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Does the future exist? In theory, is the future a 'place' that I can go to in a time-machine or does the universe alter in such a way that my desired era appears before me?

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...

I've read that as we go faster time dilates and so time slows down. So my question is that If suppose a person in a spacecraft accelerates to the speed of light. After sometime (in his prospective) he decides to decelerate finally to much much lower than the speed of light. Then during all of this how much time will have passed for everything outside? Will he be able to decelerate at all? I mean for an outside observer, who by some means, is able to see everything that is happening in the spaceship, will the person be frozen (in time) and therefore not able to push the button that decelerates the ship and ultimately travel infinitely in time and space? (again another assumption that the fuel does not run out). And (in the prospective of the space traveler) after pushing the button where will he be in time with respect to the observer? I hope I am able to convey my problem. Thanks in advance.

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.

What is time? My friend and I are having an argument about the nature of time. If I understand her position correctly my friend believes that time is simply an artefact of changes in the universe, or that change is itself synonymous with time. This seems to be a commonly accepted position, however I replied that if two spheres were moving in space in parallel to one another but one sphere was moving faster than the other then that would mean that that sphere would traverse a greater distance in less time but with an equal degree of change in the universe. Thus time and change are two separate things. I guess the physics of that claim are debatable but I suspect that it demonstrates something true in a way that is a priori irrespective of the physics because even if there were a physical difference in the amount of forces which change it would be hard to see how those imperceptible forces would contribute to a difference in how we perceive the time effect of those different objects. What do you...

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...

I was recently speaking with someone who had an argument about whether time exists or not. Time Dilation is often put forward as proof that time exists and that it is not merely a figment of imagination of mankind. But this person argued that by believing this, we are making a self-contained assumption about time. He argued that time is actually just the measurement of change that occurs in an object relative to constant natural phenomena. For instance, atomic clocks measure the microwave emissions of changing electrons, and older clocks measure the degree of the earth's rotation. He suggests that it is a huge and erroneous jump to say that these things measure anything other than what is stated i.e. emissions of changing electrons or the degree of the earth's rotation, and that to say that they measure a force or external entity that is time is simply illogical. And I would be inclined to agree. I watched a Stephen Hawking program where he discussed the possibility of time travel. He spoke about the...

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...

If time is infinite does this give us any hope for life after death? After all if time is infinite, it is inevitable that all the cells in my body (my DNA etc) will be reconstructed in some far off day and age.

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...

Is time an independent physical dimension or a human construct designed to compare events to each other ? If it is a physical entity why can we move only in one direction and at an inexorable pace? Is it theoretically possible for a time machine (Hot Tub or any other sort) could exist?

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...
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...

Kurt Gödel's incompleteness theorems represent one of the foremost achievements in mathematical logic's proud history. AskPhilosopher's very own panelist Peter Smith obviously is greatly intrigued by these theorems. Suppose out of jealously -- though I doubt he would succumb to such a vice! -- he decided to build a time machine. Imagine, moreover, that he went to a time before Gödel had proven the theorems and gave the great logician, say, the idea of Gödel numbering, the key to proving the incompleteness theorems. My question thus is as follows: Who would deserve credit for proving the incompleteness theorems? Gödel seems to have gotten the idea from Peter; Peter seems to have gotten the idea from Gödel. Is it possible that neither would deserve credit?

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,...

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. ...

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....

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...

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