Gravity: Classical or Quantum? ⚡ экспресс
If gravity is considered classical and interacts with quantum fields via semiclassical Einstein equations, then a massive non-relativistic qubit in a weak field could solve an NP-complete problem (the hardest class) in polynomial time. The reason is the nonlinearity of the dynamics. This violates the extended Church–Turing thesis (physical processes are efficiently computable), which is seen as an argument for quantum gravity. Imagine a stone that, while falling, tries out every possible path and instantly finds the right one—that's how fantastical this computational power would be.
Gravity seems simple: it keeps us on the ground and makes planets orbit. But if it were classical—described by Einstein's equations as the curvature of spacetime, while all matter follows the quantum laws of the Standard Model—then an ordinary quantum bit (qubit) falling freely would solve unsolvable problems. It turns out gravity influences the qubit so that its trajectory becomes a computation. It's like a marble rolling through a bumpy labyrinth and finding the shortest path—and that path turns out to be the answer. Problems like perfectly packing a suitcase are beyond even supercomputers.
But there's a fundamental principle: nothing physical can compute faster than an ordinary computer. If gravity were classical, a falling qubit would break that rule. Since we don't witness such magic in reality, gravity must be quantum. So it's not physics but computation theory that demands quantum gravity—an unexpected twist. Hawking, Penrose, and Wheeler grappled with this: without quantum gravity, we can't understand black holes or the birth of the Universe.
🎯 A qubit is a quantum bit that can be 0 and 1 at the same time. However, in this scenario, quantum speedup alone isn't enough—you need gravity to be tricky.