Plaintive cries from non-physicist readers asking me to exmplain the

Heisenberg joke.

Er, well. Heisenberg discovered (in 1927) the famous Uncertainty Principle

in quantum mechanics. The principle is a formal mathematical statement of

the idea that in order to observe something as small as (say) an electron,

you have to bounce a light photon (or something equivalent) off it; and that

by doing so, you disturb it, so that its subsequent behavior is not what it

otherwise would have been.

The way this works out mathematically, and putting it a bit loosely, is that

you can’t know BOTH the position AND the velocity of a particle with high

precision. The more precisely you know the one, the less precisely you’ll

know th other.

The principle actually says: (Uncertainty in measurement of position) TIMES

(Uncertainty in measurement of momentum) MUST EXCEED (a certain constant

number). Thus, if one uncertainty is verty small, the other one must be

huge, to make the principle true.

There is a decently full account here.