Quantum gravity [1]
Of the four fundamental forces in the universe, gravity is the only one that cannot be described by the laws of quantum mechanics, the theory that applies to all other forces and particles known to physics. Most physicists who now work on the problem believe that the unification occurs when we zoom in on the cosmos to what is called the Planck scale. Distances on the Planck scale are so tiny—100 trillion trillion times as small as a hydrogen atom—that spacetime itself is thought to assume quantum characteristics. A quantum spacetime would no longer be the smooth continuum described by general relativity; it would be coarse-grained, like a digital photograph that becomes pixelated when magnified. That graininess is a hallmark of quantum theory, which confines the energy, momentum and other properties of particles to discrete bits, or quanta. But what exactly is a quantum of spacetime? How could time or distance be measured if space and time themselves are fractured like broken rulers?
To understand whether gravity fits into quantum theory, physicists are designing tabletop experiments in the laboratory, such as trying to measure the gravitational fields of millimetre-wide gold spheres with extreme precision to observe gravity closer to the quantum realm for signs of quantum behaviour. Such behaviour might include “superposition” – the ability of quantum particles to occupy two places simultaneously - and “entanglement” – a kind of connection between quantum objects where their fates become intertwined. If researchers can find evidence of gravitational fields displaying superposition or entanglement, they will know that gravity has quantum properties.
Scientists have observed quantum superpositions many times in laboratories, but they are delicate states. Interactions with any nearby particles quickly cause objects in superposition to “collapse” into a single position. But while the superpositions last, what properties these particles have. Do they create their own minuscule gravitational fields, for instance? And once n a state of superposition, how does it gravitate?
In a thought experiment many years ago Richard Feynman argued that if gravity is indeed a quantum phenomenon, a superposition of a particle in two places at once would create two separate gravitational fields. According to the general theory of relativity, gravitational fields are distortions of space and time. Thus, in the case of a small mass in a quantum superposition, two different spacetimes would coexist side by side, almost like two separate mini universes, a state of affairs that should not exist in Einstein’s theory.[2], [3].
[1] The material on this page is a short summary of the article by Tim Folger, “Quantum gravity in the lab”, which appeared in the Scientific American, April 2010, 42-49.
[2] Reference should also be made to the page Black holes, the information paradox and quantum entanglement: the link between quantum physics and general relativity? under the heading "Wider ramifications of the holographic principle", containing a reference to Michael Moyer's article, “Is Space Digital? - Could foamlike fluctuations rue spacetime at the tiniest scales?", Scientific American, February 2012, 20, 25.
[3] By way of comparison, see also the page String theory, spatial expansion and the cosmological constant - the flatness problem revisited, based on an article in the New Scientist, "Welcome to the Escher verse", New Scientist, 9 June2012, 8-9, wherein it is speculated that a negative cosmological constant may eventually lead to a complete description of the universe we observe, uniting general relativity and quantum mechanics in the process.