# Time Travel

## Main Question or Discussion Point

What prompts me to write this is all the conjecture about time travel. Shouldn't we be a bit more down to earth about all this? All too many of us seem to generate concepts that are not even suggested by the original theories ... especially matters dealing with relativity. Remember all the baseless conjecture inspired by the Heisenberg Principle? It's still happening.
The only way to go back in time is to reset all particle positions and motions within a given range to some previous state. The interactions will then sequence in the same manner as they did originally. Time as we know it will repeat. A problem occurs with edge (or surface) deterioration because of particle influence outside the given range, and this may corrupt the original sequence. This could also create an initial coflict of particles. And, particles actually entering the range from outside may corrupt it. Thus, unless the given range is inclusive of the entire universe, exact re-sequencing cannot be guaranteed. For ranges smaller than the unverse, the larger the range of reset, the longer the time required for corruption to occur. We would still be restricted to the speed of light, and this poses a limit to the range of initial reset.
Obviously, to travel into the future requires that we know all the future particle positions and motions.
By the way, none of this as far as I know is supported by any established theory.

Related Special and General Relativity News on Phys.org
HallsofIvy
Homework Helper
You seem to have a very strange idea of "down to earth"! Simply making statements without any support is not "down to earth".

"The only way to go back in time is to reset all particle positions and motions within a given range to some previous state. The interactions will then sequence in the same manner as they did originally. Time as we know it will repeat. "

Do you have any reason for such a statement? How do you know this is the "only way"? You seem to be thinking that if you "reset all particle positions and motions within a given range to some previous state" then everything will happen again in the same way. In the first place that is NOT what is normally meant by "go back in time" and in the second, the uncertainty principle implies that things would NOT go forward as they did before (or, more correctly, that it is not possible to "reset all particle positions and motions" exactly).

CeeAnne said:
Obviously, to travel into the future requires that we know all the future particle positions and motions.
Not true at all, your travele'n in that direction everyday.
And as long as you don't intend to go to far - like any farther than your lifetime - You won't even need the expense and trouble of build a near light speed ship to orbit the universe.
On a cost/benefit bases it comes highly recommended.

CeeAnne said:
By the way, none of this as far as I know is supported by any established theory.
Certainly one point most all will agree on.

Quantum_Prodegy
The way I see time travel in relation to the theory of relativity is very basic. If you can travel faster than the speed of light, then you can pass light that has been reflected from, say, earth, and thus see earth as it was at the time the light was reflected. Much in the way scientists see something in far off space, however it is not the image of that 'something' in the current time frame. They are seeing the 'something' as it was in the past, how long ago depends on how far away the object is.

Ivy, if all particles and motions could be repositioned, then everything would be set to a previous time. Everthing would be where it was and exactly how it was; people, cats, cars, VISA statements ... even the virtual field ... the same sequence of events would happen again exactly as before. It wouldn't be what you normally think of as "going back in time", it would be even better, it would be true time reversal.

case 1: bob and mark are 20 years old. 5 years later, bob "travels" back for 2 years, in a way unknown to modern physics. outcome: bob-25 meats mark-23.

case 2: bob and mark are 20 years old. mark builds fast spaceship, makes a short galaxy trip, and returns in 5 years. outcome: bob-25 meats mark-23.

JesseM
Quantum_Prodegy said:
The way I see time travel in relation to the theory of relativity is very basic. If you can travel faster than the speed of light, then you can pass light that has been reflected from, say, earth, and thus see earth as it was at the time the light was reflected. Much in the way scientists see something in far off space, however it is not the image of that 'something' in the current time frame. They are seeing the 'something' as it was in the past, how long ago depends on how far away the object is.
If you could travel faster than light in any direction, or transmit FTL signals in any direction, then according to the theory of relativity you could do more than just see the past, you could actually visit your own past, or send a signal into your own past. The reason has to do with the fact that in relativity, different observers define simultaneity differently--two events that have the same time-coordinate in one reference frame will have different time-coordinates in other frames. If there is a timelike separation between two events (ie a signal travelling slower than the speed of light could bridge the two events), all observers will agree on which event came first and which came second, but if there is a spacelike separation between two events (you'd need an FTL signal to bridge them), then different frames will disagree about which came first. Thus, if I send an FTL signal from one point to another, then even if in my frame the signal was received at a later time than it was sent, there will be some other frame in which it was recieved before I sent it.

According to the principle of relativity, the laws of physics work the same in all inertial reference frames, so if I am free to send an FTL signal arbitrarily fast in any direction in my own reference frames, the same must be true of other reference frames. So, if you're travelling away from me slower than light, I can send you a signal which moves FTL in my frame and backwards in time in your frame, then as soon as you recieve it you can send a reply that moves FTL in your frame and backwards in time in my frame, which opens up the possibility that I will recieve your reply before I send my original message, violating causality. There's an illustration of this using spacetime diagrams here:

http://www.theculture.org/rich/sharpblue/archives/000089.html

If you jettison the principle of relativity, and say that the FTL signals can only go arbitrarily fast in all directions in a certain preferred reference frame, then you can avoid the possibility of causality violation. In this case, if you're not in this preferred reference frame, then it will look like the signals can go backwards in time in one direction but there's an upper limit on their speed in the opposite direction. For example, in the year 3000 I might be able to send a signal to a star 10 light-years away that would reach them in 2999 in my reference frame, but then they could only send a reply in my direction at speeds less than 10 times the speed of light (again, in my reference frame), so their reply couldn't arrive before I sent the signal.

Garth
Gold Member
CeeAnne said:
Everthing would be where it was and exactly how it was; people, cats, cars, VISA statements
I like living in the past - I can afford it!

Garth

CeeAnne
Our present perception of time has obviously evolved as a consequence of the physics of our universe and it is sufficient for our existence, within a limited range of understanding that we perceive time as we do.
I think maybe our intellectual advancement has taken us to this limit and perhaps we have to start considering our perception of time as being completely inadequate.
We even have difficulty categorising time as a philosophical or physical entity.
But what can we do?
We cannot perceive of time outside of past present and future, a one-dimensional ‘arrow of time’, or an abstract concept that links events.
Perhaps that is all there is to time, but I think that we all instinctively sense not and that the biggest challenge that faces science now is to achieve some step forward, no matter how small, towards a deeper understanding of time.
Maybe you will have considered that we may be part of a multiverse of universes. Each universe formed by the same process, be it the ‘big bang’, branes colliding, singularities exploding or whatever. Each universe would have its own unique physics because although the mechanics of its creation would be the same, the ‘unified force’ would apportion itself differently, perhaps in accordance with some law of the multiverse.
So maybe there are an infinite number of universes that cannot interact because the unique physics, and so the unique ‘arrow of time’, peculiar to each universe would prevents that
So you would have a ‘body of time’ within a multiverse..
Simply put, maybe we are not aware of the universes around us because they are at a ‘different time’ but a different ‘different time’ that we associate with the ‘arrow of time’.
But we suspect that gravity can interact between universes.
So gravity may be a fundamental force of a multiverse, interacting within a body of time.
So if gravity can travel through time…………
Maybe………

offtopic: any metric with non-zero coefficient for dt*dx suggests that different aging for things falling in different directions.

I view time as merely a comparative measurement, much as divisions on a long, flat object can be compared to to another object and we can say that the object is so many centimeters this way or that. Measurements are mathematical values.

Being able to put all the particles back to their original positions...wouldn't that violate the 2nd Law of Thermodynamics? The 2nd Law is essentially the arrow of time...

CeeAnne said:
What prompts me to write this is all the conjecture about time travel. Shouldn't we be a bit more down to earth about all this?
I travel in time all the time; fortunately it is in only one direction; that way I don't have to repeat myself. I hate having to do everything twice.

The only way to go back in time is to reset all particle positions and motions within a given range to some previous state. The interactions will then sequence in the same manner as they did originally. Time as we know it will repeat. .
I hate having to do everything twice. ( Well, usually. )

Creator

--Why did God create time? So everything wouldn't happen all at once.--

Curious3141
Homework Helper
This doesn't really belong here, it's a fringe "theory" (such as it is) and would be more at home in Theory Devt.

I am not attempting to develop a theory. I am speaking against what I view as a misapplication of other theories. There is a difference between demonstrating that clocks slow when in an accelerated frame and actually returning to, say, 1960 and having everything the way it was. The illustration at the beginning was meant to show the requirements for actually moving in time. I hardly expect a controlled repositioning of all constituents of the Universe. It's self-contradictory.

JesseM
CeeAnne said:
I hardly expect a controlled repositioning of all constituents of the Universe. It's self-contradictory.
That is not how time travel would work in general relativity. In both special and general relativity, you have to get rid of the idea of a single universal present, since these theories say that different observers have different views of whether two different events happened "at the same time" or not, and each observer's reference frame is equally valid. So instead you have to think of a single static 4-dimensional "spacetime" which contains the entire history of the universe; travelling back in time in this context means that an object's "worldline" curves back on itself and revisits a region of spacetime it already crossed through before.

Think of a block of solid ice with various 1-dimensional strings embedded in it--if you cross-section this block, you will see a collection of 0-dimensional points (the strings in cross-section) arranged in various positions on a 2-dimensional surface, and if you take pictures of successive cross-sections and arrange them into a movie, you will see the points moving around continuously relative to one another (in terms of this metaphor, the idea that there is no single universal present means you have a choice of what angle to slice the ice when you make your series of cross-sections). You shouldn't think of time travel as the points returning to precisely the same configuration they had been in at an earlier frame of the movie; instead, you should just imagine one of the strings curving around into a loop within the 3-dimensional block, what in general relativity is known as a "closed timelike curve".

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CeeAnne said:
Ivy, if all particles and motions could be repositioned, then everything would be set to a previous time. Everthing would be where it was and exactly how it was; people, cats, cars, VISA statements ... even the virtual field ... the same sequence of events would happen again exactly as before. It wouldn't be what you normally think of as "going back in time", it would be even better, it would be true time reversal.

How do you know that everything would be set to previus time? It is like saying if you roll a ball down a hill it will end up in the same place no matter how many times you try! Would it end up there always?

There probably is no way of traveling back in time but the only way we know of now is by traveling faster than the speed of light which is imposile (for something with mass anyways) If there is a mass in something as it reaches the speed of light it will increase mass. At C we would have infinet mass thus require infinet energy to move and that is imposible.

JesseM
bayan said:
There probably is no way of traveling back in time but the only way we know of now is by traveling faster than the speed of light which is imposile (for something with mass anyways) If there is a mass in something as it reaches the speed of light it will increase mass. At C we would have infinet mass thus require infinet energy to move and that is imposible.
General relativity allows other possible ways of travelling back in time, like travelling through a wormhole. Most physicists would probably guess that when we come up with a theory of quantum gravity, these possibilities will be ruled out, though.

I doubt going back in time is possible. I'm sure it's impossible. But I do believe it is possible to recreate our current world into a replica of the past. But not the universe as a whole. But with enough energy the earth can be changed. And this can be done without traveling faster then the speed of light.

Enos said:
I doubt going back in time is possible. I'm sure it's impossible. But I do believe it is possible to recreate our current world into a replica of the past. But not the universe as a whole. But with enough energy the earth can be changed. And this can be done without traveling faster then the speed of light.
Well, Enos, that is certainly "down to earth" and most refreshing. Thank you.