Time Moves Forward for Obvious Reason?

[DaveC426913]

One can only count events not measure their duration because a duration is defined as a counted number of events.

I'm not sure I follow.

Just because a duration is defined as a counted number of events, how does that mean we can't measure their duration?

Say I wanted to measure the duration of the lifetime of a neutron, from its creation to its decay. I count two events. But I could compare the duration between those two events with the lifetimes of a dozen other neutrons doing the same thing.

 

[Islam Hassan]

Another observation about time that some may have an interest in:

One can only count events not measure their duration because a duration is defined as a counted number of events.

Perhaps at an arithmetical level, but then if we make these events infinitely small, don't we tend towards a smooth, continuous definition of time? I take it that we can measure time in such a manner that it is not granular.

IH

 

[nitsuj]

Another observation about time that some may have an interest in:

One can only count events not measure their duration because a duration is defined as a counted number of events.

Wasn't it Wheeler that said time is what keeps everything from happening at once lol?

From the context I think your speaking from, can't the same comment be said of length.

 

[DaveC426913]

Perhaps at an arithmetical level, but then if we make these events infinitely small, don't we tend towards a smooth, continuous definition of time? I take it that we can measure time in such a manner that it is not granular.

IH

The lower limit is the Planck time: ~10^-43 second.

 

[Islam Hassan]

The lower limit is the Planck time: ~10^-43 second.

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]

Say I wanted to measure the duration of the lifetime of a neutron, from its creation to its decay. I count two events. But I could compare the duration between those two events with the lifetimes of a dozen other neutrons doing the same thing.

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]

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]

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]

Since spacetime itself is as static as a painting, both may be illusions.

Can you explain what you mean by this statement.

 

[James_Harford]

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]

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]

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]

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]

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:

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):

{(ds)}^{2} = {(dx)}^{2} + {(dy)}^{2}+ {(dz)}^{2} - {(cdt)}^{2}

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 +(cdt)^{2}. But it isn't, and that's the end of it.

 

[James_Harford]

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]

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]

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

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

dX4 = cdt
dX1 = vdt

 

[petm1]

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]

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]

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]

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]

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.

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]

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]

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.

 

[bobc2]

I have no way to restate that in my own words. Since incomprehension appears to be mutual, adieu.

I don't know how you could have made it more clear, James_Harford. Thanks for the comments.

 

[bobc2]

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.

That is not an altogether bad thought, sahmgeek. If you think about it, we don't really observe time directly. We are always observing successive points on a 4-dimensional object. Often the 4-dimensional object has a world line that extends for billions or trillions of miles along its 4th dimension. And the periodic squiggles back and forth in the X1 and X2 dimensions (i.e., the tip of a pendulum) offer equally spaced points along the 4-dimensional world line (a 4th dimension ruler) that we can calibrate as time, knowing that dt = dX4/c.

So, in a way, we are using rulers for measuring lengths along dX1, dX2, dX3, and also dX4.

 

[James_Harford]

I don't know how you could have made it more clear, James_Harford. Thanks for the comments.

No problem. Take care.

 

[jfy4]

James,

I was hoping for a more detailed answer before, since I was hoping to have a bit of a discussion... I hope you will come back and talk about this idea you have in more detail, even if apparently you feel there are no more details to talk about.

I will pry a little more at what confuses me with what you are saying...

Your claim is that the spacetime manifold is static, but to me you appear to draw this conclusion from premises that have little to do with your conclusion...

To be dynamic requires a second dimension of time in which it can change...

I'm understanding this to say "dynamics is about evolution in time". And you seem familiar with GR, so I 'm taking this to mean that dynamics is about evolution in time according to an observer (not a global time variable). am I correct?

The manifold itself is not embedded in another time dimension

Like a meta-time?

hence its complete description (all of space and all of time) is static.

This implication from "hence" on-ward I think is unfounded, since we didn't define dynamics as evolution in meta-time to begin with... So even if gravity doesn't evolve in some sort of meta-time, we have never thought of dynamics that way.

If you want to stick to evolution in time, The Einstein equation does say that the gravitational field strength (the observable) changes first order in the time parameter, and that change is proportional to the amount of stress-energy-momentum around it.

If I've mistook something you've said I hope you will come back and explain it again. Sorry if I'm being dense.

Thanks,

 

[nitsuj]

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...

We don't talk about measuring space (or do we?) but rather things in it. The same should be true for time.

Very well said!

 

[James_Harford]

I was hoping for a more detailed answer before, since I was hoping to have a bit of a discussion...

No problem. I myself didn't really understand your question which seemed more statement than question. My two-word response was meant to prompt a follow-up, but I guess it was a little too terse.

I will pry a little more at what confuses me with what you are saying...

Your claim is that the spacetime manifold is static, but to me you appear to draw this conclusion from premises that have little to do with your conclusion...

Just to get off to a running start, let me begin with a blatant appeal to authority -- Einstein would love that!

First quote:

People like us, who believe in physics, know that the distinction between past, present and future is only a stubbornly persistent illusion.

- Albert Einstein​

Letter of condolence to Michele Besso's family (15 Mar 1955). InTabatha Yeatts, Albert Einstein (2007), 116.
Science quotes on: *| *Physicist (53)
​

Second quote:

The four-dimensional structure (Minkowski-space) is thought of as being the carrier of matter and of the field. Inertial spaces, with their associated times, are only privileged four-dimensional coordinate systems that are linked together by the linear Lorentz transformations. Since there exist in this four-dimensional structure no longer any sections which represent “now” objectively, the concepts of happening and becoming are indeed not completely suspended, but yet complicated. It appears therefore more natural to think of physical reality as a four-dimensional existence, instead of, as hitherto, the evolution of a three-dimensional existence.

Relativity and the Problem of Space, from the revised edition of Relativity, the Special and the General Theory: A Popular Exposition. Albert Einstein. Translated by Robert W. Lawson. London: Methuen, 1954
​

I'm understanding this to say "dynamics is about evolution in time". And you seem familiar with GR, so I 'm taking this to mean that dynamics is about evolution in time according to an observer (not a global time variable). am I correct?

Like a meta-time?

Evolution, yes, in the sense that Einstein used it in the quote. The italics are his to contrast "evolution" with mere "existence", which does not evolve and hence "static".

Meta-time, no. That would be outside the physics of the spacetime, wouldn't you agree?

The only way that spacetime can evolve on its own is to be part of the physics of a larger 5-dimensional space with a second dimension of time in which it can evolve. That would be bizarre, since it suggests a past that is not static. But since this is not the case, spacetime merely "exists" and does not "evolve".

Sorry if I'm being dense.

Far from it. Judging from your statements, I can learn more from you.