# The Nature of Time

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On 12/30/2022 at 4:00 AM, swansont said:

You’ve been given examples of clocks that have no movement, or are not based on movement (i.e. the movement is incidentall)

Yes, but Genady admitted that he doesn't know how one would work or even how one would present its duration to the clock user. I think there is a problematic philosophical issue regarding what a clock with no movement would measure.

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1 hour ago, addison said:

Genady admitted that he doesn't know how one would work

No, I did not,

1 hour ago, addison said:

or even how one would present its duration to the clock user

Yes, I admitted that I don't know the answer. I didn't know it because I didn't think about it in detail. After that, @Markus Hanke and @swansont have answered more specifically.

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3 hours ago, addison said:

Yes, but Genady admitted that he doesn't know how one would work or even how one would present its duration to the clock user. I think there is a problematic philosophical issue regarding what a clock with no movement would measure.

I do; I used to build them. Spin-flip in atoms does not rely on movement.

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3 hours ago, addison said:

I think there is a problematic philosophical issue regarding what a clock with no movement would measure.

You really don’t need to bang your shinbone on a stool that’s not in the way.

The answer is simply that it measures the fact that a period of time has passed. This is in no way philosophically problematic, precisely because we don’t equate “time” with any specific clock mechanism. It’s only if you try to redefine physical time as “movement”, as you suggest, that you end up with all sorts of philosophical, mathematical, and physical issues and tautologies, because that’s simply not a good model of the world around us - there are plenty of specific examples of systems evolving without any “movement”.

The crucial point here is that this is true for all clocks, entirely irrespective of what their internal mechanisms actually are. A digital wrist watch, an atomic clock, a decaying elementary particle, or the flipping of spins all show the same fact that time passes (ie that systems evolve and age into the future), and that this is entirely separate from any specific mechanism used to measure it. It’s as true for periodic motion as it is for motionless systems. You can see this even more clearly when you compare clocks by placing them at different points within a gravitational field - gravitational time dilation affects all clocks equally, irrespective of their internal mechanisms (or lack thereof).

A vastly more interesting and pertinent question is whether - and in what sense - time (and also space, for that matter) is fundamental to the universe, or whether it is emergent from something more fundamental that is not in itself spatio-temporal in nature. This is still an open question, and very much subject to debate within the physics (and philosophy) community.

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Posted (edited)
5 hours ago, addison said:

Yes, but Genady admitted that he doesn't know how one would work or even how one would present its duration to the clock user. I think there is a problematic philosophical issue regarding what a clock with no movement would measure.

Can you decribe what you think this problem might be ?

As is so often the case thermodynamics in general and temperature in particular provides counterexamples.

A traditional mercury thermometer measures temperature by expansion movement.

A modern non contact thermometer may measure temperature colour, and a null balance electronic thermometer may measure temperature by potentiometric opposition.

BTW clock's don't measure time they measure time difference.
It is impossible to measure time with a single clock reading.

Edited by studiot
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You actually don't need movement. The depletion of material being a well-known example of clock particularly suitable to measure the age of rocks or stuff of biological origin --using the exponential function-- or a linear function --rate of mutations.

Curiously enough, it's always one kind or another of exponential --periodic phenomena are ruled by a complex exponential... Coincidence?

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On 12/29/2022 at 6:21 PM, Genady said:

A ruler does not measure distance, a ruler is distance?

A scale does not measure weight, a scale is weight?

A thermometer does not measure temperature, a thermometer is temperature?

A depth gage does not measure depth, a depth gage is depth?

A pressure gage does not measure pressure, a pressure gage is pressure?

A speedometer does not measure speed, a speedometer is speed?

Etc.

Yes to all of the above. Take time for an example. When someone asks you the time you refer to your wrist clock, you read the numbers on the dial and tell him the time therefor the clock is the time. Its the same for all of your questions.

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4 minutes ago, addison said:

therefor the clock is the time

This "therefore" is unjustified.

If somebody asks me about dinosaurs, I read the answer in Wikipedia, therefore Wikipedia is dinosaurs.

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On 12/22/2022 at 10:14 AM, Phi for All said:

Can you use your idea to calculate the height of a geosynchronous orbit?

Yes, time is valid as used in GR the only difference is that time should not be included on our graphs as a dimension. If displayed on a dimension it becomes easy to think of it as fundamental.

As far as who are the people who believe that time is fundamental I did a google search for "time as fundamental" and discovered several entries of people arguing time as fundamental. The biggest surprise to me was the following entry:  "Lee Smolin of the Perimeter Institute for Theoretical Physics in Canada, have come to believe that the fabric of reality is perhaps due to be torn into yet again. In this interview with Robert Lawrence Kuhn for the series Closer to Truth, Smolin discusses how developments in quantum mechanics have left physicists with questions that special relativity can’t seem to accommodate, and why the solution might be a conception of reality in which time is fundamental, and space emergent. "

Markus Hanke said:
The very existence of gravitational radiation depends on the reality of time (in the GR sense), so any type of gravitational wave detector is in effect an instrument that demonstrates the existence of “time” in a rather direct way.

Isn't this a statement about the existence of time as a real part of the universe?

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2 hours ago, addison said:

Yes, time is valid as used in GR the only difference is that time should not be included on our graphs as a dimension.

That’s a contradiction. Time in GR is a dimension - if you don’t include it as such, the model no longer works.

2 hours ago, addison said:

Isn't this a statement about the existence of time as a real part of the universe?

It’s a statement about time being critical to classical gravitation, ie General Relativity. You simply cannot eliminate time as a dimension, thereby reducing the dimensionality of the universe (in the classical domain) to 3, and still expect GR to provide a model of real-world gravity.

That being said, the question as to what happens once you go beyond the classical domain, into semi-classical and ultimately quantum gravity, is interesting and quite valid. Some candidate models for quantum gravity do hint to time/space, or some combination of these, not being fundamental but in some sense emergent. This is very much speculation, though.

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2 hours ago, addison said:

Yes, time is valid as used in GR the only difference is that time should not be included on our graphs as a dimension. If displayed on a dimension it becomes easy to think of it as fundamental.

As far as who are the people who believe that time is fundamental I did a google search for "time as fundamental" and discovered several entries of people arguing time as fundamental. The biggest surprise to me was the following entry:  "Lee Smolin of the Perimeter Institute for Theoretical Physics in Canada, have come to believe that the fabric of reality is perhaps due to be torn into yet again. In this interview with Robert Lawrence Kuhn for the series Closer to Truth, Smolin discusses how developments in quantum mechanics have left physicists with questions that special relativity can’t seem to accommodate, and why the solution might be a conception of reality in which time is fundamental, and space emergent. "

Markus Hanke said:
The very existence of gravitational radiation depends on the reality of time (in the GR sense), so any type of gravitational wave detector is in effect an instrument that demonstrates the existence of “time” in a rather direct way.

Isn't this a statement about the existence of time as a real part of the universe?

You may be interested to know that Lee Smolin changed his mind about this question relatively recently.

If you do a search of "time as fundamental" you're likely to find as many entries as for "time as emergent," which goes to prove that the question is far from settled.

If you propose time as fundamental, your first order of business should be answering why it is that it appears to be a dimension, and has infinitely many versions corresponding to infinitely many observers. I'm not sure that Smolin and collaborators have got around to this question as yet. It would be nice to know if they have. I don't.

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5 hours ago, addison said:

Yes, time is valid as used in GR the only difference is that time should not be included on our graphs as a dimension. If displayed on a dimension it becomes easy to think of it as fundamental.

As far as who are the people who believe that time is fundamental I did a google search for "time as fundamental" and discovered several entries of people arguing time as fundamental.

What do you mean when you say time is or is not fundamental ?

Why do you think the question is a big deal anyway ?

Of the many quantities we can calculate we currently acknowledge a set of 7 fundamental ones (including time) and derive all others from that.
But equally we could use a different set, for instance in fluid mechanics we often use force-length-time rather than mass-length-time.
But there is no fundamental argument against choosing a set excluding time.

The reason we use the setup we do is simply one of maximum convenience.

And why did you not address my previous question and comments?

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6 hours ago, addison said:

Yes, time is valid as used in GR the only difference is that time should not be included on our graphs as a dimension. If displayed on a dimension it becomes easy to think of it as fundamental.

Can you show us your calculations for the height of the orbit without references to time? How do you figure an Earth-centered orbit without taking rotation time into consideration?

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On 1/5/2023 at 10:06 AM, Markus Hanke said:

That’s a contradiction. Time in GR is a dimension - if you don’t include it as such, the model no longer works.

While it's inspired to combine time as a dimension in GR, that doesn't mean that time IS a dimension. It's very different to what we are used to calling a dimension, the three spatial dimensions.

if one of the spatial dimensions was similar to time, in that nothing could freely travel forwards or back, and the only change in that dimension was a constant inching forward at an exact fixed rate, then it would be debateable if that could be called a dimension at all.

Yes, the model no longer works without time as a dimension. But, it is a model, a scientific device to represent the world and it's physical changes.

There are three elements to time. Future, present and past. But do the future and the past actually exist, or do they just exist in our mental model ? It's the present that holds all of the details of the past, and the conditions for the future. The past and future would not exist at all, without what's contained in the present.

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7 minutes ago, mistermack said:

Yes, the model no longer works without time as a dimension. But, it is a model, a scientific device to represent the world and it's physical changes.

Actually, I formulated this badly - GR does still work, even if you take out time as a dimension. The problem is just that the kind of gravity you get then is nothing like the gravity we actually see in our universe.

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4 minutes ago, Markus Hanke said:

Actually, I formulated this badly - GR does still work, even if you take out time as a dimension. The problem is just that the kind of gravity you get then is nothing like the gravity we actually see in our universe.

Isn't it debateable if that constitutes GR working, in that case ?

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1 minute ago, mistermack said:

Isn't it debateable if that constitutes GR working, in that case ?

Of course not.

All you have to do is out dt = 0 @t=0 into the equations to see that there must still be an apparent atttraction felt between massive bodies.

It must still give measurably verifiable results in the case of static situations.

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4 minutes ago, studiot said:

Of course not.

I just meant does a model still work, if the results don't match reality?

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Posted (edited)
5 minutes ago, mistermack said:

I just meant does a model still work, if the results don't match reality?

Obviously not in respect of whatever conditions are being applied.

That is true of all models.

We don't expect elastic analysis to match reality beyond the 'elastic limit'.

Edited by studiot
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16 hours ago, studiot said:

All you have to do is out dt = 0 @t=0 into the equations to see that there must still be an apparent atttraction felt between massive bodies.

This is true, though I meant something different - my argument was that eliminating time as a dimension will reduce the dimensionality of the spacetime manifold to 3D+0, so that the indices in the field equation now only run over a range $$\mu,\nu=1…3$$. If you do this, then the Weyl tensor vanishes identically, and, because the vacuum field equations also imply $$R_{\mu\nu}=0$$, you are left with both the trace and the trace-free part of the Riemann tensor vanishing. In other words, spacetime must be Riemann-flat everywhere in vacuum. So in 3D+0, assuming validity of GR implies that there is no gravity between massive bodies in vacuum.

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What's bugging at me is that while it's hugely successful to model time as the fourth dimension, does that mean that it really is one? Every device for measuring time is simply a three dimensional object of some sort, that records three dimensional changes. So when you plot movement in 3D against time, you are really plotting movement in 3D against another movement in 3D.

In other words, the fourth dimension is just a product of the three spatial dimensions.

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A dimension is a mathematical term for a degree of freedom or some indepenendent variable.

The spatial dimensions x,y,z can each vary without the other variables varying. Time we give dimension of length by the interval (ct) the distance light travels in one second.

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Posted (edited)
58 minutes ago, mistermack said:

while it's hugely successful to model time as the fourth dimension, does that mean that it really is one?

No, it doesn't.

In relativity, time is a full-fledged dimension, the universe is a four-dimensional block, and the flow of time is considered to be an illusion. The only special thing about time (in SR, anyway) is that the sign of its coefficient in the space-time metric is the opposite of the spatial dimensions (and it's multiplied by c).

And it's often presented that way when nonphysicists ask about it. If you persist in your questioning, though, they'll eventually admit that relativity is just a model and nobody really knows what time is.

Edited by Lorentz Jr
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17 minutes ago, mistermack said:

So when you plot movement in 3D against time, you are really plotting movement in 3D against another movement in 3D.

Have a look at the following situation: this is a plot of a 3-cube that has evolved in time, but instead of using a time axis, the evolution is all plotted on the same 3D volume to just reflect “movement” (as you suggest), without reference to an external “time” at all. The movement/change here is a combination of rotations (angle not necessarily constant, and rotations not necessarily in the same direction), and a change in colour.

Without being given any information other than points within that the same 3D volume (ie only the above picture), can you tell me what the initial state of this system is, and how it evolves? What in here corresponds to past, present, future? You can’t do this, unless additional information is provided that is not itself an element of this same 3D volume.

I think you can see the issues. And this is an idealised evolution in just three discrete steps - real-world systems, especially classical ones, feature continuous evolutions, with rotations around all three axes. Try to plot that into a single 3D volume, and what you’d get in the continuum limit is a solid ball - you couldn’t even tell the original shape any more.

On the other hand, if you were to plot the evolution of the above system on a clearly labelled time axis with separate 3D plots at t=1,2,3, then there are no ambiguities at all - you can tell exactly what the original state was, and how it evolved in time.

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59 minutes ago, mistermack said:

the fourth dimension is just a product of the three spatial dimensions

What is meant by a "product"?

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