schrödinger's sink..

The other day in one of my classes, we were talking about quantum mechanics and the question of measurement - the fact that any measurement of a physical quantity involves an interaction with the object of the measurement, and that this by nature is not a purely passive act. For example, in order to find a set of keys on our nightstand, we could grope around in the dark with our hands - which might move them from their original position or knock them onto the floor. Of course we could just turn on a light. We think of this as being a passive act, but really it's not. When the light reflects off the keys it jiggles its atoms and molecules around a bit. The jiggle from the light isn't enough to knock the keys off the table, of course... it may just raise the temperature of the metal a teeny-tiny bit. For macroscopic objects like keys we can pretend that the act of looking at them (which involves poking them with light) has no effect on the object, because the effect is very very small.

But this isn't true for microscopic objects. If the object you are looking for is not a set of keys, but a single atom or a single electron, the act of bouncing light off of the object is akin to groping around in the dark. The light is apt to give the atom or electron a little *whack* and send it flying off in some direction. Thus our theories that talk about the locations and motions of microscopic objects have to deal with the inherent impossibility of making an observation of these entities without changing the very thing we are trying to measure.

It's hard to come up macroscopic analogies that illustrate the problems of quantum mechanics in a way that students can really connect to. I used some of my standard analogies in class (one of which involves a blind man with a cane trying to find a marble on the ground) and I felt pretty good about it. Then I left class and headed to the restroom to wash my hands of the accumulated dry-erase marker smoodge.

The restrooms on the 3rd floor of our building are single-service affairs, with locking doors, a single toilet, and one sink. The sink has one of those fancy automatic on-off faucets with a light sensor that turns on the water when you wave your hand under the faucet. But the sensor, being light sensitive, also turns on the water when you open the door and turn on the light! With quantum mechanics on my mind, it occurred to me that the sink was a good analogy to a quantum mechanical system. Suppose the door is closed and we wish to ask the question - is the sink on or off. We open the door... turn on the lights, and lo and behold, the faucet is running. Aah, but maybe it just came on when we turned on the lights! Maybe I could sneak into the bathroom with the lights off and stick my hand under the sink to see if I feel the water running. And wouldn't you know it, I always do!

It may be a silly example, but at the beginning of the next class I opened with this little story and the students in my class, having all used this bathroom before, immediately got it - any attempt to observe the sink changes the state of the sink. Just like an electron. Only wetter.