# Question about measuring physcial observables

1. May 11, 2012

### cragar

So my measurement of a quantum system is an eigenvalue of that operator.
And we are not able to predict what eigenvalue we will get, even if we knew the precise state vector before we make our measurement. But QM allows us to calculate the average of all these eigenvalues, if we made an infinite amount of measurements and then took the average. And we call this the expectation value. So the expectation value that we calculate might not even be a possible eigenvalue that we could measure?
Im just starting to learn QM. Any input will be much appreciated.

2. May 11, 2012

### Staff: Mentor

Correct.

3. May 11, 2012

### cragar

ok thanks for your answer. Do we just define what physical observables are and then think of ways to measure these quantities that we defined. Or does nature have infinite amount of physical observables.