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Jun. 17th, 2014 07:04 am![[personal profile]](https://www.dreamwidth.org/img/silk/identity/user.png){kind=small}
leblonNATURE and Nature’s Laws lay hid in Night:
God said, “Let Newton be!” and all was light.
(Alexander Pope)
Why do rocks fall down? Most people would say "Because the Earth attracts them" or "Because of the Earth's gravity pull." But this facile answer is unsatisfactory for many reasons. To begin with, it takes for granted that there is such a thing as the downward direction, that is, the direction towards the center of a compact gravitating body. According to current cosmological theories, such bodies arose from inhomogeneities in an otherwise fairly uniform and featureless early Universe which grew thanks to the Jeans instability. But the origin and rate of growth of these instabilities depend on the details of the cosmological model, such as inflation, as well as particle physics (the number of light neutrinos). Gravity alone is not nearly enough, and in fact current theories of structure formation do not work without postulating that most of the matter in the Universe is dark matter. (http://www.pnas.org/content/95/1/1.full.pdf)
A more serious objection is that the usual answer is essentially a tautology: the rocks fall down because there is a force which pulls them down. This is no better than Aristotle's explanation that heavy bodies move towards the center of the Earth because of their natural tendency to be there. To go beyond the tautology, one needs to explain the origin of the attractive force. While the quantitative description of the gravitational force was first found by Isaac Newton, his Law of Universal Gravitation is by no means an explanation, but a phenomenological model. Newton himself acutely realized this ("Hypothesis non fingo") and was bothered by the action-at-a-distance nature of his force.
Newton's caution was vindicated with the advent of the General Relativity Theory which dispensed with the invisible force of gravity and postulated that physical objects merely move along "shortest" paths (geodesics) in a curved space-time. Einstein also got rid of instantaneous action-at-a-distance and showed that small perturbations in the curvature of space-time propagate with the speed of light. (http://www.hs.uni-hamburg.de/DE/GNT/events/pdf/steinicke05.pdf) Einstein's beautiful theory has an air of inevitability about it which hypnotized physicists and non-physicists alike into believing that a satisfactory theory of gravity has been found. Has it though? Over the last few decades evidence has been accumulating that Einstein's theory does not work well at scales much larger than the size of our Solar System. (http://www.astr.ua.edu/keel/galaxies/darkmatter.html) Astrophysicists and cosmologists proposed the existence of dark matter and dark energy whose properties are postulated to bring Einstein's theory in agreement with observations. But despite many dedicated searches, no dark matter particles have been found, even though they are supposed to contribute 5 times more to the mass density of the Universe than ordinary matter. There are also powerful theoretical objections to dark energy (without extreme fine-tuning, its size would be 10^120 times larger than the value needed to explain the observations, http://arxiv.org/abs/astroph/0207347). Dark energy and dark matter look increasingly like epicycles whose only goal is to save the accepted theory of gravity. The level of desperation can be appreciated from the fact that some physicists seriously proposed going back to the Newtonian view of physics, with suitable modifications (Modified Newtonian Dynamics, http://arxiv.org/abs/0908.3842 ). It seems that we are no closer to understanding the nature of the gravitational force than in the times of Newton.
Why do rocks fall down?
God said, “Let Newton be!” and all was light.
(Alexander Pope)
Why do rocks fall down? Most people would say "Because the Earth attracts them" or "Because of the Earth's gravity pull." But this facile answer is unsatisfactory for many reasons. To begin with, it takes for granted that there is such a thing as the downward direction, that is, the direction towards the center of a compact gravitating body. According to current cosmological theories, such bodies arose from inhomogeneities in an otherwise fairly uniform and featureless early Universe which grew thanks to the Jeans instability. But the origin and rate of growth of these instabilities depend on the details of the cosmological model, such as inflation, as well as particle physics (the number of light neutrinos). Gravity alone is not nearly enough, and in fact current theories of structure formation do not work without postulating that most of the matter in the Universe is dark matter. (http://www.pnas.org/content/95/1/1.full.pdf)
A more serious objection is that the usual answer is essentially a tautology: the rocks fall down because there is a force which pulls them down. This is no better than Aristotle's explanation that heavy bodies move towards the center of the Earth because of their natural tendency to be there. To go beyond the tautology, one needs to explain the origin of the attractive force. While the quantitative description of the gravitational force was first found by Isaac Newton, his Law of Universal Gravitation is by no means an explanation, but a phenomenological model. Newton himself acutely realized this ("Hypothesis non fingo") and was bothered by the action-at-a-distance nature of his force.
Newton's caution was vindicated with the advent of the General Relativity Theory which dispensed with the invisible force of gravity and postulated that physical objects merely move along "shortest" paths (geodesics) in a curved space-time. Einstein also got rid of instantaneous action-at-a-distance and showed that small perturbations in the curvature of space-time propagate with the speed of light. (http://www.hs.uni-hamburg.de/DE/GNT/events/pdf/steinicke05.pdf) Einstein's beautiful theory has an air of inevitability about it which hypnotized physicists and non-physicists alike into believing that a satisfactory theory of gravity has been found. Has it though? Over the last few decades evidence has been accumulating that Einstein's theory does not work well at scales much larger than the size of our Solar System. (http://www.astr.ua.edu/keel/galaxies/darkmatter.html) Astrophysicists and cosmologists proposed the existence of dark matter and dark energy whose properties are postulated to bring Einstein's theory in agreement with observations. But despite many dedicated searches, no dark matter particles have been found, even though they are supposed to contribute 5 times more to the mass density of the Universe than ordinary matter. There are also powerful theoretical objections to dark energy (without extreme fine-tuning, its size would be 10^120 times larger than the value needed to explain the observations, http://arxiv.org/abs/astroph/0207347). Dark energy and dark matter look increasingly like epicycles whose only goal is to save the accepted theory of gravity. The level of desperation can be appreciated from the fact that some physicists seriously proposed going back to the Newtonian view of physics, with suitable modifications (Modified Newtonian Dynamics, http://arxiv.org/abs/0908.3842 ). It seems that we are no closer to understanding the nature of the gravitational force than in the times of Newton.
Why do rocks fall down?
