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Heisenberg's Uncertainty Principle


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Initial post: Aug 2, 2010 9:04:04 AM PDT
I've often wondered about those images which show how subatomic particles are deflected in a magnetic field (or by exchange particles) following a collision in an accelerator. It looks as if they record its position and momentum at the same time.

In reply to an earlier post on Aug 2, 2010 9:22:51 AM PDT
[Deleted by the author on Nov 21, 2012 7:14:05 PM PST]

Posted on Aug 2, 2010 9:36:01 AM PDT
Hi J.,

Thanks. I've always had the impression that Heisenberg's Uncertainty Principle (HUP from now on) relates only to the position and momentum of a particle. In other words, it doesn't apply to particle decay, for example.

In reply to an earlier post on Aug 2, 2010 9:38:53 AM PDT
Last edited by the author on Aug 2, 2010 9:39:10 AM PDT
Hmm, I see your point, but there's something missing here...What it is I don't know.

Posted on Aug 2, 2010 9:43:34 AM PDT
Last edited by the author on Aug 2, 2010 9:50:08 AM PDT
If the magnetic field were not fixed in one position, but was moved about randomly so that the deflection continued in a predictable way, wouldn't that negate HUP?

Posted on Aug 2, 2010 9:57:00 AM PDT
Last edited by the author on Aug 2, 2010 10:10:07 AM PDT
But you always have to make the observation (or look to see how the magnet was affecting the particle even if it is entirely predictable), the idea of which was sealed in concrete by Schrodinger's cat that says you can't know something until it is observed. Nothing paradoxical about that, although modern accounts of that and quantum physics in general make it nearly mystical.

Whereas, in fact it's pretty prosaic.

So the question is, was Bohr emulating Picasso or Matisse?

In reply to an earlier post on Aug 2, 2010 11:00:50 AM PDT
[Deleted by the author on Nov 21, 2012 7:14:22 PM PST]

Posted on Aug 2, 2010 12:28:57 PM PDT
Last edited by the author on Aug 2, 2010 12:42:14 PM PDT
I don't know what the resolution is, but go to

http://books.google.com/books?id=roDjIqHco9EC&pg=PA14&lpg=PA14&dq=quantum+collisions&source=bl&ots=qiXUyrbY1k&sig=86sLLlSG3EutUGA8QZaRtKa_OwI&hl=en&ei=4xJXTOaJBIf_nQen18GCAw&sa=X&oi=book_result&ct=result&resnum=9&ved=0CEgQ6AEwCDgU#v=onepage&q&f=false

to get an answer about HUP and perhaps, resolution, although it's difficult to tell. It's nice that Google makes things like this ("Quantum theory of high-energy ion-atom collisions" by Děevad Belkić) available, since it costs $105 on Amazon.

Quantum physicists are famous for saying, "Shut up and do the calculation." To me this means that all the flapdoodle about QM is beside the point.

Posted on Aug 2, 2010 1:17:03 PM PDT
Arno Arrak says:
Heisenberg's uncertainty principle is a consequence of the existence Planck's constant h. It can be regarded as a quantum of action which has dimensions of momentum times distance or energy times time. If you accept Planck's law you must also accept that the smallest unit of action that can exist is h. Heisenberg follows.

Posted on Aug 2, 2010 1:39:37 PM PDT
Thanks, Arno. All of the accounts I've read about HUP say it's impossible to measure the position and momentum of a subatomic particle at the same time. Yet the images from the collisions at, say, Fermilab indicate that it is easily possible to do both. J. Black has given reasons why this is so, do you have anything to add to that?

Posted on Aug 2, 2010 5:26:07 PM PDT
I hate pop-Fizick's books. Look at what it has done to you people.

Posted on Aug 2, 2010 5:35:59 PM PDT
Last edited by the author on Aug 2, 2010 7:25:23 PM PDT
Ed Raton says:
"I hate pop-Fizick's books. Look at what it has done to you people."

Not being a scientist, Stephen Lajoie (are you one?) I wonder sometimes what the view of science popularists like Sagan, Kaku, and even Stephen Hawking is amongst their peers. True, they all had great credentials before publishing their best-selling tomes, but does the science community resent the attention the popularists get?

I think at some point, it's too much. Recent example is Michio Kaku, who is on just about every other Discovery or Science channel program these days. He's getting to the point of over-exposure.

Posted on Aug 2, 2010 6:04:38 PM PDT
My profs generally said good things about Sagan and Kaku because they got people interested in physics. I always felt that they raised the expectations way too high. As a TA, I had students come to me in my lower division physics class wondering when we would be done with the blocks, incline planes and falling bodies and start doing string theory; then these students who could NOT do F= dp/dt and had no concept of quantum physics would lecture me in the most condescending way on string theory. (ARRAGH!)

I once worked as a TA for an Astronomy prof. He made watching COSMOS (which was running on TV at the time so you know how old I am) required for his Astronomy class. It was okay back then; before Jane Fonda's boyfriend got hold of it and before it was turned into a political POS by Sagan's widow.

Of all the TV physicist out there, Goldstein was my favorite because he had the best and highest physics to pop-fizix ratio.

Posted on Aug 2, 2010 6:11:32 PM PDT
Heisenberg said
delta p * delta x >= h_bar/2
Where delta p is the UNCERTAINTY in momentum, and delta X was the UNCERTAINTY in position. You could know the momentum of a particle exactly, but then all you'd know is the particle was somewhere in the universe. you could know the position exactly (at some time) but you wouldn't know it's momentum at all. Physicist like to use a nice, Gaussian wave to describe particles in QM where there is a optimum amount of uncertainty in position and momentum. Yes, you CAN measure both, but you have the Heisenberg uncertainty. You can't know both exactly. I would recommend Griffith's book for a good introduction to QM. Introduction to Quantum Mechanics (2nd Edition)

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

Posted on Aug 2, 2010 6:36:15 PM PDT
Last edited by the author on Aug 2, 2010 6:39:29 PM PDT
Stephen says, "I hate pop-Fizick's books. Look at what it has done to you people. " Heh.

I've been reading about QM for about 20 years, all of them popular physics books by Gell-Mann, Kaku, Susskind and Louisa Gilder, to name only a very few, so am familiar with the terms. I like all of them and since I wasn't a physics major and like to ask questions this is the perfect forum for that.

If the position and momentum of a particle can't be measured at the same time, then what do the images from a particle accelerator represent?

In reply to an earlier post on Aug 2, 2010 6:47:37 PM PDT
Arno Arrak says:
Position is measured in distance units. Momentum is the mass of the particle multiplied by its velocity. Call measurement error uncertainty. Then Heisenberg uncertainty is the uncertainty in the product of position uncertainty times mass uncertainty times velocity uncertainty. The dimension of this product is called action. Its minimum value is determined by the value of Planck's constant h. That is because h has the dimensions of action and is in fact a kind of an "atom" of action that is indivisible in the sense that action in a physical system cannot exist in units smaller than h. Very puzzling but that's how the universe is put together.

Posted on Aug 2, 2010 6:50:16 PM PDT
Last edited by the author on Aug 2, 2010 6:51:12 PM PDT
Please, I have to wonder, because the pattern is repeated all the time, why does the big man strut into the room calling everybody else a fool? Not talking about you Arno.

In reply to an earlier post on Aug 2, 2010 6:54:11 PM PDT
But Arno, the published images from particle accelerators show both position and momentum. What am I missing?

Posted on Aug 2, 2010 7:12:29 PM PDT
Jim Carlson - you have a really interesting question! I don't know the answer to it, but I am going to take a guess. I think that the trajectory lines DO show momentum and position, but NOT to the resolution of h-bar. The lines are 'thick'. At least, that's my guess.

Posted on Aug 2, 2010 7:15:51 PM PDT
Last edited by the author on Aug 2, 2010 7:17:01 PM PDT
I've seen several theories about the moon's orbit, each one adding a few terms or altering them, which are small corrections to the last orbital theory. Even with the three corner mirrors on the lunar surface, there are still small errors in its position. I understand about errors involved with measurements, but it looks to me that the errors involved with those images from particle accelerators are the same for both position and momentum. That particle is not on the other side of the universe. It is clearly visible on the plate.

In reply to an earlier post on Aug 2, 2010 7:23:15 PM PDT
The moon has nothing to do with Heisenberg uncertainty, unless you're talking precision out to better than 100 digits.

In reply to an earlier post on Aug 2, 2010 7:25:12 PM PDT
CivWar64 (Bob) says:
"Jim Carlson - you have a really interesting question! I don't know the answer to it, but I am going to take a guess. I think that the trajectory lines DO show momentum and position, but NOT to the resolution of h-bar. The lines are 'thick'. At least, that's my guess. "

Exactly right. The error was in thinking that the Heisenberg Uncertainty principle says you can't measure both position and momentum. Of course you can! The product of the uncertainties has to be less than 1/2 * h_bar, however.

Posted on Aug 2, 2010 7:28:59 PM PDT
Thanks, Bob, I think you're right. As Arno said earlier, "Heisenberg's uncertainty principle is a consequence of the existence Planck's constant h." But Planck's constant is so close to zero (6.626068 ◊ 10^-34 m2 kg / s) that you'd think we were back in the classical world of physics.

Posted on Aug 2, 2010 7:46:42 PM PDT
Phew, what a struggle. I thank all of you for your answers. This discussion will probably continue on the same subject or it may veer off into another one, which I heartily welcome.

In reply to an earlier post on Aug 2, 2010 10:12:38 PM PDT
You have stumbled onto exactly what the issue is.

I completely agree pop-Fizick's, for the most part suck and make physics deceptively easy. Be just imagine a person without a strong background trying to get through The Road to Reality: A Complete Guide to the Laws of the Universe or Gravitation (Physics Series). There has to be some non-pop-Fizick's way of explaining things, that don't require a physics or math degree to begin with.

Personally I think QED: The Strange Theory of Light and Matter, does a remarkable job for QED, as does Relativity: The Special and the General Theory (Classic Reprint) for relativity, but technical books on the subjects are nearly impossible to get through for anyone other than an advanced physics undergrad.

So you have to have something to get people interested, before they're ready to put 4 years into a physics program, and dragging blocks up inclined planes isn't going to do it.
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Discussion in:  Science forum
Participants:  17
Total posts:  55
Initial post:  Aug 2, 2010
Latest post:  Nov 21, 2012

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