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Not sure if I framed below question properly, let me know if anything you want me to clarify in the comments.

Implications of double slit experiment show concious observation affect the 'reality as we know it' on bigger scale of Einsteins relativity. Consider below scanarios(Lets assum the detector of type magnetic needle detector)

Scenario 1. Detectors Off + Recording Off(No paper for recording, needle movement On) => Interference pattern

Scenario 2. Detectors On + Recording On => Expected two band pattern

Scenario 3. Detectors On + Recording Off(No paper for recording + Needle movement On) => Interference pattern

When we perform the experiment with the "detectors on" but the "recording off" shows the same results as if detectors are off.

Now when recording if off(no paper for recording) the needle still moves and has effect on correcponding air molecules come in contact with the needle and as per law of conservation of information, information can still be recovered.

If law of conservation of information is correct then we would have seen the same effect for Scenario 2 and Scenario 3. So do above result contraict the law of conservation of information?

====Update

If we say magnectic needle affecting air molecules cannot be recovered that means information is lost which is against conservation of information theory.

Also if system decides(whether to produce defraction pattern or two band pattern) based on our capabilities of recovering the information then how does system know our capability of what information can be recoverd by us ? If in future we become advanced enough to be able to recover that information will the scenario 3 produce defraction pattern ?

-- The question "The Double Slit Experiment - delayed measurement " does not deal with law of conservation of information which is specifically I am acking about.

Norbert Schuch
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Xinus
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  • If you are nit recording the result then you are not recording the result: if you do some trick as you describe in your penultimate paragraph which allows you to record the result then you are, in fact, recording the result. –  May 28 '17 at 09:02
  • But as per my understanding the experiment show the same result as if no recording is done when we do not feed the paper even though magnetic needle keep moving. – Xinus May 28 '17 at 09:06
  • If the result is not recorded you do not know what it is. Not recorded means, as I said, not recorded. –  May 28 '17 at 09:23
  • Even if we do not record nature records it in airmolecules it comes in contact with and which can be recovered if we study properties of air molecules before and after the experiment, right? – Xinus May 28 '17 at 09:27
  • @Xinus I don't know how to make this clearer: if you can recover the pattern by any means then you have recorded it. –  May 28 '17 at 10:51
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    @John,tfb: are you saying even if we are not collecting any data(i.e. Non conscious observation) in scenario 3 we will get two bands instead of interference pattern? – Xinus May 28 '17 at 11:15
  • Is the statement made at 2:50 in this video correct https://www.youtube.com/watch?v=LW6Mq352f0E? – Xinus May 28 '17 at 11:23
  • @tfb: If you are talking about magnectic needle affecting air molecules cannot be recovered that means information is lost which is against conservation of information theory. Also if system decides based on our capabilities of recovering the information then how does system know our capability of what information is recoverd by us ? Also if in future we become advanced enough to be able to recover that information will the scenario 3 produce defraction pattern – Xinus May 28 '17 at 14:24
  • The video doesn't cite any kind of reference, so I'm gonna go with "it's false". Also, if they weren't recording the results, how did they know which pattern they got? – Javier May 28 '17 at 14:38
  • @Javier: They are not recording the from which slit electron passed, they are recording the pattern all the time, did you checked the video at 2:50 time – Xinus May 28 '17 at 14:40

1 Answers1

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It sounds like the guy in the video may be paraphrasing an experiment which was out of his depth.

The real answer to this would be very dependent on the particulars of the detector and where they disconnected it. In the video, he claims they just didn't load a tape to record data on, but the results he claims they got don't line up with that.

In all detectors of quantum state, one basically ends up entangling the detector with the object under observation, and then operating on that state to extract information. If you extracted the information to record it, then the waveform collapses (or whatever interpretation you prefer) and you will not see an interference pattern because the photon/electron went through one side or the other.

They key to this is the waveform collapsing, or to think of it without relying on the interpretations: the state of the particle is entangled with a large number of particles whose state is randomly distributed. If this event occurs, you will not see interference. If you take the signal from your detector, run it through a classical amplifier, and send the results to a tape drive (with the tape not installed), you will still have collapsed the waveform. You just threw the data away.

Now if you instead switched off recording by switching the detector into some high impedence mode which permits the detector to couple with the subject, but doesn't attempt to do anything with that, then there will be no collapse. The detector will interact with the subject, but it will do so according to the laws of quantum physics, and no classical information will be recoverable (because you didn't energize the detector in a way which permits the capture of information as a classical signal). In such a case, you will indeed see the interference patterns.

However, the interference patterns will be a bit lossy in this case. Why? Because of exactly what you mentioned in the comments: unintended interactions such as air particles. While the photon and the "disabled" detector are interacting, there's a finite possibility that the detector will also interact with something else, like the air around it. This couples the detector to the air molecules around it, which were certainly in a random state before the interaction. This will shape the interference pattern because "the air" observed the state of the subject. It effectively recorded the information in a tiny air current too small for you to detect, but it was there.

In practice, we design our detectors to minimize this effect (because it gets in the way of gathering good data), so you won't see a large change in the interference pattern because of it. But, if you're curious what happens regarding things like air currents, that is the mechanic that will cause a change in your observed results.

My guess, given that the speaker is apparently a book author rather than a scientist, is that he heard about someone showing that they could put their detector in such a high-impedence mode, where the detector is there but it's not actually putting out signals, and misinterpreted it.

Cort Ammon
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  • In future if we developed instruments that can detect the small currents induced in the air which could reflect the electron moving from one slit or the other that would generate two band pattern? If that is the case is the system aware of what we are capable of ? – Xinus May 28 '17 at 16:35
  • You're thinking about it backwards, from what we can detect to what happens. If you think about it forwards, either the electron couples strongly with the detector, and the detector couples strongly with the air, in which case you will always see a 2 band pattern, or you see a weak coupling with the air (which is far more likely), in which case you will mostly see the interference pattern. There is a whole class of measurements known as "weak measurements" which use these sorts of properties to glean some information about the state of an electron but not all. – Cort Ammon May 28 '17 at 16:38
  • The articles you can read on weak measurements will point out that such measurements do not violate any conservation of information laws, they are just very creative in how they choose to pull out exactly the information they want as gently as they can. – Cort Ammon May 28 '17 at 16:39
  • As an extreme example, every time an EM wave is generated by the particle/detector and it escapes outwards into the universe (never detected), it act as a weak measurement in that it carries some information away, but because it moves at the speed of light, we'll never capture that information again. It is forever lost. – Cort Ammon May 28 '17 at 16:41
  • What if the 'read out' of the 'which slit' information is achieved by an 'interaction free measurement'? Will the interference pattern disappear? – XXDD May 30 '17 at 04:19
  • @X.Dong Does an 'interaction free measurement' device exist? When I looked up the term, I saw what I expected: they only seem to occur in thought experiments, and are always surrounded by frustrating paradoxes caused by their existence. The best I have seen for their existence is "weak measurements" which are a real thing (as in they have been studied in real life experiments), but they are never truly interaction free. Instinctively, such a device could easily violate the "no cloning" theorem. – Cort Ammon May 30 '17 at 04:23
  • Yes, there is no 'interaction free measurement', the interaction must be hidden somewhere in the system. As a famous example of such kind of device, the Vaidman bomb tester seems never been explained in a satisfactory way. At least not by its inventor Vaidman himself. He always mentioned the pre/post - selected route, trace, etc. He even try to explain it with a multi-world picture. That sounds strange. Do you know if there is a clear picture of Vaidman's bomb tester? Thanks. @Cort Ammon – XXDD May 30 '17 at 14:41
  • By the way, why the existence of an interaction free measurement leads to a cloning machine? I did not know that. I only see that it can achieve something like an interaction free teleportation as explained by Vaidman https://arxiv.org/abs/1605.02181. How to clone a state with it? thanks. @Cort Ammon – XXDD May 30 '17 at 14:48
  • @X.Dong I see. I overeestimated what they would call "interaction free." What "interaction free" really means is that there was an interaction in QM, but classically it appears there was no interaction at all. I was trying to figure out how to measure the state of a particle without an interaction at all, which doesn't work due to no-cloning. – Cort Ammon May 30 '17 at 15:02
  • OK. Yes there is no real interaction free measurement. It's just a quantum trick. @Cort Ammon – XXDD May 30 '17 at 15:07