If you throw a tennis ball at a solid wall, it will strike the wall and ricochet back at you 100% of the time, just like you’d expect. In physics, a sufficiently strong barrier will prevent any incoming object from passing through it. But at the quantum level, this isn’t strictly true. If you replace a tennis ball with a quantum particle and a solid wall with any quantum mechanical barrier, there’s a finite probability that the particle will actually tunnel through the barrier, where it winds up being detected on the other side. It’s as though you threw the tennis ball at the wall and it went right through, unimpeded by the wall at all.
One of the questions you might seek to ask is whether there’s any delay in the apparent motion of that quantum particle as it moves “through” the barrier via the process of tunneling. In a 2019 experiment, scientists measured exactly that, and found that the tunneling process itself was instantaneous; there was no delay caused by the tunneling process at all. Some have questioned whether this means, in the quantum realm, that it’s possible to violate the speed limit imposed by relativity: the speed of light. Although your intuition might suggest this possibility to you, it turns out not to be the case at all. All processes, even quantum ones, still need to obey the rules of relativity, and can never exceed the speed of light. Here’s how to reconcile the instantaneous nature of quantum tunneling with the fact that no information is ever transmitted faster than the speed of light.
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