Time Moves Forward for Obvious Reason?

Islam Hassan

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

petm1

Using this same thought for the duration of a particle’s lifetime, how about a photon, because this is a simple way to describe what I "see" as space? As for duration it is always the common denominator you can count as one.

James_Harford

Time moves, we move -- is there a difference? Since spacetime itself is as static as a painting, both may be illusions.

James_Harford

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

Can you explain what you mean by this statement.

James_Harford

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

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

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

PS welcome to physicsforums!

jfy4

You're assuming a global time variable, are you not? And this is in direct contradiction to the very description you cite...

James_Harford

A very nice description. I have a minor correction and an observation:

1. Correction:
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

Am I?

James_Harford

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

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

Should be:

dX4/dt = c

petm1

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

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

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

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.