I agree with Professor George's answer, but I would like to add one thing. Suppose you are a professor of English. You take a time-machine trip into the future and learn from a reliable source that you died in dramatic fashion: in the midst of teaching a Shakespeare class. You tend to get very excited while teaching Shakespeare, and you died from cardiac arrest while giving a spirited lecture. However, you do not learn anything about the date of your death. Then you travel back to 2005 and continue your life. When your department chair asks you what you would like to teach next year, it would be perfectly rational for you to say, "Anything but Shakespeare." By not teaching Shakespeare, you cannot change the future from what it will be. (Apparently, despite your determination never to teach Shakespeare again, you end up doing so, somehow, and die in the midst of it.) But by not teaching your Shakespeare class next year, you can make it true that you lived a longer life. It is no different from your deciding today not to play in traffic. By doing so, you can make it the case that you live longer than you otherwise would.
What about the present? I can lie about the present can't I? Forexample, I could lie to you about what I am doing right now. I won't.
Andyou're mistaken to think that we cannot lie about the future. If I wereto sincerely tell you that I will be eating excellent barbecue tomorrow(I wish!) with the intention that you come to believe me, then thatwould be a lie. I won't be, and I know it. I'll be eating my heart outover not being able to get good barbecue around here. So it seemsobvious that we can and sometimes do lie about the future. Insofar aswe can talk about the future and intend to deceive others about it, wecan lie about it.
How would telling a lie about the past turn the lie into truth? (Note that your way of posing the question assumes that we cantell lies.) Suppose John lies on his job application and says that hereceived a degree from a prestigious university which he neverattended. His claim that he received the degree wouldn't make it true.Haven't we all read articles about this sort of thing happening? Sincehe intentionally deceived about some past event, it is appropriate tocharacterize his action as a lie.
On one view, time is a lot like another dimension lying alonside the three dimensions of space. On this view time doesn't move: all times are equally real at all times, just as all parts of space are equally real from all places. But do we move along time? Well, we are in different places at different times, and of course we are at different times at different times.
According to another view of time, the present is privileged. As George Santayana once said, 'the present is like the fire running along the fuse of time'. On this view, it looks like we are moving along time, and so is the present.
What we experience depends on what information about the world we receive and when we receive it. We receive information about the world through our senses, as when a ray of light arrives in one of our eyes from an event that occurred sometime in the past. (That light ray may have been launched by an event that occurred just a few millimicroseconds ago, or it might have come instead from an event in the very distant past, as when a ray of light emitted by stars many years ago enter our eyes as we look up at the sky at night. If the star is 100 light years away, then that ray of light was emitted 100 years ago.)
Since we do not receive rays of light today from events that have not happened yet, we do not experience the future (yet!).
Now light travels very quickly, as you know. The various rays of light that are arriving in my eyes right now, having previously bounced off of various objects in the room I'm in, all left those objects at very nearly the same moment. So those rays of light give me information about the states of those objects as they were at very nearly the same moment. which is also very nearly the same moment at which that light arrived in my eyes. We experience that moment as the present. (However, there is also a neurophysiological story about how long "the present" lasts, which I won't get into here.)
If the laws of physics were different, we would not experience past and present in the way that we do. Suppose, for instance, that a ray of light from an object could not enter our eyes all by itself, but instead, it could only enter our eyes as one member of a pair of light rays. In particular, suppose that the laws of physics required that a light ray wait around, cooling its heals somewhere outside of our eyes, until such time as it could enter our eyes accompanied by another ray of light that left that same region of space exactly 10 minutes later or earlier. (These would be peculiar, science-fiction laws of physics, but let's think about them just for fun!) If we could receive visual information about the world only via pairs of light rays, which left the same region of space 10 minutes apart, then presumably, we would not experience past and present as we do. (Or do you think that we would have evolved some mechanism by which to do so?)
By the same token, suppose that light travelled very very slowly. In particular, suppose that the light entering my eyes right now, having last bounced off of the keys of my computer keyboard, left minutes earlier than the light entering my eyes right now that was emitted by my computer screen (which is a little farther away from my eyes than the computer keyboard). What a drag that would be!
Newton proposed that there is "absolute time", over and above the motions of clocks and pendulums and celestial bodies that (to some degree of accuracy) measure absolute time. Newton did so in the context of a scientific theory that aimed to account for some of our observations of the motions of material bodies. The tremendous success of his theory counted as good evidence for the existence of absolute time. Roughly the same situation exists today, except that our best current theories of how and why bodies do what they do fail to use Newtonian absolute time. Those theories (roughly, quantum mechanics and relativity theory) use other notions of time -- indeed, notions that are difficult to reconcile. Still, whatever evidence we have that time exists comes primarily from evidence for our best scientific theories, which use various concepts of time to characterize the universe.
Now those theories might be mistaken, or the evidence for their accuracy might not constitute very strong evidence for what they say specifically about time. Such turned out to be the case with Newton's theory. But in any event, although we cannot "be sure" that time exists, since scientific theories are never proved for certain, the question of whether time exists is tightly bound up with questions about what theories are accurate regarding the most fundamental physical processes and interactions.
By the way, I agree with you that the fact that we say that time can be measured is not very good evidence that it exists. We also say that the magnetic field at a certain point in space can be measured -- for instance, by sticking an iron filing there and watching how it aligns itself with the magnetic field. But this measurement is not very powerful evidence that there exists a magnetic field, over and above all of the material bodies, occupying otherwise empty space throughout the universe. The behavior of the iron filing is compatible with magnetic forces operating by action at a distance rather than by way of an invisible field.
One's tempted to say that if the man has indeed murdered someone in the past, then if you were to travel back in time you could not kill him before the murder. The past is past and, contrary to Hollywood lore, it cannot be changed. For more on this, see Question 242.
Nothing happens to moments; things happen at them. After a moment passes, nothing that happens anymore happens at it.
Apast moment might, however, still be remembered and spoken of. Doesthat require that in some sense it "still exists" and is indeedeternal? For surely there is something that we are remembering and speaking of. Or is that a mistake? Could it rather be that while there was something that we are remembering and speaking of, there isn'tanything that we are remembering and speaking of? But is it coherent tosay that in addition to all the things that there are, there arealso things that that were but are no more? That sure soundsself-contradictory. How about: in addition to all the things that thereare, there are-or-were also the things that were but are nomore? Here, "there are-or-were" functions as a "quantifier" that coversthings that no longer exist. Some philosophers hold that if you use aquantifier like that, you are committed to the view that the things itcovers do exist (for there have to be the things it covers, for it to cover them). Can we reply, no! there merely have to be-or-have-been those things? Is this disagreement resolvable? Is it substantive or merely a quibble about words?
Many philosophers in the past half-century or so, especially those influenced by physics, have taken the passage of time to be wholly subjective. Physics (Newtonian as well as Einsteinian) appeals to temporal relations like 'earlier than' or 'simultaneous with', never to an ongoing now that passes. (Minkowski diagrams use 'now' as a label for a point of origin, but that's a far cry from passage of time.)
Recently, some philosophers have turned the tables, and taken 'past','present' and 'future' to be fundamental (and not merely subjective). So, there is a lively debate now about the status of the present moment.
For references on the contemporary debate, see the entry on "Time" in the Stanford Encyclopedia of Philosophy. (It's free on the internet, and a great resource.)
Kant doesn't exactly fit into the terms of the contemporary debate. I'm no Kant scholar, but I think that Kant took time to be something like succession--we experience one thing after another. Kant took time to be an a priori intuition--a necessary condition for any experience.
It's not clear that you can go back in time and kill your former self: a former self that doesn't get killed seems required for your presense at his side, contemplating the crime.
This is a version of the grandfather paradox (you can travel back and talk to your own grandfather, but not kill him); though I should say that there are some (not me!) who hold that this type of thing is in fact made possible by a multi-verse interpretation of quantum mechanics.
On the other hand, I don't see any impediments to using time-travel to kill off the final stage of yourself. (This needn't be in the relative chronological future if your life ends on a time-traveling adventure.) Whether it's suicide of merely murder seems purely terminological. It's "suicide" in one sense (you're done in by one of your own temporal stages), but not another (your final stage doesn't do itself in).