific outcomes occurred at every point.
This isn’t surprising, because timelines have to be deterministic in retrospect, otherwise the past is fluid like eschereal seems to be suggesting, and tomorrow I might wake up to find that my grandparents suddenly died of polio ninety years ago and I was never born. (Which would put a real damper on the weekend.)
Of course, when you’re talking about time travel you’re not actually talking about the many-worlds theory, where there are billions of new timelines forking off every millisecond due to the random actions of subatomic particles. You’re talking about the timeline changing becuase a bloody great big time machine just appeared in your living room yesterday. This is a qualitatively different thing.
When it comes to time travel, I picture history as a strip of videotape, where the content of a single ‘frame’ of the tape is the entire state of the universe at that given instant. And given that framing, there are only three (four) different time travel models.
A) Time travel doesn’t alter the tape. When you travel in time you just wind to a different point on the tape and pick up there, with the ‘you’ that had always been recorded at arriving at that point.
B) When you travel in time you wind to a different point in the tape and start rewriting it, changing history. Technically the entire timeline going forward from your arrival point gets changed; if you travel back in time and then return, you’ll land in a ‘future’ where your past time travel is now part of history when it wasn’t before.
C) When you travel in time, a new tape forks off that starts at the point of your arrival, leaving the previous tape unaltered as a sort of backup. This is the ‘many worlds’ model of time travel, and exists mostly so that the causality pedants can claim a timeline containing your birth still exists.
It’s worth noting that all three of these timeline models are completely immune to time paradoxes. In A you can’t kill your grandpa, and in B and C the timeline doesn’t care if you kill your grandpa - B essentially destroys all future history every time a time travel event occurs and doesn’t care, and C has a backup and doesn’t care what you do to the new timeline you made.
The thing to note about model C is that every time travel event forks off a new timeline - every time you travel to a point of time you replace the timeline where your time machine wasn’t sitting there with one where it is. This includes your trip back. You can never return to the timeline you left - in that timeline you simply disappeared forever. (But there will be a new timeline where you appeared out of nowhere, so it balances out.)
This brings us time travel model D - the narratively convenient nonsense timeline. This covers cases where…something…is trying to ‘preserve’ the timeline, so that when you come back to your ‘modified’ timeline you haven’t butterflied the world into being run by apes. The ‘something’ necessarily is sentient and aware of both timelines old and new - an all-powerful god capable of twisting events. Sometimes this god gets super-twisty and does things like have there be photos with transparent or cut-off people, despite there being no possible timeline where they looked like that.
While fun, type D timelines are narratively incoherent. And of course nobody like B and C because you can’t go home again. Which means that the best time travel model is A, and Bill and Ted’s Excellent Adventure is the best time travel movie ever.
I think you are missing something here.
Many worlds does not suggest another timeline might exist. It suggests they DO exist. All of them. At once. All possibilities exist simultaneously. You may choose eggs for breakfast, or toast and jam, or a doughnut, or go to the gym. Is it “deterministic” to say you actually did all of those things and more?
If you time travel you are just bouncing among already existing realities.
Yes, it is.
Consider a universe where there is exactly one point of possible variation in all of history. Just a single one. In this universe the timeline chugs along until it reaches that moment - and then forks into two completely separate timelines.
In timeline A the dice landed on 6. In that timeline it always landed on 6. Timeline A is determined to be the 6 timeline.
In timeline B the dice landed on 2. In that timeline it always landed on 2. Timeline B is determined to be the 2 timeline.
Viewed separately, each timeline contains only determined outcomes - it is a deterministic timeline.
Viewed collectively, the many worlds are…a collection of deterministic timelines. Among all the timelines collectively every possible outcome is represented, but each individual timeline contains only one possible outcome at each decision point.
(There’s really no escaping determinism. Best to adopt a compatiblist mindset now - it’s way better anyway.)
Like I said, the time travel timeline model is distinct from and doesn’t really overlap with the many worlds model that comes from the determinism/nondeterminism debate or physics.
In physics-style many worlds, every possible timeline exists. Every possible variation of outcomes is represented in a distinct timeline. However, a great big time machine appearing out of nowhere isn’t a ‘possible’ outcome, based on the state of things taking place immediately prior in the timeline. If time machines are appearing, it’s because of something else entirely.
The best factual answer to the OP is simply “nobody knows”. Among physicists there are some that do indeed suppose that the contradictions rule out time travel e.g. Hawking’s chronology protection conjecture.
But other theoretical physicists suggest ways in which the universe may allow time travel and not be self-inconsistent e.g. the many worlds hypothesis (not introduced only as a solution to time travel paradoxes, it just might happen to be a potential resolution to them).
(going slightly off-topic)
I suggest that that might just be a matter of framing. Regardless of whether the universe is deterministic, what we mean by “choice” is taking in prior information and using your personality and rational thought to make a decision. This process is still how you make a decision in a deterministic universe.
I think the issue many have is that the idea of there only being one future makes them consciously feel like their life is on rails and none of their decisions are “real”. But that’s not what determinism is saying at all (that would be fatalism)
Furthermore, my opinion remains that “free will” is ill-defined. It’s not a fact about our universe that free will doesn’t exist; free will doesn’t exist in any self-consistent universe because the concept makes no sense.
This is crazy talk.
You are calling all possibilities deterministic because something has to happen.
Determinism suggest you HAD to have a doughnut for breakfast today since the moment the universe started.
It is not determinism to say all possibilities for your breakfast sprung into being when the universe started. You happened to choose a doughnut. You legitimately could have chosen eggs and bacon because that possibility also exists.
Think of it like riding on a train car and you come to switches in the universe. You decide which track you want to continue on and move on to that line. But the other line is still there and still exists even though you did not choose it.
The ‘you’ that you are experiencing exists in only one timeline, though. If your timeline is the ‘doughnut’ timeline, then you will choose the donut. There are other ‘yous’ that didn’t choose the donut, but they’re not you. You’re the you in the timeline with the donut. And that timeline is been the timeline with the donut from the very start of that timeline to its very end.
The many worlds model is not actually the model you want, by the way. You want the ‘reverse fuse’ model, where the timeline that exists (that is, the past) is fixed and unchanging, but one end is ‘growing’ - new time is being created that didn’t exist before. The new time it creates is also fixed and unchanging -more ‘past’- but the future is unwritten.
Unfortunately for you, though, you’re in a time travel thread.
When time travel is on the table, your present is somebody else’s past. To a time traveler from the future jumping backwards, your time looks like past time. And again, if we exclude fluid pasts where you could find yourself unborn at any moment, the past is fixed and unchanging: determined.
Many worlds is specifically a result of quantum mechanics.
If you solve the Schrodinger equations, then you get two answers.
Many try to make some sort of dodge and invoke collapse or decoherence to say that there is only one universe, and quantum mechanics will eventually reflect the reality of that one universe.
Many worlds is simpler, in that it just follows the pure math, without trying to make an interpretation that makes sense to our perspective.
But yes, the whole point of many worlds is that the universe is constantly branching. Your idea of a universe that already has infinite variations that are independently deterministic is not one that I’ve seen very commonly.
I don’t see any real difference between the two models, particularly from a time travel thread where you have to look at all time from the end backwards. In the branching model if you scan backwards from a point in time you get one continuous timeline with a whole bunch of fixed decisions that worked out a certain way. Same as without branching. In either case you can’t climb back up the ‘tree’ via a different path and get a different history that way; your past is your past.
And in any case, there’s no part of the physics-based many worlds model that allows for time machines simply appearing out of nowhere. The many worlds model is not an answer to time travel.
What I dislike about this is that (at least I think) it requires the number of existing parallel universes to be continually increasing at an exponential rate. I feel that there should actually be a ‘conservation of reality’ law - realities cannot be created or destroyed, but can only change form
The difference is whether or not they are deterministic. Many worlds is not considered to be deterministic by those who follow the model.
If you use a radioactive random number generator to generate a 0 or a 1, where a 0 means you have waffles, and a 1 means you have eggs, you will have no way of knowing what you will have for breakfast. In fact, at this point, you can say that you will be having both for breakfast, as you will be experiencing both.
Once you have used the generator, you are now in the universe where you are either going to eat waffles, or eat eggs, and that will be the one that you experience.
As far as time travel, I think that it is almost certainly not possible, but I see the least impossible form of time travel would be traveling to a universe that is identical to a past time in your universe.
Why not?
If the arrow of time is always pointed towards the future then the only way to “time travel” is to shunt yourself into a timeline where things happened as if you time traveled.
IF (note the big “if”) time travel is possible then timelines that include events as if a time traveler visited them have to exist.
That’s the reason that many don’t like the model. But, as they say, the universe doesn’t care what we prefer, it is how it is.
There are no conservation of reality laws that have been seriously been proposed.
The math says that you get more than one result from a particle interaction. The “reality” is that we only see one.
How to reconcile those different views has been a matter of contention for as long as we’ve had equations to describe the actions of particles at the quantum level.
Ultimately, it becomes a philosophical matter, as neither can be proven, but many worlds is much simpler and more elegant, often a plus when it comes to evaluating different models.
I you presume you’re traveling to a different universe, why would it matter whether it was identical or not?
I mean, it might be more comfortable if the atmosphere was oxygenated, but other than that, if there are multiple universes to be traveled to, why should some be easier to visit than others?
But what I am saying is that those frames are a fiction. Thiere is no (multi-dimensional) flat present moment, nor any flat past moments that could be framed. If you could get it all into one frame, only a plackesque fraction of the frame would be in focus, the rest in increasing shades of indistinction.
What is happening on Utopia Planitia Mars at this exact moment is undefinable in the context of right here. A moment is an amorphous blob of varying frames of reference that cannot be cleanly reconciled with each other.
What is known about the past, the information available upon which any given entity can act, is a drop of piss in the ocean, as it were. A molecule will respond to the presence of another molecule based on its vector, charge profile and angular momentum without considering how or whence it acquired those characteristics, and we are only slightly more sophisticated than molecules.
I am saying that information is the only form in which the past exists, and what can be discerned is always, intractably, incomplete. A huge portion of that missing information is simply not meaningful. What you had for breakfast today, for example, is of vanishing significance to me or several billion other people, and whether you had jam or honey on your english muffin is of almost no importance to anyone or anything.
I am claiming that the past is not inscribed, that what exactly happened is just not knowable or meaningful beyond the most immediate local effects. It is not so much that the past is fluid, per se, it just cannot be defined in precise terms, which makes it look fluid.
The reason it would matter that it be identical is for it to be time travel. Otherwise, it wouldn’t be time travel, it would just be travel.
I didn’t say anything at all about some being easier to visit than others.
To answer your question, even though it is not related to my post, it could be said that traveling to a universe is easier the closer it is to yours, but that would be absolute conjecture based on less than even a hunch, as it is not known how to do such a thing, other than it being pretty sure that you can’t.
Because the many worlds model of physics is one where the various universes are the outcome of random particle actions. Random particle reactions are not going to be make large metal rectangular prisms (police boxes, phone booths) appear spontaneously out of nowhere. And with time travel, what you have is entire time machines (and entire people with memories of a whole other life in a different time in their brains) appearing out of nowhere.
The theory was that when the Greeks gave up and left, they left some diseased horses on the beach. After a ten year siege, the Trojans saw that the Greeks were gone and had a huge barbeque.
There was a storm and the Greeks returned for shelter and found the Trojans drunk and suffering from food poisoning and unable to hold the gates.
And I’m saying that if the past isn’t fixed, it would have consequences that materially change the present day, because whether you know what happened or not the past has consequences.
I’m here because the past happened the way it did. If the past was fluid, the present would not exist in a fixed state.