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Dumb question: "We know that the ball will bounce back every time it is thrown at a wall. A single particle, however, will sometimes pass straight through an e

Dumb question: "We know that the ball will bounce back every time it is thrown at a wall. A single particle, however, will sometimes pass straight through an equivalent barrier in its microscopic world and appear on the other side. This quantum mechanical phenomenon is called tunnelling." Is the particle just failing to collide with the wall since objects are mostly empty space? Or is something more spooky or interesting happening?

kitchi 1y ago
In quantum mechanics, the "ball" (or in this case an ideal particle) has a "wave function" associated with it. This wave function effectively describes the probability that the particle can be at a certain location. It so happens that when you solve for this problem, a ball bouncing against a wall, in this wave function paradigm then you end up with a non-zero probability that the ball appears on the other side of the wall. I'm not sure if there is a deeper explanation at play here but that's how I understand it.
bluesummers5651 1y ago
The simplest version of this problem involves a "potential barrier." Another loose classical analogy here is considering a ball rolling towards a hill. Everyone knows from experience in classical systems that with sufficient speed i.e. kinetic energy, the ball can go over the hill i.e. there is more kinetic energy in the ball when it meets the bottom of the hill than there is potential energy the ball would have at the top of the hill. If it has less energy, it will not make it past the hill. The weird thing about the analogous situation in quantum mechanics is that even if the particle has less energy than the potential barrier (the hill), it has a non-zero probability of being on the other side due to the wave function exponentially decaying in the barrier.
RigelKentaurus 1y ago
As far as I know, the "single particle" referred to here is not a "classical particle" like a ball. It's a "quantum object" that, depending on how you look at it, behaves like a wave or an object. Definitely spooky!
sbenerofe 1y ago
Not a dumb question at all. The idea that a particle could pass through a wall by luckily avoiding collisions is a classical way of thinking. In that view, a particle is a tiny solid ball and a wall is just a collection of other tiny balls with space between them. Quantum tunneling is based on a completely different concept. In quantum mechanics, the "wall" is not a physical object but a high energy barrier. Classically, a particle cannot be in a region if it doesn't have enough energy to overcome that barrier (this is why people often use the idea of a high wall and a ball that cannot make it over the wall). However, quantum mechanics treats particles as having wave-like properties. This wave is related to the probability of finding the particle at any given location. While the probability of finding the particle inside the high-energy barrier is very low, it is not zero. The wave's amplitude shrinks inside the barrier, but a small portion of it extends to the other side. This means there is a small but finite probability that if you measure the particle's position, you will find it on the other side. When that happens, we say the particle has "tunneled" through. The surprising success of the experiments that led to the Nobel Prize today is that it wasn’t just a single particle (like an electron) that they measured tunneling through a barrier, it was a macroscopic group of particles. These particles were able to tunnel through the barrier because they were kept in a coherent state that allowed them to have a wave function that coherently extended through the barrier. This meant that they had a reasonable finite amplitude on the other side of the barrier so that a measurement could show that they tunneled through the barrier.
- everdrive 1y ago
  Thanks, I'm still thinking about your answer but really appreciate your explanation. Would this mean that there is some (possibly currently unknown) maximum size for a group of particles that could be forced to maintain the correct state to pass through the wall?
  - qnleigh 1y ago
    This is another great question. It's still a matter of debate how the wave-like behavior of quantum mechanics turns into the particle-like behavior of large objects that we observe. Some people believe there should be a maximum size, some people believe that there isn't. In fact Nature did a survey a few months ago and found that physicists disagree wildly on this topic. https://www.nature.com/articles/d41586-025-02342-y https://www.nature.com/articles/d41586-025-02342-y Have you heard of Schrodinger's cat, which is hypothetically dead and alive at the same time? Schrodinger described this thought experiment to argue that quantum mechanics led to absurdities if you took it too far. Ironically, many physicists now believe that such an experiment is possible in principle, though it would be extremely difficult.
cvoss 1y ago
It's more spooky and interesting. When we switch to discussing the single particle case, the "wall" is a metaphor. You have a single particle in a low energy state at position A. There is another low energy state at position C, but the path from A to C entails (ostensibly) passing through an intermediate high energy state at position B. Without lending energy into the system, you wouldn't expect the particle to be able to move from A to C, because the particle needs a lot of energy temporarily to move from A to B (which it would give up again as it moves from B to C). Yet the particle does move from A to C, in the absence of an energy source that could explain what happened. This raises the spooky question: Did it actually pass through B on its way? It seems like it did not.
whatshisface 1y ago
Here is a one-sentence answer: Tunneling is due to incomplete reflection at a boundary mathematically akin to seeing out of your window.
IAmBroom 1y ago
Loads of good responses, but I'll just add: > Is the particle just failing to collide with the wall since objects are mostly empty space? To be frank, essentially nothing macroscopic ever reacts to impact because (subatomic) particles physically hit particles. Electrons repel with significant force long before they can "collide". The volume of space represented by "close enough" to the outermost electron shell (since electrons aren't at one single volume at any one moment in time) is somewhat large, even if the volumes of the nucleus + the sum of all electrons in the atom are much less.

kitchi 1y ago

In quantum mechanics, the "ball" (or in this case an ideal particle) has a "wave function" associated with it. This wave function effectively describes the probability that the particle can be at a certain location. It so happens that when you solve for this problem, a ball bouncing against a wall, in this wave function paradigm then you end up with a non-zero probability that the ball appears on the other side of the wall. I'm not sure if there is a deeper explanation at play here but that's how I understand it.

bluesummers5651 1y ago

The simplest version of this problem involves a "potential barrier." Another loose classical analogy here is considering a ball rolling towards a hill. Everyone knows from experience in classical systems that with sufficient speed i.e. kinetic energy, the ball can go over the hill i.e. there is more kinetic energy in the ball when it meets the bottom of the hill than there is potential energy the ball would have at the top of the hill. If it has less energy, it will not make it past the hill. The weird thing about the analogous situation in quantum mechanics is that even if the particle has less energy than the potential barrier (the hill), it has a non-zero probability of being on the other side due to the wave function exponentially decaying in the barrier.

RigelKentaurus 1y ago

As far as I know, the "single particle" referred to here is not a "classical particle" like a ball. It's a "quantum object" that, depending on how you look at it, behaves like a wave or an object. Definitely spooky!

sbenerofe 1y ago

Not a dumb question at all. The idea that a particle could pass through a wall by luckily avoiding collisions is a classical way of thinking. In that view, a particle is a tiny solid ball and a wall is just a collection of other tiny balls with space between them. Quantum tunneling is based on a completely different concept. In quantum mechanics, the "wall" is not a physical object but a high energy barrier. Classically, a particle cannot be in a region if it doesn't have enough energy to overcome that barrier (this is why people often use the idea of a high wall and a ball that cannot make it over the wall). However, quantum mechanics treats particles as having wave-like properties. This wave is related to the probability of finding the particle at any given location. While the probability of finding the particle inside the high-energy barrier is very low, it is not zero. The wave's amplitude shrinks inside the barrier, but a small portion of it extends to the other side. This means there is a small but finite probability that if you measure the particle's position, you will find it on the other side. When that happens, we say the particle has "tunneled" through. The surprising success of the experiments that led to the Nobel Prize today is that it wasn’t just a single particle (like an electron) that they measured tunneling through a barrier, it was a macroscopic group of particles. These particles were able to tunnel through the barrier because they were kept in a coherent state that allowed them to have a wave function that coherently extended through the barrier. This meant that they had a reasonable finite amplitude on the other side of the barrier so that a measurement could show that they tunneled through the barrier.

everdrive 1y ago
Thanks, I'm still thinking about your answer but really appreciate your explanation. Would this mean that there is some (possibly currently unknown) maximum size for a group of particles that could be forced to maintain the correct state to pass through the wall?
- qnleigh 1y ago
  This is another great question. It's still a matter of debate how the wave-like behavior of quantum mechanics turns into the particle-like behavior of large objects that we observe. Some people believe there should be a maximum size, some people believe that there isn't. In fact Nature did a survey a few months ago and found that physicists disagree wildly on this topic. https://www.nature.com/articles/d41586-025-02342-y https://www.nature.com/articles/d41586-025-02342-y Have you heard of Schrodinger's cat, which is hypothetically dead and alive at the same time? Schrodinger described this thought experiment to argue that quantum mechanics led to absurdities if you took it too far. Ironically, many physicists now believe that such an experiment is possible in principle, though it would be extremely difficult.

cvoss 1y ago

It's more spooky and interesting. When we switch to discussing the single particle case, the "wall" is a metaphor. You have a single particle in a low energy state at position A. There is another low energy state at position C, but the path from A to C entails (ostensibly) passing through an intermediate high energy state at position B. Without lending energy into the system, you wouldn't expect the particle to be able to move from A to C, because the particle needs a lot of energy temporarily to move from A to B (which it would give up again as it moves from B to C). Yet the particle does move from A to C, in the absence of an energy source that could explain what happened. This raises the spooky question: Did it actually pass through B on its way? It seems like it did not.

whatshisface 1y ago

Here is a one-sentence answer: Tunneling is due to incomplete reflection at a boundary mathematically akin to seeing out of your window.

IAmBroom 1y ago

Loads of good responses, but I'll just add: > Is the particle just failing to collide with the wall since objects are mostly empty space? To be frank, essentially nothing macroscopic ever reacts to impact because (subatomic) particles physically hit particles. Electrons repel with significant force long before they can "collide". The volume of space represented by "close enough" to the outermost electron shell (since electrons aren't at one single volume at any one moment in time) is somewhat large, even if the volumes of the nucleus + the sum of all electrons in the atom are much less.