Discontinuous Matter

Each time we have extended our observations to smaller length scales and higher energies, we have discovered new layers of structure. At the beginning of the century, the discovery of Brownian motion with a typical energy particle of 3 X 10 ^-2 eV showed that matter is not continuous, but is made up of atoms. Shortly thereafter, it was discovered that these supposedly indivisible atoms are made up of electrons revolving around a nucleus with energies of the order of a few electron volts. The nucleus, in turn, was found to be composed of so-called elementary particles, protons and neutrons, held together by nuclear bonds of the order of 10^6 eV. The latest episode in this story is the discovery that the proton and electron are made up of quarks held together by bonds of the order of 10^9 eV.

Our past experience might suggest that there is an infinite sequence of layers of structure at higher and higher energies. However, it seems that gravity should provide a limit, but only at the very short length scale of 10^-33 cm or the very high energy of 10^28 eV. On length scales shorter than this, one would expect that space-time would cease to behave like a smooth continuum and it would acquire a foam-like structure because of quantum fluctuations of gravitational field.

There is a very large unexplored region between our present experimental limit of about 10^10 eV and the gravitational cut-off at 10^28 eV.