# What causes a quantum state function to collapse?

We know that “when we make an observation” the “state/wave function collapses” into a particular eigenstate. But never have we defined what “making an observation” is. This seems to be a really crucial part of the whole process!

I have two theories:

1) An exchange of information. This means that any physical process or particle which can carry the state of the quantum system causes the system to collapse into an eigenstate. This implies that processes in nature are collapsing quantum states into eigenstates all the time.

2) A conscious observation ie someone somewhere is trying to make sense of the system. According to my instructor, somebody did a double-slit experiment with randomized parameters. They found that the way they attempted to analyze the data gave different answers. This implies that no quantum systems “choose an eigenstate” (exist) until someone tries to observe them consciously. Therefore, reality happens as we experience it.

*Note, I think that there is no way to tell the difference between these two theories. While case 1 may be true, we can never test if it is so because as soon as we try we have entered the conditions of case 2. Could this mean that they are actually a single case, then?

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## 18 Answers

I had always heard that the state function collapses when it becomes “entangled” with another system. This is similar to what you describe in 1) above.

That seems intuitively correct to me, actually.

@timothykinney Do you have any suggestions as to what I should read to understand your question. Something that won’t make me an alcoholic. Just above a pop up would be nice.

@O I have heard of a book called Quantum Mechanics Demystified that is supposed to not rely on mathematics, so that might help. Also, Brian Greene wrote some entertaining books touching on the topic. They are Fabric of the Cosmos and The Elegant Universe. But I should warn you that Greene is known among serious scientists for considering some of the wilder aspects of quantum theory without really discussing the serious questions. But it’s fascinating to read and think about it.

There’s also a movie called What the Bleep Do We Know? that attempts to acquaint the general public with some of the pressing issues in quantum theory. I have watched it and I can say that it is even wilder and unfounded than Greene’s books. But it is interesting to watch and think about.

I have read Fabric of the Cosmos and seen that movie, but I have only started to realize the depths of the issue by studying the mathematics of quantum mechanics via bra-ket (Dirac) notation…and it’s not particularly fun. Today, we basically proved that if quantum states are pre-determined then ¾ <= ½, which is of course wrong. This means that quantum states collapse *when we observe them* and are not determined ahead of time and then simply discovered at the moment of measurement. If you wanted to read more about this aspect, look up Bell’s Theorem.

@timothykinney Thank you. That was very kind of you. I’ll look that up.

I will also have to prop “Shrodinger’s Rabbits’ by Colin Bruce. It’s about the many worlds theory, but he goes through a good (and entertaining) overview of quantum mechanics before he gets into many worlds. In general a very good book. Also doesn’t rely on mathematics, which makes it easier to digest.

As to the original question, I always thought it was transfer of information, in many cases tied to the fact that the observation itself requires the system to undergo a change (no observation can be made without perturbing the system). But, i could be mistaken.

From what you guys are saying, I must be mistaken in thinking that you can determine the position of a particle but not it’s velocity, or vice versa, and the collapse is due to the attempt. Would someone mind ‘splaining where I’ve gone wrong?

@pdworkin You are referring to the inability to determine the position and the momentum of a particle at the same time to arbitrary precision. Since mass is usually constant, this equates to the same thing as the velocity, however. This is known as Heisenberg’s Uncertainty Principle (HUP) and is true for many such relations, including energy and time. So, you can know the velocity and the position within a certain tolerance, but you can’t know them both infinitely precisely.

In terms of a collapsed wave-function, this means that if you describe the position and velocity of a particle by a wave-function (a probability distribution), then the particle is actually not in any specific position or velocity until you make a measurement/observation. At that moment, the wave-function (probability distribution) “collapses” into a single state: the velocity/position.

So the HUP and the collapse of a wavefunction into an observation are actually slightly different concepts…but they are intimately connected.

Thanks very much for the cogent explanation .

@pdworkin The Bell equation simply proves that a theory of quantum mechanics must not depend on local hidden variables. The outcomes are considered by many to still be causal, just simply non-local. The probabilistic nature of particles in Bohmian mechanics is attributed to our disturbance of measured phenomena and an underlying set of variables that are chaotic. Bohmians tend to argue that there is an underlying determinism but we cannot observe it. Truthfully Bohmian and Copenhagen solutions are completely interchangeable so if you’re just looking for solutions either is fine for now. However, the Bohmian interpretation doesn’t require wave function collapse or the idea that particles only exist when observed.

@O No, I was referring to the “What the Bleep…” movie.

Ugh….that movie is a cult advertisement…...it has nothing to do with quantum physics and everything to do with selling the line that people have direct mind control over the universe…...

Not only is it shockingly unscientific but also downright deceitful. It used several esteemed scientists’ quotations out of context in an effort to add credibility.

Not to mention one of the main interviewees is a person channeling the spirit of an ancient Atlantian spirit-warrior.

@Shuttle128 Ah well. It could be fun. Atlantis, as it is in space and all, it makes sense. And I have been meaning to have a heart to heart with the universe for some time anyways.

When the cat is tired of being dead and alive at the same time.

The final observation made at the end of the universe I would assume. The state of you making any observation and the state of not making the observation will exist until the final state collapse at the end of the universe. Then the arrow of time goes back and builds a history with all relevant data to that final, ultimate state.

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