# Measurements in quantum mechanics

Why does measurement change things?

I read that measurement changes things because we have to bounce photons off an object to 'see' it and that changes its position, momentum etc...

But on the other hand, Griffiths' QM book seems to suggest we don't know what it is about measurement that changes the state of something. We don't know what's special about measurement, or what exactly constitutes measurement.

The photon idea sort of makes sense to me, so if it isn't actually the accepted answer, then why not?

If this question is metaphysical/doesn't have a definite answer I apologise, but I thought I'd check in case it does. It's just that the photon thing and Griffiths seem to say different things.

• You are not the only person having difficulty with Griffiths' book. Look at the related questions on the right, especially "Defining Measurement in Quantum Mechanics" physics.stackexchange.com/q/11995. – sammy gerbil May 7 '16 at 21:55
• In my opinion, there are several things going on in the quantum mechanical world. The objects that are being measured are so small that the act of measuring them disturbs them. In addition, humans live in a classical world, and as such, it is difficult to conceptualize a quantum mechanical interaction. – David White May 8 '16 at 1:19
• Every measurement process is an interaction process and by this makes the result never exact. An everyday example is the measurement of the current in an electric circuit. Then lower the resistance of the amperemeter then better the result. But since an ampere-meter without resistance is not working, a little uncertainty will be in such measurement. – HolgerFiedler May 8 '16 at 5:23

## 1 Answer

In quantum mechanics all predictions and descriptions of nature come with a probability distribution . A simple example are the orbitals of the hydrogen atom.. The probability for an electron to be found at (x,y,z,t) can be calculated and the result, is called an orbital, because it is not a classical orbit.

To compare a probability distribution with the data one has to get many samples, both for classical and quantum probability distributions. One instance which contributes to the probability distribution is a measurement ( otherwise called in graphic language "collapse of the wavefunction").

For the particular event measured so as to accumulate the probability distribution and check the theoretical model, the wavefunction will have changed due to new boundary conditions. That is what is meant as "measurement changing things". Measuring the photon from an excited energy level of the hydrogen atom, implies a change in the orbital of the electron.