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GalacticBeatDown
2009-Jul-12, 12:50 AM
Say a few thousand years in the future their are space ships that have some technology that allows them to go faster than light. As they accelerate faster and faster and time gets slower and longer, is it possible that the spaceship reaches a certain point where time stops and then it starts going into the past, i.e. -1 seconds, -2 seconds etc? Is this totally wrong or is it a possibility?
And if this is possible if they do travel into the past they would be in a (theoretically speaking) parallel universe because if they go back into the past they would not be born yet and they could not exist and the technology was never their and the machine was not yet built.

Maybe all I have said above is nonsense and the universe just does not allow you to travel at all. :surprised

kleindoofy
2009-Jul-12, 03:00 AM
... all I have said above is nonsense ...
Yup!

Well, except for:


... a few thousand years in the future their are space ships that have some technology ...
If humanity survives that long, then yes, there might be space ships, and yes, they would have some technology. ;)

eburacum45
2009-Jul-12, 08:39 AM
There have been extensive discussions of the relationship between FTL annd time travel on this forum; it is probably best to say that, yes, there is time travel involved in traveling faster than light. But the relationship is ... not straightforward, so it's not just a case of going faster-than-light = going back in time. It's more complex than that.

astromark
2009-Jul-12, 08:41 AM
And ( can not start sentence with and ) And my friend ' Kleindoofy ' is very kweva...( cleaver ), and is right on the button.
Mater can not exceed c. as a velocity. Can not move through time. It has happened already or has not. Past and future.
If you were to go into your future it becomes the present as soon as you arrive.:)
All you have done is wast the interval between. There can not be backward time shifts... Its happened already. Yes... just science fiction.

Jeff Root
2009-Jul-12, 11:05 AM
Unfortunately it's pretty clear that no spaceship can go faster than the
speed of light. And that nothing can go backward in time. Fortunately,
though, traveling at speeds close to the speed of light causes time to
slow down in a way, which would allow a spaceship to go from one place
to another so quickly that it would almost seem to be going faster than
light. But if you leave Earth now, go to a planet many light-years away,
visit the place for a short time, and return to Earth, even if you were only
gone five years according to your clocks and calendar, a hundred years
might have gone by on Earth, and everyone you ever knew will be long
dead. So it has its advantages and disadvantages.

-- Jeff, in Minneapolis

robross
2009-Jul-12, 10:42 PM
Say a few thousand years in the future their are space ships that have some technology that allows them to go faster than light.

Even with our limited understanding of the universe, we know enough about physics to know some basic limitations will prevent any matter from traveling at the speed of light. It can be easily shown that the energy requirements to accelerate a particle, let alone a spaceship, to the speed of light eventually exceed all the available energy in the entire universe.

So there really is no way to do this, and thus all the paradoxes such as those pertaining to time travel at speeds > light are easily avoided.



Maybe all I have said above is nonsense and the universe just does not allow you to travel at all. :surprised

I'm still hopeful that there may be other physics waiting to be discovered that do allow for things to move between places faster than light. This would still seem to produce numerous paradoxes, but perhaps these can be resolved as well. Any such transportation would have to be through some different mechanism other than accelerating a particle faster than light.

The only such possible idea I know of right now is the Alcubierre drive, similar to what "Warp Drive" is in the Star Trek universe:

http://en.wikipedia.org/wiki/Alcubierre_drive

It's based on the observation that space is known to be expanding, and new space being created. Thus this is already allowable in our universe. We don't know how this is happening, or if it is possible to create space ourselves, nor if it is possible to also destroy space, nor how much energy would be required for either. Also, recent papers claim that such a 'warp' field would be unstable and destroy the ship, or cause it to immediately fall out of the warp field.

However, ideas like this suggest to me a way to get around the speed of light travel limitation. But again, if this turns out to be possible, we still have to understand the implications and solutions of numerous time-travel paradoxes.

Rob

mugaliens
2009-Jul-13, 02:28 AM
A photon arriving at a location a billion light-years distant from it's origen is no older than it was a billion years ago. However, a photon can travel at c because it's rest mass is zero. Its lightspeed mometum is therefore a function of it's wavelength (energy).

If we ever figure out how to move spaceships from Point A to Point B faster than is possible to move light between those two points, I'm fairly certain it will involve physics as yet unknown, and therefore it's really impossible to say what will happen with the time arrow along the journey, except...

I don't think it will be moving backwards, however.

DrRocket
2009-Jul-13, 02:55 AM
If we ever figure out how to move spaceships from Point A to Point B faster than is possible to move light between those two points, I'm fairly certain it will involve physics as yet unknown, and therefore it's really impossible to say what will happen with the time arrow along the journey, except...

I don't think it will be moving backwards, however.

Traveling to the past should not be that difficult.

According to the information on your post, you are in Colorado Springs.

From Colorado Springs travel west northwest about 500 miles. It will seem like 1952. If you like, I'll show you around. :)

eburacum45
2009-Jul-13, 06:16 AM
One of the threads about the relationship between FTL and causality can be found here
http://www.bautforum.com/space-astronomy-questions-answers/64167-why-does-ftl-not-work.html
I found this discussion instructive. To put it much too simply, if you do manage to travel faster-than-light, there exists a range of possible observers who will see you going back in time. Do it right, and you can actually go backwards yourself.

astromark
2009-Jul-13, 09:08 AM
One of the threads about the relationship between FTL and causality can be found here
http://www.bautforum.com/space-astronomy-questions-answers/64167-why-does-ftl-not-work.html
I found this discussion instructive. To put it much too simply, if you do manage to travel faster-than-light, there exists a range of possible observers who will see you going back in time. Do it right, and you can actually go backwards yourself.

.... Try to see this from a wider point of view... Its been around the block a few times here already... With higher mathmatics you can prove most things. All you need is to know how to apply it. Much of this subject is open to some speculative meanderings... Mathmaticly we could prove that other dimensions should exist. We have no idea if that is real. I suspect not. Just as in this subject. Theoreticaly to move back through time might be a focusable probability. I have repeatedly said its not. For if an event has happened it becomes historic. Its happened. You can not go there. Its history. If you filmed the event would this time traveller suddenly appear on those films. No. No, no... Not Ever. Never. No way. To do so would break the rules that have built this universe. That's a big fat crystal clear NO !...
If you want to endorse a science fiction fantasy then yes. you can travel faster than light and can do all sorts of impossible things... but in science. The real world of coffee cake and my view of reality, sorry but, NO.

robross
2009-Jul-13, 11:06 AM
One of the threads about the relationship between FTL and causality can be found here
http://www.bautforum.com/space-astronomy-questions-answers/64167-why-does-ftl-not-work.html
I found this discussion instructive. To put it much too simply, if you do manage to travel faster-than-light, there exists a range of possible observers who will see you going back in time. Do it right, and you can actually go backwards yourself.

I've read that thread thread several times now, and found it rather unsatisfying. I think the OP had a good question, and I think most of the answers didn't even respond the original question he posed. I think when I have more time I'll start a new question on the same subject, except be more explicit and call it something like "Teleportation and Time Paradoxes" so FTL isn't even mentioned and won't pose a distraction.

Rob

eburacum45
2009-Jul-13, 12:51 PM
I find this sort of discussion fascinating, although I have to admit I shy away from the maths involved. There are diagrams involved that I think I can just about understand, however.

Here's an interesting page I found recently:
The Andromeda Paradox (http://en.wikipedia.org/wiki/Rietdijk%E2%80%93Putnam_argument)
Reading that page, you can see what the problem is with teleportation and FLT. Teleport instantly to Andromeda from a stationary location and you will arrive there 'today'; teleport there from a moving car and you will arrive there 'tomorrow'.

Basically different moving reference frames have different 'planes of simultaneity' which diverge as you get further away from where they intersect. In fact different parts of your body are in motion with respect to each other (the circulation of your blood for instance) so you yourself don't really have a single reference frame; teleport instantly to Andromeda and you would be spread out in a smear across several different simultaneous moments. I don't think that is a good idea.

Paul Beardsley
2009-Jul-13, 01:42 PM
Once again, it's the same objections, which amount to, "Time travel into the past is impossible because if it was possible you'd be able to visit events in history, which you can't, so it can't be possible." Time travel may well be impossible, but that's not a satisfying explanation. You might as well quote a peasant from the Middle Ages saying, "Jet aircraft are impossible because if they were possible you could travel from London to the Holy Land in less than a day, which is impossible, so they can't exist."

As to a time traveller "suddenly" appearing in a film - well of course he wouldn't. His presence at the event would be as much a part of history as the event itself. He would always have been in the film.

WayneFrancis
2009-Jul-13, 02:34 PM
I think we might find a way to get from one point in space to another at a rate faster then C but to do this I don't believe we will actually travel faster then C. It will be all about warping space time.

The problem of actually travelling at C is that once you are there you would not know when to stop and it would not matter. The universe would shrink to a singularity. So eburacum45 is right. This would basically mean that you would be smeared across the entire universe in your reference frame and in other peoples reference frame you'd blink out of the universe because you would be infinitely thin.

astromark
2009-Jul-13, 07:22 PM
'Paul' I have thought this through and have arrived at what can only be the reality of this ... according to me.:) That event has happened. It was filmed. I was not there. The film shows that fact. IF I were to go back through time to be in that place at that time... How would that old film be changed ? That's just not ever going to be possible. Changing the past is always going to be impossible as those events are facts of history. Warping space time to facilitate these notions of instant travel is just grasping at the unreal. Impossible.

mugaliens
2009-Jul-13, 07:36 PM
Traveling to the past should not be that difficult.

According to the information on your post, you are in Colorado Springs.

From Colorado Springs travel west northwest about 500 miles. It will seem like 1952. If you like, I'll show you around. :)

ROTFL! Near as I can tell, that puts us in Milward Slough, just NW of the town of Rexburg Ditch, ID.

I was expecting Hill Valley, but that was 1955, wasn't it?

Paul Beardsley
2009-Jul-13, 08:08 PM
'astromark', it can be stated as a fact that if the historical event was filmed, whatever else you may do, you will not be stepping in front of the camera - unless the film has already shown you to be present.

That does not rule out the possibity that you will visit a period of history that was not filmed.

What it amounts to may be put simply: If you set out to do something in the past that did not happen, then you can be sure that you will fail. This would include walking into the view of a recording camera that you know did not film you. On the other hand, if you set out to do something in the past that may or may not have happened, then you cannot be sure that you will fail.

If you think about what I have written, you will see that I don't disagree with you.

Van Rijn
2009-Jul-13, 08:48 PM
I think we might find a way to get from one point in space to another at a rate faster then C but to do this I don't believe we will actually travel faster then C. It will be all about warping space time.


Note that this has exactly the same relativity/causality problems as any other FTL travel method. Remember the rule:

Relativity, causality, FTL - pick any two.

cosmocrazy
2009-Jul-13, 08:57 PM
Say a few thousand years in the future their are space ships that have some technology that allows them to go faster than light. As they accelerate faster and faster and time gets slower and longer, is it possible that the spaceship reaches a certain point where time stops and then it starts going into the past, i.e. -1 seconds, -2 seconds etc? Is this totally wrong or is it a possibility?
And if this is possible if they do travel into the past they would be in a (theoretically speaking) parallel universe because if they go back into the past they would not be born yet and they could not exist and the technology was never their and the machine was not yet built.

Maybe all I have said above is nonsense and the universe just does not allow you to travel at all. :surprised

When you consider a question like this you have to remember that time is relative. If you managed to accelerate a spaceship up to the speed of light the instant that space ship hits the "c" speed its journey is over for any passengers on board, because according to their reference, time and space have shrunk to zero at that instant, regardless how far away that destination was originally. For any observer external to that frame of reference then the journey would have been observed to have taken x amount of years over x amount of time and no 2 different frames of reference would agree.
Now where things get complicated is when you talk about FLT. FLT for the space craft could mean arriving before they had set off which makes no sense since the time and space between start and finish would have a negative value, and would create physically impossible paradoxes. But for any external observer it would just seem that the spaceship had got there quicker than light itself and dependent how much faster than light it was, more causality paradoxes could unfold. I think the problem with time travel in using this method, is the fact that as soon as you change speed relative to something regardless of how fast then you just change your relative reference frame. Basically regardless how fast you go you always end up goin what seems like forward with the arrow of time. Time just seems to flow at different rates measured by relative reference frames.
Just goin back to FLT, i think to travel the huge distances observed in our universe then some sort of worm hole or warping of spacetime would be the only realistic possibility for interstellar space travel based on the physics we currently know of. This would give the appearance that the space craft had indeed travelled FLT when in fact it really was goin sub "C" within its own reference frame.

Van Rijn
2009-Jul-13, 09:14 PM
Just goin back to FLT, i think to travel the huge distances observed in our universe then some sort of worm hole or warping of spacetime would be the only realistic possibility for interstellar space travel based on the physics we currently know of. This would give the appearance that the space craft had indeed travelled FLT when in fact it really was goin sub "C" within its own reference frame.

Again, unless relativity is very wrong, this would allow causality violations.

Van Rijn
2009-Jul-13, 09:22 PM
'astromark', it can be stated as a fact that if the historical event was filmed, whatever else you may do, you will not be stepping in front of the camera - unless the film has already shown you to be present.


Not a fact, I'm afraid. If we allow for the possibility of time travel, that can not be ruled out. You're describing one idea (but not an exclusive one) for how time travel might work, if it were possible.

Van Rijn
2009-Jul-13, 09:31 PM
I've read that thread thread several times now, and found it rather unsatisfying. I think the OP had a good question, and I think most of the answers didn't even respond the original question he posed.


I thought they did, fairly early on in the thread. Can you explain what you thought wasn't answered?



I think when I have more time I'll start a new question on the same subject, except be more explicit and call it something like "Teleportation and Time Paradoxes" so FTL isn't even mentioned and won't pose a distraction.

Rob

As long as the hypothetical teleportation scheme operates at the speed of light or slower, causality violations wouldn't be an issue. If it does (for instance, a claimed "instantaneous" teleportation), then causality violations would be an issue. Anyway, if you don't want the FTL issue to be brought up, be sure to specify that this "teleportation" is not FTL.

Van Rijn
2009-Jul-13, 09:55 PM
Reading that page, you can see what the problem is with teleportation and FLT.


A terminology issue - I'm guessing that "FLT" stands for "Faster [than] Light Travel"? I'm used to "FTL" (Faster Than Light).

GalacticBeatDown
2009-Jul-13, 10:05 PM
Thanks a lot guys I get the understanding of most of it. I totally forgot about how once you have an object accelerating closer to the speed of light to finally get it to c you would have to user more energy that their is in the universe. But the worm hole thing like in the Star Gate series is pretty cool. What also is interesting is the hypothesis that you can move the space in front of you towards you so you could travel vast distances. Its basically like traveling say 1000 miles on a map. Fold it and you can bridge the start and end points and arrive their instantly.

Ken G
2009-Jul-14, 12:04 AM
Also, if we just assert that you can move faster than c, never mind if it's possible, then what happens is what you are asking, and it's not quite the same as going into your own past. It might be impossible to say what would "really happen" in this impossible scenario, but if we are to keep to the spirit of relativity, let us look at what would be the answer.

First of all, in all reference frames in relativity, time passes normally. So it's not really true that "time stops" for a photon, it just means that the entire age of the universe passes in zero time for the photon. The key point here is, we are talking about a comparison between two times, the observer time and the universe time. The observer time is always normal, so if you went faster than c, presumably you would say that the universe's time is doing something strange, and the universe would say your time is strange.

But the important point is, none of the clocks would tick backward-- not yours, not the universe's. That's because a local clock measures local time by using mechanisms that respond to local time. That means if local time were going backward, the clock would automatically be what you need to measure backward time, so the clock reading would also proceed forward. That is necessary, because the local observers would know the clock is moving forward. The key point is, you may interpret time going backward in other places, but their clocks will not go backward, it is your interpretation that the time is backward, and your interpretation will be that the clock reads backward also, so a backward clock reads backward time forward.

What's more, it seems to me that for an observer going faster than c with respect to the universe, following the time dilation formula gives that observer that time in the universe is imaginary, not backward. Similarly, the universe thinks the observer's time is imaginary. I don't see how anyone's time is backward. I realize people talk about tachyons going backward in time, but I don't see it, it seems to me it should be moving through imaginary time (from out point of view), and through normal time (from the tachyon's point of view).

If all this sounds confusing, then it is probably because you are still thinking in terms of absolute time. One of the most important lessons of relativity is that the time elapsed depends on the path taken. So to go faster than c, it just means that some paths accrue an imaginary time, interpreted from the other frames. But the clock that travels the path of imaginary time will not read an imaginary result, because that clock will automatically be just what you need to read imaginary time, and it will read a normal result. It is exactly the same with time dilation-- a clock participating in dilated time is automatically a slow clock (from other people's point of view), so it is automatically exactly the type of clock you need to read dilated time correctly for the local observers.

Now, does this any of this let you visit your past? No. Let's look at what happens if you travel at 2c to a star 10 Ly away, and they you turn around and come back at 2c. We know the Earth clock, and the synchronized star clock, will say the time is 5 years after you leave when you arrive at the star, and 10 years when you get back. The clock of the traveller must also read a positive result, because time proceeds normally for all observers in their own frame. The convention used in special relativity for how to extend one's own concept of time globally to other people would say that the time in the universe is imaginary, and all the universe clocks are also out of synchronization in that imaginary time. So they are clocks that read imaginary time, and they are all out of synch. The combination allows the traveler to understand why 10 years goes by on the Earth clock. That's always how relativity works, it shouldn't be any different at 2c.

How much time elapses for the traveler? It should be the same as the time dilation formula would give to people on Earth, and that will be an imaginary number (a multiple of i). So the Earth people will just say an imaginary time went by on the traveler's path, and the traveller's watch measured that imaginary time as a normal time. I've no idea why people say tachyons go backward in time.

mugaliens
2009-Jul-14, 01:34 AM
What it amounts to may be put simply: If you set out to do something in the past that did not happen, then you can be sure that you will fail.

Well, that's one theory, espoused by Heinlein in By His Bootstraps. Another, the theory involving an infinate number of parallel universes, as depicted in Jet Li's The One, says that going back in time would involve travelling to the universe where that actually happened, as opposed to remaining in this one.

Paul Beardsley
2009-Jul-14, 01:50 AM
Well, that's one theory, espoused by Heinlein in By His Bootstraps. Another, the theory involving an infinate number of parallel universes, as depicted in Jet Li's The One, says that going back in time would involve travelling to the universe where that actually happened, as opposed to remaining in this one.

Granted it's all up for grabs when one is speculating about how something that is probably impossible might work, but it seems to me that Novikov's self-consistent scenario is the only one with any merit, and the alternatives were only invented because people saw non-existent flaws in it. The parallel universe idea may address the (non-existent IMO) paradox problem, but I've never heard it explained why you'd find yourself in another universe. In which case you might as well say, "Oh, you become invisible and you can't interact with anything."

Van Rijn
2009-Jul-14, 02:35 AM
Granted it's all up for grabs when one is speculating about how something that is probably impossible might work, but it seems to me that Novikov's self-consistent scenario is the only one with any merit, and the alternatives were only invented because people saw non-existent flaws in it.


Actually, I think you have that turned around. Novikov's self-consistency was an attempt to get around time travel paradoxes.



The parallel universe idea may address the (non-existent IMO) paradox problem, but I've never heard it explained why you'd find yourself in another universe.


In a variation of the many-worlds idea, the choice of going back in time could create a branching universe. There could be one branch where you didn't go back in time, and another where you did, and appear in a '60s Doctor Who episode.

Paul Beardsley
2009-Jul-14, 03:13 AM
Actually, I think you have that turned around. Novikov's self-consistency was an attempt to get around time travel paradoxes.
I strongly disagree*, partly because I came up with it independently when I was younger. The belief in paradoxes is a product of meta-time fallacy, such as the assumption that "last time" there was no time traveller at a certain location at a certain date, whereas "this time" there is.


In a variation of the many-worlds idea, the choice of going back in time could create a branching universe. There could be one branch where you didn't go back in time, and another where you did, and appear in a '60s Doctor Who episode.
Are you thinking of a specific one?

*ETA: I mean, that might be the order of events, but your post (initially) made it sound (to me) as if Novikov was desperately plugging holes rather than discovering that the holes were never there.

Van Rijn
2009-Jul-14, 04:33 AM
The belief in paradoxes is a product of meta-time fallacy, such as the assumption that "last time" there was no time traveller at a certain location at a certain date, whereas "this time" there is.


No, paradoxes are one of the possibilities that must be considered if you are going to speculate about time travel. You obviously prefer the "single immutable timeline" idea, but that is not the only possibility.



*ETA: I mean, that might be the order of events, but your post (initially) made it sound (to me) as if Novikov was desperately plugging holes rather than discovering that the holes were never there.

Actually, I would call it more of speculation about time travel, neither plugging holes, nor discovering that holes did not exist.

robross
2009-Jul-14, 05:57 AM
No, paradoxes are one of the possibilities that must be considered if you are going to speculate about time travel. You obviously prefer the "single immutable timeline" idea, but that is not the only possibility.

Also, can you imagine all the new verb tenses we'd have to come up with to describe events in a non-causal, or time-travel possible, universe?

Rob

eburacum45
2009-Jul-14, 07:25 AM
Novikov self-consistency is only one of the possible ways in which faster-than-light travel might affect causality. Matt Visser summed them up some years ago, in a paper about traversable wormholes; here he is on the subject again;
http://homepages.mcs.vuw.ac.nz/~visser/general.shtml


The radical re-write conjecture: Grit your teeth and proceed to re-write all of modern physics from the ground up. Painful, very painful. (I'll live with this if necessary-but you'd better give me good experimental evidence before I spend too much time worrying about this possibility.)

The consistency conjecture: Since there seems to be only one universe, insist that it must be consistent no matter what. So if you try to change history you cannot succeed no matter how hard you try, because the past is already fixed. You know that you, the reader, are alive right now, so no-one can ever send a time traveller to five minutes ago to kill you as you pick up your copy of Phlogiston. If someone tries, something must go wrong: the gun must misfire, or the time machine malfumction, or the assassin miss the bus, or any of a potenially infinite list of increasingly contrived excuses. (Not my favourite way of dealing with things; it quickly begins to look like a consistency conspiracy.)

The chronology protection conjecture: A much more conservative point of view. Even though time travel seems to be absurdly easy once wormholes/warp-drives are allowed, there are reasons to expect things to go berserk just at the onset of time travel. We know that gravitational fields distort the quantum mechanical vacuum, and that this vacuum distortion heads off to infinity at the onset of time travel. We suspect that this effect destroys the wormhole/warp-bubble just as one is getting round to building a time machine. (This is my personal favourite.)

The boring physics conjecture: Forget all this nonsense. Take a good hard look at the experimental evidence, or rather lack thereof, and move on to greener pastures.
Visser suggests that some forms of FTL (or FLT if you prefer) travel might be permissible, but the universe itself conspires to forbid any use of an FTL drive that would allow reverse causality. The 'radical rewrite conjecture' includes parallel worlds and other peculiar manifolds. I think it could be made to work, but I doubt it would be useful. After all, any one individual can only live in one timeline at once...

Personally I think the 'boring physics conjecture' is the most likely, but one lives in hope...

Ken G
2009-Jul-14, 08:10 AM
Visser suggests that some forms of FTL (or FLT if you prefer) travel might be permissible, but the universe itself conspires to forbid any use of an FTL drive that would allow reverse causality.Again, I do not see how FTL drive has anything to do with going back in time in the way being talked about here. If you get on a rocket, go to a star 10 Ly away at 2c, and return, you will get back after 10 years have passed on Earth. That is not going back in time. It's true that your interpretation, using SR-like coordinates, will be that Earth time has done something funky (it seems to me you'll think Earth time went imaginary, not backward), but it doesn't matter which because it's just a coordinate thing-- you will still conclude that 10 years passes on Earth, completely normally for the Earthlings. As such, you will never visit the past by going FTL, as you knew it. I think the whole FTL business is a complete red herring in regard to the kind of time travel being discussed here-- it seems to me what is really being discussed is topological changes like wormholes, etc.

eburacum45
2009-Jul-14, 11:15 AM
Certainly topological methods of FTL travel would allow reverse time travel; but teleportation would allow this trick as well, I think.
Read Rich Baker's discussion of FTL and causality here
http://www.theculture.org/rich/sharpblue/archives/000089.html

Note that if you substitute the use of an 'ansible' (faster-than-light-communicator) in those examples with a 'faster-than-light teleport beam' the result is the same, except that the message you send into the past is yourself.

I can't quite get my head round whether one could extend this example further, and substitute the words 'faster-than-light spacecraft' for the words 'faster-than-light teleport beam' and still get the same results; I suspect you can, but that might be stretching the concept too far.

Paul Beardsley
2009-Jul-14, 08:30 PM
Novikov self-consistency is only one of the possible ways in which faster-than-light travel might affect causality.
Matt Visser's issue with it seems to be that it "seems contrived", which seems to me an absurd response. If an outcome is known, then any attempt to avoid it is obviously going to result in a convergence on that outcome. It doesn't need to be time travel - if you watch a recording of a football game in which the better players lose through sheer bad luck, and you know the outcome before you start watching, then that will seem contrived.

I think "inevitable" is a more accurate word than "contrived". People do miss buses. Including assassins, I presume. Which is more likely - a time travelling assassin misses a bus, or the whole of physics is wrong?


Personally I think the 'boring physics conjecture' is the most likely, but one lives in hope...
My feelings exactly.

Van Rijn
2009-Jul-14, 09:18 PM
Which is more likely - a time travelling assassin misses a bus, or the whole of physics is wrong?


More likely is that time travel isn't possible. However, if time travel is possible, then we are in the realm of currently unknown physics, so this isn't a valid question.

eburacum45
2009-Jul-14, 09:36 PM
One slightly unsettling thing has just come to my attention. Rich Baker (sometime OA Contributor) mentions in passing Hugo Gernsback (http://en.wikipedia.org/wiki/Hugo_Gernsback) as the originator of multiple-timeline time travel concepts. Presumably this predated Hugh Everett (http://en.wikipedia.org/wiki/Hugh_Everett)'s ideas about Many Worlds as an interpretation of quantum physics.

But reading up a little about this interesting chap (an early amateur radio pioneer, and the chap that the Hugo awards are named for) I found this anachronistic image;
http://www.sas.org/tcs/weeklyIssues/2004-10-01/feature1/art/ee.jpg
Basically Gernsback seems to have an image of Telstar (http://en.wikipedia.org/wiki/Telstar) on the cover of a magazine he published in 1915.

If anything might ever be admissible as proof of time travel or reverse causality, that image would probably qualify.
.
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GalacticBeatDown
2009-Jul-14, 09:51 PM
If anything might ever be admissible as proof of time travel or reverse causality, that image would probably qualify.

Not really, I mean it is possible but its a long shot because then someone could say well maybe the designers of the Telstar saw that cover image on that magazine and they said well that looks like a good design for what they wanted the Telstar to look like.

eburacum45
2009-Jul-14, 10:11 PM
Good point. I wonder if that did happen, one way or another.

dwnielsen
2009-Jul-14, 10:59 PM
Say a few thousand years in the future their are space ships that have some technology that allows them to go faster than light. As they accelerate faster and faster and time gets slower and longer, is it possible that the spaceship reaches a certain point where time stops and then it starts going into the past, i.e. -1 seconds, -2 seconds etc? Is this totally wrong or is it a possibility?
And if this is possible if they do travel into the past they would be in a (theoretically speaking) parallel universe because if they go back into the past they would not be born yet and they could not exist and the technology was never their and the machine was not yet built.

Maybe all I have said above is nonsense and the universe just does not allow you to travel at all. :surprised

Hi, GBD, sorry that I don't have the time to read all the replies by people who understand much more about this than I do. It is possible that they have already given the following approach or made it very evident why it is fallible, but I thought that I would take a naiive look at this, ignoring all the logical implications.
Let's just assume the 1D Lorentz transforms in the standard frame are good for v>c and that v>c is possible.
Then the transform for time becomes..
t' = ( t - v x / c^2 ) ( 1 - (v / c)^2 )^(-.5)
In this case, t' is an "imaginary" value. That would seem to imply that time does not reverse in the usual sense, but requires an entirely new conceptualization. I hope to read the other posts tonight. I'm sure they will help elucidate things for me.

Jeff Root
2009-Jul-14, 11:02 PM
Actually the father of the designer of Telstar was so dissatisfied with his
son's design that he went back in time and gave Hugo Gernsback a drawing
of what he thought Telstar should look like. Gernsback published the
drawing on the cover of 'The Electrical Experimenter'. The father of the
Telstar designer then bought a copy of the magazine as a boy, and saved
it for decades. His son read it in the 1940's, influencing the son to design
Telstar the way his father thought it should look.

-- Jeff, in Minneapolis

robross
2009-Jul-14, 11:20 PM
Actually the father of the designer of Telstar was so dissatisfied with his
son's design that he went back in time and gave Hugo Gernsback a drawing
of what he thought Telstar should look like. Gernsback published the
drawing on the cover of 'The Electrical Experimenter'. The father of the
Telstar designer then bought a copy of the magazine as a boy, and saved
it for decades. His son read it in the 1940's, influencing the son to design
Telstar the way his father thought it should look.

-- Jeff, in Minneapolis

You left out the part where the father ends up becoming the son's daughter. It's a crazy world, this time travel!

Rob

Jeff Root
2009-Jul-14, 11:33 PM
Matt Visser's issue with it seems to be that it "seems contrived",
which seems to me an absurd response. If an outcome is known, then
any attempt to avoid it is obviously going to result in a convergence
on that outcome. It doesn't need to be time travel - if you watch a
recording of a football game in which the better players lose through
sheer bad luck, and you know the outcome before you start watching,
then that will seem contrived.

I think "inevitable" is a more accurate word than "contrived". People
do miss buses. Including assassins, I presume. Which is more
likely - a time travelling assassin misses a bus, or the whole of
physics is wrong?
In addition to what Van Rijn said, you seem to miss the basic point
that this Matt Visser guy was making. One time traveller can very
well miss a bus, and so fail to accomplish what he set out to do, but
he can try again, and again, and again. The next time traveller after
him can try as well. And the next, and the next. And the next.
Could they all have bad luck? No. That's what makes it contrived.

-- Jeff, in Minneapolis

Ken G
2009-Jul-15, 02:00 AM
In this case, t' is an "imaginary" value. That would seem to imply that time does not reverse in the usual sense, but requires an entirely new conceptualization.That puzzled me too, but the post by eburacum45 about what Rich Baker said was pretty convincing. I'm still trying to sort this out.

Paul Beardsley
2009-Jul-15, 03:53 AM
In addition to what Van Rijn said, you seem to miss the basic point
that this Matt Visser guy was making. One time traveller can very
well miss a bus, and so fail to accomplish what he set out to do, but
he can try again, and again, and again. The next time traveller after
him can try as well. And the next, and the next. And the next.
Could they all have bad luck? No. That's what makes it contrived.

-- Jeff, in Minneapolis

I haven't missed the point. Time travel into the past is either easy or hard. It's probably hard, because if it was easy we'd see evidence of time travellers, but if it's easy, the hordes of time travellers are just as likely to set out to preserve history as wreck it (knowing that preserving history is possible) - or they might simply enjoy hunting assassins.

Coming back to the persistent assassin, it's trivially easy to think of barriers to his assignment - given that we know he doesn't succeed. Not least would be his loss of nerve, knowing he is taking a risk to do something that is doomed to failure. Yes, it might look a bit contrived, but again, it's only because events are converging on a known outcome.

(This assassin would also be a prime target for later time travellers who want to go on a killing holiday. "We know this guy tried to change history by assassinating someone, but he got killed by a mysterious stranger. I think I'd like to be that mysterious stranger.")

My argument is that if time travel is possible, and there are time travellers in the past, then they are no more and no less than a contributing factor to any known outcome. This is logically sound. The alternatives are not.

Coming back to astromark's example, suppose a CCTV camera pointing at a stretch of road shows that nobody walked down that road on a particular day. Now, suppose some years later a time traveller sets off from his present to that particular day. Three hours after arriving at that day, he gets to that road and walks down it, in full view of the camera. Meanwhile, back in his present, a colleague has been watching the film from the camera.

Now, when does the film of the empty road change to show the time traveller walking down it? Three hours after he set out? The instant he sets out? The instant he makes the decision to set out? These are all metatime answers.

The logical scenario is that if the film shows an empty street that day, the time traveller knows he will not be walking down the street on that day, regardless of his intentions. He might not know the reason why he won't be walking down the street, but the fact remains; were this not the case, he would already be on the film.

Ken G
2009-Jul-15, 04:16 AM
You might imagine the analogy of watching a prerecorded sporting event. Let's say you saw the final score in the newspaper, and now you want to watch the game. Will you not see the losing team doing everything in its power to win? Does it seem "contrived" that no matter what they do, they cannot win? If one adopts the "single stream" view of time, then the presence of the time travellers is already part of the history. It's not that they weren't there the first time, and then they are there but can't change anything, it's that they were there the first and only time too, and everything they did is already part of that history. The question is not then, why can't they change anything, it's why don't we have evidence that they have already played an important role in what has happened in the world. It seems that if the "single stream" is correct, then there hasn't been a whole heck of a lot of time travel so far. Maybe the first traveller will show up tomorrow, and it will get a whole lot more common after that. Remember that if no travellers arrived before 2009, then none ever can arrive before 2009-- it would be impossible to time travel to a place and time that you have not already traveled to (in the single stream model).

Another interesting issue that emerges is free will, but we already know virtually nothing about that, so we certainly don't know how it would interact with time travel. For example, time travel, if possible (unlikely of course) might scramble your brain to the point that you become a spectator in your own body. Perhaps the time continuum trumps your normal concept of free will. Perhaps free will is always pure illusion, and we only don't notice this when we don't have prior knowledge of our actions. So much is unknown, I don't see anything contrived about any of the time travel models, I just find them all quite unlikely.

astromark
2009-Jul-15, 05:41 AM
All of this is a foolish nonsense. How do I say it better ? To move through time as if it were a dimension that can be manipulated is a nonsense. All of the reasons are and, have been said.
I would like to put another fool idea out the window... Alternate realities. Every time your time traveller does some fool thing that upsets the original time line a alternate reality is created.
Only in science fiction. Surly we have agreement, Yes...?
I notice that we seem to have agreement that time travelling is not possible. We just insist on saying it our own way. and that is good.

robross
2009-Jul-15, 05:51 AM
All of this is a foolish nonsense. How do I say it better ? To move through time as if it were a dimension that can be manipulated is a nonsense. All of the reasons are and, have been said.
I would like to put another fool idea out the window... Alternate realities. Every time your time traveller does some fool thing that upsets the original time line a alternate reality is created.
Only in science fiction. Surly we have agreement, Yes...?
I notice that we seem to have agreement that time travelling is not possible. We just insist on saying it our own way. and that is good.

I would say it does appear unlikely, but that is not a sufficient argument. It's no more valid than people that say "what do you mean my clock slows down when I approach the speed of light? That's rubbish!"

Just because we have not personally observed something does not make that observation invalid. I find no compelling evidence that time travel is possible, but physics does not rule it out. The paradoxes that ensue from allowing it certainly need to be understood, but these paradoxes are not proof that time travel is impossible, any more than Zeno's paradox is proof there is no motion, nor Obler's paradox is proof the universe is not infinite.

Rob

astromark
2009-Jul-15, 06:42 AM
Quote from Robross., " but physics does not rule it out. " end Quote... I would challenge that idea. It does, and has. Do you know of such facts as to demon strait time travel ? No, you clearly say not. I do not want to hear what you might believe. That's not good enough. I want that proof.
Think of those great moments of our history. We have many to choose from. I have witnessed film of those events. The background was not filled with nameless people from the futur just witnessing the moment. The mainstream idea of a single time line starting at the moment of the Big Bang and ongoing for eternity. If you have any facts that undermine my resolve please be forth coming with it.

mugaliens
2009-Jul-15, 06:43 AM
You left out the part where the father ends up becoming the son's daughter. It's a crazy world, this time travel!

Rob

So you're saying time travel repairs the partial X chromosome?

Jetlack
2009-Jul-15, 06:46 AM
Doesn't qm prohibit travelling into our past?

For instance if one takes the delayed choice experiment seriously, then our choices now permanently and immutably affect what has gone before. So if history and qm is to be consistent there can be no mechanism allowing us to change a defined history once choices are made in the present which impact on the past.

Personally i think time travel is an impossibility.

robross
2009-Jul-15, 06:53 AM
Quote from Robross., " but physics does not rule it out. " end Quote... I would challenge that idea. It does, and has. Do you know of such facts as to demon strait time travel ? No, you clearly say not. I do not want to hear what you might believe. That's not good enough. I want that proof.
Think of those great moments of our history. We have many to choose from. I have witnessed film of those events. The background was not filled with nameless people from the futur just witnessing the moment. The mainstream idea of a single time line starting at the moment of the Big Bang and ongoing for eternity. If you have any facts that undermine my resolve please be forth coming with it.

http://en.wikipedia.org/wiki/Time_travel

"However, the theory of general relativity does suggest scientific grounds for thinking backwards time travel could be possible in certain unusual scenarios, although arguments from semiclassical gravity suggest that when quantum effects are incorporated into general relativity, these loopholes may be closed.[13] These semiclassical arguments led Hawking to formulate the chronology protection conjecture, suggesting that the fundamental laws of nature prevent time travel,[14] but physicists cannot come to a definite judgment on the issue without a theory of quantum gravity to join quantum mechanics and general relativity into a completely unified theory."

(my emphasis)


Rob

robross
2009-Jul-15, 07:20 AM
So you're saying time travel repairs the partial X chromosome?

Actually I was referring to the Heinlein short story "All You Zombies."

http://en.wikipedia.org/wiki/%E2%80%94All_You_Zombies%E2%80%94

Rob

astromark
2009-Jul-15, 07:54 AM
Yes i have read that short story:)and was amused... Science fiction can be amusing... Its still not science.
Even I have ventured down this road where I wanted to believe that 'Voyager' was real. Unfortunatly The Borg did win and we all died.:)then I woke up.
At about this time I found science was a better ally.

eburacum45
2009-Jul-15, 08:17 AM
My argument is that if time travel is possible, and there are time travellers in the past, then they are no more and no less than a contributing factor to any known outcome. This is logically sound. The alternatives are not.

Ah, but you are applying 'philosopher's logic' to the topic of time travel. I've seen philosophical arguments about time travel where philosophers 'prove' self-consistency by saying that the past is already known, so any potential time traveller *must* act consistently in order to be consistent. This is a self-referential argument, and I believe it is a fallacy.

The reality of consistency would be easily proved if time travel was possible, simply by going back in time and acting in a different way than the historical record indicates; if this is impossible, then consistency is proved to be correct. If acting differently in the past is possible, then consistency is disproved.

There is a problem though. By acting non-historically in the past you would create a new timeline- I can't see any way round that consequence. You as a time-traveller would then be commited to exist in that timeline, and would live with the consequences of your actions. Hooray! Freewill is restored.

But back in the original timeline you simply disappeared. Your colleagues back in the Time-Tunnel lab (or whatever) would see you disappear back into the past, intent on changing history- and you would never be seen again. History (to them) remains unchanged.

They would be forced to conclude that history is self-consistent, and that time-travel doesn't work. In some ways Novikov self-consistency is unfalsifiable -except for the poor time-traveller, who travels into a past she is familiar with, and by her actions creates a new future she doesn't recognise.

Of course we all create a future we don't recognise, so no change there then.

robross
2009-Jul-15, 08:28 AM
There is a problem though. By acting non-historically in the past you would create a new timeline- I can't see any way round that consequence. You as a time-traveller would then be commited to exist in that timeline, and would live with the consequences of your actions. Hooray! Freewill is restored.

But back in the original timeline you simply disappeared. Your colleagues back in the Time-Tunnel lab (or whatever) would see you disappear back into the past, intent on changing history- and you would never be seen again. History (to them) remains unchanged.

They would be forced to conclude that history is self-consistent, and that time-travel doesn't work. In some ways Novikov self-consistency is unfalsifiable -except for the poor time-traveller, who travels into a past she is familiar with, and by her actions creates a new future she doesn't recognise.


Except her actions in the past (her current 'present') could conceivably alter what her original perceptions were of that historical period. The memories in her brain would be affected by any such changes. It would seem she could not introduce significant changes in her environment that could affect her knowledge of that history, without changing her past and obviating any changes she made.

This is where it starts to get confusing!

Rob

astromark
2009-Jul-15, 09:56 AM
The present time. That what is right now. Comes at us at the speed of light, is here for an instant and is then relegated to the past. That is that. End of story. Once a moment has passed it has become history.You may tell lies about your version of history... Just listening to a politician will endorse that view...:)but, the facts as they were can not be revisited. They have passed.
All of this subjective nonsense regarding people travelling through time is fiction. The science to backup the theories does not exist. The more you mull over and imagine what ifs... just moves this closer to the realm of fiction. In the postings previous a note that it gets confusing... This is not surprising as that moment of time has come and gone. Lets make that leap of faith and say that yes you can go back through time. Now you are where you should not be, can you change history. No. Because if the answer was yes then that would suggest that events could be altered. That's the fact that makes time travel impossible. If you were not there. You were not there.

Paul Beardsley
2009-Jul-15, 10:40 AM
Ah, but you are applying 'philosopher's logic' to the topic of time travel. I've seen philosophical arguments about time travel where philosophers 'prove' self-consistency by saying that the past is already known, so any potential time traveller *must* act consistently in order to be consistent. This is a self-referential argument, and I believe it is a fallacy.
I get what you are saying, although you seem to be ignoring my justification and questions - and I would also cite Ken G's post #46.

Let's turn it around and imagine a single time traveller who pays a visit to, say, 1380 with the express intention of not changing history. He arrives, he looks around for five minutes, he returns home.

Now, is that going to cause another universe to split off? If so, why?

And if anyone is thinking, "Ah, he might not have intended to change history, but his very being there will have caused changes - he might have stepped on a butterfly," I would ask, how is that any different to a peasant stepping on a butterfly? If you did think the above, it reveals that you are thinking of the time traveller as a new addition to 1380, something that wasn't there "before". This is the fallacy I am trying to challenge.


The reality of consistency would be easily proved if time travel was possible, simply by going back in time and acting in a different way than the historical record indicates; if this is impossible, then consistency is proved to be correct. If acting differently in the past is possible, then consistency is disproved.
A failure to act inconsistently is not proof of consistency - you might simply have got unlucky. For instance, your failure to assassinate Hitler in 1933 does not mean he cannot be assassinated in 1933.

So even if time travel is invented, and proves to be fairly easy, it will take quite a few unsuccessful attempts to change history before everyone is convinced that consistency is beyond reasonable doubt.

eburacum45
2009-Jul-15, 11:28 AM
Except her actions in the past (her current 'present') could conceivably alter what her original perceptions were of that historical period. The memories in her brain would be affected by any such changes. It would seem she could not introduce significant changes in her environment that could affect her knowledge of that history, without changing her past and obviating any changes she made.

This is where it starts to get confusing!

RobAs far as I can see, her memories won't be affected by this change. It isn't like Back to the Future, where people start fading from photographs; in a Many Worlds scenario, you are either in one time-line or another, and there wouldn't be any 'fading' or alteration of anyone's memory- both timelines would be consistent. It just so happens that a person from one timeline has gone back in time for a bit and entered a different timeline.

This might even be an unavoidable consequence of time travel in a Many-Worlds universe; every time someone goes back in time they gain access to a swathe of alternate universes which were not available to them before. One of those universes would be the one where her actions follow the rules of self-consistency; the people back in the Time-lab get their time traveller back and self-consistency is apparently proven.

But a myriad other versions of that time-traveller cause a myriad other timelines to appear, and she exists in them all, but causes different things to occur. Time travel might be the only way to access such alternate universes- go back in time to before the assassination of Kennedy, and just wait for forty-odd years in real time; a myriad different versions of you will survive to the present, but only one would be in this timeline. That would be the self-consistent one. Other versions of you would be in the post-World War III scenario from Red Dwarf, or the timeline where America lands on Mars in 1986 (from Voyage by Stephen Baxter), and so on.

That's probably the answer; self-consistency does work, but only in one time-line. (Assuming that there are multiple timelines in the first place; that may very well not be the case).

eburacum45
2009-Jul-15, 12:18 PM
IA failure to act inconsistently is not proof of consistency - you might simply have got unlucky. For instance, your failure to assassinate Hitler in 1933 does not mean he cannot be assassinated in 1933.

So even if time travel is invented, and proves to be fairly easy, it will take quite a few unsuccessful attempts to change history before everyone is convinced that consistency is beyond reasonable doubt.
It may be impossible to disprove consistency; the consistent timeline is the only one we would see, in every case. Certain instances of the time traveller(s) might have a different personal experience, but we would never have contact with those alternative timelines.

This brings us on to the next question- if alternate histories exist, why don't we see time travellers from alternate futures? This is a bit like Fermi's paradox, so I think I'll just leave that particular question open...

astromark
2009-Jul-15, 07:24 PM
Quote;
That's probably the answer; self-consistency does work, but only in one time-line. (Assuming that there are multiple timelines in the first place; that may very well not be the case).

and then ...

Quote;
It may be impossible to disprove consistency; the consistent timeline is the only one we would see, in every case. Certain instances of the time traveller(s) might have a different personal experience, but we would never have contact with those alternative timelines.

This brings us on to the next question- if alternate histories exist, why don't we see time travellers from alternate futures? This is a bit like Fermi's paradox, so I think I'll just leave that particular question open... End quote.

Just think about that... Are you suggesting that any change made would create a whole new Galaxy...Universe. No this is not a open question at all. One reality, one time line. Its the only one we would see because its the only one.

Van Rijn
2009-Jul-15, 09:20 PM
Let's turn it around and imagine a single time traveller who pays a visit to, say, 1380 with the express intention of not changing history. He arrives, he looks around for five minutes, he returns home.

Now, is that going to cause another universe to split off?


Depends on the rules for time travel, but for a "many worlds" scheme, it certainly is reasonable to think it could.



If so, why?


Imagine you decide to send, say, a 2001-like monolith into deep space to a past of a "many worlds" multiverse. A universe where the monolith appeared would be different from one where it did not appear. The simple fact that there is mass where there would not otherwise be, an object that could intercept photons that would not otherwise be intercepted, would be a change. There is your split.

The change does not need to be something extremely obvious to humans. It only needs to be a change.

Ken G
2009-Jul-15, 10:29 PM
Depends on the rules for time travel, but for a "many worlds" scheme, it certainly is reasonable to think it could.Actually I think there's a pretty big flaw in both the "single stream of time" model and the "many worlds" model, interpreted as physical theories pertaining to time travel. For example, the way the "many worlds" model is normally expressed, it is as though we have one moment give rise to two moments that are "next"-- one has a time traveller appear, and one does not. That is somehow to be interpreted as two separate universes. The problem is, one of those universes would obey the laws of physics as we now apply them, and the other would not! The "single stream" is just as bad-- then the sole universe could not obey laws the way we now use them.

In other words, physics itself has its own rules that come prior to any identification of the laws of nature. Those rules look like "use the information contained in the present to predict the happenings of the future". These rules do not look like "use the information in everything that will ever happen to predict what will happen", because there's no prediction going on there. It isn't physics any more. What I'm saying is, the issue is not if time travel can be consistent with physics, it is if physics can be consistent with macroscopic time travel of physicists (it can be consistent with virtual particle time travel conceptualized by physicists). I'm saying it cannot, if one uses either the "single stream" model or the "many worlds" model as they've been expressed in this thread.

Now, of course we cannot assert the universe must follow laws of physics, but our entire approach to time travel is to imagine that it does, and discern those laws in regard to time travel. But there is an approach that science uses to do that, and it involves a continuous thread of events, which in turn defines the concept of what a prediction is. Hence, if a time traveller is going to follow the laws of physics, when they go back in time they cannot actually be returning to that time, in the sense that the previous instant to their appearance must always be the instant they left, it cannot be the instant in the past prior to their arrival. What's more, they could never affect anything in that previous time, or else there is not physics. It's one or the other, it could be possible to time travel, or it could be possible to do physics, but no "physical theory of time travel" could ever be possible, because it does not satisfy the characteristics of physics. If time travellers are allowed to appear at any point, then there are not laws of physics as we know it, there are miracles instead.

The reason it couldn't be physics is that events affected by a time traveller would either have to be interpreted as a miracle, a "poof" (an event different from the natural laws that have the time traveller not appear), or else it would require knowledge of the future to understand the appearance of the traveller, in which case there are not predictions being made. So physics is not allowed to think that way, it would lose its defining character of being a predictive science.

north
2009-Jul-15, 11:00 PM
Say a few thousand years in the future their are space ships that have some technology that allows them to go faster than light. As they accelerate faster and faster and time gets slower and longer, is it possible that the spaceship reaches a certain point where time stops and then it starts going into the past, i.e. -1 seconds, -2 seconds etc? Is this totally wrong or is it a possibility?
And if this is possible if they do travel into the past they would be in a (theoretically speaking) parallel universe because if they go back into the past they would not be born yet and they could not exist and the technology was never their and the machine was not yet built.

Maybe all I have said above is nonsense and the universe just does not allow you to travel at all. :surprised

time travel is not so much about the speed of light as it is about including the surrounding eviroment as well

to go backwards in time , means that all the physical dynamics in the Universe at that point in time , must also be included

why?

because otherwise you get a confliction between the Universe you observe then to the Universe you observe now

inotherwords , the rest of the Universe is not not carried along with you

dwnielsen
2009-Jul-16, 04:52 AM
The reason it couldn't be physics is that events affected by a time traveller would either have to be interpreted as a miracle, a "poof" (an event different from the natural laws that have the time traveller not appear), or else it would require knowledge of the future to understand the appearance of the traveller, in which case there are not predictions being made. So physics is not allowed to think that way, it would lose its defining character of being a predictive science.

Let's assume that universality of physical principles must be axiomatically inherent in the study of physics. Do events take place discretely? Do Plank values have any sort of meaning in discretizing the universe? I understand that there is a minimum energy associated with an observed "bit" of information in a system (depending on how the "bit" is defined). If we see observables as indeterminate and quantal at base, how can we claim to know that they are not spreading the field through alternative spacetimes just as likely as one's understanding of their own universe (assuming there even exists an ideal single observer)?

Ken G
2009-Jul-16, 07:06 AM
Let's assume that universality of physical principles must be axiomatically inherent in the study of physics. Do events take place discretely? Do Plank values have any sort of meaning in discretizing the universe? I understand that there is a minimum energy associated with an observed "bit" of information in a system (depending on how the "bit" is defined). If we see observables as indeterminate and quantal at base, how can we claim to know that they are not spreading the field through alternative spacetimes just as likely as one's understanding of their own universe (assuming there even exists an ideal single observer)?
It seems to me you are asking a question like "how do we know reality does not work like such-and-such", but physics doesn't answer questions like that. It answers very specific types of questions, using assertions of the form, "for model X to be useful in situation Y, then the outcome of experiment A must come out B (where B can be statistical)." If one can make assertions like that, and test them, and a certain reliability emerges, we have a good physical model. If one cannot make an assertion of that form, one is not doing physics, right from the get go. It is not possible to make assertions like that in the face of time travel, unless you can do what we already know is not possible-- determine statistical likelihoods of various future events, arbitrarily far into the future (to incorporate all possible time-travel events). If any part of the future is not included, then time travel from that future would appear as a completely miraculous event for any physical treatment, and hence falls outside said treatment. Ergo, there can be no such thing as "the physics of time travel over unpredictably long time scales". The time scales that are predictable are very short indeed, especially when we are talking about time travel of humans.

ETA: In a nutshell, I'm saying that the concepts of cause and effect are not just principles that appear in the laws of physics, they are inherent principles in the whole way that physics gets done. So if we invoke time travel, it messes with cause and effect, and we can't define physics itself in the same way any more. So if it's true, as we heard above, that "relativity, causality, time travel: choose two", what I'm saying is, "time travel, physics as we currently define it: choose one". So it doesn't matter if the "laws of physics" allow time travel, because the physics we used to derive those laws doesn't. One could assert the laws are still valid even if the way we arrived at them is not, but that would seem pretty coincidental and contrived.

Paul Beardsley
2009-Jul-16, 06:09 PM
Imagine you decide to send, say, a 2001-like monolith into deep space to a past of a "many worlds" multiverse. A universe where the monolith appeared would be different from one where it did not appear. The simple fact that there is mass where there would not otherwise be, an object that could intercept photons that would not otherwise be intercepted, would be a change. There is your split.

The change does not need to be something extremely obvious to humans. It only needs to be a change.
(My bold.)

I think this assumption is the sticking point - and this is the thing I keep mentioning but which people are avoiding answering. Why do you think there is a different version of the past in which the monolith didn't appear? Where is this idea coming from?

I content that if you were to send a 2001-like monoloth into deep space in the present, its presence has no more and no less reason to cause a split.

Van Rijn
2009-Jul-16, 06:32 PM
(My bold.)

I think this assumption is the sticking point - and this is the thing I keep mentioning but which people are avoiding answering.


I thought I did answer.




Why do you think there is a different version of the past in which the monolith didn't appear? Where is this idea coming from?


In the "many worlds" view, there could be timelines where the monolith did not appear, and ones where it did. The appearance of the monolith would be a change, would cause ongoing changes as previously stated, and could create another timeline.


I content that if you were to send a 2001-like monoloth into deep space in the present, its presence has no more and no less reason to cause a split.

Sure it could. In "many worlds," there could be universes with no monolith in deep space, and universes with monoliths in deep space.

Edit: The reasons for why there might or might not be a monolith in deep space are many. You might have not had the money for a rocket launch in one, or its guidance might have gone wrong at liftoff in another, etc. Ultimately, though, you end up with cases of monolith versus no monolith, and that's what I'm focusing on.

Ken G
2009-Jul-16, 06:41 PM
Sure it could. In "many worlds," there could be universes with no monolith in deep space, and universes with monoliths in deep space.That is so, and this is exactly my point about why "many worlds" pictures are never physics. Physics is defined by how it works, and how it works can never establish or demonstrate or predict the existence of many worlds. It's pure philosophy. Hence, if anyone is seeking a "physical theory of time travel", then any kind of "many worlds" picture is not permissible under that heading. My question is this: in a many worlds view, what is the difference between time travel, and just creating an exact replica of some earlier time, like a movie set? In what sense will that "movie set" replica that is being created have any connection to the time that is supposedly "traveled to"? Physics has no way to say that you are connecting to that earlier time and "splitting off" a new universe, instead of just altering your universe to match that early time, because the only way it could say that is if you can provide me an experiment that comes out A if all we've done is create an exact replica of the earlier time, and not A if we've actually travelled to that time and split off a new world. The distinction may be unimportant to the traveler, of course, but my point is that time travel like that would fall outside the need for physicists in past times to include it in their physics. It simply would not exist for them, or if it did, then physics could not treat it for them, it would be pure miracle. Some on here have expressed disdain for the inclusion of miracles as a possibility in physical law.

dwnielsen
2009-Jul-16, 06:41 PM
I think the idea is that there are surprising things historically that had been unobservable and unknown, but which, on discovery and further analysis, fit well with previous ideas - such as the Heisenberg effect and signal analysis.

Parsimony is fine in physical research, but for most people, I suspect, an interest in time travel is about the payoff of being able to become some kind of time voyager. There are many well-regarded physicists who at heart believe in the multiverse/many-dim interpretation. These intererested parties want to know, is there anything that precludes time travel fundamentally, not what is most likely. It is like they are handed a prize of a free lunch, and then are told that they could instead take what is behind the door - could be a car, could be nothing.

Ken G
2009-Jul-16, 06:51 PM
There are many well-regarded physicists who at heart believe in the multiverse/many-dim interpretation. These intererested parties want to know, is there anything that precludes time travel fundamentally, not what is most likely.That is certainly true, but look at the double standard there-- we are now talking about their personal beliefs about things they like to imagine because it pleases them to do so, nothing that has any scientific verification, and some that is not even scientifically verifiable (as per the absence of experimental suggestions that could test many worlds even in principle). Does that sound like something else that many people choose to do?

Jeff Root
2009-Jul-17, 02:03 AM
Why do you think there is a different version of the past in which
the monolith didn't appear? Where is this idea coming from?
Van Rijn isn't saying that he thinks there are multiple versions-- he's
saying that multiple versions is one possibility. The single version
possibility that you champion isn't the only possibility. He wants you
to accept that there are other possibilities. The multiple version
possibility among them.

If it were possible for someone to go back to an earlier time, then
their presence in that earlier time would have effects. The effects
might be very prominent or they might be tiny and insignificant.
But there would be effects.

If the time traveller appeared "the first time through", the effects
would already be in place when the traveller set out. He would see
himself walking down the road on the videotape before he left.

If the time traveller did NOT appear "the first time through" but only
appears as a change to the timeline, so that before his trip he was
not on the tape, but after he returns he IS on the tape, then I see
at least two possibilities:

1) There is only one reality, and when the traveller appeared in the
past, it changed from the one he had known.

2) When the time traveller appeared in the past, he created a new
reality, in parallel with the reality he came from.

There might be no way to distinguish between those two.

Another possibility is that the time traveller does not see himself on
the tape before the trip, and when he returns, he finds that he still
is not on the tape. In that case it appears that the traveller created
a new reality, but returned to his previous, unchanged reality.

-- Jeff, in Minneapolis

eburacum45
2009-Jul-17, 10:32 AM
Another possibility is that the time traveller does not see himself on
the tape before the trip, and when he returns, he finds that he still
is not on the tape. In that case it appears that the traveller created
a new reality, but returned to his previous, unchanged reality.

-- Jeff, in Minneapolis
This is a real problem with time travel in a many-worlds universe, Jeff. If you go back into the past, you go back to a time when there are many possible futures, including your own; you jump back in your time machine and go forwards, and you could end up in any of an uncounted number of possible versions of your original present.

Perhaps if you have something concrete, like a wormhole back to your own time, then you might be able to consistently travel back to your own version of today; but in a causality-compromised universe, I wouldn't like to bet on it.

Ken G
2009-Jul-17, 01:47 PM
Perhaps if you have something concrete, like a wormhole back to your own time, then you might be able to consistently travel back to your own version of today; but in a causality-compromised universe, I wouldn't like to bet on it.Actually, I think an assumption of many-worlds time travel is that you can not end up in the same future a second time, as that future already has a different past. That's one reason why I say it is not distinguishable from simply making a new future based on a recreation of the past. One is creating a different future that starts out in a way similar to the past, but it is not changing anything. And, there isn't going to be any physics that can address that new future, for the time traveler's presence disrupts the continuity with the past, required for physical predictions. Ergo, time travel isn't even physics at all.

Paul Beardsley
2009-Jul-17, 04:46 PM
Van Rijn isn't saying that he thinks there are multiple versions-- he's
saying that multiple versions is one possibility. The single version
possibility that you champion isn't the only possibility. He wants you
to accept that there are other possibilities. The multiple version
possibility among them.
It's possible that some confusion is arising between multiple versions of history and two versions of events - the idea that there's a 1900 (say) in which no slab ever appeared in deep space, and another 1900 in which one sent by time travellers did arrive. It is the two-versions that I consider baseless.


If it were possible for someone to go back to an earlier time, then
their presence in that earlier time would have effects. The effects
might be very prominent or they might be tiny and insignificant.
But there would be effects.
Agreed - it's not too important if the events were minor or major.


If the time traveller appeared "the first time through", the effects
would already be in place when the traveller set out. He would see
himself walking down the road on the videotape before he left.
Agreed - if he happened to look.


If the time traveller did NOT appear "the first time through" but only
appears as a change to the timeline, so that before his trip he was
not on the tape, but after he returns he IS on the tape, then I see
at least two possibilities:

1) There is only one reality, and when the traveller appeared in the
past, it changed from the one he had known.

2) When the time traveller appeared in the past, he created a new
reality, in parallel with the reality he came from.

There might be no way to distinguish between those two.
If he appeared on the tape after his trip but not before, I believe the only logical conclusion is your second one. (I note you use the word "returns" but I assume you mean he arrives in a time in the new reality equivalent to the present he'd left.)

My opinion about realities branching out is that one of two things is true:

1) It never ever happens, and never will happen, not even if people travel into the past.

2) It happens all the time, regardless of time travel.

I prefer the first of these.

My reason for rejecting the idea that he has changed something in a single time line:

Change is a function of time, so any idea of changing history (as opposed to branching off a new history) invokes some sort of metatime.

To illustrate, imagine you go back in time and meet someone who knew your father when he was a boy. [This is not a reiteration of the grandfather paradox!] You tell this person that you are from the future, and destined to be the son of his acquaintance.

During the course of the conversation, you both realise you hate each other. You decide to kill him - in fact you want to ensure he was never born. So you set off back in time to prevent his parents from meeting. Meanwhile, he's decided the same thing about you - so he's going to make sure that youth never meets your mother.

If it was possible to change history, both of these things could happen. But which would happen first? It could be years between this encounter and your parents getting together. And you could take years getting around to preventing his birth.


Another possibility is that the time traveller does not see himself on
the tape before the trip, and when he returns, he finds that he still
is not on the tape. In that case it appears that the traveller created
a new reality, but returned to his previous, unchanged reality.

-- Jeff, in Minneapolis
It's a scenario which was used in Bob Shaw's The Two-Timers. I don't think it's one of the more likely possibilities, though.

astromark
2009-Jul-18, 07:51 AM
You must be joking... Do you really think that any change to a event passed would just create some other track of reality. How can that be right ? From where would the mass of this alternate universe spring from ? How often could this happen ? That's right...NEVER.
This has become a discussion about a subject of fiction. I see no science here and wonder what you are talking about. Its Fiction. In this Forum should you argue the point of a fiction ?

eburacum45
2009-Jul-19, 06:09 PM
An overview of multiple realities is given in this .pdf by Max Tegmark.
http://space.mit.edu/home/tegmark/multiverse.pdf
In section III Tegmark suggests that all possible universes exist in some kind of metaspace, and merge and diverge as seen by an observer from within.

To be honest I can't quite imagine how different timelines can merge as easily as I can imagine them diverging, but at least such a overarching view explains how new universes can be created- they are here all along.

astromark
2009-Jul-19, 08:19 PM
An overview of multiple realities is given in this .pdf by Max Tegmark.
http://space.mit.edu/home/tegmark/multiverse.pdf
In section III Tegmark suggests that all possible universes exist in some kind of metaspace, and merge and diverge as seen by an observer from within.

To be honest I can't quite imagine how different timelines can merge as easily as I can imagine them diverging, but at least such a overarching view explains how new universes can be created- they are here all along.


As we seem to be ( mostly ) agreeable on this subject. I wonder why then I feel the need to protest so LOUDLY.---" THIS IS RUBBISH.! "
For to accept as true this ridiculous notion you are going to be creating new time lines and merging them infinitum...
Would you please show me just one that is not this one.? NO, I didn't think you could either.
This observable universe is all we know. Its not unreasonable to extrapolate from what we see ( know ) that this is not the whole thing. I harp on a bit about this but, Universe does include all of it. Even that which we as yet have no knowledge of and all. Please tell me I am not alone... I trust that the mainstream view is as I have stated. Accepting that in higher mathematics many ( multi ) universes might be factual. But in this reality we have no proof of existence other than this one. "The Universe"

Ken G
2009-Jul-19, 08:35 PM
An overview of multiple realities is given in this .pdf by Max Tegmark.
http://space.mit.edu/home/tegmark/multiverse.pdf
In section III Tegmark suggests that all possible universes exist in some kind of metaspace, and merge and diverge as seen by an observer from within.
But note that there is no suggestion of how to test this proposition. This is exactly what I mean about how one cannot be making a "physical theory of time travel" if one resorts to this kind of language. It's not right or wrong, it just isn't a physical theory. It's "not even wrong".


To be honest I can't quite imagine how different timelines can merge as easily as I can imagine them diverging, but at least such a overarching view explains how new universes can be created- they are here all along.So are invisible unicorns, and similarly untestable.

Jeff Root
2009-Jul-20, 12:54 AM
Mark,

Two things:

Creation of a "new reality" is not or would not be creation of matter or
energy or anything like that. Creation of the latter sort does not result
in a highly-organized Universe from the start. As Paul says, if a time
traveller suddenly showing up in a past time creates a new reality, then
so does *every* event. Every time a subatomic particle anywhere in
the Universe goes one direction rather than another, a new reality is
created. These realities, if they exist, do not exist in different places,
and I doubt it means anything to say that they exist in the same place.

The word "universe" generally means "everything". But sometimes things
are discovered outside the universe that you were originally talking about,
and you want to talk about those things, too. So the definition of what
the universe is might be expanded to include those newly-discovered
things, or a new term might be introduced that includes the existing
universe as a subset. There is no law requiring you to do it one way
rather than the other.

-- Jeff, in Minneapolis

Jeff Root
2009-Jul-20, 01:07 AM
Many years ago my friend said something to me. A few years later I told
him that what he said was so profound and so succinctly expressed that
he must have heard or read it somewhere-- he couldn't have come up
with it himself. :D However, he had no recollection of it at all. This is
my approximation of what he said:

There is only one path to where we are now.
From here, we can go in any direction.

Has anyone seen something like this before? I would suspect something
like an editorial in 'Analog' as the source or a major distribution point.

-- Jeff, in Minneapolis

astromark
2009-Jul-20, 03:25 AM
Jeff, I like your last post. It even makes sense.:)not everything I say does...
BUT. in your response to what I had said in post 78. ...you lost me. If its not to much trouble could you look again at this ? I fear you have me wrong. I am aware of the multi choices our reality has. But there is just one history. One time line. One universe. Where or how is that wrong ?

Ken G
2009-Jul-20, 04:00 AM
There is only one path to where we are now.
From here, we can go in any direction.

Interestingly, that's very much the spirit of the first line in my sig. The future is always unknown, all we can ever do is compare two pasts: one past where an experiment was done, and a prior past where a prediction was made about that experiment. Since they both lie along that unique path that led us here, we can compare them. The extrapolation to the future is not physics, it is philosophy-- though a rather straightforward one!

eburacum45
2009-Jul-20, 07:23 AM
If I'm reading Tegmark correctly, there is more than one past, just as there is more than one future. A kind of macroscopic sum over histories. Note that I am not necessarily supporting this position, but as Tegmark's essay was published in Sci Am, it can't be dismissed as fringe science.

astromark
2009-Jul-20, 10:13 AM
If I'm reading Tegmark correctly, there is more than one past, just as there is more than one future. A kind of macroscopic sum over histories. Note that I am not necessarily supporting this position, but as Tegmark's essay was published in Sci Am, it can't be dismissed as fringe science.

Why do you think this is not questionable ?

Every thing, fact, science, or theories... is questionable. Its the science of it.

Never stop asking, doubting, questions are the way forward.

Any acceptance of multi able time lines and or realities is foreign to my logical mind.

That according to me. In regards to matters of 'my' mind. I am the authority.

So when I say... "One time line. One past. Future as yet unknown."

I meant it. How could I possibly be wrong ?

Oh :) or so it seems...

Ken G
2009-Jul-20, 01:02 PM
If I'm reading Tegmark correctly, there is more than one past, just as there is more than one future. A kind of macroscopic sum over histories. Note that I am not necessarily supporting this position, but as Tegmark's essay was published in Sci Am, it can't be dismissed as fringe science.Yes, Tegmark is also a well established physicist and astronomer. This stuff is not his "day job". My problem with it is that it isn't even close to anything that falls under the definitions of science, so it really doesn't belong in Sci Am (not being a philosophy magazine). To support my point, I need go no farther than the first two sentences of the abstract:
"I survey physics theories involving parallel universes, which form a natural four-level hierarchy of multiverses allowing progressively greater diversity. Level I: A generic prediction of inflation is an infinite ergodic universe, which contains Hubble volumes realizing all initial conditions"

Note that this does not qualify as a "physics theory" for the very simple reason that it makes no verifiable predictions, and even if it did, it is not clear that the same predictions could not follow from assumptions that exhibit far greater parsimony. Further, he improperly uses the word "prediction" in regard to the inflationary model. He uses a kind of common-language version of the word, meaning essentially "if we adopt one philosophical and unverifiable assumption, it leads to various philosophical and unverifiable conclusions." But the word "prediction" in science has a very different meaning, relating to the outcomes of experiments prior to doing the experiment. As usual, it is very important that science stick to the precise meanings of its language, or all kinds of confusion results.

eburacum45
2009-Jul-20, 04:06 PM
Tegmark's bizarre ideas might have some testable relevance, if we ever observe any form of reverse time travel. If that is ever the case, I don't think that physicists should simply throw up their hands and say that all physics is wrong, so they might as well go home and grow cucumbers.

In a universe where time travel is permitted, then either Novikov consistency rules, or something more complex is happening; Tegmark's ideas offer one possibility.

Ken G
2009-Jul-20, 04:52 PM
Tegmark's bizarre ideas might have some testable relevance, if we ever observe any form of reverse time travel.Not good enough, it must be relevant to either observations we have now, or can imagine doing. I could just as easily say that a theory of invisible unicorns will become relevant if we ever find a way to observe invisible unicorns.


In a universe where time travel is permitted, then either Novikov consistency rules, or something more complex is happening; Tegmark's ideas offer one possibility.But I can offer a hundred more in five minutes, like, the Earth is on the back of a turtle, but you can't see it unless you travel back in time. We can't test that without traveling in time either.

eburacum45
2009-Jul-20, 05:08 PM
True; but that also reduces Novikov's ideas about self-consistency to the level of the absurd, as they are just as unfalsifiable.

Ken G
2009-Jul-20, 06:39 PM
True; but that also reduces Novikov's ideas about self-consistency to the level of the absurd, as they are just as unfalsifiable.But there are aspects in the structure of physics that are always unfalsifiable, the most obvious here being that we should require that physical theories be falsifiable. Another is that predictions can be applied to the future (see my sig). Another is that we can extrapolate laws outside of the strict limits of what has already been seen to hold. Another is that the universe (nearly) respects symmetry principles (the "nearly" part is one of the most profound aspects of physics, it teeters at the brink of failing its own assumptions. That's also related to the extrapolation problem-- how similar must two situations be before we have a right to expect a theory derived from one to apply to the other?).

Thus, one could require another unfalsifiable element in the structure of physics to be that the universe we are trying to describe with physics must not be inconsistent, even if our physics can be. The point is, we can argue about what the structure of physics needs to be, and indeed we must, all before we even come to the falsifiability of the specific theories that are subordinate to that structure. One could classify Nobokov's requirements at that level, sort of like, "if it isn't like this, then physics as we have set it up isn't going to apply". That's what I'm saying is the problem of some of the so-called "theories" of time travel that have been discussed.

Ken G
2009-Jul-20, 06:40 PM
Oops, I forgot to include the sig!