Time travel and space travel

vibhuav

First of all, my background: I am an engineer interested in physics; so I am not very well versed with the nitty-gritty details and the math of physics.

These are my thoughts and would like your comments:

In some sense, time travel, being able to move back and forward in time is the ultimate dream to be solved by scientists. Many people, including physicists compare time travel to space travel and say that since we are able to move back and forth in space, why not in time.

I have an issue with this comparison. The question I have is, are we really moving back and forth in space in a manner analogous to what we want to achieve in time travel? I don’t think so. Here’s why:

As we know, the universe is expanding continuously. In particular, space is being created and we are all moving away from each other at the large scale. The new space is being inserted between galaxies and clusters of galaxies. That is the space into which we should be able to move to. Are we able to do that? Are we able to go back to the spot in hyperspace where we were, say last year? This too may not make sense because our space (or location) has not really moved due to the expansion of the space; it is just that it got shoved out due to new space being created. That’s why I used the term hyperspace.

Similarly, just like space, time also is being created always. The one difference compared to the expanding space is that the newly created time is inserted to the edge, to the present, and we go into the future; the new time is not “inserted into the past.”
We are all “travelling” in spacetime, clutching to our personal space and time. Moving back in time is comparable to moving back in hyperspace. We know that moving back in hyperspace is humanely impossible, unless the universe were to collapse on itself – the big crunch. Therefore we should not be surprised that it is impossible to do time travel. When the big crunch happens (if it does happen), space will contract and time will reverse itself and everything will happen backwards. We just have to wait for Mother Nature to be able to go back to the past. We will then re-live our lives backwards, from the grave to old age to middle age to youth to birth to non existence. Still, we will not be able to see outside of our living duration.

From special relativity, we know that by travelling close to speed of light and coming back to earth, we are travelling into the future. So is there an analogous hyperspace travel? (I am not contradicting myself; we have a theoritical way to travel to future which may be humanely impossible. For space travel, there is no theoritical way even)

The moving around in space that we indeed do is a movement in local space. The equivalent to that in time is probably remembering the past?

Am I totally lost?

Vibhu.

Nabeshin

You mention special relativity at the end of your post, and yet you completely disregard the fact that all positions are relative.

First, your use of the term "hyperspace" is confusing. What exactly do you mean?

Second, the fact that we are able to move freely in space has nothing to do with the expansion of the universe. This capacity would be equally realized in any other scenario adhering to the same laws of physics, static universes included.

Third, the whole notion of time being created is treading awfully close to crackpottery. The analogy of space being "created" to describe the expansion should not really be taken as more than an analogy. The extension of this concept to time, then, is unwarranted. Remember, also, that time is relative to the observer. So if you want to talk about time being "created" at all, then you have to admit that time is created differently for different observers, and now we get ridiculous conversations.

Finally, when you say time will run backwards in the event of a big crunch, this sounds ridiculous to me. It reminds me of the discussion of the reverse of the laws of thermodynamics in A Brief History of Time (I think, been a while), but I don't think this is the generally accepted viewpoint. After all, all current models show an expanding universe, not one doomed to a big crunch.

Also, I don't think any physicist would claim time dilation is in any way "time travel into the future." It simply reflects the relativity of the concept of time.

vibhuav

First of all, thanks for the reply. As I mentioned, I am no physicist and therefore have used words incorrectly. I am not claiming I am correct, just trying to see if what I am thinking is correct or not.

Obviously, hyperspace is incorrect. Imagine you are looking down on the expanding universe. Where would you be standing? You can't stand in this universe, right? That's why I used the word hyperspace. Is there any other term I can use? meta-space? Not sure...

So looking down on the expanding universe, the position of our galaxy (say) has moved due to the expansion. Right? Now, by space travel, I meant we want to go back to the original position of our galaxy. THAT, I think is analogous to going back in time.

On earth we can go back and forth. We can go back and forth in the space near the earth. I call that movement in space as local movement. It is not the same as going back to the position our galaxy was, say one year ago, before the expansion happened.

Again, I was using common man's language. I know time is not a physical thing like space but for comparison, can't we say time is elapsing at the same time that space is expanding? That's all I meant.

Lets say we are talking about time in my frame.

I agree I got carried away here. I should'nt have even gone here...

The bottom line is, is it fair to compare the local movement in space to "going back in time"? I dont think so. "Going back in time," in my opinion, should be comparable to "going back to the original position of our galaxy," which was pushed out due to the expansion of the universe. Both, I think is not possible, and therefore saying "since we are able to move back and forth in space, why not in time" is incorrect. That's all.

Nabeshin

Ok thanks for defining your hyperspace. It seems you put a lot of weight on this idea, but the simple fact is you cannot do this. Seeing as the universe is defined as all that is, you cannot consider a vantage point completely removed from the universe. It simply has no meaning to talk about things in this way.

Also, you say,
Position relative to what? Without some absolute reference frame, who is to say that you didn't return to the same point in space? Certainly some observers will say you did, others will say you did not. Really digest this, because I think you'll realize a lot of your arguments hinge on it.

FayeKane

Okay, two things:

Thing 1): vibhuav said:
> Imagine you are looking down on the expanding universe. Where would you be standing?

That's like saying "Suppose god exists. What's his shoe size?" If there ARE no gods or goddesses, which appears to be the case, then the question makes no sense.

You're thinking of the universe as having a center and an edge, and it doesn't. The best way to think of it (to me) is that space on a large scale is "connected funny", meaning that one "end" wraps around and touches the other end through a higher dimension, and the effect when you're inside this finite-volume universe is that you seem to be in a space with infinite extent in all directions

The CMB power spectrum data suggest flat space, but it's still a good way to visualize how you can never be "outside the universe looking in".

Thing 2): Nabeshin said:
> Seeing as the universe is defined as all that is, you cannot consider a vantage point completely removed from the universe.

If string theory is correct, there are points in spacetime (if that's still what you call it) which are not part of what we call "the universe" (the expanding thing created at the big bang). So I think the statement "the universe is defined as all that is" is unintentionally disingenuous when used to "define away" anything external to the big bang and its decay products.

Note that those two answers contradict each other. But if Lewis' White Queen can believe 6 impossible things before breakfast, I guess I can believe two contradictory things before lunch.

-- faye kane, idiot savant

Nabeshin

From my understanding of string theory [limited], all points are a part of the universe, some of them simply happen to be located in extra dimensions beyond the 3+1 space time we normally think of. I don't think moving our vantage point to a fourth, fifth, or twelfth spatial dimension will aid the OP because there is still the problem of no absolute reference frame.

Unless of course you're referring to branes and M-theory, and all that good stuff. In which case things get a little more complicated. But I think we should use more conventional theories to field the OP's question.

JesseM

Just from a mathematical point of view, if you have a surface whose curvature is defined in an intrinsic way using the tools of differential geometry, you are also free to define the curvature in an extrinsic way by imagining a higher-dimensional uncurved "embedding space" and defining the position of points on the manifold in this higher-dimensional space (like how we can define the surface of a 2D sphere by drawing xyz axes in euclidean 3D space and defining the surface of the sphere as x^2 + y^2 + z^2 = R^2 where R is the radius). Such a higher-dimensional euclidean embedding space (or higher-dimensional minkowski embedding spacetime) would have no physical effects in GR, but just as a thought-experiment or a visualization tool it should be a well-defined idea.

FayeKane

I absolutely and sincerely agree. Sometimes I make the huge error of talking to people as if they were as familiar with certain kinds of things as myself.
Actually, I was. The OP talked about getting "outside" the expanding universe, and the only places I know of like that are branes other than the one our expanding universe is in. In fact, one reason I came here was to find someone who can tell me the metric used in higher-dimensional spaces in general. Do they all add only positive terms to the signature of the metric? Are compacted dimensions timelike, spacelike, neither, both, or something else?

I would also like to know what the typical "distance" (if that's even what you call it) between branes is. I've seen it described as everything from "a few Planck lengths" on up. As a matter of fact, I have a whole LOT of questions about ST. (Note, please don't try to answer any of them now! ).

--FLK

jeebs

I'm just a lowly physics undergraduate but here are my half-arsed musings on the topic.
I think travelling back in time is just equivalent to putting all matter and energy back to where & what state it was in at a previous time - how could you tell the difference between time 1 and time 2 if no particles had moved and no energy had been transferred?

As far as I am aware, you couldn't. So for "time travelling" in absolute time, a time machine would have to be able to completely control everything in the universe rather than some smaller isolated system, which is pretty much impossible - hence, I don't think a time travel device will never be made.

JesseM

Actually general relativity does theoretically allow for the possibility of travel back in time (closed timelike curves) in certain unusual circumstances like a rotating universe or a traversable wormhole, though there's a good chance that closed timelike curves will actually be ruled out by a theory of quantum gravity which supercedes general relativity (the so-called chronology protection conjecture), as discussed in this paper. Anyway, in a GR context travel into the past does not imply matter rearranging itself to a former configuration, rather it involves a worldline which loops around through spacetime and crosses itself at an earlier point. I discussed the concept in post #16 of this thread: