or How I Learned To Stop Worrying And Love The Foam
Based on the uncertainty principles of quantum mechanics and the general theory of relativity, there is no reason that spacetime needs to be fundamentally smooth. Instead, in a quantum theory of gravity, spacetime would consist of many small, ever-changing regions in which space and time are not definite, but fluctuate in a foam-like manner.
In quantum mechanics, and in particular in quantum field theory, the Heisenberg uncertainty principle allows energy to briefly decay into particles and antiparticles which then annihilate back to energy without violating physical conservation laws. As time and space are being probed at smaller scales, the energy of such particles, called virtual particles, increases. Combining this observation with the fact that in Einstein’s theory of general relativity energy curves spacetime, one can imagine that at sufficiently small scales the energy of these fluctuations would be large enough to cause significant departures from the smooth spacetime seen at macroscopic scales, giving spacetime a “foamy” character.