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The Elegant Universe

Brian Greene


Part IV: String Theory and the Fabric of Spacetime

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Chapter 10: Quantum Geometry

George Bernhard Riemann, a nineteenth-century German mathematician, figured out how to apply geometry to curved spaces. Einstein recognized that Rienmann’s geometry accurately described the physics of gravity, and Reinmann’s theories supplied him with the necessary mathematical foundations to analyze warped space. The curvature of spacetime, Rienmann found, is expressed mathematically as the distorted distances between its points. Einstein applied Rienmann’s discovery to the physical realm and concluded that the gravitational force felt by an object directly reflects this distortion.

String theory deals in short-distance physics, and Rienmannian geometry ceases to function at an ultramicroscopic level. This means that, for string theory to work, physicists must modify both Riemannian geometry and the general theory of relativity that Einstein derived from it. A new type of geometry is necessary to decipher tiny Planck-length scales. Physicists have called this new type of geometry quantum geometry.

Fifteen billion years ago, the universe began with the big bang. As Hubble discovered, the universe is constantly expanding, which makes it hard to measure the average density of matter in the universe. If the average matter density exceeds a so-called critical density of a hundredth of a billionth of a billionth of a billionth (10–29) of a gram per cubic centimeter, then a large gravitational force will permeate the cosmos and reverse the expansion. If the average density is less than the critical density, the gravitational expansion will be too weak to do this. (The earth is not a reliable indicator for the average density of the universe: Matter clumps, and the vast empty spaces between galaxies bring the average down.)

Conventional wisdom proclaims that the universe began with a bang from an initial zero-size state. If the universe has enough mass, it will eventually end with a “crunch” that will reduce it to a similar state of compression. String theory is required to help physicists evaluate the extremely compressed early stage; it has set Planck length as the lower limit on the size of the “Big Crunch.” It would not make sense to set this same limit for the point-particle model.

To return to the garden hose analogy for the universe: strings, unlike point particles, can “lasso” the circular part of the garden hose. When a string is in this position, it is in a winding mode of motion, which is a possibility that is inherent to strings. A string in winding mode has a minimum mass that is determined by the size of the circular dimension it is wrapping around and the number of times it is wrapped.

Wound-string configurations suggest that a string’s energy comes from two sources: vibrational motion and winding energy. All string motion is a combination of sliding and oscillating. Strings’ vibrational movements have energies that are inversely proportional to the radius of the circle they are wrapping. A small radius, for example, would confine the string more strictly and would contain more energy. But the winding mode energies are directly proportional to the radius. Greene eventually explains what this means: there is no distinction between geometrically distinct forms. The same goes for total string energies: there is no distinction between different sizes for the circular dimension! Through a complicated chain of explanations, Greene shows that there is absolutely no way to differentiate between radii that are inversely related to one another.

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