All right. Where were we? Measurements versus observations, right.
In weighing yourself or reading your speedometer you're observing a measured quantity. In the case of weighing, you don't have much influence, or not much immediate influence (oh, yeh, long term, sure), and in the case of driving and reading your speedometer you do have immediate influence. On the highway, you need to watch your speed, as they say.
Okay, you're right! People watch their weight also. I could surely stand to lose a few pounds, like ten at least. 154 at the doc's office last visit. (My digital scale in the bathroom, by the way, is inaccurate. It gives readings to the 10th of a pound [that's called precision--not the same as accuracy], but repeated measurements give different readings by as much as five pounds. The mechanical scale at the doctor's office--ancient technology--is much more accurate. A good digital scale, such as I've used in student labs many times, is both accurate and precise.)
I was going to talk about measuring cups and measuring sticks, both of which require an active role by the measurers, and contrast that with a speedometer reading. But let's just say here that a measurement is a quantitative observation, and leave it at that. No semantical difficulties there.
But sometimes a measurement doesn't give you the quantity you're looking for. Sometimes you need to make a calculation! In my very first physics lab, in the excellent year 1974 at the University of Arkansas at Little Rock (as referred to in my Intro post, August 14th), the instructor had us go out to the side of a very busy street, Universty Avenue, and measure the times it took the front bumpers of particular cars to travel a certain distance in order to calculate the speed of the cars. Rather dangerous. Probably not done these days. Anyway, there is a hierachy of observation, measurement and calculation involved in experimental physics. The calculation is not necessarily done by a person, of course.
So where does this leave us with our investigation of Schroedinger's cat? (Without the two dots on the "o", you're supposed to put in the "e".) The measurement is more like an observation because it's not obviously quantitative. But we can call the live state 1 and the dead state 0. So it's a very simple measurement, the looking in the box part anyway.
The significant and more complex thing is the meaning of a measurement in quantum mechanics. Schroedinger, in the cat thought experiment, was trying to show the ridiculous nature of the "measurement creating the reality," which is what the Copenhagen interpretation of quantum measurement says happens.
This is the same idea as the Einstein-Podolsky-Rosen "paradox," and it was the publication of the EPR paper in 1935 that set Schroedinger to thinking of his cat. Originally Einstein and Schroedinger corresponded about the wave function (or state vector) of an unexploded keg of gunpowder subject to being exploded by the decay of a radioactive nucleus (one decay from a collection of slow-decaying nuclei, actually) . Einstein pointed out that the wavefunction would have components for the exploded keg and the unexploded keg. Schroedinger changed this when he published his paper later that same year to be a cat in a box, and the components become the live cat plus dead cat combination. The measurement is considered to be performed when the state of the cat is determined.
