[dave_sullivan]

I don’t think this analogy holds up. Consider the double slit experiment: throw a bunch of basketballs at a wall and see what pattern of hits they leave by looking at where they hit the wall. If the wall is being looked at (observed), we see one pattern. If we look away, conduct the experiment, then check it, we find another.

To me that suggests the act of “observance” effects the probability distribution of likely states. If a tree falls in a forest and no one is around, then it doesn’t really fall, it just has a probability of having fallen that is not resolved until someone goes to check. How does your analogy account for those effects? For me, it looks like quantum collapse is causing the states of these objects to become “resolved” where at first they were “unresolved” and this suggests we live in a universe that knows how to save on memory and is fundamentally probabilistic.

[Tyr42]

If you ever ran into a space leak in Haskell, you would see how having unresolved thinks can use more memory than eager evaluation.

But that has some merit to it in that you can describe QC as merging equivalent paths and then sampling from a wave distribution afterwards.

One fun variant on the double slit experiment is taking a coherent laser beam (everything is in phase) and splitting it, sending it through two paths, A and B, then merging it and shining it on the wall.

If the two path lengths are equal, there is no effect from splitting it. But if we make B take slightly more time we can get a interference pattern. If we have it get shifted by half a wavelength the light will cancel out!

Now if you insert a polarizing filter along path B, when you merge the streams, you could tell with path the light came from, and the interference pattern disappears. This is not exactly measuring which path it took, but making it possible if you added a sensor to tell.

Observation is not required just making the streams distinguishable.

But now if we add another polarizing filter downstream we can erase the distinction between them, and now you get interference effects again!

[gus_massa]

Adding a polarizer is a nice variant of the experiment. I think I never heard it before. I like it, but I disagree with the expected result.

If the slit A has no polarizer and slit B has a polarizer, then in the "wall" you will the sum of 50% of the interference pattern and 50% of the diffraction pattern of A(I'm not sure about the 50%-50% split, something like that.) I.E. you will see the interference pattern, but it will not be so sharp, the black lines will not be so black, the white lines will not be so white.

I think it's better to put an horizontal polarizer on A and a vertical polarizer in B. If you don't add any other polarizer you will see the sum of the diffraction patters of A an B, without interference lines.

If you put a polarizer, the result depends on the direction:

* If it is horizontal you will see only the diffraction pattern of A (without interference lines).

* If it is vertical you will see only the diffraction pattern of B (without interference lines).

* At 45° you will see the diffraction pattern like in the original double slit experiment.

* At the other 45° you will see the inverted diffraction pattern, the black lines will be white and the white lines will be black. (All of this bounded by the diffraction pattern.)

* At other angles, you get some mix of the diffraction patterns and the interference patterns.

It would be nice to see an experimental realization of this.

[gus_massa]

There are two walls. One wall has two slits, the other wall is where the particles/waves/balls/whatever colide and form the interference pattern (or not).

You don't need someone observing the second wall to get the interference patters. You can replace the person with a photographic plate, a CCD sensor of a camera, or other equipment. All off them are more precise, reliable and even cheaper than a graduate student with paper and pencil.

The problem is if you try to add some type of equipment to first wall to collect information about how the particles/waves/balls/whatever passed thru it. Whatever equipment you add it will disturb the flow and it will kill the interference pattern.

This is not a technological problem. It is how the universe work. If you propose to use some particular method (like using light to detect the balls) you will sooner or later find that there is something that gets broken (see the former comment).

An important detail is that if you use a macroscopic object like a basketball, the slits size and the slits separation must be tiny (less than a millionth of the size of the nucleus of an atom, probably much less). So you intuition about how thinks work in the macroscopic level is not a good guide to how thinks work in the microscopic level. In the macroscopic level you can approximate the basketball as a perfect classic solid. It's just an approximation, a very good approximation.

[mercer]

> This is not a technological problem. It is how the universe work. If you propose to use some particular method (like using light to detect the balls) you will sooner or later find that there is something that gets broken (see the former comment).

what confuses me in various explanations like this is that the whole 'act of observing affects what you observe' thing seems to be rather particular in that it turns the wave-like behavior into particle-like behavior, which strikes me as rather weird/counter-intuitive. Why don't we just get slightly different interference patterns? Or some spectrum of effect between wave-like and particle-like?

Is my confusion mostly a result of the limits of the analogies presented to me as a layman?

[tylerjwilk00]

Quantum Eraser Experiment

> The double-slit quantum eraser experiment

https://en.m.wikipedia.org/wiki/Quantum_eraser_experiment

[Abishek_Muthian]

>Consider the double slit experiment: throw a bunch of basketballs at a wall and see what pattern of hits they leave by looking at where they hit the wall. If the wall is being looked at (observed), we see one pattern.

If the basketball was of energy 1 quantum, if the energy used to observe is 1 quantum or more the (shining light to see the result in realtime) then the pattern is different due to interference. If we don't use any energy to see the result in realtime, then result is different due to non-interference.

Did what I say hold up?

[dave_sullivan]

I could be wrong, but I have a different understanding on how all of that works. You keep talking about basketballs instead of waves or probability fields and I guess this is where we diverge.

[chriswarbo]

> Did what I say hold up?

No ;)

> If the basketball was of energy 1 quantum, if the energy used to observe is 1 quantum or more the (shining light to see the result in realtime)

How would we observe the light that bounced off the basket ball? Would we need to hit it with another light in order to detect where that light is? How would we detect that second particle of light; would we hit it with a third? And so on.

The answer is that we don't need to shine light to see the basketball. We can detect the basketball itself; for example, if the basketball were representing a photon of light, we could cover the wall with photomultiplier tubes ( https://en.wikipedia.org/wiki/Photomultiplier_tube )

As sibling comments have pointed out, the parent is wrong in saying that observing the wall will change the pattern. Rather, it's observing the slits that will change the pattern.

If we don't observe the slits, but we do mark the point on the wall where the basketball hits, and we do this over and over again, then the marks on the wall will show an interference pattern. Note that we're not throwing anything at the basketball: we're just waiting for it to hit the wall on its own. Also note that the marks themselves don't change anything; we could note them down on some paper instead, or type them into a spreadsheet, or whatever.

What if we do observe the slits, e.g. by putting a baseball in one and a cricket ball in the other? In this case, we'll detect the basketball hitting the wall and either a baseball or cricket ball. After many goes, the pattern on the wall made by the basketball will have two peaks (one in front of each slit), not an interference pattern. This seems analogous to your 'bounce a photon off it' explanation.

However, what if we got rid of the cricket ball? Half the time we would detect the baseball hitting the wall too, the other half we wouldn't (when the basketball went through the other slit). Yet the basketball will still make the two-peak-no-interference pattern, even though we didn't interact with it half of the time!

In fact, we could randomise which slit we put the baseball in, and mark only those goes that the basketball didn't hit the baseball, and we would still see two peaks without an interference pattern, even though those basketballs didn't hit anything (they always went through empty slits)!

This hopefully shows that your explanation (known as the observer effect) doesn't explain the interference pattern in the double-slit experiment.

[Abishek_Muthian]

Well written answer, thank you.