Double slit experiment record but not look at it I am having a tough tough stressful week trying to write an article about quantum mechanics. I know there was a question asked the same thing before, but I didn't understand nor I did not want to wake up an old thread.
The double slit experiment we all know, if we put an apparatus at the slit to detect which slit the electron has gone through but not look at the data it spits out, what happens to the screen? (Looking at the screen doesn't do any change right?)
1) If an active observer is not necessary for the wave function to collapse, the existence of a sensor will destroy the interference pattern because the sensor has interfered with the system.
2) If there needs to be an observer to read off the data from the sensor and acknowledge that the electron has acted like a particle and gone through either of the slit, the interference pattern will not be destroyed when the sensor is just there but no one has read the data.
Which one is it?
 A: The interference pattern will be destroyed even before the wavefunction-collapse.
Let's say your particle is described by the wavefunction $|\text{p}>$. When passing the double slit (I call those the up and down slits), it becomes entangled with the sensor, and your wavefunction becomes :
$|\psi> \equiv|\text{sensor}_{up}>\otimes |\text{p}_{up}>+|\text{sensor}_{down}>\otimes |\text{p}_{down}>$
When you want the probability to find the particle at a certain position on the screen, you will calculate $<\psi|\psi>$ but since $<\text{sensor}_{up}|\text{sensor}_{down}>=0$, you won't get the term $<\text{p}_{up}||\text{p}_{down}>$ which is the one you need to have an interference pattern.
Simply adding the sensor destroyed the interference, whether you look at it or not.
A: You are describing a situation equivalent to Schroedinger's Cat.  If the recording apparatus is isolated from the rest of the universe, then passage of an electron wavefunction through the double slit puts the recording apparatus and its records into a mixed state.  When you then "measure" the record of the experiment by reading it, you "collapse" the wavefunction of the apparatus, the record, and the electron.  
It is this kind of thought experiment that leads to the Many Worlds view of quantum mechanics: provide an observer to read the results of the experiment, but isolate him/her along with the apparatus and record, and the experiment puts the observer into a mixed state.
