When Gravity Breaks Down

​Newton’s Gravity Breaks Down

Discovering a new law of nature is the acme of scientific achievement, and one granted to few. Those who succeed are assured of a place in the pantheon of science, first published in 1687, his book, Mathematical Principles of Natural Philosophy, put forward his three laws of motion and his law of universal gravity. Sir Isaac Newton portrayed gravity as some kind of mysterious influence that allows masses to affect each other even through the vacuum of space. While declining to say exactly how this influence worked, Newton came up with a precise mathematical description of its effects, in the form of his celebrated inverse-square law of universal gravitation. Supposedly inspired by watching an apple fall in his mother's garden almost 350 years ago, Newton's law remained the best description of gravity until 1915, when Albert Einstein published his general theory of relativity, which gave the first detailed account of what gravity actually is.

In Newton’s view, all objects exert a force that attracts other objects. That universal law of gravitation worked pretty well for predicting the motion of planets as well as objects on Earth and it's still used, for example, when making the calculations for a rocket launch.

But Newton's view of gravity didn't work for some things, like Mercury’s peculiar orbit around the sun. The orbits of planets shift over time, and Mercury’s orbit shifted faster than Newton predicted.

While Newton was able to formulate his law of gravity in his monumental work, he was deeply uncomfortable with the notion of action at a distance that his equations implied. In 1692, in his third letter to Bentley, he wrote:

That one body may act upon another at a distance through a vacuum without the mediation of anything else, by and through which their action and force may be conveyed from one another, is to me so great an absurdity that, I believe, no man who has in philosophic matters a competent faculty of thinking could ever fall into it.

He never, in his words, assigned the cause of this power. In all other cases, he used the phenomenon of motion to explain the origin of various forces acting on bodies, but in the case of gravity, he was unable to experimentally identify the motion that produces the force of gravity. Moreover, he refused to even offer a hypothesis as to the cause of this force on grounds that to do so was contrary to sound science. He lamented that philosophers have hitherto attempted the search of nature in vain for the source of the gravitational force, as he was convinced by many reasons that there were causes hitherto unknown that were fundamental to all the phenomena of nature. These fundamental phenomena are still under investigation and, though hypotheses abound, the definitive answer has yet to be found.

I have not yet been able to discover the cause of these properties of gravity from phenomena and I feign no hypotheses.... It is enough that gravity does really exist and acts according to the laws I have explained, and that it abundantly serves to account for all the motions of celestial bodies.

When Isaac Newton put forth his universal theory of gravitation in the 1680s, it was immediately recognized for what it was: the first incredibly successful, predicatively powerful scientific theory that described the one force ruling the largest scales of all. From objects freely falling here on Earth to the planets and celestial bodies orbiting in space, Newton's theory of gravity captured their trajectories spectacularly. When the new planet Uranus was discovered, the deviations in its orbit from Newton's predictions allowed a spectacular leap: the prediction of the existence, mass and position of another new world beyond it: Neptune. The very night the Berlin Observatory received the theoretical prediction of Urbain Le Verrier working 169 years after Newton's Principia they found our Solar System's 8th planet within one degree of its predicted position. And yet, Newton's laws were about to prove insufficient for what was to come.

The problem all started not at the outer reaches of the Solar System, but in the innermost regions: with the planet Mercury, orbiting closest to the Sun. Every planet orbits the Sun not in a perfect circle, but rather in an ellipse, as Kepler noticed nearly a full century before Newton. The orbits of Venus and Earth are very close to circular, but both Mercury and Mars are noticeably more elliptical, with their closest approach to the Sun differing significantly from their greatest distance.

Mercury, in particular, reaches a distance that’s 46% greater at aphelion (its farthest point from the Sun) than at perihelion (its closest approach), as compared to just a difference of 3.4% from Earth. This doesn't have anything to do with the theory of gravity; this is merely the conditions which these planets formed under that led to these orbital properties. But the fact that these orbits aren’t perfectly circular means we can study something interesting about them. If Kepler’s laws were absolutely perfect, then a planet orbiting the Sun would return to the exact same spot with each and every orbit. When we reached perihelion one year, then if we counted out exactly one year, we’d expect to be at perihelion once again, and we’d expect the Earth to be in the same exact position in space relative to all the other stars and the Sun as it was the year