What is the Planck Scale?

In physics, the Planck scale refers to either a very large energy scale (1.22 x 10¹⁹ GeV) or a very tiny size scale (1.616 x 10^-35 meters) where the quantum effects of gravity become important in describing particle interactions. At the Planck size scale, quantum uncertainty is so intense that concepts like locality and causality become less meaningful. Today’s physicists are very interested in learning more about the Planck scale, as a quantum theory of gravity is something we currently lack. Were a physicist able to come up with a quantum theory of gravity that agrees with experiment, it would practically guarantee them a Nobel Prize.

It is a fundamental fact of the physics of light that, the more energy a photon (light particle) carries, the smaller a wavelength it has. For instance, visible light has a wavelength of around a few hundred nanometers, while the much more energetic gamma rays have a wavelength about the size of an atomic nucleus. The Planck energy and the Planck length are related in that a photon would need to have a Planck-scale energy value in order to have a wavelength as small as the Planck length.

To make things even more complicated, even if we could create a photon this energetic, we could not use it to precisely measure something at the Planck scale – it would be so energetic that the photon would collapse into a black hole before it returned any information. Thus, many physicists believe that the Planck scale represents some sort of fundamental limit on how small the distances we can probe are. The Planck length may be the smallest physically meaningful size scale there is, in which case the universe can be thought of as a tapestry of “pixels” – each a Planck length in diameter.

The Planck energy scale is almost unimaginably large, while the Planck size scale is almost unimaginably small. The Planck energy is about a quintillion times larger than the energies achievable in our very best particle accelerators, which are used to create and observe exotic subatomic particles. A particle accelerator powerful enough to probe the Planck scale directly would need to have a circumference similar in size to the orbit of Mars, constructed from about as much material as our Moon.

Since such a particle accelerator is not likely to be built in the foreseeable future, physicists look to other methods for probing the Planck scale. One is looking for gigantic “cosmic strings” which may have been created when the universe as a whole was so hot and small that it had Planck-level energies. This would have occurred in the first trillionth of a second after the Big Bang.