How a good empiricist gets to grips with gravity

Jack asked:

Aristotle, Newton and Einstein proposed radically different explanations of gravity. And yet, according to Hume, they associated their sense experiences together by using the same patterns. According to both Descartes and Hume, they accepted the same fundamental principles of logic. If Hume and Descartes are correct, why did Aristotle, Newton and Einstein disagree about gravity? Is there simply no foundation one can use to determine whether or not Aristotle, Newton or Einstein was correct?

Answer by Craig Skinner

What a very good question, bringing in key issues in both general philosophy and philosophy of science.

The short answer is that the foundation one can use is the scientific method of conjecture and testing. Put simply, we formulate a hypothesis (conjecture) as to how things work, and test it against the world by observation/experiment looking for confirmation (findings support the hypothesis, we can run with it meantime) or refutation (findings rule out the hypothesis, we must amend or replace it).

To deal first with the three hypotheses.


Hypothesis: things move to their natural place.

His conceptual framework was Earth as centre of the world surrounded by moon, sun, planets and stars going round it. He conjectured that things moved to their natural place. A stone, composed of the element earth, fell to Earth; a flame, composed of fire, moved up to the (fiery) heavens. He had no notion of gravity. If we ask why doesn’t the moon fall to Earth, he might say that it isn’t composed of earth (the moon wasn’t known to be a rock then), or that, like the planets/sun/stars, it is constrained in its orbit by a crystalline sphere. Why don’t we see the latter? Ah, it’s invisible. You can already see that the account is made resistant to refutation by what we think of as ad hoc auxiliary hypotheses such as crystalline spheres. And it appeals to purpose (things go to natural places) rather than mechanism. So it’s not a scientific hypothesis.


Hypothesis: things move under the influence of the force of gravity.

His conceptual framework was heliocentric. He saw that the movement of apples, stones, moons, planets, sun, Earth, comets and cannonballs alike could all be explained in the same way, namely as masses moving (as described by his laws of motion) under the influence of a single force (gravity). It was a wonderful synthesis of all motion, on Earth and in the heavens, and its empirical success assured its acceptance. It predicted the exact times of future eclipses and of cometary returns, and in due course Newtonian calculations were good enough to get men to the moon and back. This success rather silenced it’s philosophical critics.

Leibniz complained that, whereas Descartes proposed things stayed in orbit by being caught up in vortices, at least a coherent notion, Newton was as bad as Aristotle, proposing a force mysteriously acting at huge distances. Put simply, how could the moon know that the Earth is there, being 250,000 miles away, and so go round it. If the Earth was snatched away, how could this affect the moon – would it still go round the void where Earth had been? Newton was acutely aware of this “spooky action at a distance” criticism, and made clear he was only describing (modelling we would say these days) reality, and as regards what the force of gravity actually WAS, “framed no hypothesis (“hypothesis non fingo”). Neither did anybody else for 200 years.

So, despite the philosophical conundrum, this account is a good (excellent) scientific hypothesis, and all observations were supportive, save for anomalies in Mercury’s motion, which were just ignored.


Hypothesis: things move freely, acted on by no force, through curved space.

His conceptual framework was the same as Newton’s. In addition, he formulated the Principle of Equivalence: me, standing on Earth subject to downward gravity and feeling upward pressure on my soles is equivalent to me in outer space being accelerated by a force acting up through my soles. Similarly, me falling freely to Earth from a balloon is equivalent to me at rest in outer space – I feel no force. So, in his theory of gravity, a body falling to Earth is subject to no force. It falls freely BUT its trajectory depends on the curvature of space itself, and this depends on mass in that neighbourhood. – mass tells space how to curve, space tells mass how to move, as we can put it.

Einstein’s and Newton’s theories made different predictions as to Mercury’s orbit and as to stellar appearances at the time of a solar eclipse. Observations confirmed Einstein’s view. And no observation contrary to the theory has so far been made. And, a practical note, although Newtonian calculations get us to the moon safely, they are not good enough for accurate satnav location in city streets, the software is Einsteinian.

So, a better theory than Newton’s (which was already an excellent one), and no problem with action-at-a-distance (there is no force so acting), although one worries about whether space curving is philosophically squeaky clean.

But not the last word. Quantum mechanics is equally well established, and the two theories are incompatible at the ultra-micro level. So, much current effort in physics goes into looking for a quantum gravity theory (string theory and loop quantum gravity are contenders).

So, to summarize, the scientific method replaced Aristotle’s view with a better one (Newton’s), the latter has been replaced by an even better one (Einstein’s), and it is likely that a quantum gravity theory will replace that. No theory can ever be PROVEN to be correct (although probably many are).

To deal now with the cognitive part of your question – how come different people, using the same logic (Descartes, Hume), seeing the same things and associating their perceptions using the same patterns (Hume), come up with different explanations of, say, gravity.

Many everyday truths are manifest, such as now it’s autumn, I’m drinking a glass of wine as I type this, etc. But many truths about the world are hidden, not manifest, such as what makes the moon go round the Earth. So we must grope our way towards them by conjecture and testing, and new conjectures can occur by association/ connection of ideas that nobody thought of before. Let’s accept the Humean story that we associate ideas by contiguity, by similarity (resemblance), by cause-and-effect. Newton saw a similarity that nobody had thought of. He saw that the moon and an apple SIMILARLY fall to Earth. In the apple case, the trajectory hits Earth. In the moon case, although forever falling to Earth, this is counterbalanced by its inertial movement tending to carry it off into space, so that it orbits rather than hits the Earth.

Another famous new connection of ideas by similarity was Darwin’s. Everybody knew that plants/animals could be changed by selective breeding (artificial selection), Darwin saw that nature SIMILARLY worked by selective breeding (natural selection). Finally, Einstein’s realization that falling freely to Earth is SIMILAR to being at rest in outer space (Principle of Equivalence) helped him formulate his theory of gravity. In short new links can be made between different conceptual maps, and cognitive neuroscience has gone some way to clarifying the physical (neuronal) basis of this.


Answer by Helier Robinson

Aristotle, Newton, and Einstein were doing philosophy of nature, which later came to be called empirical science. And empirical science is an excellent foundation one can use to determine which was correct. Aristotle was weak on physics (but strong on biology); he thought gravity was simply the desire of things to reach their own place. Newton had the advantage of knowing Galileo’s principle that thins always move in a straight line unless acted upon by an outside force (he made it his first law) and of knowing Kepler’s laws of planetary motion. Also, Galileo had pointed out that the book of nature was written in the language of mathematics; and Newton was one of the greatest mathematicians who ever lived, while Aristotle disliked mathematics.

Notice that Newton did not have an explanation of gravity: he explained motion and forces, velocities and accelerations. His physics is very good for everyday purposes; it fails only under extreme conditions that he could not have known about in his day, such as very high velocity. Einstein improved on Newton, and also explained gravity as being due to the curvature of space-time. The main difference between all three is the amount of known science they had available to them.


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