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Time Moves Forward for Obvious Reason?

by PMichaud
Tags: big bang, direction of time, time
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James_Harford
#37
Feb6-12, 11:37 PM
P: 67
Quote Quote by DaveC426913 View Post
Who thinks that?

Time does not move; things move through time.
Time moves, we move -- is there a difference? Since spacetime itself is as static as a painting, both may be illusions.
James_Harford
#38
Feb7-12, 12:08 AM
P: 67
Quote Quote by Islam Hassan View Post
I wonder is the lower limit of Planck time a proposition which may be subject to experimental proof? If not, if it's a universal "given", then Passionflower's comment has some substance to it...

IH
Given the discrete nature of information, add the hypothesis that all that is real is describable, and you get a general principle akin to Passionflower's insightful observation about continuity.
jfy4
#39
Feb7-12, 12:12 AM
jfy4's Avatar
P: 647
Quote Quote by James_Harford View Post
Since spacetime itself is as static as a painting, both may be illusions.
Can you explain what you mean by this statement.
James_Harford
#40
Feb7-12, 12:29 AM
P: 67
Quote Quote by jfy4 View Post
Can you explain what you mean by this statement.
Sure.

The hypothetical spacetime manifold of which we are a part includes all of space and all of time. Its description is static. To be dynamic requires a second dimension of time in which it can change -- and that's not in its job description.
bobc2
#41
Feb7-12, 05:37 AM
P: 848
Quote Quote by jfy4 View Post
Can you explain what you mean by this statement.
James_Harford makes a very salient point here. And it certainly fits with Passionflower's comment.

Hermann Weyl's (Einstein's colleague and close friend) picture describes a static 4-dimensional universe occupied by static 4-dimensional objects (filiament-like objects strung out along the 4th dimension for billions and trillions of miles, called the world lines). Our bodies are 4-D objects of that kind with consciousnesses moving along the world lines at the speed of light (as time passes). The parametric equations for a photon straight world line is:

dX4 = cdt
dX1 = cdt

dX4/dX1 = c

The parametric equations for a normal body straight (inertial) world line are:

dX4 = cdt
dX1 = vdt


You can google for "Block Universe" to find more information on this idea. But, now we are approching a subject that may be considered to have zero physics content, subject to lock-down by the forum monitor. So, here is a summary of the concept, after which I will have no more discussion. This block universe model is in the context of a spatial 4th dimension (consciousness moves along the 4th dimension as time passes).

harrylin
#42
Feb7-12, 06:44 AM
P: 3,187
Quote Quote by PMichaud View Post
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. [..]
More or less so... To avoid misunderstanding: according to him and many other physicists, time is what we measure with clocks, and distance is what we measure with rulers. And clocks (at least, good clocks) accumulate "time" - that's how they operate.
Now about space-time, he clarified that concept as follows to non-physicists:
Space is a three-dimensional continuum. [..] Similarly, the world of physical phenomena which was briefly called “world” by Minkowski is naturally four-dimensional in the space-time sense. For it is composed of individual events, each of which is described by four numbers, namely, three space co-ordinates x, y, z and a time co-ordinate, the time-value t.
- http://www.bartleby.com/173/17.html
(emphasis mine)

PS welcome to physicsforums!
jfy4
#43
Feb7-12, 09:32 AM
jfy4's Avatar
P: 647
Quote Quote by James_Harford View Post
Sure.

The hypothetical spacetime manifold of which we are a part includes all of space and all of time. Its description is static. To be dynamic requires a second dimension of time in which it can change -- and that's not in its job description.
You're assuming a global time variable, are you not? And this is in direct contradiction to the very description you cite...
James_Harford
#44
Feb7-12, 09:37 AM
P: 67
Quote Quote by bobc2 View Post
Hermann Weyl's (Einstein's colleague and close friend) picture describes a static 4-dimensional universe occupied by static 4-dimensional objects
A very nice description. I have a minor correction and an observation:

1. Correction:
Quote Quote by bobc2 View Post
The parametric equations for a photon straight world line is:

dX4 = cdt
dX1 = cdt

dX4/dX1 = c

The parametric equations for a normal body straight (inertial) world line are:

dX4 = cdt
dX1 = vdt
These are dimensionally consistent in this form:

Photon

dX4 = cdt (change in time)
dX1 = cdt (change in position)

dX1/dX4 = 1 (speed of light)

Non-photon

dX4 = cdt (change in time)
dX1 = vdt (change in position)

dX1/dX4 = v/c (speed of non-photon)


2. Observation: Minkowski's use of the mysterious looking ict has been out of fashion for such a long time that it is probably better not to mention it at all (except for historical reasons). The crucial difference between "ordinary space" and Minkowski space is perhaps best described in terms of the formula describing the interval ("distance"), ds, between two point-events in Minkowski space (here in x,y,z,t coordinates):

[itex]{(ds)}^{2} = {(dx)}^{2} + {(dy)}^{2}+ {(dz)}^{2} - {(cdt)}^{2}[/itex]

This is almost the Pythagorian Theorem, but with a perverse minus sign in the last term. If spacetime were Euclidean (i.e. "ordinary", the last term would be [itex]+(cdt)^{2}[/itex]. But it isn't, and that's the end of it.
James_Harford
#45
Feb7-12, 09:44 AM
P: 67
Quote Quote by jfy4 View Post
You're assuming a global time variable, are you not? And this is in direct contradiction to the very description you cite...
Am I?
James_Harford
#46
Feb7-12, 10:29 AM
P: 67
Quote Quote by James_Harford View Post

Non-photon

dX4 = cdt (change in position)
dX1 = vdt (change in time)

v = dX4/dX1 (speed of non-photon)
Drat! I left out the fix to the Non-photon formulae (details, details!):

Fixed now in the earlier posting. I'm still learning. Sorry!
bobc2
#47
Feb7-12, 06:00 PM
P: 848
Quote Quote by bobc2 View Post
The parametric equations for a photon straight world line are:

dX4 = cdt
dX1 = cdt
CORRECTIONS:
dX4/dX1 = c Wrong!! (Don't know what I was thinking while typing that one)

Should be:

dX4/dt = c

Quote Quote by bobc2 View Post
The parametric equations for a normal body straight (inertial) world line are:

dX4 = cdt
dX1 = vdt
petm1
#48
Feb8-12, 11:41 AM
petm1's Avatar
P: 366
Time moves, we move -- is there a difference? Since spacetime itself is as static as a painting, both may be illusions.

Space is expanding, time is dilating, while static is an illusion of relative motion. As for the stubbornly persistent illusion of our static universe, it has more to do with the backward view of seeing photons as if at emission instead of absorption, because we do not see the motion of photons in waves we see objects.
James_Harford
#49
Feb8-12, 02:21 PM
P: 67
Quote Quote by petm1 View Post
Space is expanding, time is dilating, while static is an illusion of relative motion.
Here is a second attempt to explain why spacetime is (by definition) static:

Anything that happens in time is in the spacetime manifold. The manifold itself is not embedded in another time dimension, hence its complete description (all of space and all of time) is static.

If it were otherwise, spacetime -- all of our space and all of time -- would change along that separate time dimension, and past history would be observed to change continuously.
sahmgeek
#50
Feb8-12, 03:34 PM
P: 65
Quote Quote by harrylin View Post
More or less so... To avoid misunderstanding: according to him and many other physicists, time is what we measure with clocks, and distance is what we measure with rulers. And clocks (at least, good clocks) accumulate "time" - that's how they operate.
Now about space-time, he clarified that concept as follows to non-physicists:

Space is a three-dimensional continuum. [..] Similarly, the world of physical phenomena which was briefly called “world” by Minkowski is naturally four-dimensional in the space-time sense. For it is composed of individual events, each of which is described by four numbers, namely, three space co-ordinates x, y, z and a time co-ordinate, the time-value t.

- http://www.bartleby.com/173/17.html
(emphasis mine)

So, are we safe in asserting that "time" itself is no more measurable than "space" itself is measurable? But rather, it is the EVENTS that occur within spacetime, the dynamic nature of physical phenomena, that necessitate measurement. We don't talk about measuring space (or do we?) but rather things in it. The same should be true for time. I suspect people struggle with the dimension of time more b/c it's more challenging conceptually.

It's very possible that I am WAY off (my comprehension is mainly conceptual, not technical). If so, please be gentle.
sahmgeek
#51
Feb8-12, 03:43 PM
P: 65
sidenote: for some reason, the reference to "clocks measuring time" never sits well with me. it doesn't bother me nearly as much as "rulers measuring distance" which seems slightly more accurate. it's unclear to me what clocks are actually measuring. I find it easier to have a conversation about the nature of time when the whole notion of "clocks" is removed. Perhaps I am just lacking in proper "history of time-keeping" knowledge.
harrylin
#52
Feb8-12, 04:47 PM
P: 3,187
Quote Quote by sahmgeek View Post
So, are we safe in asserting that "time" itself is no more measurable than "space" itself is measurable? But rather, it is the EVENTS that occur within spacetime, the dynamic nature of physical phenomena, that necessitate measurement. We don't talk about measuring space (or do we?) but rather things in it. The same should be true for time.[..] It's very possible that I am WAY off (my comprehension is mainly conceptual, not technical). If so, please be gentle.
You could be way off, but if so, I don't notice it (yet): for your remark sounds insightful to me.
Early people recorded events by means of positions of Sun and moon, and thus the "time" concept emerged based on observing (and counting in one direction!) the motion of natural clocks.
Quote Quote by sahmgeek View Post
sidenote: for some reason, the reference to "clocks measuring time" never sits well with me. it doesn't bother me nearly as much as "rulers measuring distance" which seems slightly more accurate. it's unclear to me what clocks are actually measuring. I find it easier to have a conversation about the nature of time when the whole notion of "clocks" is removed. Perhaps I am just lacking in proper "history of time-keeping" knowledge.
Clocks actually are a measure of, as you already hinted at, the progress of natural processes. Thus we have the solar clock, (moon) months, water clocks and "radio clocks" such as C14 as well as "atomic clocks". However, people also have an intuition of a "flow of time" that doesn't exactly correspond to clocks; perhaps that difference in perception is what bugs people. In physics we can only deal with clocks.
petm1
#53
Feb8-12, 06:01 PM
petm1's Avatar
P: 366
for some reason, the reference to "clocks measuring time" never sits well with me. it doesn't bother me nearly as much as "rulers measuring distance" which seems slightly more accurate. it's unclear to me what clocks are actually measuring.
Clocks, using a photon for a relative measure of distance, are very accurate at explaining the duration between events at the edge of our universe that we do see.

Anything that happens in time is in the spacetime manifold. The manifold itself is not embedded in another time dimension, hence its complete description (all of space and all of time) is static.

If it were otherwise, spacetime -- all of our space and all of time -- would change along that separate time dimension, and past history would be observed to change continuously.

In the fourth dimension, time, we appear as a solid four dimensional object yet in space we see the same object as 3 dimensional and what do both of these objects have in common but the center of their own separate durations. Past history does change continuously from the future through the present into the past, if the present were the surface, or edge of matter I see, then the solid is inside with the signals from some other present waiting to get in, the future.
James_Harford
#54
Feb8-12, 08:01 PM
P: 67
Quote Quote by petm1 View Post
In the fourth dimension, time, we appear as a solid four dimensional object yet in space we see the same object as 3 dimensional and what do both of these objects have in common but the center of their own separate durations. Past history does change continuously from the future through the present into the past, if the present were the surface, or edge of matter I see, then the solid is inside with the signals from some other present waiting to get in, the future.
I have no way to restate that in my own words. Since incomprehension appears to be mutual, adieu.


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