Is Time Travel Possible and How Does it Work?

[Bugge]

Hello. I've been on this forum for a while. Though I never really bothered clicking the "register" button until now.

I am no scientist (In fact I still haven't reached university yet), so this might be a bit, well, simple-minded.

Recently I've had some thoughts and discussions with my friends about time-travel. While no one has ever drawn any conclusion, I want to bring an end to the discussion:

How is it possible? I mean, if you traveled somewhere in the past and changed it, while still being 'you', how can you really change the past? And example: If you shoot Hitler in 1937, but you still remember the events like holocaust and everything else that happened in and since the second world war (Before the time travel and after the murder), it could not possibly mean that you changed the past since there would be two different pasts for you. Will you create an instance of some sort, or how does this work?

I tried google-fu. But I ended up with many (interesting) theories - way over my head at times.

I am sorry if I'm unclear. I tried my best at 11:25 PM.

Thank you very much for reading this!Bugge

 

[loseyourname]

The Stanford Encyclopedia of Philosophy contains a fairly confusing but comprehensive introduction to paradoxes of closed causal loops. It's a frustrating discussion that isn't going to end and you're certainly not going to end it, but it's interesting to think about if you just want to pass time or write a science fiction story or something. Part of the entry suggests that physics other than relativity might make backwards causation impossible:

A possible solution may be found in a recent result which shows that the most basic features of quantum mechanics may ensure that we could never alter the past, even if it should be possible to interact with the past. The two physicists, Daniel Greenberger and Karl Svozil (2005), imagine some form of quantum mechanical feedback by introducing figurative beam splitters which are unitary, i.e., the splitters allow the feedback loop to be reversed because they have the same number of entry ports and exit ports. From quantum mechanics we know that an object may behave like a wave and that some unitary operator describes the propagation of a physical system. The system is represented by a wave function, also referred to as a path, and the time evolution of the system is calculated as a sum over all possible paths from the initial state to the final state. This calculation is usually restricted to the forward direction of time. Now, if we think of some of the paths as unfolding backwards in time, Greenberger and Svozil are able to prove that either the forward and the backward component paths of the loop cancel out, or that the propagator, which establishes the feedback in time, “wipes out the alternative possible futures, thus guaranteeing the future that has already happened.” Thus, if you could aim at something in the past, the laws of nature prohibit you to act in ways that are in conflict with what makes the future what it is (what it already turned out to be). The authors' conclusion is that if you go back in time or effect “the past quantum mechanically, you would only see those alternatives consistent with the world you left behind you.”

 

[Chronos]

I think, were time travel possible, you would probably erase the future from which you departed, effectively taking a one way trip to an alternative universe. There are, of course, other problems. I suspect the universe would object to losing mass without compensation. The gravitational effects alone would have untold consequences. Let's say, for the sake of argument, you could send the sun backwards in time. This would be catastrophic for the 'present' solar system. The planets would immediately wander off into space. And injecting another sun anywhere near the 'past' solar system would be similarly disruptive - not unlike killing everyone's grandfather. It is at best paradoxical, and probably violates any number of physical laws.

 

[nucleargirl]

I don't think time travel would work! How would you travel back in time? how would it be physically possible? I don't really understand worm holes... how can a black hole become a worm hole?
and I think if time travel did happen... then every time something traveled back in tie, then whole universe would change - its chaos! right? like a small change in the past will trigger who-knows-what in the future cos of chance?

 

[nismaratwork]

I don't think time travel would work! How would you travel back in time? how would it be physically possible? I don't really understand worm holes... how can a black hole become a worm hole?
and I think if time travel did happen... then every time something traveled back in tie, then whole universe would change - its chaos! right? like a small change in the past will trigger who-knows-what in the future cos of chance?

If you don't believe that travel in time (to a relative past) isn't possible in this universe at this time, then you're in good company. There are many conjectures and theories to explain this, ranging from the philosophical, to quantum physical explanations.

http://en.wikipedia.org/wiki/Novikov_self-consistency_principle

Tourism in time
Stephen Hawking once suggested that the absence of tourists from the future constitutes an argument against the existence of time travel—a variant of the Fermi paradox. Of course this would not prove that time travel is physically impossible, since it might be that time travel is physically possible but that it is never in fact developed (or is cautiously never used); and even if it is developed, Hawking notes elsewhere that time travel might only be possible in a region of spacetime that is warped in the right way, and that if we cannot create such a region until the future, then time travelers would not be able to travel back before that date, so "This picture would explain why we haven't been over run by tourists from the future."[17] Carl Sagan also once suggested the possibility that time travelers could be here, but are disguising their existence or are not recognized as time travelers.[18]

How you travel backwards in time would require FTL travel of some kind, or exotic geometries which achieve a similar result such as wormholes or "warp". In the end, you're still moving from point A-B-A faster than light could make the same trip.

For the rest, a black hole does not BECOME a wormhole, but in theory it could be an environment in which one could exist. That wormhole shouldn't be traversable however. http://en.wikipedia.org/wiki/Einstein-Rosen_Bridge

An example of a test particle falling into one: http://www.physics.indiana.edu/~nipoplaw/PLB_687_110.pdf

 

[Chronos]

Consider, nucleargirl, the initial universe was fine tuned to within less than one part in a septillion. Injecting [or subtracting] almost any amount of mass into the very early universe would have been catastrophic. It would have either collapsed, or been rendered incapable of producing stars, galaxies, etc.

 

[Xtensity]

I've heard an idea saying that, if one actually traveled back in time to say, kill their grandparents, they(the person) would still exist, just the act of changing the past would cause another multiverse to form. Meaning you would no longer be in your universe/[specific multiverse]. This may not be completely veritable, so excuse me if it's inaccurate since it's based on a theory.

 

[nismaratwork]

I've heard an idea saying that, if one actually traveled back in time to say, kill their grandparents, they(the person) would still exist, just the act of changing the past would cause another multiverse to form. Meaning you would no longer be in your universe/[specific multiverse]. This may not be completely veritable, so excuse me if it's inaccurate since it's based on a theory.

That would be a kind of MWI (Many Worlds Interpretation). There is thinking that: we live in the present, and that includes any influence in the past including travel from the future. There is Novikov's principle, and others that are similar in stating that traveling back in time is like rewinding a video; you can participate in it, but being "written" you cannot change anything. In that sense, traveling in time is the same as making a puppet of yourself. Then you have Hawking's view that energy densities of certain transiting particles would cause a time machine to "blow up" upon activation.

The explanation that holds the most water with me is just that the curvature needed to achieve backwards travel in time doesn't exist in the universe, outside of theory. I'm not saying you can't put the design for that topology on paper, just that it doesn't have a physical reality.

 

[Xtensity]

That would be a kind of MWI (Many Worlds Interpretation). There is thinking that: we live in the present, and that includes any influence in the past including travel from the future. There is Novikov's principle, and others that are similar in stating that traveling back in time is like rewinding a video; you can participate in it, but being "written" you cannot change anything. In that sense, traveling in time is the same as making a puppet of yourself. Then you have Hawking's view that energy densities of certain transiting particles would cause a time machine to "blow up" upon activation.

The explanation that holds the most water with me is just that the curvature needed to achieve backwards travel in time doesn't exist in the universe, outside of theory. I'm not saying you can't put the design for that topology on paper, just that it doesn't have a physical reality.

Well the way I see it, anything relating to the past and other universe is all theories. There's no mainstream science Time-Travel devices, let alone any devices which would allow for visiting other potential multiverses. Personally, if there is other universes existent other than ours with life just as intelligent as us, then I don't think it's our business to be screwing with them.. Since we don't know for a fact what would happen if we actually did travel back in time and change the past.

 

[nismaratwork]

Well the way I see it, anything relating to the past and other universe is all theories. There's no mainstream science Time-Travel devices, let alone any devices which would allow for visiting other potential multiverses. Personally, if there is other universes existent other than ours with life just as intelligent as us, then I don't think it's our business to be screwing with them.. Since we don't know for a fact what would happen if we actually did travel back in time and change the past.

I think that's a fine mix of the practical and the ethical, and it works for me.

 

[Stcloud]

If you had a time machine which you could sit in and press a panel which read : move me backward in time... you could press the panel and begin to move back in time.. until you reached the time of : before you had pushed the panel. ! Ad infinitum.. meanwhile, ' time' would be marching on.

 

[jambaugh]

Understanding first that this is rank speculation...

I had a fanciful thought once about the nature of time which I developed a bit and it offered one resolution to time travel paradoxes.

First imagine the evolution of the universe over time, and the division of time into fixed past, active present, and possible future, in the analogue of a crystal --say ice-- forming around a nucleus (the big bang).

One can view matter particles as defects in the crystal propagating according to rules but with a certain randomness (intrinsic quantum uncertainty). The surface where the solid is forming is the "now" and you might note that after freezing one can slice across an event in many different ways, the plane of simultaneous events is relative.

If any inconsistencies develop in the history then either they are isolated by being frozen over (ambiguous past quantum states never observed) or, if not, they produce stresses on the crystal instigating a thawing and refreezing until a consistent history manifests.

So given this model of history, for one to go back into the past one would have to propagate (as a collection of defects) back into past layers which necessitates thawing history back to the target time. Once this occurs then one would be propagating within a free floating piece of crystal which must remerge into the history consistently... or more likely one's personal history would melt and the past refreeze in a consistent but alternative way, or one never makes it back and spawns an alternative universe forming around ones free floating crystalline history.

Again this is beyond speculation, more a fantasy model, suitable for premises in a science fiction novel. But there's a certain beautiful consistency to it. One can imagine the God-like power of being able to swim in the ocean of possibility selectively thawing history until it happens the way you want then rejoining the evolving now.

 

[jiohdi]

why time travel is impossible:
the mass of the universe Ma-z is being left out of any possible equations when it comes to moving along spacetime lines... you may find a way to move your mass Mx to a prior space time coordinate XYZTp however you will not find a way to move the Ma-z to XYZTp because it will be where it is now.. .XYZTnow... and no machine could ever collapse the universe to a prior moment.

[XYZTnow]Mx --> XYZTp =! Ma-z --> XYZTp

 

[Lorentz Trans]

I woke up this morning and tuned in CNN television. I caught the end of a story about how a scientist in Hong Kong proved that time travel is impossible. I searched recent news stories on line and came up with this posted July 24, 2011: "The Hong Kong University of Science and Technology research team led by Du Shengwang said they had proved that a single photon, or unit of light, "obeys the traffic law of the universe."

I suppose that is what the CNN news was referring to. Does anyone out there have any knowledge of this experiment or area of study? I am thinking that he must have demonstrated that a single photon is subject to the same limitation as electromagnetic waves. Is this new information?

 

[Joncon]

Here's an article about it: -

http://www.physorg.com/news/2011-07-hk-physicists-photons.html"

 

[Lorentz Trans]

why time travel is impossible:
the mass of the universe Ma-z is being left out of any possible equations when it comes to moving along spacetime lines... you may find a way to move your mass Mx to a prior space time coordinate XYZTp however you will not find a way to move the Ma-z to XYZTp because it will be where it is now.. .XYZTnow... and no machine could ever collapse the universe to a prior moment.

[XYZTnow]Mx --> XYZTp =! Ma-z --> XYZTp

jiohdi,

say what? Can you explain that in layman's terms. I just read Einstein's 70 page explanation of Special and General relativity last night and it was loaded with metaphor and easy to grasp visual models (train tracks and lanterns etc). He even apologized in advance for repeating himself. But his discourse was beautifully written. It made it all seem at once accessible, exciting and mysterious. What do you mean by "collapse the universe to a prior moment".

 

[jiohdi]

jiohdi,

say what? Can you explain that in layman's terms. I just read Einstein's 70 page explanation of Special and General relativity last night and it was loaded with metaphor and easy to grasp visual models (train tracks and lanterns etc). He even apologized in advance for repeating himself. But his discourse was beautifully written. It made it all seem at once accessible, exciting and mysterious. What do you mean by "collapse the universe to a prior moment".

think of matter/energy as billiard balls upon a table... all the balls in motion... you can take pictures at intervals and make a grid and document where each ball was in each frame... but if you somehow grab one of the balls and place it where it as say a hundred frames ago, how will that effect all the other balls in motion? My point is that it will only move that one ball to a location it used to be in without changing the CURRENT location of any of other balls... it will not cause all of the other balls to magically move to where they were a hundred frames ago... and there can be no machine powerful enough to move all the balls back to where they were one hundred frames ago... which is what time travel would necessitate, because as far as all rational science indicates, the balls representing the energy of the universe do not leave copies of themselves where they used to be... the energy does not exist in all moments, but only in the present moment... otherwise there is no explanation for motion at all.

 

[Lorentz Trans]

jiohdi, I think that is a good intuitive analysis of what moving back in time would require. I think you are saying that going back in time is the equivalent of rewinding the big bang itself. Since the energies are unspeakably massive, it does indeed appear that it is impossible.

I think most of our fantasies about time travel implies that we are somehow slipping through some fanciful time wrinkle. The only way I can imagine approximating traveling back in time would be as a time tourist. Certainly traveling to the past as a time tourist will be possible for generations after us as long as we store the evidence of our lives. Maybe we will develop some kind of super surveillance where virtually everything is recorded, stored and perpetually kept energized in cyber space. But I travel back in time now every time I open my old yearbook, watch an old movie or watch the news for that matter.

 

[GrayGhost]

OK,

Now, we all know there is no evidence to support time travel. However ...

The Many Worlds theory assumes all possible outcomes at any instant always happen. We often see sci-fi flicks that show someone traveling back in time to their past, eg in the case of Back To The Future for example. Then, because he traveled back and interacted, when he returns to his present, everything is much different because he altered the past (a new World of many). However, if Many Worlds is the model of reality, and assuming backwards time travel possible, then shouldn't it be a foregone conclusion that when you get there "that said past would be totally different from the past that gave rise to you"? I mean, why should you return to a prior point wrt your own timeline, versus some other timeline altogether?

GrayGhost

 

[kevinfrankly]

parallel universe !

 

[GrayGhost]

parallel universe !

Of course, but you missed my point there.

Most all movies show the fellow going back in time to a past that's "100% identical to his own past". Seems to me that if one traveled back, that he could end up "in any past", ie a past of any parallel universe, and most likely a past "not of his own".

I can only recall one show presenting it right, a Star Trek Voyager episode that sent an ensign to his past ... but a past that differed a little bit from his own past. However, the concept of that episode had some holes in it as well.

GrayGhost

 

[Lorentz Trans]

Well, since any attempt to imagine ourselves personally in the past appears to be totally conjecture, I wonder if anyone who has seen the move "Source Code" has any thoughts about how the screen writers attempted to make the story idea plausible. The head scientist (in the screenplay) in charge of the project justified his theory and experiments on "Quantum Physics". This is a physics forum and I don't want to "go Hollywood" but does anyone have a sense of what "science" is applied, or more likely perverted, when the story was conceived?

And secondly, to get a little more back on track, has anyone read and cares to comment on the book by particle physicist Amit Goswam. The book is titled, Self-Aware Universe. The subtitle is: How Consciousness Creates the Material World. Amit Goswan was a physicist at the University or Oregon when I worked there 20 years ago. I don't know how well known he is or if his theories are considered crackpot or sound. He seems like an intelligent man and it is a very interesting read, but when a new physics theory is expounded by someone who is heavily influenced by Eastern religion, it does make one wary.

But that is another question. What if the rigorous physical scientist, no matter how brilliant and persistent, has now knocked on the last physical door of science and the only thing on the other side is in fact indistinguishable from religion?

My friend Don Slater who got his PHD in physics at the University of Washington, told me just after he had earned his degree that he thought physics had reached the limitation of physical exploration. He seemed disappointed, like someone who had spent his whole academic career expecting his dedication and focus to bring him to a point of understanding the physical world, only to discover that neither he, or anyone, could ever achieve it.

I don't know if he feels that way today or if he even remembers telling me so, but I still stand in awe of his effort and that of the community of physicist who apply themselves with so much rigor and honesty to their quest.

But here is the dark side of my awe of physicist:When I studied Physics at the University of Oregon, I was shocked at the level of egotism. competition and elitism in the department. Lately, I watched a Science channel program featuring Stephen Hawking's humiliation. He was basically "eating crow" for his assertion that black holes permanently lose information. It was like watching celebrity gossip. He was built up to be the second coming of Sir Issac Newton and then shot down like a fraud. His own egoistical revision of this mistake, promoted by himself as a "revelation" was apparently not that Earth shattering and largely dismissed. But what the hell is wrong with you guys? Why are you snobs? Why are you cruel and egotistical? You know who you are. The latest theories are so incredibly fanciful, I can't even take them seriously anymore. Parallel universes, string theory, duality etc, on and on and on. Something very simple is being missed. It is probably right under our noses. But beware, it may be divine and your character may come into account before you are allowed to understand it.

Go to http://www.youtube.com/watch?v=2V6SaBflpiM&playnext=1&list=PLF4837F6A720AAD90 start at 5 minutes and 30 seconds to here Amit Goswami explain "actuality".

Imagination is more important than knowledge...
Albert Einstein

 

[GrayGhost]

Lorentz Trans,

No, I haven't seen "Source Code" as yet. So I cannot comment on that story-line in relation to consistency with leading theoretical physics.

Wrt Stephen Hawking, he admitted to jumping the gun, but no one can take away his prior successes, eg unifying GR and QM wrt balck hole evaporation. That creation of physics was a work of art, maybe second to none (or very few). His ascertion of information loss from black holes was a valid statement given the knowns of that day. Susskind showed one possible solution that could save information loss, and then Hawking showed yet another implying Susskind had it wrong as well. Maybe one is right, maybe neither are correct. Time will tell. The ego is almost inescapable, and it likely stirs competition that aids in the development of correct theories by far and large. Newton was rather competitive and egotisical as well. It goes with the territory, and is human nature. Everyone wants to be right. At least Hawking admitted his as a mistake, which does not come easily. I mean, it's not as though Susskind definitively proved Hawking wrong. It was more that Susskind showed Hawking that alteratives exist, and few to none accepted deep down that information could ever be lost.

Admittedly, we do have a rather large number of fantastical competing theories floating about, each compelling in their own way. Yet, they are not all correct. I would not be surprised at all if one day we find that Einstein had it right all along ... QM was incomplete. That QM probability arises from a yet unknown certainty that lies beneath. Yet that's a hunch, and that with $3.26 buys you a grande latte at Starbucks. Another possibility is that Penrose has it right, that when the collective gravitational field of a small body is large enough, the wave function collapses and certainty reigns. Yet, one still needs to explain why uncertainty exists at the QM level. While I find Many Worlds/Parallel Universes interesting enough, and do not rule them out, I do find them rather fantastical. And, how to accept something as true that can never be proven other than by a stretch of the imagination? It's not as though time travelers are popping up proving when they come from. And, how to prove another universe exists when we are made of particles that can only discern our own dimensions.

I did view your reference for the discussion of the relation between conciousness and material entity by Amit Goswami. His video seemed more philospohy than physics. While it's interesting to ponder the relationship between conciousness and physics, it likely does not help much in the advancement of physics which to date has many compelling unanswered questions. We have superluminal theories, many worlds, parallel universe, entanglement, holographic theories, no known source of mass, no explanation as to why mass warps space, an insufficient understanding of time, etc etc etc. The double slit experiment does not IMO require a sentient observation to collapse the wavefunction. That is, it's not the fact that a sentient being made the observation, but rather that a material detector (large enough) was placed in a required location to collapse it. So IMO, Penrose probably has it right, although hen hasn't proven it as yet either.

The more we learn, the more questions we have. It will be unfortunate should we have no more questions to ever answer, so good and bad :)

GrayGhost

 

[Imax]

Possible recipe for a time machine:

1) Find a closed time-like curve (CTC), and
2) Build a vessel that can circumnavigate this CTC in space-time.

But, you could have big problems. :)

 

[GrayGhost]

Yes,

Pretty much what http://www.youtube.com/watch?v=V_beCH4TmNA&feature=related" is about. Unfortunately, this cable TV interview was to only website I could find where he discussed it "briefly", but it get's the gist of it. First thing he should do is send lottery numbers back :)

GrayGhost

 

[Imax]

First thing he should do is send lottery numbers back :)

Yes, and with the money from next weeks lottery, you may have enough to build a time machine (Wait, wouldn't that creat its own paradox?) :)

There's also:



(can't find part 1?)

 

[Imax]

Here's a fun read:

http://arxiv.org/PS_cache/arxiv/pdf/1003/1003.3828v1.pdf

It doesn't look too good for a time traveler.

 

[GrayGhost]

Here's a fun read:

http://arxiv.org/PS_cache/arxiv/pdf/1003/1003.3828v1.pdf

It doesn't look too good for a time traveler.

Indeed, how's about you go first, I'll go second?

GrayGhost :)

 

[Imax]

But it’s safe if it’s Minkowski space, so you go first. Don’t forget to send those lotto numbers. :)

 

[stefanbanev]

OK,

Now, we all know there is no evidence to support time travel. However ...

The Many Worlds theory assumes all possible outcomes at any instant always happen. We often see sci-fi flicks that show someone traveling back in time to their past, eg in the case of Back To The Future for example. Then, because he traveled back and interacted, when he returns to his present, everything is much different because he altered the past (a new World of many). However, if Many Worlds is the model of reality, and assuming backwards time travel possible, then shouldn't it be a foregone conclusion that when you get there "that said past would be totally different from the past that gave rise to you"? I mean, why should you return to a prior point wrt your own timeline, versus some other timeline altogether?

GrayGhost

Each observation of random event (like radioactive decay) does not depend on context (Markov process) thus, the "time traveler" has no advantage to predict its outcome. Therefore it is impossible to go twice along the same "time line" so, once you jump "back" to the reality you would recognize as your past (as you know it), you have no advantage to win a lottery if the lottery is based on the random process. However, it seems you "may (?)" have some advantage if the process is not purely random and has a context dependency (I may be wrong here). It seems that if ever such "tourism" may be technically possible it will be a way simpler to travel to the realities which have no resemblance to the past as you know it simply because the subset of realities similar to the history as you know it is minuscule small comparatively with all possible histories; for the same reason to get back "home" would be quite a challenge ;o(

 

[Imax]

to get back "home" would be quite a challenge

Following a Closed Timelike Curve (CTC) doesn't mean you go backwards in time. Locally, you still experience time going forwards, but as you follow this CTC you can end up at some time before you started. The problem is that it can take a very, very long time to follow a CTC, and as you're traveling along the CTC, spacetime continues to expand. It could be challenging to identify "home".