Putting the universe back as it was except for you is time travel. You've traveled your state to a different time (previous state) of the universe.

Your semantic game sounds very clever, but is actually just inconsistent and borderline ignorant of what people mean by those terms.

Traveling back in time subsequently doesn't make sense because the "past" is simply what we call previous configurations of matter.

You can't go back to those configurations (read: "go back in time") because they don't exist anymore.

Now, I'm not a physicist so I have no idea if he's correct, but semantically it makes sense and it's something I've wondered about for a while.

So? If my future goes from the world as it is now to the world as it was in 2000, in what sense am I not in 2000?

I mean, you guys seem to be under the impression that there's a single universal clock I cant reset... There's not. In whatever sense it ever was 2000, rewinding all the clocks would make it 2000 again. There's no clock besides those local ones, so if you reset them all, you're in the past (in whatever sense there was ever a past).

I just find it particularly silly to have said there's no time when you keep commenting on time happening.

(Also, does traveling a foot to the left not make sense because there's no distance, just past states Ive been in? Cause you said time was like distance, and you're right: both exist or neither exist in exactly the same sense.)

It seems like you're being obtuse about the distinction.

Amature science people are the worst: a little knowledge keeps you from seeing the picture.

The only claim I was making was to clarify what OP meant about time not being a phenomena like gravity and to passively ask if there was any evidence of time as a physical (rather than purely conceptual) force.

Sorry if my amateur curiosity offended your professional (?) understanding of such things.

To try and answer your point a bit more directly: time isn't a force, just like distance isn't a force. A force is something that induces action. If you experience a force, you're induced to act. (Some debate about if gravity is a genuine force, but that's getting messy.)

Time is what a clock does, and distance is what a ruler does. On the one hand, both time and distance are mathematical fiction to parameterize aspects of the universal state. On the other hand, Im 2m tall and my clock is making a periodic tick noise. So, time and distance really are aspects of phenomena, rather than phenomena in their own right, and as such are fiction. However, the aspects of phenomena they refer to have so far turned out to be pretty useful and appear in most phenomena we've encountered, and we think this is because both emerge from aspects of the fundamental phenomenon interacting with itself to create the other phenomena. (This is why we're trying to unify QM and GR, and why the approach is based on entanglement and information cascades.)

My point was that there's no fundamental difference between time and distance, they're either both there or both not there in exactly the same sense, we can travel through both in the same sense (if not same freedom), but only very rarely do people get insistent that distance doesnt exist, and usually only when making a highly technical point. Never have I had someone not understand what I mean by moving a foot to the left, even if they ultimately don't believe that distance exists. This comment was like that: insisting we talk in the highly technical language of distance not existing, but refusing to make the standard and well understood lift of "move a foot to the left" to that language. There's a very straightforward notion of travel, even if we're talking about around a parameter space. It's that partial use of technical machinery that frustrated me -- if you're going to have a technical discussion, don't play obtuse and refuse to make reasonable translations if other people's statements. My comment about people who know a little was mostly just venting frustration that people who are new to the highly technical aspects often can't (and don't even realize they should) make those lifts, then end up stridently talking about how the unlifted claim makes no senae in their freshly learned framework. To be fair, I was worse about that than most, and should learn to be extra patient.

My snarky comment was snarky, but had a point: time is one of the open questions in modern physics, and unless you really want to get in to the heavy stuff, less is more. About the best view to have in your head is "there's no time but what your (and everyone else's) pocket watch says". Your watch needn't agree with mine, though. (And indeed, your head is a slightly different age than your feet, from all the standing, sitting, etc in a gravity well.)

To summarize, Id say that time and distance are meta-phenomena: they're consistently behaved, widespread features of phenomena we discover, but we have good reason to expect that it's an emergent feature of a deeper phenomenon.

Again, sorry for my earlier replies, and I hope I havent put you off asking questions!

That doesn't sound right - traveling back in time should also 'replay' random events not directly affected by the position of matter, such as radioactive decay.

> Your semantic game sounds very clever, but is actually just inconsistent and borderline ignorant of what people mean by those terms.

I don't think that's how most people think of time travel.

For example, say I proceeded to invoke reactions to get all particles back to exactly how they were on Jan.1 2000. Say this took me a year, and finally, in 2018, I execute the final reaction that places the last particle, so that the universe is exactly like it was Jan.1, 2000. Have I been time travelling for a year? Does making 2018 look like 2000 what people generally think of as time travel? What about that you yourself did not change, where are those particles coming from? What about the old you existing in that time as well, there aren't enough particles in the universe to have two you's. What about the time you travelled from remaining in existence while you're gone, and the changes you invoke in the "past" propagating there?

I get that it would provide a similar experience in many ways, but it doesn't involve travelling, nor time, just changes in particle positions - which is the nuance I am highlighting.

In your example: since you erased the elapsed computation in other regions, you (by putting the particles back) are literally rewinding every clock but yours.

In whatever sense they were ever in 2000, they're back there, though you're in (your) 2018. Having your 2018 be their 2000 seems like a sensible definition of time travel.

(And yes, you'd have spent a year time traveling, though in many senses you always are time traveling forward, and you just happened to spend a year going fast "backwards".)

For the rest of your points, I agree that they're sensible objections to being able to do it, but they don't make the concept not mean anything, which was your original point.

Why would the future remain? You destroyed it when you time traveled, and any changes in the future would require time traveling forward to see. (Forward is less erasure heavy, because you're just stalling instead of rewinding.)

It's not just a similar experience, it's the literal definition of what that term means.

> nor time, just changes in particle positions

Uh... It's comments like this which makes me question your position.

Changes in particle positions is time! Literally: time is just that thing a clock does, which is tick. If particles change position, that's (by definition) time.

Whether we're talking about 60 seconds, or 6 billion years, they're both measurements of distance traveled / motion. We measure one year by the earth's relationship to the sun.

Time travel cannot exist because the universe does not keep a record of where everything was at any given point in time, such that you can rewind (adjust) it all back to how it was at every or any given point in time. There is nobody recording that, there is nobody to rewind it, there isn't enough energy to attempt either the recording or the rewinding at an independent macro scale (eg could you properly rewind/adjust a solar system while the wider galaxy acts upon it? would you have to rewind the galaxy? and on it goes up).

The closest we can get to faking time travel is by manipulating the speed of a subset of reactions (like our body's aging) by going the speed of light or freezing ourselves, etc. But it's still fake, "time" continues, you just managed to manipulate specific reactions in a peculiar way.

Also, can you explain what you think "physical reactions" are? What causes them? Why would they happen at different speeds if time doesn't exist?

Can you explain which of the two statements you disagree with and provide an explanation:

1. The speed of light is constant in all frames of reference.

2. The laws of Physics apply in all inertial frames of reference.

If you disagree with neither of these statements, then the entirety of special relativity must follow from your logic. Which means you have to explain why these physical effects happen without being able to explain it through time dilation and other effects.

How do you explain acceleration? Or things like entropy that appear to indicate that the "arrow of time" does exist. Or general relativity / cosmology that clearly shows that your "snapshots of particles" appear different in different frames and there are physical effects that change how time progresses for them all.

> Time is just a way to label snapshots of particle positions in the universe.

You're not saying anything new here, apart from saying that "time isn't real". Is any dimension real? Spacial dimensions are even weirder because they're unlabelled and you can pick and three vectors to define a basis.

That's why satellites clocks must be adjusted https://en.wikipedia.org/wiki/Time_dilation

That's why satellites clocks must be adjusted https://en.wikipedia.org/wiki/Time_dilation

Why even bother with this whole pointless facade? Just because you have a preferred model of time doesn't mean it doesn't exist. All things that are said to exist do so as models.

You're saying "time is fake, it's just labels" like somehow everything else in the universe isn't also just labels and just as real. Things are what they are. You find a useful way to talk about them. You don't spend your time laboring over meticulously irrelevant descriptions, ironically putting down 'mere labels' as though your whole argument here isn't just a grand exercise in label worship.

One could fairly trivially write a faithful version of your post on literally any subject. "Position doesn't exist, it's merely a manifestation of the relative relations between subsets of human experiences!" "Your HN post doesn't exist, it's merely electronic patterns in a transient configuration!"

Hmmmmm.