Foreigners

[Sky Walker]

So, I was thinking. The sun has been burning since the birth of our galaxy, the creation of our planet, planet Earth. The Earth and the Sun are the same old/young age of 4.6 Billion years. The Sun, halfway through its life at 4.6 Billion years old has burned up approximately half of its total energy. This leads me to believe that the Sun will burn for a further 4.6 Billion years then maybe collapse in on itself causing a black hole and sucking in our galaxy or just dying out no longer in existence to provide our energy, either way life on Earth will end.

Knowing this I assume that, if the Earth is not hit by meteorites the size of small countries or if the human race manages to somehow avoid all the other natural disasters and man made disasters, nuclear wars, alien invasions, global warming anything you can think of that will wipe out the human race, if they can sidestep all that then it is possible that humans could go on evolving and making technological advances for the next 4.6 Billion years.

Take a second to think how far we have come in the last hundred years.

So tell me, don't you think it is extremely likely and highly probable that with all that time someone will eventually have all the tools and knowledge to actually travel through time?

Thats what I think sometimes when I see foreigners, dressed different and acting different, I think, hey that could actually be a person from the future, obviously anyone traveling back through time would be under the strictest of rules and regulations and would not be aloud to tell anyone anything about the future but still you gotta ask, any members of the forum from the future?

Or has anyone ever tried to convince you that they were from the future?
If so
Did you just completely disbelieve it and mark it up to a psychological episode?

ANY comments or opinions.

[Nature Boy]

It doesn't matter how technologically advanced we get, time travel, as thought by many of the world's leading physicists, is an impossible dream that simply can't work because of the rules of physics. I think even Einstein wrote off the possibility. In the future however, I hope the human race can find a way of travelling to far away planets and galaxies in order to escape the devastation of the end of the solar system.

[Sky Walker]

Originally Posted by nature_boy_liam

It doesn't matter how technologically advanced we get, time travel, as thought by many of the world's leading physicists, is an impossible dream that simply can't work because of the rules of physics.

Do the laws of physics apply to photons?

[IHrtHalucingens]

"Scientists" also thought the earth was flat at one point. Maybe we just dont have the technology yet to even understand the possibility of time travel. I wouldnt say anything is totally impossible for the future because i have no idea what we're going to discover in the far future. And my guess is that the space travel will be closer in the future than time travel, which means that maybe we come into contact with a more advanced species that has the technology or we find a planet or something that has different laws of physics therefore making what we thought to be impossible here on earth, very possible in another solar system somewhere.

I am nowhere near knowledgable in any of the technicalities of time or space travel, i just think that anything is possible for the future and theres really no way of predicting it, too many variables. So i wouldnt rule out anything.

[falsereality]

well if time travel is possible that would prove a few other things -

time would have to be recorded by some superbeing or something like that

time would be a dimension

even if time travel did develope, if u put aside that u can travel through time. there would still be an incredible intricate process ahead of a time traveler. first over all the earth isnt all ways in the same place in space, and actually im pretty sure ive learned that the sun moves too. so u would have a hell of a time pin pointing the EXACT place in the UNIVERSE that ur destination would be and WHEN.
u may decide u want to go back to the 60s and enjoy so acid for $2 a hit. youll have to figure exactly where the sun was at that time, figure where how far away from the sun the earth was (whose can say we're alway the same radius from the sun?) where the earth would be in its revolusion, and then figure out where the Grateful Dead was on the rotation of earth, and finally where that groud would be. then ull have to find someone with balls to trust u did fuck up this coordinate.

but hey im not saying anythings impossible, i mean we did figure out the earth is round, anything is possible

[Alfa]

Originally Posted by falsereality

time would have to be recorded by some superbeing or something like that

That's like saying that if a tree falls while there is no one there, it'll make no sound. The existance natural phenomenons or dimensions do not depend upon them being recorded by anything.

Originally Posted by falsereality

first over all the earth isnt all ways in the same place in space, and actually im pretty sure ive learned that the sun moves too. so u would have a hell of a time pin pointing the EXACT place in the UNIVERSE that ur destination would be and WHEN.

The whole universe moves. In fact everyting does. There is no still point in the universe. Only the relation between objects. And on top of that gravity attracts curves both light and time.

Now bring it on...

[Alfa]

Science is only limited to imagination.

[daeron]

yea time travel is mathematically possible, certanly. you just have to either:
a)create and place a black and a "white" hole, and somehow manage to go though the black one w the speed slightly lower than the speed of light
b)make the black hole rotate somehow, so you dont really have to pass thru the singularity w the near speed of light, but just cruse in.
c)rent grease and start dressing up accordingly

[Nature Boy]

Updated 1994;
Original by Jon J. Thaler.


Time Travel - Fact or Fiction?

We define time travel to mean departure from a certain place and time followed (from the traveller's point of view) by arrival at the same place at an earlier (from the sedentary observer's point of view) time. Time travel paradoxes arise from the fact that departure occurs after arrival according to one observer and before arrival according to another. In the terminology of special relativity time travel implies that the timelike ordering of events is not invariant. This violates our intuitive notions of causality. However, intuition is not an infallible guide, so we must be careful. Is time travel really impossible, or is it merely another phenomenon where "impossible" means "nature is weirder than we think?" The answer is more interesting than you might think.
The Science Fiction Paradigm

The B-movie image of the intrepid chrononaut climbing into his time machine and watching the clock outside spin backwards while those outside the time machine watch the him revert to callow youth is, according to current theory, impossible. In current theory, the arrow of time flows in only one direction at any particular place. If this were not true, then one could not impose a 4-dimensional co-ordinate system on space-time, and many nasty consequences would result. Nevertheless, there is a scenario which is not ruled out by present knowledge. This usually requires an unusual spacetime topology (due to wormholes or strings in general relativity) which has not yet seen, but which may be possible. In this scenario the universe is well behaved in every local region; only by exploring the global properties does one discover time travel.
Conservation Laws

It is sometimes argued that time travel violates conservation laws. For example, sending mass back in time increases the amount of energy that exists at that time. Doesn't this violate conservation of energy? This argument uses the concept of a global conservation law, whereas relativistically invariant formulations of the equations of physics only imply local conservation. A local conservation law tells us that the amount of stuff inside a small volume changes only when stuff flows in or out through the surface. A global conservation law is derived from this by integrating over all space and assuming that there is no flow in or out at infinity. If this integral cannot be performed, then global conservation does not follow. So, sending mass back in time might be all right, but it implies that something strange is happening. (Why shouldn't we be able to do the integral?)
General Relativity

The possibility of time travel in GR has been known at least since 1949 (by Kurt Godel, discussed in [1], page 168). The GR spacetime found by Godel has what are now called "closed timelike curves" (CTCs). A CTC is a worldline that a particle or a person can follow which ends at the same spacetime point (the same position and time) as it started. A solution to GR which contains CTCs cannot have a spacelike embedding - space must have "holes" (as in donut holes, not holes punched in a sheet of paper). A would-be time traveller must go around or through the holes in a clever way.

The Godel solution is a curiosity, not useful for constructing a time machine. Two recent proposals, one by Morris, et al. [2] and one by Gott [3], have the possibility of actually leading to practical devices (if you believe this, I have a bridge to sell you). As with Godel, in these schemes nothing is locally strange; time travel results from the unusual topology of spacetime. The first uses a wormhole (the inner part of a black hole, see figure 1 of [2]) which is held open and manipulated by electromagnetic forces. The second uses the conical geometry generated by an infinitely long string of mass. If two strings pass by each other, a clever person can go into the past by travelling a figure-eight path around the strings. In this scenario, if the string has non-zero diameter and finite mass density, there is a CTC without any unusual topology.
Grandfather Paradoxes

With the demonstration that general relativity contains CTCs, people began studying the problem of self-consistency. Basically, the problem is that of the "grandfather paradox": What happens if our time traveller kills her grandmother before her mother was born? In more readily analyzable terms, one can ask what are the implications of the quantum mechanical interference of the particle with its future self. Boulware [5] shows that there is a problem - unitarity is violated. This is related to the question of when one can do the global conservation integral discussed above. It is an example of the "Cauchy problem" [1, chapter 7].
Other Problems (and an escape hatch?)

How does one avoid the paradox that a simple solution to GR has CTCs which QM does not like? This is not a matter of applying a theory in a domain where it is expected to fail. One relevant issue is the construction of the time machine. After all, infinite strings aren't easily obtained. In fact, it has been shown [4] that Gott's scenario implies that the total 4-momentum of spacetime must be spacelike. This seems to imply that one cannot build a time machine from any collection of non-tachyonic objects, whose 4-momentum must be timelike. There are implementation problems with the wormhole method as well.
Tachyons

Finally, a diversion on a possibly related topic.

If tachyons exist as physical objects, causality is no longer invariant. Different observers will see different causal sequences. This effect requires only special relativity (not GR), and follows from the fact that for any spacelike trajectory, reference frames can be found in which the particle moves backward or forward in time. This is illustrated by the pair of spacetime diagrams below. One must be careful about what is actually observed; a particle moving backward in time is observed to be a forward moving anti-particle, so no observer interprets this as time

t
One reference | Events A and C are at the same
frame: | place. C occurs first.
|
| Event B lies outside the causal
| B domain of events A and C.
-----------A----------- x (The intervals are spacelike).
|
C In this frame, tachyon signals
| travel from A-->B and from C-->B.
| That is, A and C are possible causes
of event B.

Another t reference | Events A and C are not at the same frame: | place. C occurs first. | | Event B lies outside the causal -----------A----------- x domain of events A and C. (The | intervals are spacelike) | | C In this frame, signals travel from | B-->A and from B-->C. B is the cause | B of both of the other two events.

The unusual situation here arises because conventional causality assumes no superluminal motion. This tachyon example is presented to demonstrate that our intuitive notion of causality may be flawed, so one must be careful when appealing to common sense. See the FAQ article on tachyons, for more about these weird hypothetical particles.
Conclusion

The possible existence of time machines remains an open question. None of the papers criticizing the two proposals are willing to categorically rule out the possibility. Nevertheless, the notion of time machines seems to carry with it a serious set of problems.
References

1. Kip S. Thorn, "Black Holes and Time Warps" Norton and Co (1994)
2. S.W. Hawking, and G.F.R. Ellis, "The Large Scale Structure of Space-Time," Cambridge University Press, (1973).
3. M.S. Morris, K.S. Thorne, and U. Yurtsever, PRL, v.61, p.1446 (1989). How wormholes can act as time machines.
4. J.R. Gott, III, PRL, v.66, p.1126 (1991). How pairs of cosmic strings can act as time machines.
5. S. Deser, R. Jackiw, and G. 't Hooft, PRL, v.66, p.267 (1992). A critique of Gott. You can't construct his machine.
6. D.G. Boulware, University of Washington preprint UW/PT-92-04. Available on the hep-th@xxx.lanl.gov bulletin board: item number 9207054. Unitarity problems in QM with closed timelike curves.
7. "Nature", May 7, 1992. Contains a very well written review with some nice figures.

[xx3]

Time travel is fiction. That for about the time travel we know from fiction.
But, if you put John in the fridge and defrost it one trillion years later, then, well, John could call that jump as time travel.
Want to travel to the past? Ok, make history and archeology perfect sciences, then compile all data to a virtual reality program, and voila, enter the matrix, lads end gents. Thats the best as time travel could be.
If not, well, drop acid or something, you can have a real time trip.

[motorhead]

This is great. Does anyone remember the Disney turd from '79, The Black Hole?http://www.space.com/sciencefiction/...ve_000602.html
Loved that movie when i was a kid. Used to be really into sci-fi, astromony and the whole relitivity thing until that Stephen Hawkins character came along with his wacky ideas. Einstein rules baby!
Found this while i was googling the flick.http://casa.colorado.edu/%7Eajsh/singularity.html talks about going near a black hole and stuff. No mention of grease though.

[daeron]

sheesh motor, you never heard of uncle hawkings grease tekk? pfff amateur.

but seriously-its a strange coincidence im reading a book that very briefly, but also in great detail goes into the subject of time travel.

it seems that there are a number of theoretical ways to accomplish this from actually slowing down the light it self to create a CTC to froming, to put it plainly a "relativistic bubble", basically to harness a shitload of negative energy to make the space curve in such a way that will enable a object to travel faster than the speed of light, which is actually allowed in the global relativistic theory.

but after some serious blah blah, the bottom line is that in every scenario you need some immence amount of energy, which is (if you respect E=mc2, ie energy proportional to mass) far more than the energy of the universe it self.

so as they have plainly put it-how to travel thru time? just do nuffin&wait


it seems that this is the modern phylosophy, the theoretical psysicists and mathematicians, completely useles, like phylosophers were most of the time, all untill the point in time when it isnt. which im afraid is far from now. unless you use the grease tekk, that is