4 ms·

I don't understand what's so surprising about only ever seeing one particular outcome. Suppose you have a robot that can observe some spin. We put the robot in

by jules 6y ago

I don't understand what's so surprising about only ever seeing one particular outcome. Suppose you have a robot that can observe some spin. We put the robot in a box together with a spin and make it measure the spin. Now the robot's mind plus the spin is a superposition of ("I measured up", spin is actually up) and ("I measured down", spin is actually down), but in no case is the robot's mind in a state "I measured both up and down". To me, the weird part is that there are phenomena (like the EPR paradox) where the final result cannot be explained by adding up the probabilities of the individual possibilities, but has to be explained by adding up of probability amplitudes of the individual possibilities and then taking the probability of that.

lisper 6y ago
> ("I measured up", spin is actually up) IMHO this is the fundamental mistake people make: conflating "I measured spin up" with "spin is actually up". Think about it: what is the evidence that there is actually a correspondence between a measurement and some external reality? There is only one possible answer to that: you can correlate this measurement with the outcomes of other measurements. For example, if you measure the same particle in the same basis twice in a row you will get the same result. But there is a problem with that: if you actually look at the details you will find that it is actually not possible to measure the same particle in the same basis twice in a row because actually making a measurement requires entangling the particle with a macroscopic system, and you can only do that once. The best you can do is run a particle through a series of filters and look at where it ended up. You can then retrodict that the particle went one way or went another way, but you cannot actually measure its trajectory. In fact, you cannot even know that there even is a particle in your apparatus until the end when you actually measure it. So the only thing you are left with at the end is a single actual measurement, and a macroscopic arrangement of some experimental setup about which you can tell a consistent story retroactively, but where you do not and cannot possibly have any direct evidence that your story is actually true, i.e. that the particle "actually" was spin up. And this is exactly what QM predicts: the consistent outcomes of measurements is not the result of the particle "actually" being spin up or spin down, but because the macroscopic systems (like humans) which compare the outcomes of their measurements are mutually entangled with each other and so decohere into classically consistent states. So if I ask you what you see as the outcome of the experiment, QM predicts that my perception of your answer will match my perception of the outcome. But it emphatically does NOT say that this outcome is actually real.
- jules 6y ago
  > it is actually not possible to measure the same particle in the same basis twice in a row If the robot's protocol is to measure the particle twice, it will measure the same outcome twice. > that the particle "actually" was spin up I didn't say that it was up, I said that it is up. Very important distinction. > But it emphatically does NOT say that this outcome is actually real. Seems like a distinction without a difference to me. > but you cannot actually measure its trajectory Bubble chambers do measure particle trajectories. https://cds.cern.ch/record/39474 https://cds.cern.ch/record/39474
  - lisper 6y ago
    > If the robot's protocol is to measure the particle twice, it will measure the same outcome twice. But you cannot measure the same particle twice. If you think you can, describe the experimental setup for me. > I didn't say that it was up, I said that it is up. Very important distinction. Indeed. But by the time you see the outcome of the experiment, the particle you measured doesn't exist any more. So what does "is up" actually mean at that point? > Seems like a distinction without a difference to me. It's a crucial difference. Our mental models of our classical universe depend heavily on the continuity of identity, i.e. that there are things in the world with properties that persist across time. This what allows us to say things like, "The vase on the table is green." This presumes that the phrase "the vase" has an actual referent, that referent is a vase, and it is on the table, and it is green. We think this makes sense because we can see the vase on the table, and we can see that it is green. But in order to see the vase, your eyes have to accumulate a lot of photons, and that takes time. So the vase on the table has to persist at least long enough for your eyes to accumulate enough reflected photons to see it. If you stop looking at the vase, the vase is still there. If you look at it again, the vase will still be on the table and it will still be green. Particle measurements are fundamentally different. It really doesn't make sense to say "the photon in the left arm of the interferometer" or "the electron in the upper branch of the Stern-Garlach apparatus. If you doubt this, read the following: http://blog.rongarret.info/2018/05/a-quantum-mechanics-puzzle.html http://blog.rongarret.info/2018/05/a-quantum-mechanics-puzzl... http://blog.rongarret.info/2018/05/a-quantum-mechanics-puzzle-part-deux.html http://blog.rongarret.info/2018/05/a-quantum-mechanics-puzzl... https://blog.rongarret.info/2018/05/a-quantum-mechanics-puzzle-part-drei.html https://blog.rongarret.info/2018/05/a-quantum-mechanics-puzz... > Bubble chambers do measure particle trajectories No, they create those trajectories. A particle that is not in a bubble chamber doesn't have a trajectory.

by jules 6y ago

lisper 6y ago

> ("I measured up", spin is actually up) IMHO this is the fundamental mistake people make: conflating "I measured spin up" with "spin is actually up". Think about it: what is the evidence that there is actually a correspondence between a measurement and some external reality? There is only one possible answer to that: you can correlate this measurement with the outcomes of other measurements. For example, if you measure the same particle in the same basis twice in a row you will get the same result. But there is a problem with that: if you actually look at the details you will find that it is actually not possible to measure the same particle in the same basis twice in a row because actually making a measurement requires entangling the particle with a macroscopic system, and you can only do that once. The best you can do is run a particle through a series of filters and look at where it ended up. You can then retrodict that the particle went one way or went another way, but you cannot actually measure its trajectory. In fact, you cannot even know that there even is a particle in your apparatus until the end when you actually measure it. So the only thing you are left with at the end is a single actual measurement, and a macroscopic arrangement of some experimental setup about which you can tell a consistent story retroactively, but where you do not and cannot possibly have any direct evidence that your story is actually true, i.e. that the particle "actually" was spin up. And this is exactly what QM predicts: the consistent outcomes of measurements is not the result of the particle "actually" being spin up or spin down, but because the macroscopic systems (like humans) which compare the outcomes of their measurements are mutually entangled with each other and so decohere into classically consistent states. So if I ask you what you see as the outcome of the experiment, QM predicts that my perception of your answer will match my perception of the outcome. But it emphatically does NOT say that this outcome is actually real.

jules 6y ago
> it is actually not possible to measure the same particle in the same basis twice in a row If the robot's protocol is to measure the particle twice, it will measure the same outcome twice. > that the particle "actually" was spin up I didn't say that it was up, I said that it is up. Very important distinction. > But it emphatically does NOT say that this outcome is actually real. Seems like a distinction without a difference to me. > but you cannot actually measure its trajectory Bubble chambers do measure particle trajectories. https://cds.cern.ch/record/39474 https://cds.cern.ch/record/39474
- lisper 6y ago
  > If the robot's protocol is to measure the particle twice, it will measure the same outcome twice. But you cannot measure the same particle twice. If you think you can, describe the experimental setup for me. > I didn't say that it was up, I said that it is up. Very important distinction. Indeed. But by the time you see the outcome of the experiment, the particle you measured doesn't exist any more. So what does "is up" actually mean at that point? > Seems like a distinction without a difference to me. It's a crucial difference. Our mental models of our classical universe depend heavily on the continuity of identity, i.e. that there are things in the world with properties that persist across time. This what allows us to say things like, "The vase on the table is green." This presumes that the phrase "the vase" has an actual referent, that referent is a vase, and it is on the table, and it is green. We think this makes sense because we can see the vase on the table, and we can see that it is green. But in order to see the vase, your eyes have to accumulate a lot of photons, and that takes time. So the vase on the table has to persist at least long enough for your eyes to accumulate enough reflected photons to see it. If you stop looking at the vase, the vase is still there. If you look at it again, the vase will still be on the table and it will still be green. Particle measurements are fundamentally different. It really doesn't make sense to say "the photon in the left arm of the interferometer" or "the electron in the upper branch of the Stern-Garlach apparatus. If you doubt this, read the following: http://blog.rongarret.info/2018/05/a-quantum-mechanics-puzzle.html http://blog.rongarret.info/2018/05/a-quantum-mechanics-puzzl... http://blog.rongarret.info/2018/05/a-quantum-mechanics-puzzle-part-deux.html http://blog.rongarret.info/2018/05/a-quantum-mechanics-puzzl... https://blog.rongarret.info/2018/05/a-quantum-mechanics-puzzle-part-drei.html https://blog.rongarret.info/2018/05/a-quantum-mechanics-puzz... > Bubble chambers do measure particle trajectories No, they create those trajectories. A particle that is not in a bubble chamber doesn't have a trajectory.