Open course ware

PROFESSOR: Last time, we spoke about photons in the context of an interferometer. The Mach-Zehnder interferometer. And we saw the very unusual properties of photons and interference, and how relatively simple interference a effect can be used to produce a very surprising measurement. Today we're going to backtrack and go from the beginning, and think about photons as physicists did 100 years ago, and how, by thinking about photons, they pretty much came up with quantum mechanics. So we want to trace this back. And the best place to start, probably, is with a photoelectric effect. The photoelectric effect is an experiment done by Hertz in 1887, in which he irradiated plates. That means shine light, high energy beams of light, on metal plate, and he found that electrons were released. Those were called photo electrons. And therefore, you would get a photoelectric current from those electrons. So this is the effect we want to discuss now, is the photoelectric effect. And...

PROFESSOR: Well, last time Gerry really let the cat out of the bag. He introduced the idea of assignment. Assignment and state. And as we started to see, the implications of introducing assignment and state into the language are absolutely frightening. First of all, the substitution model of evaluation breaks down. And we have to use this much more complicated environment model and this very mechanistic thing with diagrams, even to say what statements in the programming language mean. And that's not a mere technical point. See, it's not that we had this particular substitution model and, well, it doesn't quite work, so we have to do something else. It's that nothing like the substitution model can work. Because suddenly, a variable is not just something that stands for a value. A variable now has to somehow specify a place that holds a value. And the value that's in that place can change. Or for instance, an expression like f of x might have a side eff...

PROFESSOR: Well, last time Gerry really let the cat out of the bag. He introduced the idea of assignment. Assignment and state. And as we started to see, the implications of introducing assignment and state into the language are absolutely frightening. First of all, the substitution model of evaluation breaks down. And we have to use this much more complicated environment model and this very mechanistic thing with diagrams, even to say what statements in the programming language mean. And that's not a mere technical point. See, it's not that we had this particular substitution model and, well, it doesn't quite work, so we have to do something else. It's that nothing like the substitution model can work. Because suddenly, a variable is not just something that stands for a value. A variable now has to somehow specify a place that holds a value. And the value that's in that place can change. Or for instance, an expression like f of x might have a side eff...