# Double Slit

• B
Thanks for the answer! Yes you understand me correctly. They say that, if you stop recording the data but leave the detectors working, the interference is back, but this is unclear for me. You can say that you stop recording like switching of the recording computer, or stop recording like removing, or changing something in the system physically. So both things are different and looking the schematics of the quantum eraser experiment actually they mess around with the setup physically. So my question is how measuring with changing the experiment setup is equivalent to just observing and measuring. So how you can measure without intervening – as example you can use a cloud chamber and look the trajectories and whre the particles land, without messing with the setup – you just look literally with your eyes and record with a camera without any intervention. Maybe I miss something so I am asking. Thank you for your kindness!

Nugatory
Mentor
as example you can use a cloud chamber and look the trajectories and when the particles land without messing with the setup – you just look literally with your eyes and record with a camera without any intervention. Maybe I miss something so I am asking.
A cloud chamber intervenes enormously - every fraction of a millimeter or so the particle interacts with another droplet. It as if we had filled the entire chamber with tiny detectors always switched on.

So in a cloud chamber you do not get an interference pattern you are sure of that?

Nugatory
Mentor
So in a cloud chamber you do not get an interference pattern you are sure of that?
Yes. Any interaction with anything, whether you call the interaction an"observation" or not and whether you call the thing a "detector" or not, that allows you to assign to the particle a definite trajectory through one slit or the other will eliminate the interference pattern.

If you google for "Mott cloud chamber" you will find the detailed quantum mechanical explanation of the behavior of a particle in a cloud chamber, a problem that was first solved in 1929.

DrChinese
Gold Member
Yes you understand me correctly. They say that, if you stop recording the data but leave the detectors working, the interference is back, but this is unclear for me. ... So how you can measure without intervening – as example you can use a cloud chamber and look the trajectories and whre the particles land, without messing with the setup – you just look literally with your eyes and record with a camera without any intervention. Maybe I miss something so I am asking.
The recorder being on or off is irrelevant. If you looked with your eyes, you use light bouncing off a particle to see it.

The rule is: if it is possible to determine "which slit" information from the setup - regardless of whether you actually are aware of that information - there is NO interference.

That is why Nugatory said (correctly of course) that a cloud chamber would cause there to be NO interference. Note that you could gradually suck out the gas from the chamber until you had a near vacuum. That vacuum state would restore the interference. Anything in between a cloud and a vacuum would give you partial interference (a mixture).

So the consciousness is affecting the physical world and this is the reality, so how you can do physics – there is no point to measure anything if this is true :)

DrChinese
Gold Member
So the consciousness is affecting the physical world and this is the reality, so how you can do physics – there is no point to measure anything if this is true :)
No, we are saying the opposite: consciousness has no known role whatsoever.

Nugatory
Mentor
So the consciousness is affecting the physical world and this is the reality, so how you can do physics – there is no point to measure anything if this is true :)
You are misunderstanding something, as the conclusion you should be drawing from the above is that consciousness is completely irrelevant to whether an interference pattern forms or not.

Either there is a physical interaction that ensures that only paths through one slit can be active so there is no interference pattern; or there is not such an interaction so both paths are active and there is an interference pattern. Whether any conscious observer ever bothers to look at the experiment has nothing to do with the real fact that either there is an interference pattern or there isn't.

(One warning: Be aware that "can be active" above is a vague and dubious English-language description of what's going on. Precision comes from doing the math, and natural language is a poor substitute for mathematics.)

Thank you, but if this is true then the popular understanding "put detectors in the system and record data=no interference, then leave the detectors working but not record=interference" should be wrong, because you say "if there is any way to detect the path there is no interference". Thank you for the patience! As well what do you think about this:

and this:

Nugatory
Mentor
if this is true then the popular understanding "put detectors in the system and record data=no interference, then leave the detectors working but not record=interference" should be wrong
That popular understanding is wrong. It's one of those things that people who have never studied quantum mechanics have heard and pass on as a fact without checking first.
If you want to learn quantum mechanics, don't waste your time with them. Find a decent textbook that will give you the real thing. Be prepared to put some work into it - this is stuff that several generations of some of the smartest people who ever lived worked on for more than a century, so don't kid yourself that you'll get understanding from a few hours of reading fun stuff or watching cool videos.

If you lack the math background to take on an intro QM textbook (two years of calculus including multivariable and differential equations, and a smattering of linear algebra) you could give Giancarlo Ghirardi's book "Sneaking a look at God's cards" a try.

In fact the effect of magnetic & electric fields, radiation, gravity...etc on the two slit experiment for photons would be very easy to do.

Electrons however do couple to gravitational, electric & magnetic fields so those variables will impact on the interference pattern.

There are a lot of contradictions and the interpretations of the experiments are not clear. All experiments done are black box like for the people which are not involved directly.

[Mentor's note: Link to unsatisfactory video reference removed]

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DrChinese
Gold Member
There are a lot of contradictions and the interpretations of the experiments are not clear. All experiments done are black box like for the people which are not involved directly. So we have to accept that the reality is not objective. And as example the "put detectors in the system and record data=NO interference, then leave the detectors working but not record=interference" interpretation is from Jim Al-Khalili which is currently Professor of Theoretical Physics and Chair in the Public Engagement in Science at the University of Surrey
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Sorry, that statement is simply wrong. As mentioned previously, the recorder makes no difference. Consciousness makes no difference. You only need to turn on or off the recorder to see that it does nothing to the resulting pattern.

And your video reference falls far short of a suitable reference. You will need to post more substantive than that - a peer-reviewed paper would be appropriate to support assertions of non-standard science.

I do not make statements. I am just trying to filter the junk information with your help. So, I thank you! In fact your statement makes more sense to me. For me it means that there is a chance even a slight one, that the detection system is destroying the interference, which supports the objective reality instead the crazy consciousness-observer related paradoxes. Thank you one more time!

DrChinese
Gold Member
For me it means that there is a chance even a slight one, that the detection system is destroying the interference, which supports the objective reality instead the crazy consciousness-observer related paradoxes.
Whether there is objective reality or subjective reality is an open question in quantum physics. It is answered differently by each "interpretation" of QM.

Subjective reality - in contradiction to pop explanations - does not require a conscious observer. The "observer" is the context of the experimental setup itself, not a person (or recorder). To be specific: The EPR Paradox essentially points out that the reality of an object "here" is dependent on the nature of a measurement (observation) "there" (if you assume no action at a distance). This is a definition of subjective reality. With objective reality, each element of reality exists independently of the act of observation.

So removing consciousness from the equation does not change the debate between subjective vs objective realism in physics. There is no evidence at this time to distinguish these views as your interpretation guides your viewpoint. In Bohmian Mechanics, for example, there is objective realism.

Nugatory
Mentor
Further discussion should go into a new thread - this one has reached a natural stopping point.
It is now closed.