[thedoc (OP)]

Ken Heart  asked the Naked Scientists:
   
I listen to you on South African Radio 702 whenever I can! Great show.
 
The Heisenberg uncertainty principle states that in principle you can never know a particle's exact position and momentum simultaneously.  However, in particle accelerators such as the one at CERN, they make images of what happens at the point of collision of various streams of particles, and obtain a kind of "track" of the resultant particles.  Surely when a particle is making that "track" it's position AND momentum can be simultaneously established?  After all, there's a record of it!  I would be interested to hear how this sits with Heisenberg!
 
Regards,
 
Ken Heart

What do you think?

[lightarrow]

The track of a particle left in a vacuum chamber or other detecting medium is not infinitely thin: its width is many orders of quantity larger than the particle itself.

[yor_on]

You could also see it as a function of time. What happens in a particle accelerator is that you get a picture of outcomes, no different from outcomes macroscopically. As long as we assume that we need outcomes to define a arrow then that must be logical microscopically too. But at the time, just before the photo taken, whatever particles you might expect was still in a uncertain state as described by HUP. Then after you choose your measurement, aka photographing it, you more or less 'force' them into a definable state of outcome. To me there are no uncertainties about what's passed, ever. Unless you want to get extremely theoretical and define a history such as it falls into a 'historical fog' in where several possibilities might be constructed from hindsight, alternatively no defined possibilities at all. Myself I see histories as set parameters, from where we get quantum mechanics (and statistics).

[evan_au]

Another way of looking at Heisenberg's uncertainty principle is that the effect of a machine trying to measure a subatomic particle very accurately will disturb its position and momentum slightly.

If you measure the position accurately, the final momentum will be disturbed so much that you don't know what the momentum was before you tried to measure the position.

Similarly, if you try to measure the momentum very accurately, it disturbs the particle's position so much that you can't determine the original position as accurately.

You could measure the position accurately, then measure the momentum accurately, but you wouldn't know both of them at the same time...

So in the LHC, they try to make the best compromise between measuring both position and momentum as accurately as possible - but they don't claim to be able to beat Heisenberg! 

[Pmb]

Ken Heart  asked the Naked Scientists:
   
I listen to you on South African Radio 702 whenever I can! Great show.
 
The Heisenberg uncertainty principle states that in principle you can never know a particle's exact position and momentum simultaneously.  However, in particle accelerators such as the one at CERN, they make images of what happens at the point of collision of various streams of particles, and obtain a kind of "track" of the resultant particles.  Surely when a particle is making that "track" it's position AND momentum can be simultaneously established?  After all, there's a record of it!  I would be interested to hear how this sits with Heisenberg!
 
Regards,
 
Ken Heart

What do you think?

As lightarrow mentioned, the uncertainty in the particles position and momentum are smaller than the observed track. We can know the path the particle takes to within those limits.

I should also clarify exactly what the uncertainty principle states since its usually misinterpreted. The amount of uncertainty is a statistical quantity and not something like the error due to limited precision of an instrument. It's possible to measure the position of a particle with arbitrary precision and yet have a large degree of uncertainty.

For example: Consider tossing a fair die. The number which comes up each time is exact, i.e. a number 1 to 7. Keep tossing it and each time a number comes up it will be exact. However the uncertainty is a quantity which is calculated from the large number of tosses and the recorded number of instances each number comes up. If you were to sit down and calculate the uncertainty of the number of the face then it would come out to about 3.14

The proof is given here
http://home.comcast.net/~peter.m.brown/qm/probability.htm