Came here after the question was closed on Physics stack exchange.

An event requires 4 coordinates - spatial x,y,z, and time t. Multiple objects cannot occupy the same x,y,z at the same time t. Given this fact how can a subpart of the universe travel back in time? Won't it collide with something that was there at x,y,z at the moment t in the past? To explain it with a concrete example - cars pass through the intersection at different times. If they try to pass thru the intersection at the same time they collide. Pretty simple. Does this simple fact tell us that traveling backward in time is not possible? It may be that only the whole universe can rewind as that avoids the problem of collision.

And thinking about this more, this applies to traveling forward in time at the exact same coordinate x,y,z may not be possible. What are the implications of this to the twin paradox? Of course in the twin paradox, one twin has to travel away (change his/her x,y,z coordinates), and when they return they could avoid colliding by staying clear of objects at their original x,y,z.

What are the pitfalls of the about thought process?

BTW in Back to the Future when Marty goes to 1955, he must have implicitly adjusted the spatial coordinates to where the earth was and his town Hill Valley was on earth at that time of the day right?

BTW I am a science-literate person but not a physicist.

  • "The Moving Finger writes; and, having writ, Moves on: nor all thy Piety nor Wit Shall lure it back to cancel half a Line, Nor all thy Tears wash out a Word of it." - Omar Khayyám. Truer words have never been spoken, and no physicist can refute it.
    – user4894
    Dec 28, 2021 at 2:15
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    Won't it collide with something that was there at x,y,z at the moment t in the past? But what if the only thing at those coordinates was the object itself, having arrived in the past? I mean you could just as easily ask "how can an object that's at rest at coordinates x,y,z at time t1 endure so it's still there at those same coordinates at time t2, won't it collide with something that's already at coordinates x,y,z at time t2?" Presumably you'd agree that argument doesn't really make sense, what is it about time travel that makes the argument seem stronger to you in that case?
    – Hypnosifl
    Dec 28, 2021 at 2:20
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    @user4894 Most physicists don't think of time travel in terms of "canceling" or changing any past events, it's usually assumed that if backwards time travel is possible at all (and certain theoretical situations in general relativity could allow it), it would obey the Novikov self-consistency principle, meaning anything a time traveler did in the past would have been part of history all along. I wrote a thought-experiment that may help thinking about the idea here.
    – Hypnosifl
    Dec 28, 2021 at 2:24
  • @Hypnosifl the future is open (presentism) and thus an object can continue to stay at current x,y,z. Of course, if a large truck wants to be at x,y,z at time t2 the object will not be able to stay there at t2. Maybe I am missing your point. We know from memories that things were there at x,y,z. Dec 28, 2021 at 2:32
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    There is nothing impossible about cars colliding. And there is no "same place" and "same time", separately, in relativity, those coordinates are only locally and arbitrarily assigned. Time travel is not like decreasing one's t while keeping x,y,z fixed. If there are closed timelike curves in spacetime, which one would need to follow to "travel back in time", then the disruption of normal spacetime around them would be far greater than anything colliding might cause. But even that is not impossible, as far as we presently know.
    – Conifold
    Dec 28, 2021 at 4:11

3 Answers 3


The short answer is, we don't know. Time has not been unified with the Quantum Field Theory picture, and only Special Relativity has been integrated. We need a quantum-gravity theory in order to know. The Wheeler-DeWitt equation is an important step towards this, and it's very suggestive that it's not time-dependent.

General Relativity allows closed time-like curves, though the energies involved like for the Tipler cylinder would be large even by average galactic scales. The chronological censorship hypothesis is a subset of the cosmic censorship hypothesis, which suggests event horizons occur in ways that prevent chronology violations. 1

It is conjectured entanglement may lead to instantaneous shared effects through 'wormholes', Einstein-Rosen bridges. This conjecture is called ER = EPR. These quantum effects seem to avoid the possibility of faster-than-light signalling, summarised in the no-communucation theorem

There may only be one space and one time: How can time have a beginning when a beginning needs time?

Time going backwards may be equivalent to the universe 'forgetting' what happened: Time travel and changing the past

We need a picture that unites the thermodynamic arrow of time, with the spacetime manifold. Loop Quantum Gravity, the Purification Principle, and Universal Constructor theory, are interesting ideas about the way forward. Noether's theorem seems to indicate we can think of space-time as an emergent symmetry, rather than only as a 'neutral background'.


Since this seems to be a science-fiction question, the problem of occupied space for instant travel has been resolved in different ways in different science fiction stories.

The object to instantly move to a different place could swap places with all that was there at the destination. Or all that is at the destination could simply cease to exist to avoid the collision. Or the thing to transport could try to displace all that there is at the destination, like inflating a balloon until enough space is provided.

In any case the alternatives show that the problem of occupied space is not a philosophically interesting one for instant travel or time travel.

  • “People assume that time is a strict progression from cause to effect, but actually from a non-linear, non-subjective viewpoint, it’s more like a big ball of wibbly-wobbly, timey-wimey stuff.” - Dr Who
    – CriglCragl
    Dec 28, 2021 at 9:43

We currently have three theories of time -- a Block Universe where time is a dimension, and everything is pre-determined. Presentism, where time is just a convenient way to refer to the sequence of logic operations in a causal chain. And Growing time, where time is only the past, with the present the growing edge of time.

Historians use growing time, most people use the presentism/logic time, and physicists and engineers use block time. These three models are incompatible, and each fails to address some significant features of time.

Of them, only Block Time allows time travel at all, and that does not allow history to be changed.

To achieve any plausible time travel with causal effectiveness, one would need a much more fluid model for time. A superposition of probabalistic streams, could in principle still be probabilistic in the past as well as present and future, and time travel could change relative probabilities. This is the sort of model that time travel literature uses, and while it is not currently compatible with any of our current usage, it is plausibly capable of addressing the shortcomings of all three models. Since our current models are all inadequate, the potential for such a future model is -- possible.

You raise some other major practical issues. WE are pinned to the surface of a spinning globe taht travels through space in a helical pattern based on its solar and galactic orbits. Any time travel, to be useful, would have to somehow traverse space and also get pinned to our globe's surface. Whether THIS would be possible -- may be even more difficult of a technical challenge than getting time travel to work at all.

Additionally, yes, there is already matter occupying space, so if one moves more matter in time, then one will also have to deal with the matter that was already there. A volume exchange, where the matter in a volume is exchanged with the future time, might be the most plausible way to get around the existing matter issue. The solution to this problem seems plausibly likely to come with the time travel technology itself. This is not the biggest issue and is relatively minor compared to the other two (no current theory allows causal time travel, and if it did, we would get to a useless location in faraway space).

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