The Elegant Universe
Brian Greene
Contributed by Twanda Mangano
Chapter 5
Summary

In extreme conditions, when things are either extremely massive or extremely miniscule—for example, near the center of black holes (huge), or the entire universe at the moment of the big bang (tiny)—physicists must draw upon both general relativity and quantum mechanics for explanations. By themselves, both theories are inadequate on drastic scales. For this reason, physicists are working to develop a quantum mechanical version of general relativity. Heisenberg’s uncertainty principle marked a great revolution in the history of physics. The uncertainty principle describes the universe as more and more chaotic when examined on smaller and smaller distances and shorter and shorter time scales. The principle doesn’t only exist in experimental conditions—that is to say, it doesn’t only exist when physicists tamper with nature by trying to make measurements, as Feynman found. The uncertainty principle is intrinsic to nature and alwaysin action, even in the most serene conditions imaginable.

Quantum claustrophobia occurs even in seemingly empty regions of space. On a microscopic level, there is always a tremendous amount of activity, which becomes increasingly agitated the more distance and time scales shrink. True emptiness does not exist anywhere in the universe. Three highly successful theories form the standard model of particle physics. The only trouble with the standard model is that it conspicuously excludes gravity from its framework.

The Schrödinger wave equation, one of these theories, was approximate from the outset and did not apply to small microscopic regions. Originally, Schrödinger tried to incorporate special relativity into his conception of quantum mechanics, but he couldn’t make the pieces fit, so he simply left it out. But physicists soon understood that no quantum mechanical framework could be correct without some consideration of special relativity. Because it didn’t consider special relativity, Schrödinger’s approach ignored the malleability and constant motion of all matter. Quantum electrodynamics was developed to incorporate special relativity into quantum mechanics. Quantum electrodynamics is an early example of what came to be known as a relativistic quantum field theory: relativistic because it includes special relativity; quantum because it takes into account probability and uncertainty; and field theory because it merges quantum principles into the classical conception of a force field (Maxwell’s electromagnetic field).

Quantum electrodynamics has proven extremely successful in predicting natural phenomena. Tochiro Kinoshita has used quantum electrodynamics to calculate extremely detailed properties of electrons, which have been verified to an accuracy of better than one part in a billion. Following the model of quantum electrodynamics, physicists have tried to develop analogous frameworks for understanding the strong (quantum chromodynamics), the weak (quantum electroweak theory), and the gravitational forces. Sheldon Glashow, Abdus Salam, and Steven Weinberg formulated the quantum electroweak theory to unite the weak and the electromagnetic forces into a common form at high temperatures. At lower temperatures, the electromagnetic and weak forces crystallize in a different manner from their high-temp form. This process, called symmetry-breaking, will become important as Greene’s descriptions of string theory become more nuanced.

In the standard model, messenger particles carry the various bundles of forces (the smallest bundles of the strong force are called gluons; the bundles for the weak force are called weak gauge bosons,known as W and Z). Photons, gluons, and weak gauge bosons are the microscopic transmission mechanisms, called messenger particles. Strong, weak, and electromagnetic forces resemble each other because they are all connected by symmetries, meaning that two red quarks will interact in exactly the same way if they are substituted with two green quarks. The universe exhibits strong force symmetry, meaning that physics is completely unaffected by force-change shifts. The strong force is an example of gauge symmetry.

But what about gravity? Once again, gravity enforces the symmetry in this scenario, ensuring the equal validity of all frames of reference. Physicists have called gravity’s messenger particle graviton, though they have yet to observe it experimentally. But in order to integrate quantum mechanics into general relativity, physicists must arrive at a quantum field theory of the gravitational force. The standard model in its current form does not do this. Everything in the universe, including the gravitational field and so-called “empty space,” experiences quantum fluctuations. If the gravitational field is the same thing as the shape of space, quantum jitters mean that the shape of space fluctuates randomly. These undulations become more pronounced as the spatial focus narrows.John Wheelercame up with the term quantum foam to describe the turbulence that ultramicroscopic examination reveals. The smooth spatial geometry demanded by Einstein’s theory of general relativity ceases to exist on short-distance scales: the quantum jitters are just too violent, tearing the very fabric of space with agitated, irregular movements.

It is the presence of quantum foam that stands in the way of a theory unifying general relativity with quantum mechanics. As with most problems of quantum mechanics, these undulations are not observable in day-to-day experience; the universe appears calm and predictable. The obstacle only emerges at Planck length, which is a millionth of a billionth of a billionth of a centimeter (10–33). But however trifling this scale may seem, quantum foam poses an immense problem. In fact, it creates the central crisis of modern physics. It is clear that Einstein’s depiction of space and time as smooth was just an approximation; the real framework can only emerge at the infinitesimal scale of the quantum jitters. It is this scale that superstring theory attempts to explain.

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