# Simultaneity, and the chronon

## Recommended Posts

What evidence is there to support the suggestion that time is continuous and not granular, or vice versa, or both, or neither, or something else?

What evidence is there to suggest that relative-frame-of-referenced simultaneous events are a reality?  How do we measure two simultaneous events and declare they are simultaneous if all observables and measurements are ultimately limited by HUP?  We may synchronise two atomic clocks, but how do we know they are synchronised without measuring them and how do we know after our measurement they are still synchronised?  How can we be certain at the time of measuring two events, that the clocks were synchronised?

In a volume of space with no observations or decoherence, must time still exist?

In a volume of space with two things each observing the other, is it impossible that time ticks sequentially rather than simultaneously for both of them?

##### Share on other sites

We go where theory takes us, when it has experimental confirmation. What we have that works is time as a continuum.

You can synchronize/compare clocks before and compare after. As long as they agree within the required precision, you would say events were coincident. The limit of the HUP only matters if you are also measuring a frequency or energy at the same time.

What “simultaneous event in relative frame” experiment did you have in mind?

##### Share on other sites

7 hours ago, AbstractDreamer said:

In a volume of space with no observations or decoherence, must time still exist?

You cannot separate time and space, they always exist together as spacetime, regardless of whether the region in question is a vacuum or not.

##### Share on other sites

• 2 months later...

Sorry for the huge time gap between responses.

On 6/23/2020 at 11:16 PM, swansont said:

What “simultaneous event in relative frame” experiment did you have in mind?

Tbh, I wasnt thinking about an experiment.  I was just thinking about how everything is simultaneous (or not)!   Each and every observer in their own relative frame is simultaneously observing or "eventing" with each and all others observers within that relative frame.  The parallelism is unimaginably vast to degrees of infinity!   I'm also thinking about simultaneity across non-relative frames both due to velocity and curvature, and is it possible to measure this and say two such non-relative framed events were simultaneous at a previous point in time?

How does the mathematics work for calculating predictions, when the degree of simultaneity between interactables in reality is so vast, and more likely than not in non-relative frames.

Take a simple case of just two observers oA and oB, mutually and simultaneously interacting, over the property Ap and Bp.   Given some initial values of Apt0 and Bpt0, how would you calculate the value of ApT at time T (after some interaction)? What kind of mathematics allows such simultaneous calculations?  How do these mathematics scale to "real" events with simultaneity across many more interactables in different relative frames?

So Calculus uses an infinitesimally small change in Apt0 to calculate Bpt1, and Bpt1 is used to calculate Apt1 and so on.  But this is essentially sequential not simultaneous calculations.  It is also breaking down a continuous scale into a granular one by creating the notion of an infinitesimally small change.   The mathematics, if we use calculus, adopts a sequential and quantum model of time.

For me, this is a problem and rather contradictory.  We declare time is continuous and simultaneous in GR because the underlying mathematical framework requires it to be so.  However the same mathematics employ a granular and sequential approach to calculations.

On 6/24/2020 at 6:13 AM, Markus Hanke said:

You cannot separate time and space, they always exist together as spacetime, regardless of whether the region in question is a vacuum or not.

I understand spacetime in GR is continuous, but how does quantum mechanics reconcile spacetime if time is not quantum?

##### Share on other sites

2 minutes ago, AbstractDreamer said:

Sorry for the huge time gap between responses.

Tbh, I wasnt thinking about an experiment.  I was just thinking about how everything is simultaneous (or not)!   Each and every observer in their own relative frame is simultaneously observing or "eventing" with each and all others observers within that relative frame.  The parallelism is unimaginably vast to degrees of infinity!   I'm also thinking about simultaneity across non-relative frames both due to velocity and curvature, and is it possible to measure this and say two such non-relative framed events were simultaneous at a previous point in time?

Simultaneity is a binary condition - either two events are simultaneous in your frame, or they aren’t. This doesn’t change over time.

What do you mean by non-relative frames?

##### Share on other sites

Say two stars are co-orbiting, and there is a third star that is relatively stationary to both of them (I understand it wont remain stationary)

##### Share on other sites

26 minutes ago, AbstractDreamer said:

I understand spacetime in GR is continuous, but how does quantum mechanics reconcile spacetime if time is not quantum?

Time is continuous in QM. It's the observables that are not continuous in general. There have been attempts to define a time operator, but they have been unsuccessful.

The strongest argument I know against discrete space-time defined by elementary cells comes from reasoning in terms of continuous symmetry.

You can define translation invariance as a discrete transformation, similar to what happens in a crystal lattice. If the points of the lattice were extremely close together, you wouldn't be able to tell the difference.

But what's more difficult is to give consistency to rotation invariance and Lorentz transformation boosts (shifts from an inertial frame to another one moving at uniform speed with respect to the first, equivalent to a "hyperbolic shift") in a granular lattice. Lattice structures have nothing remotely similar mathematically to a group of such a high index of symmetry that can look like a continuous group at all scales. Keep in mind that rotations by a small angle shift far away points to arbitrarily large distances. And for all we know continuous rotations can't tell of any anisotropy of space-time at far-away distances.

Edit: x-posted with @AbstractDreamer

Edit 2: I'm having second thoughts about what I've just written. I'm sure this argument has been wielded by physicists, but... If the discrete time structure were not a regular lattice, but a random homogeneous and isotropic web of points, the symmetry argument would no longer be valid.

Edited by joigus
##### Share on other sites

I have to read your stuff a dozen times, and then see how what it means for me

I'm having trouble formalising my thoughts at the moment.  Need some time to ponder what I really mean.

##### Share on other sites

2 hours ago, AbstractDreamer said:

Say two stars are co-orbiting, and there is a third star that is relatively stationary to both of them (I understand it wont remain stationary)

How can you have this scenario? And what is its connection to my question?

##### Share on other sites

2 hours ago, swansont said:

Simultaneity is a binary condition - either two events are simultaneous in your frame, or they aren’t. This doesn’t change over time.

What do you mean by non-relative frames?

OK non-relative frames.  Not really sure what I mean to be honest, but let me try and blurt out something coherent.

We have star A,B,C mutually, simultaneously interacting with each other, lets say gravitationally affecting their velocities.    The relative frame we choose as star A.    So the gravitational "force?" between A and B can be calculated and its effect on the velocity of star B as observed from star A can be calculated.

Star C also has a gravitational effect on and affect by both Star A and B, this too can be calculated.

So the interactions between star B and C are mutually simultaneous (presumably) but not in the same frame of reference as star A.  So that is non-relative frames.  What is happening between star B and C are non-relative to measurements or observations from star A, but presumably they are still simultaneous and might even be synchronisable with frames of observations from star A.

Edited by AbstractDreamer
##### Share on other sites

3 hours ago, joigus said:

The strongest argument I know against discrete space-time defined by elementary cells comes from reasoning in terms of continuous symmetry.

Are you saying that If space and time were not continuous, but discrete, then space and time translation symmetries wouldn't hold, and momentum and energy would not be conserved ?
Even if the discrete steps were at Planck scales ?
Are we sure they are conserved at such scales ?
Maybe Noether is a macroscopic limit approximation, like all quantum effects seem continuous at large scales.
( or have I simply failed to understand what you mean ? )

One could use the argument that momentum and energy are quantized in quantum field theories, so the HUP might dictate that their non-commutative partners, space and time, are similarly quantized in a quantum field theory of gravity.

Edited by MigL
##### Share on other sites

5 hours ago, joigus said:

The strongest argument I know against discrete space-time defined by elementary cells comes from reasoning in terms of continuous symmetry.

But what about entropy? If you consider - for simplicity's sake - a Schwarzschild black hole (i.e. empty spacetime everywhere), then how would it be possible to associate a notion of entropy with the horizon surface area, if spacetime enclosed by that horizon didn't have a microstructure of some kind?

##### Share on other sites

On 6/24/2020 at 6:13 AM, Markus Hanke said:

You cannot separate time and space, they always exist together as spacetime, regardless of whether the region in question is a vacuum or not.

The word "spacetime" is interesting.  It contains two separate nouns: "space" and "time",  joined  together to create a  new noun:  "spacetime"

And as a noun generally corresponds  to a  real "thing",  we are  led to think that "spacetime" must also be a real "thing".

But ,you could take any two separate nouns, say "cheese" and "clock" . Then join them together to create a new word :"cheeseclock".

Then insist that  "cheeseclock" must  be a real thing because there's a word for it.  But no - It's just a made-up word.  It doesn't mean anything

And I suggest that so-called "spacetime" is also a made-up word that doesn't mean anything.

"Space" and "Time" are two different things.  Pushing them together into one word doesn't make them the same.   It's just a linguistic illusion.

Or am i wrong.?  I'd welcome a well-reasoned refutation!

##### Share on other sites

Yes, you're wrong.
It isn't language that links them together, but theory and math.

Is an after-thought a thing that you might want to consider ?

##### Share on other sites

4 minutes ago, Charles 3781 said:

Or am i wrong.?  I'd welcome a well-reasoned refutation!

There is nothing here that needs to be refuted - spacetime is a mathematical model, a description of certain aspects of reality, in the same way that a map is a description of some territory. Would you argue that maps don't mean anything, just because they are not identical to the territory? Of course not. Instead, you ask to what degree of accuracy they reflect the real territory, in the sense that whether or not the map can get you from A to B without you getting lost. If it can, then it is a good map, if not then it needs to be discarded or revised.

The concept of spacetime is no different - does it accurately model the dynamics of test particles under the influence of gravity (or in relative motion, or both)? Within its domain of applicability, it evidently does a pretty good job at that.

There are other examples of this in physics, like electromagnetism, or thermodynamics.

13 minutes ago, Charles 3781 said:

And as a noun generally corresponds  to a  real "thing",  we are  led to think that "spacetime" must also be a real "thing".

This is not how it is understood at all. It's a mathematical model, not a 'real thing', whatever that even means.

##### Share on other sites

2 minutes ago, Markus Hanke said:

There is nothing here that needs to be refuted - spacetime is a mathematical model, a description of certain aspects of reality, in the same way that a map is a description of some territory. Would you argue that maps don't mean anything, just because they are not identical to the territory? Of course not. Instead, you ask to what degree of accuracy they reflect the real territory, in the sense that whether or not the map can get you from A to B without you getting lost. If it can, then it is a good map, if not then it needs to be discarded or revised.

The concept of spacetime is no different - does it accurately model the dynamics of test particles under the influence of gravity (or in relative motion, or both)? Within its domain of applicability, it evidently does a pretty good job at that.

There are other examples of this in physics, like electromagnetism, or thermodynamics.

This is not how it is understood at all. It's a mathematical model, not a 'real thing', whatever that even means.

Thanks Markus for your reply.  The thrust of your argument does however  disturb me a bit,  I take your point about maps, and their usefulness. But  maps are after all, small-scale representations of reality.  In the sense that everything on the maps, does represent something that physically exists.  For example, if the map shows a church at a particular location, that church really exists.  It may be drawn on the map in a symbolic form,  but if you go there, you can see the actual physical object, with its stonework, steeple and so on.  It's not a "mathematical model". It really exists.

That's why I find it difficult to follow your argument about "spacetime".  You seem to be proposing  that "spacetime" doesn't necessarily exist - it's  only a convenient model for doing the calculations and arriving at the correct answer.

Much the same, I suppose, that when navigating a ship across the ocean, it's convenient to model that the ocean is divided into lines of Latitude and Longitude.  Even when those lines don't physically exist, stretching across the surface of the sea.

I get that.   But I still find it upsetting.  You seem, with respect, to be retreating from  the scientific quest  to discover the true nature of the Universe, to a utilitarian  approach:

In the infamous words of the physicists:

"SHUT UP AND CALCULATE".

No offence meant, and thanks again for communicating with me - I appreciate it.

##### Share on other sites

5 hours ago, MigL said:

Are you saying that If space and time were not continuous, but discrete, then space and time translation symmetries wouldn't hold, and momentum and energy would not be conserved ?

No. Momentum and energy would make little jumps, but on the average they would be conserved. It's more involved with angular momentum. I see a problem with the conservation of angular momentum and the quantities related to the boosts, which have to do with the motion of the centre of energy of the system. But I see the problem if space-time were a regular lattice. The reason is that a regular lattice is very anisotropic, and angular momentum has to do with isotropy.

5 hours ago, MigL said:

Even if the discrete steps were at Planck scales ?
Are we sure they are conserved at such scales ?

3 hours ago, Markus Hanke said:

But what about entropy? If you consider - for simplicity's sake - a Schwarzschild black hole (i.e. empty spacetime everywhere), then how would it be possible to associate a notion of entropy with the horizon surface area, if spacetime enclosed by that horizon didn't have a microstructure of some kind?

Yes, you guys are right. At Planck scales some discretization must be introduced. I'm not sure it's just dividing space-time in little cells or a more sophisticated idea, like charts must be changed around cells the same way charts must be changed around a point in a sphere (for topological reasons, not because the sphere has any kind of discrete structure). Anyway... But last night when I wrote that comment, I started thinking about that too. Plus I tried to limit the sweeping character of my assertion when I said:

9 hours ago, joigus said:

Edit 2: I'm having second thoughts about what I've just written. I'm sure this argument has been wielded by physicists, but... If the discrete time structure were not a regular lattice, but a random homogeneous and isotropic web of points, the symmetry argument would no longer be valid.

What I meant is: I'm pretty sure that the discretization of space-time in a regular lattice does not sit well with rotational/boost symmetry, but if instead of a lattice, we had a random discrete structure, right now I see no reason why it wouldn't be possible. Yet, I've heard some physicists still use that argument (last time it was Nima Arkani-Hamed).

I must come clean and say I strongly dislike the idea, but I have no robust argument against it, really.

##### Share on other sites

9 hours ago, AbstractDreamer said:

OK non-relative frames.  Not really sure what I mean to be honest, but let me try and blurt out something coherent.

We have star A,B,C mutually, simultaneously interacting with each other, lets say gravitationally affecting their velocities.    The relative frame we choose as star A.    So the gravitational "force?" between A and B can be calculated and its effect on the velocity of star B as observed from star A can be calculated.

Star C also has a gravitational effect on and affect by both Star A and B, this too can be calculated.

So the interactions between star B and C are mutually simultaneous (presumably) but not in the same frame of reference as star A.  So that is non-relative frames.  What is happening between star B and C are non-relative to measurements or observations from star A, but presumably they are still simultaneous and might even be synchronisable with frames of observations from star A.

These are not inertial frames. Time is relative in all of them, but identical clocks can’t be synchronized between these frames.

I don’t think the words you are using mean what you think they mean. “interactions between star B and C are mutually simultaneous” makes no sense. You say non-relative for something that is relative.

##### Share on other sites

8 hours ago, Charles 3781 said:

You seem, with respect, to be retreating from  the scientific quest  to discover the true nature of the Universe

Physics doesn’t seek to ‘understand the true nature’ of things (that would be more of a philosophical or at best metaphysical endeavour) - it only seeks to make descriptive models of natural phenomena, formulated in the language of mathematics. Spacetime in particular is a descriptive model of the motion of test particles under the influence of gravity, as well as the relationships between clocks and rulers in different reference frames. Oftentimes, having a description of how something works also helps to illuminate why it works that way, but that is not necessarily always the case in physics.
There really is nothing wrong with a utilitarian approach, in my opinion - even if it sometimes fails to satisfy our curiosity. I personally think that demanding physics to provide answers that are actually beyond its intended domain of applicability is far more dangerous and problematic.

##### Share on other sites

3 hours ago, swansont said:

These are not inertial frames. Time is relative in all of them, but identical clocks can’t be synchronized between these frames.

I don’t think the words you are using mean what you think they mean. “interactions between star B and C are mutually simultaneous” makes no sense. You say non-relative for something that is relative.

“interactions between star B and C are mutually simultaneous”

By this i mean Star B is affected by Star C at the same time while Star B is effecting Star C.  They are simultaneously (and not sequentially - not even by an infinitessimal small increment in time - true simultaneity - not mathematical equivalence through granularity of calculus) interacting with each other.

I do not mean inertial frames, in this instance.

If time is relative in all of them, how can any two events separated by either a space value or a time value ever be truly simultaneous?  If you cannot compare two events because time is relative to each event, by deduction, the only that can be simultaneous with an event is the event itself, no?

##### Share on other sites

10 hours ago, Charles 3781 said:

"Space" and "Time" are two different things.  Pushing them together into one word doesn't make them the same.   It's just a linguistic illusion.

Or am i wrong.?  I'd welcome a well-reasoned refutation!

No.  We used to think that "space" and "time" where unrelated.  But then we learned that they are just two ways of measuring the same thing. The term space-time was coined to reflect this revelation. The "illusion" was that time and space were distinct and unrelated.

##### Share on other sites

2 hours ago, AbstractDreamer said:

“interactions between star B and C are mutually simultaneous”

By this i mean Star B is affected by Star C at the same time while Star B is effecting Star C.  They are simultaneously (and not sequentially - not even by an infinitessimal small increment in time - true simultaneity - not mathematical equivalence through granularity of calculus) interacting with each other.

IOW they are equidistant. Such signals travel at c. Simultaneity is not really used in this scenario, AFAIK.

2 hours ago, AbstractDreamer said:

I do not mean inertial frames, in this instance.

If time is relative in all of them, how can any two events separated by either a space value or a time value ever be truly simultaneous?  If you cannot compare two events because time is relative to each event, by deduction, the only that can be simultaneous with an event is the event itself, no?

If the observer gets the signal at the same time, they are simultaneous for that observer. They would not necessarily be simultaneous for other observers.

11 hours ago, Charles 3781 said:

You seem to be proposing  that "spacetime" doesn't necessarily exist - it's  only a convenient model for doing the calculations and arriving at the correct answer.

Yes.

Charles, meet physics. Physics, Charles.

11 hours ago, Charles 3781 said:

I get that.   But I still find it upsetting.  You seem, with respect, to be retreating from  the scientific quest  to discover the true nature of the Universe, to a utilitarian  approach:

That's not the quest that science is on, but that's a discussion for a different thread (and many already exist)

##### Share on other sites

On 9/3/2020 at 3:42 PM, Markus Hanke said:

Physics doesn’t seek to ‘understand the true nature’ of things (that would be more of a philosophical or at best metaphysical endeavour) - it only seeks to make descriptive models of natural phenomena, formulated in the language of mathematics.

So -  it's back to "saving the phenomena" is it?   That was the notorious term used in Astronomy,  until the 16th and early 17th centuries.

Back then, astronomers were working on an entirely erroneous theory, descended from Aristotle.  It was that the Earth is the centre of the Universe.  With all other celestial objects, planets and stars, revolving round the Earth in perfect circles.   This is not true, physically.   But the astronomers argued this way:

Astronomy is not about physical reality.  It's only concerned with successfully predicting astronomical phenomena - such as eclipses and conjunctions.  And our geocentric theory can do this.  If we make enough tweaks to it  -  by adding imaginary devices such epicycles, deferents, and the punctum aequans.   These may not represent physical reality.  But they enable us to mathematically  "save the phenomena"!

It seems to me that modern Physics is retrogressing into a similar attitude.

Edited by Charles 3781
##### Share on other sites

On 9/3/2020 at 5:49 PM, swansont said:

If the observer gets the signal at the same time, they are simultaneous for that observer. They would not necessarily be simultaneous for other observers.

So if two stars supernova, and are recorded as simultaneous by an observer equidistant to each, then let me say (for the remainder of this thread) the supernovae were "objectively" simultaneous.  That is, there is more than one coordinate where, had a recording been made, then a simultaneous reading would have been obtained, and that all such coordinates lie on a plane perpendicular to the line between the two events.

Are there any other points or locus that is not equidistant from the two stars that might read a simultaneous reading for two "objectively simultaneous" events, either though some specific non-inertial frame or other condition?

If the two stars supernova objectively non-simultaneously (that is, any equidistant observer would not observe simultaneity), what would the shape of the locus of simultaneity of  look like for non-inertial frames as a function of degree of non-simultaneity?  That is, there would be some point where two objectively non-simultaneous supernovae would be seen as simultaneous.

Lets say the supernovae were separated by 1light year and by 1 hour.  Each produces an expanding sphere of observables.  What does shape of the intersection of the two spheres look like over some time, and does this shape change with the degree of event separation (either distance or time).  Does this shape have a pattern?  Is it still plane-like with curves, or something else?

Moving away from simultaneity, i think my original post is more about concurrency.  Interactions throughout the universe all occurring concurrently, but not observably simultaneously.  Or is there not such thing?

Edited by AbstractDreamer
##### Share on other sites

9 hours ago, Charles 3781 said:

So -  it's back to "saving the phenomena" is it?

No, it’s simply about describing the world around us in terms of suitable models.
When I do General Relativity (one of my main areas of interest within physics), I think of spacetime is a network of relationships between events; in some sense that is quite real, because we can go and (at least in principle) measure these relationships directly with clocks and rulers, and what we find is in excellent agreement with the model. I do not look at spacetime as some kind of ‘thing’, as an ephemeral substance that is somewhere out there, akin to some old ideas of aether. If anything at all, its fundamental nature is information, namely relationship networks.

Like you, I am also interested in the question as to the true nature of things - but I do not expect the answer to come from physics alone. Firstly, there is a big question mark over what the concept of ‘true nature’ actually means - how do we know that there necessarily even is such a thing, that it is a meaningful concept? Our intuition as human beings tells us that there should be, but intuition is very much fallible on such matters. Secondly, who is to say that ‘true nature’ of the universe - even if it is a meaningful concept - can be known in an epistemological sense? Perhaps it is a phenomenological endeavour, meaning it can only be known through direct experience and insight, but not through intellectual knowledge alone - which is of course what many of the Asian philosophical schools teach.

So I personally believe that a complete understanding of the human condition (which of course includes the universe as a whole, since we aren’t separate from that) requires a synthesis of all the various domains of enquiry - the physical sciences, philosophy, social sciences, spirituality etc. These domains are not separate, they just ask different questions about different aspects, and answer them using different methodologies, which is fine so long as they yield constructive answers. I would really like to see these domains supporting and informing one another more, a coherent attempt to better understand who we are within the wider universe. It seems to me that one of the great tragedies in the history of the human race is that understanding the human condition somehow has become a partisan issue - science vs philosophy vs religion vs....you get the idea. I would never expect a complete understanding to arise from anyone of these domains in isolation, and that includes physics. But I also have no doubt that physics will have a central part to play, due to the nature of what it concerns itself with.

10 hours ago, Charles 3781 said:

Astronomy is not about physical reality.  It's only concerned with successfully predicting astronomical phenomena - such as eclipses and conjunctions.  And our geocentric theory can do this.  If we make enough tweaks to it  -  by adding imaginary devices such epicycles, deferents, and the punctum aequans.   These may not represent physical reality.  But they enable us to mathematically  "save the phenomena"!

Yes, and over time more and more data became available, and people began to realise that this model just didn’t do a very good job in predicting certain phenomena that could be observed. So eventually it was abandoned for something that worked better (all the politics and religious fanaticism that was involved aside for now). So physics is a continuous process of refinement - you keep evaluating existing models against new data that becomes available, and if something doesn’t fit, you either adapt the model, or come up with a new one that can explain things better. So over time you come closer - step by little step - to the underlying ‘true reality’ you seek. This process is in full swing currently, because we already know that there are certain situations where our current models fail, for example gravitational collapse, or the very early universe. We can be fairly certain that our current understanding of spacetime and all the various phenomena in it is only an effective approximation to something more fundamental, and many efforts are underway to find out what that is.

## Create an account

Register a new account