twistor59 wrote:17 pages, and this was only episode 1.....

kennyc wrote:

twistor59 wrote:17 pages, and this was only episode 1.....

<snicker> This is the PHILOSOPHY subforum

Evolving wrote:

I’m sorry to say that this is all a bit confused.

Starting with electric charge. First: the electromagnetic attraction between the negatively charged electrons and the positively charged nucleus is all that stops the atom from instantly flying apart. The idea that an electron doesn’t acquire charge until we measure it, is completely fanciful. There wouldn’t be any atoms if that were the case.

Second: measuring the charge of an electron by measuring the electric field that it generates is not the same thing as measuring its location. A free electron travelling through space generates an electric field which we can measure as it passes a test charge by the effect that it has on that test charge.

A travelling electron can be described as a wave packet, and you may remember my description of a wave packet as like a head moving under a blanket. A very large blanket. Maybe more like a parachute. Or a mouse crawling under a sheet.

I don’t really want to imagine a mouse under my sheet, but in the interests of science I suppose I am prepared to.

Anyway, observing the effect that the electric field of a travelling electron has on a test charge does not cause the wave function of that electron to “collapse” in the way that measuring its location does. It’s still a wave packet, which in principle stretches to infinity (the sheet is never completely resting on the mattress, no matter how far away you look, at any rate in our mathematical model), but it is very much concentrated in one region, and that is where we can observe the charge.

I keep putting the word “collapse” in scare quotes because I do want to stress that it is just a word. All we are doing when we perform a measurement of location is to change the values of the wave function so that they are only not equal to zero within a certain region. The electron is still the same physical object as it was before. It hasn’t become something that it wasn’t before, nor has it acquired qualities that it hadn’t before, and it certainly hasn’t now begun to exist for the first time; all we have done is to change some of the values of its qualities and thereby have an effect on its behaviour.

All this is already in the words that we have been using in previous posts, and it would be a lot clearer if we could point to the actual maths (apart from the purely practical difficulty that without Latex it’s very difficult to post any maths on this forum). That’s all I meant with my reference to the maths: I wasn’t trying to blind you with science or do some kind of lame appeal to my own authority. (Which is not high anyway: I’m quite lowly, as physicists go.) I don’t mind trying to explain it all using words alone. It’s good practice for when my daughter is interested in quantum mechanics (we’ve already started with astrophysics).

Though it does make the whole exercise feel a bit like being cross-examined: “You are asking us to believe that… I put it to you that…”

EDIT: grammar

evolving wrote:First: the electromagnetic attraction between the negatively charged electrons and the positively charged nucleus is all that stops the atom from instantly flying apart. The idea that an electron doesn’t acquire charge until we measure it, is completely fanciful. There wouldn’t be any atoms if that were the case.

twistor59 wrote:17 pages, and this was only episode 1.....

Panderos wrote:

evolving wrote:First: the electromagnetic attraction between the negatively charged electrons and the positively charged nucleus is all that stops the atom from instantly flying apart. The idea that an electron doesn’t acquire charge until we measure it, is completely fanciful. There wouldn’t be any atoms if that were the case.

But when people talk of 'measure' or 'collapse' they don't mean human beings right? They mean any other particle. So is it not the case that the nucleus of the atom is interacting with the electron this way? When Jamest says this or that property isn't decided until a measurement is done, isn't that referring to before the nucleus interacts with the electron? Whatever that means.

campermon wrote:'Observers' are not required to be conscious agents.

Panderos wrote:

evolving wrote:First: the electromagnetic attraction between the negatively charged electrons and the positively charged nucleus is all that stops the atom from instantly flying apart. The idea that an electron doesn’t acquire charge until we measure it, is completely fanciful. There wouldn’t be any atoms if that were the case.

But when people talk of 'measure' or 'collapse' they don't mean human beings right? They mean any other particle. So is it not the case that the nucleus of the atom is interacting with the electron this way? When Jamest says this or that property isn't decided until a measurement is done, isn't that referring to before the nucleus interacts with the electron? Whatever that means.

Evolving wrote:No. Not every interaction causes wave function collapse.

twistor59 wrote:They certainly don't mean human beings.

They don't mean "just" another particle either - they mean the "measuring apparatus". To measure charge, you'd need some apparatus more complex than another microscopic particle (think Milikan's oil drop expt).

Panderos wrote:

Evolving wrote:No. Not every interaction causes wave function collapse.

twistor59 wrote:They certainly don't mean human beings.

They don't mean "just" another particle either - they mean the "measuring apparatus". To measure charge, you'd need some apparatus more complex than another microscopic particle (think Milikan's oil drop expt).

Ok so what is needed to count as 'measuring apparatus' or cause 'collapse'?

kennyc wrote:as was explained earlier there is no actual collapse, the particle potentially keeps moving, continuing to embody its wave function. The measurement is like a freeze-frame from an ongoing movie.

twistor59 wrote:People still argue about what *exactly* constitutes a measurement, but in the Copenhagen Interpretation, the apparatus has to be treatable as classical - not subject to the rules of quantum mechanics.

People also argue about what "collapse" really is. If forced to give an opinion, I suppose I'd espouse the view that it's a change in the state of knowledge about the system. i.e. I suppose I'm a Psi-epistemicist as opposed to a Psi-ontologist. But I'm only an engineer.

Panderos wrote:

twistor59 wrote:People still argue about what *exactly* constitutes a measurement, but in the Copenhagen Interpretation, the apparatus has to be treatable as classical - not subject to the rules of quantum mechanics.

People also argue about what "collapse" really is. If forced to give an opinion, I suppose I'd espouse the view that it's a change in the state of knowledge about the system. i.e. I suppose I'm a Psi-epistemicist as opposed to a Psi-ontologist. But I'm only an engineer.

Where is this knowledge? Presumably this is happening all the time in distant galaxies.

Panderos wrote:
Also, more technical, but down-to-earth question: When you measure say, the energy of a particle, do you not collapse the wavefunction into an energy eigenstate from which it then continues in a time-dependent way? But with the energy now fixed until that particle interacts with something else, or you measure some other property and thus put it into an appropriate eigenstate of that thing? [And then if you measured the energy again you might get something different]. I remember thinking that was how it worked but never being sure if I was right.

Panderos wrote:

twistor59 wrote:People still argue about what *exactly* constitutes a measurement, but in the Copenhagen Interpretation, the apparatus has to be treatable as classical - not subject to the rules of quantum mechanics.

People also argue about what "collapse" really is. If forced to give an opinion, I suppose I'd espouse the view that it's a change in the state of knowledge about the system. i.e. I suppose I'm a Psi-epistemicist as opposed to a Psi-ontologist. But I'm only an engineer.

Where is this knowledge? Presumably this is happening all the time in distant galaxies.

Also, more technical, but down-to-earth question: When you measure say, the energy of a particle, do you not collapse the wavefunction into an energy eigenstate from which it then continues in a time-dependent way? But with the energy now fixed until that particle interacts with something else, or you measure some other property and thus put it into an appropriate eigenstate of that thing? [And then if you measured the energy again you might get something different]. I remember thinking that was how it worked but never being sure if I was right.

twistor59 wrote:It's in the (classical) pointer state of the measuring apparatus. There has to be something irreversible (in the thermodynamic sense) about this sort of measuring process. So although a real measuring apparatus is actually made of quantum things, and is subject to quantum rules, it has to be capable of in some sense chucking away all those quantum features in order to "have made a measurement". The system+apparatus is surrounded by an enivornment in which it lives, and with which it interacts. It is through this interaction that all the quantum entanglements leak away. This is what is behind the decoherence program. There's all sorts of stuff there about "Einselection" and "pointer bases" which I only vaguely understand.

twistor59 wrote:Yes, the energy eigenstates are eigenstates of the Hamiltonian, and if the Hamiltonian is time dependent (which is presumably what you mean by "interacts with something else", then just after an energy measurement, you've put the particle into an energy eigenstate, but some time later, because the Hamiltonian has changed, it will no longer necessarily be an energy eigenstate.

Panderos wrote:Bit beyond me really. Would the takeaway for the thread be that particles are really interacting even before any measurement is made upon them? Then what was with J-A-K's statement about the moon not being there when you don't look?