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# Gravity

## What is Gravity?

Gravity is a force of attraction that exists between any two masses, any two bodies, any two particles. Gravity is not just the attraction between objects and the Earth. It is an attraction that exists between all objects, everywhere in the Universe. Sir Isaac Newton (1642 — 1727) discovered that a force is required to change the speed or direction of movement of an object. He also realized that the force called "gravity" must make an apple fall from a tree, or humans and animals live on the surface of our spinning planet without being flung off. Furthermore, he deduced that gravity forces exist between all objects.  Read More »

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Gravity: What is Gravity? Last Updated on 2008-11-16 00:00:00 What is Gravity? There are many articles here in the Encyclopedia of the Cosmos focusing on various aspects of gravity, as listed in "Related EoC Articles" below.  However, to begin, let's explore first some basic definitions of what gravity is, or at least our understanding of it at this point in the human quest for knowledge and understanding of the universe around us. We don't really know what gravity "is." We can define what it is as a field of influence, because we know how it operates in the Universe. Some scientists think that gravity is made up of particles called gravitons which travel at the speed of light. However, if we are to be honest, we do not know what gravity "is" in any fundamental way – we only know how it behaves. Here is what we do know... Gravity is a force of attraction that exists between any two masses, any two bodies, any two particles. Gravity is... More »
Gravity: Canonical Quantization Last Updated on 2008-06-11 00:00:00 The second program called 'Canonical Quantum Gravity' theory has a very different parentage. Following the established mathematical techniques developed by Schroedinger and Dirac, Einstein's equation for gravity gets rewritten, and it is in this new form that it can be analyzed for a consistent set of 'operators' and 'states' following conventional quantum mechanics. This forces quantum gravity theory to come out looking like a respectable quantum theory, but in which the dynamics of 3-D geometries play a key role, and the new Hamiltonian formulation describes how a system, the complete 3-D geometry of the universe, changes from state to state. Whereas quantum mechanics depends on considering the histories of particles as they move through space-time, quantum gravity must, in addition, consider all possible geometries of space as they unfold in time. Gravity: Canonical... More »
Gravity: Gauge Field Last Updated on 2008-06-11 00:00:00 Apart from the ambiguous experimental evidence that gravitational radiation exists but its quantal effects are unmeasureable, what compelling theoretical evidence do we really have that gravity should be expressable as a field theory similar to QED, QCD and Electroweak theory which form the basis of the Standard Model? We have already seen how the graviton has an upper limit to its possible rest mass which is 100 trillion times lower than the photon itself. In addition to being massless, the graviton must have a spin assignment of 2 units so that the equation that defines its action looks like Einstein's equation for the gravitational field represented by a two-index field, $g_{\mu \nu}$. Steven Weinberg also proved an important theorem in 1964 which showed that spin-2 particles have to couple to all other particles and fields with a universal strength. So, even without a single clue... More »
Gravity: String Theory Last Updated on 2008-05-02 00:00:00 There was hardly any time to wrestle with the implications of these ideas before yet another revolution in thinking hit the theoretical fan. In 1982, John Schwartz and Michael Green announced 'Superstring Theory'. Henceforth, particles would not be thought of as point-like concentrations ofenergy, but as 1-D, vibrating 'strings' of energy.Particle world lines would be fattened from spaghetti-like, 1-D tracks in spacetime, to macaronni-like tubes, and with this new structure, all infinities would vanish without any need at allfor renormalization. The only problem is that: 1) Spacetime would have to have either 10 or 26 dimensions in order that the theory was self-consistent; and 2) the theory would naturally work only at energies near 1019 GeV. The lowest "mode" of string oscillation would yield particles withno rest mass at all. The next-highest mass range would be 1019 GeV. Throughout... More »
Gravity: Quantization ca. 1990 Last Updated on 2008-03-14 00:00:00 String theory does not do away with the need for some kind of background spacetime. The equations describing the movement of strings do so from the standpoint of a string moving in a flat, Minkowski space of up to 26-D. The 1-D strings trace-out 2-D surfaces. The geometry of these surfaces have a set of internal symmetries associated with them which allow the fields to appear as different kinds of particles. There are literally millions of different topologies for these compact spaces. The number of holes in these spaces, the so-called topological genus, may have something to do with the number of different kinds of lepton types; electron, muon, tauon. There may exist regions in the universe where these spaces have chosen slightly different ways to curl-up so that differences in the fundamental particle types mayexist in our universe. How do string theorists actually think... More »