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If we move through time, then what is movement? That is to say how is movement,

If we move through time, then what is movement? That is to say how is movement, or any change for that matter, possible outside of the context of time?

One answer to your question is that there may be multiple "orders of time" and, in particular, there may exist an order of time that is separate from the one we normally experience and within which events can occur.

Thus, for example, in Western Europe around and in the centuries before 1500 certain religious rituals, ecstatic experiences, moments in liturgical calendars, may have been experienced as occurring in a special "sacred time" that constitutes a different temporal order from commonplace "secular time." In his A Secular Age (and a book I've mentioned before on this site),Charles Taylor argues that our ancestors in Western Europe possessed this bifurcated experience of two orders of time and he provides a rich account of why it is that almost everyone alive today in Western Europe and North America experiences secular time only.

Are there any good philosophical reasons for thinking that time travel is

Are there any good philosophical reasons for thinking that time travel is possible?

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. (You may dooubt that this is so; so do I. But do either of us know it beyond all doubt?) Second, however, we can say for sure that I failed. I didn't manage to kill Grandpa, because he did actually marry Grandma, and they did actually have a son who married my mother and became my father. Time travel stories have to be consistent, but consistent time travel stories are possible. So philosophers have offered reasons to think that time travel is possible in the broadest sense: it's not inherently incoherent -- it's logically possible or metaphysically possible.

If we take philosophers like Lewis to have done that part of the job correctly, however, there's another question that calls for a different sort of expertise: do the laws and conditions that hold in this universe allow for time travel? Roughly, is time travel physically possible? That's not something that philosophical reflection alone can tell us. General relativity, our best theory of space and time, apparently allows for "closed time-like curves," and following one of those would amount to time travelling. But "allows for" is pretty abstract. It doesn't follow that there's any way of arranging this that a human body could survive. And it also doesn't follow that the technology needed to generate those curves is within our capability. So on the most general question, yes: philosophers can give us reasons to think time travel is possible. But when it gets down to the nitty gritty, physicists and physiologists and engineers would have to weigh in.

(I've assumed, by the way, that you had travel into the past in mind. Ordinary "time travel" into the future is no problem; that's how we got from yesterday to today. And relativity straightforwardly allows for more interesting variants on future-oriented time travel. If you did it the right way, you could get from Times Square, Jan 1 2009 to Times Square, Jan 1 2010 in less than a year, by your reckoning. Hermann Bondi's Assumption and Myth in Physical Theory is still a friendly introduction to those ideas 40 years after it was written.)

In class, our professor discussed the impossibility of time travel. He stated

In class, our professor discussed the impossibility of time travel. He stated that if in the future, machines are made to travel back into time, then we would be seeing people from the future right now. His argument ended there but would this be true? Is this a valid argument to disprove the possibility of time traveling in the future?

I hope your professor was just trying to provoke you, because it's a terrible argument. For one thing, it's not clear why he's so sure that we aren't already seeing people from the future, who've traveled back to this time zone, as it were, and are doing a good job of blending in. And in any case, suppose that in 3008, someone figures out how to travel backward in time. Why is it so obvious that they would come to this time?" Why not a later time? Or a time when there were no humans at all? If we add the plausible conjecture that the process would be expensive, dangerous and not altogether reliable, what basis would we have at all for speculating about the likelihood that someone would have shown up somewhere that we'd know about?

More importantly, if something is actual, it's certainly possible, but the converse doesn't follow. Even if time travel is possible, it doesn't follow that it will ever actually happen. The world is and always will be pregnant with unrealized possibilities. Perhaps this is one of them.

And then there's the question "possible in what sense?" The equations of general relativity allow solutions that include so-called "closed timelike curves." Anything that followed such a curve could fairly be said to undergo time travel. But from the fact that this is physically possible, it doesn't follow at all that it's a practical possibility. The laws of physics allow many things that we'll never be in a position to do.

Though your professor doesn't seem to have this in mind, some people have argued that time travel is inherently paradoxical and therefore that the whole notion is incoherent. (For example: the idea of time travel is supposed to sanction the possibility that I could kill my own grandfather before he ever met my grandmother, which would mean that I could cause myself never to have existed.) I think this is confused, and I can do no better than recommend that you get a copy of David Lewis's paper "The Paradoxes of Time Travel" (American Philosophical Quarterly 13: 145-52. It's also reprinted in his Philosophical Papers Volume II. Oxford: Oxford University Press.)

Let's say time machines exist. What would happen if you got into a time machine

Let's say time machines exist. What would happen if you got into a time machine, went back in time, and stopped the invention of time machines? Larry 16, NJ

If time machines have been invented, then no-one can change that. Ifyou were to step into a time machine, travel back in time and try toprevent the invention time machines, you would fail.

What do we owe to people who don't yet exist? Intuitively, it seems to me that

What do we owe to people who don't yet exist? Intuitively, it seems to me that we shouldn't, say, cause widespread damage to earth because it will so valuable to our descendants. But can we really be said to be doing something wrong to someone who doesn't exist? And would it be wrong to do something that would cause them never to exist in the first place? It seems that if we can do moral harm to future people, but it isn't wrong to cause them to never exist, then it morally superior to never have children rather to bring children into the world in which you have done the *slightest* damage. (The children, of course, would disagree.) But if it is wrong to cause them to never exist-and, since they would drastically prefer to exist-then we have a tremendous burden to reproduce as much as possible. If it make any difference, I am interested in how these question relates to our burden to reduce catastrophic/existential risks to the human species (global warming, nuclear war, gray goo, etc.).

That's a lot of difficult questions! First: I think we can do wrongto people who don't yet exist. It seems unfair to be less respectful ofsomeone who will be born in, say, 2020 than someone who was born in,say, 1995. Second: it is not obvious that your second question makesmuch sense. You can't do wrong to a being who doesn't exist, never hasexisted and never will exist, simply because there are no such beings!A future being isn't yet around to be harmed, but will be later. But non-existent beings aren't there to be harmed.

You go on toconsider a conditional: 'if ... it isn't wrong to cause them never toexist ...' where 'them' is supposed to refer to future people. But ifwe cause there to be no future people then 'them' doesn't refer andthere is no issue about harming them.

Still one might wantto argue that we have a duty not to make the planet uninhabitablebecause we have a duty to our species. I am not sure how to justifythat, but the thought seems to have some intuitive appeal.

Does the law of bivalence demand that a proposition IS either true or false

Does the law of bivalence demand that a proposition IS either true or false today? What if the truth or falsity of this proposition is a correspondence to a future event that has yet to occur?

I take it that by "bivalence", you mean the principle that every proposition is either true or false. And if we take that principle in unrestricted form---we really do mean every proposition---then, well, it's hard to see how it could fail to imply that the proposition expressed by "There will be a riot in London on 13 January 2076" is either true or false.

If you don't like that conclusion, then you have to abandon bivalence---or, perhaps, the claim that the sentence in question expresses a proposition, though that seems rather worse. But note that you do not have to abandon bivalence, so to speak, across the board. You might still think that every mathematical proposition is either true or false, or that every proposition about the past is either true or false, or.... Perhaps there is something special about the future here.

As you probably know, Michael Dummett argued that one way to understand debates over "realism" takes them to turn upon our attitude towards bivalence regarding propositions about the subject matter in question: So a view that gave up bivalence for statements about the future would be a form of "anti-realism" about the future.

Great site. How does our approach to knowledge about the past differ from our

Great site. How does our approach to knowledge about the past differ from our approach to knowledge about the future?

Others may have things to add, but one obvious way is that many of our beliefs about the past are caused by things that happened in the past and produced traces, either directly or indirectly, in our brains. But on the usual view about how the universe is wired up, our beliefs about the future aren't caused by future events.

This doesn't make knowledge claims about the past uniformly more secure than knowledge claims about the future. Some facts about the past may be well nigh inaccessible; their traces may be faint or non-existent, and there may be no good general grounds for inferring. (For example: I'd guess that there's almost no hope that anyone will ever know exactly how many people were on the swath of ground now marked out by the University of Maryland campus at noon on April 3, 1808. But -- skeptical worries aside -- we can reasonably claim to know that the earth will rotate on its axis over the next 24 hours.

Still, knowledge of the past has a certain priority. Our knowledge that the earth will rotate on its axis over the next 24 hours is based on things we know about the past and generalizations that this knowledge supports. Something like this is true in general: knowledge of future events is grounded in knowledge of the past, but not vice-versa.

Please pardon the awkward structure of this question; I am afraid the

Please pardon the awkward structure of this question; I am afraid the insuperable inadequacies of autodidacticism will prevent me from asking it clearly. What I want to know is, in a nutshell: Is the Past eternal? That is to say, it makes sense to make statements about the Present (if in fact there is a present; one sometimes reads there isn't) which take the form "X is the case." It also obviously makes sense to say, where t is some point in the Past, things like, "At time t, X was the case." But I'm much less confident that I'm allowed to have sentences like (if X is no longer the case but used to be at t, which is in the past) "At time t, X will always have been the case." And in fact I want very badly to say not only that but "For any X which once obtained, is obtaining, or will obtain, at any time T, will always once have obtained." I also want to believe this not only of propositions which once held, but also of all phenomena & entities which ever occurred & existed. (That they will always once...

You've raised lots of issues, but I wanted to single out one in particular. You seemed particularly worried by the possibility that there might be some sort of influence from present to past. The worry seemed to be that if someone did the wrong sort of monkeying around now or in the future, your past might be wiped out. That's a disturbing thought, but fortunately we needn't read Gödel et al that way. The trick is to distinguish between influencing the past and changing the past. Think of it this way: reality (or physical reality, anyway) just consists of the eternally-existing set of all events. On this picture, there's no question of a sort of "moving present" with events becomingpresent and then slipping into the past. There's just the set of eventswith all their many relations. You might find it useful to read what my fellow panelists Peter Smith and Jasper Reid had to say in response to question 2032 . Among the various relations are space-time relations, but there are also causal relations. It's just that we can't think of causing in the way that the "moving present" picture suggests as a sort of bringing-into-being.

On our usual way of thinking about things, causes are always unambiguously earlier than their effects. The speculations you're considering suggest that this may not be so -- that sometimes the direction of causation isn't the same as the direction of time. But the important point is this: the total set of events just is what it is, so to speak. If someone in the future exerts an influence on what I'm typing here, it's not a matter of their changing what I type; it's a matter of certain events in the future of my typing standing in a particular relation to all the events that make up my typing this reply.

This may be a little easier to grasp if we forget about changing the past and think about affecting the future. If my typing this "now" causes something to happen in the future, that doesn't mean I'm changing the future -- that somehow the future was one thing until I decided to type what I'm typing. Rather, it's that there's a cause and effect relation between events that come after my typing and the typing itself.

This picture seems odd when you first encounter it, but I dare say that it's the one most philosophers end up settling on. And if we accept it, then your past is safe; it simply is what it is, for better or for worse. Meanwhile, if you want to read an already-classic account of what it might mean to affect but not change the past, I recommend David Lewis's article "The Paradoxes of Time Travel." It first appeared in the American Philosophical Quarterly 13:145-52 (1976), and it's reprinted in his Philosophical Papers, Volume 2 (Oxford UniversityPress, 1986). But in any case, I don't recommend eating Weltanschauungen. It tends to produce intellectual indigestion.

Are the concepts of omnipotence and omniscience mutually exclusive?

Are the concepts of omnipotence and omniscience mutually exclusive? God is generally considered to be both omnipotent and omniscient. Let’s say he created the universe. At the time of creation he knows how everything is going to play out. Doesn’t that limit his options to intervene in the future? In order to maintain his omniscience, he can’t intervene in a way that he didn’t know he was going to do beforehand. And if his actions are limited by this constraint, can he be omnipotent?

I think the difficulty here lies primarily in understanding God's knowledge and power as sort of "supersized" versions of our own knowledge and power. God's attributes are analogical at best. But the key to breaking through this kind of puzzle has to do more with the concept of time than of either omniscience or omnipotence. It's a commonplace of theology and phil of religion that God is "outside time." In other words, God's experience is not sequential, like ours is, but is eternally and universally present. So it makes no sense to talk about what God may or may not do "in the future" and what he knew "beforehand." Of course, what it means to be outside time is just as much, if not more, of a puzzle than the one you originally set out, but it seems to be closer to the target of inquiry than concerns about whether God's knowing or acting "comes first."

How can time really exist? If you think about it, threre is an immeasurably

How can time really exist? If you think about it, threre is an immeasurably short time which is the present which is ever changing. It is commonly accepted that that which cannot be measured cannot physically exsist. I think that we understand the present the way we do because of the past, and predict the future due to the past and present. But, there is effectively no actual past or future. The present doesn't even exist because the point in which it exists is so brief that by the time we perceive its existence, it is part of the past, which is impossible. So, how can time really exist?

I'd go along with Peter Smith's answer, but I figured I'd just take the occasion to point you in the direction of a couple of classic discussions in this area, which you might be interested in following up. First, your question is startlingly close to a problem raised by Saint Augustine at the end of the fourth century AD -- you're in good company! If you're not already familiar with Augustine's discussion, it's in his Confessions, book 11, paragraphs 17 to 38, pages 168 to 174 in this edition. I don't know how much his own solution to the problem would actually appeal to you, which is effectively to say that time only really exists in the mind, the past in memory, the present in sight or consideration, and the future in expectation. But another way around the problem is suggested by J.E. McTaggart's article, 'The Unreality of Time', first published in the journal Mind in 1908 and available online here. McTaggart lays out various alternative ways of thinking about time, and it's up to you to decide which you'd prefer to adopt: but, if you adopt what he calls the 'B-series' view, then the temptation to deny the reality of the past and future will fizzle out altogether. On this view, the present moment, "now", is no more or less real than other temporal moments, past or future, in precisely the same way as the present location, "here", is no more or less real than other spatial locations, in front or behind -- and that's an analogy I'd invite you to ponder.

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