## Quick Reference

The mass of a particle whose Compton wavelength is equal to the Planck length. It is given by *m*_{P} = √(*ħ**c*/*G*), where *ħ* is the rationalized Planck constant, *c* is the speed of light, and *G* is the gravitational constant. The description of an elementary particle of this mass, or particles interacting with energies per particle equivalent to it (through *E* = *mc*^{2}), requires a quantum theory of gravity. Since the Planck mass is of order 10^{−8} kg (equivalent energy 10^{19} GeV), and, for example, the proton mass is of order 10^{−27} kg and the highest energies attainable in present-day particle accelerators are of order 10^{3} GeV, quantum-gravitational effects do not arise in laboratory particle physics. However, energies equivalent to the Planck mass did occur in the early universe according to big-bang theory, and a quantum theory of gravity is important for discussing conditions there (see Planck time).

**From:**
Planck mass
in
A Dictionary of Physics »

*Subjects:*
Physics.

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