If you throw a ball at a wall, it's going to bounce back at you – that's classical physics at work. But of course, the world of quantum physics is much spookier, so if you did the same with a particle, there's a chance that it will suddenly appear on the other side. This is thanks to a phenomenon known as quantum tunneling, and now a team of physicists has measured just how long that process takes.
The phrase may conjure images of a particle phasing through a barrier, but it's not literally "tunneling" through – it's actually a product of mathematical random chance. Say, for example, you're mapping out the probability of where a particle will go after you throw it at a wall. Odds are extremely high that it will move back away from the wall – but in the quantum world you can never be 100 percent sure of something. There's a tiny, tiny chance that the particle will ignore the wall and continue its journey on the other side.
While it sounds like science fiction, quantum tunneling is a well-documented phenomenon, taking place in everyday items like electron microscopes and transistors. Elementary particles like electrons are known to "tunnel" out of their atoms on a regular basis too, which is a key driver behind radioactive decay.
But one ongoing mystery around the phenomenon is how long it takes for a particle to quantum tunnel through a barrier – or whether it takes any time at all. Some scientists believe it happens instantaneously, but that would mean it travels faster than the speed of light and may violate the principle of causality.
To try to measure tunneling time, researchers from Griffith University and the Australian National University blasted hydrogen atoms with an intense laser that fires 1,000 light pulses per second. The idea is that this sets up the right conditions for the electron to escape from the atom, allowing the team to precisely measure how long tunneling takes.
The result? Quantum tunneling seems to happen instantaneously – or at least, so incredibly quickly that it's essentially instantaneous. According to the researchers, it takes less than 1.8 attoseconds, which is a billionth of a billionth of a second.