# How to understand unitarity in QM?

• B
Ok. Let's not mention the mind as the group is not equipped to handle it.

Let's just focus on this unitarity conserving information thing. Is it like law of conservation of energy or the law of conservation of information where information cant be created nor destroyed? Then if you introduce new Hamiltonian to the system, then can't you add information to the system that can make you say couple two branches together? Ping expert Simon Phoenix and Vanhees. Need your expertise here and the math. Thanks!
After googling a lot about Unitarity and understanding Simon formulas. I understood about it more. Unitarity just says that probability equals to 100% meaning when you have 50% in one slit, you have 50% in another slit. And so if you suddenly have 30% in one slit, you have 70% in one slit (or other setups). And overlap has to do with the vectors. But all this is assuming the wave function is a probabilistic medium per Born specification. Is there solid proof at all that this is the case. No. Because one can introduce Many worlds where the wave functions are not wave of probabilities but real. And here how can you say that unitarity is preserved? They now search for Lorentz violations, is there a similar search for unitarity violation? I think the black hole firewall stuff is one of those where they explore the possibility unitarity may not be true. So this means there is no solid proof unitarity is a law in Quantum mechanics, right?

Boing3000
Gold Member
Is there solid proof at all that this is the case. No.
In science only experiment count as proof. So as of now, it is the case.

Because one can introduce Many worlds where the wave functions are not wave of probabilities but real.
Real wave of probabilities, with the same mathematics. MWI does not introduce anything, it is an interpretation.

And here how can you say that unitarity is preserved?
For two reason
1) unitary is a core component of the mathematical coherence of the theory.
2) we flip coins, and as of now, it don't turn into whale & petunias

I think the black hole firewall stuff is one of those where they explore the possibility unitarity may not be true. So this means there is no solid proof unitarity is a law in Quantum mechanics, right?
Wrong. Unitary is by definition a part of QM.
But QM don't apply to Black Hole, there is no theory yet merging Quantum & Gravity

Simon Phoenix
Gold Member
Unitarity just says that probability equals to 100% meaning when you have 50% in one slit, you have 50% in another slit.
The unitary evolution is essential to allow us to interpret things probabilistically, yes, but you should also try to appreciate Vanhees' much deeper answer above (#68) - where symmetry is the key ingredient.

I wouldn't worry too much about trying to understand all of the various interpretations (unless you want to, of course) - they're all correct in the sense that they will all allow you to make the correct predictions about experiments. Just pick one that 'speaks to you' personally.

For me I stick to the axioms I presented earlier (with the necessary technical refinements, and extensions to cope with more generalized measurements). I know that if I use these axioms correctly I'm going to be able to calculate the right predictions for experiments. Of course the strict necessity of the 'projection postulate' is disputed as we've seen in this thread, but as a tool to aid calculation I personally find it extremely useful. Interpreting things like quantum key distribution or entanglement swapping is, for me, much cleaner and easier if we adopt this postulate. I don't think it's 'wrong' as such since the more general POVM formalism contains this as a special case, but if it bothers you, then as Vanhees and Bill have argued, it's probably not strictly necessary.

Ultimately you'll find extremely smart and capable physicists defending their own particular favourite interpretation with a great degree of passion and intellect - and currently there is no clear 'winner' in any experimental sense. So if the minimal ensemble interpretation is your thing - go with it. If you prefer Bohmian approaches, or the transactional interpretation, or MWI, or Copenhagen, or consistent histories - then they're all fine too. Until we have some experimental way to distinguish between them, they're all as good as each other. Nobody can tell you which of these is actually 'correct'

But unitary evolution is an essential feature of all of them as far as I can see.

The unitary evolution is essential to allow us to interpret things probabilistically, yes, but you should also try to appreciate Vanhees' much deeper answer above (#68) - where symmetry is the key ingredient.

I wouldn't worry too much about trying to understand all of the various interpretations (unless you want to, of course) - they're all correct in the sense that they will all allow you to make the correct predictions about experiments. Just pick one that 'speaks to you' personally.

For me I stick to the axioms I presented earlier (with the necessary technical refinements, and extensions to cope with more generalized measurements). I know that if I use these axioms correctly I'm going to be able to calculate the right predictions for experiments. Of course the strict necessity of the 'projection postulate' is disputed as we've seen in this thread, but as a tool to aid calculation I personally find it extremely useful. Interpreting things like quantum key distribution or entanglement swapping is, for me, much cleaner and easier if we adopt this postulate. I don't think it's 'wrong' as such since the more general POVM formalism contains this as a special case, but if it bothers you, then as Vanhees and Bill have argued, it's probably not strictly necessary.

Ultimately you'll find extremely smart and capable physicists defending their own particular favourite interpretation with a great degree of passion and intellect - and currently there is no clear 'winner' in any experimental sense. So if the minimal ensemble interpretation is your thing - go with it. If you prefer Bohmian approaches, or the transactional interpretation, or MWI, or Copenhagen, or consistent histories - then they're all fine too. Until we have some experimental way to distinguish between them, they're all as good as each other. Nobody can tell you which of these is actually 'correct'

But unitary evolution is an essential feature of all of them as far as I can see.
I think you have come across Zurek Quantum Darwinism. I think this makes more sense than others.. But someone said: "Unitarity is not directly related to the Born rule. Unitarity is a mathematical property, Born rule is a physical law." What can you say about Quantum Darwinism with regards to unitarity and the born rule? The quantum states in quantum Darwinism is the primitive.. Zurek derives the born rule without the born rule.. but yet Zurek quantum states automatically satisfied unitarity... What for you is the relationship between unitarity and the born rule?

And by the way, did you just take up physics undergraduate course (B.S. in physics) or are you a Ph.D? I wonder if an undergraduate physics major graduate can have the vastness of your knowledge or if it requires a Ph.D?

The unitary evolution is essential to allow us to interpret things probabilistically, yes.
This is clear and yet it is remarkable that we are still always faced with two different evolution types in QM, the continuous unitary evolution bewtween measurements, wich is reversible, preserving probabilities and the measurement evolution(aka stochastic, probabilistic, acausal, irreversible, nonunitary, etc...) at each measuremnt event , and unitarity is anyway key to interpret the stochastic or nonunitary evolution of any event too and therefore its normalization of probabilities sum to 1, so it is hard not to interpret somewhat the circularity apparent in that unitarity of the theory is actually enforced by the unitary interpretation of the probability at every event. Of course this unitarity is ultimately postulated in the theory and it is essential for its consistency and this circularity is therefore granted.

The problem that always comes back is that measurements, that appear as the physical part, the empirical part with which predictions are checked, are because of the above necessarily left unexplained in its irreversible nonunitary essence that requires to undergo a new normalization after each measurement. In other words the unitarity postulate itself prevents to have any access to explain this irreversibility within the formalism, since it is actually absent of the formalism, it is a purely operational part that is integrated in the so called FAPP not formalized part of the physical practice of QM. All QM interpretations are required to respect the mathematical formalism, therefore there is no hope for them to address the irreversibility of measurements.

vanhees71