When people first hear that time is considered to be a fourth "dimension" akin to the three dimensions of space, they often get confused and think that because it is useful to consider time and space together that they're the same. In fact, there are many differences between time and space, and perhaps the most important is that time is not symmetrical and has a very definite directionality. What I mean is this: if you look at a picture it's impossible to know if the image you see is the original or if it's been reversed.
Space is symmetric in that if you consider any of its dimensions in reverse it doesn't affect the rationality of your perceptions. Space appears to be symmetric in every way. A car that gets 30 mpg driving north will get 30 mpg driving south, east, or west. A box two feet long, three feet wide, and four feet tall has the same volume as a box three feet tall, two feet wide, and four feet long.
Time is different in that it is directional, a property commonly referred to as time's arrow. The Second Law of Thermodynamics says that some events are irreversible in that once they have occured the universe cannot be put back to the way it was before. (In fact, almost every event is irreversible.)
Consider the situation in which a large container is filled with two separated liquids, for example a dye on one side and water on the other. With no barrier between the two liquids, the random jostling of their molecules will result in them becoming more mixed as time passes. However, if the dye and water are mixed then one does not expect them to separate out again when left to themselves. A movie of the mixing would seem realistic when played forwards, but unrealistic when played backwards.If the large container is observed early on in the mixing process, it might be found to be only partially mixed. It would be reasonable to conclude that, without outside intervention, the liquid reached this state because it was more ordered in the past, when there was greater separation, and will be more disordered, or mixed, in the future. ...
The thermodynamic arrow of time is provided by the Second Law of Thermodynamics, which says that in an isolated system entropy will only increase with time; it will not decrease with time. Entropy can be thought of as a measure of disorder; thus the Second Law implies that time is asymmetrical with respect to the amount of order in an isolated system: as time increases, a system will always become more disordered. This asymmetry can be used empirically to distinguish between future and past.
If you had two complete snapshots of the universe it would be possible to determine which was taken before the other by calculating the amount of entropy in each snapshot; the shot with less entropy was taken first. Thus, time is not symmetric, and traveling forward through time (as we all do) is very different from traveling backward.
Dha!
Did you spend the weekend reading Steven Hawking?
Just kidding. That was great! :>)
Actually, Relativity reveals that the relationship between time and the other 3 dimensions is even more unusual. Time is basically a fourth dimension, a fourth axis, but our movement along that axis is limited (or rather, our movement in the 4-axis space is only limited to a certain region, depending on where we start and on some other things).
Relativity also shows that dimensions are not as rock-solid as most people think. If you move really fast, you will see things contract in the direction you're moving, and things further ahead might appear to be more in the future (from the point of view of the stationary people you are moving by) than things not as far ahead.
In other words, if you go really really fast, then your destination 10 miles ahead of you might appear to be closer than a mountain five miles to your right.
And say you verified (while not moving very fast) that a clock on the street is synchronized with a big clock you can see very far away. If you were to move very fast past the clock on the street, moving in the direction of the clock very far away, you will see the clock very far away showing a later time. Once you slow down, though, you will see the two clocks appear synchronized again.
Pretty much all of this is a fairly direct consequence of the fact that the speed of any light beam is the same to anyone independent of their frame of reference. Say you're driving in your car in a straight road. Say there are two deer on the road, about 10 miles apart, and you're between them. Say that when you hit the point where you are exactly 5 miles from each deer, you turn on your car lights. Since the deer ahead of you appears to be moving towards you and the deer behind you appears to be moving away, you will see the deer ahead of you be iluminated by your headlights (it moved closer since you turned them on) before the deer behind you is illuminated by your tail-lights (it moved further away since you turned them on). However, if I'm standing by the side of the road, all I see is two light beams leave a spot towards two deer that are equidistant from that spot, so the two deer get illuminated simultaneously. You see the two beams of light move away from your car at the same speed, I see them move away from that point in the road at the same speed. So I see the effects of your headlights at the same time, you see the ones further ahead happen before the ones that happen behind. I will, additionally, probably see your headlights decrease in wavelength (the waves are "compressed") and your tail-lights increase in wavelength (the waves being "stretched") although you will not observe this.
If I laid a bunch of clocks along the side of this road, and I made sure they were all synchronized (as far as I could tell by standing there and looking at them and comparing neighboring ones), then when you drove by in your car at a good fraction of the speed of light, you would see each clock be a little further in time than the previous one. It's almost like the more "far ahead" an area is, the more "in the future" it is to you.
Because you can't go faster than the speed of light, you can't time-travel or see the future. Well, you can kinda see the future in an area way ahead of you, but it will have happened by the time you get there or by the time you manage to get a message over there, so it's not really seeing the future as much as changing the rate at which time seems to pass, in different places. You never see time flow backwards (although the closer you come to the speed of light, the more slowly you will see the flow of time around you). So in the end, the arrow of time does survive Relativity. It just might not have the same speed everywhere, and one place might experience different time-passage rates depeding on the speed and acceleration of the oberver, and I thought this probably is interesting enough to bring up (given that you think Time's Arrow is interesting enough to begin with).
I can recommend some further reading, like a Wikipedia article, and some good books.
Ah, yet another well-spent lunch break.
Bernardo: Pretty good explanation, yes! I don't really like the terminology of time as a "dimension" in the same way as space, because I think it can be confusing, but I suppose we're stuck with it.
Star Trek The Next Generation had many good episodes involving concepts of time travel, dimensions, etc. Sure it's all science fiction, but much of the science fiction in Star Trek TNG is based on current scientific understanding of those concepts, just expanded to a "reasonable" conclusion.
The infamous "transporter", for example, features a "Heisenberg Compensator".. created to address the Heisenberg Uncertainty Principle, which makes the transporter, as depicted in Star Trek, virtually impossible.
The final episode of the series (probably my favorite episode of all time) touches on time travel, anti-time, and is just a damn good piece of sci-fi and television.
Absolutely. TV does not get much better than The Next Generation.
The final episode is one of my favorites, but I also like the one where Worf switches between different quantum universes. Now THERE's a physics lesson.
Ahh yes, "Parallels". I agree.. a very good episode. Others include "Timescape", "Cause and Effect", and well pretty much every episode from the 5th, 6th, and 7th seasons.