To Richard #1129:

… where the application of quantum theory presupposes that exactly one outcome will occur.

Exactly! This is why I tend to think of the measurement problem as a pseudoproblem.

Hi Matt L.,

I’m hope I’m not joining this conversation too late. Regarding your comment in #1094:

The issue is that the protection is repreparing the system in an independent copy of the initial state, so you effectively have an ensemble of independent copies which you are measuring.

This way of describing the protection seems specific to the…[Read more]

Hi Shan,

Unfortunately your talk will be during a time slot when I will have my two young kids to look after. So I will not be able to chime in in real time. But I’ll be sure to check back later.

Also, I’m thinking that some of the points about the foundational implications of protective measurement we have talked about during Matt Pusey’s talk…[Read more]

Hi Shan,

Thank you for your reply, that’s helpful.

I will need to take a closer look at Matt’s notes myself. Tomorrow!

Best,
Max

To Shan #1028:

An example is a trapped atom, where the potential may not be known beforehand, but one does know that after a sufficiently long time the atom is to be found in the ground state.

At the risk of beating this horse to death: I would rephrase this to read “one does know that after a sufficiently long time the atom is to be found in…[Read more]

Hi all,

Just back from dinner, sorry to be joining so late.

Since you’ve been discussing the two different implementations of a PM (adiabatic and Zeno), here’s a question for Shan. Do you think a Zeno-type PM is equally indicative of the reality of the wave function as an adiabatic PM? If yes, why? If not, what do you see as the…[Read more]

Hi Matt,

Thanks for your intriguing contribution. I just managed a cursory read this afternoon, and I will still have to take a closer look, and unfortunately now is not a good time (the family is calling to dinner). But I’ll get back to it later. And I look forward to seeing what the other participants will have to say.

Best,
Max

Hi Shan,

Thank you for your reply.

As for your revised criterion, well, I think ultimately it’s a matter of taste which criterion each of us deems satisfactory for establishing the reality of the wave function. I would still say that “disturbing a system with probability arbitrarily close to zero” is not sufficient, because any nonzero…[Read more]

Dear Richard,

The kind of view of the wave-function you were suggesting (which I think is basically right) is better expressed by calling the wave-function a source of probabilities rather than as a description of the statistical properties of ensembles.

I’m happy to go with that. But what do you exactly mean by “source of probabilities”? Is…[Read more]

Hi all,

Thanks so much for all your excellent comments. I have to dash and hold office hours now –- my students are banging down the door with questions about Newtonian physics. But I’ll be back tomorrow to answer any further comments and queries.

Also, look for Matt Pusey’s and Shan’s talks on the same topic. We can continue the discussion t…[Read more]

Dear Bob,

Re your #949. Your question is essentially about how to understand probability assignments. I.e., what does a 75% chance of success mean if there’s just a single trial? And there are lots of different interpretations and ways to motivate that (see QBism for a radical and interesting take). I would say that for an ensemble person, the…[Read more]

Dear Bob,

I certainly agreee that there is some probability that you
will kick the system into a different energy eigenstate (assuming energy
provides the protection), and in that case you will draw a wrong conclusion.

Sure, and from a practical point of view, if I can make that probability small enough I might well be perfectly happy. I have…[Read more]

Bob has raised an interesting question — namely, to what extent does protective measurement challenge the viewpoint that the wave function only describes ensembles, like Ballentine et al. once suggested?

My own sense is that there is no real challenge, just as doing quantum experiments on single quantum systems does not amount to a challenge.…[Read more]

Dear Bob,

Thank you for sharing these fascinating recollections about Jeeva.

Nonetheless, wouldn’t you allow that protective measurement is an
interesting idea … ?

Absolutely! I think it’s a brilliant measurement scheme, just as brilliant as weak measurement (though the latter one seems to have received more attention and gained more e…[Read more]

Dear Bob,

Responding to your #933, you do agree that part of the time, maybe 90% of the time, I will get some information about the initial state. Well, that suggests that you would agree that there was an initial state to get information about.

Yes, certainly there “is” an initial state, in the sense that the system has been prepared in some…[Read more]

Hi Ken,

Thanks! As far as my own view of the wave function is concerned, well, this may be opening a can of worms indeed. I’m happy to say, however, that I’m partial to what people call “epistemic” approaches. That is, I like to think of the wave function as a calculational tool for organizing information/knowledge/beliefs (I’m not exactly sure…[Read more]

Richard,

Thanks, that’s a good question. In the usual formulation, protective measurement follows a von Neumann-style description of measurement. That is, Eq. (11) is taken to describe a situation in which information about the different energy eigenstates |n> is transferred to the mean position of the pointer wave packet, encoded in the…[Read more]