Double slit experiment: two persons are checking the experiment

[GiuseppeP]

Hello,

in the double slit experiment, what will be the result if two persons are checking the experiment using the same equipment to perform the experiment, but one person will take note from which slit is passing the electron while the other is not checking. Will they see the same thing?

Thanks,

Giuseppe.

 

[newjerseyrunner]

Yes, in order for anyone to see it, it must have an interaction. The photon will behave like a particle if anyone knows which path it took.

 

[Nugatory]

in the double slit experiment, what will be the result if two persons are checking the experiment using the same equipment to perform the experiment, but one person will take note from which slit is passing the electron while the other is not checking. Will they see the same thing?

It makes no difference whether a person looks at the result or not. If the equipment is set up in such a way that someone could use the measurements results to determine which slit the particle went through, there will be no interference pattern. If the equipment is set up so that there is no way of determining which slit the particle went through, then there will be an interference pattern.

 

[vanhees71]

One should also stress that a photon only in a very vague sense behaves like a "particle". It doesn't even have a position observable in the strict sense. Almost always the field picture is a better classical analogy to a photon, although of course there are important differences either. In other words, it is impossible to describe a photon accurately otherwise than using the formalism of relativistic quantum field theory (or more specifically QED in this case).

 

[GiuseppeP]

Hello,

thank all of you for the answers. For me the outcomes of this experiment are still difficult to digest.

 

[Nugatory]

For me the outcomes of this experiment are still difficult to digest.

Fair enough...

And as you trying to digest it, be sure that you're starting with a scientifically accurate presentation. A first-year QM textbook is good, or the more layman-friendly "Sneaking a look at god's cards" by Giancarlo Girardi. Be very skeptical of the popularizations (and especially anything that goes on about involving a conscious observer).

 

[vanhees71]

Well, the less math a physics books contains the more suspicious you should be ;-).

 

[GiuseppeP]

Hello,

The math part of QM theory is not a problem. But on the other hand the part of this problem is model we are creating trying to describe the phenomena and we hope to make prediction from it. The problem i have is that some how here and i haven't seen a clear explanaction that i have just to accept, is that the interference pattern is seen or not seen depending if we know ( or we don't know) from which slit the photon is passing. In the first place i thougth maybe the sensor that is sensing the passing of the photon maybe with its interaction is destroing the interference, but only recently i learned that even leaving the sensor ON , but just not collecting the data the interference will be created. So the fact that we have (or not have) the information coming from the sensor will change the result of the experiment. This is what it is difficult to understand for me. If there is something i miss-understood please let me know. Thanks

 

[Mentz114]

Hello,
..
..
In the first place i thougth maybe the sensor that is sensing the passing of the photon maybe with its interaction is destroying the interference, but only recently i learned that even leaving the sensor ON , but just not collecting the data the interference will be created. So the fact that we have (or not have) the information coming from the sensor will change the result of the experiment. This is what it is difficult to understand for me. If there is something i miss-understood please let me know. Thanks

What you thought (bold) is correct. The part that follows is wrong.

See this article https://en.wikipedia.org/wiki/Observer_effect_(physics)

This thread is also good

 

[GiuseppeP]

Hello,

ok, it is better to clarify this point . What you are saying that Tom Campbell () and and Jim Al-Khalili () is not correct? Thank you.

 

[Mentz114]

Hello,

ok, it is better to clarify this point . What you are saying that Tom Campbell [ media snipped] is not correct? Thank you.

I don't know what they are saying. Maybe you have misinterpreted what they are saying.

From the article

According to standard quantum mechanics, however, it is a matter of complete indifference whether the experimenters stay around to watch their experiment, or leave the room and delegate observing to an inanimate apparatus, instead, which amplifies the microscopic events to macroscopic[3] measurements and records them by a time-irreversible process.[4] The measured state is not interfering with the states excluded by the measurement. As Richard Feynman put it: "Nature does not know what you are looking at, and she behaves the way she is going to behave whether you bother to take down the data or not."https://en.wikipedia.org/wiki/Observer_effect_(physics)#cite_note-5

[URL='https://www.physicsforums.com/threads/quantum-physics-double-slit-delayed-observation.890131/#post-5599733https://www.physicsforums.com/threads/quantum-physics-double-slit-delayed-observation.890131/#post-5599733']Did you look at this thread ?

 

[Simon Phoenix]

But on the other hand the part of this problem is model we are creating trying to describe the phenomena and we hope to make prediction from it

You've hit the quantum nail on its rather uncertain head here.

There simply aren't any 'classical' pictures that work here - by a classical picture I mean something that is built up from our usual everyday ideas about how things work - so trying to think in terms of particles or waves, for examples.

I find the beamsplitter version of this experiment a bit cleaner in terms of pointing out the conceptual difficulties. Imagine we had a source that produced a single photon every second. We can test that we have a single photon by doing a single-photon detection (OK that's not a foolproof test because we can explain single-photon detection without ever requiring photons!). But let's not worry too much what the photon is - it would seem to be a particle, or possibly some localized wavepacket that kind of looks like a particle - so let's run with that.

Let's now see what happens at a 50:50 beamsplitter - which is just a device for, erm, splitting beams so that if we had a nice classical beam of light we'd get half the light going into one output arm (call this arm 1) and half the light going into the other output arm (call this arm 2).

What happens with our single-photon source if we point that at the beamsplitter? Well we find that if we put detectors in the output arms 1 and 2 we see that only one detector clicks, never both (we would say there is a zero coincidence count). And we've assumed we have perfect ideal detectors - which is usually far from the case. So it looks like this photon, which we imagine to be a particle, or wavepacket, or some blob of energy either goes one way or the other. Furthermore, which of the detectors fires seems to be entirely random.

That's cool, because it explains how a 'classical' beam can be thought of as containing zillions of these blobs so that roughly half of the blobs go one way and half go the other way - which gives us our half/half picture for a classical beam.

So we have a source that's producing blobs of energy and we can't split these blobs so that we get half a blob going one way and half the other way. All of the incident energy, per timeslot or second, appears at either detector 1 OR detector 2. These detectors might conceivably be many kilometres apart.

At this stage we're now entirely justified in thinking of indivisible blobs of energy going one way or another - it's hard to think of any other picture that would explain the observations (zero coincidence count, all the energy at either detector 1 or detector 2). If we tried to say that this blob is split or that a 'wave' goes along both arms then it's hard to see how all of the energy could suddenly appear at one detector at random. I don't know of any way of explaining this with just standard classical fields and quantum detectors.

OK that's experiment 1. Now we take the output arms from this beamsplitter (BS1) and we use them as the input arms to another 50:50 beamsplitter (BS2). We'd have to use mirrors to steer everything towards this second beamsplitter. Now we put detectors in the output arms of this second beamsplitter (let's call them arms 3 and 4). We call this new experiment, experiment 2.

What do we predict? Well based on experiment 1 we think that there's a blob of energy that 'makes' a random choice of which output arm (1 or 2) to go on. So that means when we run experiment 2 what we have is one of the blobs in either output arm 1 (which becomes input 1 to the second beamsplitter) or output arm 2 (which becomes input 2 to the second beamsplitter).

But based on experiment 1 if we have a single blob incident on our second beasmplitter BS2 then it has to emerge in one of the output arms 3 or 4 at random. So our 'blob' picture tells us that we should see the detectors in arm 3 and 4 fire randomly as before.

That's not what happens. If we adjust the path lengths correctly then what we see is that the detector in arm 3 fires, but the detector in arm 4 never fires. If we really had just a single blob going one way or the other this could not happen - it's inconsistent with our explanation for experiment 1. So, we conclude, something must actually be going on both output arms and when we bring the output arms from BS1 together as the inputs to BS2 we get an interference which is responsible for making detector 3 (and only detector 3) fire.

But if 'something' is going on both output arms of BS1 how do we now account for the properties of experiment 1? It's as if all of the energy goes one way or the other but some 'influence' which carries zero energy proceeds along both arms.

Furthermore, one might ask how the incident thing to BS1 (blob or wave or whatever it is) 'knows' it's going to be subject to experiment 1 or experiment 2 - in one case it has a 'natural' explanation in terms of blobs (expt 1) and in the other a 'natural' explanation in terms of waves (expt 2); but neither of these 'natural' explanations works for both experiments.

I can't see any 'natural' way of explaining this - other than recourse to the axioms of QM - and different people have different ideas about how 'natural' they are. The failure here is one of conceptualization. It's impossible to create a consistent classical picture (blobs or waves, for example) that will work for both experiment 1 and experiment 2.

So if by 'clear' explanation, you mean "without recourse to the notions of QM", then you won't find one. At some point we just have to accept that QM is the way the world works, however batshit insane it appears to us (and again, different people ascribe different levels of insanity to QM).

 

[Watson768]

Hi,

OK that's experiment 1. Now we take the output arms from this beamsplitter (BS1) and we use them as the input arms to another 50:50 beamsplitter (BS2). We'd have to use mirrors to steer everything towards this second beamsplitter. Now we put detectors in the output arms of this second beamsplitter (let's call them arms 3 and 4). We call this new experiment, experiment 2.

Just wondering if the above had been published somewhere and please let me know the reference for these experiments?

Thanks!

 

[Nugatory]

Hi,
Just wondering if the above had been published somewhere and please let me know the reference for these experiments?

Google for "Mach-Zender interferometer single photon" - that will get you started.

[Edit; But as @jtbell points out, "Mach-Zehnder interferometer single photon" would be a better search term]

 

[jtbell]

Ah, I see Google is clever enough to correct the spelling to "Mach-Zehnder".

 

[Nugatory]

Ah, I see Google is clever enough to correct the spelling to "Mach-Zehnder".

Hah - didn't even notice that - Thx