Concerning the Uncertainty Principle, what if the particle gives off its own light?


by Ralphonsicus
Tags: light, physics, quanta, quantum, uncertainty
Ralphonsicus
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#1
Feb29-12, 11:42 AM
P: 47
If a particle gives off its own light, how can we never know its position as well as its momentum, as we can see it and thus measure its speed, and know its velocity? How does the Uncertainty Principle treat this circumstance?
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zhermes
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#2
Feb29-12, 01:09 PM
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We still won't know its position exactly. There are many uncertainties in calculating a precise position based on an emitted photon. One of the biggest ones is that when a particle emits a photon, it feels a recoil (i.e. to conserve angular momentum)---this coupled with uncertainty in the particles initial energy/momentum leads to uncertainty in its new properties, and thus position.
tom.stoer
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#3
Feb29-12, 02:02 PM
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Please do not confuse uncertainty due to the HUP (Heisenberg Uncertainty Principle) with uncertainty due to recoil or inaccuracy of measurement. The HUP says that a quantum state has a intrinsic uncertainty regarding position and momentum w/o the need to refer to any experiment, measurement, recoil or something like that. Unfortunately it's often explained via measuremt or 'photons measuring the position and therefore affecting the momentum' etc., but that's missleading.

Nervous
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#4
Feb29-12, 04:07 PM
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Concerning the Uncertainty Principle, what if the particle gives off its own light?


Quote Quote by tom.stoer View Post
'photons measuring the position and therefore affecting the momentum' etc., but that's missleading.
It thought that's how it was? How do quantum particles have inherent uncertainty?
DrChinese
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#5
Feb29-12, 04:33 PM
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Quote Quote by Nervous View Post
It thought that's how it was? How do quantum particles have inherent uncertainty?
That's the point of the Heisenberg Uncertainty Principle. It specifies this as being a fundamental element of the quantum world. This has nothing to do with measurement accuracy, which is not limited in any way. You can measure any single property to any degree of experimental accuracy. It is only certain *pairs* of attributes, called non-commuting or complementary, which are affected.
Nervous
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#6
Feb29-12, 05:01 PM
P: 18
So throwing photons at an electron or any other quantum particle doesn't affect it? That explained the complementary attributes that I know of (Just momentum and position.) Since the photon would contact the electron at it's position, but that contact would effect the momentum. Of course that idea might breakdown with other complementary attributes...

I just don't see how it has inherent uncertainty and I'd appreciate if you kept trying to explain it, or point me into a source that does. Mucho gracias!
Khashishi
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#7
Mar1-12, 04:17 PM
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The effect of measurement is part of how Nature enforces that the Heisenburg uncertainty principle plays out in experiment. You can say that the particle doesn't have a precise point value for position and momentum, but it's useful to understand what happens when you TRY to measure it, and this is where the measurement effect comes into play.

When you measure a particle's position, you squeeze the wavefunction into a small range of positions, but extend it out in momentum space.

The particle DOES have a theoretical average position and average momentum based on the state wavefunction. We just can't measure it.
Nervous
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#8
Mar28-12, 04:00 PM
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Quote Quote by Khashishi View Post
When you measure a particle's position, you squeeze the wavefunction into a small range of positions, but extend it out in momentum space.

The particle DOES have a theoretical average position and average momentum based on the state wavefunction. We just can't measure it.
Yes, this is what I thought HUP meant. We have to shine a wave onto a particle to measure it. The weaker that wave is, the less it affects the momentum of the particle but the less precise the position can be known. (Since the particle could be any where between two wave crests.) Of course, if you used a higher frequency wave, with less area between the crests, you would know the position better but the momentum would be affected as you would be hitting it with more energy.

Of course, if that where true, would it mean that using a low frequency wave would eventually disturb the momentum of a particle as much as using a high frequency wave? (Since you would be adding more and more energy to it.)

If so, would the opposite be true, that using a high frequency wave would disturb the momentum of a particle as much as a low frequency wave if it only came into contact a brief enough time?

Of course, there would still have to be a limit to how briefly the wave could come into contact with the particle...
JamesOrland
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#9
Mar28-12, 05:13 PM
P: 93
That's not quite it. You're still visualising particles as if they were round little things, you're thinking classically, but that is simply not true. It's my personal belief that we should throw away the word 'particle' to describe such structures, or at least be much more careful when using it and explaining it to newcomers to subatomic theories.

The picture of a pretty little particle makes it very difficult to see why HUP should apply at all, I mean, even if a photon does affect the momentum of a particle, it still sort of makes sense that you could guess its position and a pretty close approximation to its momentum anyway. But that's not how Nature works.

Down there, the "particles" behave according to Schrödinger's Equation. They are literally described by the wavefunction, and in actuality, could be thought of as the wavefunction. If you so much as try to measure its position with some degree of accuracy, literally all information you have about its momentum starts being thrown out the window, to the point that if (don't mind the unphysical part, just the mathematical one) you get to a dirac delta to describe its position (i.e. you know exactly where it is), all information about the momentum is completely lost and you could have any momentum from zero to infinity.

It's a hard concept to grasp because we keep thinking of those pretty little balls swirling around other pretty little balls, while reality is nothing like that. A single electron can be in many places at the same time, and interact with itself (double-slit experiment, though I'm being purposefully non-technical here), and at certain temperatures certain atoms (specifically those that have an even number of nucleons in their nuclei) can even occupy the same place at the same time, crawl up walls (against gravity), exhibit properties that are called 'super' in physics. In fact, even whole molecules can exhibit those properties. So that Jimmy Neutron model of an atom we all have in our heads is pretty much a lie.

What I'm trying to say is, yes, it's very complicated, and no, there is no analogy whatsoever in our experienced world. The "particles" are just that weird.
tom.stoer
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#10
Mar29-12, 01:13 AM
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Quote Quote by JamesOrland View Post
It's my personal belief that we should throw away the word 'particle' to describe such structures, or at least be much more careful when using it and explaining it to newcomers to subatomic theories.
I completely agree with this view.


Regarding
Quote Quote by Nervous View Post
... this is what I thought HUP meant. We have to shine a wave onto a particle to measure it. The weaker that wave is, the less it affects the momentum of the particle but the less precise the position can be known.
I can only repeat what I wrote in post #3
Quote Quote by tom.stoer View Post
... do not confuse uncertainty due to the HUP with uncertainty due to inaccuracy of measurement. The HUP says that a quantum state has an intrinsic uncertainty regarding position and momentum w/o the need to refer to any experiment, measurement, ...


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