Physicists have traced three of the four forces of nature — the electromagnetic force and the strong and weak nuclear forces — to their origins in quantum particles. But the fourth fundamental force, gravity, is different.
Our current framework for understanding gravity, devised a century ago by Albert Einstein, tells us that apples fall from trees and planets orbit stars because they move along curves in the space-time continuum. These curves are gravity. According to Einstein, gravity is a feature of the space-time medium; the other forces of nature play out on that stage.
But near the center of a black hole or in the first moments of the universe, Einstein’s equations break. Physicists need a truer picture of gravity to accurately describe these extremes. This truer theory must make the same predictions Einstein’s equations make everywhere else.
Physicists think that in this truer theory, gravity must have a quantum form, like the other forces of nature. Researchers have sought the quantum theory of gravity since the 1930s. They’ve found candidate ideas — notably string theory, which says gravity and all other phenomena arise from minuscule vibrating strings — but so far these possibilities remain conjectural and incompletely understood. A working quantum theory of gravity is perhaps the loftiest goal in physics today.
What is it that makes gravity unique? What’s different about the fourth force that prevents researchers from finding its underlying quantum description? We asked four different quantum gravity researchers. We got four different answers.
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