Time and relativity (split from The Nature of Time)

[DanMP]

On 12/28/2022 at 9:09 PM, swansont said:

... 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.

https://arxiv.org/abs/1301.6145 

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 ...

The experiment I proposed is also a Hafele-Keating type experiment, but the traveling clock would be in a different gravity well than ours.

The kinematic time dilation caused by the moon rotation around the earth is the one of interest to me, not the gravitational components, so I don't think that local position invariance tests are helpful.

The clock in the moon's gravity well may be on a satellite in close orbit around the moon, so it is less costly that you may think, and doable in the near future. 

Since no such a test was ever done (as far as I know), it may be important and the benefits greater than expected. 

[Mordred]

21 minutes ago, DanMP said:

 

The kinematic time dilation caused by the moon rotation around the earth is the one of interest to me, not the gravitational components, so I don't think that local position invariance tests are helpful.

 

You can't really separate the two gravity is spacetime curvature the time dilation itself results from that curvature where inertial mass and gravitational mass have equivalency ( m_i=m_g). As I mentioned they did test the weak equivalence principle on the moon

"Lunar Laser Ranging Tests of the Equivalence Principle with the Earth and Moon"

https://arxiv.org/abs/gr-qc/0507083

the strong equivalence principle is also inclusive, however as the test involves laser ranging you are in actuality also testing time dilation via the rate of signals.

it might help to further understand that with spacetime curvature you get distortions in light rays from an object. Take two lasers fie them in parallel. If the beams stay parallel you have a flat geometry with no time dilation. If the beams converge you have positive curvature aka gravity with time dilation. 

So its quite possible to test for time dilation without having to place a clock on the moon. You can literally use lasers or other EM signals.

Edited December 30, 2022 by Mordred

[swansont]

58 minutes ago, DanMP said:

The experiment I proposed is also a Hafele-Keating type experiment, but the traveling clock would be in a different gravity well than ours.

The kinematic time dilation caused by the moon rotation around the earth is the one of interest to me, not the gravitational components, so I don't think that local position invariance tests are helpful.

How big of an effect is this? And why does it have to be on the moon?

 

[MigL]

15 hours ago, Markus Hanke said:

I feel I need to quickly summarise here what my viewpoint actually is,

Me too.

I posted an explanatory link of the blok universe model ( much to Studiot's displeasure, it seems ) only because another poster had misconceptions about where the model is applicable.
As with all models, it is not applicable in all circumstances, and I, myself, am not overly fond of it.
However, I'm even less fond of the misconceptions that get posted on this forum about it.

[Markus Hanke]

13 hours ago, DanMP said:

In the same way, when we'll understand dark matter and/or we'll learn new things, maybe we will be able to have a better understanding and to make (and test)  new predictions regarding relativity.

Yes, I definitely agree with this. This is why (as I have mentioned before) the physics community is very actively researching both quantum gravity, and alternative models of classic gravity - because eventually we would like to gain better insight into not just how gravity works, but also why it works in the specific way it does.

12 hours ago, Lorentz Jr said:

Until there's an earthquake, and then all of a sudden, knowing the answers to those "why" questions becomes VERY useful. 😉

Yes, you’re right. As mentioned above, this is something that is being worked on, and has been for some time. The only thing is that there is no guarantee that the why necessarily always falls into the domain of physics. We can never know for sure, we can just keep going forward.

[DanMP]

On 12/30/2022 at 8:10 PM, Mordred said:

You can't really separate the two

 

...

So its quite possible to test for time dilation without having to place a clock on the moon. You can literally use lasers or other EM signals.

The kinematic time dilation for the clock on the Moon I'm interested in is the one caused by the Moon rotation around the Earth. In order to calculate it we can also consider the case where the Moon is "hovering" (not rotating).

On 12/30/2022 at 8:45 PM, swansont said:

How big of an effect is this?

The difference in time between rotating and non-rotating Moon. It would be big enough, especially for long intervals. We may set the clock on the Moon to send a signal at every exact 12 hours (or any other interval) for months/years.

On 12/30/2022 at 8:45 PM, swansont said:

And why does it have to be on the moon?

It doesn't. It may be on satellites around the Moon, or on/around any massive object other than Earth (where the other clock is).

I have more GR tests in mind, most of them in space. GR is very important to us, so it needs to pass all the tests we can imagine. And I'm not talking about repeating in other locations the test we done. It's about new kind of tests. Some day I'll elaborate on this.

[swansont]

48 minutes ago, DanMP said:

The difference in time between rotating and non-rotating Moon. It would be big enough, especially for long intervals. We may set the clock on the Moon to send a signal at every exact 12 hours (or any other interval) for months/years.

“how big” is an invitation to quantify the prediction. 

I see that there is a proposal to send clocks to an orbit near the sun in a search for dark matter. If that gets approved, it will be because there is a model that predicts an effect (I know Andrei Derevianko proposed one years ago; I discussed it with him at a conference) and they will have quantified the prediction, so they would know if the effect can be measured with the clocks they sent.

You, OTOH, offer nothing beyond “send clocks to the moon” and there’s no reason to entertain such a vague proposal.

 

[Mordred]

8 hours ago, DanMP said:

The kinematic time dilation for the clock on the Moon I'm interested in is the one caused by the Moon rotation around the Earth. In order to calculate it we can also consider the case where the Moon is "hovering" (not rotating).

 

 Why would that make any difference the moons angular momentum is far too low to even make any difference however even then those same lasers can still test for it. You still seem to not understand that inertial mass is identical to gravitational mass for all practical tests.

 

8 hours ago, DanMP said:

 

I have more GR tests in mind, most of them in space. GR is very important to us, so it needs to pass all the tests we can imagine. And I'm not talking about repeating in other locations the test we done. It's about new kind of tests. Some day I'll elaborate on this.

If you ever plan on ever getting any form of funding you will require explicit mathematics to present your case. No one invests in haphazard guesses an conjecture. 

[DanMP]

20 hours ago, swansont said:

“how big” is an invitation to quantify the prediction.

...

 

You, OTOH, offer nothing beyond “send clocks to the moon” and there’s no reason to entertain such a vague proposal.

13 hours ago, Mordred said:

If you ever plan on ever getting any form of funding you will require explicit mathematics to present your case. No one invests in haphazard guesses an conjecture. 

You both seem to forget that the test I proposed is a (new kind of) test for GR, so GR specialists should make the predictions about the Moon-Earth clocks differences and see if the GR predictions are consistent with the reality/measurements. The Chinese sent a rover to the Moon, Artemis program just orbited the Moon and is planning to establish a permanent base camp on the Moon, so it would be very cheap to set up the experiment I proposed. If the scientists are not interested enough in GR (and more) in order to do it, they may lose much more time and resources going in the (possibly) wrong direction ... Remember that it doesn't matter how many test you passed if there is even one test that you don't. And I have to repeat, this kind of test was never done (as far as I know).

 

15 hours ago, Mordred said:

Why would that make any difference the moons angular momentum is far too low to even make any difference however even then those same lasers can still test for it. You still seem to not understand that inertial mass is identical to gravitational mass for all practical tests.

The speed of the planes used in the Hafele-Keating experiment was smaller than the speed of the Moon around the Earth and they registered differences, so why you consider the speed (or angular momentum) of the Moon far too low to even make any difference?

I understand that inertial mass is identical to gravitational mass and I never disputed it. What I don't understand is how the experiment I proposed (with one clock on the Moon compared for months/years with one on the Earth) can be done from Earth, with lasers.

[swansont]

21 hours ago, DanMP said:

You both seem to forget that the test I proposed is a (new kind of) test for GR, so GR specialists should make the predictions about the Moon-Earth clocks differences and see if the GR predictions are consistent with the reality/measurements.

It’s not a new kind of test, and you haven’t given any reason why some novel result would be expected.

 

21 hours ago, DanMP said:

The speed of the planes used in the Hafele-Keating experiment was smaller than the speed of the Moon around the Earth and they registered differences, so why you consider the speed (or angular momentum) of the Moon far too low to even make any difference?

GPS satellites travel even faster than that, so you aren’t covering any new ground in that regard. And the comment was specifically about the rotation of the moon (the angular momentum), which is quite slow.

[DanMP]

46 minutes ago, swansont said:

It’s not a new kind of test, and you haven’t given any reason why some novel result would be expected.

It is new because the clocks are in separate gravity wells. Never done before (as far as I know).

If we never did such a test we don't know if GR would pass it. And GR is too important to us. As I said, with so many Moon expeditions scheduled in the near future, it would be not expensive. You wrote:

On 12/28/2022 at 9:09 PM, swansont said:

(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)

 

1 hour ago, swansont said:

GPS satellites travel even faster than that, so you aren’t covering any new ground in that regard. And the comment was specifically about the rotation of the moon (the angular momentum), which is quite slow.

Faster, yes, but GPS satellites are not massive objects, with big gravity wells. So, again, on the Moon would be other conditions (the separate gravity well), never tested before in this way. The speed is higher than in the original H-K experiment, so it would be enough, especially with long intervals.

[studiot]

16 minutes ago, DanMP said:

It is new because the clocks are in separate gravity wells. Never done before (as far as I know).

I really don't understand what's special about this proposed experiment

 

Since the early days, it has been a tenet of astrospectroscopy that the pattern of spectral lines (including shifts thereto)  and aberations are determined by phenomena described by GR.
So light arising in galaxies (and therefore gravity wells) far far away mr skywalker, are calibrated using GR.

[DanMP]

8 minutes ago, studiot said:

I really don't understand what's special about this proposed experiment

 

Since the early days, it has been a tenet of astrospectroscopy that the pattern of spectral lines (including shifts thereto)  and aberations are determined by phenomena described by GR.
So light arising in galaxies (and therefore gravity wells) far far away mr skywalker, are calibrated using GR.

It is special because it was never done.

Don't expect that, if not passed, GR is/was entirely wrong. Maybe just in need for little adjustments.

[studiot]

2 minutes ago, DanMP said:

It is special because it was never done.

Don't expect that, if not passed, GR is/was entirely wrong. Maybe just in need for little adjustments.

What about the gravity well conditions I offered you ?

[DanMP]

1 minute ago, studiot said:

What about the gravity well conditions I offered you ?

I never contested gravity wells or something that you said, but how is this relevant to the experiment I proposed? There are parts/domains of GR never really tested, and this 2 clocks in 2 different gravity wells experiment/situation is one of them.

[studiot]

6 minutes ago, DanMP said:

I never contested gravity wells or something that you said, but how is this relevant to the experiment I proposed? There are parts/domains of GR never really tested, and this 2 clocks in 2 different gravity wells experiment/situation is one of them.

So what is a line spectrum from Sol and Sirius if it is not a comparison to two clocks in different widely separated gravity wells ?

Edited January 6, 2023 by studiot

[swansont]

2 hours ago, DanMP said:

Faster, yes, but GPS satellites are not massive objects, with big gravity wells. So, again, on the Moon would be other conditions (the separate gravity well), never tested before in this way. The speed is higher than in the original H-K experiment, so it would be enough, especially with long intervals.

Please stop moving the goalposts.

You cited the speed of the moon in comparison to airplanes. Nothing about gravity wells in that claim. Speed and gravitation are separate effects.

And you still haven’t explained why being in the moon’s vicinity would make a difference. 

As I noted before, we have done GR tests using variations in the sun’s gravity well in addition to the earth’s.

[Markus Hanke]

16 hours ago, DanMP said:

It is new because the clocks are in separate gravity wells. Never done before (as far as I know).

I don’t see how this is different from the Shapiro delay, which has been tested extensively in a variety of ways.

[DanMP]

On 1/6/2023 at 5:00 PM, studiot said:

So what is a line spectrum from Sol and Sirius if it is not a comparison to two clocks in different widely separated gravity wells ?

What 2 clocks? The lines in the spectrum are not from individual emitters. You don't know the exact position (altitude) in order to calculate gravitational time dilation. You don't know the exact speed in order to calculate the kinematic time dilation. And what accuracy you can get using this approach? What speed, perpendicular on the line between us and Sirius, was detected using this method that you claim it covered the test I proposed? And what other metod was used to determine that speed, in order to say that GR predicted the exact same speed?

 

On 1/6/2023 at 6:29 PM, swansont said:

Please stop moving the goalposts.

You cited the speed of the moon in comparison to airplanes. Nothing about gravity wells in that claim. Speed and gravitation are separate effects.

I wrote:

On 12/28/2022 at 6:47 PM, DanMP said:

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.

and:

On 12/30/2022 at 7:51 PM, DanMP said:

The experiment I proposed is also a Hafele-Keating type experiment, but the traveling clock would be in a different gravity well than ours.

The kinematic time dilation caused by the moon rotation around the earth is the one of interest to me, not the gravitational components, so ...

 

In H-K experiment there is kinematic time dilation and gravitational time dilation. The same in GPS time dilation, where they offered calculations and they wrote:

Quote

The SR effect is due to their constant movement and height relative to the Earth-centered, non-rotating approximately inertial reference frame. In short, the clocks on the satellites are slowed down by the velocity of the satellite.

My question is: what if we choose a GPS satellite based reference frame? This is also an approximately inertial reference frame. In this case the Earth is moving, so the clocks on Earth should be slowed down ... 

I learned from another discussion that in fact:

Quote

General relativity includes gravitational time dilation, and not just kinematic time dilation. General relativity does have formulae that allows one to compute time dilation if one specifies what one means by time dilation sufficiently well (for instance by specifying some particular coordinate system as above). However, one does not get the correct result by using the SR formulae for time dilation in this circumstance.

Given a choice of coordinate system, time dilation can conveniently be defined as the ratio of proper time to coordinate time for an observer following some specific path (worldline).

Using the metric associated with the chosen coordinate system, one can compute the proper time from the relation dτ²=g_ijdxⁱdx^k, where τ is the proper time, the xⁱ are the choosen coordinates, and g_ij are the metric coefficients associated with the particular coordinate choice. ...

so my experiment (with one clock on the Moon) is actually a test for GR not for GR+SR and the things are not that simple: what reference frame should we use and why? What exact kinematic effects would the clocks register?

 

On 1/7/2023 at 8:46 AM, Markus Hanke said:

I don’t see how this is different from the Shapiro delay, which has been tested extensively in a variety of ways.

It is different. See above.

[swansont]

1 hour ago, DanMP said:

I wrote:

and:

 

You wrote “The kinematic time dilation caused by the moon rotation around the earth is the one of interest to me, not the gravitational components” 

So why do you need a different gravity well?

What is special about the moon’s speed, that we can’t use a satellite, which can be higher?

 

1 hour ago, DanMP said:

In H-K experiment there is kinematic time dilation and gravitational time dilation. The same in GPS time dilation, where they offered calculations and they wrote:

And the moon would have both.

 

1 hour ago, DanMP said:

My question is: what if we choose a GPS satellite based reference frame? This is also an approximately inertial reference frame. In this case the Earth is moving, so the clocks on Earth should be slowed down ... 

No, it’s not an inertial frame.

 

1 hour ago, DanMP said:

I learned from another discussion that in fact:

so my experiment (with one clock on the Moon) is actually a test for GR not for GR+SR

The moon is moving relative to us. How does kinematic time dilation not occur?

1 hour ago, DanMP said:

and the things are not that simple: what reference frame should we use and why? What exact kinematic effects would the clocks register?

You can use whichever frame you wish. The adjusted clocks on GPS satellites give nominally the same time as the ground station. This is true if you are looking at it from either reference frame.

Use of a particular frame is a choice, often chosen by convenience. It’s not dictated by the physics.

 

[DanMP]

20 minutes ago, swansont said:

What is special about the moon’s speed, that we can’t use a satellite, which can be higher?

The Moon gravity well, not its speed, is special/different, compared to satellites.

 

23 minutes ago, swansont said:

No, it’s not an inertial frame.

I wrote "approximately inertial reference frame" as they said about Earth's frame. Why one is OK and the other not-OK? Is the Moon fame OK? If not, why not?

 

30 minutes ago, swansont said:

The moon is moving relative to us. How does kinematic time dilation not occur?

Did you make GR calculations, as you should, or you just applied Special Relativity? And, more important, how do you know for sure that it will occur before actually testing it? Also, the Earth is moving relative to the Moon, so according to the Moon observer the clock on Earth should be slower.

As I said, with so many expeditions scheduled for the Moon in the near future, it would be cheap to do the test. I also said that the clock can be on the orbit around the Moon, not necessarily on the ground. The "kinematic time dilation" due to the rotation of the Moon around the Earth can still be assessed and tested.

[zapatos]

37 minutes ago, DanMP said:

The Moon gravity well, not its speed, is special/different, compared to satellites.

 

Different, but not special.

What is it about the difference that makes the moon a better platform than a satellite?

[swansont]

45 minutes ago, DanMP said:

The Moon gravity well, not its speed, is special/different, compared to satellites.

You wrote “The kinematic time dilation caused by the moon rotation around the earth is the one of interest to me, not the gravitational components” 

You’ve also insisted it’s the gravity, not the speed

Which is it?

 

45 minutes ago, DanMP said:

 

I wrote "approximately inertial reference frame" as they said about Earth's frame. Why one is OK and the other not-OK? Is the Moon fame OK? If not, why not?

The earth’s frame and the GPS frame are not the same. Compare their centripetal accelerations.

Earth is not an inertial frame, but can be approximated as one under some circumstances. It can be a bad approximation in others. (who are “they” and what exactly did “they” say?)

 

45 minutes ago, DanMP said:

Did you make GR calculations, as you should, or you just applied Special Relativity? 

How is this an answer to my question: The moon is moving relative to us. How does kinematic time dilation not occur?

 

45 minutes ago, DanMP said:

And, more important, how do you know for sure that it will occur before actually testing it?

Relativity is well-tested. We’re past the point of having the necessary confidence that it’s valid within the levels of precision we’ve tested. Confident that gravity behaves the same on the moon as on earth (which we’ve tested in a number of ways)

 

45 minutes ago, DanMP said:

Also, the Earth is moving relative to the Moon, so according to the Moon observer the clock on Earth should be slower.

That symmetry only applies to inertial frames of reference.

 

45 minutes ago, DanMP said:

As I said, with so many expeditions scheduled for the Moon in the near future, it would be cheap to do the test. I also said that the clock can be on the orbit around the Moon, not necessarily on the ground. The "kinematic time dilation" due to the rotation of the Moon around the Earth can still be assessed and tested.

No, I don’t think it would necessarily be cheap. Hafele-Keating was cheap because the incremental cost was plane tickets; they already had the clocks. What you’re proposing needs space-qualified clocks, which are generally not sitting around, and more expensive. And the cost of launching payloads is significantly higher than getting on a plane.

[studiot]

3 hours ago, DanMP said:

What 2 clocks? The lines in the spectrum are not from individual emitters. You don't know the exact position (altitude) in order to calculate gravitational time dilation. You don't know the exact speed in order to calculate the kinematic time dilation. And what accuracy you can get using this approach? What speed, perpendicular on the line between us and Sirius, was detected using this method that you claim it covered the test I proposed? And what other metod was used to determine that speed, in order to say that GR predicted the exact same speed?

I don't know the exact speed of light in Earth's frame ?

What rubbish are you trying to blow to obscure the fact that you chose the Moon over a satellite because it is more massive and exerts stronger gravity (whatever way you calculate it) and I have shown you two even stronge gravity wells ?

Or are you denying that Sol and Sirius have gravity wells ?

Or are you denying that our measurement of time is tied to the frequency of some oscillator ?

Or are you denying the fact that we observe a redshift of light from Sirius, compared to Sol by observing the frequencies of certain patterns of spectral lines ?

You are just arguing for the sake of it, without any substance whatsoever.

[Ghideon]

This may be of interest to @DanMP; testing GR in the context of solar gravity by using a Viking spacecraft that landed on Mars. 

Quote

Analysis of 14 months of data obtained from radio ranging to the Viking spacecraft verified, to an estimated accuracy of 0.1%, the prediction of the general theory of relativity that the round-trip times of light signals traveling between the Earth and Mars are increased by the direct effect of solar gravity. 

https://articles.adsabs.harvard.edu/pdf/1979ApJ...234L.219R, (bold by me)

 

(I got inspired by @studiot suggestion and looked for any existing or proposed experiments within the solar system.)