what makes a quantum observer inherently different?

Question

The double slit experiment claims to prove that observing something changes the outcome.  This never made sense to me.  I've got a macro-sized experiment that shows my thinking:

I take a cardboard tube, cut a hole in the middle that I can stick my finger in, then drop a ball though the tube. 

There are 2 possible outcomes. Either the ball falls out the bottom or it doesn't. This represents the wall past the double slit. 

If I put my finger in the hole in the middle of the tube, I can feel the ball hit my finger.  This acts as a way to detect or "observe" the ball in the middle of the tube.  It also blocks, (or delays depending on finger depth) the ball from getting to the other side of the tube, because my finger is holding it back. 

Doing this cardboard tube experiment; I could come to the conclusion that observing the ball in the middle changes the outcome... or I could realize that my method of observing is flawed and I should be using my eye instead of my finger to detect if the ball got to the middle of the tube. 

Why isn't the second explanation that the detector is flawed being used in the double slit experiment? What makes "observing" at the quantum level inherently different?

Comments

The question 'what is a quantum observer?' is still controversial. But, 1, what you describe is not a quantum experiment. 2, it's not a matter of observer but of availability or not of an information.Imagine an algorithm missing a value when computing the next step of an animation. It gives to the missing measure a random value and the result is random. The way it chooses the random function is another subject. This illustrates the limits of describing with reducing sentences some physics phenomena.

---

I know what I was describing is not a quantum experiment. My entire question is around what makes the quantum experiment have different rules than the non-quantum one.  All of the uncertainty and missing information you describe in quantum mechanics seems to stem from the assumption that the double slit experiment's conclusion caused. That the act of "observing" quantum states seems to change them.

My question is why that base assumption that everything else is layered on top of was assumed in the first place.  As far as I can see, the same experimental results could be explained by a failure in experimental setup. The measuring device used was the wrong method of measuring the results.  This is obvious in my non-quantum experiment. Why was it ignored for the quantum one?

Why was the conclusion that reality itself somehow cares if it's being observed chosen instead of the conclusion that the detector chosen for the experiment was the wrong one?

Answer 1

In a way all experiments are quantum. The tube-ball-finger experiment you describe is "macroscopic", involving objects of dimensions we are acquainted to from our everyday lives. Looked at more profoundly, it involves a vast number of degrees of freedom. The tube, ball and finger consist of a large number of atoms, each in principle interacting with an even huger number of  degrees of freedom in the environment (air molecules, photons).  This environment also is a quantum system. Two quantum systems interacting with each other will form an entangled state (which is a pure quantum state). If you consider only the tube-ball-finger (TBF) system, you are disregarding the degrees of freedom of the environment. The results of experiments on TBF can statistically be described by a reduced density matrix, obtained by tracing out the degrees of freedom of the environment. This reduced density matrix will be (almost) diagonal, with off-diagonal elements, corresponding to quantum interference effects (almost) gone. As quantum interference effects, related to superpositions of states, are a specifically "quantum" phenomenon, by the coupling to the environment this "quantumness" is suppressed. This is known as local decoherence. Decoherence achieves this diagonalisation of the reduced density matrix without any additional processes, simply by treating the system (TBF) coupled to the environment as a full quantum system. You could also couple the system to a measurement apparatus and this then to an environment. The mechanism remains the same. Note: The full quantum system, i.e. TBF (+apparatus) + environment remains in a pure quantum state.

As for a collapse of the wave function (or changing the system by observing it):

It only makes sense to talk of something like that if you perform a measurement on the system and then another one (and then another one ...). If you do N measurements, you can say you have obtained N values. But that statement is incomplete. What you really have obtained is one ordered sequence of N values. One measurement result, consisting of the results of N "sub-measurements". This sequence has to be logically consistent. This may look like a collapse - but I find the word doesn't capture what is happening.

Example: Double Slit Experiment. After the double slit, not just a screen (which swallows "particles"), but a sequence of detectors following each of the slits. If the first detector after slit 1 indicates the presence of a particle, so necessarily will the 2nd, 3rd, and so on. You won't have a sequence of results where, e.g., the first and second detectors after slit 1 indicate a particle and then the 3rd detector after slit 2 (unless you have set up a mechanism to couple the paths after the slits - or you have more than one particle in the system and detectors are not perfect). 

Comments

While this looks like a lot of correct statements, it does not answer the question.  The fundamental question is why the double slit experiment's "detector" is assumed to be a perfect device for measuring if the particle was at the slit or not.  The experimental outcome obviously shows that the detector changes the results.  But why is the widely accepted conclusion that there is an inherit property to reality that "observing changes results" when the much more logical conclusion "our detector device messed up the experiment" also fits the experimental results?

---

1) Generally: If someone does / plans an experiment, but knows or expects that a certain type of detector is going to "mess up the experiment", he would discard the detector and use a more suitable one. You can also attempt to take into account how the detector interferes with the experiment - and thus separate disturbances from a specific (type of) detector from fundamental physical effects. It is therefore not in the least "the much more logical conclusion" that the detector messed up the experiments. For this conclusion to fit the experimental results this fit should be quantitative - which obviously cannot be achieved by simply stating "bad detector - messed up experiments" and not trying to understand the effects of the detector in more detail.

 2) It is not clear to me how your tube-ball-finger experiment relates to the double-slit experiment.

3) If you perform a double slit experiment with the modification that you measure which of the slits the particle is passing through, you change the setup, but the detector you are using is neither flawed nor does it "mess up" the experiment. Rather, it detects precisely what it is intended to detect (particle through slit 1 or slit 2). A changed experimental setup will usually lead to changed results. For the TBF case: finger in or not are two different setups - you should expect different results; none are messed up.

4) Local decoherence describes the seeming disappearance of "quantumness". Entangled states allow to describe the effects of interaction of systems with perfect detectors. Of course, such interactions lead to states (the entangled ones) different from the states of the system alone. But in order to perform a measurement, the system has to interact with a detector. This doesn't make the detector unsuitable, nor does it show the detector to be imperfect.  

System isolated  and (system + detector) are two different total systems. They have different states. In the second case, the states will be entangled states between the system and the detector. Thus, introducing a detector (this is not yet an observation!) changes the states (the total system even), but does not cause a collapse of the wavefunction.

---

The double slit experiment with and without the standard detector on one of the slits changes what shows up on a screen after the slit.  I think we can agree on this.

The tube-ball-finger experiment with and without a finger as the detector also changes what shows up after the tube.  But if the detector for this experiment is replaced with visually looking in the hole, instead of using a finger, then observing does NOT change what shows up after the tube.  If you want to test the experimental hypothesis "Observing a ball in the middle of the tube changes what shows up after the tube" the choice of detector matters.  Using a finger in this case breaks the point of the experiment because the finger is an active part of the system, not passive observer. The hypothesis is actually false for the observer using sight, but it is true for an "observer" blocking the tube with their finger. The definition of an observer in general parlance is someone who does not play an active part in the action but knows what happened. Thus a finger as the detector messes up the experiment because the detector does not meet the definition of an observer. It doesn't matter how the detector changes the results. The fact it changed them at all means it's flawed and should be replaced.

The entire concept of observation changing the results, superposition and a wave form collapse in quantum mechanics ties back originally to the double slit experiment's hypothesis that "Observing a particle in the middle of it's path (at the slit) changes what shows up after the slit." I'm trying to understand why that hypothesis was considered correct instead of the detector being blamed like it makes sense to do in the tube-finger-ball experiment with a very similar hypothesis.

As igael said above "'what is a quantum observer?' is still controversial." My thoughts on this are: the word "observer" is way too widely used. We should be looking at what mechanism we use to "observe" when a wave function collapses as that mechanism is doing something to force the collapse, not merely "observing".  If we could find a detector that does NOT influence the outcome on the double slit experiment, (similar to how we can use a visual detector in the tube-finger-ball experiment.) We could finally get a definition of what we are currently calling a "quantum observer" actually means, and it would tell us a lot about how wave forms collapse in the first place.

---

Using sight to observe the ball in the tube still requires the ball to interact with photons.  These photons will change the details of the outcome, albeit on a very minor scale for a macroscopic setup. 

A truly "passive" observer that does not interact with the observed system appears to be impossible.