High School Collapse and unitary evolution

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Susskind argues that information cannot be lost in quantum mechanics, emphasizing the importance of unitarity, which he believes is a fundamental law. He suggests that while the collapse interpretation of quantum mechanics implies non-unitary evolution at the moment of measurement, the evolution remains unitary prior to measurement. The discussion also touches on black hole evaporation, which raises questions about non-unitary evolution before measurement occurs. Critics point out that the AdS/CFT correspondence, while promising, remains unproven and may not apply to our universe. Ultimately, the debate centers around the implications of unitarity and the nature of measurement in quantum mechanics.
  • #61
nrqed said:
I have always wondered why the absorption of matter by a black hole could not be considered a type of measurement, which would then take care of the loss o information. I am sure this is stupid for some reason but I have never seen a clear explanation why so.
The problem is not associated with absorption of matter but with Hawking radiation.
 
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  • #62
hi demystifier
you told me that there is a loss of unitarity when there is a measurement on a black hole.
how can one say that there is no loss of information during the life or a black hole (including evaporation) if once a measurement was done?
 
  • #63
Demystifier said:
The problem is not associated with absorption of matter but with Hawking radiation.

Is that right? It seems to me that it's the combination that causes a problem. If you have a source of correlated particle pairs, and one drops into a black hole, and later the black hole evaporates, then you're left with the remaining particle from the pair. It can't be in a pure state, because it was entangled with the particle that disappeared into the black hole. But it's not an "improper" mixed state, either, because it's no longer part of a composite pure state.

So it's the combination of particle absorption and black hole evaporation that has the effect of turning pure states into mixed states.
 
  • #64
stevendaryl said:
Is that right? It seems to me that it's the combination that causes a problem. If you have a source of correlated particle pairs, and one drops into a black hole, and later the black hole evaporates, then you're left with the remaining particle from the pair. It can't be in a pure state, because it was entangled with the particle that disappeared into the black hole. But it's not an "improper" mixed state, either, because it's no longer part of a composite pure state.

So it's the combination of particle absorption and black hole evaporation that has the effect of turning pure states into mixed states.
This is an information paradox, not the information paradox. I mean yes, that's a version of the information paradox too, but dropped matter is not essential because in the standard version the dropped matter is in a pure state and the problem arises from Hawking radiation only.

By the way, I have proposed a version in which Hawking radiation is completely irrelevant.
https://arxiv.org/abs/1802.10436
 
  • #65
PaleMoon said:
hi demystifier
you told me that there is a loss of unitarity when there is a measurement on a black hole.
how can one say that there is no loss of information during the life or a black hole (including evaporation) if once a measurement was done?
Loss of unitarity by measurement and loss of information by evaporation are different things.
 
  • #66
Fra said:
If you consider instead effective laws

Reductionism has no problem with effective laws. Sure, you can formulate effective laws for, say, human social interactions. And such effective laws will not be timeless, because humans and human social interactions evolve. None of that poses any problem for reductionism.

All reductionism is saying is that, for example, the electrons inside human brains work the same as the electrons inside rocks. The difference between a human brain and a rock--or, for that matter, between a human brain now and a human brain 100,000 years ago, given the evolutionary and social changes that took place over that time span--is not that the electrons, quarks, etc. work differently; it is that there are different highly complex arrangements of the electrons, quarks, etc. All the differences that you talk about when you talk about effective laws and how they might evolve are in the arrangements, not in the electrons, quarks, etc. That's all reductionism is saying.

Fra said:
This all gives us a drastically different perspective to things. In particular one, where the premises in the semiclassical information paradox don't quite hold.

I don't see how anything you've said has anything to do with the information paradox. Can you clarify what you mean here?
 
  • #67
Demystifier said:
Loss of unitarity by measurement and loss of information by evaporation are different things.
i wondered how information could NOT be lost if there is measurement and you tell me that there are several ways to lose information. it seems that you read my question too fast :smile:
 
  • #68
Demystifier said:
This is an information paradox, not the information paradox. I mean yes, that's a version of the information paradox too, but dropped matter is not essential because in the standard version the dropped matter is in a pure state and the problem arises from Hawking radiation only.

By the way, I have proposed a version in which Hawking radiation is completely irrelevant.
https://arxiv.org/abs/1802.10436

Hmm. It seems to me that maybe there is something that I don't understand about black hole entropy. I believe it's supposed to be true that the entropy is purely a function of the black hole mass (it's something like the area of the event horizon). But if you consider an entangled electron to have a different amount of entropy than an electron in a pure state, then the entropy increase due to dropping in an electron is not just a function of the mass increase.
 
  • #69
Demystifier said:
I have proposed a version in which Hawking radiation is completely irrelevant.
https://arxiv.org/abs/1802.10436

I have a question about this argument. Basically you seem to be saying that an extremely low energy photon can "fit" inside the black hole because, as it falls in, its wavelength gets strongly blueshifted. But you are also saying that the mass added to the hole in this process is negligible, which implies that the photon's energy is not strongly blueshifted, even though its wavelength is. I don't see how you can have it both ways. If the photon's energy is not blueshifted (which I don't think it should be, since the general rule for objects falling into black holes is that the object's energy at infinity is what gets added to the hole's mass), then its wavelength should not get blueshifted either.

Also, when we talk about an ingoing photon being blueshifted, this is observer-dependent; the blueshift is relative to an observer hovering close to the horizon. But such an observer has a large outward proper acceleration. I don't think the blueshift relative to this observer can just be assumed to be relevant to the photon's interaction with the black hole itself.
 
  • #70
PeterDonis said:
This is not correct.

Consider the parallel argument: we can't "read off" from a particular configuration of chemical elements, that some particular piece of matter is a US citizen. Therefore, US citizens are not made of chemical elements.

The first question is whether a person is a quantum object in the sense that everything about them can be explained by the QM of their constituent particles.

I believe it is not possible, for example, to resolve legal issues about someone solely from the particles that make them up. Or, issues relating to their background and history, for example.

This history inasmuch as it exists physically at all, is now encoded in particles belonging to other quantum objects.

A person is, at any point in time, a set of particles, but this doesn't imply that that is all they are.

The second question is whether QM could, indeed, be used theoretically to explain everything - about a human being, human society etc. I wouldn't say this is necessarily wrong but I'd say there is a shortage of evidence. There must be reasonable doubt about this.

Then, I suggest, it's a moot point whether the "reductionist" position is accepted as there is no evidence to the contrary; or, not accepted because it involves largely untestable proposition.

Personally, I would say I'm agnostic on the reductionist position.
 
  • #71
PeroK said:
The first question is whether a person is a quantum object in the sense that everything about them can be explained by the QM of their constituent particles.

Obviously we don't have the ability to do this. But that does not show that people are not made of those constituent particles.

PeroK said:
This history inasmuch as it exists physically at all, is now encoded in particles belonging to other quantum objects.

Yes, that's true; you can't "read off" the history of a system just by looking at the system, since at least a portion of that history is encoded in the states of other systems. But once again, that does not show that the system is not made of its constituent particles.

PeroK said:
The second question is whether QM could, indeed, be used theoretically to explain everything - about a human being, human society etc. I wouldn't say this is necessarily wrong but I'd say there is a shortage of evidence.

The point @Fra makes about chaos is relevant here. If the dynamics at some level are chaotic, then it might be impossible to explain phenomena above that level in terms of fundamental constituents below that level--at least if "explain" means "model quantitatively in detail". Since it is extremely likely that there is at least one such chaotic level between humans and fundamental particles, that means it might be impossible to explain humans in terms of fundamental particles. But, once more, that does not mean humans are not made of fundamental particles.
 
  • #72
PeterDonis said:
Obviously we don't have the ability to do this. But that does not show that people are not made of those constituent particles.
Yes, that's true; you can't "read off" the history of a system just by looking at the system, since at least a portion of that history is encoded in the states of other systems. But once again, that does not show that the system is not made of its constituent particles.
The point @Fra makes about chaos is relevant here. If the dynamics at some level are chaotic, then it might be impossible to explain phenomena above that level in terms of fundamental constituents below that level--at least if "explain" means "model quantitatively in detail". Since it is extremely likely that there is at least one such chaotic level between humans and fundamental particles, that means it might be impossible to explain humans in terms of fundamental particles. But, once more, that does not mean humans are not made of fundamental particles.

I'm not sure who said people weren't made of particles. Being a quantum object suggests to me more than that. E.g. being in a superposition of states. Dead or alive; rich or poor; physicist or lawyer; US Citizen or not. The question is whether all those "real world" observables can indeed be defined in terms of quantum mechanically defined observables.
 
  • #73
PeroK said:
I'm not sure who said people weren't made of particles.

I have been saying that "reductionism" is simply the claim that all macroscopic objects, including people, are made of particles. Reductionism doesn't say we have to be able to quantitatively model people or other macroscopic objects using the equations we use to model particles. I have been making this point because it seemed like @Fra was interpreting "reductionism" to mean the latter claim, not just the former.
 
  • #74
PeterDonis said:
I don't see how anything you've said has anything to do with the information paradox. Can you clarify what you mean here?

I will try to explain in shortly with some summing hints.

QM predicts quantum states (the connection to individual measurements is only probabilisitic). Premises are initial conditions and timeless laws. From this it follows that - set aside the COMPUTATIONAL TASK to actually execut the deduction, the future is equivalent to the past. So its a "dead" system, information is of course preserved. All we have are equivalence classes of histories. And the laws governing the quantum state flow is assumed timeless. (this is like in classical mechanics)

I am suggesting that the computational procesess and chaos here are a key players. With this i don't mean human made computers, it mean natural processing. You can consider the evolution of a physical system as a computation, or decoding laws of nature from experiemntal data as computation, or scrambling data in a black hole. After all, a REAL human made computer is also a physical process, so this is just a generalisation of the computation concept. Information can be lost and then reconstructed given enough data and computational resources, you need to account for TIME, to talk about information (decoding speed etc).

So my point is that randomness, chaos, and informtion contents, must be dependent on the observer, and the observers information processing capacity and learning speed. And these parts are idealized away in QM. In fact the "equivalence of future and past"
in QM is worth nothing unless the computation is actually performed. Also except in mathemtics maybe, i see no physical rational
behind concepts like "real randomness" etc. If an observer can not distinguish a signal from noise, it will be classified as noice, and in particular TREATED as noise. Ie. you will not "save noise data", it will be discarded. So there are possible behavioural predictions from this. It also seems quite resonable that the radiation from a LARGE black hole is far more hard to decode than from a microscopic black hole.

The root cause of things here is the idea that the classical obsever in quantum mechanics, serves as a FIRM ground, to FORMULATE the quantum theory. This was also the point of the founders such as bohr etc. MY point here, is that it is TOO firm, and thus blurs of discintionc between the relative of randomness. "True randomness" requires an hypothetical infinite information processing machiney to actualy infer. This we can easily "imagine" an classical observer to have, and dismiss as practical matters. But i strongly dislike this, and it think its a deep mistake

Of course these are no formal arguments but then soley serve to briefly convey (human-to-human) the connection i see to the information paradox. Ie. i THINK (can not prove it) that it makes no sense to talk about "no-hair" or perfect infromation preservation, we need to revise the theory to account for the actual computational limits. How this relates to physical parameters is a harder question, but there are already lots of papers on where one considers black holes to be "optimal scrambler" objects etc. So without having answers, it seems the MASS for sure must constrain the computational power. An massive observer at least should ahve the physical possibility to "resolve" strucuture where a lighter observer responds with treating it like noise (and this can be OBSERVER, and VERFIED by a third observer, so there is predictive potential here)

So to sum up, it seems radiation from BH might well be random relatie to small orbiting observers, as they arent meant to be able to decode. But a large observer that can consume the black hole as it radiates away, might possible decode it. All idealisation in calculations that ignores removes my confidence in them.

Anothing think relating to this is the note that the interesting various dualities betweeen theories that many poeple research, like AdS/CFT, typically has traits that relate to computational issues. That two dual theories have different computational complexity, so that in a sense they are equivalent, from the point of view of information processing one may be preferred. This is why they are also useful as mathematical tools. Another theory "corresponds" to a different way to calculate the same thing that is easier. One might thing that, this is just a mathematical curiousoty, but i do not think so. The computational requirements has everythign to do with physical processes in nature.

/Fredrik
 
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  • #75
Is it really true considering quantum field theory that things are "made" of particles?

Many states don't admit a well defined particle number, in fact one cannot even define a particle number operator on the interacting Hilbert space in some cases.

Also in the full interacting Hilbert space of QED for example, hydrogen states cannot be broken cleanly into electron and proton states. Many complex states like this in QFTs have to be added to the scattering asymptotic Hilbert space, as if independent of the simpler particle states.

This also ignores that electrons, due to infrared renormalisation aren't truly particle states, but infraparticles.

Quarks aren't even elements of the physical Hilbert space, due to colour, so I would wonder to what extent one could halfway state protons are made of them.

Finally you can show a sort of "nuclear democracy" for many fields. Where for fields A, B, C the field algebra can have any two as its basis.

The reductionist program remains unclear to me in QFT.
 
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  • #76
DarMM said:
Is it really true considering quantum field theory that things are "made" of particles?
...
The reductionist program remains unclear to me in QFT.
I get your point and i agree.

The discussion as well as subdiscussions here are very broad and deep, and with brief comments and we all have our own special fields we´re not always on the same page in discussions and sometimes that's main soure of disagreement.

/Fredrik
 
  • #77
DarMM said:
Is it really true considering quantum field theory that things are "made" of particles?

You could say "made of quantum fields" if you want to be more precise. It doesn't change the substance of anything I said. The complications you mention are there, yes, but they are well beyond the scope of a "B" level thread.
 
  • #78
PeterDonis said:
You could say "made of quantum fields" if you want to be more precise. It doesn't change the substance of anything I said. The complications you mention are there, yes, but they are well beyond the scope of a "B" level thread.
I'll start a new thread soon, as I think it does change something of substance and I'm not sure of the degree to which "made of quantum fields" is true either.
 
  • #79
DarMM said:
I'll start a new thread soon

It probably needs to be "A" level if you really want to get into the complications you refer to.
 
  • #80
PeterDonis said:
Please give a specific reference. We can't comment on out of context quotes.The models you are talking about do not contain any measurements, so the question of whether collapse takes place or not is irrelevant. These models are just the same as, for example, the "internals" of a double slit experiment, where even collapse interpretations agree that the evolution of the wave function is unitary; the only "collapse" is at the end of the experiment when the pattern is observed on the detector screen. The equivalent of that in the models you refer to is the universe in the infinite future, when all of the black holes have evaporated and all that is left is an infinite expanse of radiation at extremely low temperature. What "unitary evolution" means in this context is that, for a hypothetical observer in that infinite future universe, they can't tell from any of their measurements whether the infinite expanse of radiation came from the evaporation of black holes or from some other process (like matter-antimatter annihilation leaving only radiation behind) that didn't involve black holes at all.

this is perhaps the only answer i received to my opening question and i can accept it as correct but...
what you say is so obvious that i wonder why there was a "war" between Hawking and Susskind.
you are talking about two slits without hits on a screen. is it so simple?
 
  • #81
PaleMoon said:
what you say is so obvious that i wonder why there was a "war" between Hawking and Susskind.

Because the issue they were having the "war" over had nothing to do with the question you are asking about collapse. (@Demystifier already pointed this out earlier in this thread.) It had to do with whether unitary evolution is truly universal in scenarios where there is no collapse, regardless of QM interpretation, because there is no measurement. The issue was that Hawking's original model of a black hole that evaporates away made it impossible for unitary evolution to apply even if no collapse or measurement ever occurred anywhere--any quantum state or portion of one that hit the singularity would be destroyed, which is a non-unitary process.
 
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  • #82
stevendaryl said:
I believe it's supposed to be true that the entropy is purely a function of the black hole mass (it's something like the area of the event horizon).
Yes, many believe that it is so.

stevendaryl said:
But if you consider an entangled electron to have a different amount of entropy than an electron in a pure state, then the entropy increase due to dropping in an electron is not just a function of the mass increase.
Exactly! In other words, I put arguments that the wide belief above might be wrong.
 
  • #83
PeterDonis said:
I have a question about this argument. Basically you seem to be saying that an extremely low energy photon can "fit" inside the black hole because, as it falls in, its wavelength gets strongly blueshifted. But you are also saying that the mass added to the hole in this process is negligible, which implies that the photon's energy is not strongly blueshifted, even though its wavelength is. I don't see how you can have it both ways. If the photon's energy is not blueshifted (which I don't think it should be, since the general rule for objects falling into black holes is that the object's energy at infinity is what gets added to the hole's mass), then its wavelength should not get blueshifted either.

Also, when we talk about an ingoing photon being blueshifted, this is observer-dependent; the blueshift is relative to an observer hovering close to the horizon. But such an observer has a large outward proper acceleration. I don't think the blueshift relative to this observer can just be assumed to be relevant to the photon's interaction with the black hole itself.
The energy, or more precisely the contribution of photon to the black hole mass, is not blueshifted from the point of observer staying at a fixed position far from the black hole.
 
  • #84
Demystifier said:
The energy, or more precisely the contribution of photon to the black hole mass, is not blueshifted from the point of observer staying at a fixed position far from the black hole.

Yes, I agree; the energy the photon adds to the hole is its energy at infinity.

What I'm questioning is whether, in the light of that, treating the photon's wavelength as blueshifted near the horizon makes sense.
 
  • #85
PeterDonis said:
Yes, I agree; the energy the photon adds to the hole is its energy at infinity.

What I'm questioning is whether, in the light of that, treating the photon's wavelength as blueshifted near the horizon makes sense.
It makes sense because this blueshift concerns the size of the wave packet. The size must be smaller than the black hole in order for the black hole to absorb it.
 
  • #86
Demystifier said:
It makes sense because this blueshift concerns the size of the wave packet. The size must be smaller than the black hole in order for the black hole to absorb it.

It seems to me that worrying about photon wavelength is sort of a red herring if the same point can be made with entangled electron/positron pairs.
 
  • #87
stevendaryl said:
It seems to me that worrying about photon wavelength is sort of a red herring if the same point can be made with entangled electron/positron pairs.
It can't, because the energy of the electron cannot be made arbitrarily small.
 
  • #88
Demystifier said:
It can't, because the energy of the electron cannot be made arbitrarily small.

Okay. So you can't drop an unlimited amount of entropy into a black hole using electrons without increasing the size of the black hole.
 
  • #89
stevendaryl said:
Okay. So you can't drop an unlimited amount of entropy into a black hole using electrons without increasing the size of the black hole.
Yes, that's why I use photons.
 
  • #90
Demystifier said:
It makes sense because this blueshift concerns the size of the wave packet. The size must be smaller than the black hole in order for the black hole to absorb it.

I understand why the wavelength is relevant. I don't understand how you can consider the photon's wavelength to be blueshifted but not its energy.
 

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