Why does an observer affect the electron?

[gptejms]

Let me just add that the measurement at the screen is that of position i.e. the spot where the particle hits---from this we infer that the momentum of the particle was 'this' at the slit.

Here's the problem with #1. You have 2 non-commuting operators A and B. If you measure A, you have only "collapsed" the wavefuction only for that observable. The value of B could still be in superposition. This is what is going on in the Schrodinger Cat-type experiments such as those done in the Delft/Stony Brook experiments. You measure a non-commuting observable in other to detect the superposition in the other.

Right, and that's why the uncertainty principle is for simultaneous measurements.

This is why I said that the momentum is undertermined until you measure it at the detector/screen. But once I measured it, I make the assumption that this particular particle made a classical trajectory from the slit to the detector to be able to calculate its momentum (more precisely, its transverse momentum).

Zz.

At the end of it all,I don't know where we differ and where we agree!

I guess the only difference is that you seem to differentiate between Heisenberg uncertainty and measurement uncertainty whereas I don't--for me all uncertainties are measurement uncertainties.Unmeasured is anyway uncertain.

 

[ZapperZ]

Let me just add that the measurement at the screen is that of position i.e. the spot where the particle hits---from this we infer that the momentum of the particle was 'this' at the slit.

Right, and that's why the uncertainty principle is for simultaneous measurements.

But I don't understand this "simultaneous" stuff. The momentum is measured AFTER the particle passed through the slit, i.e. after the position measurement.

Can you show me an example of a "simultaneous" measurement, and why this is necessary between AB or BA?

At the end of it all,I don't know where we differ and where we agree!

I guess the only difference is that you seem to differentiate between Heisenberg uncertainty and measurement uncertainty whereas I don't--for me all uncertainties are measurement uncertainties.Unmeasured is anyway uncertain.

Because I can have a perfect detector that has zero uncertainty in where the particle hits it and still have a spread of statistics as I do this repeatedly. Furthermore, and this is the major distinguising factor here, the measurement uncertainty are independent of each other. I can make them arbitrarily accurate without caring what the other is doing, because it depends on my technology.

So the instrument uncertainty and the HUP are not of the same beast.

Zz.

 

[gptejms]

But I don't understand this "simultaneous" stuff. The momentum is measured AFTER the particle passed through the slit, i.e. after the position measurement.

Momentum was not measured,but the position at which the particle hit the screen.From that it was calculated that the particle would have had 'this' momentum at the slit.So the measurement(s) refer to x & p (simultaneous)values at the slit.Because the spot on the screen that's hit varies from particle to particle(there is a \Delta p) uncertainty at the slit.

It may not always be possible however to infer the momentum in an indirect way and if you do that by some other means later,you would be disturbing the original value--in that case it would not qualify as a simultaneous measurement(measurement of simultaneous values may be a better terminology)

A gamma ray microscope may be better qualified for the job of simultaneous measurements in the literal sense.

Because I can have a perfect detector that has zero uncertainty in where the particle hits it and still have a spread of statistics as I do this repeatedly. Furthermore, and this is the major distinguising factor here, the measurement uncertainty are independent of each other. I can make them arbitrarily accurate without caring what the other is doing, because it depends on my technology.

So the instrument uncertainty and the HUP are not of the same beast.

Zz.

I see your point for 'measurement of simultaneous values' case in the sense described above.

For a simultaneous measurement(in the literal sense) as in gamma ray microscope,x & p values are uncertain (even) after measurement.

 

[ZapperZ]

Momentum was not measured,but the position at which the particle hit the screen.From that it was calculated that the particle would have had 'this' momentum at the slit.So the measurement(s) refer to x & p (simultaneous)values at the slit.Because the spot on the screen that's hit varies from particle to particle(there is a \Delta p) uncertainty at the slit.

But this is rather dicey in calling it a "simultaneous measurement". I certainly won't. I would be more comfortable in simply calling it what it is, a measurement of position, and then a measurement of momentum. That description accurately reflects what is being done, rather than what one likes it to be.

Zz.

 

[Prince of Quarkness]

as you can see, philosophical questions (about physics) are generally irrelevant to physicists.

Thanks for that, but two years of undergraduate physics here at Imperial has clued me into the vast prevalence of cool-headed empiricism in the physics community.

There's a lot to be said for your direct Popperian science - examine observations, make hypothesis to explain observations and predict new ones, test hypothesis, rinse, repeat. Indeed, this is the meat and potatoes of scientific work and I wouldn't dream of criticising scientists for not wanting to get bogged down in questions of 'reality', 'philosophy' and all the rest.

On the other hand, at the time I posted we were discussing 'interpretations' of quantum mechanics, which are inherently philosophical creatures. One day one of them might produce a testable mathematical ramification, but at this point they are on the level of 'does the good of the many outweigh the good of the few?' rather than the level of 'what result do I get if I make my laboratory setup look like this . . .'

The answer to the latter question is provided by the QM theory itself, to an enormous degree of accuracy. Philosophical questions are left to the interpretations. And yes, even Copenhagen _is_ an interpretation, because it makes an unspecified and presently untestable distinction between 'classical-like' measuring apparatus and quantum systems themselves.

Very common among physicists is a sort of empirical 'super-Copenhagen' typified by the phrase: Shut Up And Calculate.

As I have said, there is a vast amount to be said for just getting your head down and working on the actual theories, predictions et cetera without weeks of navel-gazing in the labyrinth of Quantum Conundra that so occupy popular science writers and people wanting to look smart down the pub.

On the other hand, as Roger Penrose argues in 'The Road to Reality', some physicists are willing to take a dose of philosophy along with their empiricism, and address questions of Reality rather than just the Popperian bones of hypotheses and observations. He contests that this is a useful method, and I'd recommend the book for anyone. Particularly those who have issues with quantum mechanics, because it's very entertaining to read a man who has grave doubts about quantum ontology that aren't just based on the ever-popular cliched Schrodinger's Cat/Observer Effect pseudophilosophy.

-Proteus

 

[selfAdjoint]

Proteus, may I just inject a small belief of my own. The present state of the quantum formalism, whether in it's non-relativistic or in its most extended form, does not give us enough data to base any philosophical conclusions on.

"So geographers, o'er Afric Downs
Draw elephants, for want of towns"

 

[Prince of Quarkness]

Proteus, may I just inject a small belief of my own. The present state of the quantum formalism, whether in it's non-relativistic or in its most extended form, does not give us enough data to base any philosophical conclusions on.

Aye, and that's a common and sensible opinion.

The question then becomes, might philosophical considerations be the factor that actually leads to the required development of the formalism?

The Principle of Relativity, for example, is an expression of the idea that physics 'should' be the same in all inertial reference frames. That to me sounds like a philosophical consideration (with the required empirical backup that any consideration in physics requires), and it was one of the cornerstones that led to the eminently Popperian and empirical theory of relativity.

And perhaps its a similar consideration, treated as irrelevant by Copenhagenists and those who shun 'interpretations' altogether, that will light the way in developing quantum theory.

I will admit that this path has shown few if any results thus far - QFT seems in my limited understanding to be based on practical not philosophical concerns - ie: not worrying too much about the infinities, simply making sure that one gets rid of them mathematically before trying to make an actual prediction.

So at the moment it looks to be 1-0 to the utilitarians, with the interpretationists lagging behind. :-)

-Dave (sorry for signing out with the wrong handle last post, these things become instinctive after a few years online!)