Introduction
The central issues behind these studies are deceptively simple to state: What is the quantum mechanical relationship between discrete quanta of matter embodied in their associated quark and lepton quantum fields, and spacetime? Are they like the bits of fruit suspended in a jello pudding; like flotsam adrift on some great imperceptable ocean? If we were to follow a line enscribed on the fabric of space, where do the points on that line cease to be space, and become the embedded quark or lepton? Since Clifford gave his 1879 paper On the Space Theory of Matter, and Einstein gave substance to this line of theoretical inquiry by his creation of general relativity, much effort has gone into creating a theory where matter is treated as a purely geometric phenomenon.
The seeds to this proposal can be found at least as far back as the 19th century. Boscovitch and Kant, for example, believed that matter was the result of the interactions between pure force fields acting upon each other in empty space; matter was merely an epiphenomenon tracing the interplays between far more fundamental events in the universe. Riemann, in his 1854 paper On the Hypotheses that lie at the Foundation of Geometry was convinced that space possessed a structure ( or topology) that was completely unlike the smooth, flat euclidean geometry of Newtonian physics. Some years later, the mathematician Oscar Klein in 1872 began his 'Erlanger' program from which he, too,hoped to show that all physical laws and quantities could be described by purely geometric concepts.
That there was a peculiar, ephemeral aspect to matter was already appreciated both theoretically and experimentally as early as 1901 when it was discovered that the electron behaved as though ALL of its mass were due to its electromagnetic field alone. Instead of a massive, tiny sphere embedded in its own electrostatic field, the experiments demonstrated that the way the electron behaved upon acceleration was consistent with it being a purely electromagnetic phenomenon, with no massive tiny sphere left over. Modern experimental results also show that the proton is built-up of three quarks, but that the sum of the rest masses for the quarks of 20 MeV is 46 times less than the mass of a single proton. How can a body weigh more than the sum of its parts? The answer is that the balance of the mass is in the form of the energy latent in the gluon field surrounding each quark — a totally non physical entity but a simple manifestation that Einstein's E = mc2 works. Energy and mass are just different sides of the same conceptual coin, only their total value, regardless of aportionment, is important.
Now, as you might well imagine, divesting matter of its 'mass' and considering electrons and protons as persistent 'knots' in some field has few practical applications since physicists find it useful to think of even the smallest particles as mineature spheres embedded in an otherwise implacable empty space. Yet, the idea that matter is just another form of empty space, much as Einstein showed that mass is just another form of energy, has several profound implications which reach down to the very bedrock of physical reality. Towards the latter years of his life, Einstein was so certain of the correctness of the idea that matter is not a fundamental ingredientto nature, that he wrote(1950),
"The material particle has no place as a fundamental concept in field theory. Even Maxwell's electrodynamics are not complete for this reason. Gravity as a field theory must also deny a preferred status to matter.
Another perspective on this matter is voiced by John Wheeler(1964),
"What else is there out of which to build a particle except geometry (spacetime) itself?"
The theoretical program of the geometrization of matter which began in earnest with Weyl in 1924, was subsequently developed by Wheeler and Meisner between 1955-7. They began with the proposal that electric charge was just electric lines of force that had become trapped in the contorted, knotty topology of spacetime. Spacetime at a scale of 10-33 cm was imagined not as a smooth flat sheat of paper, but one in which loops and bridges between one region and another appeared and vanished, and the geometry of space was highly curved and warped at this scale. The latest step, as we have seen, now seems to be superstring theory in which particles and geometry seem to blend imperceptably together. Assuming that the theoretical program of geometrizing matter is ultimately successful, we are still left in somewhat of a conceptual bind. To have a scientific world view that reduces existence to an unfolding of geometric spacetime curvature in Superspace, and strings of energy vibrating in 26-dimensions, only heightens the essential mystery of what space and vacuum are in themselves.
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Gravitational waves are propagating gravitational fields, "ripples" in the curvature of space-time, generated by the motion of massive particles, such as two stars or two black holes orbiting each other. Gravitational waves cause a variable strain of space-time, which result in changes in the distance between points, with the size of the changes proportional to the distance between the points. Gravitational waves can be detected by devices which measure the induced length changes. Waves of different frequencies are caused by different motions of mass, and difference in the phases of the waves allow us to perceive the direction to the source and the shape of the matter that generated them. (Source: NASA-The Laser Interferometer Space Antenna (LISA).)
Citation
Odenwald, Sten, Ph.D. (Contributing Author); Bernard Haisch (Topic Editor). 2008. "Gravity: Matter as Geometry." In: Encyclopedia of the Cosmos. Eds. Bernard Haisch and Joakim F. Lindblom (Redwood City, CA: Digital Universe Foundation). [First published February 14, 2008].
<http://www.cosmosportal.org/articles/view/135648/>
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