Uncertainty?

[peter]

Is that when in quantum physics you can't tell both the position of a sub-atomic particle and it's velocity, you can only know one or the other. Or is it something else?

If it is this then [probably exhibiting my 'dickness' for all to see but] I can't see what the big deal is. Consider a car moving down a road. I can tell you what speed it is going, but never its position - it has no position - it's moving! If I take a film of it with a clock running in the corner and I slow it down frame by frame untill the car is motionless, then the clock will be motionless as well. Now I can say what it's position is at a given time - but I can't tell you what it's velocity was untill I flick forward a frame, get a new position and a new time and do the sums based on those two readings. I can never know the cars position and velocity simultaniously; It's a logical impossibility! Why should an electron be any different!

[Is the uncertainty principle tied up intrinsically in the 'Copenhagen Explanation' and wtf is that anyway - also what other explanations are there and what are they all explaining anyway? Damned if I know! Can anyone make it all simple ]

(Answer came there aside, "Not simple enough!)

[Avatar]

Yes, the uncertainty principle forms part of the Copenhagen interpretation. Or at least, the CI assumes it is true. Heisenberg was Bohr's assistant and their work on the CI contributed to it.

Their is actually no "Copenhagen Interpretation" in text, and it was never really formalised into a paper or anything. Heisenberg only really started referring to it as a specific thing when he was criticising other people's work 20 odd years later.

--A

[peter]

Guess I've got some homework to do in this area Av!

[wayfriend]

I think the gist is that you cannot observe both the position and the velocity. Which leaves you free to know them through other means, technically.

And it's because the act of observing one of them changes the other one. Which is an issue only with sub-atomic particles, where the interaction of the observer produces a relatively large effect relative to the object itself.

So cars are right out.

Am I wrong?

[Hashi Lebwohl]

You are correct. At the quantum level we are not able to measure both position and velocity at the same time. At the macro level this limitation does not exist because the wavelike nature of macro is so very small.

[Vraith]

So cars are right out.

Am I wrong?

No, I think you're right on.
The car's velocity and position can be easily measured at the same time to within a truly tiny margin of accuracy and equally tiny effect on either measurement by the observation.
[[that's how cops can use radar guns to know you're speeding, and know where you are so they can give you a ticket for it...and the fact we can do so is why you can't go to court and cite the Uncertainty Principle to get out of paying the fine. You'll just annoy the judge. Bad idea.]]

Those tiny amounts matter when the thing being looked at are tiny, though.
Kinda like if you could only measure the car's position was by tossing walls in front of it...the crash destroying the velocity. [not totally accurate analogy, but close enough for some thought work.]

[Zarathustra]

It's not an issue with observation. It's part of the quantum nature of reality:

Historically, the uncertainty principle has been confused[6][7] with a somewhat similar effect in physics, called the observer effect, which notes that measurements of certain systems cannot be made without affecting the systems. Heisenberg offered such an observer effect at the quantum level (see below) as a physical "explanation" of quantum uncertainty.[8] It has since become clear, however, that the uncertainty principle is inherent in the properties of all wave-like systems,[9] and that it arises in quantum mechanics simply due to the matter wave nature of all quantum objects. Thus, the uncertainty principle actually states a fundamental property of quantum systems, and is not a statement about the observational success of current technology.[10] It must be emphasized that measurement does not mean only a process in which a physicist-observer takes part, but rather any interaction between classical and quantum objects regardless of any observer.[1]

https://en.wikipedia.org/wiki/Uncertainty_principle

[Vraith]

It's not an issue with observation. It's part of the quantum nature of reality:

Our understanding of the how/why has surely evolved...but there is an observational problem/difference between the car and an electron.
A gap/separation that we haven't yet bridged.

But I'm almost positive Heis. knew it was more fundamental than just a technology problem---that he thought it impossible even in theory/principle, no matter the tools...even if he, and others, were wary of saying some things were really real. [[some damn smart and respected peeps are STILL unhappy with some things being real...but both the math and experiments keep further limiting the options]].

I think not too long ago, somewhere around the 'Raat, I posted a link to an article related to what you say---a math/information paper showing duality/wave function and uncertainty are different manifestations of the same thing.

[peter]

Mmmm. Still not getting it; It seems to me that the problem of introducing only a tiny innacuracy at the scale of a car and a huge one at the scale of an electron [sufficient to render the result meaningless in the latter case] is still in effect the same problem - scale being the significant word. Am I to understand that there is no qualitative[?] difference between what occurs at the quantum level and at the macro?

[Z.s point not withstanding that it is not a 'observational issue' but relates rather to a fundamental feature of the quantum world - that being the case, what is the 'qualitative' nature of this difference?]

[Zarathustra]

Peter, it's the quantum wave function, which describes the probability that a particle will be detected in any particular place. Uncertainty is built into the system because every particle is described by this wave of probability. We can 'cheat' a little bit by reducing the uncertainty through measurement, but certain pairs of attributes are intrinsically tied (conjugate pairs), so that reducing the uncertainty of one increases it for the other. But the uncertainty isn't created by the measurement, it was already there. It's like a balloon that you squeeze, making one side bigger. You don't inherently change the balloon's nature by squeezing it, you just reduce its shape in one portion to a smaller size. And that increases the opposite side.

There are many different ways to think of this. Deutsch says that everything is actually deterministic--no probabilistic/wavelike existence for particles--but rather we're seeing the direct effects of multiverse versions of these particles as they slip in and out of the various universes (hence the uncertainty ... we can't view into other universes). This is the Many Worlds interpretation. But others think of matter as existing as a wave of probability prior to measurement, and then this wave is collapsed into something like a particle upon measurement. For conjugate pairs of attributes, this would increase the uncertainty in the probability wave for the 'other side' of that pair. So the attributes of particles are themselves things that can only be 'collapsed' partially, I suppose, and this collapsing has an inverse effect on the other side of that conjugate attribute.

But no conscious observation need be made. Merely interacting with other particles can collapse the wave function. For very large objects (i.e. 'macro' scale we deal with daily), they are made up of lots of particles which are all interacting together, therefore they've collapsed their wave functions, so they behave like particles. Their uncertainties are much smaller than the scale of the object, so they can be ignored.

[Hashi Lebwohl]

But others think of matter as existing as a wave of probability prior to measurement, and then this wave is collapsed into something like a particle upon measurement. For conjugate pairs of attributes, this would increase the uncertainty in the probability wave for the 'other side' of that pair. So the attributes of particles are themselves things that can only be 'collapsed' partially, I suppose, and this collapsing has an inverse effect on the other side of that conjugate attribute.

I am in this camp--observing reality causes an uncertain wave equation to collapse (for which I use the verb "actualize") into only one wave pattern while the other information gets "more" uncertainty.

Going back to the car example, imagine if we were using coherent radio waves in the radar gun with a wavelength significantly larger than the car. If the car is in just the right spot the coherent waves might "miss" the car entirely and register a result of "there is nothing there". The energies and wavelengths required to view quantum objects interact with those objects, putting energy into the system and destabilizing what you are looking at so that by the time you look at something what you are seeing is not what you originally wanted to see in the first place.

[peter]

Deutsch says that quotes like "If you think you undersatnd quantum mechanics you don't understand quantum mechanics" [Fennyman {attributed}] are rubbish and I agree with him. Quantum mechanics are comprehensible - just not by this puppy. I read the words and occasionally the meaning is 'actualised' albeit briefly, but then like an eel in a bucket, it slithers away from me and is gone again.

Ok - lets try again. Can I see Uncertainty as a measurable thing, like say Energy; a thing that can neither be 'created or destroyed' but only passed around in the manner described above. A 'fungible' but real thing that while not material, has actual existance? [If so has it units?]

[I've got Godel, Escher, Bach on the table in front of me and just looking at it is making me sweat; God knows what I'm going to find in there! ]

[Zarathustra]

Wheeler's delayed choice thought experiment has now been performed, and seems to imply that particle or wave nature of matter isn't determined until a measurement is made. The only other possibility seems to be that measurement changes the atom's past. However, I don't think either of these interpretations of the results takes the Many Worlds interpretation into account. I'd be curious to hear what Deutsch says about it.

[Vraith]

Can I see Uncertainty as a measurable thing, like say Energy; a thing that can neither be 'created or destroyed' but only passed around in the manner described above. A 'fungible' but real thing that while not material, has actual existance? [If so has it units?]

I'm not sure if I linked it around here or not...I thought I did...but if memory serves, a discovery was made that allows the precise calculation of how much uncertainty there will be...whereas previously there was uncertainty about the amount of uncertainty.
So, yea, it's a knowable thing, and apparently more precisely knowable thing now.
I'm not sure if it was generalized to all kinds of quantum observation/measurement...or if they just discovered the exact uncertaintly value for certain "things"...[like a photon or electron]

[peter]

I think it might have been a mistake to call it 'uncertainty': The word has too many associations in it's general usage that are really difficult to shake off, even when you are aware that you have to.