- #36
reilly
Science Advisor
- 1,077
- 2
>>>>>>>>>>>>>>>>>>>>>>>>>>>Crosson said:Actually what you measure are positions, energies, momenta. People use wavefunctions to calculate energies, and then measure energies, and the next thing you know they are saying they have measured a wavefunction.
>>>>>>>>>>>>>>>>>>
RA
Well, how else do you measure probabilities? (I'm aware of the convergence difficulties and other matters of mathematical rigor) When you have a coin toss, you measure which face of the coin is up. Then you count them. And, generally, you'll find, a 50-50 or close thereto, split. Of course, you want to make as many measurements as you can. As I'm sure you must know, physicists do a lot more with wave functions than calculate energy levels.
My thesis concerned electron-proton scattering, and I certainly used wavefunctions. When you do a scattering experiment you measure the probability that, say, an electron scattered from a proton, goes to a particular angular position. Sometimes you measure both electron and proton positions. But, in any event, you measure the modulus of the appropriate wave function. I don't expect you to believe me, but how about Bohm? He devotes over 70 pages to scattering theory in his classic book, Quantum Theory, and clearly demonstrates what I've just stated -- as do countless books and papers.
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>.
C
All I am saying is that the complex-valued wavefunction is a hidden variable that is used to predict things that we actually measure, whereas Bohm's theory always speaks in terms of things we directly measure: positions, energies and statistical Hamilton-Jacobi functions. (How would a wavefunction be measured? With what sort of complex measuring device?)
>>>>>>>>>>>>>>>>>>>>>>>>>>.
RA
Generally speaking the wave function is exactly that, one of space and time and spin and... If, in fact the wave function is a "hidden variable", in what space is this true? (Just to be precise, one normally does not consider a wave function as a variable.)
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
C
My point about orbits was obviously a miss. I am not claiming that copernican/ptolemaic (Newtonian really) astronomy does not accurately predict the positions of celestial bodies. I was simply saying that the orbits we imagine the planets to move along cannot be directly observed.
>>>>>>>>>>>>>>>>>>>>>>>>>>>>.
(RA) That's right for sure, certainly for complete orbits, or long sections thereof. It's not an issue of any consequence -- it well might be if our predictions of planetary motions were incorrect.
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>.
C
Personally I don't give a crap about observation, but if someone attacks the causal (Bohmian) interpretation of QM on the grounds that quantum trajectories are inobservable (the "in principle" part comes from that unfounded assumption of QM that I brought up earlier), then I expect that person to think about which things we discuss in physics are truly observable: the truth is that many are not.
>>>>>>>>>>>>>>>>>>>>>.
RA
The fact that you speak so disparagingly of observations says, with all due respect, you don't know nuthin' about physics. Physics has been based on observation for many centuries. Without observations physics would be the nothing more than pure speculation, with no way to discern truth, just a time-killer for restless minds. -- barring a return to the divine right of kings.
I was interested in Bohm's ideas when I was a student, albeit many years ago. My professors kindly suggested to me that. if I wanted to be a professional -- which I did -- it would be wise to learn "mainstream" physics first, and then, if I felt the urge, to return to Bohm's work. Further, I sensed that many of my Profs had great respect for Bohm, as a very bright, able and accomplished physicist. I gave my students similar advice with respect to Bohm and the whole area of alternative interpretations of QM. In short, the standard track in any profession is: pay your dues, get your union card, start you career with some strength. (Bohm certainly did) --true for docs, lawyers, jazz musicians and blues singers, academics, .. Don't fall on your sword until there's a medic around.
And, my criticism of Bohm's hidden variables is that it has not produced any new physics, unique to its precepts. I know that many physicist have a similar position. Further, as I remember, computations are more difficult with Bohm's approach. Here's a challenge: compute the Lamb Shift for hydrogen a la Bohm's approach.
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>.
C
Occam says that one should not increase, beyond what is necessary, the number of entities required to explain anything. Bohm's theory does not "increase the number of entities", it simply talks about a particle and a wave associated with the particle (the wave is a generalized Hamilton's statistical function).
>>>>>>>>>>>>>>>.
(RA)Strictly a matter of opinion.
>>>>>>>>>>>>>>>>>>>>>>>>>>>>.
C
QM talks about both these things, but it speaks of the particle in a totally inconsistent nonsensical way (as if it teleports around through places it has "no probability of being". (RA-- How so?)
If the particle and its properties do not exist other then when they are measured, what sort of particle is that? Didn't someone accuse me of claiming the empire state building does not exist when we are not looking at it? How ironic that this is the world view of standard QM which I am against.
The QM you discuss, and the QM I've known and worked with for many years appear to be rather different. Particles exist only when observed? (David Hume could make a case for that position.) Nope. Most of us do not believe that, as amatter of pragmatism. You totally misstate the "world view of standard QM", and vastly overstate the importance of the arguments about interpretation. Day-to-day physics works with a pragmatic Born-Bohr interpretation, and most physicists are involved in day-to-day. It's pretty basic to assume the real objective world exists. And day-to-day physics makes the same assumption.
I never got back to Bohm because I found high energy theory much more interesting and challenging. (But, of course, I will admit a bias toward observation, which might disqualify my intellectual seriousness.) What in the world do you mean by teleportation?
Why does QM upset you so much? The fact that we can participate in this forum is totally based on QM -- as in semiconductors,. Have you actually taken a graduate level QM course?
Are you aware that with the use of wave packets, trajectories can be defined loosely? -- a staple of formal scattering theory.
If you wish to cling to your ideas about QM, then if you want to be taken seriously, you will have to cite chapter and verse, rather than handwaving and indignation, about what you consider to be the inadequacies of QM, and give strong support based on observations and or mathematics. But if you really don't give a crap about observations, then as I said above, you are not doing nor talking about physics.
Regards,
Reilly Atkinson
PS By the way, Locrian, your definition of determinism is exactly the one used in physics. Philosophy might well talk a different talk.
Last edited: