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Comment by sigmoid10 | Hacker News Reader

This might sound true if you have a superficial understanding of relativity (and I would not be surprised to see people repeating this stuff on the internet), but it's completely false. The correction to Mercury's orbit precession from General Relativity has nothing to do with Mercury itself (its mass, speed or otherwise). It is purely based on the sun's gravitational field. If you want to interpret it as a potential field in the classical Newtonian sense, it basically picks up an additional 1/r^3 term. That's what disturbs the orbit and it's also the reason why this effect is negligible for other planets apart from Mercury, because it drops off much faster than the normal 1/r potential. Any object very close to a large mass like the sun would feel this distortion, irrespective of how fast it travels.

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Yes, this is correct. The orbital precession occurs even for a point mass.

Former physicist myself: Not sure I understand your (and parents) argument.

Given the Suns gravitational field, and the orbit that Mercurius has, the speed of the planet can be determined. So the argument that a point mass would have the same precession does not dispute the argument that it is the relativistic mass of the point mass/planet that determines the precession.

I will not venture into arguments* whether grandparents explanation is ‘correct’, ‘best’ or ‘useful’, there is an equivalence between ‘gravitational field + orbit’ being the reason and ‘(relativistic) speed + orbit’ being the reason. * A sybling comment states that relativistic mass is avoided in modern physics. As someone who did physics 30 years ago, I can not deny or corroborate this statement. And indeed, the explanation of grandparent is not the way I myself think about the perihelium precession. But that does not make me certain enough to say it is ‘wrong’.

Edit: typo ‘of’ —> ‘or’

Just think about it. If relativistic mass (a term that indeed tends to be avoided by actual physicists for good reasons, because it confuses concepts of mass and energy which are clearly defined in relativity) was the explanation, then the shape of the planet's orbit itself would influence the anomalous precession component from General Relativity. And you would see no relativistic effect at all for perfectly circular orbits with zero eccentricity. That is simply not the case.

The explanation of grandparents for the perihelium effect of Mercury tried to explain the 'overshoot'. And yes, in a perfectly circular orbit there would be no overshoot??

I do not think grandparent tried to say that all relativistic effects were caused by the increase of the 'relativistic' mass, just that the overshoot was.

So I am sorry: I cannot see how this argument disproves the explanation of grandparent. And just to be clear: I do not think in the way of grandparents argument myself.

Kind regards,

Roel

>And yes, in a perfectly circular orbit there would be no overshoot??

That's where you and the other comment are wrong. The only way to get a stable circular two body orbit is for newtonian potentials which are exactly 1/r shaped. Add any other term (like 1/r^3 for General Relativity) and you essentially get more or less chaotic movement over time, similar to the three body problem in newtonian gravity. See Betrand's theorem [1].

[1] https://en.wikipedia.org/wiki/Bertrand%27s_theorem

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