# The Double-Slit Misconception?

1. Aug 14, 2010

### MikeFromOhio

Okay so we have a series of electrons going through the famous 2 slits and the wave-nature of the electron causes an interference pattern to emerge on the back plate.

So far so good. But now we put a detector behind slit-A and the interference pattern goes away. Now I have 2 questions.

1) If we put the detector behind slit-B, does a different pattern emerge on the back plate? How is it different?

2) IF the pattern goes away simply because the photon from the detector is interfering with the wave aspect, then that seems reasonable. But then why all the fuss about "Observation" and the possibility that a "human-witness needs to be involved to collapse the wave-function" etc. IS this all just a big Misconception?

-mike

2. Aug 14, 2010

### JesseM

Ideally it should be the same, since either way you know which slit any given particle went through (if it didn't set off the detector it must have gone through the other one). Real detectors probably aren't perfect at detecting every electron that passes near them so there might be some slight difference, I'm not sure.
It's a misconception that it needs to be a human witness (or any other conscious being), if a computer records the information and then it's erased before any human can see it, there still won't be an interference pattern in the total pattern of electrons on the screen. However, it is interesting that what seems to matter is whether information is recorded by the outside world about the electron's path, the interference pattern would disappear in exactly the same way regardless of how the information was recorded (for example, you could use many different types of detectors, or even just alternate which slit was open and which had a shutter over it so that the timing of when the electrons were detected on the screen would tell you which slit they went through). Likewise, if the electron were to interact with some other particle or system in a way which didn't provide any information about which slit it went through, then there should be no loss of the interference pattern.

3. Aug 14, 2010

### juzzy

Using high energy electrons for the experiment can cause the interference pattern to disappear because high energy electrons have a higher probability of emitting photons, thereby leaking more information into the electromagnetic field. In other words the em field measures the electron and the interference pattern is lost.

another way to put it is with schrodinger's cat. We dont need to look at the cat. We could just look at the vial of poison to ascertain whether the cat was dead or alive.

I think the confusion arises because we can say that if there is any way of knowing which slit it went through then there will be no interference pattern

The truly puzzling part of the experiment is that two holes doesn't mean double the probability of the particle hitting a particular point on the screen. By having two slits the probability (even for a single particle) is either x4 or x0?

4. Aug 14, 2010

### morrobay

5. Aug 14, 2010

### MikeFromOhio

Thanks for the replies. JesseM said:

>>what seems to matter is whether information is recorded by the outside world
>>about the electron's path, the interference pattern would disappear

What if the detector is partially disabled? It's photon emitting part still works but its recording part is missing and it cannot record information.

6. Aug 15, 2010

### alxm

Doesn't matter. What matters is whether or not an interaction with the environment-at-large occurred such that the property in question (in this case, which slit the particle passed through) could be determined, if only in principle. Whether or not it's done in practice doesn't matter.

7. Aug 15, 2010

### zara90

I read Brian Greene's FABRIC OF THE COSMOS and he explains variants of the double slit experiment in which detectors are placed over the slits. He describes the detectors as adding some specific spin to the electron that passes through either slit. Obviously, we know that if detected, the electrons simply pile up in rectangles behind each slit, and without detectors, form the interference pattern.

Now, the variant he mentions is one where the detectors indeed place a spin on the electrons passing through the slit, but then before the electrons have a chance to plop down on the screen behind the slits, some machine strips the electrons of the spin which identifies which slit they went through. According to some post above, it doesn't matter if the information is destroyed after, the interference pattern won't be recovered, but Greene's example is in direct contradiction to that. He says that destroying the information before the electron hits the screen will restore the interference pattern. This is really spooky to me. If the explanation is that photons required to observe and measure the electrons are interfering, then logically we can look at the experiment causally:

1) electrons are fired through the slits
2) detectors give us the which-path information
3) before the electrons can hit the screen and take a definite position, we strip them of the which-path information via a machine we can call an anti-detector
4) interference pattern re-emerges as the electrons finally hit the screen after being stripped of the which-path information

This seems to imply that consciousness has more to do with the equation than previously thought. If someone disagrees, I can find the book tomorrow and get an exact reproduction of his words, but I'm fairly certain this is what he reports. I'm troubled by this implication, I don't like the idea of consciousness being directly associated with the perceived reality of the physical world. I know the quantum world is very strange indeed, but how can something like conscious human mind affect quantum states is really beyond me. Probably because I know next to nothing about physics, and am only interested in quantum mechanics because for whatever reason, space, time and reality are extremely interesting to me.

8. Aug 15, 2010

### alxm

Which post? Not mine, at least. If you erase the information (and the experiment in question is called a 'quantum eraser') then you can't determine (in principle) which path the particle took at the time it hits the screen. (There are many threads on this, such as https://www.physicsforums.com/showthread.php?t=158413")

I can't for the life of me see how you jumped to that conclusion. In any case, it's just not true; all the delayed-choice quantum eraser demonstrates is how strange and subtle entanglement is.

Last edited by a moderator: Apr 25, 2017
9. Aug 15, 2010

### Cthugha

In the above and any other quantum eraser scenario, you do not destroy any information. You destroy the possibility to extract information. To get some kind of information you need to perform some measurement causing some irreversible interaction. In Greene's example you just perform some reversible interactions which determine, whether you would see an interference pattern if you introduced an irreversible interaction/performed a measurement at that moment. However, you just get the information at exactly that moment you perform the measurement. And once you did that and have really extracted some which-way information, the interference pattern will not be restored. If you do not perform a measurement and destroy the possibility to extract which-way information, you will see an interference pattern. There is obviously no consciousness involved. Irreversible interactions are enough to explain all the phenomena.

10. Aug 15, 2010

### MikeFromOhio

Alxm said:

>>If you erase the information then you can't determine (in principle) which path
>>the particle took at the time it hits the screen"

Notice the phrase "you can't determine" when talking about a system response.
What is doing this "determination"?

In other words, I am getting confused by many of the responses seeming to imply that the two-slit system itself seems to "know" if information is leaking about the electron. It's like we're giving the system some intelligence here.

1) However, it is interesting that what seems to matter is whether information is recorded by the outside world about the electron's path, the interference pattern would disappear in exactly the same way regardless of how the information was recorded...

2) Likewise, if the electron were to interact with some other particle or system in a way which didn't provide any information about which slit it went through, then there should be no loss of the interference pattern.

11. Aug 15, 2010

### Dickfore

Before moving on, I have 2 questions for you:

1. How do you detect the interference pattern when the electrons go through the 2 slits?
2. Where is this detector behind slit-A exactly?

12. Aug 15, 2010

### JesseM

In the delayed choice quantum eraser, no interference pattern will be seen in the total pattern of "signal" photons on the screen, regardless of whether you record or erase the "which-path information" from the "idler" photons. If you do choose to erase the information, you do it by forcing the idlers to go to a certain pair of detectors, and only by looking at the subset of signal photons whose idlers went to one of the two detectors (a 'coincident count' between signal photons and idlers at a particular detector) can you recover an interference pattern. There have been a few threads on the delayed choice quantum eraser (DCQE), see here for example. Here was my more detailed explanation from an earlier thread:

Even in the case of the normal delayed choice quantum eraser setup where the which-path information is erased, the total pattern of photons on the screen does not show any interference, it's only when you look at the subset of signal photons matched with idler photons that ended up in a particular detector that you see an interference pattern. For reference, look at the diagram of the setup in fig. 1 of this paper:

http://arxiv.org/abs/quant-ph/9903047

In this figure, pairs of entangled photons are emitted by one of two atoms at different positions, A and B. The signal photons move to the right on the diagram, and are detected at D0--you can think of the two atoms as corresponding to the two slits in the double-slit experiment, while D0 corresponds to the screen. Meanwhile, the idler photons move to the left on the diagram. If the idler is detected at D3, then you know that it came from atom A, and thus that the signal photon came from there also; so when you look at the subset of trials where the idler was detected at D3, you will not see any interference in the distribution of positions where the signal photon was detected at D0, just as you see no interference on the screen in the double-slit experiment when you measure which slit the particle went through. Likewise, if the idler is detected at D4, then you know both it and the signal photon came from atom B, and you won't see any interference in the signal photon's distribution. But if the idler is detected at either D1 or D2, then this is equally consistent with a path where it came from atom A and was reflected by the beam-splitter BSA or a path where it came from atom B and was reflected from beam-splitter BSB, thus you have no information about which atom the signal photon came from and will get interference in the signal photon's distribution, just like in the double-slit experiment when you don't measure which slit the particle came through. Note that if you removed the beam-splitters BSA and BSB you could guarantee that the idler would be detected at D3 or D4 and thus that the path of the signal photon would be known; likewise, if you replaced the beam-splitters BSA and BSB with mirrors, then you could guarantee that the idler would be detected at D1 or D2 and thus that the path of the signal photon would be unknown. By making the distances large enough you could even choose whether to make sure the idlers go to D3&D4 or to go to D1&D2 after you have already observed the position that the signal photon was detected, so in this sense you have the choice whether or not to retroactively "erase" your opportunity to know which atom the signal photon came from, after the signal photon's position has already been detected.

This confused me for a while since it seemed like this would imply your later choice determines whether or not you observe interference in the signal photons earlier, until I got into a discussion about it online and someone showed me the "trick". In the same paper, look at the graphs in Fig. 3 and Fig. 4, Fig. 3 showing the interference pattern in the signal photons in the subset of cases where the idler was detected at D1, and Fig. 4 showing the interference pattern in the signal photons in the subset of cases where the idler was detected at D2 (the two cases where the idler's 'which-path' information is lost). They do both show interference, but if you line the graphs up you see that the peaks of one interference pattern line up with the troughs of the other--so the "trick" here is that if you add the two patterns together, you get a non-interference pattern just like if the idlers had ended up at D3 or D4. This means that even if you did replace the beam-splitters BSA and BSB with mirrors, guaranteeing that the idlers would always be detected at D1 or D2 and that their which-path information would always be erased, you still wouldn't see any interference in the total pattern of the signal photons; only after the idlers have been detected at D1 or D2, and you look at the subset of signal photons whose corresponding idlers were detected at one or the other, do you see any kind of interference.