Quantum gravity tries to combine Einstein’s general theory of relativity with quantum mechanics. Quantum corrections to classical gravity are visualized as loop diagrams, as the one shown here in white. Whether space (or time) itself is discrete or continuous is not yet decided, as is the question of whether gravity is quantized at all. (SLAC NATIONAL ACCELERATOR LAB)

This Simple Thought Experiment Shows Why We Need Quantum Gravity

If our current laws of physics can’t predict what will happen, even probabilistically, we need something new.

There are two theories we have that explain all the particles and their interactions in the known Universe: General Relativity and the Standard Model of particle physics. General Relativity describes gravity perfectly everywhere we’ve ever looked. From the smallest-scale attractions we’ve ever measured in a laboratory to the expansion and curvature of space due to Earth, the Sun, black holes, galaxies, or the entire Universe, our observations and measurements have never deviated from what we’ve observed. The Standard Model is equally successful for the other three forces: electromagnetism and the strong and weak nuclear forces. Every experiment, measurement, and observation has agreed perfectly with these two theories.

It sounds great, until you try to combine the two. If we do that, it all falls apart. The solution? We need a quantum theory of gravity. Here’s why.

The spacetime curvature around any massive object is determined by the combination of mass and distance from the center-of-mass. Other concerns, like velocity, acceleration, and other sources of energy, must be factored in. (T. PYLE/CALTECH/MIT/LIGO LAB)