With the new Star Trek out, it’s long past time (as it were) that we laid out the rules for would-be fictional time-travelers. (Spoiler: Spock travels to the past and gets a sex change and becomes Kirk’s grandfather lover.*) Not that we expect these rules to be obeyed; the dramatic demands of a work of fiction will always trump the desire to get things scientifically accurate, and Star Trek all by itself has foisted half a dozen mutually-inconsistent theories of time travel on us. But time travel isn’t magic; it may or may not be allowed by the laws of physics — we don’t know them well enough to be sure — but we do know enough to say that if time travel were possible, certain rules would have to be obeyed. And sometimes it’s more interesting to play by the rules. So if you wanted to create a fictional world involving travel through time, here are 10+1 rules by which you should try to play.
0. There are no paradoxes.
This is the overarching rule, to which all other rules are subservient. It’s not a statement about physics; it’s simply a statement about logic. In the actual world, true paradoxes — events requiring decidable propositions to be simultaneously true and false — do not occur. Anything that looks like it would be a paradox if it happened indicates either that it won’t happen, or our understanding of the laws of nature is incomplete. Whatever laws of nature the builder of fictional worlds decides to abide by, they must not allow for true paradoxes.
1. Traveling into the future is easy.
We travel into the future all the time, at a fixed rate: one second per second. Stick around, you’ll be in the future soon enough. You can even get there faster than usual, by decreasing the amount of time you experience elapsing with respect to the rest of the world — either by low-tech ways like freezing yourself, or by taking advantage of the laws of special relativity and zipping around near the speed of light. (Remember we’re talking about what is possible according to the laws of physics here, not what is plausible or technologically feasible.) It’s coming back that’s hard.
2. Traveling into the past is hard — but maybe not impossible.
If Isaac Newton’s absolute space and time had been the correct picture of nature, we could simply say that traveling backwards in time was impossible, and that would be the end of it. But in Einstein’s curved-spacetime universe, things are more flexible. From your own personal, subjective point of view, you always more forward in time — more technically, you move on a timelike curve through spacetime. But the large-scale curvature of spacetime caused by gravity could, conceivably, cause timelike curves to loop back on themselves — that is to say, become closed timelike curves — such that anyone traveling on such a path would meet themselves in the past. That’s what respectable, Einstein-approved time travel would really be like. Of course, there’s still the little difficulty of warping spacetime so severely that you actually create closed timelike curves; nobody knows a foolproof way of doing that, or even whether it’s possible, although ideas involving wormholes and cosmic strings and spinning universes have been bandied about.
3. Traveling through time is like traveling through space.
I’m only going to say this once: there would be no flashing lights. At least, there would only be flashing lights if you brought along some strobes, and decided to start them flashing as you traveled along your closed timelike curve. Likewise, there is no disappearance in a puff of smoke and re-appearing at some other time. Traveling through time is just like traveling through space: you move along a certain path, which (we are presuming) the universe has helpfully arranged so that your travels bring you to an earlier moment in time. But a time machine wouldn’t look like a booth with spinning wheels that dematerializes now and rematerializes some other time; it would look like a rocket ship. Or possibly a DeLorean, in the unlikely event that your closed timelike curve started right here on Earth and never left the road.
Think of it this way: imagine there were a race of super-intelligent trees, who could communicate with each other using abstract concepts but didn’t have the ability to walk. They might fantasize about moving through space, and in their fantasies “space travel” would resemble teleportation, with the adventurous tree disappearing in a puff of smoke and reappearing across the forest. But we know better; real travel from one point to another through space is a continuous process. Time travel would be like that.
4. Things that travel together, age together.
If you travel through time, and you bring along with you some clocks or other objects, all those things experience time in exactly the same way that you do. In particular, both you and the clocks march resolutely forward in time, from your own perspective. You don’t see clocks spinning wildly backwards, nor do you yourself “age” backwards, and you certainly don’t end up wearing the clothes you favored back in high school. Your personal experience of time is governed by clocks in your brain and body — the predictable beating of rhythmic pulses of chemical and biological processes. Whatever flow of time is being experienced by those processes — and thus by your conscious perception — is also being experienced by whatever accompanies you on your journey.
5. Black holes are not time machines.
Sadly, if you fell into a black hole, it would not spit you out at some other time. It wouldn’t spit you out at all — it would gobble you up and grow slightly more corpulent in the process. If the black hole were big enough, you might not even notice when you crossed the point of no return defined by the event horizon. But once you got close to the center of the hole, tidal forces would tug at you — gently at first, but eventually tearing you apart. The technical term is spaghettification. Not a recommended strategy for would-be time adventurers.
Wormholes — tunnels through spacetime, which in principle can connect widely-separated events — are a more promising alternative. Wormholes are to black holes as elevators are to deep wells filled with snakes and poisoned spikes. The problem is, unlike black holes, we don’t know whether wormholes exist, or even whether they can exist, or how to make them, or how to preserve them once they are made. Wormholes want to collapse and disappear, and keeping them open requires a form of negative energies. Nobody knows how to make negative energies, although they occasionally slap the name “exotic matter” on the concept and pretend it might exist.
6. If something happened, it happened.
What people want to do with time machines is to go into the past and change it. You can’t. The past already happened, and it can’t un-happen. You might wonder what’s to stop you from jumping in your time machine, finding your high-school self, and convincing them that they really shouldn’t go to the senior prom after all, thereby saving yourself all sorts of humiliation. But if you really did go to the prom, then that can’t happen. The simple way out, of course, is to suppose that travel into the past is simply impossible. But even if it’s not, you can’t change what already happened; every event in spacetime is characterized by certain things occurring, and those things are fixed once and for all once they happen. If you did manage to go back in time to your years in high school, something would prevent you from dissuading your younger self from doing anything other than what they actually did. Even if you tried really hard.
7. There is no meta-time.
The least realistic time-travel movie of all time might be Back to the Future. When Marty McFly changes the past (violating Rule 6), the future “instantaneously” changes. What the hell is that supposed to mean? Time measures the temporal interval between different events in spacetime, and can be quantified by clocks. There is no set of clocks outside the universe, with respect to which you can go muck around in the past and have effects propagate into the future “at the same time.” Likewise, your brain is not going to change to remember things differently, nor will any other record-keeping device such as diaries or photographs or embarrassing sex tapes. Sorry about that.
8. You can’t travel back to before the time machine was built.
Right now, at the particular place you are sitting, at the time when you are sitting there, one of two things is true: either there is a closed timelike curve passing through that point in spacetime, or there is not. And that situation will never change — no matter what clever engineers may do in the future, if they create closed timelike curves they cannot pass through events in spacetime through which closed timelike curves did not pass (corollary of Rule 6). Or in plain English: if you build a time machine where there wasn’t one before, it may be possible for future travelers to come back to that time, but nothing can help you go back to times before the machine was built.
9. Unless you go to a parallel universe.
Parallel universes — the kind we contemplate in the many-worlds interpretation of quantum mechanics (MWI) — provide potential loopholes for some of the above rules. According to the MWI, there exist different “branches” of the wave function of the universe, distinguished by different observed outcomes for the measurement of quantum events. In the celebrated Schrödinger’s cat thought experiment, there is a “universe” where the cat is alive, and one where it is dead. Some imaginative (but respectable) physicists, especially David Deutsch, have speculated that we could combine this idea with the possibility of closed timelike curves to contemplate travel into the past of a different universe. If time travel is unlikely, this idea is (unlikely)2, but it’s not inherently paradoxical.
If you could travel to the past in a different branch of the wave function, then we are allowed to contemplate changing that past in a self-consistent way, because it’s no longer really “your” past. So almost all cinematic invocations of time travel — where they are constantly mucking about, changing the past in crucial ways — would have to appeal to something along these lines to make any sense. But even if you can change what you thought was the past, all of the rules of continuity and sensibility still apply — no flashing lights, no disappearing, no sudden changes in the future, no re-writing of your memories, etc.
10. And even then, your old universe is still there.
Remember Rule 0: no paradoxes. If you have reliable records of having made some unwise decisions regarding your social life in high school, then those decisions were made, and can’t be un-made. Even if you go into a different branch of the wave function, where you bestow some wisdom-of-experience on your younger self, you would only be changing the history of that universe. There is still the universe you left behind, with all of your bad decisions still intact. That’s life in the multiverse for you. It remains for future scholars to write Ph.D. theses along the lines of Utility Functions and Moral Dilemmas in an Ensemble of Multiple Interacting Universes. But it’s just a matter of time.
[* Update: Spock does not actually travel backwards in time and become Kirk’s grandfather, nor lover, nor does he write Shakespeare’s plays. That was a “joke.” I am reliably informed that the Spoiler Patrol and Internet Rectitude Society does not appreciate “jokes.”]
If traveling through time is like traveling through space, then if I was able to go back in time, say 6 months, then would I still be standing on the planet Earth? The Earth would be on the other side of the solar system and I would be stranded in space.
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(“the Zany Time Travels of Warble McGorkle”)
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Okay, I admit I started skimming the responses about halfway up, but it occurs to me that given rules 1-3, 6 and 10 (which presupposes 9 and 0), any universe (fictional or otherwise) that allows for time travel to a fixed point in the past must also have a defined future. On the face of it, this violates rule 7, but rule 7 *only* applies *within* the context of a given universe.
Still, the future is only unknowable from the relative perspective of a person contained inside a specific “bubble of spacetime”. A significantly multidimensional being or, as stated, someone from a parallel universe would not be limited to the linear interpretation of time that the inhabitants of that universe experience.
To look at it another way, if spacetime is a painting, it’s one that is constantly being painted. If you think of it as a mural, the events of the past are fixed an immutable, but the events of the present are still being painted. The future is an indeterminate length of blank canvas. But only the brush sees it that way. The brush only lays down paint in a predetermined pattern.
People don’t like this aspect of time travel because they think it negates their free will. But simply knowing the past doesn’t mean that the people in the past didn’t have free will. The thought that some future (or other-universe) observer knows what decisions we choose to make doesn’t mean that we didn’t make those decisions.
Two points:
1. You can scrap all the talk about going into the past of a parallel universe. It is a parallel universe, how could you tell if you were in the past, or just the present of that universe. I suppose if you went there, came back, waited a l-o-n-g period of time then went back to same parallel universe and found it exactly the same, then maybe, but then maybe you are just at another instance of a different universe at its’ present day.
2. You can’t change the past? How might you know? Maybe someone has already gone into the past and prevented Hitler from being killed? Maybe this happens a lot, sometimes killed, sometimes saved. But each time, that line of history moves forward, and there is no record of the other line at all. Maybe it has only happened once, Hitler was saved, and the resulting timeline caused the person who created the time machine to not be born. And what if this was done because in that other future, if Hitler WAS killed, something else ocurred preventing the JIHAD from succeeding, so, some unnamed group or person went back in time, and commited a ‘man-made catastrophic act’ (not terrorist, being PC here), and let Hitler have his day. And now the coming JIHAD has at least a chance of succeeding.
i have a theory of time travel
http://www.scribd.com/doc/13158331/Trans-Dimensional-Unified-Field-Theory-509
We already know how to produce time machines, in fact the typical american has one in their kitchen.
A properly sealed refrigeration system is a perfect example of a time machine. Since the system is finite and the internal entropy fluctuates in closed cycle you effectively have a time machine. Since the molecules are indistinguishable, finite in number, and have a finite number of configurations, the system will eventually explore all possible configurations (poincare recurrence) and end up cycling back through any arbitrary starting configuration.
Hello time machine
I love fiction, especially science fiction. I have always enjoyed Star Trek in all its incarnations from the beginning to the latest movie. I like imagining warp drive, and time travel. But…
Time is just a way of keeping track of motion and change. The entire universe is constantly in motion and changing.
If you imagine traveling back in time to a specific day, let’s say the day of a solar eclipse perhaps, what has to happen? Where is the energy going to come from to stop the universe in its tracks and rewind it so that the earth, the moon, the sun, the whole galaxy, the other galaxies, and every subatomic particle of everything is right back where it was on that day? The galaxy has rotated away from that position, as has the earth, moon, sun, and everything else. Whatever physics equations may tell us about what we can do mathematically with the time variable, nothing is going to rewind and reposition every particle in the universe.
Now let’s imagine what reversing the arrow of time would mean. Gravity would now be a repulsive force. Chemical reactions would have to happen in reverse. An explosion would mean that specific gas molecules and precise little bits of things would have to assemble into a cloud and then violently assemble themselves into a stick of dynamite while absorbing a lot of heat.
No. If nothing else keeps the arrow of time firmly locked into the forward direction, the momentum of the entire universe should be enough.
Look, this “There are no paradoxes” issue is important. Many of the arguments against time travel are based on this No-Paradox Rule: A) Time travel creates paradoxes, B) The real world doesn’t allow paradoxes, C) Therefore, time travel is not possible.
Sean (the author) writes: “It’s not a statement about physics; it’s simply a statement about logic. In the actual world, true paradoxes — events requiring decidable propositions to be simultaneously true and false — do not occur.”
Well, yes, in the actual world, paradoxes do occur. In quantum mechanics, things happen that don’t make sense, yet they happen. The best-known example, which anyone can duplicate with a piece of cardboard and a razor, is the double-slit experiment, which proves that light is definitely a wave (and not a particle) and definitely a particle (and not a wave.) You’re right: that’s a contradiction, that’s illogical, that’s a paradox, and yet… that’s what happens. You can see this with your own eyes. The human mind can’t accept logical paradoxes, but the universe isn’t the human mind.
There are paradoxes in the world. Therefore, the No-Paradox Rule does not apply to time travel. Time travel may indeed be impossible, but it won’t be because the No-Paradox Rule doesn’t allow it.
For more, see “Double Slit Experiment” in Wikipedia.
Andreas: The double slit experiment would be part of the the “our understanding of the laws of nature is incomplete” bit.
Our current understanding of waves and particles might not allow for one phenomenon to be both. But obviously the double slit experiment tells us that our understanding is incomplete. Not that paradoxes are possible.
It would seem the only safe way for a human to travel in time would be in a really good space craft with lots of power available, for lots of reasons already discussed.
I would suggest any breakthrough otherwise would be in the area of communication. Some interesting experiments at the quantum level now.
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according to an as-yet unpublished Time article, this thread is “…both perky AND obsequious…” so you must have heard that Heisenberg’s wife lived a rather unsatisfying life; after all, when he had the time he didn’t have the energy, and when he had the position he didn’t have the momentum. in any case, carpe diem friends. keep yer pink matter warm
Do any of you ever think of Copernicus? Do you truly believe that we have learned even an iota of what is to be learned. Einstein suggested we use only 10% of our brain. I suggest we use less than .00001% of our brain power.(and that is a SWAG number) Within the next millennium, space ships for travel will be a laughing matter; medical doctors will be obsolete; Science, as it is now known will be spoken of like we speak of “falling off the edge of the world”, and “the earth is the center of everything.” We haven’t even learned how to learn yet. I think we are getting there, but it is a slow process and we shouldn’t take ourselves too seriously. There will be discoveries that will stand science and religion on there respective ears. We will travel anywhere in the cosmos in the time it takes to think of where we want to go. Our bodies will heal by themselves, because ‘they can.’ I think our great-great-greatgrand kids will be looking back at our science with great mirth. But, they had better watch it because learning/knowlege moves in a geometric fashion and prpbably will for a long long time.
I LOVE how this article brings up most of the things that irritate me. For example, time-travel in Stargate Continuum simply didn’t make any sense! The new Star Trek, on the other hand, definitely got things right, I think.
Note: I could be wrong about some things in what follows, since I’m not a physicist or anything like that.
Matthew said:
> (as mentioned before, if you travelled only through time, you’d be in
> a void of space since the universe is moving – though, gravity might
> still affect you as you travel through time so space movement would
> be unnecessary)
People keep saying this, and it doesn’t ring true to me. It seems to be assuming that there is an absolute coordinate system – the very thing that relativity has shown to be false. If you are on Earth and you travel back in time, you’re most likely travelling with respect to the Earth’s worldline. As an analogy, consider a river. You are at point A on the river and wish to travel upriver to point B. Do you simply travel away from the river mouth in a straight line, or do you follow the course of the river? Similarly, in time travel you would follow the Earth back through time to your new location.
It helps to think of the Earth’s “worldline” as something like the sum of its positions over the course of its history – a trail extending backwards in time. I’m not sure how accurate that is though. It also becomes confusing when you consider that, by this logic, one of two things is true: either the worldline extends into the future, or it is constantly extending as the Earth moves… neither of which quite seem right.
dmduncan Said:
>Regarding rule 9: We can’t travel to the “past” of an alternate
> universe either. What you might be trying to say is we might
> be able to travel to the present of an alternate universe which
> resembles the past of this universe, assuming that such alternate
> universes exist at all and if they do that they parallel the development
> of our own but are not parallel in time, so that a lateral shift from
> this one to the other brings us into a similar universe at an earlier
> stage of development. But that’s not time travel. It’s weird, but not time travel.
…what? In all honesty, I don’t see how this is any different from travelling into the past of a parallel universe. After all, there isn’t really an ordering of times in different parallel universes – if it looks like a past of your universe of origin, for all intents and purposes it IS a past of your universe of origin.
Keanna Said:
> There should be no reason we can’t go faster than the speed of light.
> It seems simple, really. I invite people to actually ponder this and to
> comment on it. And please, I don’t want
> to hear “you’re wrong” or “that’s a stupid idea”. Explain to me why I
> might be wrong but also look and try to explain why I could be right.
No reason? I could name a few, in theory. Really, it probably depends on the attainability of infinity. In order for anything with mass to reach the speed of light, you need to apply an infinite force to it. It’s generally accepted that this is impossible.
Of course, an alternative would be to somehow suppress one’s mass, which would allow you to travel at the speed of light. Or rotate it 90° in the complex plane (whatever that means) – then you could travel through space without regard to time, moving backwards and forwards through time as easily as you can normally move backwards and forwards through space… but at the cost of never being able to stop – with imaginary mass you can never travel slower than the speed of light. If the Higgs field turns out to be true, I suppose suppressing mass MIGHT be possible; the other, I can’t even begin to say (probably not though).
chris said:
> Doesn’t time travel imply greater then 3 dimensional space?
Yes, indeed it does. In fact, it implies a minimum of 4 dimensions – the fourth being the time dimension (which is mostly just like the three spatial dimensions, except that when calculating distances between two points you subtract rather than add the square of the time difference).
Emanla Eraton said:
> A black hole itself may not be a time machine, but you can theoretically
> use its properties to acheive time travel. If you somehow find a way to
> hover right above the event horizon of the black hole, you will travel
> forward through time at a much faster rate than normal.
It’s called “orbiting”. 😉
Science_Boy Said:
> If you imagine traveling back in time to a specific day,
> let’s say the day of a solar eclipse perhaps, what has to
> happen? Where is the energy going to come from to stop
> the universe in its tracks and rewind it so that the earth, the
> moon, the sun, the whole galaxy, the other galaxies, and every
> subatomic particle of everything is right back where it was on that
> day? The galaxy has rotated away from that position, as has the earth,
> moon, sun, and everything else. Whatever physics equations may
> tell us about what we can do mathematically with the time variable,
> nothing is going to rewind and reposition every particle in the universe.
But why is rewinding necessary? Think of the Earth as an object that is very large in the temporal direction. If you consider spacetime to be static from an outside perspective, what would such an observer see from outside the universe? They would see Earth as an enormous spirally object, something like a helix, extending back through time and “fraying” as it pokes back beyond the point of its formation. If you travel back in time, you’re just moving “down” that helix to an earlier point.
Trouble still arises when you extend that analogy to the future; the many-worlds interpretation helps there though.
Science_Boy Said:
> Now let’s imagine what reversing the arrow of time would mean.
> Gravity would now be a repulsive force. Chemical reactions would
> have to happen in reverse. An explosion would mean that specific
> gas molecules and precise little bits of things would have to assemble
> into a cloud and then violently assemble themselves into a stick of
> dynamite while absorbing a lot of heat.
All possible, except for the bit about gravity, which is false. Run a chemical reaction backwards in time, and it still follows all the laws of physics; same with the explosion. The reason we don’t ever see a reverse explosion is simply because the starting state is more complex than the ending state; we don’t have the resources or patience to set it up. That doesn’t mean it’s breaking physical laws. As for gravity, no, it wouldn’t be repulsive. However, consider a meteor heading for the Earth. It has a certain velocity as it approaches, and that velocity increase as it comes closer. Now reverse time. Suddenly, its velocity is reversed – it’s moving AWAY from the Earth. So gravity is repelling it, right? No, it’s not. As it moves away from Earth, it SLOWS DOWN due to the ATTRACTIVE force of gravity – just like we would expect. It doesn’t fall to Earth because its velocity exceeds escape velocity.
Whoa, long post. I hope I didn’t say anything stupid…
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Intuite says: How does time exist outside of the cogent mind? Is it only a creative tool that works well for the measured mind? Imagine a universe with no cogent mind, no time pieces; what part of that universe, micro or macro would be effected as to where it is, is it moving or stationary, what speed, would speed exist? Eternity has no measurement, it has no fixed place. Space is no different. If it is infinite every point is the center, there is no alternative. Without our handy tools, of which one is vision, every point may be the same point; like the eternal now.
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Obviously, IF time exists in the manner in which we perceive it (or, I should say, in the way we understand our own and each others’ perception of it), and IF we could travel from one point to another within it, then Bill and Ted had the right idea. If we need a set of keys, we just need remember to get them in the future, travel back to before we need them, and place them in the bushes. If we had seen them in the bushes first, we wouldn’t have brought them back, so they wouldn’t have been there and we would have brought them back. Paradox? Naah. We didn’t look in the bushes until we thought to put them there.
IF the ability to do this in a controllable manners exists/existed/will exist, spacial location is not at all a problem. All we would need to do is be able to perceive one extra dimension and move through it, heading for a desired 4D point just as we would head for a 3D point within the confines of our current limitations.
Of course, then we would start trying to figure out how to move from one 5D point to another…
To CelticMinstrel. You’re right about the gravity. I should have left that out because reversed time would not make it behave like a repulsive force. Your description of how the meteor would move is more accurate.
My point was simply that time travel, as pictured in popular fiction, is completely, utterly, totally impossible. The past no longer exists. You can’t go back to a previous point in time because there is nowhere to go to. Any past time you can name represents a previous arrangement of all the particles in the entire universe. You can’t put the entire universe back into the previous arrangement.
We can *imagine* the earth as a spirally object in 4 dimensions, but that is all it is: a way of imagining it.
Sean’s article has it right. Time travel is something that happens in a “fictional world”. That’s the fun of science fiction. We say what if something impossible were possible, then what? When we do that, we need rules to keep the internal logic of the story consistent, given the original premise for the fiction, that time travel can happen.
ScienceBoy: I don’t think you and I are going to agree; you have more of a “presentist”” view whereas I have more of an “eternalist” view. I don’t know of significant evidence to support either viewpoint; however, one of the reasons I take the eternalist stance is because general relativity treats time as just another dimension (albeit a dimension with slightly different properties), and if it’s just another dimension, then theoretically it would be possible to travel backwards through it and hence return to an earlier moment.
Of course, general relativity is not a perfect model. Perhaps the perfect model will agree about time being a dimension; perhaps it won’t. Only time ( 😉 ) will tell.
If time travel to a time before a time machine was invented were possible we would already have evidence of such travel. Humans are way too careless not to leave evidence behind. The same goes for travel to another dimensional timeline. (Not us going there – but them coming here.) Since we see no evidence of interdimensional time travelers from “there” it’s probably not possible. Imagine the evidence of a Tourist Time Spot (significant events in our history both natural and human oriented). Temporal Archeologists would have blown their cover “long ago.” 😉