Can anyone explain quantum uncertainty/probability?

I have read many times in many places, on the quantum level electrons and other things that small, don’t actually exist in any definite location until they are observed? Their location can only be described as a probability.

Now I understand Heisenbergs uncertainty principle, that you can never measure the exact location and momentum of anything that small, because anything that touched such things would immediately impact the location and momentum. But this doesn’t mean to me that an object such as an electron doesn’t exist in a particular place at a particular time.

I just don’t get the whole “observer” part of this.

Are there any experiments you can refer me to, that show such bizarre results that you need this “the act of observation changes the results” phenomena is evident?

Just because we can not know the exact location and momentum of an electron, doesn’t entail the electron has no definite location and momentum? Right? Please educate me. Or if you know of a particularly brilliant explanation for this somewhere please direct me.

I believe the experiment you’re looking for is the double-slit experiment. Search for that on google for a precise explanation. I don’t want to risk explaining it incorrectly, or I’d try.

No. The electron does NOT have a definite location and momentum in quantum mechanics. Here is an analogy:

Christina Aquilera (I’m trying to think of someone hip and hot from a younger generation) may go out with you. Then again, maybe not. However, she is not sitting there thinking “No way I’ll date that Beetle99”. She doesn’t know you exist. It’s not like she has already decided and you just don’t know her decision. You could walk up and ask her to date you, and you’d probably get an exact answer, but the answer doesn’t exist until the question has been formed. Until then, it doesn’t make sense to speculate what is in Christina’s mind.

Same with quantum mechanics. Let’s start with the particle-wave duality. You probably think of a particle as a thing with edges and a specific location in space. Like this: /__. A wave you think of as something like this: ////////////\ extending to infinity on both sides. Here is the corker: momentum is related to the wavelength of the wave function.

OK, suppose you have something like this: _____////_. It is not really a wave because it doesn’t extend to infinity in both directions. And it is not really a particle because it doesn’t have a precise location. But you could see how it has some concept of a wavelength and some concept of location. AHA! Uncertainty Principle: if the wave extends to infinity, then you know the wavelength (and thus the momentum) REALLY WELL, but of course the wave/particle is spread out to infinity, so not so localized. If you know the location really well (/_____), the wavelength/momentum is ill-defined.

So let’s say you want to take this _////_ and measure the location. You could send it through a slit and (I’m getting pretty unscientific here) and chop bits off: __/_ and voila, you have the location, but you’ve affected the wavelength/momentum.

So think of particles as their wave functions: sort of fuzzy location and sort of fuzzy wavelength/momentum.

This site has a pretty cool “demonstration” of the double slit experiment. Although you won’t get too many technical details.

Personally, I highly recommend The Dancing Wu Li Masters by Gary Zukav, which is perhaps a bit dated, but very good reading for quantum mechanic concepts.

It may not answer your question entirely, but it is particularly brilliant :

The Master at his very best.

I’ll have a go. At the very least, someone more knowledgable might come along and put me right if I’ve munged it.

As someone already mentioned, the famous “Young’s Slits” experiment is pretty weird (I think Feynmann described it as a phenomenon where, if you’re not blown away by the conclusion, you haven’t understood it).

Basically, imagine a source of (say) electrons. You can vary the intensity of the source down from bazillions of electrons a second to 1 single electron per hour. This source is pointed at a phosphorescent tube (like your TV set) but with a very long decay time (so that ‘hits’ on the tube stay lit for much longer than your TV). In between the source and the detector, you put a plate with 2 small, parallel slits in it.

Now if electrons were particles and you fired a whole bunch of them at the plate, you can imagine most of them hitting the plate, with some going through slit-A and some through slit-B. You’d probably expect the detector to show two blobs of hits, each opposite the corresponding slit.

What you actually see (and I emphasise that this has been proven by experiment, time and time again), is what’s called an interference pattern. This is analagous to two sources of waves in (for instance) water - you get peaks (where the high points of two waves intersect and you get a higher-than-average peak) and troughs (where the low points of two waves intersect and you get a lower-than-average trough). This process is called interference (because the waves ‘interfere’ with each other). You may also have heard the term constructive (peaks) and destructive (troughs) interference.

So, what you see on the detector is a series of light and dark lines, corresponding to the peaks (more electron ‘hits’) and troughs (less hits). This implies that, at least at some point, electrons are behaving as waves (particles can’t interfere with each other and produce interference).

So far, so good?

Well, here, it starts to get interesting.

Suppose you slow the rate of electron emission down. Let’s say you slow it down to 1 per hour (again, this has been done, it’s not idle conjecture). Clearly, there isn’t a stream of nearby electrons for our recently fired electron to interfere with (the last one was an hour ago), so we’d expect the electron - if it went through either slit at all - to go through one, and one only, and we’d end up with the two-blob result rather than the interference result.

Well, you don’t. You still get interference. This points to the (single) electron having passed through both slits - as a wave entity -, interfering with itself but making a single (particle) dot when it hits the detector.

It gets better.

Suppose now that you put a detector on one (or both, it doesn’t matter) of the slits, so that you can tell which one of them a certain electron passes through. The moment you do that, you lose the interference pattern and get the two-blobs result.

The explanation (as I understand it) is that the wave function collapses at the point of observance; that is, the detector has ‘forced’ the electron to decide what it is (and which slit to pass through). [sub]Actually, my understanding is that rather than force the electron to decide what it is, the detector forces the electron to behave in a manner decided by what the detector is looking for (which is even weirder). That is, if you set up a detector to detect particles, the electron will behave as a particle. If you’re looking for wave-like behaviour, it will behave as a wave).[/sub]

I hope I’ve got the basics right here (there’s a whole lot more, like delayed action observance where the detector is between the plate and the detector tube, but that’s probably enough for now).

Gah. I hope I’ve got at least some of this right. This stuff blows me away, I have to admit.

Karen Too, adding /_/_/_ and ///_///__ completes all the stitch patterns of my mom’s sewing machine… :slight_smile:

Even the double-slit experiment doesn’t get at the full extent of quantum weirdness. The reason physicists say the electron “doesn’t have” a position until you measure it (rather than “we don’t know”) is illustrated by the EPR Paradox.

I’m not going to try to explain it here. Try to find Speakable and Unspeakable in Quantum Mechanics by John S. Bell, and look at the article “Bertlmann’s Socks and the Nature of Reality” (I wish I’d come up with that title!) Another (maybe easier) book is Einstein’s Moon by F. David Peat.

Good luck!