Quantum Physics: Observers and double slit experiment.

Factual Questions
1

Disclaimer: Please feel free to direct me to websites or books that answer my questions or address my examples. I am interested in clear easy to understand experimental evidence.

I have a novice question about the Observer effects on quantum events in experiments such as the electron two slit experiment. I have read many descriptions of the experiment, and I understand it has been replicated many times. But every time I read an explanation of the experiment I have this huge desire to scream, “But…did they try this? And what happened?”

Now I am going to explain my screaming need to know, and hopefully someone out there will respond, “Yes of course they tried that and if you want to read all about it go here.”

My screaming need to know:

So you shoot an electron beam at two slits and you get an interference pattern.
Then you shoot electrons at two slits one at a time and you STILL get an interference pattern. Wow.
But then you put a detector at each of the slits so you can see which slit each electron actually went through. And then the interference pattern stops. BIG WOW.

Conclusion usually expressed: The act of observing the electrons “choice” of slits forces the electron to act in a different way. (collapsing possibility waves and what not). In other words, the simple act of having a conscious observer of the electron changes how the electron behaves. Wow.

So here is my screaming need to know. I am sure this is a stupid question, so please direct me to really well explained easy to comprehend experimental evidence that I can peruse. My question is, are they sure? Are they sure the detector isn’t the cause of the electrons change in behavior? There are certainly experiments that could verify whether the simple act of conscious observation truly is the cause. I would like to get them to set up an experiment that shows if the observer is facing the apparatus, the electron goes one way. But if the observer closes his/her eyes then the electron behaves differently. If this was demonstrated that would really be impressive…at least to me. (and as I’ve said, I hope this has been done and you can just tell me where to go read about it!)

Here is my very rudimentary example of such an experiment.

  1. Put the detector in place and have it detect the electrons “choice” but not record the choice anywhere. Then have noone watching the “read out” on the detector. No conscious observer of the electron. Does it create an interference pattern or not?
  2. Now have an observer watching the read out. Now how does the electron behave?
  3. Have the detector watching and recording the results on a single media. Have no observer watching the read out AND have no one looking at the results on the media. Then after the test immediately destroy the media before any observer has observed the results. Again in this example, there would have been NO conscious observation of the electrons “choice”. Now what was the result on the target? An interference pattern or not?

Experiment number 3 is particularly interesting to me…because it will create paradoxes. But if what matters really is whether or not a conscious observer witnesses the electron’s choice of slits, then this should be a valid experiment. If the recording device alone affects the electrons choices then the observer is ruled out as a relevant cause.

Ok let me have it. Please go slowly.

2

As physicist John Wheeler has noted, the way this is expressed is bad. By “observer” you really mean the detector system that measures the event. You don’t need a cognitive mind to “force” a wavefunction to collapse to a particular state (although there are New Age types who would like to believe this, since it suggests that the human mind helps create the universe). So the results of your experiments would all be the same.
See the book Where does the Weirdness go? (I forget the author, but there can’t be that many books out there with a title like that) for more on quantum weirdness, and the answers to a few quantum conundrums.

3

Thanks CalMeacham.

But if the detector alone causes the collapse of the wavefunction…how do we know that the detector is not interfering with the electrons behavior?

After all, for a detector to do any detecting…there must be some sort of interaction between the electron and the detector…and if there is an interaction…there could be some sort of affect from the interaction.

Does this make sense? Have there been any experiments that solidly eliminate the possiblity that the detector may be causing the electron to change behavior?

Thanks again.

4

It’s not the consciousness, it’s the detector. There are plenty of things you can do to collapse the wavefunction without yielding any useful information; it’s just that anything you can do that does give useful information will collapse the wavefunction. No aspect of any branch of physics says anything about consciousness or intelligence.

5

The best I can tell you is that it has more to do with the “wave nature” of electrons and photons (It’s a particle! It’s a wave! It’s both! But that’s impossible. I know!) and a quantum phemonenon know as superposition, than it has to do with consciousness. The Uncertainty Principle may give you better info than I. Here’s a quote from the link:

So the best I can explain it is that there is no interference. It just seems that way because the states interfere with each other. Other more knowledgeable dopers will probably tell you that I missed the mark a bit. They will probably be right.

Now, if you excuse me, I need to report this Shrodinger guy to the SPCA.

6

At this page: http://www.pipeline.com/~rhodesr/bottom/reality/chap2.html

The author explains (regarding whether it’s the detector itself or something else): “We would like to think that the particle detectors at the slits are affecting the passage of the electron - - perhaps deflecting it, or modifying it’s 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.”

Then goes on to say "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.”

There is even wierder stuff after this. Is the author of this page full of it? Is he lying about what experiments have been done?

7

Nope, that guy is full of it. Consciousness has nothing to do with it. It’s the interaction of the detector with the wave function that changes the outcome of the experiment, not the fact that some mammal will eventually read and understand them.

8

Wonderful site CurtC thank YOU!

Here is a quote which seems to be very close to one of my experiments:

“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 ([pp. 15-17]), 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 a completed 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.”

This experimental evidence seems to disagree with what CalMeacham and Chronos said. The result seems to change depending upon the knowledge of a conscious observer.

Again though these experiments leave me wanting. I want them to get deeper into that last experiment.

What if you look at the pattern on the wall and then delete the data before looking at it? What will the result be? Will it always be the same?

This site says that the result of the past experiment changes depending upon an act in the present. Wild. Can anyone direct me to further experiments that explore experiments that thouroughly test this?

Thanks again!

9

Ok, well if the guy is full of it then that is good to know. Can anyone direct me to solid academically accepted experimental discussion like this? How about someone that reproduces this guys exact experiments?

Pretty please. (sorry to doubt you guys!)

10

The sad thing is that the site linked by CurtC is full of accurate information. At first. It’s only when he starts talking about leaving the detectors on but not reading the results that things go bad.

I also recommend Where Does the Weirdness Go?, by David Lindley. It’s accurate, clearly written and focuses on why quantum mechanics is exciting as a science, without resorting to metaphysics and philosophy.

11

ready29003, I would suggest going out and checking out any book that is written by someone in the physics profession about quantum mechanics. If they are at all mainstream, they will reiterate with varying degrees of eloquence what others in this thread have said. If you are really interested in the experimental evidence, they will have the citations of relevant literature for you to peruse at your leisure.

The site CurtC has given is indeed a bit off-kilter. The site’s author’s stated goal is to “explore the hypothesis that our world is, in fact, a computer simulation”. I would give it a crank.net rating of “fringe” at best.

12

Here’s a question for the anti-consciousness guys.

Since any detector is ultimately composed of quantum particles, what is it that prevents these particle’s wavefunctions from simply forming a superposition with the electron’s wavefunction?

Superposed wavefunctions are not collapsed wavefunctions.

13

hmm…well i will check that book out. But if anyone can direct me to a good summary on the web that clearly explains experiments done that eliminate the detectors as the cause of the wavefunction collapse I would appreciate it.

(for instance a detector that does not record any data. I still would like to know if that would or would not cause a collapse.)

14

Ring, I myself am not the most gung-ho fan of collapsing wavefunctions, largely because I think that they mislead us into believing that there is something “magical” about observation which there truly isn’t. But I can see how such a formulation can be useful descriptively. The whole point in using this idea with the double-slit experiment is that you’ve build your detector “ideally” which explains the double-slit interference (or, more precisely, its lack thereof). The model is purely theoretical and divorced from the practical considerations you are trying to impose on it.

That said, your idea does hold some merit. In principle, there is nothing that prevents superposition of the wavefunctions of the detector constituents and what we wish to measure. Indeed, this need not involve any “collapsing” of wavefunctions. In fact, it may be that all interactions in nature are this way. We really don’t have the capability yet to say for certain whether nature behaves that way or not.

The response to the paradox of having no collapsing wavefunctions can be one of two, that such an effect either (a) is not be important or (b) is somehow prevented. I think that probably (okay, okay, this is a bit of a WAG, but I’ll try to back it up as well as I can), when it gets right down to it, (a) is the most appropriate explanation.

In order to have a measurement, we still have to have a mechanistic determination of one of the waveparticles’ observables. If your detector is built such that all that occurs is superposition of the waves and no other practicable measurement, then you really don’t have a detector at the slit. Measurement is not taking place, rather simply another type of operation (not necessarily Hermetian) is happening. To look at this another way, the screens in front of the slit are themselves detectors, they just aren’t confined in type to one slit or the other and as such can “see” the superposition of the measured waveparticles that go on at the slits without a detector. That is the only evidence we have for superposition going on at the slits! The screen itself forces an observable measurement of “yes” or “no” indications of where there is an event or not, but the interpretation of the pattern on the screen leads us to surmise the superposition that occurred at the double-slit. Now, truly, there is some process going on at the screen whereby the measurement is obtained that may or may not involve superpositions of the quantum particles that make up the screen. What prevents a superposition of the constituent waveparticles of the beams and those at the screen? In principle, again, nothing… only we don’t measure a “fuzzy” image of the particle at the screen, we measure an absolute image. It’s this “absolutism” that is the source of this collapsing wavefunction bit that is somewhat discomforting for those who are happy living in the world of mathematical formalism. The fact of the matter is, we can never directly observe a superposition of wavefunctions, but can only see one of the eigenstates. How does the waveparticle choose? Well, mathematically, of course, but nevertheless the particle chooses. If this happens at the screen, it must also happen when we place the detector at one of the slits. Thus my take on the solution to the quandary of the double-slit experiment.

Pointing a suggestive finger at superposition being something like the “true way” everything interacts might be a provacative way of throwing consciousness back in the mix, but it fundamentally fails to take into account what exactly the detector is doing; that is measuring an observable: a Hermetian operator.

For more I suggest:
Ed. J. Wheeler, W. Zurek, Quantum theory and measurement, Princeton University Press, 1983.

15

ready29003, as I alluded to above, detectors basically are changing the electrons behavior. That’s what the experiment shows and was proposed in the first place to show. There are many ways to think about this seemingly problematic way the world works. One of them is to say that there is no such thing as an objective observation (that you always somehow end up modifying the world when you try to observe it). This is disturbing to some in physics who really strive to understand the way the world is, not as they influence it to be. I would say that this is perhaps not the right way to look at the problem. That rather, there’s nothing to say that mathematical formalism that can derive observations is more objective than the actual measurements we take, it just makes things easier to understand. Take, by analogy, electromagnetism. Maxwell was absolutely convinced that these forces were results of fields and not particles. His belief led him to discover some very fundamental things about the way the world is. We now know that we can take electromagnetism into the realm of waveparticles (specifically photons) by using various principles in QED, but Maxwell’s fields often remain a better way of figuring out what is going on.

In effect, QM tells us that wavefunctions are a formalism for the way things exist. This is bizarre and weird, but the consequences of this theory are well-thought out (as off-the-wall as they seem) and in every case we’ve managed to dream up so far, the bizarre and weird consequences turn out to be what actually happens.

In terms of “web summary”, I’m afraid I’m not quite sure what you are looking for. There are plenty of websites (course lecture notes from Universities, book excerpts, etc.) available on the web that go into the various intricacies of the role of observation in quantum mechanics. A Google search will turn many of them up. Yes, they even go into detail about what happens if you don’t record the measurement or if you do. Of course, you have to be careful you don’t run into crackpots as the one discovered by CurtC, but basically the information is out there. (As a general but not universal guideline: the best sources of information will have .edu in the URL.) As I don’t know your level of expertise, I hesitate to refer you to some of the more technical papers on the subject, and I don’t know if you have access to them anyway as some are found in pay-for-access journals. Also, I’m pretty sure there are some threads on the SDMB that have dealt with the subject. Doing a search for them might be fruitful. All in all, I think that the answers you seek have been well-provided by Chronos and CalMeacham.

16

I really appreciate all these answers, but I cannot seem to get a definitive answer to this one part.

"Measurements, once made, can also be erased, altering the outcome of an experiment that has already occurred. "

This is one of the questions posed by one of the experiments in my opening post and CalMeacham said that no interference pattern will exist even if the detector’s data is destroyed. Yet I am reading lots of stuff that says measurment alone will not stop the interference pattern.

So again, apologizing for being ignorant and having not finished my research:
If you shoot electrons at two slits and you have a detector at each slit, recording data. But you erase the data before examining the pattern…will you or will you not get an interference pattern?

I am researching…but I keep reading stuff that says you will get an interference pattern if you delete the data, but you won’t if you read the data.

17

I don’t know what stuff you’re reading, ready29003, but it is wrong. It is just more of this magic of measurement nonsense that people like to equate with quantum mechanics. If you have something that is able to make the measurement and then you throw the measurement away then you have still, in effect, taken the measurement. It will not change the results. Anyone who says that if you have a device that gives you perfectly good measurements but then don’t read the measurements or delete the data and it causes the interference pattern to come back is just plain incorrect.

In any case, I did a Google search for you and the very first website I found was a fabulous discussion that explains how the detectors (the actual “observers”) are changing the the measurements. Without further ado, then here it is!

18

So after looking at JS Princeton’s link, I want to make sure I’ve got this right:

It is not that the observation removes the interference pattern.

It is the electron-photon collision that removes the interference pattern. However, if the results of the electron-photon collision carry enough information to theoretically determine which slit the electron passed through, then the collision will also remove the interference pattern. If the results of the collision (or if there is no collision) do not carry enough information to determine the path of the electron as being through one slit and not the other, then the interference pattern persists for that electron.

So it is the character of any collisions that cause either the interference pattern or no interference pattern. If the electron collides with a photon of sufficient wavelength that the electron could still have gone through either slit, then the results of the collision do not contain enough information to determine which slit the electron passed through, so the interference pattern will persist.

Now, in the other case, where the electron-photon collision is theoretically capable of carrying enough information to determine the path of the electron, a scientist might set up an experiment carefully enough to actually determine the path of the electron. But whether he does or not, there will be no interference pattern on the back screen.

19

The quote:

"Measurements, once made, can also be erased, altering the outcome of an experiment that has already occurred. "

is apparently from Scientific American Jul 1992. I am trying to verify that…

http://www.fortunecity.com/emachines/e11/86/qphil.html

20

Yeah, but why does measurement cause the interference pattern to vanish?

My brain hurts.