The Planck
Era

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
Quantity Value
Planck Mass 1.2 x 1019 GeV/c2
Planck Length 1.6 x 10-33 cm
Planck Time 5.4 x 10-44 s
Planck Temperature 1.4 x 1032 K
The Planck scale corresponds to incredibly small distances (or equivalently, incredibly large energies). The corresponding lengths, energies, temperatures, and times are displayed in the adjacent table (the unit GeV stands for 1 billion electron volts of energy).

Quantum Gravitation

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 quantum gravitation. 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.


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