Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: Kryptid on 21/02/2019 20:58:47
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After reading discussion in this topic, https://www.thenakedscientists.com/forum/index.php?topic=76187.0, I have become genuinely curious about the mechanism behind relativistic precession of perihelion. I have heard different explanations, but I want to know whether those explanations are correct or simply misconceptions. I did find one explanation that seemed intuitive to me: that the curvature of space near the Sun causes the ratio between the circumference of a planet's orbit and its semi-major axis to diverge from the expected 2*pi*r. This means that the planet actually travels a little bit further than expected each time it orbits, resulting in precession of perihelion. I found this explanation in the following YouTube video:
The following webpage seems to agree with this notion: https://www.relativity.li/en/https://physics.stackexchange.com/questions/26408/what-did-general-relativity-clarify-about-mercury/read/i0_en/i1_en
To quote the relevant comments:
Another way of looking at it is that the circumference of a circular orbit near the Sun is slightly less than 2*pi*r because of the positive curvature of space-time. – Pete Jackson Aug 3 '11 at 22:25
@PeteJackson: You can't downvote comments, but this is not a good way of looking at it, since this assumes that the effect is independent of velocity, which it isn't. – Ron Maimon Jun 3 '12 at 18:56
Is this correct? Is a divergence from 2*pi*r a bad way of understanding the precession?
Then there is the following YouTube video that says time dilation is responsible:
Although he says that time dilation is responsible for the precession of perihelion, he doesn't explain how it causes the precession. I'm looking specifically for the how. Can anyone help throw light on this matter?
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Time dilation wouldn't do it unless the gravitational field time dilates it's own action.
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Maybe this will help Kryptid?
http://www.gravity.gatech.edu/P4147/wp-content/uploads/2014/06/Oct27b.pdf
For the rest of us maybe this?
http://everything.explained.today/Precession/
Quite thought provoking, as f.ex
" The behavior of a spinning object simply obeys laws of inertia by resisting any change in direction. A spinning object possesses a property known as rigidity in space, meaning the spin axis resists any change in orientation. It is the inertia of matter comprising the object as it resists any change in direction that provides this property. Of course, the direction this matter travels constantly changes as the object spins, but any further change in direction is resisted. If a force is applied to the surface of a spinning disc, for example, matter experiences no change in direction at the place the force was applied (or 180 degrees from that place). But 90 degrees before and 90 degrees after that place, matter is forced to change direction. This causes the object to behave as if the force was applied at those places instead. When a force is applied to anything, the object exerts an equal force back but in the opposite direction. Since no actual force was applied 90 degrees before or after, nothing prevents the reaction from taking place, and the object causes itself to move in response. "
starting from a beginning.
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This one is maybe a little more substantial? With a conclusion to which I agree furthermore :)
'Aberration and the Speed of Gravity' by Steven Carlip.
https://arxiv.org/abs/gr-qc/9909087
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There also exist a quite readable book discussing how Einstein reached his conclusion.
" The Hunt for Vulcan " by Thomas Levenson
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I think that we should be looking for 4 explanations, not 1. Here is what VanFlandern said (BF means basic function)......
In GR, the correct multiplier of BF is arrived at by combining three contributions.
The first is the effect of "time dilation", which contributes +4BF .
The second is the effect of "space contraction", which contributes -2BF.
The third is the effect of mass or momentum increase with speed, which contributes+1BF .
The sum of these three contributions gives the observed amount, +3BF.
The 4th explanation might be why +4 & -2 & +1, if it doesnt fall out in the other 3 explanations.
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Then there is the following YouTube video that says time dilation is responsible:
Although he says that time dilation is responsible for the precession of perihelion, he doesn't explain how it causes the precession. I'm looking specifically for the how. Can anyone help throw light on this matter?
I had a look at that video. I agree that ticking of clocks on Mercury slows during bits of orbits closer to the Sun. And i agree that ticking of clocks on Mercury slows during faster bits of orbits. And these look additive to me (they didnt actually say).
But i dont understand how any of that slowing of ticking on Mercury stuff could affect the measured transits of Mercury (indicating that the perihelion of the orbit changes at 5600"/century) as measured/timed on Earth using clocks on Earth.
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This one is maybe a little more substantial? With a conclusion to which I agree furthermore :) 'Aberration and the Speed of Gravity' by Steven Carlip. https://arxiv.org/abs/gr-qc/9909087
There also exist a quite readable book discussing how Einstein reached his conclusion. " The Hunt for Vulcan " by Thomas Levenson.
I had a quick look at Carlip. I think that he says that there is zero gravitational aberration (or that there is no such thing as gravitational aberration)(please yourself), in which case i think that Carlip would say that the 43" is not due to gravitational aberration (or that 00" of the 43" is due to gravitational aberration)(please yourself).
But if gravitational aberration exists, ie if gravity is not instantaneous, then i think that this would gradually pull Mercury into a faster orbit, & in which case by my reckoning the effect on precession would be negative, ie going the wrong way, ie the precession would be say 5,999" per century instead of 5,600" per century, in which case Einstein's SR/GR would need to account for 44" per century (no i didnt make a mistake)(no, 42" is not the correct number)(have a think).
Praps someone could work out the exact predicted effect on precession based on various values for the speed of gravity. If it is say 0.01" arcsec per century then it could be included in the list of effects. If it is 1" per century then it might show that accepting that Einstein's 43" is true & correct then there is some other effect that has been entirely overlooked, or that the well known theoretical Newtonian 532.3" is miscalculated (it would need to be 1" more)(& praps such miscalculation might be due to that there same kind of gravitational aberration itself).
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Mercury responds to the forces that it feels in it's frame of reference. That includes the radial gravitational attraction of the Sun and the tangential velocity of Mercury.
From the viewpoint of someone on Earth, time will pass more slowly on Mercury, especially when it is closest to the Sun: Deepest in the gravitational well, and travelling at the highest speed.
This means that the tangential distance travelled will be slightly different from Newton's predictions, when viewed by a distant observer. This will result in a precession of perihelion due to relativistic effects.
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I think that we should be looking for 4 explanations, not 1.
You think that.
Science, on the other hand, is trying to make things simpler.
Why are you trying to make them more complex than they need to be?
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Mercury responds to the forces that it feels in it's frame of reference. That includes the radial gravitational attraction of the Sun and the tangential velocity of Mercury.
Correct me if i am wrong. GR due to nearness to the Sun contracts metre-rods on Mercury in the radial direction -- hencely Mercury reckons that R is larger than the R seen by Earth. In addition clocks on Mercury are ticking slower due to the higher orbit speed when near to the Sun -- hencely Mercury reckons that the orbit speed is larger than the speed seen by Earth.
If the R feels larger, & the V feels larger, & if Mercury responds to feel, then the orbit will extend, & the turn-back point will be further away from the Sun, which in its frame of reference will feel much further away due to the radial metre-rods being contracted, & the time taken to reach the turn-back point will feel greater.
Will all of this advance the axis of the ellipse or retard it?
If Mercury feels that the axis has advanced then wouldnt a star-shot put them right, ie tell them that they were imagining things? From the viewpoint of someone on Earth, time will pass more slowly on Mercury, especially when it is closest to the Sun: Deepest in the gravitational well, and travelling at the highest speed.
This means that the tangential distance travelled will be slightly different from Newton's predictions, when viewed by a distant observer. This will result in a precession of perihelion due to relativistic effects.
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Mercury responds to the forces that it feels in it's frame of reference. That includes the radial gravitational attraction of the Sun and the tangential velocity of Mercury.
Correct me if i am wrong. GR due to nearness to the Sun contracts metre-rods on Mercury in the radial direction -- hencely Mercury reckons that R is larger than the R seen by Earth. In addition clocks on Mercury are ticking slower due to the higher orbit speed when near to the Sun -- hencely Mercury reckons that the orbit speed is larger than the speed seen by Earth.
If the R feels larger, & the V feels larger, & if Mercury responds to feel, then the orbit will extend, & the turn-back point will be further away from the Sun, which in its frame of reference will feel much further away due to the radial metre-rods being contracted, & the time taken to reach the turn-back point will feel greater.
Will all of this advance the axis of the ellipse or retard it?
If Mercury feels that the axis has advanced then wouldnt a star-shot put them right, ie tell them that they were imagining things? From the viewpoint of someone on Earth, time will pass more slowly on Mercury, especially when it is closest to the Sun: Deepest in the gravitational well, and travelling at the highest speed.
This means that the tangential distance travelled will be slightly different from Newton's predictions, when viewed by a distant observer. This will result in a precession of perihelion due to relativistic effects.
I think you rather missed the point.
In Newtonian physics, the orbit would not precess at the rate it does (and not at all if it were not for the other planets etc).
But it does.
So there is clearly something which affects the orbit.
If you say it isn't relativity, you need to explain what it is.
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Mercury responds to the forces that it feels in it's frame of reference. That includes the radial gravitational attraction of the Sun and the tangential velocity of Mercury.
Correct me if i am wrong. GR due to nearness to the Sun contracts metre-rods on Mercury in the radial direction -- hencely Mercury reckons that R is larger than the R seen by Earth. In addition clocks on Mercury are ticking slower due to the higher orbit speed when near to the Sun -- hencely Mercury reckons that the orbit speed is larger than the speed seen by Earth.
If the R feels larger, & the V feels larger, & if Mercury responds to feel, then the orbit will extend, & the turn-back point will be further away from the Sun, which in its frame of reference will feel much further away due to the radial metre-rods being contracted, & the time taken to reach the turn-back point will feel greater.
Will all of this advance the axis of the ellipse or retard it?
If Mercury feels that the axis has advanced then wouldnt a star-shot put them right, ie tell them that they were imagining things? From the viewpoint of someone on Earth, time will pass more slowly on Mercury, especially when it is closest to the Sun: Deepest in the gravitational well, and travelling at the highest speed.
This means that the tangential distance travelled will be slightly different from Newton's predictions, when viewed by a distant observer. This will result in a precession of perihelion due to relativistic effects.
I think you rather missed the point. In Newtonian physics, the orbit would not precess at the rate it does (and not at all if it were not for the other planets etc). But it does. So there is clearly something which affects the orbit. If you say it isn't relativity, you need to explain what it is.
I cant say that it isnt due to Einstein's SR & GR until i find out the what-where-how-why of SR & GR, which is what this thread is all about. I will then decide whether the explanation(s) are convincing. If not convincing then of course that doesnt necessarily mean that SR & GR fail, it might in the first instance simply mean that no-one around here understands it.
I have got a few novel ideas re possible causes which i have already mentioned in New Theories.
But i too am keen on understanding the mechanics of the SR & GR explanation.
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Science doesn't generally do "what-where-how-why"
If you are looking for a "why" you probably need to study theology.
If you didn't already know that, then you need to study the philosophy of science.
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Science doesn't generally do "what-where-how-why"
If you are looking for a "why" you probably need to study theology.
If you didn't already know that, then you need to study the philosophy of science.
Of the what where how why of SR & GR the why is the key to the whole shemozzle.