Sean Carroll writes in Discover magazine that many of the paradoxes presented in time travel science fiction simply wouldn't occur if time travel were made possible. Paradoxes are logical contradictions, and logical contradictions can't exist.As we will see, the time travel paradox—the possibility of changing our past—seems intractable only because it conflicts with our notion of ourselves as beings with free will. Consistent stories are possible, even in space-times with closed timelike curves.
To illustrate this point, imagine that you stumble upon a time machine in the form of a gate. When you pass through it in one direction, it takes you exactly one day into the past; if you pass through in the other direction, it takes you exactly one day into the future. You walk up to the gate, where you see an older version of yourself waiting for you. The two of you exchange pleasantries. Then you leave your other self behind as you walk through the gate into yesterday. But instead of obstinately wandering off, you wait around a day to meet up with the younger version of yourself (you have now aged into the older version you saw the day before) with whom you exchange pleasantries before going on your way. Everyone’s version of every event would be completely consistent.
We can have much more dramatic stories that are nevertheless consistent. Imagine that we have been appointed Guardian of the Gate, and our job is to keep vigilant watch over who passes through. One day, as we are standing off to the side, we see a person walk out of the rear side of the gate, emerging from one day in the future. That’s no surprise; it just means that you will see that person enter the front side of the gate tomorrow. But as you keep watch, you notice that he simply loiters around for one day, and when precisely 24 hours have passed, the traveler walks calmly through the front of the gate. Nobody ever approached from elsewhere. That 24-hour period constitutes the entire life span of this time traveler. He experiences the same thing over and over again, although he doesn’t realize it himself, since he does not accumulate new memories along the way. Every trip through the gate is precisely the same to him. That may strike you as weird or unlikely, but there is nothing paradoxical or logically inconsistent about it.
So could you mess up your own past, so that you ceased to exist? Carroll writes:
We know what the answer is: That cannot happen. If you met up with an older version of yourself, we know with absolute certainty that once you age into that older self, you will be there to meet your younger self. That is because, from your personal point of view, that meet-up happened, and there is no way to make it un-happen, any more than we can change the past without any time travel complications. There may be more than one consistent set of things that could happen at the various events in space-time, but one and only one set of things actually does occur. Consistent stories happen; inconsistent ones do not. The vexing part is understanding what forces us to play along.
All of which suggests that, since time is a closed curve, linking the future to the past, we are predestined to do whatever we do, or experience whatever we will experience:
Our concept of free will is intimately related to the idea that the past may be set in stone, but the future is up for grabs. Even if we believe that the laws of physics in principle determine the evolution of some particular state of the universe with perfect fidelity, we don’t know what that state is, and in the real world the increase of entropy is consistent with any number of possible futures. A closed timelike curve seems to imply predestination: We know what is going to happen to us in the future because we witnessed it in our past.
Closed timelike curves, in other words, make the future resemble the past. It is set in stone, not up for grabs at all. The reason we think the past is fixed once and for all is that there is a boundary condition at the beginning of time. The entropy of the universe started very small (at the time of the Big Bang) and has been growing ever since. Ordinarily we do not imagine that there is any analogous boundary condition in the future—entropy continues to grow, but we cannot use that information to draw any conclusions. If we use a closed timelike curve to observe something about our future actions, those actions become predestined. That’s extra information about the history of the universe, over and above what we normally glean from the laws of physics, and it makes us uncomfortable.
Link via Gizmodo
8 comments:
This makes me want to read Einstein's Dreams again. Having fun with time concepts never gets "old."
It's nice to hear someone talking sensibly about time travel for a change :-) Sadly, there are a couple of nits I'd like to pick in what you wrote.
Your time traveller in the 24-hour loop is, in fact, a paradox. You still want to hang on to causality in your cosmology, I suppose, so you need to have a chain of events that led up to this person being a fully grown adult and standing in front of the gate so he could step through. Saying he sprang into being when he stepped through the gate 24 hours ago, just isn't good enough. Once he's in the loop, fine. He will keep going round. But somehow, he got there and that's the paradox.
It's also the case that the guy will age - 24 hours for every 24 hours he 'loiters' around the gate. As does the Guardian who is observing it. So saying this is a loop is wrong too. It is more like a chain of loops progressing into the future, one new loop each time the guy steps through the portal again.
Finally, since I and 24-hours-older me can chat through the gate, information about the future is flowing back to me which I should be able to use to make decisions about my future. If, while we're chatting, I see future me being stabbed to death, I might decide to get away from the gate and never go near it again. This case doesn't necessarily break your predestination hypothesis, but the kind of convoluted story you'd have to come up with to explain why I'm there, 24-hours later, chatting amiably, even though I know I'm going to be attacked and have done my best to avoid it, certainly makes it look suspect.
Oooh, I hadn't heard of Einstein's Dreams. That looks like a neat idea for a novel.
Graham wrote:
Your time traveller in the 24-hour loop is, in fact, a paradox. You still want to hang on to causality in your cosmology, I suppose, so you need to have a chain of events that led up to this person being a fully grown adult and standing in front of the gate so he could step through. Saying he sprang into being when he stepped through the gate 24 hours ago, just isn't good enough. Once he's in the loop, fine. He will keep going round. But somehow, he got there and that's the paradox.
So even envisioning the paradox is, itself, a paradox?
It's also the case that the guy will age - 24 hours for every 24 hours he 'loiters' around the gate. As does the Guardian who is observing it. So saying this is a loop is wrong too. It is more like a chain of loops progressing into the future, one new loop each time the guy steps through the portal again.
This makes a lot of sense, and I suspect that only through the creation of multiple, independent timelines could one account for the endless duplication that would result from starting the loop on another cycle.
My theory is that time travel is possible, and a natural development of any rational progression of technological civilization. And, therefore, all of history -- past, present and future -- is a solid mass of time paradoxes. The time paradox is the building block of reality.
This follows from Niven's Law, which states: if time travel is possible in the universe of discourse, it cannot be invented. If it is invented, then people will travel into the past and change the past, until they introduce a change that prevents the invention of time travel. Then they can't introduce any more changes.
But if the invention is a natural, even easy development, preventing Marconi from inventing it only means that Dr. Wang will invent it a bit later, and when he's prevented, Edison will do it a bit later yet. And so on. Preventing time travel is a never-ending job.
This theory explains why technological civilization does not progress rationally. That's a side effect of preventing time travel. History has been systematically distorted from its natural progression.
You have a better explanation?
If time travel is possible in our future (and therefore something that could impact us today), would there be a way of knowing? If one were in an altered timeline, how would one know?
I have usually found Discover magazine to be a diverting and thoughtful read, although perhaps taken with a few grains of salt. It is one subscription I intend to seek one day, if I build up the financial capacity.
I used to have Discover in my RSS reader, but it was so political all the time that it left a bad taste in my mouth.
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