Time travel, and what happens to you - and you

Factual Questions
1

I am reading How to Build a Time Machine by Paul Davies (and very good it is too), and it has got me thinking about a few aspects of time travel.

The second you embark on your time travel journey what happens in the present you have just left? Do you disappear from that present, or are you still there? If you are, that would mean that there are two of you existing in history. The one that you left behind would therefore be living in his present and creating a past and would be moving into his future. When you, the traveller, return, would you return to the moment that you left, and if so, what would have happen to the other you that is now further ahead in time and continuing to move forward in his present? Or could you join up with the you that has moved forward in time and somehow meld together.

I suppose an alternative would be that as you started your travels, the other you could disappear. If that happens, your life would have a gap in it, which would seem to break quite a lot of laws of physics.

I have read a lot of objections to the practicalities of time travel, but I haven’t come across the questions I have asked above. Does anyone have any ideas about possible answers?

2

An awful lot about Time Travel isn’t well thought out, or logically consistent.

In H.G. wells’ novel (and in the George Pal movie made from it), the Chronic Argonaut sees things changing around him, as if in a speeded-up film. That seems to imply that he’s always there, in the same physical location, but that time simply passes more slowly for him. If you walked into a room with a time traveller, you ought, then, to see what looks like a statue of a time traveller and his machine, unmoving and virtually invulnerable.
But, of course, you never see this in any time travel movies, and it’s never described that way in the stories or novels. The time traveller pushes the button, and he’s gone. Where? Although they sometimes talk about “what must be happening in these ages”, and speak about stopping at a certain time, the time traveller and his machine simply seem to wink out of one time and appear instantaneously in another time. The “Back to the Future” movies actually seem to show this, and it’s one of the few that does.
What happens if two of you end up in the same time? If the first explanation is correct, you couldn’t avoid having more than one of you in a certain time. In fact, make enough time trips, and you can fill a room or a building with versions of you travelling through time, forward or backward. I’ve never bought or understood the idea that two of you can’t exist simultaneously without creating awful effects, as in Timecop, or that Dr. Who episode. Of course, if two of you end up somewhere you get a variation of that Grandfather Paradox – if Future You kills Less Future You, what happens? Robert Forward disliked the whole idea of such time paradoxes, claiming that one thing happened – there is only One Reality for all time travellers. In his novel, Time Master, different temporal versions of the same character interact in interesting ways. More interesting, aruably, than in Heinlein – his two stories, “By His Bootstraps” and “All you Zombies…” are wonderfully convoluted cases of people interacting with future selves, and he took pains to keep them consistent.
Of course, everything is much simpler if easy time travel of the sort imagined by such stories doesn’t and can’t exist. My belief in a perverse universe makes me favor that sort of reality.

3

There is a concept in time-space called the worldline. The worldline of an object is a four-dimensional plot through space and time. Part of the concept of a worldline is to remove the concept of time as moving, and show the world as though you could see everything from all time “at the same time” (not a very elegant description but it’s difficult to explain without pictures).

For a simpler example, imagine a “bug” (a point, really) who lives on a one-dimensional line, moving back and forth. To plot a simple two-dimensional worldline, imagine the y axis is the bug’s position on the line, and the x axis is time. The worldline of the bug would zig-zag up and down but the plot would continuously move to the right. That is, it would be a continuous function; a given value of x would produce exactly one value of y.

Now imagine the bug could travel in time. The worldline is no longer a continuous function but a relation. It can hop backwards or forwards. That implies that the bug can be two places at the same time (but it’s the bug at two different ages) or there can be a time where is does not exist at all.

The bug could jump back in time, hang out for a year, then return to the point where he departed, although now he’d have aged a year, biologically. So there is no gap where it doesn’t exist, but there is an interval where he existed in two places at the same time. His world line has a discontinuity.

I do not know what current physics theories have to say about the possibility of time travel, problems with causality paradoxes (let’s say you go back in time and kill your younger self, you could not have survived to age into the future and go back in time and kill yourself, so you didn’t, but then you *did * survive, and so…), or the possibility of the same matter existing in two places at the same time. But in a science fiction story the author can set his own ground rules (I’m not familiar with the story you’re reading). These questions you are asking have been asked by science fiction readers (and scientists) for decades.

Another problem that is not brought up as often is if you can travel in time, say to back six months, that causes certain paradoxes about *where * you pop out. The Earth has gone halfway around the sun, rotated about 185 times, the sun has been zooming through the galaxy, the galaxy is doing I-don’t-know-what, and all of space-time is expanding. So it’s pretty sure you wouldn’t end up standing in the same spot you were when you pushed the button.

4

Great post, I neglected to review it when I previewed my own. I seem to remember reading a Heinlein story where the main character was also both of his own parents, IIRC.

5

That’s All you Zombies…

6

There are all sorts of answers to questions about time travel:

  1. You can go back and affect past events (common; my favorite example is David Gerrold’s The Man Who Folded Himself).
  2. You affect past events, but not anything you do deliberately (the Twilight Zone episode where they tried to save Lincoln).
  3. You cannot affect past events (common).
  4. You can affect past events, but only with a great deal of effort (Fritz Leiber’s “Try to Change the Past.”
  5. You can only affect your own past; it doesn’t affect anything else (Alfred Bester’s “The Men Who Murdered Muhammed”).
  6. The past is a group mental construct (George Alec Effinger’s, “The Bird of Time Bears Bitter Fruit.”)
  7. You can meet yourself in the past and change things (my favorite is Lester Del Rey’s “. . . And It Comes Out Here.”)
  8. You can meet your past self, but if you come into contact, there will be an explosion (Doctor Who episode “Mawdryn Undead”).

But ultimately, there really isn’t any scientific answer to the question.

7

Here’s a picture (expand to 100% when viewing). Note that points 1 and 4 could also be made to be the same point. Or point 4 could even occur earlier than point 1, in which case you arrive back in the future just in time to see yourself travel back in time.

8

I can’t say that I’ve given a lot of thought as to the mechanics, repercusions, ethics, whatever, of time travel, but I have given it some thought.

One would think that if there ever will be time travel that we would have encountered a visitor by now. All conspiracy theories aside where the government does know that we have had visitors or has one stashed in Area 51, someone should have noticed someone who fit in even less that everyone else.

Theory one. Time travel is possible, but the traveler is out of phase with the current reality and can only observe but not interact or effect events in the period being visited. Technological ghosts.

Theory two. Time jumping spawns alternate timelines to protect itself from causality. Insufficient caffeine on board to fully explore this one.

Theory three. Time travel will never be possible. At least on a scale to allow individuals to move back and forth. Maybe something on the lines of allowing a spaceship to jump. I would expect the power required would be ginormous to say the least.

Theory four. We just live in boring times that no one will ever want to visit.

9

CookingWithGas: thanks for your reply – I’m still grappling with the worldline scenario, but from what little that I do understand about it, it still doesn’t seem to solve the two bugs existing at once problem.

Another thought has just occurred to me: when travelling in time, is the amount of time you spend travelling instantaneous – which seems to me would transgress the the laws of relativity. Or do you travel at a presumably very high-speed and therefore take a finite amount of time, in which case the laws of relativity will suggest a time difference caused by the speed travelling between the ‘now’ you left behind, and the past you’re going to.

(We may have difficulty achieving time travel purely due to the sheer complexity of it!)

10

I should re-iterate that I mean here “Most of what’s written as Time Travel in stories, literature, movies, and comic books isn’t well-thought out.” I’m not running down possible time travel derived from General Rel. But, as I say, it seems to me that, logically, H.G. wells’ Time Traveller ought to look like a statue in the room. He ought not to simply disappear, as he’s shown to do in all the movies and comic books, and in the imitations of his work. People have worked out a number of dodges to get around this image and time travel logical problems, and there’s a vast literature on this – see Larry Niven’s “Theory and Practice of Time Travel” and L. Sprague de Camp’s “Language for Time Travellers”, and all the books entitled “Time Travel”, and the entry in The Science Fiction Encyclopedia on it.

11

Try watching a film called Primer. It will mess with your head and I really didn’t understand the whole plot even after watching it twice, but the “time machine” there works at the speed of one hour per hour. If you want to go back to this time yesterday, it’ll take you 24 hours. And you have to have switched the machine on at the time you want to go back to. Got that?

Say it’s Monday at 10am, and I want to return to this time from tomorrow (so I can make a killing on the stock market, say). I switch on the machine now, then, at 10pm I climb into the machine and wait for 12 hours, before climbing out at 10am. Time inside the machine is going backwards, but at the same speed, one hour per hour.

Alternatively, I could switch it on at 10am, and only wait until 11am before climbing in, then one hour after that climb out at 10am.

BUT, when you go back in time, the “you” that was already there is still there, so now there are two of you! To avoid running into themselves, the characters in the film used a time-delay switch to start up the machine, and then locked themselves in a motel room for the duration that their future selves would be present.

Still got that?

Now, what would happen if you took another time machine back in time through the first time machine…? :slight_smile:

12

CalMeacham:

The problem, as I see it, would be that there are two of you both experiencing a different life. When you, the traveller, returned to your present, you would have a particular set of memories. When the younger you reaches the age that you were at your present, he will have a different set of memories. In addition, it would also mean that, from the moment you arrive in your past, there will be two of you living in the same timeline, each with different experiences. How can that be reconciled?

13

I don’t see the contradiction. Older yopu has all the same memories as younger you. When you run into each other, older you knows what’s going to happen (unless he has a bad memory), so younger you will think he’s being smug. But there’s no inconsistency. Older you will have experienced the scene twice – once when younger, a second time when older. It’d be like acting in a play twice, as two different characters.

14

But what if the older you decided not to follow the script?

15

Without the notion of some absolute reference frame though, it’s difficult to resolve this one.

16

Indeed – that’s the whole “Grandfather Paradox” thing in a different guise. But that’s not itself a problem with two of you being in the same place at the same time. As long as the older You doesn’t deviate from the script, there’s nothing physically impossible, and no logical inconsistency.

As for that problem of “What if Older You Breaks Script?”, possible answers are:
1.)You can’t. There’s only one reality. Whatever Future You decides to do was always in the script (Forward’s version of things)

2.) If Future You breaks script, he creates a New Reality. This is “Many Worlds Interpretation.” Now you’re limited in the futures you can travel to

3.) Time Cops rush in to restore order. Time Cops are a conservative bunch. They hate people screwing with reality, and they’ll come in to stop or minimize any changes. They LIKE the future the way it is (Lotsa examples – TimeCop, End of Eternity, Up the Line)

4.) We only have the illusion of Free Will, and can’t really break script – really a variation of #1. See Doctor Manhattan in Watchmen.
5.) Something (not necessarily human TimeCops) keeps time from changing. as Larry Niven says in his essay, “If you go back to the first century and try to shoot Christ, your machine gun will positively jam.” No good reason, but it does maintain causality.
and there’s always:
6.) You Can’t go back to greet youir future self. The Time Machine is limited to trips only exactly 10,000 years ago, or something. Avoid the problem altogether.

Try to think up your own dodge! It’s fun! Don’t mistake this for reality.

17

I just wanted to point out that it’s not only the subsequent portrayals of Wells’ time traveler that show him disappearing – in the original book, he disappears from the view of his guests as well.

The entire story is online here

18

My Theory:

Time Travel is indeed possible, but the above theories have ignored a very important and very fatal problem: Relativistic Movement of Bodies in Space.

Sure, go back 5,000 years. The only problem is that the Earth of 5,000 years ago is not anywhere near where you were at the moment you left. Congratulations, you’re in the Past, but you’re also a trillion miles or so out in empty space. Don’t worry, in 5,000 years the Earth will once again pass through your present location. :smiley:

Go back to yesterday. Sorry, the Earth has moved nearly a million miles in it’s orbit around the Sun, plus the Sun has moved N distance in orbit of the Milky Way, etcetera.

Go back one minute. The Earth has moved roughly 700 miles in orbit around the Sun and your previous position on Earth has moved as much as 16 miles, plus all other relative movements. Oh, and your angular momentum no longer matches either. Oops.

Nowhere in Time Travel Theories have I seen any attempt to correct for Relative Location and Momentum.

19

This has been mentioned in this thread, starting with #3. I’ve seen it pop up occasionally in stories, so the issue is recognized. (Heck, I thought of it when I was a kid, and wrote a story where the Time Traveller turned on his machine, went into the future, and promptly asphyxiated. I’ll bet that story’s been written a couple of thousand times by now. SF editors probably have a special box for it. )
The problem is, of course, that it’s BORING. There are much more interesting things to do with the idea of Time Travel, so every story ignores the issue, or hand-waves that their machine compensates for it. I’m sure that’s one reason Dr. Who’s crib is Time And Relative Dimension In Space, instead of just Time Travel (although, of course, being able to go anywhere is a great power to have, even if your interest isn’t just keeping your feet on the ground)

20

If you’re dealing with fictional time travel, of course it’s fun and not real. But, don’t forget, relativity has proved that future time travel actually happens. And there are no contradictions to time travel to the future, because you do actually move very fast to somewhere and then return: it’s just that your time and their time have travelled at different speeds.

So I was trying to extrapolate that reality to the unknown of time travel to the past, but still remaining within the realms of science.