[Sylos]

> This is a common error. Macroscopic "everyday" objects don't have a definite position and momentum. Macroscopic objects are quantum objects. But when the mass is big enough, the position and momentum can be defined simultaneously with an error that is so small that you can just ignore the uncertainty and approximate them as classical objects.

To put this into simpler terms:

Whenever we measure something, we need to throw something at it and then have that something rebound and hit us again. In most experiments, we throw photons and have them rebound into our eyes.

Throwing a photon against a "classical object" - a chair, a ladder, bacteria - is like throwing a tennis ball against a skyscraper. You throwing that does not have no effect at all, but it's very much negligible.

But when trying to measure quanta, you're now throwing your tennis ball at a football, or at another tennis ball. You're gonna be lucky, if it rebounds at all, instead of just pushing the object that you're trying to measure out of the way. (You also don't have any smaller balls to throw.)

That's why when you measure something in quantum physics, you only know that it has this exact value in the moment that you measure it. It's going to be pushed away because you threw something at it, so after your measurement it has a different value.

You also can't observe it over a longer period, so there's no way to know whether it was only in that moment at your measured position or a long time beforehand.

[l33tman]

This is not a correct description at all of QM complementary observables. This is a purely classical explanation (and was one of the first layman "explanations" back in 1920, but that was 100 years ago and QM is much better understood now).

[mercer]

Could you elaborate on that? From my extremely limited knowledge it does seem like a just-so explanation (what you're responding to), but I'm not sure why.

[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]

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.

[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?

[david927]

That's a nice explanation but doesn't it give the impression that if we could find a better way to do that experiment, we could find a way around the problem, when instead it's a fundamental limit on what we can know about a quantum system?

[wodenokoto]

But isn't the reason why it is a fundamental problem, that fundamentally there is nothing smaller to throw?

[gus_massa]

I agree with both. This explanation is easier to understand, but it makes it look like a technological problem that can be solved, instead of a fundamental property of the universe.

[cygx]

The observer effect and the uncertainty principle are not the same thing.