Can the naked eye change the outcome of the double slit experiment?

1. Nov 10, 2014

Thedavui

Are there any actual experiments that have been made where it has established if the naked eye can kill the interference pattern on the detector screen in the double slit experiment, that is, can the naked eye somehow function as a detector in this experiement? Let me give an example to clarify my point.

Consider one double slit experiment (E1) that results in an interference pattern on the detector screen behind. The experiment E1 has the following properties. E1(a): E1 is performed in an enclosed room with no communication being able to pass in or out. E1(b) There is only technical equipment in the room where the experiment E1 is conducted, no humans or other sentient beings. E1(c): E1 produces an interference pattern on the detector screen.

Now concider another double slit experiment E2. E2 is equivalent in E1 in every way, with 2 exceptions, there is a human in the same room looking at a certain aspect of the experiment and the experiment does not produce an interference pattern on the detector screen. So the conditions for E2 would be: E2(d): E2 is performed in an enclosed room with no communication being able to pass in or out. E2(e) There is only technical equipment in the room where the experiment E2 is conducted AND one human being looking at a certain aspect of the expriment, but not interfering in any other way. E2(f): E2 does NOT produce an interference pattern on the detector screen.

Is it possible to set up the 2 experiments E1 and E2 so that they completely comply with the conditions above? That is, can E1 and E2(d) & E2(e) be set up in a way so that the following implication is possible:

E1(a)=true & E1(b)=true & E1(c)=true & E2(d) =true & E2(e) =true ======> E2(f)

To summerize, my question is the following: Can the outcome of the double slit experiment (interference pattern or not) be changed by adding a human observer to the scenario whose vision is directed on a certain aspect of the experiment but not making any other changes in the experimental setup.

If you have the answer, kindly provide references to actual experiments that have been published and peer-reviewed.

2. Nov 10, 2014

Staff: Mentor

The presence or absence of a conscious observer is completely irrelevant to the outcome of any quantum mechanical experiment. There have been many previous threads on this topic.

3. Nov 10, 2014

Staff: Mentor

I can only reiterate the very important point.

Conciousness has nothing to do with it except in some very backwater and way out of favour interpretations. It was originally proposed by the great mathematical physicist John Von Neumann for reasons that are no longer relevant.

QM is a theory about observations that appear here in a common-sense classical world that exists independent of us. The real issue is how QM explains that world. Much progress has been made - but a few issues remain without going into what they are. 100% for sure though its very unlikely conciousness has nothing to do with it.

Thanks
Bill

Last edited: Nov 11, 2014
4. Nov 11, 2014

Thedavui

Thank you for your answer. I agree that consciousness per se is irrelevant for the outcome and apologize that my question wasn't clear enough on this point. What matters is the primary detection generating which-slit information, not if this information is made available to a conscious observer or not. So everything related to consiousness should be removed from my original question, e.g. remove "other sentient beings " from E1(b). What is important, and the aspect I want to focus on, is the function of and detection made by of the human eye, not human consiousness (I apologize that my question wasn't clear enough on this point). That being said, let me clarify my point further.

In an article by Philip Gibbs from 1996 it is concluded that under low-light conditions (scotopic conditions) , rods (a type of receptors on the retina of the eye) are able to respond to single photons. I quote:
So if the article is correct and the eye is indeed able to respond to single photons, wouldn't that mean that the human eye would be able to make the which-slit detection in a perfectly dark room (as said above, I agree that it isn't relevant whether the consiousness is involved or not. For the exepriment we could imagine a perfect model of the human eye with all receptors perfectly replicated without any consciousness involved).

Finally I would like to take this argument a step further. Suppose that we, in our example, replace the human (or perfect model of a human eye) with a cat's eye with all receptors intact (let's assume that we are using one of the eyes from a cat that has been killed in a previous quantum experiment (see Schrödinger's[/PLAIN] [Broken] cat). Would it be possible to position this eye in a way to detect which slit each photon passes through and make the interference pattern go away?

To summerize the modified question.

Can the outcome of the double slit experiment (interference pattern or not) be changed by adding a perfect model of the human eye (alternatively, a disected eye from a cat killed in a previous quantum experiment with all receptors intact) to the scenario, directed on a certain aspect of the experiment but not making any other changes in the experimental setup?

Last edited by a moderator: May 7, 2017
5. Nov 11, 2014

Staff: Mentor

No. Its got nothing to do with it.

The pattern you get is the same regardless of if you observe it with an eye or develop a photographic plate later - exactly as you would expect with photographing anything instead of viewing it directly.

It must be stressed QM is a theory about observations that appear in a common-sense classical world. Common-sense means it makes no difference whether its observed or not, photographed or not, recorded on a computer and the data examined 100 years later or not. Everything you assume about the world from everyday experience applies to the world the observations appear in.

Thanks
Bill

6. Nov 11, 2014

Thedavui

Thanks for your reply. Please note! The imagined perfect model of the human eye would only be watching the 2 slits, not the detector screen behind where the interference pattern may or may not emerge.

So if I understand correctly one CANNOT make the interference pattern go away by placing an eyeball, camera or telescope etc focused on one of the slits to see if the photon passes through. Lenses like these can never consitute which-slit detectors, no matter how powerful, and can never make the interference pattern disappear. Is this correct?

There are however (as I gather), various measuring devices that successfully CAN be used to accomplish this (collapsing the wave function): polarized filters on the slits (perpendicular to each other) would be one. A photon detector in combination with some kind of photo amplifier another, somehow being able to detect the photon's energy as it pass through one of the slits, thus being able to determine which slit. Please correct me if I am wrong.

So my conclusion is that you would need some indirect way of determining which path (such as polarized filters in the case of photons making passage through both slits impossible if they are perpendicular in relation to each other) or a particle detector detecting the electron or photon's energy passing one of the slits. If no detection is made, the electron will exhibit wave properties and generate the interference pattern. If which-slit detection is made: no interference pattern. Lenses or films (eyball, camera, telescope) can NOT be detectors in this sense, because the are not able to gather information which slit the electron or photon passes through, hence cannot produce any which-slit information. Is this conclusion right?

It is possible to apply which-slit detectors to make the interference pattern disappear as I believe everybody agrees on. But optical instruments or lenses from the biological organisms' eyes lack the necessary properties to function as such.

Feel free to correct or comment whatever in this post isn't accurate.

Thanks,

7. Nov 11, 2014

naima

If you add an eye behind the slits you will have two screens and if the photon hits the retina it will not hit the first screen. The other ones will build the unchanged pattern on the screen.
You could wonder if an interference pattern appears on the retina.

Things are different when what passes thru the slits are not photons but particles.
if they can emit photons the pattern will be changed (with or without an eye).
And if you look at them you will be able to get information (maybe imperfect) on the path. but your eye is just a witness.

Last edited: Nov 11, 2014
8. Nov 11, 2014

Staff: Mentor

What do you mean by focussed on one of the slits? This usually means shooting some light at it and seeing what reflects. Exactly how do you observe light by shooting light at it?

Thanks
Bill

9. Nov 11, 2014

DrChinese

No. The eye is a photon detector, true enough, but that will not in and of itself allow interference to be destroyed unless it is simply blocking a slit (which does not require detection).

Also, please note bhobba's question in post #8.

10. Nov 11, 2014

Thedavui

Thanks. I agree. A camera, eyeball etc focused this way would not be able to detect or observe the photon for the reason you stated. As I understand, one needs some other way of checking the photon's path such as polarized films on each slit (90 degree difference in angle between each film) when firing one photon at a time with random polarization for each photon. That would make it impossible for a photon to pass both slits and interfere with itself. Or one would need to use some type of particle detector, being able to track the photon's enegry or the electrical field of an electron triggered by the photon. Such an approach or detector would also be able cancel the interference pattern. However, I doubt that my technical understanding of particle detectors is correct. But I gather that the primary which-slit detection (collapsing the wave function) cannot be optically based, it is rather about checking energies or electrical fields in close proximity to one of the slits in order to figure out which path the particle (photon or electron) has taken, thus being able to make the interference pattern on the back screen disappear.

11. Nov 11, 2014

Staff: Mentor

It cannot be optically based, because optics is the science of the collective statistical behavior of very large numbers of photons so is irrelevant to the observation of single photons - it would be like trying to use a barometer to track the motion of a single molecule in the earth's atmosphere.

I think you still may be tripping over that word "detector". A "detection" is any interaction between the quantum object and a macroscopic system which causes the macroscopic system to irreversibly change state. It might have been better if the words "observation" and "detection" had never made it into common usage in QM, but for historical reasons they're here to stay - the pioneers were of course particularly interested in the interactions that happened in their pieces of lab equipment called "detectors", and it seemed natural to call these "detections".

12. Nov 11, 2014

Thedavui

Thanks, I agree. I believe that some of the confusion originates from the term "Observation" in reference to which slit a particle has chosen. I am not saying that the term is wrong, on the contrary, it seems adequate. But it can easily be associated with visual observation, and apperantly, in quantum mechanics, the primary observation (collapsing the wave function) can never be visual or optical. It must be either a detection based on electical fields / energies in the proximity to one of the slits (by means of a particle detector) or a purely logical observation and implicit conclusion which way the particle must have taken based on the whole setup and result of the experiment (there might be other alternatives as well that aren't mentioned here). But it helps my understanding of the whole subject to think about a non-optical, non-visual and non human consciousness related observation with respect to the collapsing of the quantum wave function. It is either a purely logical which-path deduction or a non-visual, non-optical detection that triggers the collapse. But many online sources such as the Dr Quantum video are adding to the confusion. In this video (4.9), it is said that "they decided to peek" and the measuring device in 4.14 looks very much like an eye. The whole presentation gives the impresion that the detection is visually/optically based whereas in actuality, a primary quantum which-slit detection can never be that.

Last edited: Nov 11, 2014
13. Nov 11, 2014

Staff: Mentor

Your reasoning for such a statement escapes me.

Of course it can be - it happens all around us all the time.

What you cant do is observe a photon going through a hole by other photons because photons interact only very weakly - but things like dust particles for example are another matter. In fact a few stray photons from the cosmic microwave background radiation is enough to decohere a dust particle and give it a definite position. The answer to Einstein's question of if the moon is there when you are not looking is that the moon is always being observed by the environment.

You seem to be tied in knots about what an observation is. There is nothing hard about it (except at a deep level where you want to define it Quantum Mechanically - but that's another story). Its the simple common-sense idea we all have. To observe a photon going through a slit you, for example, put some kind of device like a photo multiplier in there and see if it flashes - that sort of thing ie you interact with it in some way. But once you do that the interference pattern disappears.

Technically what happens is when it interacts with whatever you are observing it with you get decoherence:
http://www.ipod.org.uk/reality/reality_decoherence.asp [Broken]

In fact that's responsible for a very weird effect called the quantum eraser:
http://en.wikipedia.org/wiki/Quantum_eraser_experiment

The reason is in simple cases decoherence can be reversed ie you can 'unobserve' something. Weird hey. And just as an aside decoherence is what you use to define an observation quantum mechanically - but that's just by the by.

BTW don't feel bad if this isn't gelling - it does take a while to get used to quantum weirdness.

Thanks
Bill

Last edited by a moderator: May 7, 2017
14. Nov 11, 2014

Thedavui

Just to be clear, with "observation" I am only referring to the primary which-slit observation / detection in the double slit experiment, not observation /detection in general. With "primary" I mean that by just removing this which-slit observation / detection from the experimental setup, the interference pattern will reemerge on the back screen when fireing particles one at a time. On the other hand, with this which-slit observation /detection present, there will be no interference pattern. So I am using the expression "primary observation" in this very narrow sense as a type of measurement whose presence determines whether there will be an interference pattern or not on the back screen. Please limit the discussion to this double-slit experiment setup as I am not referring to observation or detection in any other sense. In this sense, introducing a primary which-slit observation in the experiment that is visual or optical (such as an eye, a telescope, a camera etc) without making any other modifications to the experiment can never cause an interference pattern on the back screen to disappear if there was one before.

If you still disagree despite this attempt to clarification, please present a reference to an actual experiment that has been published and peer reviewed demonstrating how a visual or optical which-slit observation /detection alone can cause the quantum wave function to collapse (i.e. destroy the interference pattern on the back screen). Unless provided with actual proof I cannot believe it.

You may also check the post by Nugatory (post #11) for further explanations why the which-slit observation / detection cannot be optically based.

Last edited: Nov 11, 2014
15. Nov 11, 2014

Staff: Mentor

I must confess that I am at a loss to understand your point then. You started by asking whether a naked eye can change the outcome of the double slit experiment. The answer to that question is that it will if it interacts with the photon in a way that allows us to localize the photon, and not otherwise. In the double-slit experiment the only way I can think of to produce such an interaction is to use the eye to block one of the slits, and for that purpose I could as easily use a chunk of ground meat instead of an eye (and for hygienic reasons I would prefer a piece of metal foil over either of of these).

This has nothing to do with the eye's ability to detect some frequencies of light in other situations.

16. Nov 11, 2014

naima

You do not seem to be interested in the concept of observation.
Your definition of "primary observation" as something that make interferences reappear when it is removed is obviously insufficient. Even if you add that they disappear when there is an observation.

17. Nov 11, 2014

Thedavui

From what I am reading in this and other posts, I don't see that there is any critical disagreement on the subject matter. I am not referring to other situations at all, only the double slit experiment. And I am not trying to make any "point". I am trying to get a better understanding of this experiment and that is the reason I started this thread. As far as I can see there is consensus between us, and it might be a good idea to close the thread (except I don't know how). I feel I got a better understanding of it now, partly thanks to you Nugatory, thank you very much! A lot of information on the Internet on this subject is very misleading, contradictory and confusing and this discussion has actually clarified the matter a great deal. But as I said, I don't see the point of continuing the discussion, simply because there is consensus as far as I am concerned and therefore nothing further to discuss. My questions have been answered. If there is still disagreement about details but not about important aspects I think we can leave them. So, as I have said, I want to thank everybody for helping me out to better understand this matter. /Thedavui

18. Nov 11, 2014

Staff: Mentor

That's why we're here!

Glad to have been able to help.