In classical physics, if a ball does not have enough energy to roll over a hill, it stops. Simple enough.
But quantum particles do not behave like tiny billiard balls following fixed paths. In quantum mechanics, a particle is described by a wavefunction: a rational model that gives us probabilities for where the particle may be found.
Sometimes, that model predicts a small chance the particle will be found on the other side of an energy barrier, even though classical physics says it should not have enough energy to get there. When that happens, we call it quantum tunneling. And yes, we’ve verified it! This is not just math on a chalkboard. This part is empirical.
And this a strange idea has real effects. It helps explain nuclear fusion in stars, certain types of radioactive decay, and enables technologies like scanning tunneling microscopes.
So did the particle literally pass through the barrier? Did it “borrow” energy? Did it exist in several places until measured?
That deeper meaning is still debated.
The TST distinction helps: quantum tunneling is empirical in its observed effects, rational in its mathematical description, and speculative in its metaphysical meaning. The phenomenon is real. The math works. But the deeper explanation of what kind of reality allows it remains unsettled.