To start off, a terrible pun.
What does a physicist eat with soup? Quarkers.
And this one is one of the best of a whole category of jokes, "The Mathematician, the Physicist, and the Engineer [and sometimes the Sociologist]," in which the mathematician is technically correct but ridiculously precise, the physicist is focused on the real world but the wrong part of the real world, the engineer is relentlessly practical, and the sociologist, if present, is vague, over-general, and not very scientific:
A mathematician, a physicist, a sociologist, and an engineer are traveling by train into Scotland. They see a brown cow. “Oh look,” says the sociologist, “Scottish cows are brown.” “No, no,” replies the physicist, “we perceive one of the cows in Scotland to be brown.” “You're both wrong,” replies the mathematician, shaking his head at his friends' muddled reasoning. “There is at least one cow in Scotland, of which at least one side appears to be brown.” “Oh, who cares?” asks the engineer. “Let's get out and milk the cow.”
Another MP&E[&S] (hey, that sounded like an IT abbreviation -- wonder if any of these involve computer scientists) joke, this time without the sociologist:
A mathematician, a physicist, and an engineer are staying in a hotel (perhaps the Hotel Infinity?). The physicist gets up in the middle of the night and sees a fire in the hall. Rubbing his chin, he concocts a method for manufacturing carbon dioxide from the nearby bucket of water to use in a makeshift fire extinguisher. While he's thinking, the engineer smells smoke, runs out, and dumps the bucket of water on the fire, effectively putting it out. In the morning, they discuss the events of the night with the mathematician. “Did you see the fire?” asks the physicist. “Oh, yes,” replies the mathematician, “I saw it before either of you did.” “Then why didn't you put it out?” says the engineer. The mathematician explains, “Well, there was a fire and a bucket of water – obviously a solution existed!”
An infinite number of mathematicians walk into a bar. "I want a pint of beer," says the first mathematician. "I'll have half of what he's having," says the second. "I'll have half of what he's having," says the third, and so all the way down, halving all the way. "OK," says the bartender, and gets two pints of beer. [Optional ending: "Separate tabs," say the mathematicians. An infinite number of mathematicians get thrown out of a bar.]
An infinite number of mathematicians walk into a bar. That's to be expected -- they never do watch where they're going. [Inspired by the joke, "Two men walk into a bar. The third one ducks."]
OK, enough walking into a bar jokes. They're not that great anyway, and besides, those are about mathematicians. Now for one that's true:
Physicist P.A.M. Dirac (he always used the initals) was famously untalkative. So much so, in fact, that his colleagues invented a unit of information flow – the dirac, equal to one word per hour.
Here's a one-word piece of physics-related humor: Feynman. If you've heard about his crazy escapades (say, picking all the locks at Los Alamos), you'll know why.