Afterward, physicists and mathematicians tried to come up with a mathematical way of describing what entanglement really meant. David Nield. Proof of concept verifies physics that could enable quantum batteries. If black holes have Hilbert spaces that mature isometrically, as Cotler and Strominger hypothesize, physicists may face a somewhat different puzzle than they thought. Harder problems require more computational resources to solve — more running time, more memory. But a century ago, the pioneers of quantum mechanics made a surprising discovery—one that elevated unitarity from common sense to a hallowed principle.