Help a layperson understand the term "information" in quantum mechanics
March 26, 2023 7:22 AM Subscribe
There's been articles going around about the possibility of black holes destroying information, so I've been poking around to find out what the term "information" means, which I tentatively think is quantum states, and what the conservation of information means. Here's what I (a layperson that took a total of one college physics course decades ago) gathered. Can you tell me if I am understanding this correctly? Because I am very uncertain.
A) I see that quantum mechanics is built around unitarity, which I take to mean that the transformation performed over time on a quantum state (which seems to be like a table of probabilities for possible outcomes for measurements on everything in a given system) always produces a new state in which every cell in the new table corresponds to a cell in the previous state.
B) Further, the transformation is reversible. That's how quantum states are used to determine quantum states from the past. I've read that "information" is what you use to map back to a system's past. So I think that's the same thing as the quantum states. But the quantum states are called "information" because they inform physicists what quantum states were there before.
So, in the context of physics articles, am I correct in assuming that quantum states are the same thing as "information"?
Bonus questions:
C) There is a theory that quantum information cannot be created or destroyed. There's also decoherence, which moves information out of a system into its enclosing environment. I can see how observing a system would "settle" the probabilities into something definite, thereby getting rid of the information (quantum state) for that point in time, but I'm not quite sure how the quantum state would move out into the enclosing environment.
D) Black holes might decohere quantum states into definite states. This is very hazy to me. In the article, there's this idea that a black hole can be an observer. (If so, can anything be an observer?) So, it moves quantum states from outside the black hole to within the black hole. Then, they decay via Hawking radiation. But why isn't it letting the quantum states back out via the radiation?
I understand that layers upon layers of my understanding may be wrong here, so try your best! I will appreciate it.
A) I see that quantum mechanics is built around unitarity, which I take to mean that the transformation performed over time on a quantum state (which seems to be like a table of probabilities for possible outcomes for measurements on everything in a given system) always produces a new state in which every cell in the new table corresponds to a cell in the previous state.
B) Further, the transformation is reversible. That's how quantum states are used to determine quantum states from the past. I've read that "information" is what you use to map back to a system's past. So I think that's the same thing as the quantum states. But the quantum states are called "information" because they inform physicists what quantum states were there before.
So, in the context of physics articles, am I correct in assuming that quantum states are the same thing as "information"?
Bonus questions:
C) There is a theory that quantum information cannot be created or destroyed. There's also decoherence, which moves information out of a system into its enclosing environment. I can see how observing a system would "settle" the probabilities into something definite, thereby getting rid of the information (quantum state) for that point in time, but I'm not quite sure how the quantum state would move out into the enclosing environment.
D) Black holes might decohere quantum states into definite states. This is very hazy to me. In the article, there's this idea that a black hole can be an observer. (If so, can anything be an observer?) So, it moves quantum states from outside the black hole to within the black hole. Then, they decay via Hawking radiation. But why isn't it letting the quantum states back out via the radiation?
I understand that layers upon layers of my understanding may be wrong here, so try your best! I will appreciate it.
I'm no physicist, just another layman fascinated by QM. Have you tried Feynman's Lectures on Physics, vol. 3? It's all on QM and you may not get the math (I didn't) but there's a lot about how to interpret, and how not to interpret, what the math means. One reason I mention it is that I'm pretty sure he gives examples where information is destroyed, or maybe "destroyed". E.g. you choose to measure property X and that means you cannot measure property Y any more. (Not just Heisenberg's position vs momentum; it might be a matter of spin polarization.)
But this gets into interpretation, which is contentious. You mention "observers", which come up in some but not all interpretations of QM.
I can't help you with black holes, but I'd throw in a reminder that they are a consequence of gravity, and gravity has never been completely integrated with QM.
posted by zompist at 3:25 PM on March 26, 2023
But this gets into interpretation, which is contentious. You mention "observers", which come up in some but not all interpretations of QM.
I can't help you with black holes, but I'd throw in a reminder that they are a consequence of gravity, and gravity has never been completely integrated with QM.
posted by zompist at 3:25 PM on March 26, 2023
It might be useful to read the Wikipedia article on Information Theory. In doesn't t go 8nto QM, and least not in the first paragraph or so which is all I looked at. It focuses on the concept of entropy.
An illustration might be a bag of Legos. If the prices are all loose, and you shake the bag, the pieces get mixed up in a different way, but nothing has really changed, but if the pieces are put together as a model rocket and you shake the bag and it falls apart, you've changed the entropy state, and destroyed some information.
posted by SemiSalt at 5:00 AM on March 27, 2023
An illustration might be a bag of Legos. If the prices are all loose, and you shake the bag, the pieces get mixed up in a different way, but nothing has really changed, but if the pieces are put together as a model rocket and you shake the bag and it falls apart, you've changed the entropy state, and destroyed some information.
posted by SemiSalt at 5:00 AM on March 27, 2023
The link to Information Theory comes via thermodynamics, heat and entropy. An atom or collection of atoms has a few ways you can describe what it does, one being pool-ball type classical physics, another being quantum wave-type behaviour, and a third as thermodynamic energy distinct from the cosmic background.
Those pool-ball atoms have definite energy levels (heat and motion and possibly height from gravity) but a bucket of them can only be summarised by statistics. The quantum equations give a better grasp of the probabilities of these summary statistics of the information, which you use to help you to predict what happens next.
Up until that information crosses the boundary of a gravitational singularity (which we commonly call a Black Hole) and then the predictability of the heat and position information is ... deemed lost.
At present ...but there are some insights into black holes which say they evaporate over time via Hawking Radiaton, and so would need to recreate the information previously lost to us over the singluarity's threshold. Consevation of energy and mass (and so position, momentum, temperature and more) are important parts of Physics predicting what happens next in the cosmos we're in, so matter hidden behind the edge of a black hole should emerge with conserved information, they say.
posted by k3ninho at 12:38 PM on March 31, 2023
Those pool-ball atoms have definite energy levels (heat and motion and possibly height from gravity) but a bucket of them can only be summarised by statistics. The quantum equations give a better grasp of the probabilities of these summary statistics of the information, which you use to help you to predict what happens next.
Up until that information crosses the boundary of a gravitational singularity (which we commonly call a Black Hole) and then the predictability of the heat and position information is ... deemed lost.
At present ...but there are some insights into black holes which say they evaporate over time via Hawking Radiaton, and so would need to recreate the information previously lost to us over the singluarity's threshold. Consevation of energy and mass (and so position, momentum, temperature and more) are important parts of Physics predicting what happens next in the cosmos we're in, so matter hidden behind the edge of a black hole should emerge with conserved information, they say.
posted by k3ninho at 12:38 PM on March 31, 2023
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Certainty is one way to describe information, as a measure of how surprised you will be to see something.
Let's say you have a bucket of balls, which you expect to have different colors. You pull one ball out after another and they are all of the same color.
Over time, you could call this a "low information" ball — you are not surprised when a ball comes out that is of that color. You have high certainty that the next ball you get will be of that color.
But then you pull a ball out of the bucket that is a color you haven't seen before. This is a "high information" ball. You would be surprised to see this color of ball.
You could also describe this as odds or probabilities of seeing one or another color of ball. As a general numerical value, information is a function of these probabilities and describes the contents of the bucket.
Quantum superposition is where the state of a particle can have properties like position and spin with some probabilities. This set of probabilities is a function of the spins and positions, all the states it can have over time. But you won't know those properties of a particle at a given point in time until you measure or observe them. This observation is like pulling a ball out of the bucket and it carries information to you about the universe/bucket the particle resides in.
posted by They sucked his brains out! at 10:46 AM on March 26, 2023 [2 favorites]