Monday, September 24, 2012

Are black holes surrounded by firewalls?

Dilaton has noticed a new, extremely provocative concept that was introduced among the quantum gravity researchers two months ago: the firewall.

For decades, people teaching general relativity – including your humble correspondent (e.g. here) – have been explaining that nothing special happens to an infalling observer when she crosses a black hole event horizon. The curvature is usually pretty small there – the curvature radius is close to the black hole radius – and you only get torn apart once you approach the black hole singularity which may be much later.
Advertisement of a future text: Read also Raphael Bousso is right about firewalls
The event horizon is just a coordinate singularity; with a better choice of coordinates, the vicinity of the horizon (including a region below and a region above the horizon) looks like a nearly flat piece of the Minkowski spacetime. These coordinates may be "extremely distorted" functions of some other coordinates you may use for other purposes but they exist. Because the laws of general relativity are local, the (nearly) flat geometry of the region implies that there will be (nearly) the same phenomena there as in the flat space.

Later, some quantum properties of black holes have been pretty much established, too. The picture has made sense to everyone who has ever been considered a top expert in quantum gravity. That was the case until July 2012.

Let me first say what the quantum insights about black holes have been. The black holes evaporate and, as seen in AdS/CFT and Matrix Theory, it's still possible without any violation of the principles of quantum mechanics. So pure states evolve into pure states. From the viewpoint of the observers at infinity, a black hole is just another object with a discrete energy spectrum (well, the levels aren't really sharp because the black hole is unstable: they have a width) that effectively exists outside the event horizon only.

There may be an apparent contradiction between all these things and the validity of the effective field theory for low-energy processes but it's been believed that the contradictions go away because of the "black hole complementarity" paradigm, an opinion that the degrees of freedom (fields) inside the black hole aren't quite independent from those that are outside. They are complicated, scrambled functionals of them.

Now, in mid July 2012, four authors – two of whom are already established as quantum gravity black hole experts you don't want to overlook – published an explosive preprint called
Black Holes: Complementarity or Firewalls?
Joseph Polchinski, Donald Marolf, James Sully, and Ahmed Almheiri – sorry that I sorted the names from the most famous ones – decided to claim that after they have investigated some "detailed models" what happens with the information during the black hole evaporation, they concluded that the usual assumptions are mutually inconsistent, after all.

(See Joe Polchinski's guest blog at Cosmic Variance.)

They considered some thought experiments about entangled qubits that fall into the black hole - constructed out of the \(s\)-wave or other waves in the spherical harmonic decomposition – and decided that the only sensible conclusion is that when a black hole becomes "old" (i.e. when it emits or loses one-half of its initial Bekenstein-Hawking entropy), its event horizon gets transformed into a firewall that destroys everything that gets there.

(If you want to do an experiment, note that you will only be able to say "how you feel" after you cross the event horizon to those people who also fell into a black hole and whose lives are as doomed as yours.)

A song on several observers approaching a firewall. The musician suggests that it's only burning in the observer's eyes.

Within days, an emotional Leonard Susskind replied. The horizon may be kept intact; it's only the singularity of an old black hole that may need a make-up. In another day, Susskind released the second version of the manuscript. Two weeks later, he withdrew the paper because he "no longer believed the argument was right".

A week after the initial provocative paper, Raphael Bousso replied with a rather intelligent paper arguing why Polchinski et al. are wrong. It's clear that Raphael Bousso had to think it was wrong because he's really closer to classical general relativity and Polchinski et al. wanted to question its validity in environments that seem completely mundane! Bousso pointed out that Polchinski et al. were sloppy about the information that various observers, especially the infalling one, may access. When one realizes that they can only evaluate the "causal diamond", all the proofs of contradictions (which typically claim that one may xerox a quantum bit which must be impossible – or which clearly is impossible, depending on your goals – in every consistent quantum theory) become impossible.

Bousso's talk at Strings 2012 about this issue

Daniel Harlow posted another seemingly intelligent reply four days after Bousso. Polchinski et al. were sloppy when they were converting the observations from one observer's reference frame to another. However, Donald Marolf, a co-author of the original paper, kept on fighting and convinced Harlow that there was a hole in his argument. So Harlow withdrew the paper, just like Susskind. The topic of the black hole firewalls surely seems to be a firewall when it comes to burning the actual papers. ;-)

Great Firewall of China, by Ryan McLaughlin

But the fight for freedom and against firewalls continued. Yasunori Nomura, Jaime Varela, and Sean J. Weinberg argued in a way that is somewhat similar to Harlow: one must be careful when she constructs the map between the unitary quantum mechanics with the qubits on one side and the semiclassical world on the other side. The paper exists in the version v3 as well but unlike Harlow's paper, it hasn't been withdrawn yet.

Samir D. Mathur and David Turton "paradoxically" disagree with the firewall, too. I say it's "paradoxical" because Mathur is the father of fuzzballs which also "brutally change" the appearance of the black hole interior. However, they actually believe that the infalling observer has a complementary "nothing happens" description. Their explanation why Polchinski et al. are wrong is seemingly different again: Polchinski et al. assumed that an observer near the event horizon may say lots about the Hawking radiation even if he only looks outside the stretched horizon. Mathur and Turton say that he must actually go all the way to the real horizon and all the answers therefore depend on the Planckian physics.

Borun D. Chowdhury and Andrea Puhm picked catchy words for the same question: Is Alice burning of fuzzing? ;-) Among the followups, they're the closest ones so far to the original paper. They claim that all the critics of Polchinski et al. are just babbling irrelevant nonsense. The only exception are the fuzzball guys from the previous paragraph. Chowdhury and Puhm declare that it's important to get rid of the observer-centric description and talk about decoherence. When it's done, Alice burns when she is a low-energy packet but she may keep on living in the complementary fuzzball picture when she is a high-energy excitation. I suppose that for real people falling into a large, old black hole, this means that they're burned at stake.

In mid August, Leonard Susskind posted a new preprint, unusually similar to the previous one that was withdrawn weeks earlier. It's only the singularity that is modified for an old black hole. However, in the new paper, the evolution of the singularity is rather dramatic because it is – thanks to the growing entanglement – growing towards the event horizon and it ultimately overlaps with it. So Polchinski et al. are right that there's a firewall that burns you at that place; Susskind just says that it's more natural to call it a grown-up singularity, not an event horizon. He at least hopes that this only happens to old black holes (after the Page time, half entropy etc.), not after a much shorter scrambling time (which is just by a logarithmic factor longer than the black hole radius).

The debate wasn't stopped, of course. A day later, Iosif Bena, Andrea Puhm, and Bert Vercnocke formulated the question in yet another way: Non-extremal Black Hole Microstates: Fuzzballs of Fire or Fuzzballs of Fuzz? They take the fuzzball picture as a dogma and try to figure out how the interior looks to an infalling observer. Their conclusions seem inconclusive to me but they surely say lots of general and vacuous things that it could be an important research. ;-)

Amit Giveon and Nissan Itzhaki became supporters of the firewall when they decided to publish a related provocative concept: they think that string theory adds an extra degree of freedom, a zero mode, to the tip of the cigar (the counterpart of the event horizon in simple 1+1-dimensional examples of black holes) relatively to general relativity and this extra degree (or these extra degrees) of freedom may get generalized to a firewall that kills you what you fall into a higher-dimensional black hole.

Tom Banks and Willy Fischler use Tom's somewhat incomprehensible axiomatic framework, the holographic spacetime (I've been exposed to very intensely to as Tom's student), and they conclude that this axiomatic framework doesn't imply any firewalls.

Amos Ori prefers to assume that the semiclassical physics simply has to hold and adjusts any claims about the quantum information as necessary to agree with the primary assumption. With this attitude, he reaches a nearly comparably dramatic conclusion about the black hole information. Most of the information remains trapped throughout most of the evaporation process. Effectively, a small black hole behaves as a black hole remnant.

Ram Brustein wrote so far the most recent followup. The author chooses some very conservative language but arguably proposes a much more radical departure from the lore. The event horizon is a wrong concept; it only exists in the classical theory. In the quantum theory, the black hole's Compton wavelength is nonzero which, the author believes, creates a region near the horizon where the densities are inevitably high and quantum gravity is needed to predict what happens in this new extreme region.

I guess that hasn't hit a firewall yet so new and new followups will keep on emerging.

Your humble correspondent has an opinion what happens but I don't want to extend this cacophony. You must already feel it's crazy. There's surely no consensus here at all and if there were any majority, you would manifestly see that it's irrelevant. The researchers don't seem to agree about anything at all! ;-) Some of the papers are potentially compatible with some of the other papers but you won't find a pair of papers that are really answering the question by Polchinski et al. in equivalent ways.

It's plausible that the reason is that all the questions "what an infalling observer sees and feels" is ill-defined. He may feel "nothing special" but the transformation of the quantum information needed to produce his future state may become arbitrarily contrived once he crosses the horizon, with no need to have any simple relation to perceptions by other observers. After all, extremely singular coordinate transformations are bound to translate to extreme transformations on the Hilbert space, especially if it includes some Planckian degrees of freedom (well, degrees of freedom interpreted as "Planckian" by some of the near-horizon observers). Well, one of the papers above was making a similar point. Perceptions and observations depend on the sensory system's being described by a predictable Hilbert space that reacts in predictable ways. If you can't isolate the Hilbert space that behaves as an "ordinary Hilbert space for the sensory system", it makes no sense to talk about someone's perceptions. (I don't really need to reconstruct eyes; what may get destroyed at the event horizon are much more brute pieces of material, too.) On the other hand, when you redefine the degrees of freedom and evolve them by an ad hoc evolution you would expect outside the black hole, it's not a problem and it won't lead to real contradictions with the things outside because the infalling observer is never going to liberate herself, anyway.

I also think it's problematic to assume that the radiation may be described as a pure state even before the black hole evaporates. The state of the radiation may be obtained by tracing over the interior and the horizon degrees of freedom. Even if the strictly internal degrees of freedom are reshuffled outside degrees of freedom, the influence of the near-horizon degrees of freedom could still make the state of the "radiation only" mixed. One may only be sure about the purity when the black hole is really gone.

Well, I actually think that Polchinski et al. and many others are doing exactly the opposite mistake, too. They think that the radiation is maximally entangled with the black hole so it must be described by a heavily mixed state and can't be maximally entangled with someone else. However, the very point of complementarity, as I understand it, is that the black hole interior's degrees of freedom are just "scrambled copies" of the external ones so you shouldn't double count them (which would be spurious quantum xeroxing). The radiation without the interior is nearly or entirely in a pure state at the Page time! I realize this paragraph says exactly the opposite than the previous one but whichever way it goes, I feel they're not being careful about these important considerations.

At any rate, it surely looks bizarre that the quantum gravity folks can't agree about such a seemingly elementary question, namely the existence and character of the hypothetical firewalls. Many of them are excellent folks but maybe they have focused on too ill-defined questions. Maybe this huge cacophony is a warning sign that the research into these "excessively conceptual" questions got stuck in a swampland observed in a letter by Richard Feynman to his wife after he visited the 1962 conference on (general) relativity in Warsaw:
"I am not getting anything out of the meeting. I am learning nothing. Because there are no experiments, this field is not an active one, so few of the best men are doing work in it. The result is that there are hosts of dopes here (126) and it is not good for my blood pressure. Remind me not to come to any more gravity conferences!"
Are we there again? The black hole interior will always be a mostly inaccessible place for most lucky people so these questions will remain theoretical. But are they meaningful as theoretical questions at all? When you look at the amplitudes that string theory allows you to naturally calculate, such as the S-matrix in various Minkowski spaces, you will find out that the "perceptions of an infalling observer" are not among these calculable things. Maybe string theory has a very good reason why it's trying to hide those would-be observables from us! When I wrote about the reincarnation of the infalling observer, it wasn't quite a joke. I really feel that questions about the infalling observer may be somewhat analogous to various spiritual questions about near-death experiences etc. Some of them may be inaccessible to science – and really ill-defined from a scientific viewpoint.


  1. Seth Lloyd has already defined this in what is left.

  2. PlatoHagel Seth Lloyd has already defined this in what is left. The Reference Frame: Are black holes surrounded by firewalls? · 3 minutes ago

  3. Dear Lumo,

    for an outside observer (pun intended :-P ...), the actual agitation going on in the Arxiv as you nicely describe it in this article, is some kind of interesting fun to watch. Maybe we just have to wait until the dust has settled down a bit, I dont know ... (?).

    Or could it be that some kind of a "shut up and calculate" rule should be applied and one should just try not too hard to imagine what happens to an infalling observer in analogy to what I always think about all this quantum interpretation business (more accurately: nonsense in my opinion ;-) ...)? And the means to caclculate in the case of firewalls and infalling observers would be the holographic principle ?

    Nevertheless, I always thought that trying to find out what the microscopic degrees of freedom of a black hole really are and how the information comes out again is some kind of a difficult but real and interesting topic :-/ ...?

  4. Holy crap I didn't know so many papers had been published on this. It will be interesting to look at it when the dust settles.

  5. Seth Lloyd is a joke...

  6. As an ignoramus (in respect of relevant details) but nevertheless interested bystander, I enjoyed and felt satisfied reading this overview/analysis and its concluding comment [one that _I interpret_ to mean that the 'Firewall idea' is an anomalous hypothesis that ought to have self-incinerated before it was hatched ;>].

  7. The study of black holes has had an interesting trajectory. Back when I was working on them around 1971, people were looking for exact solutions to Einstein's equations for various matter and charge distributions, stationary or in example would be the Kerr solution for an uncharged rotating black hole. Another would be the eponymous solution :).
    The next phase has been a transition to numerical relativity (Matt Choptuik, Franz Praetorius etc), still
    focusing on classical GR, but with the evolution of computing power, various simulations are now possible.
    Now, with what Lubos has reviewed, we seem to be getting into a quantum (gravity) quagmire applied to black holes. The situation is reminding me of ancient Greek philosophers who had untrammeled speculation not constrained by Parmenides,Zeno-- nothing changes, block time vs Heraclitus---everything changes...I am not criticizing this, just a comment.
    An (admittedly uninformed) suggestion from me would be to emulate Dirac's invention of his equation and come up with models for quantum gravity and simply see where the math leads. If the equations lead to a contradiction or inconsistency, that would still be useful. If they don't seem to, explore what the results mean. We seem to have moved beyond the Descartes/Galileo/Newton model of experimental theoretical physics and back to the Greeks armed with much more powerful tools (math).

  8. Yes, my rather long blather above does sort of boil down to "play with beautiful equations, and shut up and calculate". Einstein and Dirac and Heisenberg all did this, and the genesis of their insight often seemed like magic and not reason from experiment.

  9. Dear Gordon, physics switched into the research of quantum physics of black holes in 1974, due to Hawking's groundbreaking discovery, so it's clearly invalid to suggest that "today" we are making the transition.

    Also, many people today are working on analytic solutions to GR. One could even say that not much has changed about the composition of the research. So your suggestion that the 1970s were about the exact solutions and "today" is about the quantum properties is just bullshit. You may have only worked on the less revolutionary part of these GR-related insights but you were not all of GR research, sorry to say.
    If you would dismiss even Hawking's discovery and similar theoretical ones just because they're theoretical ones, I couldn't disagree more because it's one of the greatest discoveries of the 20th century science.

    I think it's complete nonsense, a Smolin-style nonsense, and a kind of insulting nonsense, that we have moved "back to the Greeks". Greeks were asking and (usually incorrectly) answering ambitious questions because they're the most attractive ones and they couldn't answer any questions really correctly, so among the possible questions to work on, they chose according to the audacity.

    We are solving ambitious questions because the less ambitious ones have been genuinely solved and our knowledge and tools are marginally enough to attack the ambitious ones. This boundary moving towards the previously "hopelessly detached" questions is what defines the progress in science and it's been moving largely uniformly in the positive direction, so your suggestion that 2,000 years have been undone is just shit.

  10. I have never seen a more COMPLETE misunderstanding of what I have said ever, anywhere. Geez, Lubos. Get someone else to scan stuff before you rant. I was certainly not calling for a modification of the Dirac equation---I was suggesting the method he used---playing with beautiful equations and following the consequences, could prove a fruitful strategy. So it goes with ALL the rest.
    Certainly I dont think that nothing quantum was done before the present and I didnt say that. I have never come across anyone so black and white as you, or someone who seems incapable of getting the sense of a post---I am not suggesting we go back to the Greeks---just suggesting that speculative theory not tied to experiment may be forced on us by inability to experiment. And the ancient Greeks were virtuosos at that.
    For a smart person, you are pretty dumb.
    Yes, string theory is the best candidate quantum gravity theory, but is it presently a canonical qgt?

  11. Where, oh where am I denigrating Hawking's contributions? We referenced chats with him in our paper.
    About slinging mud on modern science---total bullshit. I was trying to praise it, not bury it. And I am not suggesting we go back to Galileo or Dirac. I am beginning to understand how people get into frustrating disagreements with you.

  12. Dear Gordon, you always complain that you were misunderstood. How can it be misunderstood? You just repeated the same thing.

    What does "playing with beautiful equations" have to do with answering what the infalling observer observes when he crosses the event horizon? Try to play with the Dirac equation or equations of string theory and answer the question. Others have tried. It hasn't been possible. The Dirac equation clearly has nothing to say about it and it seems that string theory doesn't allow one to calculate "exact values" of any observables for an infalling observer, either. One has to use different methods than just "playing with beautiful equations" to find out what happens when the horizon is crossed. For the required answer, something conceptual – and perhaps some equations – are missing so the equations would first have to be found if the answer boiled down to equations.

    So why are you pumping this completely irrelevant junk about playing with beautiful equation etc. if you must know that this has nothing whatsoever to do with the essence of the question here? Why are you introducing Dirac or Galileo into these debates who have *absolutely* nothing to do with these matters and who wouldn't really understand any of the papers? And what about the ancient Greeks? WTF?

    If a theory is "forced on us", it is no longer speculative, so this part of your comment is internally inconsistent, too. Ancient Greeks were never properly forced to accept any theory – their arguments have never really worked. Why are you comparing their situation with the situation in science, especially modern science? They have nothing to do with each other. The ancient Greeks weren't really doing science, except for some very elementary branches of it.

    String theory is not only the "canonical" theory of quantum gravity but it's also the only mathematically possible consistent one. Never heard of us, huh!!?

  13. You were denigrating Hawking's – and many others' – contribution in your comment containing the sentence:

    Now, with what Lubos has reviewed, we seem to be getting into a quantum (gravity) quagmire applied to black holes.

    First, this uses a negatively sounding word "quagmire" for an exciting – and largely understood as of today – science about the quantum properties of black holes. Second, this sentence is saying that quantum properties of black holes are only starting to be studied "now" (your word) which implies that you think that what Hawking realized and people started to study in 1975 is either not a research of quantum properties of black holes or it should be ignored. You did ignore it in all your comments, which is why you're denigrating this pillar of the field. You did the same to string theory.

  14. Any reaction of S. Hawking regarding this matter yet?

  15. Oh Lumo,

    I'm so sorry that the discussion below your very nice answer to my question has gone bad and I hope I did not make it worse :-/

    Since I'm here on TRF I always thought that Gordon likes and appreciates modern fundamental physics a lot too. And I still dont think that he is aligned with the sourballs who want the physics wisdom we have today to be thrown out of the window by a next Newton, Einstein, etc ... Maybe Gorden was just a bit clumsy in choosing his formulations (which I agree look partly some kind of similar to what Sabine Hossenfelder could say for example ...) to explain what he wanted to say. Maybe the dust between you and Gordon has to settle too a little bit too... ;-) ?

    Anyway, I think the discussion by papers in the Arxiv among your colleagues about the firewalls is interesting and I'm curious about if some deeper insights (about the microscopic degrees of freedom of a black hole or how the information can come out again for example) will result from this when the dust has settled :-)


  16. Dear Lubos,

    can I ask some questions about the standard view on quantum black holes?

    You wrote:
    "The black holes evaporate and, as seen in AdS/CFT and Matrix Theory,
    it's still possible without any violation of the principles of quantum

    I thought that presently, string theory can only describe the thermodynamics of extremal black holes. But the temperature of extremal black holes is zero, so they don't evaporate. Or is this an outdated view?

    "So pure states evolve into pure states."

    If I describe the black hole evaporation in the "QFT in classical curved spacetime" framework, I think that the result should be a mixed state, because it is a finite temperature state. So am I right, that in quantum gravity, this low energy effective mixed state is produced by entanglement between low and high energy degrees of freedom?

    Thank you,

  17. There is no doubt we are all still blind individuals when it comes to the subject here. Us layman more so then others. :)

    The real issue here is a progressive approach to the questions revealed by any theoretical approach and open discussions. This provides for framework and basis for that discussion.

    This has been historically verify by information already processed so by laying out Susskinds thought experiment as Gedanken Experiments Involving Black Holes I have provided for similarity of discussions on that basis alone. Adding Seth Lloyd to the question of entanglement shows this progressive connection.

  18. Dear Rezso, the view is hugely outdated. It was already outdated in 1996. Only the first paper by Strominger and Vafa focused on a particular extremal, supersymmetric black hole. It has over 2,000 followups at this moment, many of which are computing thermodynamic properties (the right values) of near-extremal or completely non-extremal black holes, including rotating non-supersymmetric Kerr, and 7-parameter families, and infinitely many higher-order corrections to various black holes. Many things can't be calculated analytically. However, it's still possible to prove that AdS/CFT and Matrix theory contain non-extremal black holes as they should, and so on, and so on. There's no reasonable doubt that string theory describes thermodynamics of all black holes correctly (and all of their behavior outside the event horizon, to make the possible gap very explicit).

    Yes, the mixed/thermal state of the Hawking radiation is just an approximation, and in any exact theory of quantum gravity, which realistically means in any implementation or vacuum of string theory we know today, it may be seen that a pure state always evolves into a pure state, whether there is a black hole or not. The detailed information about the initial state is imprinted into subtle correlations and entanglement between all the degrees of freedom.

  19. Ah, I see---you took the word "quagmire" and conflated it into an all out attack by me on quantum mechanics and modern science.
    Also, my reference to Galileo, if you actually read it for the sense, was to say that Galileo and Newton were instrumental in tying physics to actual experiments and hard data--ie, the scientific method. This is not a speculation by me, and is not either promoting Galileo or wanting to return to him (or Dirac, whom I do admire).
    What you have done is to focus on one or two words and look for negative connotations.
    Yes, when there is experimental evidence, direct or indirect, theory needs to conform to it. I was simply TRYING to point out that these wars over a firewall remind me of the old Greek philosophers throwing up speculative theories---that in modern astrophysics, like in high energy particle physics, more theories will be inaccessible to many direct or maybe indirect--this does not mean that I attack these theories at all if they have explanatory power like string theory does (and also I am not saying that string theory is inaccessible to confirmation either, like Woit, whose attitudes are deplorable.)

    I do apologize for flaming you, and admit that the way I wrote the initial post made it seem like QM just recently entered into black hole theory. But my post wasn't meant to be critical, certainly of Hawking, whom I admire extremely and with whom my supervisor co-edited two books and spent a sabbatical year with. Also, I in no way challenge your authority in scientific matters---I have, as I indicated, been out of any active physics activity since 1972. So please lighten up. Calling me Smolian or whatever is a total insult and will simply result in my packing it in.
    Of course I haven't read any of the firewall papers---I would be incapable of following them at this point. I assume that you don't want to limit your audience to only active theoretical physicists.
    As for being off topic, I don't think so. Just like me saying I am misunderstood, your saying "off topic" doesn't make it so...maybe a bit tangential.
    Anyway, thats it for at least this post. I just got back from two weeks at Cambridge and am horribly jet lagged from delays, and the initial post was quickly written after scanning your blog post (no, I didn't pay enough attention to it) but I would suggest you assume I am an ally, and if what I say sounds stupid, see if you may be over-analyzing something or taking a word literally and missing nuance or irony or maybe misuse of a word due to fatigue.

  20. Dear Lubos,

    thank you for the answer. However, I'm still sceptical about the claim, that the evaporation of non extremal black holes is fully understood in string theory.

    1. I found the following Strominger paper from 1996 about "Nonextremal Black Hole Microstates":

    Below equation (1.1):
    "We have not been able to obtain a stringy
    derivation of the full expression."

    Below equation (3.7):
    "We do not know of a systematic derivation of this formula using D-brane technology.
    However miraculously it agrees with the Bekenstein-Hawking entropy calculated in the previous section from the area of the event horizon."

    As it seems, my view was not outdated in 1996. :)

    2. I looked at a recent CMS collaboration paper which is based on the ADD model (TeV scale quantum gravity).

    "Search for Microscopic Black Hole Signatures at the Large Hadron Collider"

    It sais:
    "The parton-level cross section of black hole production is derived from geometrical considerations ...... The exact cross section cannot be calculated without knowledge of the underlying theory of quantum gravity and is subject to significant uncertainty."

    It seems to me that black hole evaporation is only understood in the semiclassical approximation.

  21. an expert's opinion would be good.

  22. Dear George,

    the Alcubierre warp drive is a solution of General Relativity and it was proposed by the mexican physicist Miguel Alcubierre.

    The basic idea is that if you can create a special spacetime geometry (warp-bubble), where space expands behind a spaceship, and contracts in front of it, than it can lead to faster than light travel. Note that this solution doesn't contradict the basic principle of Einstein's theory, because the speed of light is never exceeded in the LOCAL reference frame. This means that a light beam within the warp-bubble would still always move faster than the ship.

    However, it is very very very hard ( if not completely impossible ) to create spacetimes like this, because it requires negative energy density to be present at various locations. But there is an experimentally verified quantum phenomena, the Casimir effect, where negative energy density exists in Nature, so the solution is not completely excluded.


  23. Wow, Lubos. If it was not for Joseph Polchinski's name on the paper (and the names of several other serious physicists discussing it) I would think this is just a crackpot paper which does not deserve a minute of attention. We may just be falling through the Rindler horizon of some aliens with their space ship so we might all be burned to death soon. :-)
    Does nobody have an idea how to calculate the experience of an infalling observer of a black hole within string theory or is the calculation to hard?

  24. Dear Rezso, I am confident that such solutions are impossible in the real world, while the technical reason is probably that they violate an energy (positivity) condition. Alternatively, one may say that the warp bubble before and behind the spaceship is a gravitational wave and it is not allowed to move superluminally, another constraint you violate.

    The local speed limit at "c" isn't really the only constraint that may be derived from a special relativity limit of GR. There's another ramification. If the spacetime is asymptotically Minkowski flat, then impulses in it can't propagate superluminally, either, whether or not you find an excuse in the form of modified local geometry.

  25. Dear Mikael, their quantum-information argument is highly nontrivial, even if it is ultimately wrong. It may look like a crackpot paper but it's not.

    I think that string theory doesn't allow us to calculate observations by an observer inside even in principle. Because of the finite lifetime in front of her, there are no really exact observables known that could be computed and verified. As I said, I suspect that string theory has a good reason why it keeps silence about these matters.

  26. Dear Lubos,
    the lifetime may be finite but with a big enough black hole you can make it as big as you want. Physics suddenly breaking down in a binary way is just not plausible for me. The answers should become less sharp in a continuous way when making the horizon smaller. Also all the paradoxes of the falling observer appearing frozen at the horizon and the horizon appearing hot for the distant observer already exist for the Rinder horizon of an accelerated observer in Minkowski space.

  27. Just read the guest blog of Polchinski. Really exciting stuff going on,