What is time? (Again)

[geordief]

This boundary of a boundary as applied to 4d spacetime ,would the frame of reference be at the centre of an expanding set of potential events?

Ie suppose we choose the 9/11 attack on the Twin Towers  and set a time period of 1,000,000 light seconds is there a 4d spherical volume  created which encloses the set of all possible physical events  following the attack?

And the boundary of this 4d volume is a number of 3d  volumes ?

(Do they have to be purely spatial or can they be 2d +1?)

[joigus]

On 8/15/2020 at 3:22 PM, MigL said:

Don't know much about AdS/CFT, but always thought that although the boundary of AdS space has a lower dimensionality than the space itself, it must still encode all the higher dimensional information on that surface ( holographic principle ), and the boundary, locally around any point, reduces to Minkowsky space. IOW, still include time.

Or have I misunderstood AdS/CFT all this time ?

I don't think you have, necessarily. But much of the difficulty is that the status of the "theory" is heavily laden with guesswork. People who work on this area frequently talk about a "dictionary": A set of rules to translate from the language of the gauge theory to the language of gravity. It is by no means clear what time is in the Chern-Simons theory, for example, which is a favourite gauge-theory choice for 1+2 gravity, and you must make assumptions to get to known physics.

7 hours ago, studiot said:

@joigus   and @Markus Hanke

So how would you gentlemen regard

7 hours ago, studiot said:

the following simple example

Consider a sphere divided into  many conducting lands, insulated from each other, and each charged to some different electrical potential.

The Field within the sphere can be determined from a knowledge of the position and potentials of the surface lands alone.

Interesting problem, but quite complicated, and I'm not sure it can be related to the gravity-gauge duality. I would start with two symmetric lands, one positive and one negative (for simplicity). I would also assume the metric to be trivial. You can solve for a half-sphere of charge with the help of the superposition principle and by assuming the negative potential function for a negative hemi-sphere of charge being the symmetric (in charge & space) of a positive hemi-sphere of charge.

If, as Markus suggests, you assume permeability and permitivity, you're throwing in two more (scalar) degrees of freedom: You've got two more scalar fields. You need dielectrics to insulate the conductors. Properties of matter are additional DOFs, the same as metric properties of ST.

The simple argument we were talking about has to do with fields in the vacuum, and nothing but gravity inside. If you throw in matter, it's much more complicated.

I haven't thought very seriously about this problem really.

2 hours ago, geordief said:

Ie suppose we choose the 9/11 attack on the Twin Towers  and set a time period of 1,000,000 light seconds is there a 4d spherical volume  created which encloses the set of all possible physical events  following the attack?

1,000,000 light seconds is a distance. Do you mean 1,000,000 seconds? Or a distance radius of 1,000,000 light seconds?

Something like 9/11 attack I don't think would be well (or easily) described by smooth fields...

The AdS/CFT duality assumes pure gravity inside and I'm sure analyticity of the fields is playing a role in all of it. Mixing everything inside and outside doesn't let one (me, at least) think clearly of what we're talking about.

[geordief]

1 hour ago, joigus said:

 

1,000,000 light seconds is a distance. Do you mean 1,000,000 seconds? Or a distance radius of 1,000,000 light seconds?

Something like 9/11 attack I don't think would be well (or easily) described by smooth fields...

 

Yes,my mistake ,but I was trying to describe a spacetime volume  centred on a physical event such as the 9/11 attack.

The volume would enclose all possible subsequent events which I understand would look like a light cone centred on that physical event.

In my example light would travel  1,000,000 light seconds  and the spacetime volume would be the set of all possible event between  the initial emission  and its destination  some 1,000,000 light seconds distant.

I was trying to represent a spacetime volume such that Markus could point out to me  what it's boundary was and how it was "zero"

 

Actually ,on reflection ,I have been wondering if the volume in question might be an incremental increase in the spacetime volume ,but that may just represent an additional layer to my confusion

[Edgard Neuman]

hi,

there's a thing that i think is a good candidate for "time", and that has been under our noses the whole time : decoherence..
Decoherence doesn't work backward.. a choice is made..
Any thoughts about that ? 

thks

Edited August 17, 2020 by Edgard Neuman

[studiot]

20 hours ago, Markus Hanke said:

No, it also depends on the metric of the underlying manifold, and obviously permeability and permittivity of the vacuum. 
But provided the above things are given, an interesting question arises - are the relative positions and potentials on the surface of the sphere sufficient boundary conditions in order to fix a unique solution to Maxwell's equations? I don't know the answer, but my guess would be no, because there is a gauge freedom in where the zero point of the electric potential is, so I think we need precisely one additional boundary condition that specifies whether or not there is a background potential/field permeating the space. The relative positions of the charges alone don't uniquely fix this. But that's just a guess now, without having done any actual maths.

 

10 hours ago, joigus said:

Interesting problem, but quite complicated, and I'm not sure it can be related to the gravity-gauge duality. I would start with two symmetric lands, one positive and one negative (for simplicity). I would also assume the metric to be trivial. You can solve for a half-sphere of charge with the help of the superposition principle and by assuming the negative potential function for a negative hemi-sphere of charge being the symmetric (in charge & space) of a positive hemi-sphere of charge.

If, as Markus suggests, you assume permeability and permitivity, you're throwing in two more (scalar) degrees of freedom: You've got two more scalar fields. You need dielectrics to insulate the conductors. Properties of matter are additional DOFs, the same as metric properties of ST.

The simple argument we were talking about has to do with fields in the vacuum, and nothing but gravity inside. If you throw in matter, it's much more complicated.

I haven't thought very seriously about this problem really.

Thank you for your replies.

I particularly like joigus' idea of simplifying to two hemispheres to start with.
I had considered that.

The reason for suggesting this is to return to the thread question what is time.

I posed a static time free situation.

The next logical step is to examine the question

What extra properties would that adding a 'time' axis (dimension) would be conferred on the system, that adding an extra space axis would not ?

The answer to this question would go a long way towards the thread question in Physics.

[geordief]

Is there  a problem if we just go the whole hog and accept the  interpretation that time is indeed  just the flip side of spatial distance?

Does our experience of time differ from our experience of spatial  distance in any way?

It is said that you can go "back" in space but not in time ,but is that really so?

 

Maybe we can only go forward in space as well.That is how I look at it,anyway;we never return to the sane place in space and always keep moving forward ,never back.

 

But are there ways where it can be correctly said that time is fundamentally different from space.?

 

When we die ,we are out of time but also out of space

[swansont]

43 minutes ago, geordief said:

Is there  a problem if we just go the whole hog and accept the  interpretation that time is indeed  just the flip side of spatial distance?

Does our experience of time differ from our experience of spatial  distance in any way?

You can visualize distance. I think that's the big hangup. Time is experienced in a different way.

 

[geordief]

 

27 minutes ago, swansont said:

You can visualize distance. I think that's the big hangup. Time is experienced in a different way.

 

https://www.spacetimetravel.org/ueberblick/ueberblick1.html

 

Markus showed me these simulations a while back in the thread.

Do they allow us to visualize time in the same way as distance?

Iow it just the fact that  our experience of the unfolding of events is  at such an extreme remove from the signal transmission  that   our experience of time takes a back seat.

 

If our experiences were at light speed (approx) would our experience of spatial distances take   a back seat in the same way?

Would the subjective experiences of time and space  be reversed?

 

Or is your point different?

[joigus]

44 minutes ago, geordief said:

It is said that you can go "back" in space but not in time ,but is that really so?

There is a "no space inversion symmetry" property of sorts, if you think about it. Because CPT is the robust ST-inversion symmetry of Nature, this suggests that CP are twin properties of T. That all these properties go together in building up one entity.

The hard question about these symmetries is (IMO) that they do not allow you to think about them in terms of active transformations (actually changing anything in a system), but as passive transformations (re-labelling of everything).

Thanks for bringing these animations to our attention. +1

 

[Markus Hanke]

21 hours ago, geordief said:

This boundary of a boundary as applied to 4d spacetime ,would the frame of reference be at the centre of an expanding set of potential events?

The reference frame (i.e. coordinate choice) is arbitrary here; the entire formalism used in the post I made is covariant under coordinate transformations. So the 4D volume (which is thought of as sufficiently small) can be anywhere and anytime.

21 hours ago, geordief said:

Ie suppose we choose the 9/11 attack on the Twin Towers  and set a time period of 1,000,000 light seconds is there a 4d spherical volume  created which encloses the set of all possible physical events  following the attack?

Yes, you can define such a 4D volume.

21 hours ago, geordief said:

And the boundary of this 4d volume is a number of 3d  volumes ?

Yes, the boundary of any n-dimensional volume is a (n-1)-dimensional volume.

15 hours ago, geordief said:

I was trying to represent a spacetime volume such that Markus could point out to me  what it's boundary was and how it was "zero"

It isn't the boundary that is zero, but the boundary of the boundary. 
It is easier to visualise this in fewer dimensions. Suppose you have a ball in normal 3D space - its boundary is just its surface, which is a 2D sphere. And what is the boundary of the 2D sphere (which is the boundary of the boundary of the 3D ball)? It doesn't have any - there's no edge or discontinuity to it, you could draw continuous lines on it in any direction you choose.

The boundary of a boundary is zero.

The same is true for the 4D spacetime hypercube, but it's much less obvious, because the sides of the 3D cubes that make up its boundary are oriented surfaces, so each side is counted twice, but with opposite sign, giving a net total of zero. 

[swansont]

2 hours ago, geordief said:

 

https://www.spacetimetravel.org/ueberblick/ueberblick1.html

 

Markus showed me these simulations a while back in the thread.

Do they allow us to visualize time in the same way as distance?

Those aren't really visualizing distance, they are visualizing objects moving at high speed.

 

Quote

Iow it just the fact that  our experience of the unfolding of events is  at such an extreme remove from the signal transmission  that   our experience of time takes a back seat.

 

If our experiences were at light speed (approx) would our experience of spatial distances take   a back seat in the same way?

Would the subjective experiences of time and space  be reversed?

 

Or is your point different?

You use your eyes to visualize distance. You are literally visualizing it.

I don't know about you, but I don't experience time the same way. I can see a meter stick, and I can picture something a meter away, or even a point where nothing that exists that is a meter away (well, these days it's more like two meters). Not the same with time.

[geordief]

3 minutes ago, swansont said:

 

 

You use your eyes to visualize distance. You are literally visualizing it.

I don't know about you, but I don't experience time the same way.

Do you think if you were moving at approx c wrt the object you would visualize distances  in the same way?

Would you ,in that scenario (and just taking you and the object into account)  experience distance and time intervals  in the same way as you experience them under normal conditions?

 

Lets say there was  a column of objects all passing in front of your vision (at almost  c wrt you)  and the objects were changing in  a similar way to   the way you make a movie picture out of a pack of cards** would  the images created be any different  to the images we experience in everyday life?

 

** you know the way, if you have a pack of cards,each one with a picture of the same horse in different stages of movement and you "flick" the pack ... you can see the horse move.

[swansont]

Just now, geordief said:

Do you think if you were moving at approx c wrt the object you would visualize distances  in the same way?

That's a different question. We know things will looks different, but I don't think that really changes the fact that we don't experience time and distance the same way.

 

 

[geordief]

9 minutes ago, swansont said:

That's a different question. We know things will looks different, but I don't think that really changes the fact that we don't experience time and distance the same way.

Is it possible that our experience of time  and space intervals might be mirror images of each other depending on our states of relative motion wrt to objects we observe?

Edited August 17, 2020 by geordief

[Strange]

2 hours ago, geordief said:

Is it possible that our experience of time  and space intervals might be mirror images of each other depending on our states of relative motion wrt to objects we observe?

Not so much a mirror image, but they are related by a rotation that depends on your relative speed.

[geordief]

5 minutes ago, Strange said:

Not so much a mirror image, but they are related by a rotation that depends on your relative speed.

 So at the 2 extremes one does not effectively become the other ?

 

One extreme being our normal low relative velocity world and the other being a world where all interactions involved relative velocities of very close to c....

 

I think I have come across the idea of space transforming into time ,but perhaps I imagined it (since I would not have understood how that could work)

Edited August 17, 2020 by geordief

[Strange]

Just now, geordief said:

So at the 2 extremes one does not effectively become the other ?

Yes. An extreme example is that beyond the event horizon of a black hole, the radial direction becomes time so the singularity is in your future, instead of ahead of you (in some coordinate systems, anyway).

[geordief]

8 minutes ago, Strange said:

Yes. An extreme example is that beyond the event horizon of a black hole, the radial direction becomes time so the singularity is in your future, instead of ahead of you (in some coordinate systems, anyway).

Wouldn't "mirror image" describe that scenario? The two extremes are related in a way that the spatial dimensions perform the role of the temporal dimension(and vice versa)

 

 

 

[swansont]

8 hours ago, geordief said:

Is it possible that our experience of time  and space intervals might be mirror images of each other depending on our states of relative motion wrt to objects we observe?

Does this matter? Time and distance in a single frame - no relative motion - are experienced differently. Motion doesn’t enter into it.

[geordief]

37 minutes ago, swansont said:

Does this matter? Time and distance in a single frame -  no relative motion - are experienced differently. Motion doesn’t enter into it.

Well , "no relative motion" is one extreme and ,as you say time  and spatial distance are experienced differently.

If ,under the other extreme of almost c-like relative motion they are again experienced differently  but with roles reversed  would it not put them(Time and Space) on an equal and complementary footing.

If the OP asks "What is Time"  then might that not be a large part of the answer?

Temporal  distances  would be an aspect of Spatial distances and Time itself would be an aspect of Space.

So Time would not depend on motion to exist (as I have read claimed)  but it would depend on the existence of Space.

Has it ,I wonder been suggested that ,in the inflationary epoch that Time was  also affected?  (if Space was expanding exponentially ,then according to the way I am thinking it would be reasonable to suppose that Time somehow might "contract" in some kind of a relationship)

 

Happy to be told if that is word salad as I am just following the logic of my idea and logic is very  far from my strong point.

 

[swansont]

12 hours ago, geordief said:

Well , "no relative motion" is one extreme and ,as you say time  and spatial distance are experienced differently.

It’s the usual situation one is in. And the corrections of relativity are usually not noticeable under virtually all circumstances the average person encounters.

 

Quote

If ,under the other extreme of almost c-like relative motion they are again experienced differently  but with roles reversed  would it not put them(Time and Space) on an equal and complementary footing.

How are the “roles” reversed? Do you suddenly see time? Does stereoscopic vision assist with time perception?

Quote

So Time would not depend on motion to exist (as I have read claimed)  but it would depend on the existence of Space.

Claimed, but it never stands up to scrutiny. Not a good notion on which to base anything 

[Markus Hanke]

2 minutes ago, swansont said:

And the corrections of relativity are usually not noticeable under virtually all circumstances the average person encounters.

Well, there are at least some relativistic effects that are directly observable in everyday life - two examples that immediately spring to me mind would be the colour of gold (it would be silvery without relativistic effects), and the effectiveness of lead-acid batteries used in cars etc.

[swansont]

1 minute ago, Markus Hanke said:

Well, there are at least some relativistic effects that are directly observable in everyday life - two examples that immediately spring to me mind would be the colour of gold (it would be silvery without relativistic effects), and the effectiveness of lead-acid batteries used in cars etc.

True but I didn’t say we don’t notice results of relativistic effects. I said we don’t see the corrections - the differences between Newton and Einstein as relative speed changes (the context of the discussion). Neither of these are manifestations of time vs distance. They are energy corrections, and it’s not something that changes with relative motion.  

[Markus Hanke]

1 minute ago, swansont said:

I said we don’t see the corrections - the differences between Newton and Einstein as relative speed changes (the context of the discussion). Neither of these are manifestations of time vs distance.

You're right, I didn't read it carefully enough  

[geordief]

1 hour ago, swansont said:

How are the “roles” reversed? Do you suddenly see time? Does stereoscopic vision assist with time perception?

I was wondering whether that might be the case. Yes I was wondering whether one might indeed "see time" and experience spatial distances  in the same kind of a way that we experience time normally.

 

 

Strange said ,about 7 posts above "so the singularity is in your future, instead of ahead of you" and that is what may have prompted my mind to wander down this path.

 

I have nothing to back this up

 

As for your stereoscopic vision point ,that is too complicated for me to  give an opinion . Maybe  it will become clearer(or muddier)  to me  at some later point.

 

Edited August 18, 2020 by geordief