Why is heisenberg uncertainty not a limit of technology?

[zonde]

You don't have anything like a particle for sound waves, water waves, classical electromagnetic waves etc.

I think that this is the key point (instead of arguments about x and p). Or in other words we don't have straight forward model for quantization of classical wave packet.

 

[TrickyDicky]

No, for classical waves you don't have x as a position of a particle neither do you have p as the momentum of a particle. You don't have anything like a particle for sound waves, water waves, classical electromagnetic waves etc. So formally you can derive an "uncertainty relation" using Fourier analysis but it's not to be interpreted as something affecting particles.

The difference is that you interpret a QM wave function as something representing a particle; this you never do with classical waves.

But aren't photons kind of interpreted that way (interpreting the classical EM wave as something particle-like), or how about phonons for sound, or solitons for water waves...

 

[tom.stoer]

Photons are particles (particle-like excitations) of the quantized electromagnetic field in QED; they are not particles of classical electromagnetic waves of Maxwell's theory; Maxwell's theory explicitly rules out particle-like behavior of the electromagnetic field!

 

[TrickyDicky]

Photons are particles (particle-like excitations) of the quantized electromagnetic field in QED; they are not particles of classical electromagnetic waves of Maxwell's theory; Maxwell's theory explicitly rules out particle-like behavior of the electromagnetic field!

You completely missed my point. I was not pointing to the evident ifferences between QED and Maxwell's EM, besides note we have the notion of photons since much earlier than QFT was imagined, or even QM-the Einstein photon- and you could even say from Newton and his "corpuscles".
I was giving examples of wave phenomena like light, sound and water waves (the three of them can still be regarded as "classical waves" in their macroscopic behaviour, can't they?) and when you say something like "The problem which does not exist with classical waves is that you don't interpret them as particles, so you don't have something like x and p" , I don't think is so clear cut that under certain very specific circumstances like the ones I mentioned you don't have something like x and p.

 

[Whovian]

How do we know that the uncertainty principle is a property of an electron and not a limit of our measuring ability? I understand that photons striking an electron alter its momentum, but imagine an electron that is not being observed. Couldn't it have both a position and a momentum at a given point in time?

I know this has been answered, but I feel like answering anyway.

That's what Einstein thought, if I'm remembering my history right (which I probably am not). Then we just get repeated experiments, proving his hypothesis wrong. There's even a famous misquote "God does not play dice with the Universe." (Again, a misquote.)

Even Einstein's susceptible to mistakes. (In fact, science is based around mistakes!)

 

[TrickyDicky]

Photons are particles (particle-like excitations) of the quantized electromagnetic field in QED; they are not particles of classical electromagnetic waves of Maxwell's theory; Maxwell's theory explicitly rules out particle-like behavior of the electromagnetic field!

You completely missed my point...

Anyway I don't think there is probably much point in arguing about this since if you were just referring to the classical wave theory of course I agree with you, I was more trying to confront the macroscopic wave behaviour with what we know about the microscopic behaviour of matter and fields. Certainly not trying to say here that the classical theory picture applies in the quantum realm.


To go back on topic, I think what jtbell was saying about waves, HUP and Fourier analysis was spot on. Quantization(and therefore the introducing of noncommutation /HUP) of classical fields and matter has much more to do with the microscopic wave nature (in the modern sense) of matter and fields than it is usually acknowledged. The kind of counterintuitive thing is that at the same time quantization is what allows also the quantum particle behaviour of matter and fields, the fact that unlike the classical particles we cannot know simultaneously momentum and position is then an unavoidable consequence of the quantum wave nature.

 

[Rosen]

Hello, This is my first entrance into the Physics Forum so I'll ask a naive question. If an electron wave packet collapses into a particle, wouldn't it have to have a definite position and momentum in a single instant? Particularly if we don't observe it.

 

[DrChinese]

Hello, This is my first entrance into the Physics Forum so I'll ask a naive question. If an electron wave packet collapses into a particle, wouldn't it have to have a definite position and momentum in a single instant? Particularly if we don't observe it.

Welcome to PhysicsForums, Rosen!

If you don't observe it, it doesn't collapse. But you can choose what basis you want to observe it.

The collapse of the wave function places the electron into an eigenstate on that specific basis. Let's say it is position. Then the non-commuting partner - momentum - is undefined. Same for other pairs of non-commuting properties. So no, you cannot meaningfully say it had both position and momentum at any time.

Notice that I said "non-commuting" above. Commuting properties CAN have simultaneously well-defined values. So for example: spin and momentum can both be known, but not position and momentum.

 

[Ger]

Isn't the basics of it all that we observe differences? A completely static object would have a delta of 0 for all its observables, meaning there is nothing there to observe. A constant can be added and removed from any differential or integration, even if the constant to add would be infinite: renormalization. If one is stuck with such a non-removable singularity, it might have a physical meaning or the interpretation is wrong.