Three Roads To Quantum Gravity

POSTSCRIPT 219
This is good news indeed, for as soon as the light of experiment
is turned on, sociological forces such as govern
academic politics and fashion must slink back to the shadows,
as the judgement of nature supercedes the judgements
of professors.
This is not the only place where cosmological observation
and fundamental theory are confronting each other. An even
more exciting—and for some disturbing—case has to do with
the cosmological constant. This refers to the possibility—
first realized by Einstein—that empty space might have a
non-zero energy density. This energy density would be observable
in the effect it has on the expansion of the universe.
Once this possibility was accepted, it led to a major crisis
in theoretical physics. The reason is that the most natural
possibility allowed for the value of this empty space energy
density is that it should be huge—more than a hundred powers
of ten larger than is compatible with observation. The
exact value—which is what the name cosmological constant
refers to—cannot be predicted by current theory. In fact, we
can adjust a parameter to get any value for the cosmological
constant we want. The problem is that to avoid a huge cosmological
constant, the parameter has to be adjusted to an
accuracy of at least 120 decimal places. How such a precise
adjustment is to be obtained is a mystery.
This is perhaps the most serious problem facing fundamental
physics, and it recently got worse. Until a few years
ago, it was almost universally believed that even if it required
a very precise adjustment, in the end the cosmological
constant would be exactly zero. We had no idea why the
cosmological constant would be zero, but at least zero is a
simple answer. However, recent observations have suggested
that the cosmological constant is not zero; it has instead a
very small, but positive, value. This value is tiny on the
scales of fundamental physics; in Planck units it is around
10 –120 (or .0000. . . .) with 120 zeros before one encounters a
non-zero digit.
But even though tiny when measured in fundamental
units, this value is large enough to have a profound effect on