Not the magazine, but the concept so important to physics, science … heck, everything.

I always figured that time was a means of measuring motion, or, to put it another way, a way to make sense of effects and their causes. I got into a discussion tonight with people who argued that time is finite (it had a beginning) and that time was created. To my mind, any act of creation is a cause that has an effect (the created thing) and is therefore measurable by time. To say that something existed or occured before something else is to say that it exists within the realm of “time” as we understand it, does it not?

If anyone can expound upon (or even correct) my understanding of the concept of time, I’d appreciate it. Thanks!

(Moderators: I corrected a minor typo and changed the notification settings in this post. Please feel free to delete the duplicate. Thank you!)

I think the question may be more philosphical than physical but that’s never stopped me.

For starters it may help to look at time as not a different thing than space but for both those two things to be aspects of the space-time continuum. We have no way of answering the question because we have no way to conceptualize where something would be if it wasn’t in space-time. Everythign has to be somwhere, right?

We know we can distort space-time by moving really fast. We can actually observe it in the orbit of mercury and GPS satellites. They zip around so fast they no longer follow Newton’s laws of physics like slower things do. We can observe these things becuase we are in a different frame of reference. We don’t have a way of observing space-time from an outside frame of reference. God can but that changes the question.

For ideas about how time fits into physics, search on “general relativity” in the fora.

As for time being “created”, one inescapable consequence of the geometrical equations of general relativity is that if we run time backwards there must eventually be a point all paths must pass arbitrarily close to. That is, an “initial singularity”. We really can’t predict (or observe) what came before that so for all practical purposes, that was the beginning of time. To say the universe was “created” does imply an agental action, but looking from within the system itself (as physics says we must) we can describe the shape of spacetime as a whole, and it must include an initial singularity.

[quote]
[ul]
[li]“Big Bang” was a term coined originally to ridicule the idea of an expanding universe. It is misleading in that it implies an event rather than simply a “place”. Perhaps a better term is timeless singularity.[/li][li]If we travel “back through time” we come to a point where our normal laws of the universe break down (the Planck epoch). We find that as we go back we reach a point where “time” has less and less meaning, essentially “becoming” another dimension of space. The point at which this “occurs” is the timeless singularity.[/li][li]The timeless singularity did not have a “cause”, and it is an “origin” only in the strict mathematical sense: It simply exists. Now, what shall we call this system comprising a timeless singularity to which is attached a “line of time”? Let us call it the Universe.[/li][li]Our universe is the place in the Universe where time exists.[/li][li]That’s so important it needs repeating: Our universe is the place in the Universe where time (or, at least, the illusion of time) exists.[/ul][/li][/quote]

Basically, saying that time exists because X happened “before” Y is a bit like saying length in 3D space exists because A is longer than B. It’s true that A is longer than B, but that’s not the reason why length exists, length exists, therefore A is longer than B. Can length exist without A being longer than B? Why, of course.

Same for time - time has always existed, and only became a seperate dimension from 3D (10D?) space at the big bang. There is no need for a cause and effect to “create” time. Time is another dimension altogether, and the only difference is that we are always moving in one direction in time. You might as well say that “length” was created when the first measurable length was formed after the big bang.

Anyway, IANAPhysicst, so take everything with a pinch of salt. Hopefully, a REAL physicist will be along shortly.

The idea that time branches off into multiple timelines has come up in some physics theories. The “many-worlds” interpretation of quantum mechanics says that whenever two outcomes are possible, the universe splits such that one outcome happens in each universe. This interpretation has a lot of problems, though.

In cosmology, it has been suggested that when a black hole forms, a new “universe” is spun off. In our universe, looking back to the big bang (BB) is actually looking into the black hole that connects our region of spacetime to the region ours was born from, like an umbilical cord linking us to Mom.

Some older cosmologies suggested that the BB happened when a previous universe had a big crunch, and that the cycle of bangs and crunches will repeat indefinitely. This idea is no longer popular. For one thing, current measurements suggest that the universe will continue to expand forever, and so there will never be a crunch.

Well, serious physics journal articles have been written about it, but IMO these ideas are all untestable and therefore outside of science.

As regards the “big bang”: it helps if you think of time on a logarithmic scale. As time --> 0, ln(time) --> -inf. In other words, you can never reach time 0 in the same way that you can never position ln(0).

As regards what time “is”, I suppose that you first have to figure out what length “is”. If you can do that, I suspect you will have your answer.

There’s an article on this month’s Scientific American web site: The Myth of the Beginning of Time. It takes what string theory hints at, and draws up a potential scenario where our Big Bang was just the rebound from a previous Big Crunch. It notes that the existing Big Bang models can’t be trusted too far, since we don’t have a quantum theory of gravity yet, and this would be extremely significant in the early universe.

On an entirely different, metaphysical rather than physical tack, some people have suggested that God exists “out of Time” – that he doesn’t experience our existence sequentially as we do, but looks upon it from outside, kind of like you looking at a comic strip, seeing all instants at once. It eliminates alll those philosophical questions about God not knowing what will happen in the future, or being bound by Time Him(Her?)Self. An interesting point of view, and one I’d subscribe to, were I a Deity.

Of course, if your tastes run to Non-Benevolent Deities, you can always have Lovecraft’s Shadows Out of Time.

I think **encom[/] pretty much has it right in his OP - time is simply a theoretical construct that does a decent job of “explaining” motion. As some of the previous posts suggest, time probably hasn’t been defined completely - there seem to be a few “loose ends.”

So, I’d say time is merely a theoretical construct, and thus having no “objective existence” in the way that, say, a turd or a Hollywood agent does.

One could say that time was created by mathematicians, physicists, etc. I suspect your friends are just fishing for something “scientific” to bolster their Theism (witness the way so many Christian apologists splash about in the Second Law of Thermodynamics as if it were the River Jordan).

Okay, lets put it this way. Can a turd exist without the time dimension?

In fact, it would be easier to have an object exist in one space and one time dimension, rather than 3 space dimensions and no time demension. “theoretical construct”? I fail to get what you mean.

You seem to be under the impression that “the direction of time” is well-defined. While it’s true that in spacetime at any one point you can say “those directions are future, those are past, and those are ‘sideways’”, there is no well-defined direction in which time increases, or even a well-defined notion of the difference in times between two points in spacetime.

I think you’re caught up on the word time in the (misleading, shorthand) phrase “time dimension”. To put it more accurately, spacetime in GR has a symmtric 4x4 matrix g at every point called the metric. The actual entries of the matrix change when we change coordinate systems, but the geometric meaning stays the same. Specifically, they change by taking an invertible 4x4 matrix m and sending g to mgm[sup]T[/sup]. Spacetime in GR is “locally Lorentzian”, meaning that there exists a matrix m sending the metric to the diagonal matrix n with entries (-1,1,1,1). That “-1” in the metric says that in this coordinate system, the 0th basis vector (in GR you index from 0 to 3) is the direction of increasing time. This is very much not unique, since there exist matrices s such that sns[sup]T[/sup]=n, so there are coordinate changes giving different directions of increasing time, though always (geometrically, independant of any choice of coordinates) this direction will be “timelike” (tangent vectors have negative squared-length).

I fail to understand how this affects the existence of a time value in an object. With regards to a particular non-moving frame, there has to be x[sup]1[/sup], x[sup]2[/sup], x[sup]3[/sup], x[sup]4[/sup], yes? And x[sup]4[/sup] is conventionally taken to be time? Otherwise, wouldn’t the matrix be 3X3 with regards to any particular vector instead?

If I understand you correctly, you are saying that the matrix of a vector need not necessarity have to have the -1 entry in any particular position, but there needs to be one, am I right?

With the one-dimensional object, I was thinking of strings with ends fixed in 9 dimensions.

I must admit that I have only the barest understanding of GR, though, so feel free to educate me.

Yes, given a choice of reference frame there is a “direction of increasing time”, but given any timelike vector at a point p, there is a reference frame with that vector pointing in the direction of increasing time. Further, given two points in spacetime that are timelike-separated one can construct different coordinate systems that give rise to different paths – each always traveling in the direction of increasing time for its coordinate system – with different amounts of elapsed time between the points. If two observers start at point A and each follows a different one of these paths, their watches will not agree when they meet up at point B. Thus there is no coordinate-independant function that could serve as a universal time coordinate.

No, there must be a 4x4 invertible matrix m such that mgm[sup]T[/sup] is that diagonal matrix n. Alternatively, g must be of the form mnm[sup]T[/sup]. Try plugging in various invertible matrices m to see what various g matrices can look like.

As near as I can tell, this is meaningless without a lot of extra background structure you need to pecify.

[QUOTE[I must admit that I have only the barest understanding of GR, though, so feel free to educate me.[/QUOTE]

As I said earlier, search the fora on “general relativity”. I myself have made three or four extensive posts on the ideas within the last few days.

Right, however, the theists I am currently debating want God to exist out of Time, seeing all instants at once, while also allowing humans free will.

Just as Calvin cannot change his actions in the last frame of Bill Watterson’s strip, neither can humans truly change their future in a universe run by a perfectly omniscient, infallibly prescient God.