A little over a hundred years ago, Albert Einstein revolutionized our understanding of gravity with his general theory of relativity. "According to Einstein’s theory, gravity is not a force but emerges due to the geometry of the fourdimensional spacetime continuum, or spacetime for short," says Heefer. "And it’s central to the emergence of fascinating phenomena in our universe such as gravitational waves."
Massive objects, such as the Sun or galaxies, warp spacetime around them, and other objects then move along the straightest possible paths – otherwise known as geodesics – through this curved spacetime.
Due to the curvature, however, these geodesics are not straight in the usual sense at all. In the case of the planets in the solar system, for instance, they describe elliptical orbits around the sun. In this way, general relativity elegantly explains the movement of the planets as well as numerous other gravitational phenomena, ranging from everyday situations to black holes and the big bang. As such it remains a cornerstone of modern physics.
"Quantum mechanics suggests that particles (like electrons or muons) exist in multiple states at the same time until they are measured or observed," says Heefer. "Once measured, they randomly select a state due to a mysterious effect referred to as the ‘collapse of the wave function.’"
In quantum mechanics, a wave function is a mathematical expression that describes the position and state of a particle, such as an electron. And the square of the wave function leads to a collection of probabilities of where the particle might be located. The larger the square of the wave function at a particular location, the higher the probability that a particle will be located at that location once it is observed.
"All matter in our universe appears to be subject to the strange probabilistic laws of quantum mechanics," Heefer notes. "And the same is true for all forces of nature – except for gravity. This discrepancy leads to deep philosophical and mathematical paradoxes, and resolving these is one of the primary challenges in fundamental physics today."
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