Gravity: Gauge Field
Gravity: Gauge Field
| Topics: |  Cosmology Gravity General Relativity |
Introduction
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 to what such a theory ought to consist of, we can already specify with some certainty what the properties of the quantum of gravity ought to look like. The graviton has all the right properties as a theoretical particle to be the carrier of gravity, now all we have to do is create a field theory to go along with the particle.
This takes us smack into two hard questions: can such a theory of gravitons be described by the same class of theories that have proven so effective in the Standard Model, namely, the "non-Abelian gauge field theories? Also, can it be shown that graviton quantum field theory is free of infinite answers just as QED and QCD are now known to be thanks to the renormalization technique.
In 1956, Ryoyu Utiyama at the University of Osaka showed that, just as in the case of electromagnetism and Yang-Mills theory, gravity can also be expressed as a similar kind of gauge field. This was a significant new finding for gravity since it confirmed that gravity was of the same class of theory as ones that were already proving to be important as the foundation for understanding the other three forces. When you think about it, this is a rather astonishing result because gravity sure seems to look different than the other forces.
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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: Gauge Field." In: Encyclopedia of the Cosmos. Eds. Bernard Haisch and Joakim F. Lindblom (Redwood City, CA: Digital Universe Foundation). [First published June 11, 2008].
<http://www.cosmosportal.org/articles/view/135645/>
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