I remember reading here a physicist mentioning that laymen almost always interpret the HUP as a statement “can’t look at it and find it at the same time,” and that that is certainly missing the whole point, separate from the measurement issue.
(Note to self: post query on what the “measurement problem” in QM means)
Could someone help me out here?
Well, the layman version isn’t that. It’s “can’t determine position and velocity at the same time”. But yeah, it’s more complex than that and beyond me. Something about probability smears and fundamental laws, and definitely not about precision of instruments or disturbing the particle by looking at it.
Except, as far as I can tell, saying anything beyond the popular determination is not science but philosophy. I have yet to see anyone show an experiment that disproves the idea that both position and momentum exist in any particle but are simultaneously unmeasurable.
The HUP comes out of the mathematics of wave mechanics. A wave with a precise position of probability has no definable frequency. Give the particle a wave long enough to measure its frequency and the probability function no longer has a defined location. It isn’t about proving that position and momentum don’t exist, it’s about proving that they do.
WarmNPrickly has it correct – if you buy deBroglie’s hypothesis that matter is a wave, then the Uncertainty Principle fals right out of that – You can perfectly specify the position of a particle as a superposition of an infinite number of wavelengths, but then you have no idea of the particle’s frequency. the frequency is proportional to the particle’s momentum Similarly, you can precisely determine its frequency, but then you have no idea of its location.
The Uncertainty Principle applies to other pairs, ass well – angular momentum and angle, Energy and time. One manifestation is that you cannot simultaneously know the total angular momentum of a particle and its individual components. One early experiment in quantum mechanics that demonstrated this was the 1922 Stern-Gerlach experiment, probably the first experimental dmonstration of uncertanty:
I am a physicist. I can explain it.
Let’s think of a particle. The properties of a particle are edges. There are places inside which the particle is and outside of which the particle is not. If we describe that particle mathematically it looks like this: /___
Now let’s think of a wave. The properties of a wave are wavelength, or frequency. A wave is described mathematically like a sine wave and it kinda looks like this: //////////\
OK: here is where you have to take a leap of faith. In particle physics, the momentum of a particle is related to its wavelength.
So look again at the mathematical drawing of the particle: it has edges, it is well localized, but it doesn’t have a wavelength, so you don’t know its momentum.
Look at the wave – no edges, it is not well localized, but it has a great wavelength, so you know its momentum really well.
And those aren’t the only two functions on the planet, what about something like this: _/////_ Kinda localized, kinda has a wavelength.
So, localization and wavelength (momentum) are not compatible with each other. Heisenberg.
I just read de Broglie Nobel lecture.
Which part of what you wrote are his ideas and which H’s? They seem similar to my pea brain.
The Quantities of position and direction:velocity are there, but…due to the inherent destructive nature of the measurement process…are unavailable to us.
Take an asteroid hurtling through space.
Where is it? Where’s it going?
One can track it using mere radar or optical means…light won’t do much to such a massive object.
However, the smaller the object, the smaller the wavelength needed to detect the object: wavelengths cannot resolve an object when the object is smaller than the wavelength.
The smaller the object, the less massive it is.
The smaller the wavelength, the more energy it has.
At a certain microscopic point, the measuring particle/wave can knock the measured particle off it’s position/trajectory.
This is the line of demarcation between the world of classical Newtonian physics and the world of quantum mechanics.
I think there was. I’m trying to remember stuff from the book The Fabric of the Cosmos, and it’s tough, because the uncertainty principle doesn’t sit well in my brain either.
I think that by shooting off thousands of particles, and getting them to have mirror images (which can be done, somehow - creating two identical particles except traveling in opposite directions), and measuring the velocity of one, and the location of the other, you can show that these ‘identical but opposite’ particles don’t have a precise location until it’s measured. I forget the details, but there’s a long piece in there with a thought experiment with Scully and Mulder from the X-Files and a box that doesn’t decide which of three colors to show until you open it.
How about this? Check out the EPR Paradox, a thought experiment where you attempt to ‘cheat’ the uncertainty by measuring the complementary value of one particle to find out the value of a different particle that it has just rubbed shoulders with. That is you may destroy the state of the one you just looked at but you find out something exact about the one you left alone. Turns out it doesn’t work, dicking with one particle still affects the other one.
How the frack does that work? Good luck answering that.
I don’t know what the OP’s physicist was trying to say, but it’s possible that this the kind of thing that bothered him. The point is that in some ways, it’s not just about an inability to measure. According to quantum mechanics, the quantities of position and direction really aren’t there, at least not in any precise way. It’s not just a measurement issue, but an electron really is sort-of here and sort-of over there, at the same time. So if you have an electron cooped up inside a very small box, it’s actually only mostly within the box, and there’s a chance you could find it interacting with something outside of the box. It’s not about imprecise measurements: you could detect the electron inside the box perfectly well, and come back later and find the electron outside of the box, because it was only ever mostly inside.
If you’re ready to have your brain turn to scrambled egg…
This from a great book, Boojums All the Way Through.
I like this: “The mystery… is that it presents us with a set of correlations for which there is simply no explanation.”
The HUP is sometimes confused with the Observer Effect, that in certain situations it’s difficult-to-impossible to measure or observe what’s going on without influencing it in some way by your observation.
This thought experiment is called Heisenberg’s Microscope and is flawed as a description of the uncertainly principle, mainly because it treats it like it is a flaw in our ability to measure, rather than a fundamental property of the particle like the math says it is.
The UP is a much stronger result that this – it is not “we can not measure these things together because the act of measuring it by hitting it with a photon changes it”, but instead, “there is no way, even in principle, using any technique whatsoever, to measure both of these, or both of any other number of pairs of quantities, at the same time”.
I would direct folks back to Karen Lingel’s post in this thread. As others are noting, the uncertainty (between, for example, position and momentum) is not just a practical difficulty owing to the messy consequences of the measurement process, but is rather fundamental to the quantum mechanical description of nature. This can be seen in Karen’s example, showing how the wave describing a particle cannot have both a definite momentum and position: the only way to have a definite momentum is to have a definite wavelength, and the only way to have a definite wavelength is to consistently cycle up and down …//////.… for a good distance, which means spreading out, not being localized in position!
It’s flawed, but only in the sense of incomplete, not because it’s wrong.
I think of it as being analogous to finding the flaws in perpetual motion machines. For any given machine, you can inspect it and figure out what’s wrong: they miscalculated something regarding gravity or magnetism, say.
Of course the more general principle is that conservation of energy holds and that we can dismiss machines based on that alone. Nevertheless, it can be instructive to inspect a particular machine and see precisely how it breaks down.
Likewise, “Heisenberg’s Microscope” looks at a specific case of the HUP in action. It’s not the only way that the universe prevents us from violating the HUP, but it’s an important and illuminating one.
I probably just remembered this wrong, but isn’t it theoretically possible for the particle to be anywhere in the universe? IIRC, the probability would be so infinitesimal you’d probably need scientific notation for the exponent, but at least in theory . . . If so, then at least technically, there really isn’t any in or out - yes, no, maybe?
I think the problem most people have with the wave-particle duality business is that a wave implies a medium and QM, being the rude boy that it is, doesn’t even extend us the courtesy of a medium (or did I get that wrong too? 
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entanglement? superposition? both? (since I think one is redundant of the other)