Time Travel

[Obi2Kenobi]

Hmm... Haven't heard that one before. (The not being able to go back in time before the machine is built) :) Interesting.

[gekko]

I don't think some "power" would "stop" you from killing your grandfather; I stated things unclearly.  What I meant to say was, *even before* you go back in time, your exploits in the past are already part of history.  When you go back in time, you fulfill your role in history, which is how it originally happened.

That doesn't solve the problem though -- my grandfather WAS NOT killed before I was born. He wasn't. So for myself or anyone else to go back and kill him changes history. But if anyone EVER from now until the end of time were to do that, it would have happened already! And it hasn't, nor have other such changes occured.

Saying that you go back and fulfill your role in history is essentially agreeing with predestination -- history is already set and done, and nothing we do can change the future. Since that's another whole can of worms, I'll just mention it and run for the hills...

gekko

[--Pete]

Hi,

There is a theory, now not much in fashion, that every event that had more than one possible outcome came out in all possible ways. This was possible because each such event caused a bifurcation of the universe and there was an infinite "sheaf" of universes, some "closer" and some further apart.

This, BTW, was postulated to go down to (and indeed be dominated by) the quantum level. It was proposed as a serious attempt to explain some quantum weirdness (such as the Schrodinger's cat and the EPR paradox). The only "useful" application of his AFAIK was in some sf stories.

But, according to that theory, everything that could happen did happen. And so, if you did go back in time, that would create a new universe. If you did kill a direct ancestor before s/he had kids, that would create a new universe -- one in which you would no longer exist.

Fun? Maybe.

--Pete

[Guest]

A lack of understanding can be an inane thing.

[Albion Child]

I have heard speculationd that areas of immense gravity actually warp time, allowing matter to exist in 2 times at the same.....uh....time......Me quit, me hurt head

[channel1]

Or, "in an infinite universe, whatever is not impossible is inevitable".

The simple-minded Grandfather Paradox relies upon the existence of something (a paradox) that actually cannot exist in reality. A paradox is by definition impossible, just like the immovable object and the irresistible force.

There is an amusing detail with the idea of a new universe being formed when event possibilities diverge (i.e., "changing the past"). Since the sum of the universe should remain constant, the additional universe should come from somewhere.

One possible source would be time. The universe, if nothing else destroys it first, should evaporate after about 10^100 years. A new, divergent universe could get it's existence (matter, energy, et al) by reducing the total lifetime of the universe by half. In other words, you would have two universes that would evaporate after 10^100/2 years.

But, eventually, the two universes would become identical. After a sufficiently long period of time, even an event that would have a profound effect upon humanity would be forgotten. At that time, the reverse would occur: the two universes would recombine when they become identical, and the total time that the recombined universe would exist would be returned to 10^100 years.

I'm surprised that I never ran into this used as a concept for a story. Imagine, a malevolent entity, that continually changes the past, reducing the life of the divided universes by half with every change.

With that in mind, if it ever DID become possible to change the past, it would be necessary to pass a law prohibiting time travel. Time Cops would have to pursue their quarry with caution, lest they do more damage than the criminal.

-rcv-

[gekko]

Fun?  Maybe.

Fun, perhaps, however, there's no point discussing or debating such theories. Obviously, when discussing something like time travel, we can't expect only concrete examples to be used. However, if we start arguing with possibilities whos basis is basially no more than speculation and imagination, any serious discussion goes out the window. We can't debate such things because there is nothing to debate.

What do I mean? Consider existentiallism (or whatever it's called). Boiled down to the most basic level, just deny everything exists. When you simply don't accept ANY givens, any argument becomes moot.

gekko

[--Pete]

Hi,

Since the sum of the universe should remain constant

There is no reason to postulate this. The number of universes could be growing without bounds. The rules that apply (as if we even knew what they were :) ) *in* one universe don't necessarily apply *between* two universes.

Or are you introducing that as a postulate for the remainder of your argument? In which case it is a nice, but somewhat shaky hypothesis. After all, if the number of universes is finite, then why should it be any particular number other than one (or zero)? They are the only two "special" numbers. ;)

--Pete

[--Pete]

Hi,

There are two types of discussion of time travel. One is the pure BS type that is going on in this thread. Of that type, I agree that "there's no point discussing or debating such theories." In Fact, I believe I said it first.

There is a much more interesting type of discussion of time travel. That is the one where a physical theory that comes close to explaining the observable universe includes within itself the possibility of some form of time travel. A number of such theories exist and are being studied and their ramifications examined. That's how the scientific method works, a hypothesis is postulated that explains all known facts. The hypothesis gives birth to new possibilities which are tested. On the basis of the results of those tests, the hypothesis is (1) provisionally accepted, (2) rejected outright, or (3) modified and tested again.

However, the second type of discussion cannot be held on most fora simply because very few people are conversant enough with the details of the theories to say anything of real use. Most of the "speculation" is really nothing more wild guesses based on ignorance or misunderstandings of the underlying theories. Or, more often, pure moonshine.

Ah, well.

--Pete

[gekko]

Of that type, I agree that "there's no point discussing or debating such theories."  In Fact, I believe I said it first.

There is a much more interesting type of discussion of time travel. [...] However, the second type of discussion cannot be held on most fora simply because very few people are conversant enough with the details of the theories to say anything of real use.

You're right there; we're actually on the same page, it seems. I'm going to take this moment to point out that in this case, the poll option wasn't to blame so much as the topic -- i.e. the thread itself. Just a poke at anyone who's also in the poll thread. :D

gekko

[channel1]

Since the sum of the universe should remain constant

There is no reason to postulate this.

Sure there is. Conservation of energy, the whole is equal to the sum of it's parts, things that are the basis for the physical universe as we know it would point that way.

But, you are certainly correct that it may not be the case. The physical laws that govern something as large as the universe may not be the same as what we expect. That's why I said "should remain constant", not "will remain constant".


... if the number of universes is finite, then why should it be any particular number other than one (or zero)? They are the only two "special" numbers.

I'm certainly not saying that. Although, it is probably right, that the number of universes is finite. :)

Certainly not necessarily one, but we have a hard time seeing more than one.

Under some concepts, there would be a finite number of universes that could be created from one universe, assuming that the division of the universes reduces the total time for the universes' existence.

Taking the current life expectancy of our universe as about 10^100 years, and the smallest unit of time as 3x10^-43 seconds (call that a quantum second?), there will be a total of about 10^150 quantum seconds to the life of the universe.

Although the period of the quantum second will change over time (getting longer), the number will remain constant. So, when a universe is divided to the point where it will evaporate after only one quantum second, it can't be divided any more. Of course, you'd have to be pretty quick to even try.

Based on that, there would be a maximum of 10^150 universes that could derive from the one that we can see.

Of course, the universe we can see may be a daughter universe, that split from a larger universe. Off the top of my head, I can't think of any way to consider what the limit would be in that direction, unless...

The time required to split a universe should be at least 1 quantum second. We're back on the Time travel / changing the past idea. "Changing the past" is an event, and an event takes time, and time as short as a quantum second is indivisible.

So, the maximum number of universes that could exist at this time would be the number of quantum seconds since the birth of the universe. 12-20 billion years worth of quantum seconds.

Of course, if you want to consider universes that existed before the birth of the universe, I give up. :)

-rcv-

[Mikedok]

Hail Pete,

First I want to say that I enjoy reading your posts. You always have a good way with words, and your ideas/arguements are well thought out. I'm writting to respond to some of your comments. I'm not really disagreeing, just commenting.

There are two types of discussion of time travel. One is the pure BS type that is going on in this thread. Of that type, I agree that "there's no point discussing or debating such theories." In Fact, I believe I said it first.

I agree that the type on this thread is BS, but there is a point: Entertainment. I enjoy the BS ideas here. I think that in a fantasy game forum BS theories are fun and appropriate. Real theories based on the little information known about this topic are also interesting, but I don't think we need to limit the thread to them. Maybe future posters should clarify if they are talking about BS or real ideas.

Most of the "speculation" is really nothing more wild guesses based on ignorance or misunderstandings of the underlying theories. Or, more often, pure moonshine.

My post certainly was speculation, but not ignorance. It was meant to be a fun BS theory. Just something for entertainment's sake. Maybe it would have been more entertaining if moonshine was involved. ;) (we need a drunk smilie.) I am well aware that I don't know enough about physics and such to even comment on possible real theories. I didn't state in my post that it was BS. Next time I post for entertainment's sake, I'll make sure to say it.

[rholst1]

Hi,

I would like to contribute to the conversation. I have a basic university background in the sciences, so I should be able to say somethings worth reading.

Lets look at some semantics first. If time travel is possible, we would need to establish some kind of communication with the distant future to let them know we want them to come visit us. Imagine that we are the time travellers in the future that are waiting for this message from the past. Think about how something from the past communicates with us now. The data that I type here is going to communicate something to everyone on the forum in the very near future. By recording something in the present, we can assume that these records will be available in the future. Communicating with the future is a technology we've been using for a few thousand years. (and built in biologically if we want to get technical, but I'll stay away from the whole biological aspect of time...)

The problem is how we can communicate with the past. The first thing we need to know is how a single particle behaves when motionless (or without time). Wow, what a task! The variables involved in this problem boggle the mind, and I cannot even begin to suggest what would happen. I guess I'll just throw some questions out there and see what kind of answers I get from people with knowledge of experiments related to my questions/suggestions.

If we consider time to be a perpetually moving entity, something moving "faster" than the maximum speed allowed in the present would be propelled into the future. Something moving "slower" than the slowest speed allowed in the present would be repelled into the past. Assuming there are indeed these limits associated with the "present", I've come up with some thoughts.

I've always had the understanding that a complete lack of motion was reflected by absolute zero. Approaching absolute zero would seem to be the slowest speed in the present relative to everything else. The upper limit of speed would be approaching the speed of light... ... (clip)

You know what? I just realized that I have way too many unanswered questions to formulate more precise questions. I would need to see results from experients relating to motion before I can question the way things work. I give up! Time for me to communicate with everyone in the future by hitting the reply button, hehe.

[--Pete]

Hi,

I agree that the type on this thread is BS, but there is a point: Entertainment. I enjoy the BS ideas here. I think that in a fantasy game forum BS theories are fun and appropriate. Real theories based on the little information known about this topic are also interesting, but I don't think we need to limit the thread to them. Maybe future posters should clarify if they are talking about BS or real ideas.

Oh, I agree completely on the potential entertainment value of speculation. Sitting around the hostility (actually, hospitality) suite and BSing about that type of topic till sunrise was one of my favorite pastimes at cons. The point is that this thread was started with a post where some noodling on time travel was presented. It was almost, but not quite, science based. That took it out of the realm of fantasy. But the science was poorly understood or not understood at all. That took it out of the realm of science. And the speculations of how it would work were not really developed. That took it out of the realm of sf.

My post certainly was speculation, but not ignorance. It was meant to be a fun BS theory. Just something for entertainment's sake.

I presume you mean your reply to MEAT? There was nothing wrong with that post. You do not discuss the "how" of time travel, just the "what". Discussions of the form "if time travel (or FTL travel, or immortality, or a person was raised by Martians) was possible, then . . ." are fine. That is the basis of all good science fiction. It is a fun exercise in logic to present a hypothetical case and then see what the probable outcome of the hypothetical is. The only fault (and it hardly qualifies as a fault, really) I could find with your post is that that viewpoint has been expressed many, many times.

Basically, if time travel were possible then: either you could change the past or you can't. If you can't, then you have the "the universe will stop you" and the "what has happened *will* happen" camps. If you can, then you have the "the universe splits off into a new branch" and you have "everything in the 'future' changes" lines. In all the years that I've heard of these ideas, those are about the only ones that have come up. Sure, there are an infinite number of details that vary from person to person (or story to story), but they all pretty much fall into one of those four categories.

It's for that reason that, you'll notice, I don't respond to the "if time travel were possible then . . ." speculations. Been there. What I find much more interesting and enjoyable is the "how" of time travel. When I started reading about science in the early '60s, time travel wasn't even considered. The official dogma was that time flowed in one direction and that nothing could be done beyond what special and general relativity allowed, i.e., time dilation and contraction in one frame relative to another. The first scientific speculation that I'm aware of that this might not be absolute was the search for tachyons from that period. While most physicists didn't subscribe to the possibility (and apparently they were right), a few did point out that special relativity did *not* say that material objects could not travel faster than light, simply that they could not travel at the speed of light.

Even earlier than this, but later in my education, I ran into the concept that anti-particles could in some cases be the particle moving backwards in time. Of course, the anti-particle could also be a "hole" in a dense background sea of particles :)

So, there is indeed a sense of excitement in the various possibilities for time travel, for time travel and FTL travel are one and the same. And without FTL, we are destined to never venture much beyond the local neighborhood of the Sun. Perhaps a realistic fate, but a boring one ;)

And that is why I prefer good, informed discussion on the "how" ;)

--Pete

[--Pete]

Hi,

If time travel is possible, we would need to establish some kind of communication with the distant future to let them know we want them to come visit us.

Why? I don't see this at all. Consider "space" travel. If I want to go to the grocery store, do I have to wait for a clerk to call me and invite me? No. I simply hop into my car and go. Why do you think that time travel would be different in this respect?

The first thing we need to know is how a single particle behaves when motionless (or without time).

We know this, quite well. Also "when motionless" and "without time" are not the same thing. Being motionless simply means that in a given inertial reference frame the position does not change over time. To consider a particles world line without time is meaningless nonsense. As opposed to, say, Jabberwocky which is meaningful nonsense.

If we consider time to be a perpetually moving entity,

If we do that, then we've generated a model that does not work right and is too limited for useful speculation. If we did that, then our premise would be wrong, and from it we could generate any form of nonsense. The problem with "a perpetually moving entity" is in the very definition of motion: the change in position over time. Thus, your consideration boils down to "if we consider time to change it's position over time". I'm not even sure how to scan that sentence.

something moving "faster" than the maximum speed allowed in the present would be propelled into the future.

Total confusion. If by "speed" you mean what is conventionally meant, then you are totally backwards. Something moving relative to you faster than the speed of light would be traveling into your past.

I've always had the understanding that a complete lack of motion was reflected by absolute zero.

Again, relative. If I'm on a spaceship doing, say, 0.5c relative to you, and I have an atom in its ground state trapped in a laser well, that atom is at absolute zero relative to me. Its internal energy, relative to you, is also zero but its total energy is not. Which is why temperature, and thus absolute zero, can hardly be defined and is relatively useless in such a discussion.

Approaching absolute zero would seem to be the slowest speed in the present relative to everything else.

What "everything else"? There is no background structure of fixed rods with which to measure. *That* is the fundamental basis of relativity. All measurements are relative to some reference frame. That is especially true of measurements of speed. The only speed that is the same in all frames is the speed of light. And that is where the weirdness begins.

Sorry for picking your post apart like this, but yours was an almost perfect example of the worthless speculation based on concepts that are totally misunderstood by the poster.

--Pete

[whyBish]

"I'm surprised that I never ran into this used as a concept for a story. Imagine, a malevolent entity, that continually changes the past, reducing the life of the divided universes by half with every change.
"

I remember something like that a year or so back, with martial arts stuff in it. Can't remember the name though, sorry :(

[kier]

The usual theory put forward for time travel is to break the speed of light. agreed?

That way, you theoreticaly can time travel. Allready, when we break the sound barrier, it projects slightly into the future. there is a teeny weeny draw back though.

We cant travel faster than light, it is quite imposible. This is due to the fact it allways takes the fastest route (and yes, we can to, but light allways does) and because it has no mass, its power to weight ratio is a bit too good to beat :P .

So far this sort of time travel is for traveling back in time. forward in time would, i think, mean going slower than 0. Or slowing/stopping time. slowing would mean you would get there before you theoreticaly should of, and stopping it would get there instantaniously. Now, here's the million $ question. How the &%$ do you do that?

Some black hole theory posibly could, but i dont fancy the chances of surviving in a black hole. There maybe other ways, but i dont know of them.

kier

[whereagles]

Hi, I'm a physicist. Sometime ago I was at a national physics conference where I listened to a review talk on the issue of time-traveling.

In a nutshell, time-traveling requires existance of wormholes. If Einstein's equations of General Relativity have wormhole solutions, there is in principle no theoretical obstruction to time-traveling. Einstein's equations are hard to solve, but as far as I know, there is no theorem forbidding wormhole solutions.

If time-traveling happens, you have a bunch of paradoxes in your hands, like for instance the `killing your grandpa before being born' thing. There are only two plausible ways out.

1. A wormhole is a gateway to another universe, so when you time-travel, you're also traveling between universes.

2. Wormholes simply don't exist and it is impossible to time-travel, period.

Well, nr.1 is a beautiful romantic thought, but most scientists believe nr.2 is the right answer.

So what could be the obstruction that forbids wormhole formation? The answer is not known, but there is a strong hint. General Relativity is a classical, large-distance theory of gravity. It does not explain what happens to gravity at short-distances. The theory of short-distance gravity is called `Quantum Gravity', but unfortunately no one knows what it is. By the way, the prize for finding it is immortality ;)

Nowadays, it is common belief among scientists that wormhole formation should get some obstruction from Quantum Gravity effects.

Well, that was it. Hope it's clear.

[--Pete]

Hi,

In a nutshell, time-traveling requires existance of wormholes.

Not quite. Wormholes do provide some of the models which permit time travel. So does an infinitely long massive rod (and the "infinite" requirement is there because we can't handle the equations in three dimensions, thus can't handle end effects. A suitably long rod with correctly shaped end pieces would probably work.) So do two strings which are moving relative to each other. And the possibilities in M theory are pretty well wide open.

If time-traveling happens, you have a bunch of paradoxes in your hands, like for instance the `killing your grandpa before being born' thing. There are only two plausible ways out.

1. A wormhole is a gateway to another universe, so when you time-travel, you're also traveling between universes.

2. Wormholes simply don't exist and it is impossible to time-travel, period.

Nonsense. Not that these two aren't "plausible" but that they are not the only two ways out. There are closed loop theories of time travel (and the universe might have been in such a loop prior to the "big bang". That permits a universe with a past that is infinite and yet bounded.) Indeed, any attempt to apply our present "logic" in the question of time travel is as likely to prove wrong as Newton's application of his "logic" on the questions of space and time. It is intuitively clear that if two people have relative motion along the path of a photon, they will measure a different speed for that photon. Unfortunately, intuition is wrong. All the arguments for or against time travel are nothing but intuition.

Nowadays, it is common belief among scientists that wormhole formation should get some obstruction from Quantum Gravity effects.

As you pointed out, quantum gravity is not a field of study. It is a postulated field of study. Until either someone figures out a new way to re-normalize the fields (and that seems to be impossible since the infinities seem to be essential) or comes up with a totally different approach, all that can be said about quantum gravity is that people "believe" it will behave one way or another. Nature has had scant concern for the beliefs of physicists till now, I doubt that that will change in the future.

Sorry, I have as much respect for the prevalent opinion on time travel (which includes FTL travel) as I do for the Copenhagen Interpretation. Which is to say, nearly none.

--Pete

[Occhidiangela]

I thought this was self explanatory, but maybe I missed a nuance here and there.

Certainly not necessarily one, {universe} but we have a hard time seeing more than one.

Edit: Apologies to all, I am not sure who I am replying to here, I think the quote I took is from one of channel1's posts, and here I have replied to someone else. I blame it on the caffeine, and the phone call that interrupeted my post. Oops.

If I understand Hawking correctly, the reason you can only see one universe is that a given universe is bounded by space-time, whose limiting dimension (constant?) is the speed of light. Thus, if you can see it, it is by default within your space-time universe, so you could never 'see' a second universe. You could 'be in a second universe' but then you'd only see that one. While that makes sense to me, I might have tripped on the physics somewhere.

This brings me to the head scratching problem of 'how do you deal with and study something that you can't see?' Do you use some quantum version of the electron microscope, one that can peek through wormholes?

And now to the problem for the engineers out there: How do you grind the lens for that device? If you thought tolerances were tough in the manufacture of slip rings or ball bearings, how about the tolerances for the manufacture of quantum tools? :lol: