Here is a sampling of views on time that you'll find in recent literature: Presentism: Only the present moment is real; neither the past nor the future exist. Among other things, this view is supposed to help make sense of our sense that time really passes, but it does that (to the extent that it does) by treating the present as an infinitely thin slice and not as something eternal.On the contrary: the present is utterly evanescent. The Block Universe: On this view, all events exist eternally, as it were. That includes all events before my writing this post, all events simultaneous (in some frame of reference) with my writing, and all events after my writing. This view is claimed to fit best with the understanding of time that Relativity provides, but there is no eternal now . "Now" has no metaphysical interest whatsoever. "Now" is like "here": it's what is sometimes called an indexical term, picking out what it picks out only relative to its use. When you say "I am here" and I say "I am...

There are interpretations of quantum mechanics that make related claims. There's the transactional interpretation, proposed by John Cramer and developed more recently by Ruth Kastner. It holds that quantum events such as measurement results occur when there is a "handshake" between an advanced wave, traveling from future to past, and a retarded wave, traveling from past to future. The so-called two-state vector formalism, pursued in recent years by Yakir Aharanov and Lev Vaidman, is in some ways similar. Huw Price has long argued that if we allow for backward causation, we can avoid having to posit faster-than-light action at a distance. Some people have argued that in certain cases, quantum teleportation involves information moving from future to past. But all of this is controversial and it would be hard to argue that a consistent understanding of quantum mechanics requires backward causation. To which we should add: these interpretations do not claim that quantum mechanics can exploit any...

A timely topic, if you'll pardon the pun. It's very much a live issue in contemporary philosophy and as you'd guess, there is more than one camp. Very roughly, we can carve the territory up this way: Presentists say that only the present is real. The future does not exist, and will only come into being after the present has slipped away. One reason why some people defend presentism is that they believe it does more justice to our sense of the passage of time. On this view, things "become." Eternalists think that all events are equally real. There is no special moment that counts as "the present." Rather, for any given moment, there are earlier and later moments. One reason (though not the only one) why some people defend eternalism is that it seems to fit better with the understanding of space and time that we get from the theory of relativity. Think of it this way. Suppose an event happens outside my window as I'm writing this—say, a car backfires. And suppose that on a planet far...

A good question. The nub of the matter is this: if something is moving literally at light speed, then the amount of time between two points along its trajectory, measured in its frame, is 0. So your hunch is right in one way: at light speed (along the edge of a light cone), time doesn't pass. However, if we try to accelerate a massive body (for example, a spaceship) to light speed, we'll fail. Close enough for present purposes, the reason is that as the body accelerates, it takes on mass, and that mass approaches infinity as the speed approaches c . So your space traveler will never arrive at "frozen" time.

Some good questions. One view is that if there is change, there is time. However, it doesn't follow from that that time has a "metric" -- that there is an answer to questions of the sort "how much time?" If all that existed were two solid spheres in relative motion, then someone might say that there's no answer to the question "What's the relative velocity?" (hence how much time has passed between varying degrees of separation) even though there is change going on, and hence there's time. More generally we can at least imagine a universe where time _order_ among events -- what happens before, after or simultaneously with what -- is definite, but without there being any correct answers to questions such as "How much time passed between event x and event y?" Insofar as that's right, it provides a way around your objection. Whether time requires change is yet another question. Not everyone agrees that it does. Sidney Shoemaker, in a paper from some years ago, argues that there could be...

As you may sense yourself, part of the problem is to decide just what question we're asking and what would count as an answer. Start with space. One way of claiming that space is real is to say that it's a thing -- a "substance", as the jargon of philosophy would have it. (In this sense, a substance is something that _has_ properties or qualities and stands in various relations, but isn't a property or a relation. An atom is a substance in this sense; so is an animal.) The "substantival" view of space-time has its adherents, but also has its detractors. There are many philosophers who don't think space is real if that would imply that it's some kind of thing. Typically such philosophers would say that instead of talking about space as such, we should talk about spatial relations. Things can be between other things, or beside them or above the, or longer than them or a certain distance away from them. What's real, on this view, are the things that stand in the various relations; space isn't...

I'm not quite ready to go along with my colleague's answer, but my answer isn't any more hopeful. If time has the structure of the real line (as we usually think) then even if it's infinite, every moment is only a finite time away from now. (Compare: every real number is only a finite distance from 0.) But even if time is infinite in the way the real number line is, it doesn't follow that there will be a duplicate of you somewhere off in the future. To get that conclusion woud take a lot of extra and optional premises. More important, even if there will be a duplicate of you someday, there's no good reason to think it would be you , nor is there any good reason to think that you could look forward to its experiences. (These two aren't quite the same issue, as it turns out.) Clearly there's a lot in the background here. If you're interested in more reading on the core problem, i.e., the problem personal identity, you might have a look at Martin and Barresi's anthology, called Personal...

Just a footnote on Jonathan's reply on the matter of direction. Length is a measure of a property of things, and it has a natural 'direction' from shorter to longer. As Jonathan suggests, it wouldn't make any sense to say that the difference between one direction and another on the length scale is just a matter of convention. But it may be that position coordinates are closer to your worry. We can assign position coordinate so that heading north from my desk gives us bigger numbers. Or we could do it the other way: going north fives us smaller coordinates. And in this case, we'd say that the choice really is just convention. Nature doesn't favor one direction in space over another. But time seems to be different. There seems to be a real difference between the direction we label with increasing numbers and the opposite direction. As it turns out, physicists and philosophers have written a great deal about this asymmetry. As it also turns out, there isn't a consensus about the best way to think of it. Cups...

Just to continue the conversation - Jonathan and I agree that time is a dimension and not a force. It's just that this still leaves room for interesting questions about the relationship between thermodynamics and fundamental physical laws. We don't agree, it seems, about time travel, but we may agree for "all practical purposes." I'm quite satisfied with David Lewis's treatment of the grandparent paradox. Lewis agrees: you can't travel back in time and kill grandpa or grandma. Any such story is inconsistent, and inconsistent stories are guaranteed to be false. In fact, loosely put, you can't go back in time period unless it's actually a part of the world's history that it happened. Unless it's "already" a fact about the world that an adult Allen Stairs was wondering around the streets of his boyhood home in the 50s and 60s, then we are guaranteed that I will never do any such thing. Lewis's point was that in spite of this, we can tell consistent time travel stories. They just require very...

What a lovely question! The first thing to ask is whether the story is internally consistent -- unlike a story in which John kills his grandfather before Grandfather fathers John's mother. That appears not to be a problem here; there's no obvious hint of an event having to have happened and not happened. Instead, we simply have surprising set of internally consistent occurrences. On, then, to responsibility. Since we have a causal loop here, there's no clear way to say which of Peter or Kurt is causally responsible for the theorem's having come to be stated and proved. If pressed, we might say that each gets equal causal credit,m though your mileage might vary on the apportionment. Indeed, neither is the originator or initiator; the most that can be said is that both were crucial parts of the process through which the theorem came to be. What of intellectual credit? Well, Peter didn't think the ideas up on his own. He learned them by reading about them, having been taught them,...

Yes and no. Let me explain. Some people think that is flat-out impossible. They appeal, for example, to puzzles like the Grandfather Paradox: if time travel were possible, the argument goes, I would be able to go back to 19xx and kill my Grandfather before he met my Grandmother. This would mean that I would never exist, and so the scenario requires both that I do and that I don't exist: contradiction. This is meant to show that time travel is impossible. Philosophers can help us sort through that sort of problem, and in fact some have. David Lewis's "The Paradoxes of Time Travel" (you can find it in his Philosophical Papers , volume II) is still a lucid and useful sorting of the issues. Lewis argues -- correctly, I think -- that the Grandfather Paradox doesn't show what it's meant to. Roughly, the idea is this: if I do someday travel back to 19xx gunning for Grandpa, then first, it's true even now in 2008 that there was a deranged philosopher lurking around in those days gunning for Earl Stairs. ...