Schrodinger and Heisenberg are driving in a car that is pulled over by a police officer. The officer walks up to Heisenberg, who was driving, and asks "Do you know how fast you were going?" To which Heisenberg replies, "No, but I know where I am." The officer decides to search the cars trunk. After opening it, he walks back over to the car window and asks, "Are you aware that there is a dead cat in your trunk?" Schrodinger replies to this saying, "Now I am."
I have not heard the cat in the trunk part. That's new to me.
The way I've heard the joke is that to officer asks if they know how fast they were going, Heisenberg says, "No, but I know where I am." The kneficer replies, "You were traveling at 90 miles per hour." Heisenberg replies, "Great, now I have no idea where I'm going!"
Explanation: With the uncertainty principle, you can not simultaneously know both location and momentum. However, momentum is mass time velocity, and velocity includes direction. Assuming the car's mass is known, if you know a particle's location, you CAN also know it's exact speed, but then you can not know the direction, and thus, you only know one component of the velocity, and therefore, you don't know the velocity.
Here's an excellent one I heard recently:
Einstein, Pascal, and Newton are playing hide and seek. It's Einstein's turn to count. He covers his eyes and Pascal runs and hides. Newton takes out a meter stick and uses it to draw a square in the dirt. He then stands inside it, right in front of Einstein. When Einstein opens his eyes, he sees Newton instantly. "Isaac, you are not very good at this game...I found you." Newton replies, "No, you did not find a Newton. You found a Newton over a square meter. Thus, you found a Pascal."
"It is far better to grasp the universe as it really is than to persist in delusion, however satisfying and reassuring." -- Carl Sagan
"The first principle is that you must not fool yourself and you are the easiest person to fool." -- Richard Feynman