[TyroJack (OP)]

Is there a currently accepted theory about time? - What it is; and how it works?

[Colin2B]

I’ve split this off because it is a much broader topic than time dilation.

At the beginning of last century there were 2 philosophical theories A and B, but they have been overtaken by modern views eg:
Presentism - only the present is objectively real
Eternalism/block universe -  more or less a 4D stage we all move through.
Smolin suggests a many paths view where we all make our way between events via different paths.

Most physicists would say that we measure time using arbitrary units to place events in spacetime and measure the time between events. 

[TyroJack (OP)]

Thank you. I didn't want to be told it should have been part of the other thread!  (I'm learning all the time).

Are those really theories, or interpretations of how we perceive (or may be perceiving) things?
It seems to me that Science follows strict basic rules; though it sometimes takes scientists a while to understand what those rules are - as with relativity and the fact that time - and space - are not absolute.

Can time really be a dimension in all those theories? It certainly doesn't behave like the spatial dimensions
.
But the big questions to me are:
How does time work?
Why does it work that way?
What is the logic of time in those views?


 

[Halc]

Are those really theories, or interpretations of how we perceive (or may be perceiving) things?

They're interpretations.  Neither predicts an empirical distinction.  There are attempted arguments that one or the other is the correct one, but I've never seen one that wasn't fallacious.

Can time really be a dimension in all those theories? It certainly doesn't behave like the spatial dimensions

It isn't a spatial dimension, even if it is very much orthogonal and can be measured in meters and such.  It is a temporal dimension, and yes, it behaves different, but like the three axes in a model of space, the orientation of all four spacetime axes is pretty arbitrary so long as they remain orthogonal.

But the big questions to me are:
How does time work?
Why does it work that way?
What is the logic of time in those views?

I suspect each interpretation would answer those differently, so I'll not attempt it.

I will say that the block view doesn't have anything moving through it.  Time is part of the structure, so the structure as a whole doesn't change.  Motion is represented as worldlines, not by travel.

[Colin2B]

Learning is good

They aren’t theories in the scientific sense ie experiment, results etc, more hypotheses. They arn’t philosophical ‘how do we percieve’ ie sense of time.
In most hypotheses time is assumed to be a dimension that we move along, although not voluntarily!

It would be hard to answer your questions here because it would be like writing a textbook, or two!
Probably best if you read up on the ideas and come back with questions - or post your own ideas in new theories.
You won’t get many answers in this section because most ‘theories’ are conjecture ie new theories. The only real theories are around relativity, and measurement + use of time in physical laws and prediction.

[TyroJack (OP)]

Can time really be a dimension in all those theories? It certainly doesn't behave like the spatial dimensions

It isn't a spatial dimension, even if it is very much orthogonal and can be measured in meters and such.  It is a temporal dimension, and yes, it behaves different, but like the three axes in a model of space, the orientation of all four spacetime axes is pretty arbitrary so long as they remain orthogonal.

Yes, I understand it isn't a spatial dimension. I wonder why, if it behaves differently - has different properties from spatial dimensions - why it is included as a fourth orthogonal dimension? Why orthogonal?

I find it difficult to see the justification for that as time has no orientation with respect to space.
I can certainly see how time can be a dimension, but making it orthogonal with respect to the spatial dimensions seems like a step too far.
I wonder if it could be presented as a different sort of additional dimension on a Cartesian, three dimensional, space diagram?

As I said, time has no spatial orientation; time only moves in one direction; time can be measured from any starting point in space.  It would effectively be those lines that are labelled of the form ct...

I am envisaging it to be a spherical or circular dimension.
Any point in space could then have different times measured from different initial events...
But , hey-ho, I am probably just daydreaming...

[Halc]

Yes, I understand it isn't a spatial dimension. I wonder why, if it behaves differently - has different properties from spatial dimensions - why it is included as a fourth orthogonal dimension? Why orthogonal?

I find it difficult to see the justification for that as time has no orientation with respect to space.

Newton's view was it being independent (space + time, not spacetime), thus any change to the time dimension would have no effect on the spatial dimensions. That view was falsified eventually.
Making them orthogonal makes for one geometric space with valid concepts like intervals and such.  A change to the time dimension necessarily also moves the orientation of the space axes, resulting in relativity of simultaneity.

I wonder if it could be presented as a different sort of additional dimension on a Cartesian, three dimensional, space diagram?

Sort of, but it isn't Cartesian since that coordinate system does have for instance Lortentz transformations when moving the axes around.  As long as the axes are stable, yes, all of spacetime can be referenced via 4D Cartesian coordinates.  Every event has 4 values to identify its location relative to a given origin (which can be almost anywhere).

As I said, time has no spatial orientation; time only moves in one direction; time can be measured from any starting point in space.

The distance along the x axis also can be measured from any starting point in spacetime, so the statement, while true, isn't particularly profound.  The x axis is arbitrarily assigned just like the other 3, but they all should be orthogonal, else much of the mathematics gets far more complicated.

I can reference any point on a sheet of paper with X and Y coordinates along axes that are say 40% from each other, but distances are hard to compute with parallelogram coordinates like that.  I say this just to illustrate that non-orthogonal axes are not strictly forbidden.  They just add needless complication.

Any point in space could then have...

Points in space are defined with only 3 of the 4 dimensions, so they define a straight line (parallel to the t axis) in 4D spacetime, not a point.  That line would correspond to the worldline of an inertial object in Euclidean spacetime.  Since 1) I cannot think of any real inertial objects, and 2) spacetime is not Euclidean, the worldlines of real objects tend not to be straight in any Euclidean sense.

[jeffreyH]

Time can only be orthogonal to the other axes in a local inertial for frame. The time in a frame moving relative to the observer can never be orthogonal from the observers perspective. Hence mathematical time dilation.

[TyroJack (OP)]

Any point in space could then have...

Points in space are defined with only 3 of the 4 dimensions, so they define a straight line (parallel to the t axis) in 4D spacetime, not a point. 

Yes indeed I should have said any event in Spacetime...

[TyroJack (OP)]

Spacetime comprises 4 dimensions, 3 of space and 1 of time.
In Special Relativity we are concerned with the movement of bodies, in particular clocks, obeying simple mechanical laws e.g. v=d/t.
Why then are Time Dilation, Length Contraction and the relativity of simultaneity not derived from the simple mechanics of Spacetime?
What I mean is: shouldn't time dilation be a natural result of the movement of a clock in spacetime?  Without any need for Lorentz transformation equations or the Lorentz factor.
We are only dealing with distances times and directions, all components of classical mechanics the only extra is the constant speed of light...

[Halc]

What I mean is: shouldn't time dilation be a natural result of the movement of a clock in spacetime?  Without any need for Lorentz transformation equations or the Lorentz factor.
We are only dealing with distances times and directions, all components of classical mechanics the only extra is the constant speed of light...

The Lorentz transformations follow from the premise of constant light speed.  In the universe with a different (more simple) motion description (say with fixed rather than arbitrary temporal axis orientation), time would flow at an objective rate and light would be measured at different speeds by moving observers..  This was empirically falsified.

[TyroJack (OP)]

The Lorentz transformations follow from the premise of constant light speed.  In the universe with a different (more simple) motion description (say with fixed rather than arbitrary temporal axis orientation), time would flow at an objective rate and light would be measured at different speeds by moving observers..  This was empirically falsified.

Hmm. I'm not sure what you are implying here. Lots of things may be different in a universe with a different motion - but we are discussing this universe and the way it works.
The Lorentz transformation equations do follow from the premise of constant light speed, but I am thinking that we should be able to reach the same conclusions from the mechanics of spacetime.  If they don't follow form that what are the properties of space and time or the rules for dealing with them, what are the scientific principles, laws, behaviours that make them more complicated?
What I am saying is that built upon such straightforward entities as space and time, why can we not see the mechaics of it?

[Colin2B]

The Lorentz transformation equations do follow from the premise of constant light speed, but I am thinking that we should be able to reach the same conclusions from the mechanics of spacetime.  ....
What I am saying is that built upon such straightforward entities as space and time, why can we not see the mechaics of it?

I’m not clear what you mean by the mechanics of spacetime.
At a basic level spacetime is just a way of describing how we measure position and movement. We can also view it as a stage for various properties - which in shorthand we refer to as properties of space eg permittivity - and indeed Maxwell used these properties/behaviours of eg electric fields to derive c. So in a way you could say it is derived from the mechanics of spacetime.

[TyroJack (OP)]

In his little book,

Relativity: The Special and General Theory.

Einstein worked from classical Newtonian Mechanics.  He added but two postulates: the principle of relativity and the invariance of the speed of light.
He goes on to build his insight into his theories based on these principles.

Can we not derive the same  outcomes from classical mechanics by complying with rules of Newtonian mechanics as we know them with the addition of these postulates?

Could not time dilation, length contraction and simultaneity be then demonstrated in such a diagram?
 

[Colin2B]

......He added but two postulates: the principle of relativity and the invariance of the speed of light.
He goes on to build his insight into his theories based on these principles.

Can we not derive the same  outcomes from classical mechanics by complying with rules of Newtonian mechanics as we know them with the addition of these postulates?

The first postulate is in fact “The laws of physics are the same and can be stated in their simplest form in all inertial frames of reference.”
Einstein takes Maxwell’s equations (and along with them the physical laws of Coulomb, Gauss, Faraday and Ampere) which state that the speed of light is c. Under Newtons laws, based on Galilean relativity, the speed of light is not the same in all inertial frames, hence the laws of Coulomb, Gauss, Faraday and Ampere are not the same in all inertial frames.
Effectively, Einstein did take Newton’s laws and made them the same in all inertial frames.