Gravity is unique among the known fundamental forces of the universe: Currently, there is no experimental result whose explanation requires a quantum theory of gravity. But physicists, including Albert Einstein, 1 have still argued on empirical grounds that quantum theory must modify theories of gravitation. As early as the 1920s, dimensional-analysis arguments suggested that observing any phenomena that meaningfully involved quantum dynamics of the gravitational field is beyond the scope of what’s experimentally possible. Thus, the hope of directly testing ideas about quantum gravity with experiments languished for nearly a century.
Today, the simplest expectation for a quantum theory of gravity is that, at a minimum, states of weak gravitational fields can be in quantum superpositions and be described in terms of gravitons. The hypothesized particles quantize the gravitational field akin to how photons quantize the electromagnetic field. Beyond that simple picture, physicists have quantum gravity models, such as string theory, that support the existence of gravitons at low energy but also work at vastly higher energy scales, where the graviton picture breaks down. How can researchers know if that quantum description accurately models how gravity operates in the real world?
To answer that, experiments are needed. That may be possible now that scientists have made significant advances in quantum-state control and measurement, which provide not only unprecedented spatial and temporal sensitivity but also new ways to prepare quantum states of macroscopic objects. A prime example is using the kilogram-scale mirrors of the Laser Interferometer Gravitational-Wave Observatory (LIGO) to measure gravitational waves, a measurement that is at the limit of sensitivity set by the Heisenberg uncertainty relation.
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