‘Spooky action at a distance’ is now used to describe quantum entanglement. But forces, like gravity, appear in the form of action at a distance too. Are forces spooky too? Physics professor, Sverre Holm, journeys the occult origins of forces, and the mysteries still looming over modern science.
Isaac Newton is well known for having added, "I frame no hypotheses" to the second edition of Philosophiæ Naturalis Principia Mathematica in 1713, meaning that he could not explain the cause of gravitation.
Gottfried Leibniz’ view was that if such attraction at a distance is not explainable then it is a perpetual miracle, and added that it is “a chimerical thing, a scholastic occult quality.”
Leibniz’ dismissal is all the more strange in light of Newton’s seeming agreement with Leibniz. Newton himself had after all dismissed the medieval scholastics for their belief in substantial forms, like “sympathies” between similar objects. He had written that “to tell us that every Species of Things is endow'd with an occult specifick Quality by which it acts and produces manifest Effects, is to tell us nothing.”
How could Newton be so sure that his theory of gravitation did not fall under the category of such a scholastic form, and thus that Leibniz' arguments were not valid?
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Newton called the presence of forces without an intervening medium an absurdity.
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In retrospect, we know that Newton was right and Leibniz wrong. The field concept, which plays such an important role in today’s physics, was well established by the end of the 19th century. Michael Faraday and James Clerk Maxwell played major roles through their work with electric and magnetic fields. Mary Hesses’s classical book from 1961 is the definite guide to this history.
Action at a distance, sympathies, and tides
Leibniz was right that Newton’s gravitational action at a distance shares the property of the medieval substantial form in that it is fundamentally inexplicable. The main difference is that the gravitational law is quantitative, and Newton could demonstrate for the first time that the same law describes the moon’s orbit and a falling apple on earth. This was the definite break with the Greek separation between a heavenly and an earthly physics.
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From antiquity, it had also been known that the moon and the tides were linked. The scholastic way was to say that the moon had a “watery nature” and thus attracted seawater on the Earth through joint “sympathy”. This could explain the tidal bulge on the side facing the moon, but not the bulge on the opposite side of the Earth. Galileo had sought for decades to develop an alternative model that was meant to prove that tides instead were caused by the combined motion of the Earth around the Sun and the Earth’s rotation. It was all a part of his failed attempt to prove Copernican heliocentrism. Newton, however, could show that gravitational attraction would explain both tidal bulges.
Three important aspects
To understand the novelty of Newton’s approach one needs to see that there was a subtle change in the meaning of the word “occult” in the early modern period. Over the course of the 17th century, the meaning changed from encompassing both lack of sensibility and lack of intelligibility, to mean only unintelligible. Historian Keith Hutchison emphasizes three important aspects to consider for the Leibniz-Newton debate to be comprehensible:
- Experience or detectability is the fact that although we cannot sense the gravitational field from the moon with our five senses, we experience its effects indirectly in the form of the tides.
- Sensibility is how we directly sense properties like mass, extent, and colour with our eyes or ears, or as touch, smell, or taste.
- Intelligibility is the extent to which a rational explanation can be found for why two bodies attract each other.
Kepler’s second law from 1609, which says that planets in their elliptical orbit move faster when closer to the sun, was an important step on the way to understanding the central role of the attraction of the sun. Newton completed the picture with his gravitational law. It precisely describes the first aspect, the experience of gravitation.
Hutchison describes medieval scholastics as too focused on sensation, making them too sceptical to accept phenomena which could only be experienced indirectly. It is surprising that strictly speaking the Aristotelian scholastics were more logical than following generations, not less. Descartes, one generation before Newton, had however pointed out that even the sensible does not really enter the intellect directly. Senses, which are there to safeguard our bodies, are not necessarily reliable for picturing reality. This would put less emphasis on sensibility.
Leibniz, when saying that what is not explicable is an occult quality, had reacted to the lack of the third aspect, the intelligibility. For the mainly British empiricists it was, however, enough that one could describe the experience of gravitation, while most continental natural philosophers initially demanded explanations at a deeper level to accept Newton’s theory.
Newton and natural law
Newton’s explanation was in effect a substitution of the medieval concept of form with a natural law, the law of gravitational attraction. His justification is in the second edition of Principia:
“Surely, this World – so beautifully diversified in all its forms and motions – could not have arisen except from the perfectly free will of God, who provides and governs all things. From this source, then, have all the laws that are called laws of nature come …”
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The scholastic way was to say that the moon had a “watery nature” and thus attracted seawater on the Earth through joint “sympathy”.
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Newton’s idea of a lawgiver is very different from the modern concept of a natural law, which is just a brute fact about nature. Philosopher of science Nancy Cartwright has understood the limitations of the modern view as she so aptly says, “that in the end the concept of a law does not make sense without the supposition of a law-giver.” Consequently, she seeks to base natural science on a different foundation than laws of nature. I think she is right in pinpointing the weakness of the brute fact concept of law. However, at present, this seems not to engage practicing scientists, who build on centuries of empirical evidence that the concept of law is a useful tool.
Different levels of explanation
Judging from the use of the word “mathematical” in the title, the intention of the Principia was not to explain gravitation, but to describe it with mathematics. With time, however, the gravitational law was even seen to explain gravitation. Philosopher George Berkely in De Motu (1721) is right in saying that it is the laws of motion, not the cause, which have been found.
Historian Ernan McMullin points out that this means that we in fact have two levels of explanation, strong and weak. The gravitational law is of the weak kind as it just explains the effects of gravitation. It may give the impression that we know all there is to know, but on a second look, it does not explain the cause of gravitation itself. The latter is what one would require from a natural law of the strong form.
Mechanical theories
Leibniz’ problem with gravitation was based on notions like the old adage that ‘‘matter cannot act where it is not”. His model for matter was based on continuity and not finite atoms and implied that there had to be a continuum of matter between the moon and the earth, for action to occur. A similar model was Descartes’ vortex theory where matter particles due to their proximity to each other, move in circles. The vortices could be of all sizes and even as large as planetary systems.
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| Ether vortices around celestial bodies, after Descartes (Wikipedia) |
It is harder to figure out what Newton’s views were. On the one hand, he must have been satisfied with the success of the mathematical description. On the other hand, Newton called the presence of forces without an intervening medium an absurdity. He did discuss a material ether which had a vague resemblance with the quintessence, the element which occupied the heavenly realm in the Greek world picture. It was the fifth element, different from the four substances of the material world, earth, water, air, and fire. The ether was a kind of corpuscular theory, consistent with Newton’s optical theory where light was considered to consist of particles.
As the atomic model for matter gained more support through the 18th century, it became apparent that even this model contains tiny voids, and thus implied some form of action at a distance, but at the microscopic level.
Michael Faraday: forces constitute matter
The modern picture comes from Michael Faraday’s experiments with magnetic and electric forces. The fact that changes in the intervening medium between magnetic poles curve the force lines and thus influence the field was significant. Faraday, the experimentalist, was sceptical to philosophical theories, but in 1844 his experiments led to his, at first, hesitant belief “that particles are only centres of force; that the force or forces constitute the matter”.
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| Magnetic field lines as shown by iron filings sprinkled on paper above a bar magnet (Wikipedia) |
In the same year, Faraday wrote an interesting note-to-self, which was much more subjective. Here he explained how this view of forces also fitted with his religious world picture. T.H. Levere considers this Faraday’s main reason for accepting the new model. Faraday writes, “that the Creator governs his material works by definite laws resulting from the forces impressed on matter, and, that matter is that of which we take cognizance by our external senses”, and goes on to say, “I cannot imagine physical force without matter, or matter without force.” First Faraday expresses here a similar conviction to Newton’s about the origin of natural law. Second, in agreement with his mentor Humphry Davy’s conviction that God was always active in creation due to his omnipotence and omnipresence, forces everywhere would be perfectly consistent. Levere also explains how convictions such as these do not necessarily shape scientific theories, but they do play a role in selection of theories. After all Newton and Leibniz shared similar notions without using them to advocate field theories.
James Clerk Maxwell: energy is in the field
It was up to James Clerk Maxwell, the greatest physicist between Newton and Einstein, to complete the transition from a mechanical to a field view. Initially he formulated his electromagnetic theory in the framework of the mechanical vortex atom theory, which was so popular in the last half of the 19th century, but with “measured scepticism” as Helge Kragh puts it. At this point, he considered vortices just to be illustrative, but not explanatory. In 1864, he made it clear that when speaking of the energy of the field he wished to be understood literally. The field was now more than a mathematical convenience and had become a physical reality, through which energy travels with a finite velocity.
Finally, we had arrived at what Freeman Dyson calls a world with two layers. The first layer is the truly fundamental one and consists of fields which are hidden from our view. The second layer can be sensed, touched, and measured, and consists of mechanical stresses, energies, and forces.
In this way, the foundation was laid for developments in gravitation and quantum physics. In general relativity, it is space-time itself which is the field. Newton would probably have marvelled at the discovery of gravitational waves and he might have supported the search for a hypothetical elementary particle of gravitation, the graviton.
Would he also have been satisfied with the intelligibility that the curvature of space-time gives, or would he still have thought that the fundamental question remained unanswered: What causes gravitation?
Last year Norwegian actor Svein Tindberg played a curious observer of quantum physics in the popular play “The God particle”. Critics praised him for pondering existential questions with humour and seriousness.
In the play, Tindberg scratched his head over the fact that a particle can go in two directions at the same time, and simultaneously be here and not be here. I think the classical field concept is just as mysterious as quantum properties are and well worth pondering. Regrettably, our awe and wonder over it was overshadowed by last century’s other developments in physics.
References
- Cartwright, N. (1993). Is natural science ‘natural’ enough? A reply to Philip Allport. Synthese: Int. J. Epistemology, Methodology and Philosophy of Science, 291–301.
- Dyson, F. J. (2007). Why is Maxwell's theory so hard to understand? In 2nd Eur. Conf. Antennas and Propagation, EuCAP 2007, IET, 1-6.
- Hesse, M. B. (1961). Forces and fields: The concept of action at a distance in the history of physics. Dover (2005).
- Hutchison, K. (1982). What happened to occult qualities in the scientific revolution? Isis, 73(2), 233-253.
- Kragh, H. (2002). The vortex atom: A Victorian theory of everything. Centaurus, 44(1‐2), 32-114.
- Leibniz, G. W. (2007). Exchange of papers between Leibniz and Clarke (1715-1716). Translated by Jonathan Bennett.
- Levere, T. H. (1968). Faraday, matter, and natural theology—reflections on an unpublished manuscript. The British Journal for the History of Science, 4(2), 95-107.
- McMullin, E. (2002). The origins of the field concept in physics. Physics in Perspective, 4, 13-39.
- Newton, I. (1704), Opticks, Query 31.
Images from:
- https://en.wikipedia.org/wiki/Tide
- https://en.wikipedia.org/wiki/Mechanical_explanations_of_gravitation
- https://en.wikipedia.org/wiki/Magnetic_field
