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Comment by canjobear | Hacker News Reader

> if one simply accepts that a conscious observer can be in a superposition like any other piece of matter, the paradox is resolved.

It’s not so simple. Suppose you’re in a box and observe a quantum experiment, and then I open the box and observe you. Then before I open the box you’re in a superposed state |x> + |y>, corresponding to the two possible outcomes x and y. Fine, no problem so far. But what is your own subjective experience? You, subjectively, inside the box, will only ever observe yourself to be in |x> or |y>, never |x> + |y>. Even if from the outside your brain can be said to be in a superposed state, your experience of the world is not superposed.

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The |y> superposition of you has one experience, and the |x> superposition of you has a different experience. The |y> version has no information about the |x> experience, and vice versa, because those two states are orthogonal in Hilbert space. This is essentially the prediction that results in the Everett many worlds interpretation.

If you somehow had an experience of "both outcomes simultaneously", that would violate the quantum mechanical prediction that there is no mutual information between the two superpositions. There are two brain states, an x state and a y state, and they know nothing about each other.

To each brain state (with the limited information available to it) it would appear that something "definite" had happened, even though in the global picture, a superposition still exists.

Either you believe in collapse, so there are two of you, who each observed different things, and one of those will at some point cease to exist, or you embrace the Everett interpretation, and so there are two of you who each observe different things and go on to live their separate lives.

Both paths you laid out are built upon the idea that superposition happens. Is there any serious consideration these days that superposition itself is dead end?

There’s kind of a massive amount of experimental evidence that superpositions happen.

I didn't know that. If you have more information I'd love to learn about it. I was under the impression that collapse wiped out all information about any other states.

In a word, no.

When people compare QM theories they often do so on basis of the original experiments that were used to develop them. But as a discipline physics has moved well past those. Superposition has been proven and tested to exist, and we have long since moved past that phase and started building things, like quantum computers, on top of it. At this point, pretty much the only way superposition doesn't exist is that every time a physicist does something that superposition needs to work, a devil figures out what kind of result is needed to fake it and does that.

Trying to challenge superposition would kind of get same kind of results from physicists as trying to challenge the existence of electrons would get from people who build circuits. Like, if you have some interesting new theory, I am intrigued, but do understand that to get people to follow it you will need to explain how it replicates the results of what superposition would do in so many different cases that you won't be able to enumerate them in a week.

Many-worlds interpretation takes care of this. Anyway, how do you know your experience of the world cannot be superposed? Have you ever been in superposition? We make up stories to maintain consistent histories all the time.

I'm not convinced it does. Which branch was taken within the box now becomes a local hidden variable so Bell rules this out, no?

did you just yada-yada Many-worlds?

> how do you know your experience of the world cannot be superposed?

For your experience of the world to be superposed it would mean that you carry out a quantum experiment with two mutually exclusive outcomes |x> and |y> and you actually experience the superposed result |x>+|y>. This would be like opening the box in the Schroedinger's cat experiment and actually observing the cat to be |alive>+|dead>, instead of either alive or dead. Maybe it's possible, but such an experience has never been reported.

It's "possible", but not really possible. The larger the object is, the harder it is to maintain superposition. Even the slightest nudge will tend to push it all into one state or other. Seeing the superposition directly would be like standing twenty octillion strands of spaghetti on end.

So it's "possible" but would never happen in a trillion lifetimes of the universe. You can tell the difference between that and "impossible" by watching the quantum mechanics work for a few isolated particles, and observing what it means for them to fall out of superposition equilibrium. But in practical terms, it's equivalent to impossible.

If you embrace many worlds all these issues go away.

Or if you embrace nonlocality.

Wouldn’t that imply FTL travel, at least for information?

No, non-locality simply means that effects happen at infinite speeds, not that they carry information.

For example, the Copenhagen interpretation of QM abandons both locality and realism - particles don't have definite states, and they also communicate at infinite speed (but in a way that can't carry information).

What does it mean to communicate without information? That genuinely sounds like an oxymoron.

Basically when you measure the state of two particles that are entangled, you find that their state is correlated in some way. For example, they may be entangled in such a way that their spins are the same. So if you measure one to be spin up (when measured along any axis), the other will also be spin up (when measured along the same axis). This happens regardless of the separation distance between the two particles. By carefully adjusting the axes along which you do your measurements, you can prove that the spins that you get are not predetermined (this is Bell's Theorem) and yet the particles are not communicating at the speed of light with each other either (the correlation remains even if you move the particles far apart and then do the measurements at the same time). Note that the actual spin value itself is random, and once you do the measurements the entanglement is broken, so you can't use this to transfer information faster than light. Hence what the parent poster means by "communication without information": each particle individually appears to be completely random, and yet when you compare them you see both particles are random _in the same way_ (or in the opposite way, particles can be anti-correlated too).

Thanks for the detailed answer. I guess the word “communication” threw me off.

Nonlocality basically means that the universe has a global RNG state. If you write some code that uses a global RNG and no other global variables, that code will still be "local" in the sense that functions cannot communicate information between each other using the RNG. But the outcomes of the functions might still be correlated in interesting ways (corresponding to entanglement).

Only in your world do the issues go away, but in those worlds in which "many worlds" is not true it is maintained as a standing issue.

Multiple words <----> modal logic; possibility mode <----> monad.

Your friend is in the monad, now you are too. No big deal.

I always thought Schoedingers cat should have been a person instead, just to point out this very problem.

Schrödinger's cat was also supposed to show a paradox not be a cool quantum magic physics thing.

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