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Quantum Mechanics

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Editor’s note: The information below is extremely important relative to understanding the famous Double Slit. To review the essentials of this seminal experiment, please see the videos listed on the “Double Slit” page.

From the website http://www.bottomlayer.com/bottom/reality/RealityFrame1.html

Do particle detectors affect the patterns on the screen?

We would like to think that the particle detectors at the slits are affecting the passage of the electron -- perhaps deflecting it, or modifying its path, or in some other way influencing the experiment. We could accept such an explanation. But that does not seem to be the case. A series of experiments have been conducted to test just such a hypothesis, and the results are uniformly negative. I will quickly run through some of the more ingenious attempts to isolate and remove any possible influence stemming from the detectors located at the slits.

1.  Turn off the electron detectors at the slits. Suppose we take our modified double slit set-up -- with electron detectors at the slits -- and leave everything intact. But, we will conduct the experiment with the electron detectors at the slits turned off, so that we will not actually detect any electrons at the slits.

The result upon analysis: an interference pattern at the back wall. So it seems that mere passage through the electron detectors at the slits does not affect the electron, so long as those electron detectors are not functioning.

2.  Leave the electron detectors on, but don't gather the information. Suppose we take our modified double slit set-up -- with electron detectors at the slits -- and still leave everything intact. And we will keep the electron detectors at the slits turned on, so that they will be doing whatever they do to detect electrons at the slits. But, we will not actually look at the count of electrons at the slits, nor will we record the count at the slits in any way, so that we will not be able to obtain any results from these fully-functioning electron detectors.

The result upon analysis: an interference pattern at the back wall. So it seems that the electron detectors located at the slits do not themselves affect the electron, even when the equipment is fully functioning and detecting (in a mechanical sense) the electrons, so long as we don't obtain the results of these measurements.

3.  Record the measurements at the slits, but then erase it before analyzing the results at the back wall. Suppose we take our modified double slit set up -- with electron detectors at the slits -- and still leave everything intact. And we will still keep the electron detectors at the slits turned on, so that they will be doing whatever they do to detect electrons at the slits. And we will record the count at the slits, so that we will be able to obtain the results. But, we will erase the data obtained from the electron detectors at the slits before we analyze the data from the back wall.

The result upon analysis: an interference pattern at the back wall. Notice that, in this variation, the double slit experiment with detectors at the slits is completed in every respect by the time we choose to erase the recorded data. Up to that point, there is no difference in our procedure here and in our initial procedure, which yielded the puzzling clumping pattern.

Yet, it seems that if we, in a sense, retroactively remove the electron detectors at the slits (not by going back in time to physically remove them, but only by removing the information they have gathered so that it is not available from the time of the erasure going forward into the future), we can "change" the results of what we presume is a mechanically complete experiment, so far as those results are determined by a later analysis, to produce an interference pattern instead of a clumping pattern. This is mind-boggling.

4.  Arrange the experiment so that we can make an arbitrary choice at some later time, after the experiment is "complete," whether or not to use the information gathered by the electron detectors at the slits.  Suppose we take our modified double slit set up -- with electron detectors at the slits -- and still leave everything intact. And we will still keep the electron detectors at the slits turned on, so that they will be doing whatever they do to detect electrons at the slits. And we will record the count at the slits, so that we will be able to obtain the results. But (this gets a little complicated), we will 

(1) mix the data from the slits with additional, irrelevant garbage data, and record the combined (and incomprehensible) data; 

(2) design a program to analyze data coming from the slits in one of two ways, either 

(a) filtering out the garbage data so that we will be able to obtain clean results of electrons going through the slits, or

(b) analyzing the mixed-up data so that we will not be able to obtain the results of electrons going through the slits; and 

(3) leave it up to a visiting politician which way we actually analyze the data from the slits.

The result upon final analysis by method (2)(a): a particle clumping pattern appears from the data.

The result upon final analysis by method (2)(b): an interference pattern appears from the data. 

So it seems that an arbitrary choice … made hours, days, months, or even years after the experiment is "complete," will change the result of that completed experiment. And, by changing the result, we mean that this arbitrary, delayed choice will affect the actual location of the electron hits as recorded by the electron detector at the back wall, representing an event that was supposed to have happened days, months, or even years in the past.  An event that we suppose has taken place in the past (impingement of the electron on the detector) will turn out to be correlated to a choice that we make in the present.  Imagine that.

The proverbial tree has already fallen in the forest, and we can later choose whether or not to listen. And if we choose to listen then the falling tree will have made a noise, and if we choose not to listen then the falling tree will not have made a noise.

What is the difference? It turns out that, so far as experimentalists have been able to determine, the difference is not whether electrons were run through an electron detector at the slits. It turns out that, so far as experimentalists have been able to determine, the difference is whether the analysis of the results at the back wall is conducted when information about the electrons' positions at the slits is available, or not.

In searching for the wave-like phenomenon that must, it simply must be taking place in the unmodified electron double slit experiment…

The measurement effect. With sublime understatement, this phenomenon is referred to as "the measurement effect." When we measure (or detect, or see, or quantify, or determine, or otherwise gain knowledge of) something at the quantum level, the very act of measurement will have an effect on the thing itself. To all intents and purposes, the act of a sentient being in seeking a measurement will cause the thing to have a property which can be measured, and thereby produce a definite property that can be measured.

Since around 1927, the standard quantum mechanical explanation for the difference between results in the double slit experiments particularly, and for the measurement effect generally, is that in one set of experiments, we know (or more precisely, we can in principle know) which slit the electron went through; and in the other set of experiments, we don't know (i.e., we cannot know even in principle) which slit the electron went through. This conclusion is one facet of the "Copenhagen interpretation" of quantum mechanics (so named because it was developed by Niels Bohr's institute located in Copenhagen, Denmark), which represents the closest thing to a consensus among physicists for the last seventy years or so.

The difference is whether we know. The difference is whether we choose to have the information available.

If we demand to know which slit the particle went through, then a particle must appear at one slit or the other so that we will have an answer to our question; and so our curiosity has caused there to be a particle at one of the slits, and now there is a particle; and if there is a particle at one slit or the other, it must obey the rules for particle motion, and so it does.

Conversely, if we do not demand to know which slit the particle went through, no particle need appear at either slit; and so we have not caused there to be any particle, and now there is no particle; and if there is no particle at either slit, the system remains free to roam the universe in whatever form seems most pleasing to itself.

And all of this is determined at the time we demand the knowledge, not at the time we institute any mechanical processes for obtaining the information.