## No meaning to go backwards in time...

Look at it this way. If we treat time as a dimension just like space then there is no meaning to go backwards into time. You cannot go "back" in time just like you cannot go "back" in space. Think about it. No matter what direction an object is moving, it's direction of motion is always forward. You can say the object may turn around and move the opposite way, but it's direction of motion is still forward! No matter which direction the object moves, it's direction of motion will always be forward! Get it? To technically go "backwards" in space means to arrive at your destination before you left (see the connection?). No matter which direction you travel in space, your direction of motion is always forward. Therefore, no matter which direction you travel in time, you are always moving "forward!"

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 "To technically go "backwards" in space means to arrive at your destination before you left" No. To technically go backwards in space is to arrive at your starting point in space, not in time. You can easily do this by turning around and going "forward" to your starting point. You're arguing semantics. You are playing on an ambiguous use of the word "forward". Sometimes you use it to mean "direction my vehicle is travelling" and sometimes you use it to mean "direction in which I make headway". Just becasue your vehicle is always moving forward does not mean that you are always making headway. If you are pointed back towards where you came from, you'll end up there, not "forward". So you're going forward through space, and you turn your ship around and go forward in the opposite direction until you end up at your starting point again. You're going forward through time, and you ...what ? turn your ship around and go forward in the opposite direction through time until you end up back at your starting time again?

 Quote by DaveC426913No. To technically go backwards in [I space[/I] is to arrive at your starting point in space, not in time. You can easily do this by turning around and going "forward" to your starting point.
I agree, and it is similar with time.

I think there is a definite meaning for to go backwards in time.
Another question of course is if that is at all possible.

In my opinion it is not possible at all to go backwards in time.
Even in the case of a hypothetical closed loop situation in GR would we univocally have to conclude that something went back in time? I do not think so, since we could alternatively interpret the results as an effect of non-locality.

## No meaning to go backwards in time...

I agree with the emerging view that Time is a QUANTUM concept- the arrow of time in classical physics is invariant- the Page and Wooters idea being advanced by quantum computer science works better: that the universe is an ensemble of all possable static states and some are related by being the output of specific physical rule-systems [ Page, D.N. and Wooters, W.K. (1983), Phys. Rev. D 27, 2885-2892]- and in the case of our universe our states are the output of a quantum algorithm determined by Entropy- which emerges to observer states within the larger world-states as the illusory 'flow' of time into the future-

this would mean that flowing time is wholly determined by the causal structure of related states- and certainly quantum histories with 'backward' or 'sideways' [i.e. parallel time-like degrees of freedom] are likely in the ensemble- but these worlds would be nothing like ours-

 Quote by DaveC426913 "To technically go "backwards" in space means to arrive at your destination before you left" No. To technically go backwards in space is to arrive at your starting point in space, not in time.
That does not make sense. If you arrive at your starting point, you still went forwards in motion. For example, I can walk around the planet earth and arrive back at my starting point without ever having to turn around. Does that mean I went backwards?

 Quote by DaveC426913 "You can easily do this by turning around and going "forward" to your starting point.
Exactly, you still went "forward" to your starting point. You didn't go backwards, you went forwards. My point precisely.

 Quote by DaveC426913 "You're arguing semantics. You are playing on an ambiguous use of the word "forward". Sometimes you use it to mean "direction my vehicle is travelling" and sometimes you use it to mean "direction in which I make headway".
In this case, we are talking about direction of motion. Please read my original post, I have stated so many times.

 Quote by DaveC426913 You're going forward through time, and you ...what ? turn your ship around and go forward in the opposite direction through time until you end up back at your starting time again?
Uhh that's what I'm trying to say...you can't do that. Am I missing something?

 Recognitions: Science Advisor Staff Emeritus In modern langualge, "going backwards in time" would be replaced with "a closed timelike curve" for the reasons you describe. The two concepts arent quite totally equivalent, a CTC is more like a time-loop the jargon of time-travel.

Mentor
 Quote by MeJennifer I think there is a definite meaning for to go backwards in time.
Going backwards in time means going into the past while traveling forwards in time.

Let p be an event in spacetime, Event q is in the (chronological) past of p if there exists a future-directed timelilke curve from q to p.

Suppose that event p is on the worldline of an observer, and that there is an event q is in the past of p such that a future-directed timelike curve from p to q. Then, it is possible for an observer to travel into his own past.

Joining the future-directed timelike curve form p to to q with the future-directed timelike curve from q to p, shows that, as already mentioned by pervect, this is completely equivalent to the existence of a closed timelike curve.

 Another question of course is if that is at all possible.
Its certainly allowed by general relativity, as there are numerous solutions to Einsten's equations that have closed timelike curves.

1. Radically rerwite physics from the ground up;

2. Permit time travel, but also invoke consistency constraints;

3. Quantum physics intervenes to prevent time travel;

4. the Boring Physics Conjecture, where we assume (until forced not) that our particular universe is globally hyperbolic, and thus doesn't have closed timelike curves.

In the past 4. was often assumed, but since global hyperbolicity is a very strong global condition and Einstein's equations are (local) differential equations, many physicists have moved to 2. and 3. Stephen Hawking likes 3., for example, and has formulated the Chronology Protection Conjecture, "It seems that there is a Chronology Protection Agency wich prevents the appearance of closed timelike curves and so makes the universe safe for historians."

This roughly states that near a chronology horizon (horizon at which spacetime becomes causally ill-behaved), expectation values of stress-energy tensors for quantum fields blow up, thus preventing (by wall-of-fire barriers) physical objects from crossing chronology horizons. There seems to be some semi-classical evidence for this conjecture, but a more refined analysis by Kay, Radzikowski, and Wald muddies the picture a bit. Their analysis shows that the semi-classical stress-energy tensor is ill-defined, but not necessarily infinite, at a chronology horizon.

This may be just an indication that the semi-classical theory breaks down at chronology horizons, and that full quantum gravity is needed for definitive predictions.

 This discussion shows why we need mathematics to clarify discussion, if we consider time now as Tn anything less than Tn is the past anything greater than Tn is the future we can now at least describe which direction in time we are travelling. Time travel is possible in some ways even with our limited knowledge, the "twins paradox" describes a form of time travel. Relative to each other one twin is travelling forward in time faster than the other, however both will arrive at a time greater than Tn but one will appear to have "jumped" forward to that time.

Quote by Flatland
 Originally Posted by DaveC426913 "To technically go "backwards" in space means to arrive at your destination before you left" No. To technically go backwards in space is to arrive at your starting point in space, not in time.
That does not make sense. If you arrive at your starting point, you still went forwards in motion. For example, I can walk around the planet earth and arrive back at my starting point without ever having to turn around. Does that mean I went backwards?
What you suggest is another way of returning to your starting point (and an excellent exmaple of how it IS possible to return to your starting point without going backwards through the medium). Nobody said there was only one way. But this has nothing to do with the example originally posed.

What exactly does not make sense to you about returning to your starting point in space?

Again, you are playing on an ambiguous definition of the words "forward" and "backward". Sometimes you mean "relative to the ship" and sometimes you mean "relative to the medium".

If any doubt, replace the ambiguous words in your statements with more succinct terminology. You'll find the paradox goes away (mostly because the claim makes no sense anymore, you'd have to change the wording, and that would make the paradox go away):

* This makes no sense now.
** This statement is still plain false, as pointed out previously.
*** And this says nothing we didn't already know.

Disambiguation of words shows the logical flaws.

 To technically go "backwards" in space means to arrive at your destination before you left (see the connection?). No matter which direction you travel in space, your direction of motion is always forward. Therefore, no matter which direction you travel in time, you are always moving "forward!"
Yup. Love it. Nice one Flatland.

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 Quote by Flatland Look at it this way. If we treat time as a dimension just like space then there is no meaning to go backwards into time. You cannot go "back" in time just like you cannot go "back" in space. Think about it. No matter what direction an object is moving, it's direction of motion is always forward.
I would say that if you really "treat time as a dimension just like space", then from the perspective of spacetime nothing is "moving" forward or backwards in time, you just have a bunch of worldlines on 4D spacetime which don't move any more than lines drawn on a piece of paper. If the paper has x and y axes drawn on you can say that as you vary the y-coordinate, the x-coordinate of the corresponding point on the line may change, and likewise you can say that as you vary the t-coordinate in spacetime, the space coordinates x,y,z of a given worldline may change, but that's all.

Really, all "travelling backwards in time" means in relativity is that a worldline's path enters the past light cone of an earlier point on the worldline. If you want to argue that I can meet my own past self and yet I have not really "travelled backward in time" because from my perspective the event of meeting my younger self still lies in the future of actually having been that younger self, then you're just not using language in the way physicists would use it.

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 Quote by Flatland Therefore, no matter which direction you travel in time, you are always moving "forward!"
"Going forwards in (coordinate) time" simply means:

$$\frac{d\tau}{dt} > 0$$

similarly, "Going backwards in (coordinate) time" simply means:

$$\frac{d\tau}{dt} < 0$$

That's all there is to it.

 Quote by George Jones Let p be an event in spacetime, Event q is in the (chronological) past of p if there exists a future-directed timelilke curve from q to p. Suppose that event p is on the worldline of an observer, and that there is an event q is in the past of p such that a future-directed timelike curve from p to q. Then, it is possible for an observer to travel into his own past. Joining the future-directed timelike curve form p to to q with the future-directed timelike curve from q to p, shows that, as already mentioned by pervect, this is completely equivalent to the existence of a closed timelike curve.
I know what you are saying but that is simply a particular interpretation.
Spacelike becoming timelike and vice versa is a coordinate dependent interpretation.
After all the square root of a negative number is not a negative number.
Furthermore as I already wrote we could alternatively interpret this case as a non-local interaction.

 - I think you're missing something.... "time travel" at classical or semiclassical scale perhaps is impossible since you're dealing with Geodesic Tensor and other "Geommetric" entities.. - HOwever it's different at the scale of quantum world , where time and space are just "Eigenstates" of a certain Unknown operator, so if you are on an "state" let's call $$|t_0 >$$ and you want to travel forward or backwards in time to an state $$|t_1 >$$ you should use some kind of "teleportation" to reconstruct a reality that's on state $$|t_1 >$$ then the "time travel probability" is just $$| |^{2}$$ unfortunately in most of cases this (semiclassical limit) will be almost 0 ...i think that perhaps if "time travel" exist or will exist perhaps it will be made similar to "teleportation" so they pick up you that are on an initial state and "reconstruct" you on a final state... similar to electron teleportation.

Mentor
 Quote by MeJennifer Spacelike becoming timelike and vice versa is a coordinate dependent interpretation.
Whether a curve is timelike, lightlike, or spacelike at a particular event is independent of coordinates.

 Quote by George Jones Whether a curve is timelike, lightlike, or spacelike at a particular event is independent of coordinates.
Yes, but what is the relevance of that comment?

 Quote by MeJennifer Yes, but what is the relevance of that comment?
Its relevance is that it negates your earlier comment in Post #13.