It's a prediction of other things, too. Even just sheer every-day quantum mechanics sets a limit, too; eventually you're asking to nail down a photon's origin w

It's a prediction of other things, too. Even just sheer every-day quantum mechanics sets a limit, too; eventually you're asking to nail down a photon's origin with so much precision that you lose the frequency or something, things you don't normally think about even in the most extreme astronomy today, but become relevant when discussing bouncing photons around a black hole 30 million light years away.