Picturing Electrons

Most recent answer: 10/22/2007

Q:
How fast do electrons move? I have heard that when an electron gains enough energy it can jump from an orbit to another one without actually crossing the space in between, is this true? And finally, do electrons go round an orbit whose center is the nucleus, or do they have a different pattern of movement, like an electron cloud perhaps?
- Alp Oztek (age 17)
Turkey
A:
Electrons can move at any speed up to, but not including, the speed of light. In atoms, typical speeds relative to the nucleus are on the order of a few percent of the speed of light.
The picture you hint at, with electrons gaining energy then suddenly appearing at a new place is suggested by the common phrase ’quantum jump’, but it doesn’t accurately describe how they behave. The electron states are sort of cloud-like. They neither are located at particular places nor do they have sharply-defined velocities. The ’wave functions’ describing these clouds can have surprising properties, including being non-zero in two regions even though they fall to zero in between. It wouldn’t make sense to think of an electron hopping between the two regions without going in between.

The quantum states change in time according to something like Schrödinger’s equation. It gives purely continuous changes, with no leaps at all. That’s true not just for electrons but for all microscopic collections of particles.

There is a mystery to quantum mechanics, however. If we look at what should come out of the Schrödinger equation for big collections of particles, say like a cat, it can be states which are composed of parts representing a live cat and a dead cat. We never see anything like that, just one or the other. So it’s tempting to think that somehow the large-scale world does ’jump’ into a particular state rather than just obeying the Schrödinger equation. People argue about all sorts of different ways to understand this issue.

Mike W.

(published on 10/22/2007)