Is there such a thing as at rest ?

[JesseM]

There is no justification for declaring time as a "dimension".

The justification is that it is simplest to treat spacetime as a unified 4D geometrical structure in relativity.

Not only does our daily experience indicate that entities are 3D in the sense that they have extent in three mutually perpendicular directions

As long as relativity makes correct predictions about all the specific events that actually make up our daily experience, and relativity is compatible with the notion of time as a dimension, then there can be no logical way that our daily experience can contradict this idea.

but mathematically what we insert and label "time" behaves entirely differently. There is no reason to think that time is anything other than: relative motion i.e. motion+observer.

Time is obviously different from spatial dimensions (this is true in relativity for numerous reasons), but there's nothing illogical about the idea of time being a different sort of dimension in 4D spacetime. It seems to me you are failing to take your own advice here:

Why should Nature conform to what a human being is limited to doing? Of course Nature doesn't bend to our will, so it would behoove us to try to use our imagination.

Indeed one cannot even make a substantial claim for time as a dimension without defining both time and dimension unambiguously. The only consistent definitions I have seen summarily rule out "time" as a dimension.

Mathematically the idea of a 4D spacetime manifold, which is distinct from a 4D spatial manifold because of a difference in the metric signature, is quite well-defined in differential geoemetry, it's a type of psuedo-Riemannian manifold (with a Riemannian manifold being a purely spatial one).

Objectively, all we have before us is the slowing of cyclical mechanisms in particular scenarios. There is no reason to think this not purely a mechanical effect that is not understood. Indeed, since radioactive decay is still considered "random" one cannot deduce any definitive conclusions about "the nature of time" from SR experiments. When the process for measuring time itself, the clock, is understood we can begin to make meaningful judgments on this matter. In the meantime we are still studying the clock itself.

Nature should not conform to my ideas, and I welcome skepticism, but not for skepticism's sake. One needs to pose an alternative definition/conception or, as a bare minimum, try to argue why the one posed is wrong.

Why does your "series of 3D moments" view get to be treated as the default while "4D spacetime" is treated as an "alternative" that can only be considered if your view is shown wrong? Since relativity offers no preferred definition of simultaneity, any notion of absolute simultaneity must either assume new physics will eventually justify it, or it must admit to being a philosophical idea that can never be verified by experiment. If the latter, then the "4D spacetime" view and the "series of 3D moments" view are competing philosophical notions, so any argument as to why one is "wrong" would have to be a purely philosophical argument. I don't claim there are any clear-cut philosophical arguments that prove the "series of 3D moments" view is wrong, just that there are also no philosophical arguments that prove the "4D spacetime" view wrong either, so there is no basis for your seeming total confidence that the first is right and the second is wrong; it seems that your only basis for this is that you find the first more aesthetically appealing, or less counterintuitive. Given current physics, I personally find the "4D spacetime" view more elegant for the simple reason that it doesn't require us to postulate undetectable metaphysical entities beyond those described by physics. You don't have to agree, but I hope you will at least admit that there is no logical contradiction of philosophical impossibility in this view.

 

[map19]

Motion in space is measured by the frequency shift seen in the CMB. Compound motion causes the CMB frequenct to show a quadripole image. As noticed by the COBE satellite, because the Earth has motion in all axes relative to the Cosmos.
It might be possible to be "stopped in space" as mentioned on Startrek if a spaceship could manoevre to where the CMB showed no frequency shifts in any of the three spatial axes.

 

[altonhare]

Motion is a change in position over a given time. Position is the spatial dimension measured with respect to some reference frame. (My, and I think most people's, definition of motion).

Let's examine your concept of "location" which you seem to think is invariant. Let's assume you have a universe of three objects, A, B, and C. If I am situated on A, I will measure B and C to be some distance away at some moment in time. (Say I get B=5 and C=6 units away from me). This is your concept of location. However, if I am situated on B and I attempt to measure the same quantities, namely, the distances from A to B and from A to C, I may very well arrive at B=3 and C=2. There really is no reason for you to think these numbers will be the same (and they aren't).

Here we have A lying "5" away from B and "6" away from C:

000000000000000000000
0000000000A0000000000
000000000000000000000
000000000000000000000
000000000000000000000
000000000000000000000
000000000000000000000
0000000000B0000000000
0000000000C0000000000
000000000000000000000

Did I make the diagram wrong? No matter how I look at it I get AB=5 and AC=6.

 

[altonhare]

The justification is that it is simplest to treat spacetime as a unified 4D geometrical structure in relativity.

As long as relativity makes correct predictions about all the specific events that actually make up our daily experience, and relativity is compatible with the notion of time as a dimension, then there can be no logical way that our daily experience can contradict this idea.

Time is obviously different from spatial dimensions (this is true in relativity for numerous reasons), but there's nothing illogical about the idea of time being a different sort of dimension in 4D spacetime. It seems to me you are failing to take your own advice here:


Mathematically the idea of a 4D spacetime manifold, which is distinct from a 4D spatial manifold because of a difference in the metric signature, is quite well-defined in differential geoemetry, it's a type of psuedo-Riemannian manifold (with a Riemannian manifold being a purely spatial one).

Objectively, all we have before us is the slowing of cyclical mechanisms in particular scenarios. There is no reason to think this not purely a mechanical effect that is not understood. Indeed, since radioactive decay is still considered "random" one cannot deduce any definitive conclusions about "the nature of time" from SR experiments. When the process for measuring time itself, the clock, is understood we can begin to make meaningful judgments on this matter. In the meantime we are still studying the clock itself.

Why does your "series of 3D moments" view get to be treated as the default while "4D spacetime" is treated as an "alternative" that can only be considered if your view is shown wrong? Since relativity offers no preferred definition of simultaneity, any notion of absolute simultaneity must either assume new physics will eventually justify it, or it must admit to being a philosophical idea that can never be verified by experiment. If the latter, then the "4D spacetime" view and the "series of 3D moments" view are competing philosophical notions, so any argument as to why one is "wrong" would have to be a purely philosophical argument. I don't claim there are any clear-cut philosophical arguments that prove the "series of 3D moments" view is wrong, just that there are also no philosophical arguments that prove the "4D spacetime" view wrong either, so there is no basis for your seeming total confidence that the first is right and the second is wrong; it seems that your only basis for this is that you find the first more aesthetically appealing, or less counterintuitive. Given current physics, I personally find the "4D spacetime" view more elegant for the simple reason that it doesn't require us to postulate undetectable metaphysical entities beyond those described by physics. You don't have to agree, but I hope you will at least admit that there is no logical contradiction of philosophical impossibility in this view.

Fundamentally, what is your justification for this symbol "time" referring to a "dimension"? An equation can have as many parameters in it as we want, they don't equal dimensions. This is not an issue that can be resolved mathematically. Mathematics deals with useful descriptive models. Time is a useful parameter to humans so we put it in our equations. When you reduce this down to essential language "time" is no more than relative motion and is, in fact, unnecessary though convenient.

 

[Nabeshin]

Here we have A lying "5" away from B and "6" away from C:

000000000000000000000
0000000000A0000000000
000000000000000000000
000000000000000000000
000000000000000000000
000000000000000000000
000000000000000000000
0000000000B0000000000
0000000000C0000000000
000000000000000000000

Did I make the diagram wrong? No matter how I look at it I get AB=5 and AC=6.

Your diagram is with respect to some reference frame. In another frame, it might look like this:
000000000000000000000
0000000000A0000000000
000000000000000000000
000000000000000000000
000000000000000000000
0000000000B0000000000
000000000000000000000
000000000000000000000
0000000000C0000000000
000000000000000000000

(not the same as the numbers I gave earlier but yeah).
Just because you put a grid onto something does not make it an absolute!

 

[JesseM]

Fundamentally, what is your justification for this symbol "time" referring to a "dimension"? An equation can have as many parameters in it as we want, they don't equal dimensions.

I don't know what significance you are imparting to the word "dimension", but for me any parameter can in fact be treated as a dimension in the phase space of a system.

This is not an issue that can be resolved mathematically.

What is not an issue that can be resolved mathematically?

Mathematics deals with useful descriptive models. Time is a useful parameter to humans so we put it in our equations.

And would you not say the same thing about spatial dimensions? If not, why not?

The idea of eternalism is not that "time is a dimension", but that all events throughout space and time have the same ontological status, that there is no unique subset of events that are specially marked out as being in the objective "present". Mathematically you can model the relationship between these events in terms of their relative positions in a 4D pseudo-Riemann manifold, but as you say this is a descriptive model, and this model could be used by a presentist too. The point is that if we can make all the correct predictions about physical events using a model that does not include any notion of objective truths about simultaneity, then philosophically there is no irrefutable argument for believing there must be any absolute truth about simultaneity, even though you are still free to take the presentist position which says there is. Do you think this is wrong? If so, please present whatever irrefutable philosophical argument for absolute simultaneity that you think you have.

 

[Mentz114]

altonhare:

Fundamentally, what is your justification for this symbol "time" referring to a "dimension"? An equation can have as many parameters in it as we want, they don't equal dimensions. This is not an issue that can be resolved mathematically. Mathematics deals with useful descriptive models. Time is a useful parameter to humans so we put it in our equations. When you reduce this down to essential language "time" is no more than relative motion and is, in fact, unnecessary though convenient.

(my emphasis)
Time is essential in doing physics, just as the passage of time is an inescapable fact of existence. This is a physics forum. You are talking something else. The word 'nonsense' comes to mind.

 

[altonhare]

Your diagram is with respect to some reference frame. In another frame, it might look like this:
000000000000000000000
0000000000A0000000000
000000000000000000000
000000000000000000000
000000000000000000000
0000000000B0000000000
000000000000000000000
000000000000000000000
0000000000C0000000000
000000000000000000000

(not the same as the numbers I gave earlier but yeah).
Just because you put a grid onto something does not make it an absolute!

All you did was move the B a little closer to the A and away from the C. You didn't change reference frames, you just inexplicably and unjustifiably moved the objects under study around.

You're going to have to justify to me how, at an instant, one person can measure 5 and another 3. So far you have just stated this and "justified" it by showing A, B, and C at different locations.

In the "grid" I showed, I can sit on the A and walk downward. I count 5. I can sit on the B and walk upward, I get 5.

I don't know what significance you are imparting to the word "dimension", but for me any parameter can in fact be treated as a dimension in the phase space of a system.

Dimension: extent of an entity in a direction mutually perpendicular to every other direction

If instead we're just talking about how many mathematical parameters we can assign in our model, one can make an argument for an arbitrary # of dimensions.

What is not an issue that can be resolved mathematically?

And would you not say the same thing about spatial dimensions? If not, why not?

"What is a dimension?" is not resolved mathematically because you don't prove a definition with a measurement or a mathematical model. You just define the word "dimension".

The idea of eternalism is not that "time is a dimension", but that all events throughout space and time have the same ontological status, that there is no unique subset of events that are specially marked out as being in the objective "present".

Entities only exist in present-mode, i.e. they just have location. They are eternal in this sense, that they have no past or future.

Mathematically you can model the relationship between these events in terms of their relative positions in a 4D pseudo-Riemann manifold, but as you say this is a descriptive model, and this model could be used by a presentist too. The point is that if we can make all the correct predictions about physical events using a model that does not include any notion of objective truths about simultaneity, then philosophically there is no irrefutable argument for believing there must be any absolute truth about simultaneity, even though you are still free to take the presentist position which says there is. Do you think this is wrong? If so, please present whatever irrefutable philosophical argument for absolute simultaneity that you think you have.

There are no ontological contradictions.

altonhare:
(my emphasis)
Time is essential in doing physics, just as the passage of time is an inescapable fact of existence. This is a physics forum. You are talking something else. The word 'nonsense' comes to mind.

Passage of time? Like the passing of a car? The hand on my clock rotated, did time go by me? Time is simply relative motion. We could extract time from our equations and model reality just as accurately, although it would be a bit more tedious. All our equations have to do is to tell us successive locations.

 

[Mentz114]

altonhare:

Time is simply relative motion. We could extract time from our equations and model reality just as accurately, although it would be a bit more tedious. All our equations have to do is to tell us successive locations.

If you have successive locations you are including time. Time is what stopped everything being everywhere at once.

Time is not just relative motion. Does time stop in a universe containing only one distinguishable piece of matter ? Thermodynamical time is certainly not merely relative motion.