Time Travel

[Obi2Kenobi]

Time Travel
Some people believe that time travel is impossible. Others disagree. In the following paragraphs I will attempt to enlighten you to a subject extremely difficult to understand. I am of the group that really isn’t certain (however I am leaning towards no).
I don’t believe in the “Time Paradox” of killing ones grandmother at birth thus causing one to never be born, meaning one never would have killed his/her grandmother. There are three possible solutions to this (that I can think of...); The first is that one would move in the fifth dimension into another universe, from which one could never return to his/her current dimension, the second, which is less commonly thought of, is that one would age backwards, meaning one wouldn’t be born by the time s/he goes back that far. The last one I can think of is that your attempts to do so are doomed to fail, since you (according to some) can't change the past.
However, another possibility is that time for everyone else would go forward. The reason for this is that if one could achieve the speed of light, then one could travel anywhere instantly, at least so it would seem for the person doing the time travel, (but if s/he really did accelerate instantaneously, s/he would get crushed due to the fact that matter gains mass when accelerated) because time would stop for the time traveler, but for everyone else, it would seem to have taken longer (plus there is the inconvenience that you would need to somehow make the machine able to stop itself right before reaching light speed, or it would go on forever [in an instant...] due to the fact that time would be stopped for it).
If I were to travel across to Andromeda, which is 2.2 million light years away from Earth, and if I were to assume that the speed I would accelerate is the same of the gravity of Earth (9.81 m/s^2) then it would take about 354 days to reach the maximum speed (the speed of light). The reason that is the maximum speed here is that it is commonly believed that it would require an infinite amount of energy, since mass increases as speed does, to reach the speed of light. Also, if you were to reach the speed of light, you would have no way of stopping your spacecraft, since time for you would stop. So if I could stop the acceleration instantly before I hit the speed of light it would take me about two years to reach Andromeda (1 year to reach the midpoint, then 1 year decelerating. Then multiply that by 2 to find out my roundtrip time) However, for the poor people restrained to Earth during this, 4.4 million years would have passed (2.2 there, 2.2 back). The same holds true to traveling anywhere. You could travel anywhere in the universe in just under two years in such a manner (at least for you it would be two years, but for others it could seem like who knows how long). However, for everyone else it would take much, much, longer.
For some reason, some scientists believe that if you manage to get to a speed greater than that of light, time for everyone would go backwards. I think that you would just age in reverse, however the rest of the world would keep moving. Thus you wouldn’t really travel back in time, but it would instead be a “Fountain of Youth” of sorts.
Now where would we get such a vast amount of energy? Black holes. If we were to find a rapidly spinning black hole, we could pass a rod through the center, due to the fact that once rapidly spinning it would turn into a torus (doughnut). The rod would then be out of the event horizon from which nothing can escape, Then you could send particles from the rod with a certain charge, then others with an opposite charge in a different area, then you have a charged black hole to draw your energy from. However, we must first find a black hole spinning fast enough. Most black holes are not, and are merely spherical.


This isn't as good as I would like it to be, but due to time constraints, I had to shorten it a little, and not expand on some issues...

[Kryn]

Well, if space is infinite, then theoretically, shouldn't everything exist? Also, you wouldn't be able to travel everywhere in just under two years because there isn't an end. So, you could travel for an eternity and not even get halfway through. Of course, if everything exists in infinite space, you get some crazy paradoxes.

[Kasreyn]

First off, what does the poll mean? I'm totally confused as to what the subject of the poll is. And don't say "time travel, duh!". Time travel is a technology (also a very unlikely one). One doesn't make emotional judgements about technology; technology is neutral. Only its uses are up for debate.

Also:

if one could achieve the speed of light, then one could travel anywhere instantly

Incorrect. You would instead be travelling at the speed of light, which as you may or may not know is far, far slower than "instantly". A paltry 186,200 miles per second!

Another problem with time travel I haven't seen mentioned recently: Say you have a normal universe, going about its business next Thursday. Suddenly, a time traveller from the year 30,000 AD appears out of nowhere, in a time machine / spaceship. How can we reconcile the fact that universal entropy has suddenly reversed a tiny amount? Certainly we must assume that the universe is a closed system, at least until we can discover if there is anything beyond it. So how could a person travel to the past, thus decreasing entropy at a specific point in the past? Or would it be balanced by his absence in the future?

Finally, as to the "kill your own grandmother" paradox, I'm leaning toward the belief that the timestream is more flexible than that. If you go back in time and somehow affect the events leading to your existence, I predict you will actually CAUSE yourself to exist, rather than the opposite. IMO paradox is something that would simply never occur. If you went back in time with the purpose of proving me wrong and murdering your ancestor, something would happen to stop you. Why? Because otherwise you would not have existed. Paradox would be used by the universe, to prevent paradox.

-Kasreyn

[yangman]

Of course time travel is possible: we are doing it, in the forward direction, right at this moment. :P

As far as the "grandfather paradox" goes, there are a few theories which explain how it cannot occur or is insignificant, one of which is the
multiverse theory. (extremely long article)

A quick google search should yield you many sites and articles regarding this issue.

the second, which is less commonly thought of, is that one would age backwards, meaning one wouldn’t be born by the time s/he goes back that far.

Oh? But that wouldn't mean only an "observer" has really travelled back in time, and the the subject has not?

As far as the universe being infinite, there exists a counterpoint where space is indeed infinite, but the universe is not.
Imagine a sphere: although one may travel along the surface of the sphere indefinitely without reaching an end, the sphere itself is quite finite. IIRC, it is quite possible that gravity warps 3D space into a sphere of sorts, allowing infinite space in an finite universe.

Again, you should be able to find many related articles on the web.

Age in reverse?
How do you suppose this can happen?
Once the traveller exceedes "the speed of light", the laws governing the particles which makes up his body is reversed? And, since everything is relative, wouldn't that be the same as the rest of the universe going "back in time"?

The rod would then be out of the event horizon from which nothing can escape, Then you could send particles from the rod with a certain charge, then others with an opposite charge in a different area, then you have a charged black hole to draw your energy from.

I find this quite interesting. How would this work?
The common basic assumption is that once particles crosses the event horizon, it is essentially "lost" to the black-hole. This includes all matter and electromagnetic energy. Charged particles are matter, and hence cannot escape from the event-horizon. How can the energy of a black hole be harnessed through the means of extracting charged particles?

[WarLocke]

So how could a person travel to the past, thus decreasing entropy at a specific point in the past? Or would it be balanced by his absence in the future?

I'm no expert, but if the universe was bounded (or closed), then shouldn't there be a limit to time as well as space? In that case, the effect of entropy wouldn't be reduced so much as err... shifted? To another point in time.

The best concept of time travel I've come across is from a James P. Hogan book, Thrice Upon a Time. In it, he postulates that it's impossible to send matter through time, only data; and only into the past. Then, from the moment you sent the data back to, ripples travel back upstream (towards the future) along the timeline, basically rewriting history. It's a novel approach, and my bumbling description doesn't do it justice.

- WL

[TaMeOlta]

:blink: Didn't we already go over all this ? ..... Oh wait ...... that hasn't happened yet :blink: ...

[--Pete]

Hi,

if one could achieve the speed of light, then one could travel anywhere instantly

Incorrect. You would instead be travelling at the speed of light, which as you may or may not know is far, far slower than "instantly". A paltry 186,200 miles per second!

This is the problem with people who don't know what they are talking about discussing subjects like this. You are both, almost, right. Photons, being light, travel at the speed of light (duh). Now, the Earth and Sun are almost at rest with each other (30 km/s (IIRC) orbital velocity as compared to 300,000 km/s speed of light). So we can consider the Earth to be in the Sun's (almost) inertial frame. Now, measurements are only valid relative to the frame they are measured in. In the Sun's reference frame it takes approximately 8 minutes for a photon to cross from the Sun to the Earth. But in the Photon's reference frame it takes no time at all. The space contraction in the direction of the photon's motion is complete. So the point at which the photon is emitted and the point at which it is absorbed are "in contact".

As to the rest of the subject, it is very interesting. Unfortunately, the actual ideas that fit within existing or even possible theories of how the universe works are not included nor discussed. Trying to rationally discuss them in a group that has not had the necessary background is like a group of people born blind trying to discuss Renaissance painting -- it can be done, but most of what is said will be nonsense and none of it worth the time.

--Pete

[kandrathe]

Pete, I have to agree with you. I would like to discuss the topic, but without everyone having a good basis of understanding on state of thinking on general relativity and quantum mechanics it is pointless.

Obi and others -- If you are *really* interested in this you should read and fully understand at least the following authors works. From what I have read, many of you are misunderstanding the general principles of relativity and in understanding the continuum of space time. That is, you don't need to travel at or even near the speed of light to experience temporal distortion.

Stephan Hawking --
A Brief History of Time
The Future of Spacetime

Einstein --
Relativity (Routledge Classics) by Albert Einstein, Robert W. Lawson (Translator)
Ideas and Opinions by Albert Einstein

Another one to consider would be;
Black Holes and Time Warps: Einstein's Outrageous Legacy by Kip S. Thorne, Frederick Seitz

...and I would add something dealing with String Theory like,
The Elegant Universe: Superstrings, Hidden Dimensions, and the Quest for the Ultimate Theory by Brian Greene

Here is more of a philosophical poser for you. If time travel were possible, wouldn't someone from the future have come back to tell us. So if they haven't then maybe it isn't possible, or maybe in the future when time travel is possible they exercise wisdom along with their knowledge.

[ShadowHM]

Here is more of a philosophical poser for you.  If time travel were possible, wouldn't someone from the future have come back to tell us.  So if they haven't then maybe it isn't possible, or maybe in the future when time travel is possible they exercise wisdom along with their knowledge.

I just read "Bones of the Earth" by Michael Swanwick

A bit of a spoiler for the story, but the reason the 'people' from the future came along and told about time travel was because it didn't matter for our civilization - it was due to be annihilated soon enough from other causes. Thus our tinkering with time travel would make no difference. :P

[gekko]

Consider this -- if time travel were to be discovered, at ANY TIME from now until the end of time...

why hasn't anyone from the future come back?

Think on that -- no one, from now until the end of time, has come back to anytime from the begining of time until now. Why not?

That's not "proof", but it's a pretty darned hard question to answer.

A more direct reason why it can't possibly happen is that time travel could essentially un-make reality. The paradox of killing your own grandmother would result in reality constantly flip-flopping between two realities. Obviously, that isn't happening.

As for travelling at (or near) the speed of light, that's a different kind of "time travel" and one that could, in theory, occur. Moving AT the speed of light is theoretically impossible, since to accelerate to that speed would give a person (or object) infinite mass and therefore require infinite energy. Since neither of those (mass or energy) can get to infinity...

Moving near the speed of light, however, causes time to contract -- move more slowly for the object or person moving so quickly. So in a way, you are "time travelling" -- while you can't go backwards, you will be moving more slowly than those around you. So if you made a few hundred loops around the earth a .9 times the speed of light, you would age a very small ammount compared to those on earth.

gekko

[gekko]

If you went back in time with the purpose of proving me wrong and murdering your ancestor, something would happen to stop you.

So you're now suggesting that not only is time travel possible, there is some sort of power that stops you from doing anything that would create a paradox.

*cough cough*

I'm sorry, that's a little too far-fetched for me (even in a thread about time travel). Particularly since that paradox you're refering to is far more complicated that simply "killing your ancestor."

Each and every individual person is affect by every person they meet; every event they witness; everything that happens to them. A person changing ANY event in the past would alter things not only for their future selves; not only for everyone they met while in the past; not only for everyone those people ever met... etc. It's a ripple effect. It's the idea that if you go back and squash an insect, in the future Germany wins the second world war. Sure, it seems a little ridiculous, but forget about big changes -- even tiny changes are paradoxes. Why?

We're talking about time travel. So if anyone in the future EVER went back in time, he'd already have been in the past. So those changes would be made, but would then affect the future, thus changing the present the time traveller left from.

See what I mean?

gekko

<<edit: for those of you using linear view, my second post was a reply to a seperate post; sorry for the confusion >>

[Sheep]

<conspiracy theory>Leonardo Davinci...</conspiracy theory>

Anyways, I liked the Hitchhikers Guide th the Galaxy approach. :D

[ithil]

> So you're now suggesting that not only is time travel possible, there is some sort of power that stops you
> from doing anything that would create a paradox.

No power is necessary. It won't happen because it didn't happen. Think of spacetime as a sheet that we only see slices of. The whole is a static tapestry.

> We're talking about time travel. So if anyone in the future EVER went back in time, he'd already have been in
> the past. So those changes would be made, but would then affect the future, thus changing the present the
> time traveller left from.

So? Nature solves the three-body problem just fine, too. The universe is the solution to a universal equation, the Theory of Everything. Well, probably. Physicists are still working on that detail. ;-)

I glossed over two points: quantum uncertainty (spacetime may be static but its exact state doesn't seem to be pinned), and the existence of different reference frame views that may also see different slices of the universe. (Thinking of the universe as a single, absolute sheet is incorrect. Thinking of it from a relative perspective that gives us our own particular view of the sheet is not.)

[Taem]

RE: Kill your parents;

Einsteins theory of relativity says energy cannot simply cease to exist, but rather is transformed. If you went back in time and killed your parents, or even yourself, you COULD NOT simply cease to exist, because that would mean the theory of relativity was mute, which would make just about as much sense as saying I was a traveler from the future. Perhaps his body would disintegrate and his energy would become... your guess here. In any case, IF you could travel back in time and IF you did kill your parents, you COULD NOT simply cease to exist in the past (I mean, the you from the future).

[Obi2Kenobi]

Thanks for pointing that out... I should have clarified. For you it would seem like two years, because of the quantom machanics stuff. :P For others it would seem like a different amount of time.

To whoever asked what the poll was reffering to, I was asking what you thought of this, since I want to know if I conveyed my point well enough. And if it was interesting.

[Mikedok]

Edit: Oops, some of the ideas in my post are already expressed. Somehow I missed some of the posts while reading this thread. :unsure: Feel free to skip it, but I hope you read it anyway. It expands a little on some ideas.

Think about larger paradoxes. What if you went back in time and killed Hitler or George Washington or Jesus Christ for that matter. These events would cause huge problems with time. If it was possible to do these things then it would send a huge ripple throughout the time stream affecting almost everything. I don't think such a thing would be possible.

My take on it has been that you can't change the past. This doesn't mean that time travel is impossible. I think that if you went back in time to a certain time/event, you would have been there when the event/time first happened. Any change you make would have had to be made the first time that history happened. Therefore any attempt to go back in time and kill your grandfather is doomed to failure because you know he wasn't killed. The theory breaks down a little because I can't explain how or why you failed, but for whatever reason you do fail. The same applies to trying to kill Hitler. We know the he lived, so that means that nobody went back and killed him. Maybe someone will go back and try to kill him, but they will not succeed.

This means that maybe someone went/goes back in time and kills JFK. Of course, this means that the future is kind of already planned. We know that JFK was killed, so if he was killed by a time traveller that traveller will definitely at some point go back and do this.

Most past changes would probably happen by accident, because why would you go back in time and try to change something, if you know it already happened differently?

This does mean that you can go to the future and make changes as well, because when the future happens it happens with you there.

This theory does have holes, but I think it is more believable then a theory that says you erase the time stream that already happened and make a new one.

I think anyone who discovers enough about the nature of time to build a time machine would also have learned how this type of thing works by default. Meaning that the knowledge needed to build a time machine has the knowledge of how time travel works imbedded within it.

I apologize that this post is a little confusing, but I'm not great with words and this is a confusing topic to explain.

[Swarmalicious]

Consider this -- if time travel were to be discovered, at ANY TIME from now until the end of time...

why hasn't anyone from the future come back?


- Some folks have suggested that the "Greys" (the aliens that abduct people and carry out notorious anal probe experiments) are actually highly evolved human beings who have travelled back in time, for whatever reason. I think its a neat idea.

Concerning time (travel) and all that goes with it - I believe that the human race's entire concept of time is probably off - linear existence has the boundary of its own composition. The way we interperet the universe is limited by our ability to comprehend it (for instance - there is only so much of the light spectrum we can see with the naked eye, and then only so much we can detect with the aid of technology - that doesn't mean that that's all there is, or even that light exists the way we percieve it to). Why then, should our interpertation of time be any different? Its very likely that our tools (science, mathematics) are limited simply because they exist in the 3rd (er... 4th) dimension.

Wasn't long ago that we thought the sun orbitted the earth - we should allow the same gigantic margin for error in our current thinking. While I do have faith in the work of Stephen H, and take great interest in what he and his constituents are doing to smoke out the truth about our reality, its possible everything we've 'proven' so far is absolutley false.

People are flawed that way with everything - from fashion to fusion - we think that whatever the current crest of the wave is now is the truth. The prisoners in Plato's cave are no different.

I don't think we've even begun to comprehend the truth. ...but please - I'm no pessimist here - what I say is actually very exciting.
For me, anyway. :)

[--Pete]

Hi,

Here is more of a philosophical poser for you. If time travel were possible, wouldn't someone from the future have come back to tell us.

J. Richard Gott wrote a very accessible book on time travel (Time Travel in Einstein's Universe"). One of the things he points out is that all time travel theories that have a decent physical basis require some form of "time machine" (e.g., a stretched wormhole, an infinite rod, moving crossed strings). The time travel is such that one can only go from sometime when the machine is in existence to some other time the machine is in existence (some of these machines permit only a given delta, others are continuous). Thus, time travel to *before* the machine is built is not possible.

I found Gott's book fascinating, especially his last chapter, "Report from the Future" where he discusses how to make predictions ;) Another nice discussion in that book is the time loop at the beginning, which makes the age of the universe both bounded and infinite.

As I said (and as you agreed), this is a fascinating topic, but one that requires some background or it devolves into error and/or fantasy.

--Pete

[--Pete]

Hi,

Einsteins theory of relativity says energy cannot simply cease to exist

No, it does not. It does not address that question at all. What it does say is that mass is a form of energy.

The conservation of energy does not come from any more fundamental concepts. It is simply an observational "fact". Some cynics say that it isn't a fact at all, it is a construct of physicists. As the old saw goes: "Energy: a quantity that every time some of it seems to disappear physicists invent a new rule to conserve it." :)

--Pete

[Kasreyn]

What if JFK was killed directly because of the actions of someone who went back in time to try to STOP the assassination? I.e., he tried to change history and wound up realizing that if not for his actions, history wouldn't have worked out that way in the first place.

It sure would give a neat explanation for the "magic bullet". =P

In reply to an earlier poster: 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. The past cannot be changed, not because it resists intervention, but because time is made out of whole cloth and any "changes" you try to make are already part of the picture before you even decide to make them.

Now, if someone went back in time to *ensure* JFK's assassination, that might make sense, and he might even have been the shooter (one of the shooters? let's not get into conspiracy theory though).

-Kasreyn