So what is it? Maybe its a stoopid question but in what form does time exist other than the movement of a clock. My old physics teacher tried to explain it by using energy levels within chemical reactions as an example. It is easier to strike a match and ignite it than take a burnt match and wait for it to reassemble itself into and unburnt match because the energy supplied to ignite the match is so much lower than that needed to reassemble it in its unstruck form. So is time just a concept we use to get over a more confusing way in which the universe works? Or is it something more tangible?
It’s a dimension, silly.* Try asking, ‘What is length?’ and see how far you get. Anything you can come up with, if you can apply it to length, you can apply it to time, too. (Except maybe the bit about going forwards and backwards.)
Next question?
*[sub]Not entirely a glib answer. Time is devilishly tough to pin down in a definition.[/sub]
Oh, and your old physics teacher was talking about entropy, not time. Entropy is not time, although it ‘flows’ in one direction along the time axis.
The time as a dimension thing always gets me. You often hear time described as the 4[sup]th[/sup] dimension yet you can also talk of a 4[sup]th[/sup] dimension in a spacelike sense. A line is 1 dimension, a second line perpindicular to the first is 2 dimensions, a third line perpindicular to the first two lines is 3 dimensions, a fourth line perpindicular to the first three lines is 4 dimensions (and so on). Don’t worry if that last one is hard to imagine. If you truly could visualize that (or access the 4[sup]th[/sup] dimension) you’d be capable of all sorts of nifty tricks that would seem magical to anyone else.
For time as a 4[sup]th[/sup] dimension think of it like this:
For three dimensions you have an X, Y and Z coordinate that defines an objects location in space. For example, I could say, “Meet me at the pub which is 1 mile north of me, 2 miles west of me and on the second floor.” This fixes the pub’s location in space relative me. However, I can add a 4[sup]th[/sup] coordinate and say, “Meet me at the pub described above at 5 p.m. on Tuesday, May 29, 2001.”
Of course none of this defines time. Everything above is relative to me using measurement conventions I am used to using but you could substitute whatever you want in their place.
MrDeath is correct about your teacher showing you entropy. However, while entropy is not time I think it might be used as a tautologous argument for time if not a proper definition.
That entropy always increases seems a supremely fundamental feature of our universe. Time is what we call the measurments used mark the increase of entropy. Whatever units you want to use there is absolutely a ‘before’ when entropy was less and a ‘later’ when entropy was greater than ‘before’. The measurements themselves are extremely malleable (your units used and the person or thing observing this all affect the measurement) but they can all agree on the entropy bit regardless of other factors.
One thing I know about physicists: We’re lazy. We don’t care to define time, so long as we know how to measure it. If you like, I suppose you could define time as “that which is measured by clocks”, but that gets circular as soon as you try to define “clock”: A device for measuring time. We think up some pretty nutty ways of making clocks (have you ever tried to tell time by counting the bounces of a laser beam between mirrors?), but that doesn’t really get us any closer to definitions.
The significance of entropy in all of this is that it’s one of the three known “Arrows of Time”, and the only one most folks commonly encounter. That is, it’s a way to tell the difference between the future and the past: The future is when there’s more entropy than now, and the past is when there’s less. This is actually the basis for more familiar arrows of time, such as the fact that you can remember the past but not the future. The second Arrow of Time is the expansion of the Universe: The past is when the Universe is smaller, and the future is when the Universe is bigger. This obviously runs into problems if the Universe ever stops expanding and contracts again. The third Arrow of Time is a very subtle effect concerning the decay of the neutral mixed-state K meson: The reaction is more difficult by about one part in a thousand in one direction than the other. It’s believed that these three arrows are all independent, i.e., that it would be possible to have the kaon arrow reversed, but not the entropy arrow.
Whack-a-Mole, I can usually visualize in 4-d when I set my mind to it, but aside from it making it a bit easier to do some physics problems, I’ve never been able to use such visualizations for any “nifty tricks”. What did you have in mind?
Time and space are basic foundations of the universe and I do not think you can find satisfactory definitions in terms of other concepts because they are veryt basic and primary concepts. You can say tboth are components of movement. Movement cannot exist without time and space.
Time is like pornography: I can’t define it but I know it when I see it. Speaking of which… why does time go by faster when you are “reading” Playboy than when you are studying for finals? is time “elastic”?
Time is what keeps everything from happening all at once.
Actually I’m not sure that visualizing the 4[sup]th[/sup] spacelike dimension will get you anything. That said I can’t imagine how you can truly visualize a seeming impossibility. I’ve seen tesseracts but they were still 3-D analouges to the 4-D real thing (like the 2-D picture on your TV emulates the 3-D real world). I don’t know if this equates to a tesseract but I’ve also seem representations of what a shadow of a 4-D object looks like (just like your 3-D self casts a 2-D shadow on the wall). But getting my head to visualize 4[sup]th[/sup] line pepindicular to X, Y and Z axis is absolutely beyond me. I guess that’s why you study phisics and I don’t (are you sure you’re not speaking about 4-D in a mathematical sense?).
The nifty tricks department come if you could actually access the 4[sup]th[/sup] dimension. You could reverse the spiral in a conch shell for instance. Want something more useful? How about being able to walk into any bank vault anywhere no matter how tightly locked up it is? Sure this would make you a criminal but you needn’t worry since this also means there isn’t a prison anywhere that could hold you. Don’t like being a criminal? I’m sure our government could find some uses for that talent and would probably pay nicely.
(To understand getting into and out of anywhere imagine you are standing on a large sheet of paper inhabited by 2-D creatures [they’re flat…no height]. These creatures only see you as the bit that is touching the paper…they can’t perceive in any way, shape or form, the rest of your 3-D self. Being a bad guy the 2-D people decide to throw you in prison. In 2-D land a prison is made by drawing a circle around you so you can’t get out. Needless to say this prison is laughable to you since you just walk out of the circle. To the 2-D inhabitants you just disappeared and reappeared outside the prison at which point they set about burning at the stake…uhhh line…for being a witch. Anyway…a person with access to the 4[sup]th[/sup] dimension could do the same thing to our prisons.)
How about:
Time is the underlying phenomenon associated with the ordering of events as understood in the context of causality. That is, we generally accept that events can have causes that preceed them and/or effects that follow them; and we find that we can construct elaborate sequences of events related by cause and effect, such that they proceed in one certain order, and that the passage of some supposed essence can be associated with the progression through this order, and can even be measured with repeatably high precision. More properly, while the above appears to be correct in certain limits that typify human experience, it is more accurate in a more general or less constrained sense to say that time is interrelated with the linear dimensions of space, such that a four dimensional time space continuum behaves according to Einstein’s theories of special and general relativity.
I like Websters definition:
b : a nonspatial continuum that is measured in terms of events which succeed one another from past through present to future.
As Chronos said, it doesn’t matter what it is just that it is measureable.
It’s an intangible part of the very fabric of the universe (spacetime). One of the four basic dimensions (3space+time) of the universe. It is not known what time actually IS anymore than we know what space IS or what gravity IS. Perhaps this is a fundamental lack of understanding of physics, but we can certainly measure it well enough to make accurate predictions about the way things work.
It’s a good question without a satisfactory answer.
Another interesting question about time is whether it flows continuously like a river or whether it flips like movie frames (i.e., quantized chronons or something). Modern physics does not allow inspection of this (or of quantized space - - gravitons?) beyond the Planck Limits.
Get a watch with a second hand that sweeps… or, alternately, a watch with a digital display that counts off seconds. Sit down in a comfortable, well-lit spot. Watch the watch (as it were), and do nothing else but watch the seconds tick off, for two full minutes.
OK?
THAT was time.
(Thank you, Severin Dardin)
That hardly seems fair to your profession. Surely there must be some cosmologists & GUT/String theorists who like to ponder this question. But in the end they probably have to use the magic wand to define “space” and “time” and then get on with the more testable bits of the rest of their work.
Perhaps space and time are kind of like a mathematical axiom…a self defined truth that cannot be proven/explained within the context of its own universe.
Or a waste of time. 
*from *The Devil’s DP Dictionary by Stan Kelly-Bootle
Time* n.* That which tries to prevent everyting happening at once.
Oh, those tricks. Yeah, I’m quite aware of all the nifty things you could do if you could manipulate the fouth dimension in the same way as the first three. It’s a little more complicated than you make out, though, since our prisons, etc, aren’t really three dimensional. Realizing that is the key to being able to visualize four dimensions.
For example, visualize a cube. Got that? Good. Now, when you visualized that cube, how long did it last? A few seconds? Uh, oh, that wasn’t a cube, then, since it had duration, in addition to length, width, and height, and hence was four-dimensional. If you want a hypercube, then you’d first visualize empty space, then all of a sudden, a cube appears. Instantly, everything except the corners disappears, but the corners stick around for a little while. Then, the whole cube appears again momentarily, and everything disappears. That’s a hypercube: You’ve got 16 vertices (eight each at the first appearance and the second, 32 edges (12 spatial ones each, at the first and second appearances, and eight temporal ones, the spatial points which have a duration), 24 faces made from those edges, and eight three-dimensional cubical “hyperfaces”.
I agree, Chronos, that that would be visualizing a hypercube by “sweeping” through it timewise. This sweep would be orthoganal to one of its “hyperfaces.” However, to really understand a hypercube, one would need to be able to visualize sweeping through it from a different angle.
Clearly this is possible, and I doubt a properly-programmed computer would have much trouble coming up with an animation, but our brains just aren’t set up to do it. If a person truly comprehended the fourth dimension, imagining what a hypercube would like like from a different perspective shouldn’t be much more difficult than visualizing what a cube looks like when rotated.
I found an animated 3D shadow of a rotating 4D hypercube. They suggest that Big Bang is some sort of shock wave in 5D space. I don’t get that, but the animation is cool. I found the animation looking through the previous pictures at the Astronomy Picture of the Day, that sford posted on another thread. Thanks, sford.
Try this:
Time is progression. Progress is change. Change is as good as a rest. The rest is history.
Therefore: Time is History. QED.
Hey Chronos, you haven’t by any chance read CH Hinton’s treatise on visualizing 4D space, have you? I managed to get a color xerox copy, I started doing the exercises, which are something like a Rubik’s cube where all the colored faces are on the INSIDE of the cube. But I lost interest along the way, it was too incomprehensible.
My favorite cyberpunk author and mathematician Rudy Rucker wrote a treatise on Hinton, but I’ve searched in vain for 20 years and never found it. In one of his general math books, he described Hinton’s exercises as “a recipe for insanity.” Another reason I quit doing the exercises.
Anyway, I’m just curious if you’d ever read this and had any educated opinions to offer.