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# Einsteins theory of general relativity explained the orbit of planet. . .?

The physics teacher worded this question weird... sorry

### 5 Answers

- Dr SpockLv 61 decade agoFavorite Answer
It would have been better to give us more --- at the moment there isn't even a question! But nevertheless, let me try to complete the thought:

Einsteins theory of general relativity explained the orbit of planet Mercury. How so? Because it PREDICTED, PERFECTLY, a stubborn oddity about Mercury's orbit that had defied a Newtonian explanation for almost 50 years.

If that was all you were after, we're done.

However, this was one of the greatest triumphs in modern science. So, should you want to know more about this, I give you a more complete answer, below. (All my own work; no pasting from any web-site.)

Since the last third of the 19th century, the orbit of the planet Mercury had appeared to be behaving oddly. After accounting for the gravitational effects of all the other planets (which all affect each other's orbits by a calculable amount), Mercury had something unexplained left over: instead of its orbital "ellipse" staying put in space, it was VERY SLOWLY "advancing" (in the same general direction as Mercury orbited) at the astonishingly small rate of 43" (43 seconds of arc) a CENTURY (!!) This is something like the speed of the motion across the diameter of a penny as seen from 3 miles away --- in a hundred years!

This extremely slow rotation of the whole, repeated elliptical orbit was known as the "advance of Mercury's perihelion." ("Perihelion"means the "closest point to the Sun in an orbit"; it could just as well have had "Aphelion" --- the furthest point --- in it, instead. The important point is that the whole orbit is "precessing" forward --- another common usage in the alternative phrase, "the precession of Mercury's perihelion," that is actually misleading. It's not "precessing" as that term is usually used, it's simply rotating around, if very, very slowly.)

Lots of attempts were made to explain why this might be happening. A much favoured one was to have yet one more hitherto undiscovered planet, interior to Mercury's orbit, whose own orbital motion would drag Mercury around just that little bit more. Indeed, false sightings of just such a hypothesized planet were reported, several times, by normally competent observers, as well as some quacks hoping for glory. Several names were already proposed, in anticipation of such a discovery: Vulcan and Falcon were two. (Another "explanation" was that for some reason, gravity ceased behaving as an inverse square law inside Venus's orbit --- it had to be more like an inverse 2.00003 [or some number like that] power law there!)

Meanwhile, in something apparently unrelated, Einstein first changed our understanding of the supposed immutability of time and space with Special Relativity (1905). He then laboured for some ten years, with lots of false moves largely unkown and/or ignored today, to try to include a "natural" (to him!) extension of his new ideas, to incorporate gravity into the properties of both space and time, following from the effects of the presence of matter on this augmented "space-time" around them.

Finally, in December, 1915, he published the final "field equations" that he had tortuously derived from this incredibly long process. Since he was trying to explain how gravity behaved, a natural thing was to apply his new theory to the problem of Mercury's orbit.

(Let me emphasize once again: he had been following a very esoteric, almost philosophical/mathematical approach to generalise how space and time would together form a "continuum." That approach was guided by certain aesthetic ideals; it was deliberately designed to employ very general considerations. But who knew if it would really fit the Universe we live in? That's where the early so-called "Tests" of the theory came in: phenomena not used in constructing the theory would test if this audacious theory worked.

In arriving at the final form of the theory, he had already had to require that his theory reduce to the well-known Newtonian results in the grossest, first pass at its implications. (Newtonian gravity did indeed emerge from his theory, but he had to

be sure of the value of an adjustable constant in the theory. That constant involved the value of 'G,' the "Gravitational Constant," that appeared in one of his equations.)

But what must have been initially intriguing to him, and later quite astonishingly gratifying, was that his theory said that the equations of motion in HIS theory didn't end with the Newtonian terms --- there was something extra. When he applied that "extra bit" to Mercury, lo and behold:

Mercury's orbit advanced at 43" per century!

It's extremely difficult for all the rest of us merely ordinary mortals to appreciate how he must have felt when that number appeared at the end of his calculation. Stunned, stupified, exultant ... ?:

HE HAD HIT A TARGET THAT HE HADN'T EXPLICITLY BEEN AIMING AT !!

I don't know of any other single discovery in science that comes remotely close to this one.

To my mind, compared to this, even the other two of the classical predictions/tests of General Relativity --- the "bending of light" or the "gravitational redshift" --- pale in comparison. Those are subtle new phenomena with predictions that might or might not prove true and verifiable. But here was a 40 or 50 year old puzzle with lots of failed "explanations," yet Einstein had "hit a hole in one" without really looking where the swing of his golf club was aimed!

This may be far more than you either wanted or expected, but at least you now know something of the full story.

Live long and prosper.

Source(s): Originally, fascinating courses in General Relativity taken from leading astrophysicists, supplemented by much outside reading. - 1 decade ago
Einstien's theory of general relativity explained gravities affect on space-time. It viewed gravity as a warping of space by any mass. It is an alternative way of looking at gravity compared to the classical interpretation of a force transmitted by particles or 'gravitons'. I suppose you could say it explains the orbit of planets, but no more so than the classical approach.

- 1 decade ago
Mercury. Before Einstein, Mercury's orbit was hard to pin down with newtonian math. It just kept wandering away from the predictions. With general relativity it's orbit could be accurately described.

- Anonymous1 decade ago
XenoFlux is correct. As an aside, another planet was assumed to exist to try to explain the problems with Mecury's orbit. The "planet" was called Vulcan.

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- Anonymous1 decade ago
may be Ur physics teacher flirts with U? ...