Hello, I'm gonna ramble about the logic of literal past time travel. I posted this elsewhere but I'm wondering if I might get more interesting responses here.

Okay preface. This is not a scientific essay. I am not a scientist, I barely passed every science class I've been in and remember nothing from them. No sources will be cited, and all of these ideas come from my own thoughts. I do not claim to be the first to think of anything, however, I do not see others speaking on any of this. I am most likely just not looking hard enough, but I'd just love to see the thoughts of others here and the input of people who know more than me on the subject.

Preface over, I'm going to be going over possible complications and mechanics that would have to be answered for if one were to build a real time machine, at least in my mind. The general problem, I believe, is bridging the gap between the mechanics of the universe and our social constructs. If that sounds confusing, I will elaborate.

Social v. Material

Shout in your room right now "Teleport me to London on the 15th of March in 1963."

Nothing will happen. You look ridiculous. For one, the universe doesn't know what a "London" is, we made that word up, nor does it know what that date is because we made up all these terms for time, nor would it know the notion of teleporting. Beyond that, who would hear you? Your neighbors, roommates, perhaps. The universe? Who knows. It won't respond though.

So if your goal is to use a time machine to go back to London on the 15th of March in 1963, the main or at least significant hurdle to overcome would be figuring out how to translate those terms into something a machine can understand. You can't hack the universe if reality isn't composed of any language you comprehend.

Location

One possible problem with a teleporting time machine, a part of the last issue, is setting a location for it to move to. There's no "universal coordinates" for anything to ever understand, so the instantaneous movement would have to do more with distance ("Move 5000 miles in this direction") than a name ("Move to 8.436 at the Fart Quadrant"). Then after that, how could you ever tell it a direction? The Earth constantly rotates and the universe knows no ups and downs. A computer can move you up if it understood the concept, but could it adjust for the spinning Earth's movement, in real time and constantly? Even then, think, London in 1963 isn't in the same place it is now, at least not in a specific technical sense that'd get your nerd glasses broken if you mentioned it to a varsity jacket jock. Sure, it is the same place on Earth, but everything in the solar system and everything else around it is moving constantly. That is to say, even if you solved the other problems and went to where London is now in 1963, you could just end up stranded in the galactic void waiting for a planet to hit you in sixty years.

Time

How do you make a computer understand the concept of 1963 A.D.? You can't, it could read you the definition if programmed but it could not in any meaningful capacity take you back to that time just by the phrase alone.

Okay, so instead of asking it to drop you into a year, what about pushing you back? Asking it to go back 60 years from the present 2023 to 1963. Such a thing would require us to find and observe a measurable unit of time that can send you back in time a certain amount for each of the unit. The further back you want to go, the more of the units. Also remember, the present is constantly moving forward. You may have to make a system where the units increase for each second the time machine isn't travelling. Or not, you wouldn't need to care if you're not the type to fuss over the exact second in a given day you time travel to. It would honestly be a little dumb to, anyways.

No Teleportation

So what we can conclude from those first two sections is that a hypothetical time machine could not teleport. It would have to involve some sort of pushing back in time. You cannot tell something to drop into a coordinate, but you can workshop it to move in the direction of where you want. How that'd work is beyond me, at least at the moment. It's like telling your car to "be" in New York instead of driving to New York, you'd have to drive it or nothing would happen.

The one big exception that would make it possible would be if we were able to harness something beyond our current understanding like a wormhole or a fantastical portal or something.

Past

If you want to build a time machine, you have to ask yourself at some point, to what extent does the past exist? Going back to a past implies our very reality remembers or records. For all we know, it only exists as far as our memories and social constructs around it allow. Imagine you're playing a video game and you never save and you're at the final boss. You're at the final boss and you've unlocked everything, that proves that you actually played the game this far. However, you couldn't go to a previous part of your playthrough because there are no previous save states. Does the universe "save" anything? How could one go back to a past that does not exist in any meaningful way beyond memory and explaining the present? I'm saying, you can't go back if there's nothing to go back to.

Time Travelers

Something that must eventually be asked on the question of real time travel is: where are all the time travelers? The idea is that if past time travel were possible, someone in the future could invent time travel and make it evident that it's possible by going to the past. Perhaps it is possible and yet nobody was able to invent a time machine due to the doom of humanity as the last advanced civilization on Earth before we could even grasp the mechanics of such a device. Maybe we're in an objective present as opposed to a relative one where a future hasn't been recorded yet for any supposed time travelers to even come from, which if false would have a lot of audacity as a theory. Maybe it's stupid fucking science fiction novel bullshit where we haven't seen evidence of time travelers because of a secret time-keeping organization strictly regulating such a device and also trying to prevent paradoxes or whatever the fuck and doing their job really well, that one is my least favorite.

Relativity

I'm gonna do this one assuming you all know the gist of the theory of relativity and the idea that time is relative to where you are, though I very well may have no idea what I'm talking about. Imagine this, we have Not-Earth and Earth. Not-Earth is an inconceivably long distance away from Earth to the extent that it goes through time slower comparatively than Earth does, though the human consciousness on both planets experiences both at the same rate. Remember the hypothetical measurable time units? My question is, would relativity suggest the hypothetical time units needed to go back a given amount of years would vary on location, or would it just need the same amount regardless of location?

The Science

This section is going to be brief because I don't know jack shit. From what I've gathered, past time travel can be possible according to certain scientific laws. Knowing whether or not past time travel can be achieved for certain, at least as it seems to me, seems like such a fantastical thing to know that it has to be out of our grasp for a long time. I'm not gonna link the article so either google it or call me a liar, but I saw an article about how scientists successfully simulated sending a particle back in time through a quantum computer. That suggests nothing on the possibility of physical particles time traveling back in time though, only that a computer can show us fantastical things which I know already from watching videos of horse dressage. That's a joke, I don't watch that.

In Conclusion

So, time machines capable of backwards time travel, what a mouthful. Basically, if you wanted to make such a thing you'd have to think of something so immensely clever and beyond the imagination and science we currently have. If it's ever done I'd love to learn how they solve these issues, at least the parts I could wrap my head. Hell, maybe even science fiction writers know how to jump the hurdles, even if they lack the technical fortitude.

Hope this is a fun read!

Well......I have always enjoyed time travel films. I don't have any clever replies for ya sorry.

The concept of "miles" was also constructed by humans, too. I feel like it would be equally as difficult for a computer to understand like time and location.

I do have an affinity for sci-fi, and I like time travel. My favourite Futurama episodes involve time travel because I find the concept interesting. One of my favourite movies is Back to the Future, which is unlikely a coincidence. There's obvious issues with time travel, but I like the idea.

Nice idea for a thread.

I agree with a lot of your points, especially about the universe not having a memory of events. I think the biggest problem with time travel is we don't even know that time is a real, absolute phenomenon. Is my experience of a second the same as yours? (aside from the influences of time dilation or relativity) I'm inclined to think it's not. I think, at least in terms of physics, that the universe is like an etch-a-sketch - once something's happened, it's erased and there's no going back. But I'm open to being wrong about that.

And subjectively, as the observer? Well, just like we can take a bunch of drugs and find that the physics of space don't apply, we can probably do the same with time and traveling through it. But I don't think that's what you want to talk about in this thread.

School is a lot different as an interested adult. You should look into some local physics courses at a community college! I think you'd be intrigued by it all.

As for finding a way to "undo" every physical process which has occurred in a specific amount of time (without being affected by it yourself) is generally within the realm of science fiction, unfortunately, and for many of the reasons which you have already discovered.

Indeed. I'd also suggest philosophy or maybe metaphysics specifically, though most likely (I'm guessing) the latter would require some intro course(s) as prerequisites...?

Or, in any case, if you're interested in continuing education as an adult, as someone who's already graduated, or as someone whose purpose is just personal interest/enrichment rather than credit, then many colleges offer that sort of thing on a formal or informal basis. It's a lot less expensive than enrolling as a student; sometimes, if you're already on decent terms with a professor (or if they happen to be easy-going and relaxed about it anyway), you can simply reach out personally and ask to sit in completely unofficially at no cost.

As for time travel... I don't have anything to throw in that isn't well-covered in this overview: https://plato.stanford.edu/entries/time-travel/ - personally I incline to think it conceptually impossible (basically because of the grandfather paradox), but I haven't really looked at it in any depth.

Let me put something to you, as a totally hypothetical thought experiment. What if I told you there have been time travelers and I had met them. Or what if you came across a book in the non-fiction section of a bookshop which described encounters with time-travelers. What would your reaction be?

Or let's go further and say someone was seen walking around the streets of a major city in weird clothes announcing to everyone that they're from the future, what would the general reaction be to that?

^Tangentially related, but running through that scenario put me in mind of Hume's argument concerning miracle testimony. Roughly stated, in three steps...

(1) By definition, miraculous events are violations of the laws of nature, hence (as far as we're rationally entitled to believe) maximally improbable.

(2) If a human being claims with evident sincerity to have witnessed a certain event, there is always a non-zero probability that their claim is false (or true), perhaps a very low (or very high) probability that their claim is false, but never a maximal (im-)probability that their claim is false. (It isn't even clear how to make sense of the last option.)

(3) Therefore, when a human being claims to have witnessed a miracle, we can never be rationally entitled to accept their testimony.

And with that, "revealed religion" is done away with. (Despite the sarcasm, I think it is an interesting and important argument!)

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EDIT: Sorry, rereading this, I didn't write (2) clearly at all. Another way to phrase this whole thing is: "Whenever a human being claims to have witnessed something 'truly' extraordinary (i.e., maximally improbable), the less extraordinary (i.e., more probable) alternative would have it that they are an unreliable witness in this case."

As was pointed out in the opening post (the 'location' section) the practicality of travelling back in time is severely limited as there is no such thing as a cosmic coordinate and you'd probably end up in the void between stars.

It may be that our perspective on the issue is similarly limited however, because to us time seems only linear. If you were a heptapod who experiences past, present and future all at once, 'time travel' would be meaningless. I think there may yet be a lot to discover about the nature of time, beyond the scope of our own intuitive understanding of it.

This thread is proving to be as zany as time travel itself And I love it!

Agreed. On one hand, we experience only the present, while past is just a memory/record and future - an anticipation. On the other hand - the present itself is the infinitely short instant where the future becomes the past. Add to that the fact that our perception of reality isn't instantaneous - several nanoseconds have already flown by while our brain processes the external stimuli that comprise our awareness of the world around us, thus our perceived present is objectively already in the past.

To get any closer to the answer on probability of time travel, we must first get to the basics of understanding what time essentially is.

I would say this depends on the nature of perception. What determines what we perceive? Is it an objective, standalone reality that is common to all, or do we filter everything we perceive through our own thoughts, emotions, beliefs and our biochemistry? Experience itself is extremely variable, to the point where some people are convinced they've witnessed miracles, while others are convinced they're bullshit.

Even assuming an objective reality and the low likelihood of extraordinary events like time travelers, that's not what I was suggesting. Although it's kinda relevant. What I was saying was that if time travelers from the future are a possibility, the reason we haven't seen them could be that no one would have believed them if they told us they were time travelers from the future. Or at least, very few people. And if people in the future have any level of intelligence, they would be aware of this, and avoid time travel rather than haphazardly time-jumping into the past making fools of themselves.

Right, sorry for the tangent there, I was just tooling around with the idea that "What if someone told you they had encountered a time traveler?" might be analogous in a way to "What if someone told you they had witnessed a miracle?" - just suggesting that Hume-style misgivings with miracle testimony might be applicable to time-travel reports in a similar way. (Compare: "Which is less unlikely: that Jesus rose from the dead, or that the Bible is inaccurate in reporting that he did?" versus, "Which is less implausible: that time-travel is possible, has actually taken place, and has taken place in Bill's neighborhood; or that Bill's claim to have met a time-traveler may be dismissed out of hand?")

Not that Hume-style misgivings are necessarily decisive in the way he makes them out to be (I tend to doubt they are), but it seems an interesting exercise to explore his argument and try to decide on its limitations, or figure out how its reasoning fails.

Can folks here give some of their favorite time travel movies; tv shows if there are any tv shows about TT.
12 Monkeys comes to mind

ALso I always thought it was funny in Superman how he flew around the world backwards and made time reverse LMAO!

I like Terminator 2.

I was trying to think of an example of a causal loop. Maybe: John Connor sending his father back in time to secure the sequence of events resulting in John Connor's conception, birth, and survival into adulthood (to once again send his father back in time, and so on).

I think it fails in a lot of ways. If you think about every paradigm shift in the history of the world, of science, how many of those would have been accepted by Hume's reasoning prior to their occurrence? The Earth moving around the Sun and not the other way round, the world not being flat - those had an extremely low likelihood based on the way we perceived things to be, to a degree that the vast majority would have thought them impossible. So I think that's the biggest flaw - how do we determine these probabilities? If we are measuring them by our current, "correct" understanding of the world, then so many discoveries and shifts in thinking of history would have had a probability close to zero. It also ignores that in nearly every (if not every) kind of thinking there are multiple views and perspectives, making it extremely difficult to boil something as complex as the occurrence of an event (especially one as vague as a "miracle") down to a function of a single probability.

Anyone watched DARK on nyetflix? German show about TT that's pretty cool.
Also, Back to the Future will always be a classic.

Those are very good points. I hadn't thought about the argument in a while, and now find myself at a loss to say what's valuable about it, apart from whatever historical interest and lines of philosophical analysis/debate it influenced (concerning how to evaluate testimony, how to conceptualize miracles).

It's funny (and a bit sad, I guess) - when I first read Hume's argument, as someone just getting into philosophy, I was extremely impressed, and for a long period of time remained convinced that it was a rock-solid argument (and that its most perplexing feature was people's "willful refusal" to accept it as such). Hume's just such a charming writer, for one thing. And there was definitely some motivated reasoning going on in my case - like, how cool and vindicating (and relieving!) I thought it would be if someone could deduce from the armchair, in a few simple lines of reasoning, that there's never any need to actually investigate the historical/scientific merits of any sufficiently "wild" claim (e.g. miracles, the paranormal, "fringe science").

Well this stalled out.......as it seems 90% of most threads do.

Time travel is endlessly interesting to me, but when I heard the explanation that if you did time travel you'd be going to an alternate timeline that really took the wind out of my sails.

can we have a time machine that's completely selective, and only brings its users to the good parts of each decade? Can we go back in time and erase the choices that gave us the 2017-2023 period?