## Monday, October 28, 2013 ... /////

### Raju, Papadodimas isolate the reasons why there aren't firewalls

Wonderful new papers refine the black hole complementarity and show that the black hole interior operators are included in the CFT and other descriptions of the bulk while locality holds more exactly than previously thought (by most experts)

Kyriakos Papadodimas and Suvrat Raju, two careful and bright researchers with a refined Harvard pedigree, wrote their first paper about the black hole information puzzle in November 2012.

Yesterday, they released two new papers that make the incorporation of the infalling observer's observations into a description of quantum gravity – especially the AdS/CFT correspondence – clearer than ever before. But before I will discuss their new insights, let me look at a fresh hype by Clara Moskowitz in Scientific American,

which seems extremely unfortunate to me. My memory isn't too bad but I really don't remember the last article in Scientific American about theoretical physics, climatology, or sociology of science that didn't look painful to me.

The basic premise of Moskowitz's article is that a significant contradiction was found by Joe Polchinski et al. in quantum gravity, the firewall paradox, and most of the insights that were claimed to have been settled became unsettled again. While Scientific American would promote the "consensus" in other situations, now the fact that a majority of the AMPS followups say that there is no firewall paradox is not only ignored but it is being pretended that the papers don't exist at all.

Moskowitz quotes several physicists who say extreme things. Those things may be divided to two opposite extreme camps – extremely excited ones and extremely disgusted ones – but every reader who can read in between the lines understands that these two camps really want the reader to make the very same conclusion: something is seriously wrong with the field.

Raphael Bousso says that the firewall arguments are "by far the most shocking and surprising things of his career". Different people clearly think very differently because they surely don't make it to my top 1,000 list. Joe Polchinski says that he's uncertain about the firewalls half of his life but no one has found a flaw in their arguments – which, as far as I can say, is a lie, in fact a collection of about 5 lies packaged into one package. Several independent serious enough flaws in the AMPS arguments (enough to invalidate them) are known.

Donald Marolf does admit that many people do say that their pro-firewall arguments are flawed but he dismisses all these papers that disagree with AMPS and MP as "red herrings". And Brian Greene says that this whole ludicrous firewall fad (my words) is "what we live for". Not exactly the measured wording that I used to praise Brian for.

On the other hand, Moskowitz quotes Matt Strassler who indefensibly claims that "quantum gravity is stuck". It even prints sentences by a hardcore crackpot, a teaching assistant at Columbia University, about "people lulled themselves to sleep" and now "shaken out of their dogmatic slumber" while Donald Marolf is said to "agree" with those insane propositions. Make no doubts about it: sleeping are those who haven't noticed how wonderfully consistent the previously accumulated wisdom has been shown by the research of quantum gravity in the recent 2 decades or so.

While Bousso & Greene and others are presented as "folks excited about the developments", it must be very clear that in reality, they play the role of something that Joseph Stalin would call "useful idiots" for the basic propositions of demagogues like Strassler and the teaching assistant at Columbia University that quantum gravity sucks.

All these people are wrong and, when it comes to their more general proclamations, a more accurate assessment is that they are all deluded. It is sort of sad to look at several chaps whom I used to value so highly as they are joining this decadent "everything is stuck and, at the same moment, in the state of permanent revolution" movement. I emphasize that this comment is mine and shouldn't be automatically associated with other folks who think that there aren't valid arguments showing that firewalls exist.

Now the clever and careful papers: why there aren't firewalls

Fine, that was the negative part of this blog post. Now the positive one. I had the pleasure to see Kyriakos Papadodimas' and Suvrat Raju's new papers in advance; they appeared yesterday in the morning. Their titles are
The Black Hole Interior in AdS/CFT and the Information Paradox (5 pages)

State-Dependent Bulk-Boundary Maps and Black Hole Complementarity (92 pages)
The short paper is a summary of the long one but both of them are excellent. Suvrat and Kyriakos demonstrate that the AdS/CFT-based (and probably other) descriptions of quantum gravity do contain sufficient degrees of freedom to account for all the infalling observer's observations while the unitarity is preserved and all the paradoxes in the literature are resolved.

The two features of their description that contradict some of the AMPS assumptions (and assumptions of other firewall advocates) are the usual notion of complementarity – namely that the degrees of freedom inside the black hole are a reshuffled collection of observables that are in principle already included in the observations outside; and that the definition of the black hole interior operators is state-dependent.

Well, perhaps one should use a less provocative adjective than "state-dependent", perhaps "environment-dependent", to emphasize that there is a sense in which the change of the black hole interior operators is "small" if the state upon which the operators depend is changed "a little" (by an action of a few operators). This is the reason why the operators are still "large enough matrices" with many matrix elements. We're effectively not fixing a ket vector exactly; we're choosing a subspace of the Hilbert space that contains states that are "similar" to this ket vector – where the similarity means "obtainable by a limited number of actions of local operators".

With these disclaimers or generalized assumptions – which are perfectly compatible with all the postulates of quantum mechanics and all existing knowledge of string/M-theory or AdS/CFT, they explicitly show how all the paradoxes in the literature known to Suvrat and Kyriakos are resolved, including:
• the strong subadditivity puzzle by Mathur and by AMPS
• the $[E,\tilde B]\neq 0$ paradox
• the non-existence of a left inverse paradox by AMPS
• the Marolf-Polchinski $N_a\neq 0$ paradox
• the objections against unitarization of the Hawking radiation by small corrections
• the frozen vacuum criticism by Bousso
and several others.

When unnecessary complicated things from the pro-firewall arguments are stripped off, their basic claim may be formulated in a very simple way: The pro-firewall folks always claim that the CFT in AdS/CFT or any consistent, unitary theory of quantum gravity cannot contain enough degrees of freedom to define the operators inside black holes which means that the perceptions of the infalling observers just can't be predicted from the CFT. It follows that the babes who fall into a black hole must have a "universal perception" – namely a death caused by their contact with the black hole event horizon where a deadly "firewall" should consequently reside.

Papadodimas and Raju show that this claim against the completeness of the AdS/CFT correspondence (and probably other descriptions) is invalid by explicitly constructing the black hole interior operators that have all the desired properties and obey all the consistency conditions. More precisely, they carefully summarize what these conditions are and they prove that the conditions required from the black hole interior operators have solutions.

In their first paper, the black hole interior operators depended on the separation of the degrees of freedom into fine-grained and coarse-grained. This dependence looked puzzling to me (and probably others) and it was eliminated in the new papers. Instead, the potential non-uniqueness of the resulting operators was extended by finding all solutions for the conditions that the set of black hole interior operators have to obey.

The different constructions don't substantially differ and it's likely that an infalling observer doesn't have enough time (before she's killed in the singularity) to figure out which of the possible sets of operators is realized in her actual universe.

OK, what are the basic objects and the key conditions? The basic reason why the operators inside the black hole are included in the description of the AdS physics by the CFT is that for a particular state $\ket{\Psi}$ which is pure but effectively indistinguishable from a thermal one (and for the similar states obtained by an action of a few natural/local operators on $\ket{\Psi}$), the set of local operators may be doubled.

In other words, one may define the so-called "mirror operators" $\tilde A_p$ that satisfy$\eq{ \tilde A_p \ket{\Psi} &= \exp(-\beta H / 2) A^\dagger_p \exp(+\beta H/2) \ket{\Psi}\\ \tilde A_p A_m \ket{\Psi} &= A_m \tilde A_p \ket{\Psi}. }$ So the tilded operator to an annihilation operator is conjugate to the corresponding Hermitian conjugate by the evolution by $\exp(\beta H /2)$, the operator shifting you by one-half of the thermal circle (that drives you to the other side of the cigar in the Euclideanized black hole solution), at least when it acts on $\ket{\Psi}$.

The second condition is that the tilded operators "seem to commute" with the untilded ones when they act on $\ket{\Psi}$. This is enough for the commutator to "effectively vanish". This effective vanishing means that if you calculate the commutator's matrix elements – or, more generally, the matrix elements of this commutator multiplied by a limited number of other operators from both sides – in the state $\ket{\Psi}$ and/or a "small enough excitation" of $\ket{\Psi}$ (which may be constructed by a polynomial action on $\ket{\Psi}$), these matrix elements vanish.

Those two conditions reproduced as eqn (3.8) on the longer paper's page 12 represent the (almost?) only displayed $\rm\LaTeX$ equation in this blog post because I believe that by these conditions, Papadodimas and Raju really got to the essence of the problem of the black hole interior and found a much more accurate formalization of the black hole complementarity than one that we have vaguely believed for two decades.

What do I mean?

Since the early 1990s, people who took complementarity for granted (including your humble correspondent who was arguably affected by my ex-adviser Tom Banks who claims to have learned these things directly from Lenny Susskind) would usually say that the commutator of the interior and exterior operators "is not exactly zero" although "it is effectively zero for all measurable purposes". Such a comment seemed to directly follow from the assumption that the black hole interior operators are "redundant" in the sense that they're already encoded in the physics that is generated by the black hole exterior operators.

Papadodimas and Raju explain why the commutator is "effectively zero" in a completely new, more satisfactory, more constraining, more well-defined way. They actually say that the commutator of the interior and exterior operators vanishes exactly as long as it acts on the state $\ket{\Psi}$ for which the interior operators were defined – or its low-excitation siblings.

The fact that this "effective vanishing" isn't "completely vanishing" and ultimately breaks down if you insert the product of too many operators in front of the ket vector (or if you try to make too many measurements, operationally speaking) is the underlying reason why many of the paradoxes mentioned in the literature are resolved.

Kyriakos and Suvrat show that their conditions for the mirror (and black hole interior) operators have a solution by counting dimensions of the available space and the number of constraints. But they also identify the right operators in various numerical ways (I got a very transparent toy model Mathematica code doing it for the spin chains) and even analytic ways – even in cases that aren't the simplest ones.

As I said, there are over 50 papers that use various constructions and observations to argue that there aren't any firewalls. These 50+ papers aren't exactly equivalent to each other. Do they agree? I believe that the new Papadodimas-Raju paper defines the most general framework for addressing the problem.

What do I mean by this generality? Well, I believe that any picture that solves the problem arising from the infalling observer's observations does have to claim that the black hole interior operators exist in some sense and they may be expressed as functionals of some operators that exist in a theory that doesn't need to talk about the black hole interior at all – the interior operators are effectively encoded in the exterior ones but in a complicated way.

Any satisfactory solution to the black hole interior problems should probably obey the Raju-Papadodimas conjugation-and-commutation consistency conditions, at least approximately. But because they show that the conditions may be required exactly and we don't get an overdetermined system, it really seems that a good solution obeys their conditions exactly, in the form they have stated. At most, you have the freedom to choose one particular solution for the interior operators from the Papadodimas-Raju set of solutions.

So for these reasons, I believe that even Maldacena-Susskind's ER-EPR correspondence should be viewed as a special solution or Ansatz to the new Papadodimas-Raju rules of the game – although we might be forced to resolve the problems for the operators inside the Einstein-Rosen bridges rather than inside ordinary black holes.

But I believe that even though it hasn't been written in the literature yet, the Papadodimas-Raju way of thinking implies a rather simple way to prove that the ER-EPR correspondence is true. How do we prove it? Well, we just prove that if we have the Hilbert space of states with two black holes, it is possible to find the "Einstein-Rosen-bridge interior operator" that obey all the conjugation-and-commutation conditions that are needed for the locality and that connect to the two black holes smoothly. So we may literally show that the two black holes' horizons may be connected by a non-traversable wormhole by defining the operators in this wormhole that smoothly connect to the black hole exterior operators at the two horizons, that obey all the locality etc. conditions we expect, at least when restricted to the neighborhood of a state $\ket{\Psi}$, and that only use the Hilbert space of the two (seemingly disconnected) black holes we started with.

I find it almost obvious that this will work and the paper with all the details may be written down. ;-) If it doesn't work for some reason I can't foresee now, I think that the Raju-Papadodimas' methodology is general enough to allow you to disprove the ER-EPR correspondence.

There are other ideas that these papers led me to and I wrote some of them to Kyriakos and Suvrat. They will surely have lots of their own new ideas. At any rate, I think that you have seen the text that demonstrates that all the claims that the firewalls are supported by solid arguments and no flaws in these arguments have been known are simply untrue. There exist papers that are much more careful, constructive, and explicit than papers by Marolf, Polchinski, their collaborators, and their followers, and it's these papers that deserve to be read because they really show how the things work.

The research in quantum gravity isn't stuck and the frameworks allowing black holes to be incorporated in unitary quantum theories have been around for 2 decades or so. The provocation by AMPS has arguably led to these more explicit proofs that all the apparent paradoxes one might invent are illusions due to the subtly coherent internal structure of the theory. But it's important to say that these paradoxes were never there and many quantum gravity folks always "knew" the reasons although only Kyriakos and Suvrat (and perhaps others) were able to disprove the existence of the paradoxes this clearly. Yes, I realize that these comments of mine are completely analogous to what I said about the discovery of quantum mechanics and various would-be paradoxes that some people starting from EPR (incorrectly) "saw" inside quantum mechanics.

Incidentally, a month ago, there was a Google+ Hangout featuring Bousso, Maldacena, Polchinski, and Susskind – and a host, Bruce Lieberman. It's fun but I think that Juan was by far the wisest guy in this 53-minute video. He would compare the AMPS-like paradoxes to the "many" paradoxes that people would be "finding" in statistical physics as a theory of thermodynamics – such as Maxwell's Demon. When one looks carefully, no paradoxes are actually there. For example, we must carefully impose the laws of thermodynamics and/or statistical physics to Maxwell's Demon himself and when we do so, his miraculous abilities to create a paradox evaporate.

It's just an analogy, he stressed, but a damn good one, I would add, because the geometry in general relativity is a similar "emergent" structure in a theory of quantum gravity as the thermodynamic variables and the coarse-grained descriptions are "emergent" features of a system in statistical physics. Well, I had the feeling that the other three guys' faces indicated that Juan's comments looked like a form of heresy to them. It's not a heresy, it's almost certainly the truth. And by the way, the analogy isn't insulting. Maxwell's demon was coined by a guy called Maxwell who wasn't exactly a loser. He had found many true important things and sometimes added an aether or a demon on top of them. ;-)

But to be sure that I am not recommending you wear pink glasses either, Maxwell was probably getting senile when he began with the demon in 1873 although he was just 42 then. He had found the Maxwell-Boltzmann distribution in 1857 and completed Maxwell's equations by 1865, years before the demon.

#### snail feedback (71) :

Wow, the two papers you linked to (Raju and Papa dodimas) are amazing!

I finished reading the 5-page paper (understood just some bits and pieces of it) and was so happy when I could actually understand Section IV : )

So AdS/CFT is just dependent on the existence of those funny O^i's in the CFT, nice!

By the way, did you notice? The Reference [11] in the 5-page paper, the "6335" became an "nnnn".

Thanks for your interest! And good that you noticed the untranslated nnnn.

I wanted to say that "nnnn" should automatically translate to the right number at the arXiv server but this comment would be nonsense because that could at most generate the right "nnnn" of the same paper where "nnnn" is written but they won't to refer to the other one, right? ;-)

Black holes don't exist as shown by Stephen Crothers.

This is spam, can it be removed :-( ?

Thanks for this summary, Lubos, it really exercises the little grey cells.

How to answer stupid critics of string theory and Leonard Susskind black hole in less than a word?

Are you sure that you are using little grey cells as opposed to little white cells white are more abundant in a female brain?

This comment of mine was written on my tablet ASUS ME301T with the FolioKey keyboard. For the generic long-text writing, I find this combination almost as fast as a laptop keyboard. There are certain tasks that are much slower with the tablet if not impossible, however.

this white cell business escaped me, I was just quoting Hercule Poirot from Agatha Christie.

Mayne I am color blind and all cells are grey :)

There is nothing like white and gray cells. There is only white and gray brain matter. The gray matter is formed of cell bodies and white matter is formed of myelinated cell axons (myelinated = insulated like a copper wire). Males have relatively more gray matter, females white matter. More white matter can mean more processing speed because the action potentials can travel faster.

I note that Papadodimas and Raju reserve special thanks for Lubos Motl among their many acknowledgements.

An 18 billion solar mass static black hole (observed OJ 287) has its event horizon radius 2.95 km/solar mass = 355 astronomical units. That is nine times Pluto's semi-major orbital radius. Said event horizon is locally zero curvature (OK, an epsilon - still no tidal spaghettification upon entry). What major event (aside from inescapability) informs you of entry, say dangled from a cable?

From where would the firewall suddenly arise upon crossing such a gentle event horizon?

You mention the thermodynamic paradoxes. Maxwel's daemon is imho the less interesting one. Loschmidt's paradox is more interesting. It is interesting that you disagree with Phil Gibbs about it

The logical arrow of time is some invention of yours or does any other physicist believe in it? It seems like an ad hoc axiom invented to perform the time symmetry breaking mentioned in the discussion. It essentially says that time is asymmetric because our logic says so which sounds like tautology and fallacy to me. Our logic is the product of time asymmetry and not the other way round.

It's really bizarre what many people including Phil want to believe about such things.

You say that Loschmidt's paradox is more interesting and you probably want to say it's unresolved, too. But it's nonsense.

There is a sense in which Loschmidt's and Maxwell's demon non-paradoxes are completely equivalent. Both of them claim that it is possible to design processes with decreasing entropy

The logical arrow of time is not my invention, it's a standard term used by competent physicists when they discuss connections between various manifestations of the arrow of time.

Among the papers that use it

(some of them say "cosmological arrow of time" and one has to eliminate these hits), I recommend you the paper by Omnes on the truth in quantum mechanics.

It looks like a fallacy to you because you're dumb. Your comment that the logic is a result of a pre-existing asymmetry is the same idiocy that leads you to believe in the objective reality behind quantum mechanics. There is no objective reality. Quantum mechanics - and even classical statistical physics - has the purpose of deciding about the validity of propositions. They're deciding by assigning probabilities to propositions which is why the logic is always primary.

The primary laws of physics that make these calculations possible have to assume that the future properties are probabilistically calculable effects of the past and not vice versa. It follows that the past may only be reconstructed from the future indirectly, via Bayesian inference, and the results always depend on arbitrary priors. All objectively calculable probabilities in physics are probabilities of implications whose assumptions are properties that are evaluated at times before the properties in the claims of the propositions.

Isn't grey/white matter composed of grey/white cells?

I just heard from a Czech Swedish amateur physicist in the pub that individual gold atoms are "green". ;-)

Thermodynamic paradoxes are not really paradoxes, are they? Entropy can only be slowed down. It looks like reversed entropy from a *classical* point of view but in reality it is just a manipulation within the experiment to make it look like reversed entropy.
Am I right?

Symmetry, breaking ooooh! A hermetically isolated hard vacuum envelope contains two closely
spaced but not touching, in-register and parallel, electrically conductive
plates having micro-spiked inner surfaces.
The plates are connected with a wire, perhaps containing a dissipative load
(small motor). One plate has a large vacuum
work function material inner surface (e.g., osmium at 5.93 eV). The other plate has a small vacuum work
function material inner surface (e.g., n-doped diamond "carbon
nitride" at 0.1 eV). Above 0
kelvin, spontaneous cold cathode emission runs the closed isolated system. The plates never come into thermal
equilibrium when electrically shorted.
The motor runs forever. Find the
error. (Not a Shottky barrier - heavy
doping, ohmic contact for the diamond.)

Dear Lubos,
I think psychological arrow of time is a better name because time is not really a part of logic or mathematics for that matter. I understand your reply in a way that first you have to state how the results your theory links the observations before it even makes sense to talk about a physical theory. Quantum mechanics talks about probabilities of future events so one has to already know what one means by that. Still one may ask where is the psychological arrow of time is coming from but I think no one has a good answer,

II see that you had implicitly anticipated and disposed of the paradox, long ago. In context of eternal inflation in 2011:
"we should also admit that vastly different environments belonging to different vacua have non-convertible time coordinates (which are linked to the spatial ones by the respective Lorentz symmetries) and the unification of the patches to a bigger spacetime may be meaningless. "
That was almost a year before the discussion in the context of firewalls. You made this in that co text already clear by a year ago. Please do not Blacklist me for ths but seems that the firstvfive page paper is right but should have Given credit to Dirac And you. It treats Time as an operator not merely a fixed dependence that is time does not merely dilate but does not commute inside and outside the black hole radius. Once one stops imposing a single time operator on space based on the outside degrees of freedom outside and ditto for the

Hmmm again this IPad declines to edit. Sort of like a black hole once the information crosses the threshold it is in a different spacetime

Lubos wrote: There is no objective reality.

Can I ask what may be a stupid question? AIUI, a photon, say, has certain observable properties, such as spin. Said spin has to be measured to know what it is. As you said, it has no objective reality until it is measured.

But. what about the photon itself? Does it not exist until it is measured? I'm getting hung up on the distinction between the thing (which may, or may not, be objectively real) and the properties of the thing (which are not objectively real).

Do they have more of it or more of it works when doing the same problem compared to the other sex?

From a newer paper by the same author:

http://www.sciencedirect.com/science/article/pii/S0160289611001206

you can search for the pdf, males have both more gray matter and white matter and even the ratios are similar.

NOT a stupid question. There are many stupid answers by people who have not a clue. So you are lucky that Lubos is the one place to study and come to clarity on this. See:

http://motls.blogspot.com/2012/11/why-subjective-quantum-mechanics-allows.html

Yes. I had missed that but not that this is what he explained long back.

Wonderful. Quality and progress by traditional standards. Makes me very happy and excited. Thanks for promoting this type of work here. I'll dedicate the rest of the day to studying their papers. :-)

Susskind around 50:30 says that the problem is that we don’t have a theory of QG (“we thought we had a theory”) to answer these questions since String theory is not powerful enough to confront such problems. I’m not sure why you need a theory of QG to deal with the black hole horizon physics. Isn't the semi classical approximation good enough? We don’t have strong curvatures in that regime and Quantization of fields on curved background geometry should be adequate.

Do we need another revolution to supplement AdS/CFT? I don’t think so and Papadodimas and Raju prove that. The CFT on the boundary gives the complete dual description of everything that is happening in the bulk, you just need to look carefully…

Thanks for this nice article Lumo, even though I am a bit late to the party ... :-)

Just forget about the Scientific American, at least concerning (fundamental) physics topics it is not worth readidng (anymore?) ... :-/

What the rest of the TRF article says I like a lot, as far as I understand it ;-)

So can we look forward to a Lumo/Raju/Papadodimas paper (the order of the names bears no meaning) applying the new way of thinking to (I rather bet) prove the EPR-ER correspondance (and maybe other arguments against the firewall) ...?

Huh ...?

This is off topic and I have no idea why and what you are talking about...

Maybe Lumo could even appear on the list of authors too next time ...?

Me neither. Both Mephisto's and Uncale's comments look like randomly generated nonsense with no connection to the article, nor within the comment itself.

Mikael did understand my point. You didn't. The point is that our brains evolved in an asymmetric time, our logic is product of our brain, hence you cannot use "logical arrow of time" as a reason for time asymmetry because you are using an effect to argue for the cause (circular reasoning = tautology). We simply need an aditional axiom to perform time symmetry breaking - one such axiom is the second law of thermodynamics. And the Loschmidt paradox is that you cannot derive the second law of thermodynamics from underlying laws of physics which are time symmetric (you simply need to state that entropy was low at the beginning as an axiom).

What was the relation of my comment to this blog? Mosty a free assotiation to termodynamic paradoxes discussed at the end of the blog.
At least I am trying to discuss physics but you seem to be more interested in sociological issues which I am not.

I did understand your point but your point is just wrong. The logic relating the initial knowledge and the final predictions is primary, the physical phenomena are reflections of this logic. There is no objective reality that would be "above logic".

You seem to have the same problem with QM that I had (and still have, although I supressed the problem for the moment, but I will come back to it after studying QFT some more)
It is the question of objective reality. It makes absolutely no sense to claim that there is no reality prior to measurement. There are several hints that something must exist prior to measurement. If you prepare particles in some pure state and after that you measure the particles again (you prepare them in spin up state relative to z-axis and later you measure them again relative to z-axis), you always get the same result - that means nature need some mechanism how she "remembers" the state of the particles (that they were prepared in the up state). The outcomes of measurements are not created out of nothing by some random generator during the act of measurement. Nature remembers quantum correlations, remembers if the particle is pure state or not etc. The point is that this "remembering" must exist somehere in reality.
The real problem with QM is that we describe this reality with wave functions but we know that the wave functions are not the reality. That is the most difficult conceptual step with QM - understand that the wave function is not real. What is real, what is really there prior to measurement nobody knows. There is surely something, some mechanism how nature remembers correlations. But where is this mechanism stored, where does it exist? Nobody knows.

I didn't mean you, I meant Dimension10 who wrote that my comment was "randomly generated nonsense".
BTW, I remember that as a kid I read my first book about physics - The Brief History of Time by Stephen Hawking. It was my first book about physics and it was the book that got me insterested in physics for the first time. I remember reading there that the arrow of time is linked to cosmic expansion - the universe expands, time flows forward and entropy increases, and it claimed that if unverse contracted time would run backwards and entropy would decrease. Does it make any sense? Was it just some fancy idea of Hawking or does it have any justification? Why would time run backwards if the universe contracted?

Dear Lubos,
there may not be an objective reality on the most fundamental level but I think you would also not go to the other extreme of solipsism. You must be familiar with the argument of evolutionary biology that our eyes reflect the laws of optics because they got adopted to this over millions of years. Similarly our brain may have gotten adjusted to an arrow of time which exists in the outside world. Or time may just be a psychological illusion,who knows. I am not claiming that these arguments are a part of physics, just things I ask myself.

"That is the most
difficult conceptual step with QM - understand that the wave function is not real. What is real, what is really there prior to measurement nobody knows"

I don’t understand you. That sort of problems are greatly resolved (i believe) by reading papers and book on consistent histories interpretation by e.g. Robert B. Griffiths and you must know it, cause I saw your earlier posts on decoherence etc.

http://quantum.phys.cmu.edu/CQT/

As I understand it, neither measurement nor wave function are fundamental and it isn’t big deal.

I may point simple paper that answers "conceptual problems" based on toy models http://arxiv.org/abs/quant-ph/0612065

"There is surely something, some mechanism how nature remembers correlations. But where is this mechanism stored, where does it exist? Nobody knows."

But isn't it just hidden variables idea? Isn't the first thing you learn in QM is that there is no such mechanism? You may present it as axiom (and it works perfectly as seen by experiment) or sort of "derive" it by Consistent Histories. You just want to be UNCONFUSED observer and everything follows from normal logic and rules of QM.

I must either not see something elementary (because that whole confusion is about the most elementary QM) or you say smth wrong. I confess that I didn’t have proper QM course yet, but from reading Dirac book and Griffiths’s one (and Omnes too), I think I understand those issues. Ultimately, aren’t they something you should learn at first 2-3
lectures of QM? That stuff is rather simple comparing to hydrogen atom or perturbation theory, I believe – I still have to do that stuff in Dirac book :)

The outcomes of observations are not created out of nothing by a random number generator, Mephisto, but a random number generator does, indeed, participate deeply in any observation. The wave function represents our state of knowledge and it precisely specifies the relative probabilities of a measurement.
Whether reality exists prior to the observation is not a scientific question, it is a philosophical or religious one.
Until you utterly abandon your futile quest for reality you will never get it. If you do get it you will understand that there are no mysteries in QM. Nothing whatsoever is missing; QM is complete and exact.

Hi, thanks for your answer. I had a proper course in QM, but I must admit that I did not visit the lectures and I am mostly autodidact because I studied while working and could not attend. I passed the course by learning how to use the mathematics - at my exam I had to compute the spectrum of 2D harmonic oscillator and probabilities of quantum transitions in magnetic resonance (perturbation theory), but the interpretation of the equations I supplied myself.
I never studied the consistent histories interpretation. It is definitely on my to-do list. But you are quite good if you can read Dirac's book without even having had a course in QM. So you might understand it better than me and it might be that I am confused. IMHO the main confusion concerning QM comes from the Copenhagen school - all these concepts like collapse of the wave function, there is no reality, measurement problem etc. are terribly misleading. I will read the paper you linked in another post - from Griffiths.

You are right, Shannon; there are no paradoxes in thermodynamics and the growth of entropy (in a closed system) is inexorable. Any experiment that shows an entropy reduction is dealing with an open system and there exists elsewhere an entropy increase greater than the observed loss.

I have to say that this wonderful article doesn't quite deserve this philosophical discussion about quantum mechanics and thermodynamics. But it is too late because the discussion already started.

Yes, I am sorry to spam it here. The problem is that the firewall problem is very advanced and not many readers can follow.

Not sure either but it looks like he is announcing that he has invented a perpetuum mobile. Such geniuses, giving away their precious knowledge for free on the Internet.

Hi, thanks for your answer, too. I also wanted to say that i admire your passion for physics - not many people would be willing to study for degree as a hobby, while working at the same time. I also can say that i feel kind of respect for any inteligent physician, because majority of my family are ones. :)

Well, i kind of lied, last summer i dedicated to those "conceptual" stuff + i watched Oxford course at http://www-thphys.physics.ox.ac.uk/people/JamesBinney/qm_synopsis.html
and did simplest problems from that, so that I would be ready for Dirac book that summer. Unfortunately, instructor there, while excelent in technical stuff (which is the most important, btw), subscribes to "copenhagen is wrong". Fortunately, I 've read Lubos posts about such issues earlier so i just skipped it ;)
Also, my high-school teacher (who used to be assistant-prof at local univ) gave me sort of pre-QM course - classic papers by Planck, Sommerfeld and Bohr. Then, when I learned enough calculus etc. to understand Schrod.eq in simplest cases, we did reading of initial chapters from Dirac book. And later I read some papers by Omnes and Griffiths on Consistent Histories and later a book by latter. And that's it - I don't think "conceptual" problems are that important to spend more time on it.

Well, the second problem is cool that, as i said, don't know how to solve because I didn't go that far with Dirac book. I have to do much more problems in easy QM, before that, so that the knowledge stays forever.

You either have a really good random text generator, or are really just babbling off-topic garbage. Probably the second.

You' are not trying to discuss physics. You' are trying to test out a really good garbage generator or are trying to teach people how to babble garbage. Probably the second.

And what sociological issues?

Mephisto, could you please immediately stop with this repetitive crackpot gibberish? This is a thread about the black hole information puzzle, not a place for aggressive cranks to promote their delusions about/against quantum mechanics that they have written about 500 times already.

I am ready to close my eyes and agree that you shouldn't be euthanized for this mental defect of yours - but that's it.

Mikael, in science, one can't a priori decide which opinion is extreme. Things may look extreme to an ignorant person - or billions of persons - but they may be completely correct.

With this said, I didn't promote any solipsism and I didn't even remotely suggest that anything in quantum mechanics or statistical physics depends on the shape and inner organization of our eyes or other senses.

This is just a plain demagogy you add to this discussion to create the impression that there is anything wrong about the super important things I am trying to teach you and other ignorant readers. But as long as you will invent demagogical excuses why you don't want to understand the basic principles of physics, you will never understand the basic principles of physics.

Someone's lack of knowledge needed for a certain topic isn't an excuse to contaminate the thread about an important topic by delusions about basic physics that you have written about 500 times already.

I hopefully removed you from the whitelist because your presence here isn't possible without a pre-moderation.

I would say: locality holds more exact than previously thought *by some* :D

As you wish Lubos. If you prefer a blog where dumb immature agressive kids like Dimension10 spam you blog with constant idiotic comments about forums on the internet and other users there then it is your call (I did not find a single comment of his that would be about physics). I guess I will take a break from this blog. I hardly need it for my internal hapiness

I read the shorter paper and, yes, it is correct although rather unsurprisingly so... Even I said something similar on this blog some time ago and had a small discussion with Lubos about that... I will definitely have to read the longer paper as well as it may be instructive but the main idea is the same... and again, if thinking in the context of consistent histories this is really no surprise. What I found very deep, intelligent and interesting are some remarks by Lubos in the previous post about the ER-EPR duality. Mainly those related to the background choice and how this procedure can be seen from the perspective of complexity... I still wait for that damn PRL to review my paper and it's been 5 months already... At that moment there was no ER-EPR conjecture... they actually posted that on arxiv 2 weeks after I sent the paper to PRL but I made an explicit use of this kind of freedom of choice... with a bit of luck you will see it in PRL by the end of the next century and we can talk about it... of course, unless until then arxiv will be oversaturated with it...

for god's sake people: it is not that there is no objective reality... there is no objective choice of a framework to describe that reality in the same way there is no objective choice of a reference frame or of a preferred color... the questions you ask with one choice of observables you cannot ask with another choice, that's why there is quantum entanglement and that's why some operators do not commute (although the logic could be reversed here) ... all these statements about "reality", "life", "mind", "objectivity", "counter...factu...ality..." and other complicated words are just po-science nonsense

I was saying on a pop-science group about the "triviality" of the area law connection to entropy... I mean by that that it is funny that the same entropy law is obtained while avoiding to look in the interior of a region (by the choice of the observer and not because some gravitational horizons)... Now, I listen to Susskind's talk about the cosmological horizon (the one of the universe) and it seems pretty clear to me that there should be nothing special happening "on the horizon"... I also read the paper about generating spacetime out of "entanglement" and I keep looking at entanglement as the result of our choice of looking at some observables instead of others... There will always be some "entanglement" (call it non-factorability if you want) if one keeps "asking" a specific class of questions... it is kind of logically necessary to have entanglement... I would just avoid putting entanglement and spacetime in a too "instrumental" category and think at them from the perspective of the logician ... they seem to be inseparable from a logical perspective... it's like taking a manifold of logical propositions and identifying some propositions (creating classes) and avoiding others (creating 'holes' that should be visible via homology groups)... my latest question is related to the role of the axiom of choice in this context... (for the more logically oriented people around here :p )

I mean, for example cohomology can detect when the axiom of choice fails...

I cannot avoid the feeling that I speak alone when discussing these things...

As I stated, of course it is nonsense, however, YOU CANNOT DISPROVE IT. Redirected stupidity is not intelligence, whatever its talents for presentation.

The paradox is beautifully unraveled by Laszlo Kish modeling two wires. Go find it - or present the three line algebraic disproof yourself. Put up or shut up.

If we make a hypothesis that BHs are just ordinary "elementary" particles (mighty big thou), many headaches will be avoided. Here's my two cents -> http://www.toebi.com/blog/theory-of-everything-by-illusion/black-holes-by-toebi/

To think that logic is something emergent is as stupid as to think that mathematics in general is something that is not needed in physics and only emerges out of the biological jellyfish junk
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Lubos you surprised me with this statement. I wouldn't think that you could be so vehement about a philosophical e.g non scientific point of view.
Indeed your statement seems to show that you strongly believe in the platonician interpretation of mathematics.
Even if this faith represents the majority of the mathematic community, it is just this. Faith.
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The alternative faith is that indeed mathematics are a brain made construct, e.g they are a result of interaction of neuronal cells. This philosophy considers that the natural evolution of the brain(s) made them so that they are able to detect and interpret efficiently the regularities in the environment so that predictions about the future are possible.
I have a cat and read several papers that deal with processes in cats' brains.
Just observe a cat catching a fly.
It is obvious that the cat's brain uses a 4D coordinate system and efficiently "computes" the fly's trajectory so that the jump puts it at a point in space where the fly will be at a later time. This is nothing else than making a prediction even if it is neither verbal nor written.
The difference between its brain and yours is that you are effectively able to do this computation explicitely and justify it by an elaborate set of abstract concepts that exist only in your (or any human) brain.
On the other hand you catch flies much less efficiently than a cat does :)
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This suggests that the differences are mostly quantitative but not qualitative. Give the cats (or chimps !) a few millions of years of evolution and their brains might very well add new specialsed cells and interconnections providing new and more efficient functions.
I believe that this point of view is not an idiocy and that it doesn't even really contradict the platonic point of view.
The distinctions seem to be mostly about semantics.
Indeed if one admits that a brain is just a material subset of the world whose specialized function is to make valid predictions about the world, then it is not surprising that the evolution selects brains that do this function better than others.
Eventually (but it is not guaranteed) given some additional abilities like language, opposing thumbs etc this predictive function gets more and more sophisticated untill it becomes able to express itself autoreferentially, e.g describe its own processes symbolically.
In this view mathematics are precisely this autoreferential description which applies both to the brain processes and to the Universe of which the brain is just a part.
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Personnaly I find this evolutionary view legitimate and actually more "scientific" than the platonician view.
Btw if you find this comment wildly off topic, I'd understand that you erase it.

Dear Tom, I don't really understand how your alternative, non-Platonic viewpoint could be true. It sounds like a straight oxymoron to me and it always did. I am convinced that none of these things are a matter of a belief or a philosophy - it's about elementary science.

Jellyfish and brains are made out of physical objects that are described and have to be described by well-defined maths and logic. As long as one agrees that the Universe obeys any laws at all, and it apparently does due to the success of science in the last millennia, the previous statement is an indisputable tautology because the laws' being "mathematical" and "logical" just means that we're not satisfied with an incoherent or approximate understanding.

The sorry state of physics is reflected in how it took many theorists many months to come up with some sort of hand wave to explain away the firewall, when its obvious as you point out that it can't be there.

Sorry, Uncle Al's comment surely doesn't contain anything that would settle the firewall question in one way or another.

The sorry state of the brains of the two us is that you aren't even capable of seeing this self-evident fact.

The non-existence of firewall is not a "super high tech" insight but it's still vastly more complex than the two of you are imagining.

Moreover, Raju's and Papadodimas' paper is no handwaving, it's as rigorous a piece of work as you can never even dream of. So please don't be this excessively arrogant if you're scientifically challenged.

"I wouldn't think that you could be so vehement about a philosophical e.g non scientific point of view."

This is also what surprises me.
Lubos, could you maybe explain how the existence of time fits into your viewpoint.

Dear Mikael, the "existence of time" sounds like a bit vague and general question encouraging one to drown in meaningless philosophizing flapdoodle.

However, this very blog entry and the previous one in the same category

http://motls.blogspot.com/2013/10/is-space-and-time-emergent-er-epr.html?m=1

was exactly dedicated to the question whether time may be emergent and what the modern research actually implies about this question.

It surprises me that my attitude to reductionism may surprise anyone. I've been confident about the validity of reductionism for every single minute since my 3rd birthday. I really don't know where your surprise may be coming from.

Dear Lubos,
ok, let me say it a bit more direct. The logical arrow of time is an extremely important concept and as you pointed out, it is needed to get even started on quantum mechanics. Just you seem to place it in the realm of mathematics. But since time itself for me is not part of mathematics the arrow of time can't be part of mathematics. I would call the arrow of time an axiom of physics. But since it is an axiom one may try to question it at least over a beer. This activity I wouldn't call science but philosophy of science. You may call it also "meaningless philosophizing flapdoodle" but you shouldn't assume that all people who sometimes like it couldn't distinguish it from science.

Dear Mikael, the logical arrow of time is just a name for the asymmetry between the two directions of time that inevitably arises whenever one talks about propositions about properties/objects/events that are positioned in time and whose basic relationships are dictated by causal laws (past influences the future). For example, we may remember the past because the past influences the state of the brain according to some laws but we can't remember the future.

You can't do any physics or science without that. After you drink a sufficient number of beers, you may question that but because the existence of this asymmetry is more unquestionable than pretty much any other claim in science, you will need many more beers than e.g. if you just question relatively shaky things such as the existence of the Moon, gravity, the American continent, or evolution.

http://arxiv.org/abs/1310.4771
New paper, may interest you I thought.
"Black holes may radiate, but wont evaporate"

I saw it but sorry, I won't read it beyond the first sentence of the abstract because there are no remnants.

Dear Lubos
Brains couldn't exist, they couldn't evolve, the rules of Darwin's evolution couldn't work if they were not realized by a particular technical setup i.e. if there were no fundamental laws described by maths.
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I of course agree with that and actually said the same thing in other words : Indeed if one admits that a brain is just a material subset of the world whose specialized function is to make valid predictions about the world, then it is not surprising that the evolution selects brains that do this function better than others.
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Of course I think (like 99% of people) that the Universe would exist even if brains didn't.
This of course implies that the quarks and electrons cannot be an "emergent" feature of brains.
That's also why I said that the platonician and anti-platonician views about mathematics (not the the Universe !) are mostly a question of semantics.
The platonician view postulates the existence of mathematical beings like e or 2/3 in some non material realm that the brains "explore". Every possible theorem already exists in this realm and it is only a matter of discovering it through this mysterious non physical "interaction" between brains and the platonic realm of mathematics
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The anti-platonician view says that the Universe.took opportunity of existence of quarks and electrons to invent brains (through the evolution process) whose primary purpose is to make predictions.
Now, obviously, the Universe would not (could not) do that if the predictions were impossible, e.g if everything was chaos.
Therefore the very existence of brains is a proof of the existence of some rules in the Universe making some predictions possible. Among others, like you wrote a bit above, it also implies the necessity of an arrow of time because it is fundamentally necessary for predictions too.
That's why this view says that a cat's brain does not do much less "mathematics" than a Lumo's brain and that the difference is mainly that the former is not (yet) rightly configured to feel the necessity of non euclidian geometries and make a blog post about it :)
But given enough time and a right set of additional conditions (like language), the cats will want to demonstrate the Riemann's conjecture one day too.
In this sense the mathematics understood as a dual of the Universe, (by definition) imperfectly realized by a finite material tool called brain are indeed an "emerging" process.
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The interesting conclusion is that the mathematic of cats will be conceptually identical to the one of humans because they would also have to make valid predictions about the Universe which is exactly the same for cats and for humans.
This last shows that the differences between platonician and anti platonician views of mathematics are actually much less than what most people think.

Excellent, Tom.

Well, the "primary purpose of the brain" is a bit moral question and "making predictions" is a fair general description although it's arguably more abstract than what the cavemen would say as the explanation what their brain was good for (did they know that the thinking took place in the skull? How do we actually know that? Do we feel it?).

The paper forbids anyone from reading it's abstract; since there always needs to be something remaining : )

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