Thread: The observer effect and storing information.

In the double slit experiment, if you try to observe which slit a single electron (or photo) takes through the slit. It will behave like a particle and it will not draw an interference pattern. As i understand if the detector is short circuit so it can not record any information, the electron is able to interact with itself and draw a wave pattern on the screen it hits.

Let's say that the screen that the electrons hits are of film however i have not seen the result yet. If i decide to erase the "through-which slit recorder" so the information is destroyed, before i look at the film screen, will i always see an interfence pattern?

No, you will not see an interference pattern. The instrument you use to detect the electron disturbs its wave function enough to destroy the pattern, whether you read the instrument or not.

Incidentally, the electron does not act as a particle just because you detect which slit it went through. What happens is that the wave function is modified, but it is still there. If, for instance, you put another double-slit apparatus behind one slit of the first one, the second one would produce its interference pattern. Unless, of course, you had proven that the electron had not gone through the slit in front of the section experiment at all. If the electron could have gone electron that slit, even though the wave-function was disturbed enough to destroy the first interference pattern, the interference pattern of the second apparatus will survive.

Hmmm...

Delayed choice quantum eraser - Wikipedia, the free encyclopedia

If the "which path" information is contained in the system, there is no interference pattern. If the "which path" information is erased from the system, an interference pattern can be recovered from the recorded data.

Originally Posted by SpeedFreek

Hmmm...

Delayed choice quantum eraser - Wikipedia, the free encyclopedia

If the "which path" information is contained in the system, there is no interference pattern. If the "which path" information is erased from the system, an interference pattern can be recovered from the recorded data.

The delayed choice experiments are very complicated and take some analysis to understand. In those that I have worked through, the determination of which slit was gone through is delayed rather than being done right at the slit. Once that determination is made, the interference pattern is destroyed. I personally feel that the conventional terminology of "erased" is misleading. What happens in fact is that the actual determination is delayed, and the decision to make or not make a determination can be made after the electron has passed through the slits. But it is always before the observation of the interference pattern. "Observation" always means when the apparatus has made a measurement; the results do not have to be seen by a human to destroy the pattern. There is no magical connection to a mind that settles the existence of an interference pattern.

Prediction: this thread is going to go on and on and on but will change nobody's mind.

I wasn't for a moment suggesting that there is any magical connection to a mind that settles the existence of an interference pattern. There is no requirement for a conscious entity to be involved whatsoever.

The interference pattern does not exist if the "which path" information exists in the universe. If the "which path" information has been erased from the universe, an interference pattern can be recovered from the signal data collected at the final detector screen, even if that data was collected before the choice was made to erase the "which path" information. That choice need not be made by a conscious mind, either.

But in that case why are so many "scholars" putting so much in to the importance of consciousness and the observer?

And why did Einstein say as a reaction to QM that he would like to think the moon was there when he was not observing it?

Originally Posted by bluez

But in that case why are so many "scholars" putting so much in to the importance of consciousness and the observer?

I think that the problem is mostly philosophical. The wave function in quantum mechanics is determined mathematically and deterministically, but it tells you only what the probability distribution is for your experiment. This is fine if you are looking at an interference pattern, which is essentially a plot of the probability function. However, that pattern builds up point by point, and where within the overall pattern the points arrive is completely random. I don't see how anyone whose senses work classically could fail to be uncomfortable with that situation. It is natural to ask "where was that electron before I saw that flash?" and very uncomfortable to answer "there is no way to tell, we only know how they will be distributed after the pattern builds up." How can the probability be determined when the individual behaviors as seen are not?

Personally, I take the wave function to be the basic physical object and say you should use it to calculate the distribution when everything is finished. Many, probably most people don't like to take that position, so other (philosophical) options are explored. Everyone agrees on the physics, but there is no agreement on what philosophical attitude to take. You can tell how uncomfortable people are with the situation by the labelling of the viewpoint I have described: "the shut up and calculate" interpretation.