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# Clocks, rulers... and an issue for relativity

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I will ask you what I asked robinpike elsewhere in the thread. Two objects, A and B, are in relative motion

In A's frame it is is rest and has no kinetic energy. B is moving, and has KEB. KEB > KEA (=0)

In B's frame it is is rest and has no kinetic energy. A is moving, and has KEA. (0=) KEB < KEA

Can you accept this? Or put another way, that they each see themselves as moving slower than the other one, since they see themselves as at rest?

Yes I accept this. Very enlightening post. +1 again.

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Yes I accept this. Very enlightening post. +1 again.

The next step: Length is another property that's relative.

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The next step: Length is another property that's relative.

The problem is that that doesn't explain anything. That makes everything mysterious. Kinetic Energy, length, distance, time become elusive.

----------------------------

edit

That is why I am searching a way to ground all these. In the FOR of the object itself, length doesn't change, time doesn't change, and KE is null.

Perfect.

Edited by michel123456
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bold emphasis mine:

Are you not complicating things unnecessarily? I'm just asking, as I don't really know what you have in mind, but the simplest case is a traveling clock that is passing the Earth's clock at high speed and later turns around and passes the Earth's clock again. Only one acceleration step involved, and roughly the same prediction about the difference in clock times between the two events.

I agree, this is a good case to study first.

After subsequently addressing the full twins scenario, its also good to compare the all-inertial time-handoff scenario to the other 2 cases.

Best regards,

Celeritas

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The problem is that that doesn't explain anything. That makes everything mysterious. Kinetic Energy, length, distance, time become elusive.

Complain to mother nature if it's confusing. But you only have to worry about one frame, anyway.

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Sure there is.

Because in my examples nobody is arguing that B becomes "really" shorter because there is a distance between A & B.

In Relativity some argue that B is "really" shorter and that time is "really" dilated because these are not optical effects.

You must understand that it is really troubling. Especially when it is linked to the symmetrical effect.

IOW any layman will accept that if B is "really" shorter than A, the A is "really" bigger than B.

And a layman will not accept easily that they are "both really shorter than the other".

Let's hope this will help:

If you cut a loaf of '3D' bread, all '2D' slices are equally 'real'. But depending on the angle you cut, the slice will contain a different set of grains, with spatial distances between those grains.

If a frame makes a 3D section through 4D spacetime, all sections are equally 'real'. But depending on the angle you cut (relative speed) you will have a different set of simultaneous events, and spatial distances between those events.

A slice of bread at one angle is not less or more real than a slice at another angle. And the shorter slice is not a 'contracted' one of a slice at another angle, because the slices consist of different set of grains.

Same thing with the 3D sections of simultaneous events through a 4D train (or car, or ruler!) spread out in 4D spacetime.

Edited by VandD
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Complain to mother nature if it's confusing. But you only have to worry about one frame, anyway.

Great we can all run around in circles yelling " My Frame,My Frame.."

Edited by Mordred
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Complain to mother nature if it's confusing.

I am doing that all the time since the age of twelve.

Let's hope this will help:

If you cut a loaf of '3D' bread, all '2D' slices are equally 'real'. But depending on the angle you cut, the slice will contain a different set of grains, with spatial distances between those grains.

If a frame makes a 3D section through 4D spacetime, all sections are equally 'real'. But depending on the angle you cut (relative speed) you will have a different set of simultaneous events, and spatial distances between those events.

A slice of bread at one angle is not less or more real than a slice at another angle. And the shorter slice is not a 'contracted' one of a slice at another angle, because the slices consist of different set of grains.

Same thing with the 3D sections of simultaneous events through a 4D train (or car, or ruler!) spread out in 4D spacetime.

If you replace the cut with a scan, yes that may help.

Because when you make a cut, it is done and you cannot make it again in another way. A scan does not disturb the bread, it is simply an image.

Everything is great and fully comprehensible about the relativity of observation from each frame.

The comprehensibility gets difficult when it comes to make the image a "real" thing. As if the image from the scan suddenly transformed into a real slice of bread.

And as Swansont posted, the same utterly incomprehensible phenomenon appears with kinetic energy.

Figure that the Earth is hit tomorrow by some gigantic asteroid, where was the KE hidden all this time? Inside motion? The KE reveals with the collision the same way the image of the scan reveals into a slice of bread.

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[..] the simplest case is a traveling clock that is passing the Earth's clock at high speed and later turns around and passes the Earth's clock again. Only one acceleration step involved, and roughly the same prediction about the difference in clock times between the two events.

Perhaps, but I would prefer not to use a scenario where different observers may measure the travelling clock in different ways. If I keep to when the travelling clock begins next to the stay at home clock, and ends its round trip, again next to the stay at home clock, then at least all observers can agree that the travelling clock lost time compared to the stay at home clock. That may make it easier to find if a logical contradiction has occurred? Also it is the acceleration of those two events that results in the travelling clock making different progress through space-time (shorter / as opposed to back to not shorter).

Robin I perfectly understood that you want to stick to a case in which all observers can agree that the travelling clock "lost time" compared to the stay at home clock. In my simplified version with only one acceleration step, that is the case. As several of us clarified earlier, acceleration matters for arriving at a different velocity (so that the traveler is not continuously at rest in an inertial reference system), as well for being able to return to the stay-at-home.

And note that, in order to make it even clearer that the Lorentz transformations relate to velocity, there is also a version without acceleration:

Instead of turning around, the traveler may send a time signal to another traveler passing nearby and who travels in opposite direction. If that other traveler adopts the time of the first traveler at the instant of passing, the SR prediction is just the same for the predicted clock difference when that last traveler passes the Earth clock.

PS: Probably that is what Celeritas meant with "the all-inertial time-handoff scenario".

Sure there is.

Because in my examples nobody is arguing that B becomes "really" shorter because there is a distance between A & B.

In Relativity some argue that B is "really" shorter and that time is "really" dilated because these are not optical effects.

You must understand that it is really troubling. Especially when it is linked to the symmetrical effect.

IOW any layman will accept that if B is "really" shorter than A, the A is "really" bigger than B.

And a layman will not accept easily that they are "both really shorter than the other".

Hi Michael, after "knowing" SR for a long time (that is , knowing how to calculate), I was confronted with the same issues some 15 years ago and in my opinion this is due to the purposeful lack of a physical model in SR so that models automatically are "metaphysics". Happily Scienceforums has a philosophy forum on which currently already a thread is open on a related topic. If nobody else does it then I'll soon start a spin-off topic that should be helpful for you (and probably also Robin), discussing two possible models to "make sense" of SR.

Edited by Tim88
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Hi Michael, after "knowing" SR for a long time (that is , knowing how to calculate), I was confronted with the same issues some 15 years ago and in my opinion this is due to the purposeful lack of a physical model in SR so that models automatically are "metaphysics". Happily Scienceforums has a philosophy forum on which currently already a thread is open on a related topic. If nobody else does it then I'll soon start a spin-off topic that should be helpful for you (and probably also Robin), discussing two possible models to "make sense" of SR.

I cannot disagree more. If you need metaphysics to "make sense" of physics it is a huge defeat. I will not step into this kind of discussion. To me, any model of physics must be self explainable and self comprehensible. You don't need metaphysics to explain physics. You don't need metaphysics to understand physics

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I cannot disagree more. If you need metaphysics to "make sense" of physics it is a huge defeat. I will not step into this kind of discussion. To me, any model of physics must be self explainable and self comprehensible. You don't need metaphysics to explain physics. You don't need metaphysics to understand physics

You no doubt disagreed with a misunderstanding of what I said: it's hard to believe that you deem physical models useless for understanding!

For example the atomic model could not be proven until the last century and was therefore still metaphysics; Mach did not believe it. But it facilitated physical understanding.

As a matter of fact, many of your questions here fit Wikipedia's introduction description of metaphysics.

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You no doubt disagreed with a misunderstanding of what I said: it's hard to believe that you deem physical models useless for understanding!

For example the atomic model could not be proven until the last century and was therefore still metaphysics; Mach did not believe it. But it facilitated physical understanding.

As a matter of fact, many of your questions here fit Wikipedia's introduction description of metaphysics.

Atomic models were demonstrably wrong until they could be compared to experiment and refined. But even e.g. the plum pudding model was informed by experiment — we knew electrons existed. They all came crashing down when Rutherford's students fire alpha particles at a gold foil and saw some backscatter, indicating a dense nucleus. Then they got better.

I'm not sure why you are labeling SR as metaphysics. It's a tested model that agrees with nature. And we know the source of the effects: the invariance of c, which appears in electrodynamics and is also well-tested.

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Atomic models were demonstrably wrong until they could be compared to experiment and refined. But even e.g. the plum pudding model was informed by experiment — we knew electrons existed. They all came crashing down when Rutherford's students fire alpha particles at a gold foil and saw some backscatter, indicating a dense nucleus. Then they got better.

I'm not sure why you are labeling SR as metaphysics. It's a tested model that agrees with nature. And we know the source of the effects: the invariance of c, which appears in electrodynamics and is also well-tested.

I wrote "until"; you read "after". Once more, Mach did not believe in atoms. Atoms were a conceptual model for many centuries, but only last century we could really observe them. And I'm not labeling SR as metaphysics; it's unclear why you would think so. SR is a theory based on postulates which were presented as principles instead of physical models.

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I wrote "until"; you read "after". Once more, Mach did not believe in atoms. Atoms were a conceptual model for many centuries, but only last century we could really observe them. And I'm not labeling SR as metaphysics; it's unclear why you would think so. SR is a theory based on postulates which were presented as principles instead of physical models.

I was focusing on your statement that they facilitated understanding.

You implied SR was metaphysics a few posts back, owing to its lack of a ohysical model. At least, that's how it reads to me.

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In Relativity some argue that B is "really" shorter and that time is "really" dilated because these are not optical effects.

You must understand that it is really troubling. Especially when it is linked to the symmetrical effect.

IOW any layman will accept that if B is "really" shorter than A, the A is "really" bigger than B.

And a layman will not accept easily that they are "both really shorter than the other".

Michel123456,

The thread is presumedly about "interpretation of correct relativity statements". You have registered a complaint, that forum members are trying to explain relativity instead. Yet, the only reason folks interpret the traditional statements of relativity incorrectly, is because they do not really understand SR yet. That alone, makes it impossible to ever understand the much more complex twins scenario.

Here's another example. robinpike has been trying to build a proof that relativity is not self contradictory. He's doing this, by collecting all the relativity related buzz-phrases that many relativists make, and that most relativists agree about. If he attains all the required buzz phrases, he believes his collection will stand on its own, and in a way such that anyone (who does not yet understand relativity) will easily "see the light". However, this is about the worst way in which one can come to understand the theory, to always interpret proper statements of relativity correctly. One must first learn the math, that is, the LTs. Run some calculations for the basic scenarios. Best, would be to derive the LTs. A good understanding of relativity is expedited immensely, with a good understanding of spacetime diagrams.

The relativistic effects are reciprocal. The only time that time-dilation is not reciprocal, is when one transitions inertial frames of reference (ie properly accelerates). He who does so, undergoes a change that impacts his predictions of others. I could make a simple statement ... twin B 's POV dynamically changes while properly accelerating, and this causes others to shift in his spacetime system in a way that inertial POVs never experience. But, that will help little to none, for one who does not yet understand SR well enough.

In post 266 I provided a very simple description wrt the classic twins scenario as to why B ages less than A, and all agree. There, it was all about the A experience without consideration of B, and the B experience without consideration of A. However, to understand how B can predict the A experience, and vice versa, there is much more to that story. One cannot possibly understand that, without first understanding SR. For example ...

Let's say twin B is also always inertial, and he executes a flyby of twin A on earth. B coasts to a flyby of planet X (at rest with earth) a proper distance LP away. One must understand that BOTH record the other moving, and his moving clock ticking slower. Yet A records more time between those 2 events (A/B flyby B/X flyby) than does B (both events on the B worldline). Here, we consider the A experience per A and the B experience per B . ← And, this is an all-inertial scenario, and it does not violate reciprocal time-dilation. This is the first thing that needs understood. Then, one considers A per B and B per A. Then one considers 2 events on the A worldline to 2 events on the B worldline, in which case reciprocal time dilation is made clear. Only after doing that, does one have the tools to tackle the twins scenario of acceleration. (of course, all clocks of the earth frame are always synchronized in their own frame).

EDIT: The red highlight was added to the original post.

Best regards,

Celeritas

Edited by Celeritas
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Let's say twin B is also always inertial, and he executes a flyby of twin A on earth. B coasts to a flyby of planet X (at rest with earth) a proper distance LP away. One must understand that BOTH record the other moving, and his moving clock ticking slower. Yet A records more time between those 2 events (A/B flyby B/X flyby) than does B. Here, we consider the A experience per A and the B experience per B .

If B records less time between both events because he is flying from A to X, isn't that the B experience per A?

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Hi bvr,

I addressed the B experience per B, and the A experience per A. Intentionally, I omitted the A experience per B and the B experience per A. I just wanted to ensure the first case was understood first, before addressing the other ...

However yes, you are of course correct ... the B experience per B does match the B experience per A. But the A experience per A does not match the A experience per B, given the 2 specific events identified.

EDIT: I made an edit in red highlight on my prior post.

Best regards,

Celeritas

Edited by Celeritas
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I was focusing on your statement that they facilitated understanding.

You implied SR was metaphysics a few posts back, owing to its lack of a ohysical model. At least, that's how it reads to me.

I implied with that the exact contrary of what you read in it. As you may have clarified yourself, physical models such as for example virtual photons cannot be directly observed, they are part of metaphysics as defined in wikipedia (not everyone groups metaphysics the same, but let's not quibble about words). In contrast, QM and SR are founded on mathematical models which in turn are directly based on observed phenomena.

[..] One must first learn the math, that is, the LTs. Run some calculations for the basic scenarios. Best, would be to derive the LTs.[..]

Yes, absolutely!

Most revealing is next the derivation of time dilation and length contraction, which is one way or the inverse depending on which time interval is put to zero. That simple, basic mathematical understanding cannot be compensated by innumerous space-time diagrams or hundreds of words.

[..]

[1.] Everything is great and fully comprehensible about the relativity of observation from each frame.

The comprehensibility gets difficult when it comes to make the image a "real" thing. As if the image from the scan suddenly transformed into a real slice of bread.

[2.] And as Swansont posted, the same utterly incomprehensible phenomenon appears with kinetic energy.

Figure that the Earth is hit tomorrow by some gigantic asteroid, where was the KE hidden all this time? Inside motion? The KE reveals with the collision the same way the image of the scan reveals into a slice of bread.

In the new General philosophy thread I intended to discuss two competing (and very different) models for making sense of, among other things, your topic no.1. Coincidentally, someone else introduced the topic of energies in there, and so we'll likely also discuss that mystery.

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To reiterate what Tim88 stated, the measurements are relative, so you can only state that one clock ran slow as compared to another, which means that the other ran fast in comparison. There is no absolute time that you can use as a basis for the analysis.

There is no contradiction here. Again, the issue is that clock A runs slow relative to clock B in clock B's frame, and B runs slow relative to A is A's frame.

Your error is ignoring that clock rates are not invariant values. Your invalid assumption in your logic is that they should be. A simpler example:

In A's frame it is is rest and has no kinetic energy. B is moving, and has KEB. KEB > KEA (=0)

In B's frame it is is rest and has no kinetic energy. A is moving, and has KEA. (0=) KEB < KEA

Is there a contradiction in those statements? You can argue that they can't be true simultaneously, but you can't be in two frames at once, so that's not a problem.

Putting together the above and all the other points people have made to help me, I believe the following shows how two clocks moving relative to each other can both lose time with respect to each other...

So that we do not have to worry about clocks accelerating / decelerating, as someone mentioned previously, passing clocks can be used to communicate / compare times.

So the stay at home / travelling clock example can be stated like this...

The stay at home clock (a) is at rest and a travelling clock (b) flies past the stay at home clock, say from right to left at speed '1s'. As the travelling clock passes the stay at home clock, the two clocks are synchronized. The travelling clock continues on its way to the left at speed '1s'.

A second travelling clock ( c ) that is moving from left to right at speed '1s', passes the first travelling clock, and at the moment of passing synchronizes its clock with the first travelling clock. This second travelling clock continues left to right at speed '1s' until it passes the stay at home clock, at which point its time is compared to the stay at home clock's time. It is found that the travelling clock ( c ) has lost time compared to the stay at home clock (a).

But who is to say which clock is the 'travelling' clock and which clock is the 'stay at home' clock? To demonstrate that point, here is a slightly extended version of the above scenario...

This time, instead of describing the stay at home clock's reference frame as stationary (i.e. speed '0s'), it will be considered to be moving from left to right at speed '0.5s' and the travelling clock (b) moving at speed '0.5s' from right to left. This is still the same scenario as before, as the relative speed between the two clocks is still '1s'. [Any reference frame could be used to state the relative speeds of the clocks, but this one conveniently emphasizes the symmetry between the stay at home clock and the travelling clock.] As before, when the travelling clock (b) passes the stay at home clock (a), the two clocks are synchronized.

The second travelling clock ( c ) that is moving from left to right at speed '1.5s', passes the first travelling clock, and at the moment of passing synchronizes its clock with the first travelling clock. The second travelling clock continues left to right at speed '1.5s' until it passes the stay at home clock (a), at which point its time is compared to the stay at home clock's time. It is found that the travelling clock ( c ) has lost time compared to the stay at home clock (a).

And to show that it is equally valid to consider that it is the stay at home clock (a) that loses time compared to the travelling clock (b), a fourth clock (d) travelling from right to left at speed '1.5s' can pass the stay at home clock (a) and synchronize its time with the stay at home clock.

When this travelling clock catches up with the travelling clock (b) and their times compared, it is found that the stay at home clock (a) has lost time compared to the travelling clock (b).

So if I have got the above correct, then what I need to demonstrate is does the above lead to a logical contradiction?

Edited by robinpike
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[..]

So the stay at home / travelling clock example can be stated like this...

The stay at home clock (a) is at rest and a travelling clock (b) flies past the stay at home clock, say from right to left at speed '1s'. As the travelling clock passes the stay at home clock, the two clocks are synchronized. The travelling clock continues on its way to the left at speed '1s'.

[..]

instead of describing the stay at home clock's reference frame as stationary (i.e. speed '0s'), it will be considered to be moving from left to right at speed '0.5s' and the travelling clock (b) moving at speed '0.5s' from right to left. This is still the same scenario as before, as the relative speed between the two clocks is still '1s'. [Any reference frame could be used to state the relative speeds of the clocks, but this one conveniently emphasizes the symmetry between the stay at home clock and the travelling clock.] As before, when the travelling clock (b) passes the stay at home clock (a), the two clocks are synchronized.

[..]

After a hasty read I see that you made a quantitative beginner mistake, which you can easily spot if you plug in 0.5c for "0.5s" in your symmetrical point of view, and then transform back to your first view, using your calculus: you will find that clocks are supposed to move at the speed of light! Transformations of relative velocities are more complex in SR than in classical mechanics.

If you won't use the exact numbers then it's an irrelevant glitch, but else it needs correcting.

Edited by Tim88
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After a hasty read I see that you made a quantitative beginner mistake, which you can easily spot if you plug in 0.5c for "0.5s" in your symmetrical point of view, and then transform back to your first view, using your calculus: you will find that clocks are supposed to move at the speed of light! Transformations of relative velocities are more complex in SR than in classical mechanics.

If you won't use the exact numbers then it's an irrelevant glitch, but else it needs correcting.

I thought it would be easier to follow if I plugged some numbers in! Never mind, in that case best to just ignore those example speeds.

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And to show that it is equally valid to consider that it is the stay at home clock (a) that loses time compared to the travelling clock (b), a fourth clock (d) travelling from right to left at speed '1.5s' can pass the stay at home clock (a) and synchronize its time with the stay at home clock.

When this travelling clock catches up with the travelling clock (b) and their times compared, it is found that the stay at home clock (a) has lost time compared to the travelling clock (b).

So if I have got the above correct, then what I need to demonstrate is does the above lead to a logical contradiction?

You got it correct, except for the last sentence: when d catches up with b, it doesn't show anything about a.

After being synchronized with a, d lost time wrt b.

a[0] ->

<- b[0]

----------------------------------------

<- b[1] a[1] ->

c[1] ->>

<<- d[1]

----------------------------------------

<- b[3] a[3] ->

<<- d[2] c[2] ->>

Note that, according to b, a will also lose time, but b will have to calculate that, or send EM signals to verify. And according to a, b will lose time.

You will never find a contradiction, but I understand your search. Those concepts are frustrating, and SR doesn't explain. It's more: "Don't think about. It works, so accept it."

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You got it correct, except for the last sentence: when d catches up with b, it doesn't show anything about a.

After being synchronized with a, d lost time wrt b.

a[0] ->

<- b[0]

----------------------------------------

<- b[1] a[1] ->

c[1] ->>

<<- d[1]

----------------------------------------

<- b[3] a[3] ->

<<- d[2] c[2] ->>

Note that, according to b, a will also lose time, but b will have to calculate that, or send EM signals to verify. And according to a, b will lose time.

You will never find a contradiction, but I understand your search. Those concepts are frustrating, and SR doesn't explain. It's more: "Don't think about. It works, so accept it."

Thanks for spotting that. Even though relativity isn't an explanation (nor does it have to be in order for its equations to work and to be useful), I am still thinking about whether there is a logical contradiction in there somewhere. There is something that doesn't seem to be quite right when trying to apply an explanation of what happens to a clock when it changes from one reference frame to another. But since relativity is not an explanation per se, it could well be that any such issues would be irrelevant to relativity anyway.

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Thanks for spotting that. Even though relativity isn't an explanation (nor does it have to be in order for its equations to work and to be useful), I am still thinking about whether there is a logical contradiction in there somewhere. There is something that doesn't seem to be quite right when trying to apply an explanation of what happens to a clock when it changes from one reference frame to another. But since relativity is not an explanation per se, it could well be that any such issues would be irrelevant to relativity anyway.

At its core, relativity is math. It's a transform between reference frames. It is mathematically self-consistent. It has to be logically consistent. Any inconsistency comes from assumptions that disagree with relativity itself.

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swansont... This was in another thread a couple of weeks ago (can't find it now) but I think it will fit this thread too so I will ask you here as I presume you don't have time to give me a lecture in a private message.
You stated in that other thread that "gravity stems from spacetime curvature" and later that "gravity is spacetime curvature, you can't have one without the other"
I've been thinking about your statements and as much as I think I understand them I have a feeling that something is missing or something is not accurate most probably due to lack of my knowledge.

Is the aproach stating that gravity is curvature of spacetime really correct? Gravity seems to be the consequence of spacetime curvature or at least one of the implications of mass effecting spacetime seems to be that of gravity but is that equivalent to your statement?

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