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> Many quantum information theorists don’t even believe the wave function is “real” but just a mathematical tool for making predictions about measurement outcom

> Many quantum information theorists don’t even believe the wave function is “real” but just a mathematical tool for making predictions about measurement outcomes You are correct that many say this, but this is a constant source of frustration for me (a physicist who does believe the wave function is the only real thing). These physicists never seem to articulate what then is supposed to actually be “real” (under their definition) or what laws govern the “real” things as opposed to the wave function. Implicit seems to be that there is some kind of separate classical realm that obeys non quantum mechanical laws. Is this separate classical realm to be understood as a macroscopic limit of the quantum realm? But if so then the whole picture is circular since wavefunctions are supposed to be mere bookkeeping devices for classical things. Or to put this more succinctly, if the wave function is a mere bookkeeping device, then bookkeeping for __what__? (I should mention that yes I know about QBism and all that and my confusion is not for lack of talking to QBists about such things — I just can’t make heads or tails of what they tell me!)

ranguna 3y ago
> bookkeeping for __what__? Bookiping for things that happen so fast and at such a small scale that our current technological tools cannot fully capture data with enough precision and accuracy for us to make an good model of the underlying behaviour. Imagine you put a spinning ball on top of a frozen lake on a windy day. You are going to observe the ball spin continously towards the same orientation until several gusts of wind make it spin the other way around, and this goes on and on for hours. From the measurements of the wind and current spin of the ball, you can make a model that accurately and precisely predicts the spin of the ball on the almost frictionless frozen lake. Now imagine your spinning ball is extremely small you can't even see it or any "wind" with any tools that you currently have, but you can still measure its spin to a certain precision. Now imagine this "wind" is so strong and volatile that the tools you have sometimes takes a somewhat accurate physical snapshot of it and sometimes it just misses the gust. You take a look at the somewhat accurate measured spin and it seems to have changed without any reason, but it was just because your tooling is not accurate and precise enough to capture all the quantum gusts of wind that influence the quantum ball, so it looks like the ball is changing its spin randomly, whist in reality, we just can't precisely measure whatever is influencing the ball's spin with our current tooling. The influences are there, it's just that our tooling can't capture them precisely and accurately enough. To combat that, we continously measure the ball's spin and we are able to figure out a pattern, not a precise one (because again, our tooling is not precise and accurate enough), but a pattern based on probability of the ball being in a specific spin state, we can even combine this with our inaccurate measurements of the quantum wind and further improve the accuracy of the probability. But never to a precise pattern, because our tooling sometimes misses certain wind states that it looks like the ball chagend spin randomly. If we had tools that precisely and accurately measured the ball's spin and the quantum wind, we would be able to build a precise and accurate model of the spin based on those measurements. But we can't, although, we still want to make science around these inaccurate measurements, and probability based patterns are just enough for the science we want to make. The wave function is just the result of our lack of precise and accurate measuring tools and measuring methods at this quantum scale. And for now, it's good enough for the science we want to do.
- superposeur 3y ago
  You describe a local hidden variable theory such as has been definitively ruled out by Bell/CHSH inequality experiments (see, e.g. (2022 Nobel Prize)[https://www.nobelprize.org/all-nobel-prizes-2022/ https://www.nobelprize.org/all-nobel-prizes-2022/] ). But even if you could make some baroque version of a model like this (with the position of one ball instantaneously reacting to other far away balls perhaps and some pilot waves) invoking it still wouldn’t answer my question about ontology. Implicit in this description seems to be the existence of a separate non-quantum realm (little balls that spin, jostled by “wind”). What are these balls supposed to be made of? If not atoms (since atoms are stable by virtue of quantum mechanics, which you seek to explain), then why don’t they suffer from ultraviolet catastrophes? Hopefully you see my point.
  - ranguna 3y ago
    Maybe it's things smaller than whatever smallest thing we have been able to measure. I'll read up the inequality experiment.

ranguna 3y ago

> bookkeeping for __what__? Bookiping for things that happen so fast and at such a small scale that our current technological tools cannot fully capture data with enough precision and accuracy for us to make an good model of the underlying behaviour. Imagine you put a spinning ball on top of a frozen lake on a windy day. You are going to observe the ball spin continously towards the same orientation until several gusts of wind make it spin the other way around, and this goes on and on for hours. From the measurements of the wind and current spin of the ball, you can make a model that accurately and precisely predicts the spin of the ball on the almost frictionless frozen lake. Now imagine your spinning ball is extremely small you can't even see it or any "wind" with any tools that you currently have, but you can still measure its spin to a certain precision. Now imagine this "wind" is so strong and volatile that the tools you have sometimes takes a somewhat accurate physical snapshot of it and sometimes it just misses the gust. You take a look at the somewhat accurate measured spin and it seems to have changed without any reason, but it was just because your tooling is not accurate and precise enough to capture all the quantum gusts of wind that influence the quantum ball, so it looks like the ball is changing its spin randomly, whist in reality, we just can't precisely measure whatever is influencing the ball's spin with our current tooling. The influences are there, it's just that our tooling can't capture them precisely and accurately enough. To combat that, we continously measure the ball's spin and we are able to figure out a pattern, not a precise one (because again, our tooling is not precise and accurate enough), but a pattern based on probability of the ball being in a specific spin state, we can even combine this with our inaccurate measurements of the quantum wind and further improve the accuracy of the probability. But never to a precise pattern, because our tooling sometimes misses certain wind states that it looks like the ball chagend spin randomly. If we had tools that precisely and accurately measured the ball's spin and the quantum wind, we would be able to build a precise and accurate model of the spin based on those measurements. But we can't, although, we still want to make science around these inaccurate measurements, and probability based patterns are just enough for the science we want to make. The wave function is just the result of our lack of precise and accurate measuring tools and measuring methods at this quantum scale. And for now, it's good enough for the science we want to do.

superposeur 3y ago
You describe a local hidden variable theory such as has been definitively ruled out by Bell/CHSH inequality experiments (see, e.g. (2022 Nobel Prize)[https://www.nobelprize.org/all-nobel-prizes-2022/ https://www.nobelprize.org/all-nobel-prizes-2022/] ). But even if you could make some baroque version of a model like this (with the position of one ball instantaneously reacting to other far away balls perhaps and some pilot waves) invoking it still wouldn’t answer my question about ontology. Implicit in this description seems to be the existence of a separate non-quantum realm (little balls that spin, jostled by “wind”). What are these balls supposed to be made of? If not atoms (since atoms are stable by virtue of quantum mechanics, which you seek to explain), then why don’t they suffer from ultraviolet catastrophes? Hopefully you see my point.
- ranguna 3y ago
  Maybe it's things smaller than whatever smallest thing we have been able to measure. I'll read up the inequality experiment.