As we imagine extrapolating the history of the Universe backward in time, the big
bang theory tells us that the Universe becomes more dense and hotter, and the
relevant distance scales become shorter and shorter.
The Planck Scale
But we have already seen that
if the distance scales become short enough (of atomic dimensions or smaller), the
theory of quantum mechanics must be used. Therefore, as we extrapolate back in
time to the beginning of the Universe,
eventually one would reach a
state of sufficient temperature and density that a fully quantum mechanical
theory of gravitation would be required. This is called the
Planck era, and the corresponding scales of distance, energy, and time
are called the Planck scale.
The Planck scale corresponds to incredibly small distances (or equivalently,
incredibly large energies). The corresponding lengths, energies, temperatures,
are displayed in the adjacent table (the unit GeV stands for 1 billion electron
volts of energy).
The Planck Scale
||1.2 x 1019
||1.6 x 10-33 cm
||5.4 x 10-44 s
||1.4 x 1032 K
But the General Theory of Relativity does not respect the principles of quantum
mechanics. What is required then is a theory of gravitation that also is
consistent with quantum mechanics. This could be termed a theory of
Unfortunately, no one has yet understood how to
accomplish this very difficult task, and we do not yet have an internally
consistent theory of quantum gravity. The most promising present alternative is
called superstring theory, but it is not yet clear whether it can
provide a correct picture of quantum gravitation.
The Breakdown of Our Current Laws of Physics
Therefore, since we do not yet have a
consistent wedding of general relativity to quantum mechanics,
the presently understood laws of physics may be expected
to break down on the Planck scale,
and our standard picture of inflation followed by the big bang
says nothing about the Universe at those very early
times (which would precede inflation).
In this respect then, we are absolutely certain that our present laws of
physics are not complete. However, the Planck scale is so incredibly small that
this presumably only had meaning in the initial instants of the creation of the
Universe. We, for example, have no hope of doing experiments to test the Planck
scale in any present or conceivable future experiment.