# Time Moves Forward for Obvious Reason?

by PMichaud
Tags: big bang, direction of time, time
 P: 1 I tried to pick the most appropriate subforum, feel free to move this if there's a better one! I'm not a physicist, but I had an epiphany recently that I've never see anywhere else, and it seems so incredibly obvious that I think it's either the accepted theory of time directionality, or I'm missing something huge. According to Einstein we have a thing called spacetime which is 4 dimensional. In much the same way travelling North West will slow down progress toward the North (given the same travel speed), travelling through space slows down your progress through time proportional to how much of your motion you're diverting into motion through space instead of motion through time. So the first part is that we're living in this 4 dimension spacetime stuff, and we can travel along any of the 4 dimensions. We'll come back to that. Consider a particle flying away from a big explosion, like a chunk of MacGuffinium. Just for the sake of argument, say this chunk of stuff can consider its situation. It notices as it's living its life that it can move pretty freely along a 2D plane, but that if it "stops" trying to move, it's still moving very quickly in a particular, outward direction (we know the direction is away from the explosion, but the chunk doesn't). Whenever he moves from side to side he can measure that he's going infinitesimally more slowly in that outward direction. He can't seem to go back in that direction--the force is too great by far--but he can make (barely) measurable impact on the velocity by moving through the 2 dimensions he's free to move around. So back to spacetime. If we live in a universe that began with an enormous explosion into a 4 dimensional substrate, doesn't it make sense to consider that time is the "outward" direction of that explosion, and that we are travelling along that axis, in this direction, because of the force of that explosion? Wouldn't that explain why we can move relatively freely in the other 3 dimensions, but not so much in the 4th dimension? That would explain why time is moving the direction it seems to be moving, even though the equations don't demand it, and it explains why we can't really go back or forward on that dimension (extreme velocity from the explosion, relative to our puniness). It may also explain why we perceive time as qualitatively different than the other 3 dimensions: we're travelling super duper fast in that dimension, so we're experiencing it it a really distorted, compressed way in comparison to the other 3 dimensions. So... is this an accepted thing, am I missing something huge, or did I just blow open an old problem with an obvious explanation?
 PF Patron P: 4,163 The issue you are talking about is usually called "The Arrow of Time" and it has nothing to do with your epiphany. It has to do with the fact that all the equations describing physical process that have time in them can be solved with time flowing in the negative direction just as easily as in the positive direction. To talk about the time dimension being just like the three spatial dimensions in the context of motion ignores the fact that speed is distance divided time--isn't it obvious that they are different? Distance is the numerator, time is the denominator. In all the equations involving spacetime, the time coordinate is always treated differently, usually with a negative sign in front of it. Of course, that doesn't keep a lot of people from insisting that there's no difference between space and time, as you'll soon see by some of the posts that will follow on this thread.
P: 846
 Quote by PMichaud ...So back to spacetime. If we live in a universe that began with an enormous explosion into a 4 dimensional substrate, doesn't it make sense to consider that time is the "outward" direction of that explosion, and that we are travelling along that axis, in this direction, because of the force of that explosion? Wouldn't that explain why we can move relatively freely in the other 3 dimensions, but not so much in the 4th dimension? That would explain why time is moving the direction it seems to be moving, even though the equations don't demand it, and it explains why we can't really go back or forward on that dimension (extreme velocity from the explosion, relative to our puniness).
I agree with ghwellsjr; our concept of time is totally different from our concept of space (although our concept of time seems to be limited at a fundamental philosophical level compared to our concept of space).

I'll just mention a couple of considerations that might have a bearing on some of your ideas about motion through space-time. Observers having different relativistic velocities with respect to each other all measure the same speed for light. This idea is illustrated in the sketches below, a sequence of space-time diagrams depicting observers moving at increasing velocities with respect to a black rest frame of reference. Each space-time diagram is kind of like a picture of a piece of a 4-dimensional universe, where one observer (black rest system) moves along his X4 coordinate and another observer (blue system) moves along his X4 coordinate (the path that a given observer follows in the 4-D universe is referred to as his world line).

Notice that the 45-degree photon world line always bisects the angle between the X4 axis and the X1 axis for all observers depicted in the sketches below. Thus, the ratio of a change in distance along the X4 to a change in distance along the X1 is: dX4/dX1 = 1 for all observers (that's why everyone measures the same speed for light). That means that dX4 = dX1. Speed along X1 is always c. Thus, dX1 = dX4 = cdt. It is often said that the observer moves along his own X4 axis at the speed of light:

dX4/dt=c.

Hermann Weyl expresses the situation as 4-dimensional objects (with their world lines) existing in a 4-D universe with the consciousness of an observer (3-dimensional) moving along its world line at the speed of light ("Mind and Nature", selected writings by Hermann Weyl, edited by Peter Pesic, Princeton University Press, Chapter 3, hardback pg 32). This picture is one of a static 4-dimensional universe--no motion associated with objects, thus no actual explosion (no big bang, other than its description as a point on a 4-dimensional universe structure).

P: 131

## Time Moves Forward for Obvious Reason?

 Quote by ghwellsjr The issue you are talking about is usually called "The Arrow of Time" and it has nothing to do with your epiphany. Of course, that doesn't keep a lot of people from insisting that there's no difference between space and time, as you'll soon see by some of the posts that will follow on this thread.
I thought that the arrow of time was a macro-matter of entropy and not relativity, the fact that the most likely outcome of a macro-physical interaction is one which will increase disorder, which we interpret as "forward" in time. On the other hand, I understand that at the elementary particle level, the arrow of time cannot be ascertained, that we perceive all outcomes are equally likely.

As for space, my understanding is that it is distinct from time but indispensable for time to be defined since time can be thought of as the pacing of any movement in space with respect to a standard movement like a clock.

Don't know if that helps though...

IH
 P: 1,065 yea that's what I thought too Islam Hassan, that the "arrow of time" is nearly another term for entropy. And that in classical physics there is no "arrow of time". not sure what it means to say; the arrow of time means that the it makes no difference to "physical processes" which way time flows. In fact, clearly arrow of time reffers to entropy, where there is an apparent direction of physical processes as a whole.
 P: 422 If you disregard the details of what we know about uncertainty, quantum behavior, discontinuities, determinism, and other things, you can do some simple thought experiments that bear on the direction of time... If you open a bottle of perfume in a still room, the molecules in the bottle will eventually fill the room (and the bottle in the room) with an even balance of distribution. If you could take a snapshot of the room after this happens, you would have a "configuration" of the molecules' positions and speed directions. Assuming continuity and determinism, there will be an infinite number of these configurations between any two snapshots, and every one of them could be reversed and you would expect that at some point later all the molecules would get back into the bottle. If you took a particular snapshot 4 hours after opening the bottle and reversed all the molecules, you would see the molecules all back in the bottle after four hours... So the number of possible configurations of molecules in the room that end up with them back in the bottle is infinite, but the number that don't seem to outnumber those that do by a huge margin. Think about this in cases where the number of molecules starting inside the bottle is only 1, or 2 or 3... and then many more. With just one molecule, you don't have to wait very long for it to make its way back into the bottle, for two, a long time, for three, much longer... Likewise, if you imagine every possible configuration of the universe, each can be mapped to a point within a complex entity called phase space. Two points infinitely near each other in this space will represent very similar configurations. Changes from one configuration to another are represented by continuous line paths from one point in this space to another. The paths may not "jump", but must be continuous - and the allowable paths are interpreted as the "laws of physics". In terms of entropy, the vast majority of points (configurations) in this phase space would be characterized as very high entropy regions, and very few points would be characterized as being in a low entropy region. Statistically, the overwhelming likelihood is that allowable paths from one point to another (one configuration to another) will result in the new point location (configuration) being in a region of same or more entropy. It is possible for a relocation in phase space to end up in a lower entropy region following allowable paths, just hugely unlikely. "Time" sort of becomes the alignment of direction from less to more entropy in the local entropy gradient.
P: 846
 Quote by bahamagreen ...Statistically, the overwhelming likelihood is that allowable paths from one point to another (one configuration to another) will result in the new point location (configuration) being in a region of same or more entropy. It is possible for a relocation in phase space to end up in a lower entropy region following allowable paths, just hugely unlikely. "Time" sort of becomes the alignment of direction from less to more entropy in the local entropy gradient.
Very nice analysis, bahamagreen. Thanks.
 P: 1,555 I think time really does not move at all. Time for any observer is simply the path distance between two events in spacetime. However the order in which events happen may not be the same for different observers. So I do not see why we even need the concept of time moving forward or backward whatever that might mean.
P: 846
 Quote by Passionflower I think time really does not move at all. Time for any observer is simply the path distance between two events in spacetime. However the order in which events happen may not be the same for different observers. So I do not see why we even need the concept of time moving forward or backward whatever that might mean.
After all, it is not as though time was some kind of object capable of motion. I remember you mentioning once the idea that passing of time is an illusion. You are probably in pretty good company with that idea--people like Kurt Godel. --and Einstein (a stubborn persistent illusion)? But, I think you are skating close to getting the thread locked, Passionflower.
P: 15,294
 Quote by Passionflower I think time really does not move at all.
Who thinks that?

Time does not move; things move through time.
PF Patron
P: 1,366
 Quote by bahamagreen If you took a particular snapshot 4 hours after opening the bottle and reversed all the molecules, you would see the molecules all back in the bottle after four hours...
Does not seem very plausible. Molecules are bouncing from the walls. And walls in turn are emitting black body radiation. And gas of molecules is emitting some tiny amount of black body radiation as well.
So I would say that this won't happen if you do not reverse environment as well.
P: 131
 Quote by DaveC426913 Time does not move; things move through time.

Correct me if I'm wrong but I believe you mean that in a metaphorical sense. To my mind, things move through space and we pace or perceive them as moving through "time" through a comparison with another thing moving through space. That second thing is taken as a standard movement reference which we call a clock.

Make sense or not?

IH
P: 422
If you took a particular snapshot 4 hours after opening the bottle and reversed all the molecules, you would see the molecules all back in the bottle after four hours...

IH[/QUOTE]

 Quote by zonde Does not seem very plausible. Molecules are bouncing from the walls. And walls in turn are emitting black body radiation. And gas of molecules is emitting some tiny amount of black body radiation as well. So I would say that this won't happen if you do not reverse environment as well.
I know... the thought experiment implies lots of simplifications and assumptions.
But the point was to see that there are configurations that do lead to all of them back in the bottle - in the thought experiment the symmetric reversal of any snapshot after opening the bottle is just a way of noticing that every possible reversed snapshot throughout the remaining history of the universe after opening the bottle IS one of those snapshots that gets them back in the bottle.
If you drop the assumptions about determinism, uncertainty, and include everything (like the black body radiation, neutrino and cosmic alpha bombardment, tidal effects, the vibration of the walls etc.) you STILL have snapshots that will result in getting it all back in the bottle - those that DO include every possible influence. The symmetry reversal of getting out and getting in to the bottle was just a means of seeing this.
For example, even if you did include all effects and influences, if you observed all of it getting back into the bottle... then you could reverse those historical paths from there and they would include any and all effects and influences back to the time of opening the bottle (long time) within which would be an infinite collections of snapshots that when reversed would get to the bottle refilled result.
P: 846
 Quote by DaveC426913 Who thinks that? Time does not move; things move through time.
What kind of medium is time that things would move through it?
P: 1,065
 Quote by bobc2 What kind of medium is time that things would move through it?
Jumping in, sorry Dave.

I think DaveC426913 maybe saying that things move through space, which has the property of a limited, invariat, isotropic speed. Time is not it's own "medium".
P: 15,294
 Quote by bobc2 What kind of medium is time that things would move through it?
Not a medium, a dimension. You agree that things move through space, yes?
 PF Patron P: 647 The general consensus between objects that we all travel in the same "direction" of time has to do with the fact that almost everything has positive energy, which is the component of temporal momentum in the momentum 4-vector. That is, everything for some reason has the same sign of temporal momentum (except some might argue, anti-particles). The structure of the "direction" though is not a property of our energy content, its a property of the gravitational field.
P: 1,065
 Quote by jfy4 The structure of the "direction" though is not a property of our energy content, its a property of the gravitational field.
What does that mean?

Is it in the same sense that mathematically time in "reverse" doesn't mean gravity becomes "anti-gravity"?

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