constants and variables in time travel

[jer_j]

Disclaimer: The following discusses time travel using an idea the show Lost as an example. It contains vague references to the show that could be considered spoilers. I post it recognizing that only those interested in questions of how time travel works (I find we are a people group limited in number) are likely to be interested in this post.

If anybody here is familiar with Lost, it (so far) is one of the few TV shows intelligent enough to adopt the “self-consistency principle” when dealing with time travel. (See italicized explanation below if you’re unfamiliar with this principle).

On the show, Dr. Faraday repeatedly informs the show’s time travelers that logic prohibits them from changing the past – that is, until he later decides that there is a “variable” in the equation. That variable is human behavior, which due to its unpredictable nature and people’s ability to make choices, can alter timelines. I find this concept to be nonsense, and I’m trusting that Dr. Faraday’s “variable” theory was only a fit of madness. However, it does lead to an interesting point, and when one considers what a variable actually is and how it works, it illustrates how the past/future cannot be changed.

In the equations below, which are extremely simplified, each term represents a broad segment in a sequence of time.


Basic Example:

3 + 4 = 7

• 3 represents the fact that, two days ago, a tree was standing in my yard.
• 4 represents that I chopped it down yesterday.
• 7 would be the consequence of four added to three, which is that this morning there is not tree in my yard.

I would have to have observed all three events (in other words, they are in my relative past) to be fill in the equations with the constants three, four, and seven.


Humans as "variables"

If an event is in the future, it is a variable, because I don’t know what will happen. And human behavior is an exceptionally unpredictable variable. For example, let’s look back to when I was a teacher:

2 + x = y

• 2 represents that, tonight, I did an exceptional job planning a lesson.
• x represents how the lesson will go tomorrow.
• y represents how my students will perform on a test the day after tomorrow.

Because my lesson was well-planned, it seems the lesson should go well and by extension that students should do well on their test. However, their human behavior makes the equation unpredictable. I don’t know if they will come in tired or distracted, or if they will simply misbehave throughout the lesson. What’s more, if I wished to, I could decide to purposefully mess up the lesson just to assert my will over fate. Therefore, the outcome of “y” is very hard to predict, largely because of the randomness of human behavior in determining “x.” This is where Faraday’s description of human behavior as a variable contains a seed of truth.


Misuse in time travel

However, the idea does not hold up when one has observed the future. Take this example based on Kennedy’s assassination:

5 + x = 9

• 5 represents the sum total of events the day before the assassination.
• x represents a time traveler’s actions the morning before the assassination.
• 9 represents the sum total of events that includes Kennedy’s assassination that afternoon.

Now we don’t know what the time traveler will do, because we haven’t observed him directly. And he is a human, with the ability to make independent decisions – thus he is a “variable.” However, let’s look at this equation for a moment. We have 5, an observed constant. Because we have observed through the historical record that Kennedy was shot, we know that yesterday (5) plus the time traveler’s actions must add up to 9. This means that, while the time traveler’s actions are a variable as long as we haven’t observed them, they are still restricted in that they must fall within the number 4, where “4” represents any set of actions in which he does not foil Lee Harvey Oswald. If we said tried saying he could do anything he wants because he is a variable, including "7" where 7 represents stopping Oswald, we would have to claim that 5 + 7 = 9.


Thus, even the analogy to human action as a variable demonstrates that time travel cannot result in timeline inconsistencies. Any thoughts?



The self consistency principle posits that a person cannot go back into the past and change the present (or future). For example, if you decided to go back in time and kill your grandfather when he was a child, you wouldn’t erase your existence, nor would you set off a universe-ending paradox. What would happen is you would simply fail to kill your grandfather. You would have to, because (barring technobabble explanations about “divergent timelines” and alternate universes, and assuming you’re involved in literal time travel) the death of your grandfather would be inconsistent with observed reality.

[Cuddles]

I'd say there are two problems here. The first is that, as always with this kind of thing, it's all completely made up. A story on a TV show is just as valid as your speculation because none of it actually has any connection to reality. Until we can demonstrate time travel and see what actually happens, arguing about which version is more "real" is really pretty pointless.

Secondly, and completely ignoring what I just said, I disagree with your statement of the self consistency principle. Your version says that if you go back in time, you can't change anything. However, the usual version says that if you go back in time, you can't change the consequences already observed in the "present".

It's an important distinction. Your version says that you can't go back in time and kill your own grandfather because you'll fail. That's fine as far as it goes, but if you follow that logic, it turns out that you can't actually do anything at all. Anything you do must affect something. Even simply standing still and breathing will alter things. Maybe only completely inconsequential things, but your principle says that that's not possible at all. In trying to avoid paradox, your version of self consistency actually creates a much worse one - you can't change anything, but the mere act of travelling in time must inevitably do so.

One of the usual versions avoids this by being less strict. If you go back in time to kill your own grandfather (Why do people always want to do this anyway? I quite liked my grandfather.), you may well be able to do so. However, doing so won't change any of the observed consequences. It will turn out that he wasn't your grandfather after all, or something like that. Even more extreme cases can be argued - you know your grandfather is alive today, but you go back one day and kill him. Obviously you can't argue that away by becoming your own grandfather, but it will turn out that the one you thought was still alive was actually a clever imposter out to steal your grandmother's fortune. Or whatever.

The important point is that it allows you to actually exist in the past without screwing things up in the present. To go with your maths, although I'm not sure that really helped things, you have 5 + x = 9. Without your time travel, that may have been 5 + 4 = 9. Afterwards it may be 5 + 3 + 1 = 9. The end point is the same so the present hasn't been changed, but things still happened due to time travel.

It's still not perfect, and requires a hard separation between the present and past that isn't at all obvious when you can act perfectly normally while in the past, but I think it's a lot better than the restrictions you have.

Another of the usual versions, which is probably the best in avoiding paradoxes and inconsistency but the worst for storylines and the concept of free will, is simply that the past is fixed. If you do something in the past, that's possible because you'd already done it. Anyone travelling back in time is already a part of history, so they can't do anything different. The obvious problem is that that implies everything you do while in the past is predetermined (pre? whatever). Either you have to assume that the act of time travel removes free will, or that it doesn't exist in the first place. Depending on your opinions on the subject of free will that may or may not be a problem.

Finally, I'll just note that my personal favourite version of time travel is Doctor Who. "Hey, this will make a good story. Logic and consistency be damned!"

[jer_j]

I think we actually agree for the most part. I'm not saying you can't interact with ANYTHING in the past. I'm just saying you can't change anything you've observed - which is pretty much the same thing you argued.

If the assassination of JFK could be summed up as 5 + x = 9, the result of the equation is a constant because we have a historical record showing JFK was killed. Of course there are alternatives, such as the traveler saving Kennedy and finding out that recorded history was a lie. But I would consider that possibility ridiculously small, even compared to outlandish obstacles to the time traveler's attempts at intervening (such as his gun jamming repeatedly or a brick falling on his head and knocking him out).

But let's say the time traveler met a woman in the past and fell in love with her. And let's say she was in danger of being murdered. Given that the time traveler has no memory of her being killed (or of accurate records indicating the same), it would be possible for him to save her. In other words, the equation would be something like 8 + x = y (where 8 is the condition in which he meets her, x is his interaction with her, and y is the future question of whether or not she survives). The future is the variable y in this case, where it is the constant 9 in the case of JFK, because of the traveler's foreknowledge. So he could save her, even do things as consequential as marry her and have children with her in the past, as long as those actions didn't contradict the historical record as he had observed it. In other words, the possibilities for x are wide open, since there's no knowledge of y to contradict.

Does this mean the traveler has more or less free will depending on what he's observed before? No! In any undertaking we attempt, there are always obstacles that could get in our way. Previous knowledge only reveals with certainty whether or not we might succeed at something ahead of time.

Here's the foundational issue at hand: When we recall a specific event from the past through memory or record, we are observing the result of all factors that influenced that event. If a person were to travel back in time (again, in the literal sense of time travel, not of alternate universes, etc), his actions may have been factors in what he's already observed - but it can't produce anything different. It's as fundamental to reality as saying it can't be raining and not raining at the same time and in the same place.

Now as to the relevance of this topic, I admit it has no practical application - outside of expanding one's mind and thinking things through logically. And I know the interest in this topic is limited, which is why I gave warning in my disclaimer. The use of Lost does not mean I build an entire argument on a work of fiction. I just used it to illustrate an existing point.

Thank you for your feedback!

[Speed of Light]

How do you get round the 'infinite series' problem?
E.g. If you travel just one minute into the past, and you have been in the same room for a minute, then there will be two of you in the room (!) -
but your 'past self' will also have done the same thing and travelled into the past, with the result that there will then be three of you -
and so on ad infinitum - So the room will end up a little crowded (!)

I wondered if just 'rolling back time' as in 'Superman', is the least paradoxical - but even this also seems to have paradoxes (?)

[jer_j]

Originally Posted by Speed of Light

How do you get round the 'infinite series' problem?
E.g. If you travel just one minute into the past, and you have been in the same room for a minute, then there will be two of you in the room (!) -
but your 'past self' will also have done the same thing and travelled into the past, with the result that there will then be three of you -
and so on ad infinitum - So the room will end up a little crowded (!)

I wondered if just 'rolling back time' as in 'Superman', is the least paradoxical - but even this also seems to have paradoxes (?)

Unless I'm visualizing this problem incorrectly, there would only be two of me occupying the same room for one minute. I'm sitting in the room, I see my future self for a minute, then I travel back in time and see my past self for a minute, then he travels back in time and I'm alone.

I'm not sure what you mean about the Superman one... All I know is that spinning the earth backward would destroy it instead of spinning time backward.

[Speed of Light]

Originally Posted by Speed of Light

How do you get round the 'infinite series' problem?
E.g. If you travel just one minute into the past, and you have been in the same room for a minute, then there will be two of you in the room (!) -
but your 'past self' will also have done the same thing and travelled into the past, with the result that there will then be three of you -
and so on ad infinitum - So the room will end up a little crowded (!)

Originally Posted by jer_j

Unless I'm visualizing this problem incorrectly, there would only be two of me occupying the same room for one minute. I'm sitting in the room, I see my future self for a minute, then I travel back in time and see my past self for a minute, then he travels back in time and I'm alone.

If we call the original time 'T', then when you travel one minute into the past, you end up at time (T - 1 minute) -
You will then find that your past self is also there - So there will be two of you.
After being in the past for one minute, the time will then be 'T' again. Your past self will then do what you did, and travel to (T - 1 minute).
When your past self arrives at (T - 1 minute), after travelling back in time, he will find (a) You, and (b) your past self, before he travelled back in time, so then there will be three of you!
It is very hard to describe it verbally if you don't understand - The best way might be to do a computer simulation, and see what happens!
Basically, I think that each time one of your selves (past self) travels back one minute, it creates a duplicate, and this process will repeat forever, creating an infinite number of people in the room at (T - 1 minute)

The last part of what you said 'then he travels back in time and I'm alone' I think is incorrect -
When he travels back in time, he travels to time (T- 1 minute) -
and we already agreed that at that time there were two of you (you and your past self), so now there would be three of you - (a) You (b) Your past self, and now (c) You past self traveling into the past -
It's like an exponential progression

I suppose, one way of simply explaining it is to say that each time you travel into the past, you create a duplicate of yourself (in the room) at time (T - 1 minute).
But it doesn't stop there, as your duplicate will also repeat the process and itself create a duplicate, and it goes on forever!

To complicate matters more, they'll also be occupying the same space

[Speed of Light]

Originally Posted by Speed of Light

I wondered if just 'rolling back time' as in 'Superman', is the least paradoxical - but even this also seems to have paradoxes (?)

Originally Posted by jer_j

I'm not sure what you mean about the Superman one... All I know is that spinning the earth backward would destroy it instead of spinning time backward.

By 'Rolling back time', I mean solving a disaster by simply removing the last 24 hours and starting again. The best analogy I can think of, is if you've had an accident and lost your memory of the last 24 hours, then it's as if time has rolled back for you - The last 24 hours has, in effect, been removed from your memory.
In the Supeman films, he often solves disasters by simply turning back time, in effect, giving things a second chance.
For example, in the first film, Lois Lane was killed, so Supeman simply reversed time to the point before she was killed, in effect, saving her life.
This seems a less paradoxical way of travelling into the past - simply removing the future and starting again, but it too seems to have its paradoxes.
Another example is in one of the Twilight zone films, where someone was able to win at poker by continually reversing time by up to five minutes, every time he had a losing hand, and then playing it differently.

[Dave Rogers]

Originally Posted by Speed of Light

If we call the original time 'T', then when you travel one minute into the past, you end up at time (T - 1 minute) -
You will then find that your past self is also there - So there will be two of you.
After being in the past for one minute, the time will then be 'T' again. Your past self will then do what you did, and travel to (T - 1 minute).
When your past self arrives at (T - 1 minute), after travelling back in time, he will find (a) You, and (b) your past self, before he travelled back in time, so then there will be three of you!

No, he won't.

The basis of time travel is that we treat time as just another spatial dimension, in which we can move at will, although the default is to travel forwards in time at 1 second per second. So let's take the analogy of a car that normally moves at a constant speed of 30mph along one lane of a road. At some point it stops, reverses diagonally 100 yards into the other carriageway, then starts moving at 30mph again. Imagine, now, that it has a paint brush trailing behind it, painting a line along the road, so we can follow its track. We get to the point where it reversed to, and note that there are three lines from that point forwards. However, when we get to the place where the car first stopped and reversed, from there onwards there's only one line again. But at no point are there an infinite number of lines. The car follows a single path; go forward, stop, go back, stop, go forward again - and that's all it does.

The difference between this and time travel is that travelling back in time is usually imagined as a sudden jump, so let's imagine that the brush on the car only makes a mark when travelling forwards. Then we get a single line up to the first point, two lines up to the second, then one line again.

To rephrase it as time travel, suppose you jump back from 12:00 to 11:59. Up till 11:59, you're on your own. From 11:59 to 12:00 there appear to be two of you, but it's simply you experiencing the same events twice over. From 12:00 onwards you're on your own again.

Your mistake is that you're viewing yourself and your past self as different people. They aren't, no more than you are a different person than you were a minute ago.

Trust me. I've read a lot of Fifties sci-fi. I know this stuff.

Dave

[Brian-M]

The effects of any action you are going to perform in the past would already have already taken place before you go back into the past, so no matter what you do in the past, things will have already turned out like they were before you went back in time.

Basically, if you are going to go back in time, then your future is in the past. Looking at how things turned out in the present, and what happened after you arrived in the past is no different in looking into your future.

So the only way you would be able to change things is if going back in time creates a different timeline in which different events can occur. Otherwise, you wouldn't be able to change anything.

(And if you can't change anything, that means it would be impossible to travel to the past unless a future version of you already appeared in the past.)

Does this make things any clearer?

[Speed of Light]

Originally Posted by Brian-M

Does this make things any clearer?

It might do!

Perhaps if you made clearer what it is you're trying to make clearer, then it would

[GlennB]

It would be a terrible mistake to build a time machine powered by electricity, then travel back to a time where there's nowhere to plug it in. Please bear this in mind in your design phase.

[Speed of Light]

Originally Posted by Dave Rogers

To rephrase it as time travel, suppose you jump back from 12:00 to 11:59. Up till 11:59, you're on your own. From 11:59 to 12:00 there appear to be two of you, but it's simply you experiencing the same events twice over. From 12:00 onwards you're on your own again.

Your mistake is that you're viewing yourself and your past self as different people. They aren't, no more than you are a different person than you were a minute ago.

Why does there just 'appear to be two of you'?
Surely, when you reach 11:59, there will definitely be two of you!

You cannot divorce the 'you' from the equation - 'You' are part of 11:59, and when you reach 11:59, the 'you' fom 11:59 will be different from the 'you' from 12:00
Your 12:00 'you' will have experienced different memories to your 11:59 version -
From 11:59 to 12:00, there will be two distinct you's, having different memories and experiences.
the act of going back in time, in effect, duplicates you -
The problem is, that your duplicate will also duplicate himself in the same way, thus causing a never ending paradox

Originally Posted by Dave Rogers

Trust me. I've read a lot of Fifties sci-fi. I know this stuff.

I'm sure you know your sci-fi stuff, but I'm not clear what the connection is with trusting you??

[Dave Rogers]

Originally Posted by Speed of Light

Why does there just 'appear to be two of you'?
Surely, when you reach 11:59, there will definitely be two of you!

You cannot divorce the 'you' from the equation - 'You' are part of 11:59, and when you reach 11:59, the 'you' fom 11:59 will be different from the 'you' from 12:00
Your 12:00 'you' will have experienced different memories to your 11:59 version -
From 11:59 to 12:00, there will be two distinct you's, having different memories and experiences.
the act of going back in time, in effect, duplicates you -
The problem is, that your duplicate will also duplicate himself in the same way, thus causing a never ending paradox

No, it doesn't. Let's try again. Suppose we give you subscripts.

Up to 11:59, you are you₀. At 11:59, you₂ pops into existence, and you become you₁. For one minute, you₁ and you₂ co-exist. At 12:00, what happens then?

What happens is that you₁ goes back one minute in time, and becomes you₂. You₂ continues to exist, becoming you₀.

It's the bit in bold type that I think you're missing. The first version of you, on going back in time, becomes the second version of you. That second version doesn't go back in time to find the second version already there, because he is the version that will have been there. It's the same as in the car analogy; there's a single line of progression through time, that just involves traversing the same bit of it twice. That doesn't imply any infinite loops; you just experience time from birth to 12:00, then the bit from 11:59 to 12:00 again from a second point of view, then from 12:00 until death. Put another way, although you've 'duplicated' yourself, the duplicate doesn't necessarily duplicate himself.

I really think you should read some time travel literature. Heinlein's short story, "By His Bootstraps", is an excellent starting point, if you can find it. Harry Harrison's "The Stainless Steel Rat Saves the World" is another.

Dave

[Speed of Light]

Originally Posted by Dave Rogers

No, he won't.

The basis of time travel is that we treat time as just another spatial dimension, in which we can move at will, although the default is to travel forwards in time at 1 second per second. So let's take the analogy of a car that normally moves at a constant speed of 30mph along one lane of a road. At some point it stops, reverses diagonally 100 yards into the other carriageway, then starts moving at 30mph again. Imagine, now, that it has a paint brush trailing behind it, painting a line along the road, so we can follow its track. We get to the point where it reversed to, and note that there are three lines from that point forwards. However, when we get to the place where the car first stopped and reversed, from there onwards there's only one line again. But at no point are there an infinite number of lines. The car follows a single path; go forward, stop, go back, stop, go forward again - and that's all it does.

The difference between this and time travel is that travelling back in time is usually imagined as a sudden jump, so let's imagine that the brush on the car only makes a mark when travelling forwards. Then we get a single line up to the first point, two lines up to the second, then one line again.

Your analogy is not a true analogy, as you don't duplicate the actual car -
Even the line is not duplicated, as it uses different paint, and may be slightly different - So no real duplication occurs.

It seems to me that trying to understand purely by analogy, might not be the best idea, especially taking into account the relevant differences which make the analogy incorrect.

I would have thought that the best way is to try to analyse and understand the actual situation, which is 'unique', as it is difficult to find a 'proper' analogy to 'time travel', in the 'real' world

[Roboramma]

No duplication is required.

For instance, in one way of seeing things, if an electron encounters a photon, it can knock it back in time. After travelling backward through time for a while, it may encounter another photon, which affects its trajectory in such a way as it is now moving forward through time again.

So, from the perspective of someone moving forward through time, what do we see? A photon becomes an electron-positron pair (pair-production), and the positron goes on to encounter an electron and annihilate in a burst of radiation (the radiation is the photon we saw above which knocked the electron back in time).

Note that we don't see an infinite number of electrons, even though this situation is pretty much the same (minus teleportation) as the time travel scenario that you described above.

[Dave Rogers]

Originally Posted by Speed of Light

Your analogy is not a true analogy, as you don't duplicate the actual car -
Even the line is not duplicated, as it uses different paint, and may be slightly different - So no real duplication occurs.

That is exactly the point - no real duplication occurs in the time travel instance either. The two versions of you between 11:59 and 12:00 are not duplicates; one of them has a memory of the events of that minute, and the other does not. Which translates to the different paint on the two lines. At 12:00, it's only the one who doesn't have the second set of memories of the time from 11:59 to 12:00 that disappears, and at 11:59 it's only the one that does have the first set that appears.

Dave

[Dave Rogers]

Originally Posted by Roboramma

Note that we don't see an infinite number of electrons, even though this situation is pretty much the same (minus teleportation) as the time travel scenario that you described above.

We probably don't need the teleportation either, because your location at 12:00 isn't your location at 11:59. That's another problem with the instantaneous jump concept of time travel, BTW - it requires an absolute frame of reference, otherwise the emergence point can't be defined.

Dave

[Speed of Light]

Originally Posted by Dave Rogers

No, it doesn't. Let's try again. Suppose we give you subscripts.

Up to 11:59, you are you₀. At 11:59, you₂ pops into existence, and you become you₁. For one minute, you₁ and you₂ co-exist. At 12:00, what happens then?

What happens is that you₁ goes back one minute in time, and becomes you₂. You₂ continues to exist, becoming you₀.

It's the bit in bold type that I think you're missing. The first version of you, on going back in time, becomes the second version of you. That second version doesn't go back in time to find the second version already there, because he is the version that will have been there. It's the same as in the car analogy; there's a single line of progression through time, that just involves traversing the same bit of it twice. That doesn't imply any infinite loops; you just experience time from birth to 12:00, then the bit from 11:59 to 12:00 again from a second point of view, then from 12:00 until death. Put another way, although you've 'duplicated' yourself, the duplicate doesn't necessarily duplicate himself.

I really think you should read some time travel literature. Heinlein's short story, "By His Bootstraps", is an excellent starting point, if you can find it. Harry Harrison's "The Stainless Steel Rat Saves the World" is another.

Dave

I must apologize -
I do believe you are right
I've never thought about it that deeply - I just assumed there'd be a paradox.
Do you think that Superman style time travel, just rolling back time, giving yourself a second chance to get things right, has paradoxes?

[Roboramma]

Originally Posted by Speed of Light

I must apologize -
I do believe you are right
I've never thought about it that deeply - I just assumed there'd be a paradox.
Do you think that Superman style time travel, just rolling back time, giving yourself a second chance to get things right, has paradoxes?

Well, one that I see is, "Where does the information come from?"

For instance, say superman rolls back time in order to save Lois Lane. Given that the future in which Lois dies no longer exists, how does he know to save her?
Are there two supermen now? The one from the past and the one who rolled back time? If there's only one, its the one from the past (otherwise, what happens to him?) and he has no idea that Lois is in trouble (otherwise future superman wouldn't need to roll back time).
If they both exist, the question "where did superman₂ come from?" has no meaningful answer, because that future doesn't exist.

But I don't think any of that is concept destroying. In fact, it just gives you something interesting to think about. The real problem comes when more than one person is capable of doing it.

[dlorde]

Originally Posted by jer_j

What’s more, if I wished to, I could decide to purposefully mess up the lesson just to assert my will over fate.

That's exactly what fate said you'd do...

Quote:

... Therefore, the outcome of “y” is very hard to predict, largely because of the randomness unpredictability of human behavior in determining “x.”

Fixed it for you. Unpredictability may look like randomness, but is qualitatively different.

Quote:

... And he is a human, with the ability to make independent decisions ...

Independent of what?

Quote:

(barring technobabble explanations about “divergent timelines” and alternate universes, and assuming you’re involved in literal time travel)

What about non-technobabble explanations about “divergent timelines” and alternate universes, such as the 'Many Worlds' interpretation of QM ?

What do you mean by 'literal' time travel?

[Dave Rogers]

Originally Posted by Speed of Light

Do you think that Superman style time travel, just rolling back time, giving yourself a second chance to get things right, has paradoxes?

What Roboramma said, basically. But in fact that's the same as the Grandfather paradox. If I go back in time and kill my grandfather before he meets my grandmother, I destroy the possibility of my having existed in the future I came from; but Superman is doing the same with the information that Lois Lane needs saving, and I don't see why I'm more important to the universe than any other piece of information. So, there tend to be three approaches to this:

(1) Every time anyone goes back in time, they create a branching in the timeline, and return not to the reality they start from but to the reality they create. Of course, this means that there could be an infinite number of realities, and there's no reason - if time travel were possible - why travel between probability universes shouldn't also be possible. Bob Shaw's "The Two-Timers", Larry Niven's "Flight of the Horse" series and, with an added plot twist, Isaac Asimov's "The End of Eternity" are good examples of this approach.

(2) Whatever people's actions, history is resilient and springs back into shape despite the attempts of individuals to change it. This seems to me to be less poplar because it essentially negates free will, and determinism just doesn't feel so good in fiction.

(3) Don't worry about continuity; the Dr. Who approach. Popular because, quite simply, it's so much easier to write.

Dave

[VulcanWay]

You would still need to teleport to survive.

If you jumped back even a minute in time, you would die very quickly. You're only changing one dimension - time. Your axes of reference in space are otherwise unchanged. So in that minute, the Earth has moved leaving you dead in space when you arrive at those same coordinates, save time, in your time jump.

[Dave Rogers]

Originally Posted by VulcanWay

You would still need to teleport to survive.

If you jumped back even a minute in time, you would die very quickly. You're only changing one dimension - time. Your axes of reference in space are otherwise unchanged. So in that minute, the Earth has moved leaving you dead in space when you arrive at those same coordinates, save time, in your time jump.

I thought the Vulcan Science Council had determined that time travel was impossible.

But seriously, that's why instantaneous time travel doesn't square with special relativity. If you're still at the same spacial co-ordinates, what are you at the same co-ordinates with reference to? In effect, the difference between your position before and after the time jump would give you the information to determine the absolute rate of movement of an object relative to the universal frame of reference.

Dave

[VulcanWay]

Originally Posted by Dave Rogers

I thought the Vulcan Science Council had determined that time travel was impossible.

Yeah, then Kirk did that slingshot around the Earth's sun thing and screwed everything up!

[wuschel]

I just invented the ultimate Free Energy Device, comprised of:

- An ordinary battery
- A time machine

1.) Discharge battery (use electricity as you see fit)
2.) Use time machine on entire Universe, except battery
3.) Rinse, repeat!

[AvalonXQ]

Originally Posted by wuschel

I just invented the ultimate Free Energy Device, comprised of:

- An ordinary battery
- A time machine

1.) Discharge battery (use electricity as you see fit)
2.) Use time machine on entire Universe, except battery
3.) Rinse, repeat!

For a great use of batteries and time travel, see the text game "All Things Devours".

[wuschel]

Originally Posted by AvalonXQ

For a great use of batteries and time travel, see the text game "All Things Devours".

How creepy of you, using a time machine in order to establish prior art, to my otherwise perfectly patentable invention!

[Speed of Light]

Originally Posted by Dave Rogers

But seriously, that's why instantaneous time travel doesn't square with special relativity. If you're still at the same spacial co-ordinates, what are you at the same co-ordinates with reference to? In effect, the difference between your position before and after the time jump would give you the information to determine the absolute rate of movement of an object relative to the universal frame of reference.

Dave

Wouldn't you just retain your original state of motion - according to Newton's first law - You're changing only the time dimension, nothing else

[AvalonXQ]

Originally Posted by wuschel

How creepy of you, using a time machine in order to establish prior art, to my otherwise perfectly patentable invention!

Creepy indeed.

[jer_j]

Originally Posted by dlorde

That's exactly what fate said you'd do...

I agree with you 100%. Saying I was going to "assert my will over fate" had ironic undertones, as anybody deciding to do so was only acting out a script they didn't know was already written. If 2 + x = y and I wanted to mess up the lesson (making x = 5) and I succeeded, it would only work because y = 7 all along. I only FELT I was changing fate because I was altering the expected outcome.

Originally Posted by dlorde

Fixed it for you. Unpredictability may look like randomness, but is qualitatively different.

True. I only meant random in the apparent sense.

Originally Posted by dlorde

Independent of what?

Again, I meant this in the sense that human action APPEARS independent. Our minds can take in information, process it, and make decisions in ways that defy the easy predictability of other phenomenon. Thus we are unpredictable, and any place where a human was acting and we didn't yet know the outcome would be a "variable." We wouldn't have seen everything the time traveler did, so within "x" he might be able to kick someone in the shins and throw a rock through a window - but he could not prevent JFK's assassination, because that outcome had been observed.

Originally Posted by dlorde

What about non-technobabble explanations about “divergent timelines” and alternate universes, such as the 'Many Worlds' interpretation of QM ?

What do you mean by 'literal' time travel?

By "literal" time travel, I mean traveling to the past (a time that has acted on our present) or to the future (a time our past and present will act upon). The sum total of all events in the past has worked to make our present what it is - so if a person engages to "literal" travel to that past and acts within it, he becomes part of that sum total, and his actions logically cannot contradict the resulting outcome. "Divergent timelines" and "alternate universes" are usually used in time travel discussions (often in movies such as Back to the Future) to explain things like why you can go back and, by causing your parents not to get married, create an "alternate timeline" in which you were never born. That, of course, is only an excuse for pushing forward a comedy/action plot (fine for what it is, and BTTF is very fun) and is completely illogical (why, for example, would Marty's parents not getting married cause him to be born, live for 18 years, and then vanish?). So I prefer what I call "literal" time travel: that which falls into the rules of cause and effect such as we observe them on a day to day basis. I also use the word literal to avoid scenarios in which someone says they had a vision of a "possible future."

[GlennB]

Another terrible thing would be to travel back/forward in time and materialise somewhere with a brick wall passing through your body.

[AvalonXQ]

Originally Posted by GlennB

Another terrible thing would be to travel back/forward in time and materialise somewhere with a brick wall passing through your body.

Once we get here, we have the problem of actually catching up to the Earth itself. It's moving pretty fast.
I guess it depends on our time travel mechanism.

[Roboramma]

Originally Posted by VulcanWay

You would still need to teleport to survive.

If you jumped back even a minute in time, you would die very quickly. You're only changing one dimension - time. Your axes of reference in space are otherwise unchanged. So in that minute, the Earth has moved leaving you dead in space when you arrive at those same coordinates, save time, in your time jump.

I can only real see a few ways that time travel could work (none of which are likely :P ). One is a wormhole of some sort (or series of them) after travelling through which, you end up in the past. That's a sort of teleportation time travel.
The thing about this is that it teleports you to a specific place which has a specific reference frame. Maybe you need to match your velocity to its velocity (I don't know), but it isn't some universal background frame, because none exists.

Another is a machine that somehow can alter your motion through time. This doesn't look much like most time travel stories you see, because to get to any particular point in the past, it still has to travel through all the intervening points.
But, I don't see why the laws of motion would be suspended for this machine. It would continue to have inertia, for instance, and so, in the same way that if you jump you don't see the spinning earth go flying beneath your feet, you wouldn't find yourself thrown off the earth and left behind in space.

Of course, the best way to alter your motion through time is to change your velocity. A quick flight around our stellar neighbourhood with a warp drive or something, arriving back at the earth before you left, would seem to do it.

Mind you, none of the above is likely to be physically possible. :P

[Brian-M]

Originally Posted by Roboramma

Well, one that I see is, "Where does the information come from?"

For instance, say superman rolls back time in order to save Lois Lane. Given that the future in which Lois dies no longer exists, how does he know to save her?
Are there two supermen now? The one from the past and the one who rolled back time? If there's only one, its the one from the past (otherwise, what happens to him?) and he has no idea that Lois is in trouble (otherwise future superman wouldn't need to roll back time).
If they both exist, the question "where did superman₂ come from?" has no meaningful answer, because that future doesn't exist.

In this case after saving Lois, Supermanfuture would inform Supermanpresent that he needs to save Louispast. Supermanpresent would then go back in time, saving Lois and becoming Supermanfuture in the process.

That means that Lois would never have died... Superman would just have been told to go back and save her by his future self, and the movie wouldn't be anywhere near as poignant as it would be if Lois had actually died.

[Roboramma]

Originally Posted by Brian-M

In this case after saving Lois, Supermanfuture would inform Supermanpresent that he needs to save Louispast. Supermanpresent would then go back in time, saving Lois and becoming Supermanfuture in the process.

Why would superman_future bother, since in his time Lois is alive and well and has already been saved?

Quote:

That means that Lois would never have died... Superman would just have been told to go back and save her by his future self, and the movie wouldn't be anywhere near as poignant as it would be if Lois had actually died.

Agreed.

[Dave Rogers]

Originally Posted by Speed of Light

Wouldn't you just retain your original state of motion - according to Newton's first law - You're changing only the time dimension, nothing else

No, because your position in space varies with time.

Let me explain in more detail.

Suppose you're in a spacecraft, moving at 1km per second, and you jump back in time one minute. You'll rematerialise 60km ahead of yourself.

Now, suppose you're in a spacecraft that's completely stationary, and you jump back in time one minute. You'll rematerialise in the same place you already were, with consequences that can only be guessed at, but which are likely to be somewhat unfortunate.

So it's a good idea to be moving when you make the time jump. But - and here's the killer problem - moving relative to what? According to special relativity, there isn't any difference between moving at 60km/s and not moving at all; both are inertial frames of reference, so there's no way to tell which one you're in.

So, when you jump back, it makes a big difference to the outcome which of two inertial frames of reference you're in, which means that you can distinguish between them. That means that instantaneous-jump time travel is incompatible with special relativity.

That doesn't affect the validity of a particle travelling backwards in time in a Feynman diagram, because it doesn't jump discontinuously back in tim, but simply travels the wrong way. So there's no fundamental reason you can't travel backwards in time; it's just not possible to jump instantaneously.

Dave

[dlorde]

Originally Posted by jer_j

By "literal" time travel, I mean traveling to the past (a time that has acted on our present) or to the future (a time our past and present will act upon). The sum total of all events in the past has worked to make our present what it is - so if a person engages to "literal" travel to that past and acts within it, he becomes part of that sum total, and his actions logically cannot contradict the resulting outcome.

Yes, if we assume a single 'time stream'. But this model seems to have a 'censorship' problem which is tied up with determinism, free will, and what you mean by "his actions logically cannot contradict the resulting outcome".

It's easy enough to say that once in the past, the time-traveller will find that he/she can't affect the future in a contradictory way because the future (at least up to the point of the time-traveller's departure) has already happened, and so the time-traveller must 'already' be part of it - which also means that the time-traveller's actions in the past (which is also his future) are pre-determined. This is disconcerting enough because it means that a time-traveller whose intention on travelling back is to introduce himself to his past self (for example), will not be able to achieve that goal unless he already remembers meeting his future self - but it also means that the time-traveller's actions are pre-determined even before he travels back in time - otherwise he could decide to travel back to the point he entered the lab that morning and meet himself with certainty (and it doesn't make sense to me that his/her actions might be deterministic or not simply depending on when he/she decides to time-travel). The alternative is some kind of on/off determinism and/or a kind of 'cosmic censorship' principle that somehow stops the machine from sending the time-traveller into any situation where the past-future could be 'compromised'.

So not only does it appear to require determinism (which is fine by me, but may upset some people), but it also suggests that we already know that future time-travellers will not be able decide to compromise the past-future by travelling back into a situation they were not previously/already in... Unless, of course, there is a 'cosmic censorship' principle ready to pull the plug on any such attempts ('God', anyone? ).

[Speed of Light]

Originally Posted by Speed of Light

Wouldn't you just retain your original state of motion - according to Newton's first law - You're changing only the time dimension, nothing else

Originally Posted by Dave Rogers

No, because your position in space varies with time.

Let me explain in more detail.

Suppose you're in a spacecraft, moving at 1km per second, and you jump back in time one minute. You'll rematerialise 60km ahead of yourself.

Now, suppose you're in a spacecraft that's completely stationary, and you jump back in time one minute. You'll rematerialise in the same place you already were, with consequences that can only be guessed at, but which are likely to be somewhat unfortunate.

So it's a good idea to be moving when you make the time jump. But - and here's the killer problem - moving relative to what? According to special relativity, there isn't any difference between moving at 60km/s and not moving at all; both are inertial frames of reference, so there's no way to tell which one you're in.

So, when you jump back, it makes a big difference to the outcome which of two inertial frames of reference you're in, which means that you can distinguish between them. That means that instantaneous-jump time travel is incompatible with special relativity.

I'm not sure I'd go along with that.

Thee is no intrinsic difference between a body travelling at 1 Km/sec relative to us, or a body which is stationary, relative to us.
Either of them would be thought of as stationary, in its own frame of reference.
Remember that, absolute position in space, as well as motion, is also a relative concept - so it is meaningless to talk about materialising 60 Km's away from your original position, in 'absolute' terms, as it depends on the observer's state of motion.
As all motion is relative, and can be defined only in relation to other bodies, if you are travelling at 1 Km/sec, as far as you're concerned, you are stationary.
If you jump back a minute, you will rematerialise in the exact position that you were one minute ago (as this is your perceived stationary, but is moving only relative to us).
It might be wise to sit in a different part of the cabin while doing this, to avoid materialising in the exact position

I see no ambiguity or paradox, as the time traveller will always see themselves as stationary, whatever their state of motion is, to us

[Brian-M]

Originally Posted by Roboramma

Why would superman_future bother, since in his time Lois is alive and well and has already been saved?

Because Superman_future and Superman_past don't want to compete with each-other for Louis Lane's attention. This way, they both get the girl.

Originally Posted by Speed of Light

If you jump back a minute, you will rematerialise in the exact position that you were one minute ago (as this is your perceived stationary, but is moving only relative to us).

And what happens if the spaceship changed direction during this minute?

[sol invictus]

Originally Posted by jer_j

Thus, even the analogy to human action as a variable demonstrates that time travel cannot result in timeline inconsistencies. Any thoughts?

Time travel of the type you're describing violates essentially every law of physics, from conservation of energy to conservation of momentum to unitarity. It is inconsistent with cause and effect regardless of what you're "allowed" to do. This "variable" idea is silly; there is nothing more or less variable about human behavior than there is about the behavior of an electron.

So there's not much point in discussing this in any kind of logical context based on the known laws of physics. Of course you can ask what would happen if the laws of physics were different, but they'd have to be so different that the discussion isn't likely to be very fruitful.