Quantum foam

Quantum foam (also referred to as space-time foam) is a concept in quantum mechanics devised by John Wheeler in 1955. The foam is conceptualized as the foundation of the fabric of the universe.

Based on some general principles of quantum mechanics and general theory of relativity, one can argue that space-time is fundamentally not smooth. Instead, in a quantum theory of gravity space-time would have a foamy, jittery nature and would consist of many small, ever-changing, regions in which space and time are not definite, but fluctuate.

In quantum mechanics, and in particular in quantum field theory, 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 space-time, one can imagine that at sufficiently small scales the energy of these fluctuations would be large enough to cause significant departures from the smooth space-time seen at macroscopic scales, giving space-time a "foamy" character.

Ordinarily, however, quantum field theory does not deal with virtual particles of sufficient energy to curve space-time significantly, so quantum foam is a speculative extension of these concepts which imagines the consequences of such high-energy virtual particles at very short distances and times.

With an incomplete theory of quantum gravity, it is impossible to be certain what space-time would look like at small scales. Also, our understanding of the quantum foam will necessarily be ambiguous as long as there are many competing proposals for a theory of quantum gravity.

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