The Hidden Reality_ Parallel Universes and the Deep Laws of the Cosmos - Brian Greene [60]
GOAL: Incorporate key breakthroughs from past research
IS GOAL REQUIRED?: No.
In principle, a successful theory need bear little resemblance to successful theories from the past.
STATUS: Excellent.
Though progress isn’t necessarily incremental, history shows that it usually is; successful new theories typically embrace past successes as limiting cases. String theory incorporates the essential key breakthroughs from previously successful physical frameworks.
GOAL: Explain particle properties
IS GOAL REQUIRED?: No.
This is a noble goal, and if achieved would provide a profound level of explanation—but it is not required of a successful theory of quantum gravity.
STATUS: Indeterminate; no predictions.
Going beyond quantum field theory, string theory offers a framework for explaining particle properties. But to date, this potential remains unrealized; the many different possible forms for the extra dimensions imply many different possible collections of particle properties. There is no currently available means to pick one shape from the many.
GOAL: Experimental confirmation
IS GOAL REQUIRED?: Yes.
This is the only way to determine whether a theory is a correct description of nature.
STATUS: Indeterminate; no predictions.
This is the most important criterion; to date, string theory has not been tested on it. Optimists hope that experiments at the Large Hadron Collider and observations by satellite-borne telescopes have the capacity to bring string theory much closer to data. But there’s no guarantee that current technology is sufficiently refined to reach this goal.
GOAL: Cure singularities
IS GOAL REQUIRED?: Yes.
A quantum theory of gravity should make sense of singularities arising in situations that are, even just in principle, physically realizable.
STATUS: Excellent.
Tremendous progress; many kinds of singularities have been resolved by string theory. The theory still needs to address black hole and big bang singularities.
GOAL: Black hole entropy
IS GOAL REQUIRED?: Yes.
A black hole’s entropy provides a hallmark context in which general relativity and quantum mechanics interface.
STATUS: Excellent.
String theory has succeeded in explicitly calculating, and confirming, the entropy formula proposed in the 1970s.
GOAL: Mathematical contributions
IS GOAL REQUIRED?: No.
There’s no requirement that correct theories of nature yield mathematical insights.
STATUS: Excellent.
Although mathematical insights aren’t necessary to validate string theory, significant ones have emerged from the theory, revealing the profound reach of its mathematical underpinnings.
*If you’d like to know how string theory surmounts the problems that blocked earlier attempts to join gravity and quantum mechanics, see The Elegant Universe, Chapter 6; for a sketch, see note 8. For an even briefer summary, note that whereas a point particle exists at a single location, a string, because it has length, is slightly spread out. This spreading, in turn, dilutes the raucous short-distance quantum jitters that stymied previous attempts. By the late 1980s, there was strong evidence that string theory successfully melds general relativity and quantum mechanics; more recent developments (Chapter 9) make the case overwhelming.
CHAPTER 5
Hovering Universes in Nearby Dimensions
The Brane and Cyclic Multiverses
Late one night many years ago, I was in my office at Cornell University putting together the freshman physics final exam that would be given the following morning. Since this was the honors class, I wanted to enliven things a little by giving them one somewhat more challenging problem. But it was late and I was hungry, so rather than carefully working through various possibilities, I quickly modified a standard problem that most of them had already encountered, wrote it into the exam, and headed home. (The details hardly matter, but the problem had to do with predicting the motion of a ladder, leaning against a wall, as it loses its footing and falls. I modified the standard problem