Exploring quantum gravity—for whom the pendulum swings.

Exploring quantum gravity—for whom the pendulum swings.

When it comes to a marriage with quantum theory, gravity is the lone holdout among the four fundamental forces in nature. The three others—the electromagnetic force, the weak force, which is responsible for radioactive decay, and the strong force, which binds neutrons and protons together within the atomic nucleus—have all merged with quantum theory to successfully describe the universe on the tiniest of scales, where the laws of quantum mechanics must play a leading role.


Although Einstein’s theory of general relativity, which describes gravity as a curvature of space-time, explains a multitude of gravitational phenomena, it fails within the tiniest of volumes—the center of a black hole or the universe at its explosive birth, when it was less than an atomic diameter in size. That’s where quantum mechanics ought to dominate.


Yet over the past eight decades, expert after expert, including Einstein, have been unable to unite quantum theory with gravity. So, is gravity truly a quantum force?


Researchers at the National Institute of Standards and Technology (NIST) and their colleagues have now proposed an experiment that may help settle the question.




(1) In an atomic interferometer, the atom's wave function is split into left and right arms. The left and right arms are then recombined, producing an interference pattern.


Credit: S. Kelley/NIST




(2) When the experiment begins, the atom's wave function is unaffected by the pendulum. This means the two arms of the single atom interfere fully with each other.


Credit: S. Kelley/NIST






(3) If gravitational attraction indeed causes an entanglement between the pendulum and the atom, the pendulum will partially measure the position of the atom, concentrating it into one arm or the other.


Credit: S. Kelley/ ..

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