Is there a lost information paradox for quantum physics?

[Heidi]

Hi Pfs,
When Stephen Hawking proposed the idea of black hole information it appeared that information could be lost. it was a problem in GR which is a dererminitic theory. Knowing initial data and Hamiltonian tells you what was and will be.
It is not the case in quantum physics. things evolve unitarily before and after measurements but the results are random. Information is then erased and replaced by new information.
I wonder why the first (maybe wrong) attempt to solve the black hole paradox was not to say: it is not a paradox , it is a quantum process.

 

[PeterDonis]

it was a problem in GR which is a dererminitic theory. Knowing initial data and Hamiltonian tells you what was and will be.

No, this is not a correct statement of the paradox. Classical GR is known to predict singularities under certain conditions, of which black hole collapse is one. This in no way violates determinism; it just means the deterministic prediction of GR under certain conditions is that there will be singularities. There is no paradox here at all.

It is not the case in quantum physics. things evolve unitarily before and after measurements but the results are random. Information is then erased and replaced by new information.
I wonder why the first (maybe wrong) attempt to solve the black hole paradox was not to say: it is not a paradox , it is a quantum process.

This is not solving the paradox, this is stating the paradox. The paradox is the inconsistency between classical GR, which predicts singularities, and QM, which says that singularities violate unitarity.

 

[Heidi]

Do you agree that information is lost during measurements in QM?
so the loss of information after a black hole evaporation would not surprise quantum physicists who are accustomed to these losses

 

[Heidi]

The title is about quantum physicists.
can we say that in QM the paradox is to have two different processes (unitary and non unitary) in the same theory?

 

[PeterDonis]

Do you agree that information is lost during measurements in QM?

It depends on what you mean by "information", "lost", and "measurements". It also depends on what interpretation of QM you adopt (which means that discussion of this question probably belongs in the QM interpretations subforum, not this one).

can we say that in QM the paradox is to have two different processes (unitary and non unitary) in the same theory?

This is certainly one of the possible issues with QM, at least under certain interpretations (the ones where "collapse" is a real physical process), but it is not the same as the black hole information paradox (although solving the QM issue might possibly point at a solution to the black hole information paradox).

 

[PeterDonis]

The title is about quantum physicists.

The OP of this thread asks about the black hole information paradox. If that isn't what you wanted to talk about, why did you ask about it?

 

[Nugatory]

Do you agree that information is lost during measurements in QM?

Information is lost only if we introduce the interpretation-dependent notion of non-unitary collapse. In collapse-free interpretations such as MWI the evolution of the wave function is always unitary and there is no loss of information.

 

[Demystifier]

Do you agree that information is lost during measurements in QM?
so the loss of information after a black hole evaporation would not surprise quantum physicists who are accustomed to these losses

There is a big difference between those two "losses" of information.

The quantum collapse transforms a pure state into a pure state, i.e. a zero-entropy state into a zero-entropy state, so the amount of information remains the same. (This is somewhat analogous to erasing the data on USB and replacing it with another data.)

The black hole information loss transforms a pure state into a mixed state, i.e. zero-entropy state into a non-zero-entropy state, so the amount of information changes. (This is somewhat analogous to permanently destroying the USB device with data.)

So no, quantum physicists are not accustomed to black-hole type of information loss.

 

[Cthugha]

The title is about quantum physicists.
can we say that in QM the paradox is to have two different processes (unitary and non unitary) in the same theory?

There is no paradox here. Stinespring's dilation theorem tells us that we can always go to the "church of the larger Hilbert space" as Smolin called it. Any non-unitary channel (e.g. a von-Neumann measurement) can always be considered as a unitary map in a larger Hilbert space (e.g. including the environment), where the additional part (e.g. the environment) has been traced out.

 

[Heidi]

can we say that if we include the environement there is no measurement result? that what we observe is only something due to our ignorance?

 

[Heidi]

I would like to see what happens in this enlargement process if we make successive measurements. i suppose that we have still unitarity but what about tracing out on the big system and the the successive possible results?
Do we get probabilities on possible histories of measurements or only on the last result?
sorry if all that is a nonsense. i began with the idea that there was loss of information in QM. this church of larger hilbert spaces is new to me.
and i am also skeptical (as Demystifier)
I see a "like" button. where is the button for skeptical?

 

[Demystifier]

this church of larger hilbert spaces is new to me.
and i am also skeptical (as Demystifier)

I am a member of the church of larger Hilbert space too, but this church alone cannot solve all our problems. In particular, it is not sufficient to solve the single measurement outcome in quantum mechanics and the information loss problem of black holes.

I see a "like" button. where is the button for skeptical?

Move the cursor to the like button without clicking it, and then click the last button.

 

[Heidi]

what do you think about the consistent histories interpretation?
of course it does not solve the probleme of one measurement but it seems to avoid collapses , and give the probability of sequences of results (it was one of my questions)

 

[Heidi]

I am ready to join this church too but what will i have to give up?

 

[Demystifier]

what do you think about the consistent histories interpretation?
of course it does not solve the probleme of one measurement but it seems to avoid collapses , and give the probability of sequences of results (it was one of my questions)

It gives the probability of sequences of results, but it is based on the Wigner formula which does not depend on interpretation. See e.g. Appendix B in my recent https://arxiv.org/abs/2107.08777.

As long as it does not solve the problem of one measurement as you say, I think that this interpretation does not serve a purpose that an interpretation of QM is supposed to serve.

 

[Demystifier]

I am ready to join this church too but what will i have to give up?

The idea that a small quantum system can be understood on its own, without its environment. This church promotes a kind of pantheism, so to speak. See e.g. Sec. 4.3 in my recent https://arxiv.org/abs/2010.07575.

 

[*now*]

There seem good reasons for scepticism regarding explanations relying on ignorance but there could be alternatives for coarse-graining that lack such problems, eg the paper linked post #15 https://www.physicsforums.com/threads/is-there-a-generalized-second-law-of-thermodynamics.1015122/