# Uncertainty Principle?

## Main Question or Discussion Point

Could someone explain the Uncertianty Principle?

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Heisenberg's uncertainty principle states that we can never be completely certain of both the velocity and position of a particle, as to observe it, we must affect the particle (usually by shining photons onto it).

DrChinese
Gold Member
Could someone explain the Uncertianty Principle?
It would probably make sense if you check out wikipedia and the PhysicsForums faqs on the subject:

Uncertainty Principle

After, you might find it beneficial to ask some specific questions.

Please note that the principle has nothing directly to do with the fact that to observe a particle, you must disturb it. (The principle applies even when there is no disturbance.) It has more to do with the nature of reality and/or limits to knowledge.

If we can affect the particle by shining photons onto it, then can we affect it in a predictable/measurable way?

ZapperZ
Staff Emeritus
If we can affect the particle by shining photons onto it, then can we affect it in a predictable/measurable way?
Note that this has nothing to do with the HUP. The HUP isn't a "measurement technique" limitation.

Zz.

I got my definition from 'A brief history of time', which I have just got out again to see if I remembered it correctly. It clearly states here, that it is only due to the fundamental uncertainty created from bouncing photons off the particle. Are you saying it's wrong, or that it is not giving the full picture?

I know that in quantum theory, we have to assume that an electron/other particle is in all states until it is observed (like schrodinger's cat), but I assumed that the uncertainty principle was only a restriction on the measuring of that particle.

DrChinese
Gold Member
I got my definition from 'A brief history of time', which I have just got out again to see if I remembered it correctly. It clearly states here, that it is only due to the fundamental uncertainty created from bouncing photons off the particle. Are you saying it's wrong, or that it is not giving the full picture?

I know that in quantum theory, we have to assume that an electron/other particle is in all states until it is observed (like schrodinger's cat), but I assumed that the uncertainty principle was only a restriction on the measuring of that particle.
It is wrong, this is a leftover explanation (from the early days of QM) that is thrown around as a simple way to avoid further explanation. But still wrong. There are several ways to see this.

First, observing particle spin - for example - does not affect other commuting attributes (say momentum). So obviously the observation is not the issue, it is only whether the observables are non-commuting.

Second, and more recently studied in detail (last 20+ years): entangled particles obey the HUP. Even at distances of 10 km, non-commuting observations on a pair (say vertical polatization for Alice and diagonal polarization for Bob) obey the HUP. So again, the issue is not the observation itself disturbing the particle.

And keep in mind, there is no prohibition against measuring non-commuting attributes to any degree of accuracy. The issue is that the values you obtain are NOT indicative of the particle at any single point in time. It is best to simply say that the value you get is indicative of the particle at that time. If you make a second, identical measurement, you will get the same value. If you make a non-commuting measurement, and then go back to the first again, all bets are off.

-DrC

Matterwave
Gold Member
Well it is wrong, but it's still a useful way of thinking about the HUP in practice. When you observe a particle, you collapse the state of that particle to an eigenstate of the observable which erases the information you have of non-compatible (commuting) observables. When you go to measure the momentum, after you measured where the particle is, you get a different value than before you measured where the particle is.

In practice this is sort of a "disturbance" to the particle. In reality it's a disturbance to the wavefunction, but since wavefunctions are very abstract, it's sometimes useful to think of it as a disturbance to the particle. I suppose this view may not work for entangled pairs but that doesn't diminish the usefulness of this way of thinking.

I am sorry you have to forgive my limited understanding of Uncertianty Principle.

The way I understand it is that if someone watches a subatomic particle or a piece of light as it passes though slits ( The 2 Slit experiment ) the particle behaves like a bullet fired from gun going from one to the other. However if no one is watching it then can behave in all manner of ways, like a wave going through both slits at the same time.

Now is this only our perception of the particle as it passes though or does our perception of the particle make it this way.