Precession of Mercury

Nickelodeon

Could someone explain in layman terms the current thinking on why the orbit of Mercury precesses? Presumably, it is not precessing in a gyroscope sense but the perihelion of the orbit just advances in the same plane.

mathman

Precession of Mercury's orbit, as well as other planets, results from the gravitational effects of all planets on each other.

Janus

As mathman pointed out, the other planet's effect Mercury's orbit, as can the rotation of the Sun itself.

You can calculate these effect from equations such as found here:
http://scienceworld.wolfram.com/phys...recession.html

Back in the 19th century such calculations were done, but it was found that Mercury's precession was greater than that which could be accounted for. This led to the idea that there was an undiscovered planet orbiting closer to the Sun than Mercury that accounted for the extra precession. They even went as far as giving this planet the name of "Vulcan". A search was made for this planet, and despite a few false alarms, no planet Vulcan was found.
Then Einstein, in his General Theory of Relativity, showed that the extra precession could be explained without an inner planet(in fact, the theory required the extra precession to exist), and the need for the planet Vulcan disappeared.(At least until the mid 1960's, where it provided a home for Mr. Spock)

Nickelodeon

I'm having difficulty visualising how the planets could consistently nudge the Mercury's perihelion in the same direction. I would have thought that they could pull it one way then perhaps the other way depending on their relative positions.

The next thing I would like to know is regard to Einstein and how he explains the mechanics of what causes the small discrepancy. If the sun wasn't rotating would there be no discrepancy or is there something else going on?

Jonathan Scott

The additional perihelion precession in Einstein's General Relativity is nothing to do with the rotation of the sun. It is to do with the way in which a massive body modifies the shape of space-time.

If you draw an ellipse on a flat sheet of paper, but then remove a segment of the paper (a narrow wedge extending out from the centre) and join it up again, making it slightly cone-shaped, the ellipse no longer meets up correctly, and if continued it does not repeat until slightly after going all the way round. The GR effect on Mercury's perihelion precession is very similar to this, although it involves curvature in time as well as space.

Nickelodeon

This worries me because Janus says that the Sun's rotation does have an impact on Mercury's precession which introduces some doubt as to there being a definitive answer.

Jonathan Scott

The Sun's rotation does indeed have a effect, but it only gives a very tiny correction to the original Newtonian plus Einstein calculation. It is only relatively recently that the Sun's rotation rate and the resulting oblateness were known well enough to make an accurate calculation, and up to that point there was still some speculation that Einstein might be wrong when the rotation was taken into account.

For some actual figures, see the Wikipedia entry on Tests of general relativity.

Nickelodeon

Is it the oblateness of the sun or frame dragging due to the rotation or both which causes the discrepancy?

D H

If you followed the link in Janus' post you would have seen stuff involving J₂, J₄, etc. These are classical, non-spherical mass moments of the Sun about its center of mass. The Sun, because it rotates very slowly, is very close to spherical and thus has a very small J2 value: 2×10^-7. There is no doubt that Mercury's relativistic recession results from relativity rather than the Sun's oblateness.

Nickelodeon

My maths is appalling so I'm struggling with the formula in Janus's link. The only thing that the formula doesn't seem to take into account is the mass of the sun, big M ?, reducing each year. Do you think, over the century, this would have any effect on Mercuy's orbit?

*** Just looked up the sun's mass loss per year and it seems negligible so please disregard last question ***

Ian

Nickelodeon,
When calculating the precession of any planet make sure you calculate the length the body advances along the path of orbit - don't calculate the angular advance as Einstein did, or everyone else for that matter.
If the orbit of mercury was circular you would not observe an advance because Einstein's indicator (the perhelion) would not exist if the orbit was circular but the orbit would still advance.

Nickelodeon

That sounds sense.

I'm still no nearer understanding the mechanical effect that causes the advance though. Einstein obviously had a feel for it and, presumably, devised his fomulae based on that insight. Does anyone know what that insight was?

RandallB

Nothing wrong with converting to, or caculating for angular advance.
"Length" can be confusing as it requires defining where the distance was measured, periapsis or apoapsis, while both will convert to the same angular advance.

Ian

RandallB,
If you look at Einstein's original equation when he calculated the advance of mercury's perhelion you will see that he first deduced for a circular orbit and then he added the 'correction' for an elliptical orbit.
The units of the advance for a circular orbit are 'metres', as shown below:

(24pi³r³)/(c²t²) Metres.

and after adding the r(1-e²) correction for elliptical motion the equation becomes:

(24pi³r²)/(c²t²(1-e²)) radians

If you calculate the advance for any planet assuming the orbit is circular then the advance must be expressed in metres along the length of orbit. This figure will always be the same (~27833 metres) as the sun's mass is the only contributing factor.
The correction factor and change of units have made you think that the advance is different for all planets.
Besides, Einstein didn't actually explain the advance, he only produced a calculation that agreed with observation.

Jonathan Scott

Yes, the precession distance is the same for all circular orbits around the sun. I'm not quite clear on your units; I'd write it as 6 pi Gm/c^2 where m is the mass of the sun, and Google Calculator gives me 27.835km which agrees well enough with your numeric result.

A similar related feature applies to the curvature of space, in that in isotropic coordinates the "length deficit" in the circumference of any circle around the sun is one third of that precession distance.

That's absurd. He produced an equation which was extremely simple in concept but mathematically very tricky, saying that the curvature of space-time is determined by the distribution of matter, and the only free parameter (ignoring cosmological considerations) was effectively the strength of the gravitational potential, which is simply matched up against Newtonian theory in the limit. The perihelion precession of Mercury depends on a second-order term in the complicated maths from that theory, yet came out right first time. This may have been a fluke (and sometimes I subscribe to that point of view) but it certainly wasn't contrived to match the observations.

Nickelodeon

The thinking man would look at the phenomenum (the precession discrepancy over and above planetary influencies) and decide on a probably cause, be it relative time, space curvature or relative mass variances then apply the maths accordingly. I'm sure Einstein did this but is there anyone out there who can shed some light on what those mechanical variance might be?

Jonathan Scott

What do you mean "mechanical variance"?

Einstein's model is simply that free falling objects follow "geodesics" in space-time (the local equivalent of straight lines), but mass curves space-time with the result that near to a large mass those geodesics describe accelerated motion relative to a static coordinate system. Although this is completely different from Newton's description of gravity, it not only reproduces the same effects, but it also explains why Newton's theory doesn't quite get Mercury right.

The shape of the orbit can be calculated in Einstein's theory in a very similar way to Newton's, by describing the equation of motion relative to a coordinate system. Einstein's result is basically the same as Newton's except that relativistic corrections mean that any oscillation about the average orbit radius has a period which is a tiny fraction longer than the the time it takes to complete an orbit, so the shape of the orbit precesses forwards.

Since Einstein's theory can be approximated by Newton's, the conventional method of computing the total precession of Mercury's orbit is simply to add the Einstein correction to the classical Newtonian precession. However, it should be noted that Einstein's theory is effectively being used to calculate the whole orbit including the correction, not just the relativistic correction.