# Time and relativity (split from The Nature of Time)

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On 12/24/2022 at 8:18 AM, Markus Hanke said:

My post referred to gravity as we experience it (in the real world around us). Yes, you would also have some version of gravity in a universe without the GR concept of time (see my last post), but it would be very different from what we actually see in our universe.

l wrote:

On 12/23/2022 at 4:48 PM, DanMP said:

Gravity does exist regardless on how we define or understand time.

and you can't deny that 😄

You are very attached to the current understanding/definition of time, the one used in GR, but you really think that this is the final/ultimate theory that we can have in order to explain gravity with all its aspects? There are already many complaints about it since dark "stuff" appeared. Some even say that dark matter is not real and is used to maintain GR valid (I don't agree but it is possible). There are MOND theories proposed. There are other attempts also, including my theory, based on dark matter.

Moreover, if the GR definition/notion of time is "the one and only", please explain how is this particular definition (the notion of space-time) used in quantum physics.

Also please explain why, and how exactly, gravity wells are formed around massive objects and why exactly is the speed of light invariant? If you don't have an explanation, how can you be so sure that GR is the ultimate theory? With my theory I explained them fairly easy.

My opinion about time is that it is something we cannot see, touch, feel in any way. What we can see/observe is change. Because there is change, we can invent/define time as an useful notion/tool. We need it to compare changes (faster/slower), both in position (movement, speed, acceleration) and in structure (ageing, decay). We need it to make our theories, for our equations, for accurate predictions. And we also need it to write our history in chronological order. Last but not least we need it to function, to catch a train, to meet someone, to plan a trip, etc.

So time is as real as density, or pressure, or temperature, but more important.

3 hours ago, 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.

Gravitational radiation? You mean gravitational waves? If so, what is the explanation for them? The space-time vibrates? How?

You are absolutely sure that there is no other possibility to explain gravitational waves (outside GR)? How can you be? ( I have one, not the one I wrote here, that one was wrong).

Edited by DanMP

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20 minutes ago, DanMP said:

we can invent/define time as an useful notion/tool

I think that the notion of time evolved with us as we evolved as species. We needed it to survive, and it had to evolve as our environment changed. It evolved to deal with short intervals, order of .1s, important for fast responses, and with long intervals, of years, important for events on our life scale. Biological evolution didn't need to deal with nanoseconds, billions of years, star masses, or speed of light. But we deal with such environments now and thus the notion of time continues to evolve to include them as well.

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33 minutes ago, DanMP said:

If you don't have an explanation, how can you be so sure that GR is the ultimate theory? With my theory I explained them fairly easy.

It isn't the 'ultimate theory, but it is the one best supported by observational evidence.
Do I need to go through the 100 years of evidence accumulated in support of GR ?

Theories don't explain why, they explain how.
And IIRC, your 'theory' did neither, other than conjecture, and had no supporting observational evidence.

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2 minutes ago, MigL said:

Theories don't explain why, they explain how

Yes, but when you don't know how to explain why, you should be aware that your theory is very probable not the final/ultimate one. The more you can't explain, the more weak/vulnerable is your theory.

8 minutes ago, MigL said:

[...] and had no supporting observational evidence

GR also lack in evidence. Did we tested it using atomic clocks on the Moon? There are many tests to be done outside Earth.

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49 minutes ago, DanMP said:

GR also lack in evidence.

I suggest you get a new calendar; the year is 2022, soon to be 2023, and not 1916.
There has been over 100 years of observational evidence.
I suggest you look it up; I'm not here to spoon feed you.

And if you wish to discuss the failings of GR, which are also well documented, start another thread.

Edited by MigL
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2 hours ago, DanMP said:

I didn't entered in details regarding my theories and I won't (not even offer links), in order to respect the rules.

!

Moderator Note

There is no need to mention it since it’s not relevant. No need to respond to the modnote.

2 hours ago, DanMP said:

GR also lack in evidence. Did we tested it using atomic clocks on the Moon? There are many tests to be done outside Earth.

Why is this required? I’m sure there are places in Antarctica where g has not been measured. Is there any serious doubt that gravity exists there?

Quote

Yes, but when you don't know how to explain why, you should be aware that your theory is very probable not the final/ultimate one. The more you can't explain, the more weak/vulnerable is your theory.

It’s not part of theory, so how is it a weakness? GR doesn’t explain evolution, either. It doesn’t matter, since it’s not expected to.

And being the ultimate theory is not a criterion, either. That said, GR has passed every experimental rest that’s been done.

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A curious observation about past, present, and future as reflected in a language.

English has a simple system that only distinguishes between past, present, and future, just like SR. However, some languages have a system that obligates a speaker to make a finer distinction regarding when an event occurred. For example, whenever a speaker of Yagua (Peru) wants to refer to an event in the past, they must specify one of five different degrees of remoteness from the present: a few hours / couple of days / weeks up to one month / months up to a couple of years / distant or legendary past. Looks like not only the past is subdivided, but also the scale is "logarithmic" rather than linear.

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14 hours ago, DanMP said:

and you can't deny that 😄

Yes I can. As I have already pointed out, the kind of gravity we observe around us requires a very specific concept of time, which is the Einsteinian one. You can’t do away with it, or else the whole thing will no longer work.

15 hours ago, DanMP said:

You are very attached to the current understanding/definition of time, the one used in GR

No, I’m not attached to anything (in fact, modified and alternative theories of gravity are a special interest of mine). I’m simply considering what works within a given domain of applicability, and what doesn’t. If you look at the classical low-energy regime, then clearly GR works very well in that it provides an excellent description of the phenomenology of gravity as we see and measure it, within its domain of applicability. The experimental evidence for this from the past 100+ years is incontrovertible and overwhelming. As I have said multiple times, this requires a specific notion of time - if you do away with that notion, or replace it with something different, then the model can no longer function. That’s all there is to it.

15 hours ago, DanMP said:

but you really think that this is the final/ultimate theory that we can have in order to explain gravity with all its aspects?

Of course not. How could you think that? We already know that GR has a limited domain of applicability - at a minimum it cannot incorporate any quantum effects, and it is possible that even within the classical regime some phenomena might require modifications to ordinary GR. The jury is still out on that one.

But that’s not the point here at all. This thread deals with your claim that time is not required for classical gravity to exist in the way we see it around us, and that’s manifestly false.

15 hours ago, DanMP said:

There are MOND theories proposed.

MOND is only one example - there is in a fact a large number of alternative theories of gravity out there (none of which does away with time, btw!). The problem is that once you consider all available data (not just some isolated phenomenon), then none of the other models provide nearly as good a fit as GR does. We already know that it won’t be the final theory of gravity, but it really is the best one we have at present.

15 hours ago, DanMP said:

Moreover, if the GR definition/notion of time is "the one and only"

It’s not - no one said such a thing. There are many different ways to define a concept of “time”, both in physics and in philosophy. The point we are making here is that classical gravity as we observe it around us requires a specific concept of time, being the Einsteinian one.

15 hours ago, DanMP said:

please explain how is this particular definition (the notion of space-time) used in quantum physics

Quantum field theory - which is the most fundamental theory we currently have - requires a Minkowski space-time background in order to work; the entirety of the Standard Model is formulated against this background, so Einsteinian time is as fundamental to QFT as it is to GR. To be precise, the Lorentz symmetry we find in local patches of Minkowski spacetime implies the CPT invariance of the Standard Model Lagrangian, and vice versa.

One example of a model where a different notion of time is used is ordinary quantum mechanics - which is a low-energy, low-velocity, non-relativistic approximation to QFT that works only for systems where particle numbers are conserved. Here, time is simply a free parameter that is used to describe the evolution of a given system, it is not an observable of the theory, ie it can’t be consistently written as a Hermitian operator.

15 hours ago, DanMP said:

Also please explain why, and how exactly, gravity wells are formed around massive objects and why exactly is the speed of light invariant?

GR and SR are models that describe aspects of the universe - as such they answer mostly only the how questions, but not the why ones, in the same manner as a map describes the topography of some territory without providing an explanation of the geological processes that gave rise to that territory. That doesn’t make the map any less useful, if you’re trying to find your way from A to B.
The invariance of the speed of light is equivalent to saying that all inertial observers experience the same laws of physics; that’s an empirical finding about the world we find ourselves in. We don’t know yet why this is so, but if you really think about it, you’ll realise very quickly that the absence of this symmetry would immediately lead to physical and logical paradoxes that cannot be resolved, so at a minimum it is a matter of logical consistency.

As for the gravity wells, the answer is in my signature. There is a well defined relationship between local sources of energy-momentum, and certain aspects of local spacetime geometry, as described by the Einstein equations. Thus, the relevant aspects of gravity in the interior of things like planets, stars etc are directly given by the distribution of energy-momentum there. But because spacetime as a manifold is everywhere smooth and continuous, the exterior vacuum geometry must match up with the interior geometry in a way that guarantees smoothness and continuity at the boundary. This provides a boundary condition that - along with the asymptotic behaviour far away from the central body - determines the exterior geometry. In most ordinary cases the embedding diagram belonging to this exterior geometry will look roughly like the “gravity well” you are referring to.

15 hours ago, DanMP said:

You are absolutely sure that there is no other possibility to explain gravitational waves (outside GR)?

This wasn’t the point. The issue is whether - given a geometric theory of gravity, be that GR or some of its viable alternatives - you can have gravitational radiation without there being time. The answer is clearly no. Thus, the presence of gravitational radiation implies some level of physical reality for time.

16 hours ago, DanMP said:

The space-time vibrates? How?

Spacetime is not a physical medium, so there is nothing there that “vibrates” - that’s the kind of unfortunate misconception that is spread in pop-sci media. In reality, local geometry within some region of spacetime is determined not just by local energy-momentum, but also by distant sources. These do not appear explicitly in the field equations, but have to be provided in the form of initial and boundary conditions when solving them (they are differential equations after all). If it so happens that there is a distant source that has some form of non-vanishing quadrupole or higher multipole moment, then the local geometry will not be invariant under time translations, meaning you get tidal components that are explicitly time-dependent, even though all your local sources are stationary. This is how you get the characteristic effects of gravitational radiation.

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A curious observation about past, present, and future as reflected in a language.

English has a simple system that only distinguishes between past, present, and future, just like SR. However, some languages have a system that obligates a speaker to make a finer distinction regarding when an event occurred. For example, whenever a speaker of Yagua (Peru) wants to refer to an event in the past, they must specify one of five different degrees of remoteness from the present: a few hours / couple of days / weeks up to one month / months up to a couple of years / distant or legendary past. Looks like not only the past is subdivided, but also the scale is "logarithmic" rather than linear.

FYI, English is more subtle than that.

It offers the means to distinguish the order of events, with or without duration measurement.

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4 minutes ago, studiot said:

It offers the means to distinguish the order of events, with or without duration measurement.

It does indeed.

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21 hours ago, MigL said:

I suggest you get a new calendar; the year is 2022, soon to be 2023, and not 1916.
There has been over 100 years of observational evidence.
I suggest you look it up; I'm not here to spoon feed you.

I'm aware about the observational evidence and I wouldn't propose a theory inconsistent with it.

I have to make a correction: the test with the clock on the Moon (and one on the Earth) is a test for both GR and SR. The clocks would be subjected to both kinematic and gravitational time dilation.

21 hours ago, swansont said:

Why is this required? I’m sure there are places in Antarctica where g has not been measured. Is there any serious doubt that gravity exists there?

The test with a clock on the Moon I suggested, is a kind of test that was never done, so don't treat it as trivial. As far as I know we never tested GR+SR measuring and computing time dilation for 2 clocks situated in separate gravitational wells. All our tests were inside Earth's gravity well, including the GPS clocks on orbit. The Moon is orbiting the Earth but it is massive, having its own gravitational well. This is the novelty. And it can be done in the next years. Why we shouldn't do it?

8 hours ago, Markus Hanke said:

the kind of gravity we observe around us requires a very specific concept of time, which is the Einsteinian one. You can’t do away with it, or else the whole thing will no longer work.

8 hours ago, Markus Hanke said:

As I have said multiple times, this requires a specific notion of time - if you do away with that notion, or replace it with something different, then the model can no longer function.

Aha, there you said it, the model can no longer function, so GR/Einsteinian definition of time is required for the model, not for the gravity itself.

You also:

8 hours ago, Markus Hanke said:
On 12/27/2022 at 5:05 PM, DanMP said:

but you really think that this is the final/ultimate theory that we can have in order to explain gravity with all its aspects?

Of course not. How could you think that?

admitted that the current relativity is not the ultimate theory for gravity, so you are admitting that it is conceivable to have a new & better theory/model, maybe with a different concept/understanding of time.

So, as long as there is change, we can (and we kind of need) to define time. The way we define and use it can vary, so there is no need to cling on the current GR definition or any particular definition. As long as we can use it successfully, any definition works.

I just remembered something: if GR definition of time is the "true" one, our future is already "written"?

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12 minutes ago, DanMP said:

The test with a clock on the Moon I suggested, is a kind of test that was never done, so don't treat it as trivial. As far as I know we never tested GR+SR measuring and computing time dilation for 2 clocks situated in separate gravitational wells. All our tests were inside Earth's gravity well, including the GPS clocks on orbit. The Moon is orbiting the Earth but it is massive, having its own gravitational well. This is the novelty. And it can be done in the next years. Why we shouldn't do it?

We have tested the weak equivalence principle on the moon and found it holds as $m_i=m_g$ held on the moon test it follows that the time dilation aspects of GR will also hold

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34 minutes ago, DanMP said:

if GR definition of time is the "true" one, our future is already "written"?

Yes, that's the block universe theory. Einstein has been quoted* as calling the flow of time a "persistent illusion".

* Or seen on video. Sorry, I don't remember the source.

Edited by Lorentz Jr
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48 minutes ago, DanMP said:

is it not?

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2 hours ago, DanMP said:

The test with a clock on the Moon I suggested, is a kind of test that was never done, so don't treat it as trivial. As far as I know we never tested GR+SR measuring and computing time dilation for 2 clocks situated in separate gravitational wells. All our tests were inside Earth's gravity well, including the GPS clocks on orbit. The Moon is orbiting the Earth but it is massive, having its own gravitational well. This is the novelty. And it can be done in the next years. Why we shouldn't do it?

We didn’t have space-ready atomic clocks back when we were doing moon missions. Further, we are in the sun’s gravity well, and we’ve done measurements that used the different potential we sample as we orbit the sun (since the earth’s orbit isn’t a circle) in tests of local position invariance.

As for reasons not to do it, it’s a matter of cost/benefit. Without a compelling theoretical reason to think we’d find something new, it’s doubtful anyone would fund it. (The reason they did the Hafele-Keating experiment, since nobody was expecting a novel result, was that it was cheap - \$8k - so the ONR funded it)

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3 hours ago, DanMP said:

I just remembered something: if GR definition of time is the "true" one, our future is already "written"?

A lot of misunderstandings about the 'block' universe and what it represents.

This is a good explanation ...

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1 hour ago, MigL said:

A lot of misunderstandings about the 'block' universe and what it represents.

This is a good explanation ...

I agree there are a lot of misunderstandings, but omar khayyam and I can think of another interpretation of the block universe than the either of ones put forward in your vid.

4 hours ago, DanMP said:

I just remembered something: if GR definition of time is the "true" one, our future is already "written"?

The Migl's video does make the very important point about the meaning (or lack of it) of 'simultaneity'.

However the whole idea depends upon which you consider more fundamental, the floor you are standing on, or some x,y,z,t coordinate system.

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3 hours ago, studiot said:

omar khayyam and I can think of another interpretation of the block universe than the either of ones put forward in your vid.

Ok, I'm always interested in your view.

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14 hours ago, DanMP said:

As far as I know we never tested GR+SR measuring and computing time dilation for 2 clocks situated in separate gravitational wells.

You cannot ‘separate’ nearby gravitational wells neatly, because gravity is non-linear - spacetime in the Earth-Moon system is not simply the sum of ‘Earth gravity well’ + ‘Moon gravity well’, but something more complicated (though GR effects are quite small here). The same is true for all other planets in our solar system, since they are all subject to the gravity of the Sun.

14 hours ago, DanMP said:

Aha, there you said it, the model can no longer function, so GR/Einsteinian definition of time is required for the model, not for the gravity itself.

Of course, that goes without saying - the entirety of physics is about making models that describe aspects of the world around us. We don’t do philosophy or metaphysics or religion here, so everything that is being talked about in the physics section of this forum has to do with models. And whatever idea you have in mind about gravity is also a hypothesis about gravity, and not gravity itself.

But here’s the point - GR has been extensively tested, and found to be in excellent agreement with experiment and observation. So we can regard the model as valid within its domain of applicability, and thus extract predictions from it. Now, if you take the ‘time’ out of spacetime, thus reducing the universe to three dimensions, then GR tells us very clearly what would happen so far as gravity is concerned. As such, if you claim that gravity still works in 3D in the exact manner as we can observe around us, then the onus will be on you to present a mathematically self-consistent model that shows this, and we shall be happy to take a look at it.

14 hours ago, DanMP said:

admitted that the current relativity is not the ultimate theory for gravity

Of course it isn’t. That’s been known for at least the past 50+ years!

14 hours ago, DanMP said:

so you are admitting that it is conceivable to have a new & better theory/model

I don’t know why you use the word ‘admitting’, as if this is something that secretive and kept hidden. It isn’t. Alternative theories of classical gravity, as well as theories of quantum gravity, are probably the two most active areas of research in modern theoretical physics. Just search for some related terms of arXiv, and see yourself the amount of hits you get. So yes - the idea that the domain of applicability of GR might be limited even within the classical regime, and thus that it might only be a special case of something more general, is most definitely conceivable, and is being actively researched; there is a very large number of alternative models in existence. Do note though that when doing a direct comparison, none of these models - with the possible exceptions of relativistic MOND and Einstein-Cartan gravity - come even close to the versatility and success of GR.

Also do note - and this is important - that the domain of applicability of a model being limited is not the same as that model being wrong. We still use Newtonian gravity extensively to model scenarios where relativistic effects can be neglected, so the model continues to be right within its domain of applicability, even though we have had GR for over a century. It’s just that this domain is limited, and we have a pretty good idea exactly where those limits are. The same will eventually also be true for GR, though at the moment we have only a very rough idea where the limits might be, and only some educated guesswork about what lies beyond these.

14 hours ago, DanMP said:

maybe with a different concept/understanding of time.

Within the classical domain, it is very unlikely that any successful model of gravity will have a notion of time that is substantially different from that of Einsteinian spacetime, because any such theory will need to preserve local Lorentz invariance (and thus CPT invariance), which puts stringent bounds on what such models can look like. There are also more fundamental reasons based in topology that dictate why GR looks the way it does, so the notion of spacetime isn’t just a wild guess. The domain of quantum gravity is another matter entirely though - it is not just possible, but nearly certain, that we will have to fundamentally rethink our notions of both time and space to make quantised gravity work. Note that GR would be the classical limit of any such model.

14 hours ago, DanMP said:

As long as we can use it successfully, any definition works.

So then, why is the GR definition a problem for you? It works really well.

Edited by Markus Hanke
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11 hours ago, MigL said:

Ok, I'm always interested in your view.

I don't know how my offerings on the nature of time got mixed up with a discussion of gravitation or even why gravitation is being discussed.

Gravitation is not dependent upon time or models of how time works.

I would argue that starting off with a thread by linking relativity to the nature of time is likely to lead to confusion since you cannot effective discuss the former without an effective model of the latter.

So establishing that model is the first step and somehow the three should really be separated, but time, relativity and gravitation fit that song

(the nature of) Time, Relativity

Gravity makes three.

In terms of Omar Khayyam, and the block universe, I am thinking in terms of the operation of twinrod scrolls, corresponding to past, present and future.

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1 hour ago, studiot said:

somehow the three should really be separated, but time, relativity and gravitation fit that song

(the nature of) Time, Relativity

Gravity makes three.

As @Markus Hanke has pointed, the gravity (as we know it) would not exist without time. Would time (as we know it) exist without gravity? Without space?

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As @Markus Hanke has pointed, the gravity (as we know it) would not exist without time. Would time (as we know it) exist without gravity? Without space?

As time is a property that describes rate of change you must have the requirement of some state, object, field, space (as volume) that changes. as long as you can measure something then time is applicable. Obviously measurement isn't a requirement for time to be involved either as things change even without being measured. However you must have something that changes even if that something is another property.

Edited by Mordred
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21 minutes ago, Mordred said:

As time is a property that describes rate of change you must have the requirement of some state, object, field, space (as volume) that changes. as long as you can measure something then time is applicable. Obviously measurement isn't a requirement for time to be involved either as things change even without being measured. However you must have something that changes even if that something is another property.

In other words, yes, time could exist without gravity and without space, if I understand your statement correctly.

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without gravity certainly however you can't really have something that changes if that something has no spatial dimension. Can any object or state exist without a spatial dimension of some form. Time isn't an entity unto itself, so under that in order to have time you must something that changes

Edited by Mordred
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42 minutes ago, Mordred said:

you can't really have something that changes if that something has no spatial dimension

Why not? Some kind akin oscillating neutrinos?

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