Time Travel: Will it ever be possible, how soon, how far, etc.

[mlw]

Hello everyone,
I am new to this forum and would greatly appreciate your expertise in answering (or attempting to) a few questions for me. I don't care if you focus on one or all of them, but any answers are appreciated! These are about time travel, with a focus on traveling BACK in time...

1. Will time travel ever be possible?
2. If so, how soon? Also, how far back would we be able to travel?
3. Would our bodies be able to survive time travel?
4. Would time travellers be able to communicate with other humans, or would they be on a separate plane of time/dimension?
5. Is it possible that spaceship sitings are time travel machines from the future?

Also, if anyone can answer #4 (at least in theory), I'd be grateful.

 

[Mephisto]

probably not. but possibly yes. in any case, if would most likely be time travel that would not be exciting to you at all - moving at most fundamental particles like electrons around in time.

 

[mlw]

Time Travel

Hello everyone,
I am new to this forum and would greatly appreciate your expertise in answering (or attempting to) a few questions for me. I don't care if you focus on one or all of them, but any answers are appreciated! These are about time travel, with a focus on traveling BACK in time...

1. Will time travel ever be possible?
2. If so, how soon? Also, how far back would we be able to travel?
3. Would our bodies be able to survive time travel?
4. Would time travellers be able to communicate with other humans, or would they be on a separate plane of time/dimension?
5. Is it possible that spaceship sitings are time travel machines from the future?

Also, if anyone can answer #4 (at least in theory), I'd be grateful.

 

[FunkyDwarf]

Welcome to PF mlw!

1.) Never say never but it doesn't look good. Wormholes are probably your best bet, pending discovery of some crazy other dimension we can enter.
2.) See one :P
3.) Depends how its done.
4.) Theres really only one dimension of time (although i recently read about two dimensional time but i think the jurys still out on that one) so i see no problem there
5.) Sure, why not =P

All in all there is next to no serious science, that I am aware of, pointing to feasible time travel.

 

[pervect]

It's unclear at this point if time travel is theoretically possible or not (in the context of General Relativity).

Given this state of ignorance, the answers to 2 and 3 are unknown. There's no reason to believe that if time travel were to be possible that there would be any such thing as "separate planes or dimensions" as far as I know.

If your main interest is in #5, I (or another moderator) can move this thread to the skepticism and debunking forum where most talk on UFO's occurs.

 

[oldtobor]

Hello everyone,
I am new to this forum and would greatly appreciate your expertise in answering (or attempting to) a few questions for me. I don't care if you focus on one or all of them, but any answers are appreciated! These are about time travel, with a focus on traveling BACK in time...

1. Will time travel ever be possible?
2. If so, how soon? Also, how far back would we be able to travel?
3. Would our bodies be able to survive time travel?
4. Would time travellers be able to communicate with other humans, or would they be on a separate plane of time/dimension?
5. Is it possible that spaceship sitings are time travel machines from the future?

Also, if anyone can answer #4 (at least in theory), I'd be grateful.

In theory yes. Picture your life or any life as a sequence of events that a combination of atoms, a mass-energy combination performs through a fixed number of steps. If this exact sequence can be replicated then you can have time travel to any destination or any configuration. Look at it as a sequence of energy states, then just reproducing the sequence let's the same events happen all over again.

 

[ΔxΔp≥ћ/2]

I am not a physicist. I have read however that theoretically, it is possible. All possibilities that I am aware of would never transport humans or be remotely economically feasible. Unless, of course something totally ridiculous and illogical that "defies the know laws of physics" comes along. I wouldn't hold my breath. Information is one thing, but as far as transporting people I would say never.

 

[elcabin]

All I know is that if time travel was possible, it wouldn't be made public. Do we know anyone from the past or future?

 

[Math Jeans]

Well, I would generally say that if time travel did exist, then the chances of some messed up guy going back to say...the 20th century and showing us the time machine and how it works would be highly probable.

That is, unless you bring up number four. Unless the traveler is in a different plane, I would deem it highly unlikely.

 

[Ivan Seeking]

If your main interest is in #5, I (or another moderator) can move this thread to the skepticism and debunking forum where most talk on UFO's occurs.

Eh, not really within the scope of the forum. This would require that we assume that time travel is possible, and assumptions like this are not allowed. In S&D we are limited to discussing the evidence [anecdotal or scientific] from specific events, summaries of data and observations, etc. All technical discussions are limited to established scientific principles.

 

[pervect]

Some of this thread has gotten rather wildly speculative, as one might perhaps expect from the topic. Still, there are serious papers about time travel in the physics literature.

In an attempt to get the thread "back on track", I'm going to quote some semi-popular and more professional papers about time travel in the context of General Relativity to try and give interested readers a feeling for the current state of the physics in this frontier area as it is actually written about by the physics community.

John Cramer's "Alternate view" column is written by a physicist, and tends to stick reasonably closely to the facts (Cramer is a physicist himself).

http://www.npl.washington.edu/AV/altvw33.html is, for instance, a discussion of one of the main ways that time travel might be possible in GR, through wormholes. A collection of Cramer's alternate view columns is online, and is a good place to start looking for semi-popular accounts about time travel.

Kip Thorne's book: "Wormholes and time machines: Einstein's outrageous legacy" is another good readable popularization which covers this material.

Now, for some of the published papers (published papers are the best way to understand the topic, but the most work - that's why I opened the thread with some of the better popular media discussions which stick close to the facts.)

http://prola.aps.org/abstract/PRL/v61/i13/p1446_1 is Moris and Thorne's famous paper. The abstract reads:

It is argued that, if the laws of physics permit an advanced civilization to create and maintain a wormhole in space for interstellar travel, then that wormhole can be converted into a time machine with which causality might be violatable. Whether wormholes can be created and maintained entails deep, ill-understood issues about cosmic censorship, quantum gravity, and quantum field theory, including the question of whether field theory enforces an averaged version of the weak energy condition.

This is still a good summary today though the paper is one of the earliest in the field and there have been many developments since then.

http://prola.aps.org/abstract/PRD/v46/i2/p603_1 Hawking's original "chronology protection" paper.

It has been suggested that an advanced civilization might have the technology to warp spacetime so that closed timelike curves would appear, allowing travel into the past. This paper examines this possibility in the case that the causality violations appear in a finite region of spacetime without curvature singularities. There will be a Cauchy horizon that is compactly generated and that in general contains one or more closed null geodesics which will be incomplete. One can define geometrical quantities that measure the Lorentz boost and area increase on going round these closed null geodesics. If the causality violation developed from a noncompact initial surface, the averaged weak energy condition must be violated on the Cauchy horizon. This shows that one cannot create closed timelike curves with finite lengths of cosmic string. Even if violations of the weak energy condition are allowed by quantum theory, the expectation value of the energy-momentum tensor would get very large if timelike curves become almost closed. It seems the back reaction would prevent closed timelike curves from appearing. These results strongly support the chronology protection conjecture: The laws of physics do not allow the appearance of closed timelike curves.

Note that chronology protection is still a conjecture, not a fact, but some particular classes of time machines have been disproved by Hawking's paper, such as:

http://prola.aps.org/abstract/PRD/v9/i8/p2203_1 Tipler's time machine. One of the earlier papers about time machine, this machine has been shown not to work unless the rotating cylinder is actually infinite by Hawking's paper above. This means it can't be built.

Gott's time machine with cosmic strings is another example of a historical time machine proposal which has fallen by the wayside due to Hawking's paper.

http://arxiv.org/abs/gr-qc/9703024 Some debate about the chronology protection postulate. One can probably find many more in this vein, this particular example was taken from the Wikipedia (and checked to make sure it was published).

http://adsabs.harvard.edu/abs/1993PhRvD..47.1432M talks about the "Grandfather paradox" of time travel using billiard balls

http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=PRLTAO000095000002021101000001&idtype=cvips&gifs=yes
Ori's time machine proposal, which is one of the few non-wormhole time machines that gets around Hawking theorem. (I don't really understand the details, though, and I suspect many other readers will be in the same boat. It's still of some interest, though).

One can find also information on Mallet's time machine, in the media and in past threads. I'm not going to link to it because I personally believe Mallet's proposal is flawed - basically I think it falls afoul of Hawking's theorem, and other authors have published critical papers about it (see the past threads).

Mallet doesn't seem to agree with his critics (but I agree with the critics and disagree with Mallet).

There is probably more, but this should be enough reading to get people started in the right direction and away from some of the more overheated media speculation.

 

[mace2]

Here's a question, in the same vein...
I know a conference was held and the idea was to say "Okay, if there's anyone traveling back in time, appear NOW." No one appeared. The idea being that if time travel were accomplished in the future, they could come back to this time to inform us.

Well, does that mean to imply the future has already occurred (in some weird way)? Or is it accepted that the events occurring moment by moment are happening for the first time?
Or is it even impossible to deduce?

Sorry if this is hard to comprehend, it's late and I'm tired but I'm curious! Thanks.

 

[DaveC426913]

Well, I would generally say that if time travel did exist, then the chances of some messed up guy going back to say...the 20th century and showing us the time machine and how it works would be highly probable.

I question this line of thinking. There are a lot of assumptions in it about who gets hold of the technology and what they do with it and how widespread the effect would be.

It seems to me, it is analagous to any potentially highly-disruptive technology such as nuclear weaponry.

"Well, I would generally say that if atomic bombs were invented, then the chances of some messed up guy using them to blow up the planet would be highly probable."

 

[Math Jeans]

I question this line of thinking. There are a lot of assumptions in it about who gets hold of the technology and what they do with it and how widespread the effect would be.

It seems to me, it is analagous to any potentially highly-disruptive technology such as nuclear weaponry.

"Well, I would generally say that if atomic bombs were invented, then the chances of some messed up guy using them to blow up the planet would be highly probable."

However, the circumstances of an Atomic bomb are different. My example with a time machine is this: If a time machine were invented, would it be a mass-produced thing, or would they only be used in labs? Also, would the materials used to create it be rare or things you could get at a hardware store?

The atomic bomb is different. The reason that no messed up guys have been blowing nukes is because 1) Atomic bombs arn't mass-produced and 2) the materials used to make them are NOT found easily.

 

[ZapperZ]

This thread has meandered in many different ways. Please keep in mind that this thread is still in the physics section of PF. Pervect and a few others have tried to keep this issue within the realm of a physics discussion. However, if it continues to meander into guessing about the nature of time machines and wild speculation, this then no longer belongs in here.

So either we get back to the physics discussion, or this is the end of this thread.

Zz.

 

[mlw]

Thanks for the help, Pervect!

I was also wondering how certain most physicists are that wormholes do exist.

I'm assuming if time travel were ever possible, and someone did visit this century, we of course wouln't know about it. I would think by that time the human race would have evolved at least enough to know that that would have an effect on their own place in time. If they were to announce, "Hey, we're from the future!" it would only serve to endanger their own lives.

My main concern is the question as to whether time has already been altered (in other words, has any living creature already traveled back in time?). To me, the only way to be sure is to assume that time travel (no matter how far back) will one day (no matter how far in the future) be possible. Also, is it true that the future has already occurred? I assume this would mean that the past, present, and future are occurring simultaneously (correct me if I'm wrong)?

 

[I-C-US]

Has anybody else ever considered the possibility that time travel has already been made possible by future generations of the human race? The unexplained similarity of extra terestrial beings to the human form may suggest that this could be the case. The physical description from various (unfounded) sightings of Mr.Grey are the same ( depending on individual interpretation) across the globe. Thier ( Mr.Grey) illusiveness is another indication of the possibility of a space, time disruption that could have detremental affects on their current (future) enviroment. It is evolutionary possible that future generations of the human race would look similar to the reported physical appearance of Mr.Grey. Increased levels of inteligence would indicate an increase in the size of the cranium, labour saving devices could indicate a reduction in the need for phisical strength and the complete loss of hair on the human body is already a proven fact ( If Darwin's theory is the basis for thought)...

 

[DaveC426913]

Has anybody else ever considered the possibility that time travel has already been made possible by future generations of the human race? The unexplained similarity of extra terestrial beings to the human form may suggest that this could be the case. The physical description from various (unfounded) sightings of Mr.Grey are the same ( depending on individual interpretation) across the globe. Thier ( Mr.Grey) illusiveness is another indication of the possibility of a space, time disruption that could have detremental affects on their current (future) enviroment. It is evolutionary possible that future generations of the human race would look similar to the reported physical appearance of Mr.Grey. Increased levels of inteligence would indicate an increase in the size of the cranium, labour saving devices could indicate a reduction in the need for phisical strength and the complete loss of hair on the human body is already a proven fact ( If Darwin's theory is the basis for thought)...

Yes it has been thought of before. But this is a science forum, not a fanciful thinking forum.