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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...

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...

Is time a philosophical concept or a scientific concept?

How about neither? Or both? (Or both neither and both?) Put another way... Time is just one of our many concepts. By far most people who use the concept of time aren't philosophers and aren't scientists either. And so the concept of time as such isn't a peculiarly philosophical concept, nor a peculiarly scientific one. That said, time has a special place in science as a fundamental parameter. We can do a lot of science without the concept of sex, for example, even though there's a place in science for the study of sex. (And of course, if there were no sex, science would grind to a halt in a few decades!) But outside of mathematics, we can't do much science without the concept of time. Moreover, physicists have things to say about time that are deep and surprising and were mostly beyond the imagination of the philosophers and the folk until relatively recently. Philosophers have long taken an interest in time as well, and have taken it as a special subject for philosophical analysis. They've...

Does a proposition about the future have to be true today? If so does this preclude contingency and is every proposition of the future necessary?

Let's start with an analogy and see how far it gets us. Suppose I consider a proposition about some distant place. Suppose I consider the proposition that the population of Woodstock, New Brunswick (my home town in Canada) is over 6,000. [To keep things simple, assume that I mean the population today, August 5 2007.] I'm contemplating this "here" in Washington DC. But it's a proposition about some other place -- "there," not "here." And now consider the question: "Does this proposition about Woodstock have to be true or false here in Washington?" The question seems a little odd. What the proposition asserts refers to a particular place, but the idea that the truth of the proposition is, as it were, tied to the place where it's being contemplated seems off. We might put it this way: the proposition picked out by my use of the sentence "Woodstock has a population over 6,000" is true if the population of Woodstock really is over 6,000 and false otherwise. Asking if the proposition is true ...

Space and time are measured in hours and metres, value is measured in utility. In these three fundamental scales, I have read that zero and the unit are arbitrary. I can see that there is no beginning of time, and no bottom to the universe and no absolutely valueless state of affairs, but it seems perfectly sensible to talk of two states of affairs being of equal value, in which case the difference in value would be zero. Two durations could be of equal length, as could two bodies. So is there a non-arbitrary zero in space, time and value that corresponds to the difference in length, duration or utility between the equally long, enduring or valuable?

It may be that there are two questions hidden here. You're right: if we can compare things in terms of length or duration or utility, then we'll sometimes be able to say that they're the same on this scale -- that if we subtract one value from the other, we get zero. But there's another question: is there such a thing as a thing's having zero length, taking zero time or possessing zero utility? Length and duration are not quite the same sorts of scales as utility. Length and duration are ratio scales. It makes sense to say that this stick of wood is twice as long as that one. Turns out that this goes with the fact that there is such a thing as having no length or lasting for no time. In these cases, we have a natural zero. However, it may not make sense to say that one thing has twice as much utility as another. Utility scales are interval scales. All that matters are the ratios of the differences. Let's make this a bit more concrete. I might rate the utility of a cup of coffee at 1,...

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