# Gravity: Quantum

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## Introduction

Taking a clue from theories we have for the other three forces, it stands to reason that in order to create a Theory of Everything that includes gravity, we are going to have to create something like a quantum theory for the gravitational forces and its field. It does no good to unify three forces only to leave the Big One an orphan with its own separate theory. Electromagnetism and gravity were the only two forces known to physicists up until 1930. But, while the electromagnetic interaction was rapidly evolving from a classical field theory into a relativistic quantum field theory, gravity during this same time seemed to languish. Of course one cannot deny the formidable advances in the mathematical representation of gravity as a consequence of Einstein's general relativity, however, these advances seemed to reinforce the belief among physicists at that time that gravity was so unlike electromagnetism that the unification of the two was an unreachable goal.

Much of the impetus for developing gravitation theory seemed to come more from the ranks of mathematicians enthralled by the abstract properties of Riemannian spaces. In contrast, much of the stimulus for advancing quantum electrodynamics during its later years, came from physicists trying to understand a number of experimental results in atomic physics. With the exception of a handfull of distinct experiments, general relativity remained, and still remains, a theory beyond the reach of modern experimental techniques. As Einstein once reflected,

"...The theoretical scientist is compelled in an increasing degree to be guided by purely mathematical, formal considerations in his search for a theory, because the physical experience of the experimenter cannot lead him up to the regions of highest abstraction..."

Relativists weren't the only ones thinking about what to do with gravity.

Other physicists steeped in the language of quantum mechanics were taking a very differentand perhaps more pragmatic approach. By the 1930's a quantum theory for the electron had been fashioned by Dirac. More importantly perhaps is that, with the discovery of the strong and weak nuclear forces, nature no longer looked quite as simple as it had with only gravity and electromagnetism to worry about. With a quantum theory for gravity no where in sight, attention turned away from gravity unification and towards more profitable areas of investigation. Still, some physicists struggled onward as best they could with only a sketchy guide of the landscape of quantum gravity to guide them. One of those was Leon Rosenfeld who, in 1930, attempted to apply the fledgling techniques of quantum field theory to the gravitational force by calculating the gravitational self-energy of the photon. He was the first to identify many of the technical problems that such quantum gravity calculations would have to overcome. Not the least of which was an even more terrible plague of infinities that anyone had ever seen in quantum electrodynamics. As Bryce DeWitta at Princeton pointed out in his 1967 paper "__Quantum Theory of Gravity__,"

...Rosenfeld's result [was] a forecast that quantum gravidynamics was destined, from the very beginning, to be inextricably linked with the difficult issues lying at the theoretical foundation of particle physics."

Bryce DeWitt while a graduate student at Princeton, re-performed Rosenfeld's calculations 20 years later as part of his PhD thesis at Harvard University, spurred-on by the significant advancements that had been made in QED since the time Rosenfeld attempted to tackle the same calculations. Even though the QED infinities had been shackled using the renormalization technique, DeWitt's results only uncovered still more problems with nievely applying QED techniques to gravity. Renormalization would have to be applied not just twice as in the case of QED, but over and over again an infinite number of times just to keep the gravitational calculations from blowing up. These early efforts by Rosenfeld and DeWitt led to a vastly improved understanding of what a quantum gravity theory would have to consist of, and a first reconnisance of this theoretical landscape, but they also yielded not a shred of evidence for what a true theory should look like that could consistently explain gravity as a quantum field. * *

"The quantization of the Einstin theory ... raises fundamental questions about the meaning of geometry, of spacetime, of manifolds, and of the relationship of gravitation to the rest of physics."

Although theoreticians working in this difficult area of physics are quite bullish aboutits chances for success at least eventually, thecritics of the quantum gravity program are not as certain about the inevitability of finding such a Mother of All Theories. RichardFeynman has stated that,

"The extreme weakness of quantum gravitational effects now poses some philisophical problems: maybe nature is trying to tell us something new here,maybe we should not try to quantize gravity...it is still possible thatquantum theory does not absolutely guarantee that gravity HAS to be quantized."

An increasingly vocal minority of theoreticians have gone on record as proposing that nothing could be more un-natural that to quantize gravity, and that such a program is logically inconsistent with experimental quantum mechanics.

Because of the intimate relationship between gravity and spacetime, it seems almost obscene to propose that at some level spacetime is quantized and its shape uncertain. It is bad enough that we have had to put up with a vacuum populated by virtual particles and false vacuua, but then to say that the whole shebang is based on a foundation for spacetime as changeable as the shifting sands ona beach is, well, simply perverse. These physicists argue that quantum mechanics works well when you have test particles like electrons that are vastly smaller than the scale of the quantum phenomena. But when you reach the Planck scale at 10^{-33} centimeters, you run out of test particles and clocks that are smaller than the phenomena you are attempting to formulate quantum laws for.This is like trying to cut a 1/2 inch hole with a 2 inch chisel, or like trying to feel the texture of beach sand through a pair of winter mittens. By the time you arrive at the Planck scale, you have reached an epistemological horizon to nature, beyond measurement and mathematical formulation in terms of known quantum principles. If the world trembles from moment to moment in a frenzy of quantum indecision, is there any evidence forsuch a phenomenon? Two areas of investigation seem to point in this direction. Thefirst is the so-called EPR experiment, the second is the many-Worlds interpretation of quantum mechanics.

Although the experimental results for the existence of a quantum of gravity are dissappointing, and some theoreticians seem to have powerful biases against such a concept, Julian Schwinger has been substantially more upbeat about the prospects forthe quantum gravity program eventually succeeding, believing that the evidence for the existence of the graviton, though indirect,is actually 'impressive'. We should keep in mind that the absense of evidence for a quantum nature to gravity is not the same as evidence of absense. There was a time when the W, Z and top quark had not yet been detected, and a time when the Lamb Shift had not been measured. Theories have to play both to the audience of the future as well as to the critics of the present. So, let's just suppose along with no less a luminary in physics as Julian Schwinger, that the theoreticians are right and that as Plato might have said, a quantum theory of gravity exists 'out there' waiting to be discovered. What would it look like?

Theoretically, the shape of this theory is uncertain, but at least now the fortunes of quantum gravity theory have been linked up with the unification of the other forces.A tremendous symbiotic relationship has evolved over the last decade between the quantum gravity program and the search for a Grand Unification Theory. As we saw in the last chapter, GUTs are a powerful but apparently incomplete way of looking at how the strong, weak and electromagnetic forces operate. They also seem to suffer from numerous technical problems that some theoreticians now feel can only be remedied by including gravity explicitly. Evidently, the search for a Theory of Everything seems destined to proceed by our having to be forced to understand all of the forces and particles in a unified way right from the start. Nature seems uninterested in offering us any short cuts where you might be allowed to add one field at a time to a steadily rising pyramid of self-consistent understanding. This makes the search for such a theory easy in some ways, because the theory will either work for all of the forces together, or it will fail for all of the forces.

## Related EoC Articles

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- Gravity: Quantum
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- Gravity: What is Gravity?

## Image Preview

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: Quantum." In: Encyclopedia of the Cosmos. Eds. Bernard Haisch and Joakim F. Lindblom (Redwood City, CA: Digital Universe Foundation). [First published January 17, 2008].

<http://www.cosmosportal.org/articles/view/135653/>

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