The amount of energy infusing empty space seems too small to explain without a multiverse. But physicists have at least one alternative left to explore.
A recently proposed experiment would confirm that gravity is a quantum force.
Two methods of measuring the neutron’s longevity give different answers, creating uncertainty in cosmological models. But no one has a clue what the problem is.
A complete classification could lead to a wealth of new materials and technologies. But some exotic phases continue to resist understanding.
The mother of all string theories passes a litmus test that, so far, no other candidate theory of quantum gravity has been able to match.
Edward Witten reflects on the meaning of dualities in physics and math, emergent space-time, and the pursuit of a complete description of nature.