# Real Time Travel

1. Apr 29, 2004

### strubenuff

Does anyone know whatever came of Dr. Mallet and his time machine? I've searched online, but I can't find anything more rceent than 2002, when they started to build the machine. For those of you that have no idea what I'm talking about, the professor wanted to use the distortion of space time produced by high-intensity, spiraling lasers to send subatomic particles bacj in time.

2. Apr 30, 2004

### gnl

I have always had the suspect that time-machines belonged to the realm of crackpot theories. They never give a satisfactory answer to the fundamental question of causality (I believe a non-causal theory to be totally meaningless).

3. Apr 30, 2004

### jackle

I built a time machine tomorrow. Go figure.

4. Apr 30, 2004

### jcsd

He went back to the Jurassic period, unfortunately he ran out of fuel and was unable to generate the 1.21 gigawatts required to the flux capacitor to get back and was eaten by a Tyrannosaurus Rex.

I'm pretty sure he's still working on it, but don't think anyone seriouslly thinks he's going to able to do the sort of thing he wants to at such low energies, but he's not a total crackpot as the kind of distortions he is seeking to create are not theoretically impossible.

5. May 4, 2004

### gnl

I see. However, if mathematical consistency and physical reality are non necessarily the same, I suppose time travel can still be exluded on the basis of its unphysical nature. After all, this is done with Goedel's solutions of GR equations that involve closed time-like curves, time travel again. He should keep working on that, but I remain very skeptical.

6. May 4, 2004

### Antonio Lao

As what Kip Thorne would say, time travel can be remotely possible if wormholes can be created. The other problem seems to be entering the hole.

I think the real problem with real time travel is "how to change the physical dimension while keeping the time dimension at almost constant."

The spacetime is warped at every point of the wormhole, so the logical thing to say is that the spacetime interval is practically zero. This could mean that the physical dimension is zero but in superstring and M-theory this is where the nine or ten physical dimensions are hidden. So does physical dimension increase or decrease at smaller and smaller volume? What is the volume at the local infinitesimal region of space? How gradual is the dimensional transition? Or is the transition abrupt?

7. May 5, 2004

### gnl

I still cannot avoid thinking of all the paradoxa. I think the REAL question is: is causality a physical law? Should it be? I think so. I also think that even small violations of causality can lead to disaster. In other words, I think it would be very useful to look for rigorous physical motivations to prove why time travel is NOT possible.

8. May 5, 2004

### alpha_wolf

1. Define causality.
2. Are you sure your definition is not too limited?
3. Most if not all paradoxes can be eliminated by assuming the existance of paralel universes and what I cal "multi-spacial" timelines. By multi-spacial timelines, I mean event sequences that loop back in time, diverge into a paralel space, possibly repeat the process a few times, and then optionally return to the original space.
4. If it just so happens that everything is predetermined, then paradoxes are inherently impossible. And no, QM does not prohibit predeterminism, but this question is more suited for philosophycal discussion at the present time, rather than a physical discussion...

9. May 5, 2004

### gnl

1. Causality: the order of time-like separated events is not reversible for any observer. Different approach: what if I travel back in time and kill my father as a child??? nobody ever gives a satisfactory answer to this.

2. the existence of parallel universes is not supported by any kind of evidence whatsoever. It might be good maths or science fiction, but not physics. Not yet.

4. if everything is pre-determined, how can you change something that has already happened?

10. May 5, 2004

### alpha_wolf

1. Paralel universes does solve this. If you go back in time from "our" space (paralel universe), the moment you kill your father, you divert into an alternative space. Thus your personal history becomes a what I refered to as a multi spacial timeline. Prederminism can also resolve this, even without paralel universes (see 4).

3. True, it is not supported, but to the best of my knowledge it is not disputed either.

4. That's exactly the point - if everything is predermined, then all event have essentially already occured, including all time travel etc. Therefore events which cancel themselves out (like you going back in time to kill you father) do not exist. If you traveled back in time, you must have been born first, which means your father must have survived long enough to empregnate your mother, which means you have failed to execute your mallicious plan. In a predetrmined universe, there is simply no maneuvering space to create paradoxes.

EDIT: I did not phrase #1 very well. You would actually divert into the alternative space just before you kill "your" father, so what should really happen is that you would actually be killing the father of your alternate self, which therefore doesn't get born. If you then manage to return to the space you started from, you should find that your father is still very much alive.

Last edited: May 6, 2004
11. May 6, 2004

### gnl

1. Let us assume parallel universes exist. The moment I kill my father I get diverted to a new universe. Question: since I can think of infinite variations, do infinitely many universes exist - one for every variation I can imagine like killing my mother instead or my mother and someone else at random - or do I create a new universe in the moment I "bifurcate"? Both options seem very strange. In one case all versions of history you can think of exist somewhere, in the other a single act produces a new reality...

1a. what is "prederminism" ?

3. you can think of many absurd things that are not proved and yet not even proven false. Prove that there is not a race of telepath aliens who live on a planet on some distant galaxy

4. if events have all occurred including time-travel, you need more than one time dimension (I suspect).

12. May 7, 2004

### alpha_wolf

1. This is of course nothing more than speculation, but my guess is that spacetime would be spilt into the two versions at the moment where there occurs a missmatch of event sequences (i.e. kill vs. not kill). I doubt there would be another version for you killing your mother, unless you actually try to kill her... What's more, it may only split "temporarily" (i.e. remerge when any differences cancel out), and/or only locally... In any case, for reasons of energy conservation, I'd guess that you won't actually be creating anything. Instead, my beleif is that the mass you see in each version of the universe is a cross-section of the same total mass. And you don't really need a paralel universe for every version of history, only for where multi spacial timelines are required.

1a. I don't know the formal definition, but basically predeterminism will mean that there is only one possible history of the universe, from begining to end.

3. True. I'm am not saying that paralel universes do in fact exist, nor am I trying to prove their existance. But IMO they aren't absurd at all, and are quite possible. After all, it has been said that reality is far stranger than fiction...

4. Perhaps.

13. May 10, 2004

### gnl

I do not really understand this predeterminism... but I would like to. It does not make any sense to me as I understand it now.

14. May 10, 2004

### Antonio Lao

To me, "predeterminism" means giving the answer before the question. Maybe similar to precognition? Or similar to potentiality in contrast to actuality.

But in physics, the fundamental principle is the action. The Principle of Least Action turns out just to be the difference between potential and kinetic energy. The difference does not indicate any directional property. The same symmetry is implied in time. If a direction can be incorporated into the energy difference then time direction can be determined, e.g., going into the past or going into the future. In any case, the maximum number of direction is two. The state of no motion has no direction but has the potentiality to choose two directions for one dimensional motion, infinite directions for two dimensional motion, and infinite directions for three dimensional motion.

15. May 10, 2004

### alpha_wolf

Back in Newton's days, people said that if we knew the exact position and velocity of every particle in the universe, we could accurately predict any event that will ever happen, as well as calculate every event that had ever occurred in the past (I suppose one would also need to know other properties, such as mass, charge, etc). The situation today is not much different, except that according to today's physics, you'd need to know the exact wave function of every particle. More precisely, you'd need to know the total 4-dimentoinal wave function of the entire universe. Not very practical, I admit. But there is no pysical prevention (that I know of) that this entire wave function is can be described mathematically, even if it is a superposition of multiple extremely complex base functions, and even if we aren't able to do that.

If it can indeed be described accurately, that would mean that all events - past, present, and future - are basically known, even though we don't have access to that knowledge. Everything you or I will ever do, think, and feel, everything that will ever happen to us or elsewhere, all of history - all of that is already "decided", for lack of a better word. That is predetminism. The other option, is that the universe's wave function is not yet defined for time coordinates that are in the future, and we are creating and shaping the future with every dicision that we make.

Another way of thinking about it is this:
From our current perspective, all past events have already occured, and we can tell exactly what they were, to the limits of our historical records. 10000 years from now, we'd be able to say the same about the events during those 10000 years, which are currently ahead of us. Near the end of the universe (assuming that it will ever end, and that we will survive that long), we'll be able to say the same about (almost) all events. If it's possible to travel back in time, than the past and present must somehow coexist, since you cannot physically travel to a place that doesn't exist. In that case, from the perspective of the people back then, none of their future has occured yet. But from our perspective, some of it already has. So if time travel is possible, and our present coexists with other times, what makes you think none of our future has occured yet?

Last edited: May 10, 2004
16. May 10, 2004

### Antonio Lao

The point that I'm trying to get across is the difference between potentiality and actuality and from this difference surmise about the flow of time.

Potentiality is analogous to nonexistence and actuality is analogous to existence. Something can only exist when it starts doing something. If what it is doing cannot be detected then we can say it does not exist.

It does seem to indicate that past and future events coexist as infinite potentiality until a course of action is chosen. The direction of this action is then the flow of time.

17. May 10, 2004

### alpha_wolf

True, we can say that it doesn't exist, but does it really not exist? Personaly, I doubt that is always the case. Consider this thought experiment: you are stuck on an uninhabitted island, without any technology whatsoever. Surely, you cannot detect the continent that is some 1000 km away, but does the continent not exist? I'm sure that the billion or more people that live there would claim it does.
If potentiality is analogous to nonexistence, doesn't this imply that only the now exists, and everything else is little more than our imagination? (And this is where it makes a sharp turn into philosphy land.. )

18. May 10, 2004

### Antonio Lao

Empirical methods cannot determine whether reality is purely potential or purely actual because experiments are only possible to measure the difference. This difference is called the Lagrangian and is given by $L = T - V$, where T is the kinetic energy and V is the potential energy. Then the action is

$$A = \int L dt$$

I am trying to work out a justification for possibility of the square of action given by

$$A^2 = \int \int L^2 dtdt$$

Last edited: May 10, 2004
19. May 10, 2004

### Antonio Lao

If L is exactly zero then the sum over time of the action is also zero. When L is zero it implies that T and V are equal. It does not necessarily mean they don't exist.

Last edited: May 10, 2004
20. May 12, 2004

### gnl

OK, I see. Sounds a bit "fantastic" to me, in the sense that I see no convincing reason to find this theory more acceptable than any other I can come up with to explain time travel. Where is evidence?