Two days from now, on April 8, a total solar eclipse will sweep across North America. A number of us who write for Sacred Space Astronomy will be in Bloomington, Indiana as part of a Faith & Science retreat organized by Fr. Timothy Sauppé, a friend of the Vatican Observatory and pastor of St. Mary’s Church in Westville, Illinois (not too far from Bloomington). We will see that eclipse, the Good Lord willing and the clouds don’t come. Two of us in particular, Br. Guy and I, are scheduled to be interviewed on the day of the eclipse via video by Fr. James Martin, S.J., for students at Jesuit high schools. We were kicking around ideas about what topics to discuss in that interview, and Br. Guy mentioned that we might talk about the use of a solar eclipse to test Albert Einstein’s theory of gravity, General Relativity. That got me thinking of another gravity test.
General Relativity says that gravity is a warping of space and time caused by massive objects, and that it can therefore bend light. One of the first tests of the theory was at a 1919 eclipse of the sun. The positions of stars seen near the sun in the sky were shown to be slightly displaced, thanks to their light being bent by the sun’s gravity.
Human beings have been familiar with gravity since the first person ever dropped something: “Gronk let rock go. Rock go to ground. Gronk drop rock again. Rock go to ground again. Gronk discover… gravity!” By contrast, Einstein’s theory, which tells us why the rock falls, is pretty new. It has only been around for a little more than a century. A much longer-lasting theory of gravity was Aristotle’s. It was accepted for almost two thousand years.
Aristotle theorized that gravity was a natural tendency of “earth” and “water” elements to move toward the lowest point, or center of the universe. That is why the rock falls. The Earth we walk on exists, according to Aristotle, because these elements pile together at the center. The Earth’s central position was not privileged — its position was akin to that of the crud that accumulates around a clogged basement floor drain.
The person whose ideas finally overthrew Aristotle’s was Isaac Newton. He theorized that gravity was a universal force of attraction between all mass-having objects. The rock falls, in this case, because of a force of attraction between the rock and the Earth as a whole.
Note the difference between Aristotle and Newton. According to Aristotle, the rock moves to the center — to the “drain”. If you could take away the Earth, the rock would still fall, because the Earth, being just the accumulation of all the crud that sank toward the “drain”, is not part of the gravitational picture. According to Newton, the rock moves toward the Earth. If you could take away the Earth, the rock would not fall.
In the eighteenth century, an important gravity experiment directly confirmed that Newton was right, and Aristotle was wrong. It involved the displacement, not of light, but of a plumb line — by a mountain. A plumb line is a dense, heavy mass, called a “bob”, on the end of a string, all constructed with some precision, and used in construction to determine vertical (“plumb” comes from the Latin word for “lead” — the dense stuff that the bob might be made of).
If gravity worked like Aristotle said, mountains would have no effect on the bob of a line, because the bob would simply seek the center. If gravity was a universal attraction between all mass-having objects, on the other hand, the mass of a mountain would exert a pull on a plumb bob, deflecting the line away from vertical as measured using the stars and the center of the Earth.
The first experiment was in 1738. Pierre Bouguer detected a displacement of a plumb line from vertical at the volcano Chimborazo in Equador, although his results were shaky. Neville Maskelyne did the same at a steep granite mountain in Scotland called Schehallian in 1774, with better results.
Here are some of Maskelyne’s words regarding his detection of a displacement of a plumb line away from the vertical:
If the attraction of gravity be exerted, as Sir Isaac Newton supposes, not only between the large bodies of the universe, but between the minutest particles of which these bodies are composed, or into which the mind can imagine them to be divided, acting universally according to that law by which the force which carries on the celestial motions is regulated; namely, that the accelerative force of each particle of matter, towards every other particle, decreases as the squares of the distances increase; it will necessarily follow, that every hill must, by its attraction, alter the direction of gravitation in heavy bodies in its neighbourhood, from what it would have been from the attraction of the earth alone, considered as bounded by a smooth and even surface…. Hence the plumb-line of a quadrant, or any other astronomical instrument, must be deflected from its proper situation, by a small quantity towards the mountain; and the apparent altitudes of the stars, taken with the instrument, will be altered accordingly….
[The results from Bouguer are] insufficient to prove the reality of an attraction of the mountain Chimboraço, though the general result from them is in favour of it…. It appears from this [my] experiment that the mountain Schehallian exerts a sensible attraction; therefore, from the rules of philosophizing we are to conclude that every mountain, and indeed every particle of earth, is endued with the same property….
So Newton was correct — or, at least more correct than Aristotle. After all, not even 150 years after Maskelyne’s experiment, measurements of the displacement of starlight at that 1919 total eclipse would show that Newton was wrong. According to Newtonian physics, light, having no mass, cannot feel a force of gravitational attraction, and thus should not react to the sun’s gravity.
And in the future, some other theory of gravity may displace Einstein’s!
