A beautiful piece on why 'Grand Unified Theory' is so elusive; because to detect a “graviton” – the hypothetical particle making up part of a gravitational field – would require a particle collider the size of the Milky Way or a detector with a mass of the planet Jupiter.

Sabine Hossenfelder writes on the centenary of Albert Einstein’s general theory of relativity, his masterwork describing gravity as the curvature of space and time. Clearly, there is something about the combination of quantum theory and gravity that remains unknown, and our understanding of space, time, and matter hinges on unraveling this connection.

Einstein’s theory not only describes our universe, from the Big Bang to black holes; it has also taught physicists the relevance of geometry and symmetry – lessons that spread from particle physics to crystallography. But, despite the similarities that Einstein’s theory has with other theories in physics, it stands apart by its refusal to fit together with quantum mechanics, the theory that explains the dominant behavior of matter at the atomic and subatomic scale.

According to Einstein’s theory, gravity, unlike all other physical forces known to man, is not quantized. It is not subject to Heisenberg’s famed uncertainty principle.

Finding a description of gravity that is compatible with our understanding of quantum physics would revolutionize cosmology, yield new insights into the first moments of our universe, and provide a deeper understanding of the theories on which all of modern physics is based.

But, despite the enormous potential impact of such a breakthrough and the efforts of generations of physicists to achieve it, we still do not know which theory is the right one. 'Detecting a “graviton” – the hypothetical particle making up part of a gravitational field – would require a particle collider the size of the Milky Way or a detector with a mass of the planet Jupiter.