## information loss in black holes

<jabberwocky><div class="vbmenu_control"><a href="jabberwocky:;" onClick="newWindow=window.open('','usenetCode','toolbar=no,location=no, scrollbars=yes,resizable=yes,status=no,width=650,height=400'); newWindow.document.write('<HTML><HEAD><TITLE>Usenet ASCII</TITLE></HEAD><BODY topmargin=0 leftmargin=0 BGCOLOR=#F1F1F1><table border=0 width=625><td bgcolor=midnightblue><font color=#F1F1F1>This Usenet message\'s original ASCII form: </font></td></tr><tr><td width=449><br><br><font face=courier><UL><PRE>If I burned a book the information in it would be lost forever with\nrespect to all possible observers.This is because the burning of the\nbook is about as irreversible a process as can be imagined.Processes\nin which information is retrieved depend on the idea of reversibility:\nI can put information on a hard disc of a computer and take it off; I\ncan put a book on a shelf and take it off the shelf etc.So if\ninformation which has gone into a black hole is to be retrieved the\nprocess by which the information went into the black hole must have\nsomething inherently reversible about it.Suppose a book fell into a\nblack hole.I could not physically intervene to get it out again - as I\ncould in the case of a book on a shelf - I could not grab the book and\nreverse the path it has taken. And just as I wouldn\'t expect a book on\na shelf to spontaneously jump off the shelf into my hands, I wouldn\'t\nexpect the black hole to spontaneously yield the information in the\nbook in the form of radiation.\nBut all my ideas of the likelihood of processes occurring is based on\nmy experience of living outside a black hole.If I could live inside a\nblack hole\n( where there is no three space) would I reach the same conclusion\nabout the loss of information - would I conclude that information that\ncomes into the hole is destroyed irreversibly?\n\n</UL></PRE></font></td></tr></table></BODY><HTML>');"> <IMG SRC=/images/buttons/ip.gif BORDER=0 ALIGN=CENTER ALT="View this Usenet post in original ASCII form">&nbsp;&nbsp;View this Usenet post in original ASCII form </a></div><P></jabberwocky>If I burned a book the information in it would be lost forever with
respect to all possible observers.This is because the burning of the
book is about as irreversible a process as can be imagined.Processes
in which information is retrieved depend on the idea of reversibility:
I can put information on a hard disc of a computer and take it off; I
can put a book on a shelf and take it off the shelf etc.So if
information which has gone into a black hole is to be retrieved the
process by which the information went into the black hole must have
something inherently reversible about it.Suppose a book fell into a
black hole.I could not physically intervene to get it out again - as I
could in the case of a book on a shelf - I could not grab the book and
reverse the path it has taken. And just as I wouldn't expect a book on
a shelf to spontaneously jump off the shelf into my hands, I wouldn't
expect the black hole to spontaneously yield the information in the
book in the form of radiation.
But all my ideas of the likelihood of processes occurring is based on
my experience of living outside a black hole.If I could live inside a
black hole
( where there is no three space) would I reach the same conclusion
about the loss of information - would I conclude that information that
comes into the hole is destroyed irreversibly?

 PhysOrg.com physics news on PhysOrg.com >> A quantum simulator for magnetic materials>> Atomic-scale investigations solve key puzzle of LED efficiency>> Error sought & found: State-of-the-art measurement technique optimised


Black holes can have angular momentum, mass and charge.It is often pointed out that these three variables on their own cannot reflect the large number of ways in which a large number of unique black holes could be made. But how do we know that black holes are made in lots of different ways?



On Tue, 22 Jun 2004, alistair wrote: > But all my ideas of the likelihood of processes occurring is based on my > experience of living outside a black hole. If I could live inside a > black hole (where there is no three space) I don't know what you mean by this, but it sounds like a misconception. Here is some information which may help to clarify the situation: 1. Conventional Lorentzian spacetimes M are everywhere four dimensional, which means that it in each local neighborhood U of an event P (think of a small four dimensional "box" containing P--- if you like, you can now distort the "box" diffeomorphically without moving P, and call the result a new neighborhood U' of P), we can find an infinite variety of "slicings". Such a slicing consists of a family of three dimensional spacelike "hypersurfaces" which fill up the "box", such that no two hypersurfaces intersect one another. IOW: the hypersurfaces are the level surfaces of an appropriate function f:U-->R. 2. A spacetime containing an isolated black hole has (with some additional assumptions which need not concern us) "interior" and "exterior" regions. These are four dimensional pieces, separated by the "event horizon". (Describing the "dimension" of the horizon itself, or rather its "world sheet", gets us into further technical issues which I'll ignore here). 3. In particular, events P in the interior region admit infinitely many three dimensional spacelike hypersurfaces containing P. In a small enough neighborhood of P, each of these corresponds (many-one), approximately, to a "local Lorentz frame" at P. 4. Physically, this means that any observer in the interior region will experience three dimensional space just like we do, they will just observe some pretty extreme physics like large tidal stresses, weird optical effects, etc. "T. Essel" (hiding somewhere in cyberspace)

## information loss in black holes

<jabberwocky><div class="vbmenu_control"><a href="jabberwocky:;" onClick="newWindow=window.open('','usenetCode','toolbar=no,location=no, scrollbars=yes,resizable=yes,status=no,width=650,height=400'); newWindow.document.write('<HTML><HEAD><TITLE>Usenet ASCII</TITLE></HEAD><BODY topmargin=0 leftmargin=0 BGCOLOR=#F1F1F1><table border=0 width=625><td bgcolor=midnightblue><font color=#F1F1F1>This Usenet message\'s original ASCII form: </font></td></tr><tr><td width=449><br><br><font face=courier><UL><PRE>\nTESSEL@TUM.BOT replied to ALISTAIR with respect to the loss of\ninformation in black holes:\n\n&gt;any observer in the interior region will\n&gt;experience three dimensional space just like we do, they will just\nobserve\n&gt;some pretty extreme physics like large tidal stresses, weird optical\n&gt;effects, etc.\n\nALISTAIR writes:\n\nIf I put a sheet of paper in my hands with writing on it, and\npulled it until it ripped, is this what tidal forces inside a black\nhole\'s event horizon would do to the sheet of paper? Pulling a sheet\nof paper apart with my hands\nwill irreversibly destroy information on the sheet so if a black\nhole\'s tidal forces tears the sheet apart then surely this means\ninformation is lost forever in a black hole?\n</UL></PRE></font></td></tr></table></BODY><HTML>');"> <IMG SRC=/images/buttons/ip.gif BORDER=0 ALIGN=CENTER ALT="View this Usenet post in original ASCII form">&nbsp;&nbsp;View this Usenet post in original ASCII form </a></div><P></jabberwocky>TESSEL@TUM.BOT replied to ALISTAIR with respect to the loss of
information in black holes:

>any observer in the interior region will
>experience three dimensional space just like we do, they will just

observe
>some pretty extreme physics like large tidal stresses, weird optical
>effects, etc.

ALISTAIR writes:

If I put a sheet of paper in my hands with writing on it, and
pulled it until it ripped, is this what tidal forces inside a black
hole's event horizon would do to the sheet of paper? Pulling a sheet
of paper apart with my hands
will irreversibly destroy information on the sheet so if a black
hole's tidal forces tears the sheet apart then surely this means
information is lost forever in a black hole?


Alistair asked about information loss; his post included the dubious claim > If I could live inside a black hole (where there is no three space) I replied only to correct this apparent misconception, saying in part: > >any observer in the interior region will experience three dimensional > >space just like we do, they will just observe some pretty extreme > >physics like large tidal stresses, weird optical effects, etc. > > ALISTAIR writes: > > If I put a sheet of paper in my hands with writing on it, and > pulled it until it ripped, is this what tidal forces inside a black > hole's event horizon would do to the sheet of paper? Some relevant points: 1. In gtr--- and in any other theory which has a chance of comparing fairly well with observation/experiment, including good old Newtonian gravitation--- any gravitational field will produce tidal forces on infalling objects (even free falling objects). Indeed, in gtr, one could even say that describing the tidal forces experienced by arbitrarily falling small objects is exactly the same thing as describing "the gravitational field". 2. Small falling objects near a -compact massive object- will experience tidal forces having a "Coulomb character". This means that they will be -stretched- along the direction of fastest increase of the strength of the field (i.e. the direction pointing toward the compact object) and also -compressed- transversely. In particular, a sheet of paper falling through a Coulomb tidal force field will torn apart most efficiently if it is oriented so that a diagonal drawn on the sheet points in the direction of fastest increase of the magnitude of the field (i.e. points toward the compact object). The nature of this tearing is not much different from what you will observe if you hold a piece of paper by the tips and pull very hard. As a little experimentation quickly suggests, if you could observe a sheet of paper falling in a really strong Coulomb tidal force field, you would expect to see "transverse crumpling" long before you see "radial tearing"! 3. It doesn't matter all that much precisely how your object is "falling", e.g. an -orbiting- object will also experience tidal forces with a Coulomb character similar to those experienced by a -radially infalling- object, although the details may differ slightly, at least in relativistic gravitation theories like gtr. (In past posts, I've compared in detail the tidal forces for small objects in stable circular orbits around a Schwarzschild hole with the tidal forces on small objects which are radially and freely falling into the hole.) 4. In any reasonable gravitation theory--- including good old Newtonian gravitation--- of two equally massive objects, the more compact one will allow for stronger fields in its -exterior- simply because falling objects can approach more closely before reaching the "surface" (if any). So in a sense, the strong gravitational fields experienced (according to gtr) near a neutron star are due to the fact that one can approach much more closely to a stellar mass object without running into anything than one can for an ordinary star. Similarly, of course, for stellar mass black holes--- which are not that much more compact than neutron stars, incidentally! So none of this depends upon whether your compact massive object is "made of", in particular, whether it is some kind of star or black hole, or even upon which gravitation theory you are using to model the situation, as long as your theory has the correct Newtonian limit. 5. Tidal forces are -local-; they can be measured within a small idealized "laboratory" without "looking out the window". For example, in (2), in principle one can determine the "direction of fastest increase of magnitude of tidal forces" without looking out the window, so this is a local concept. Contrast this with the "direction pointing at the massive compact object which is the source of the field"--- determining this direction presumably requires looking out the window! In this case, these phrases describe two (not entirely well-defined) methods (one local and one not) for determining a certain spatial direction somewhere outside a Schwarzschild hole. But I am certainly not claiming that every local concept can be alternatively expressed in nonlocal terms, or vice versa. In particular, "event horizon" is a -global- concept which we should not expect to be expressible in local terms. (If this seems confusing, the reason is probably that the "local/global distinction" is difficult to really explain without first explaining certain concepts from manifold theory.) 6. Since "tidal force" is a -local- concept, but "event horizon" is a -global- concept, the nature of tidal forces must be identical on either side of any horizon. This is not invalidated by the fact that deep inside the horizon of some black hole solutions admitted by the EFE (Einstein field equation), the tidal forces measured by at least some observers may assume a "non-Coulomb character". 7. I see now I missed another possible objection to the phrase quoted above--- you can "live" inside a Schwarzschild hole for only a limited time, since (according to gtr) you cannot possibly avoid falling further in, and in fact there is a specific upper bound to your remaining proper lifetime. It is true that for more exotic black hole solutions of the EFE, it is in principle possible for at least some observers to avoid striking a "strong scalar curvature singularity", but this may be an artifact of assuming too much symmetry or of neglecting the effect of infalling radiation and "stuff". (This question is a whole 'nother can of worms, however--- the true nature of the interior of "generic" black hole solutions in gtr remains very vexed, and this problem is the focus of a good deal of current research. In various previous posts I have said much more about this.) 8. All the above concerns purely classical gravitation physics; no statistical, thermodynamical, or quantum mechanical ideas are involved (although I did make some modest nongravitational assumptions concerning how material like paper responds to tensile and compressive forces). The "black hole information loss problem", however, concerns quantum field theory as well as gravitation physics, in (apparently) an essential way. Furthermore, the (apparently) relevant notions of "information/entropy" employ thermodynamical or statistical concepts. > Pulling a sheet of paper apart with my hands will irreversibly destroy > information on the sheet so if a black hole's tidal forces tears the > sheet apart then surely this means information is lost forever in a > black hole? You are trying to invoke an alleged "destruction" of some kind of "information" which you think should happen when you shred paper, but even if you can invent precise mathematical definitions which capture this intuition, I doubt your notion of "shredable information" will turn out to be readily comparable with the kind(s) of "information" physicists have in mind when they discuss the "information loss problem". But again, I'll leave it to others to discuss the "information loss problem" itself; I have only been attempting to correct various apparent misconceptions concerning classical gravitation physics. "T. Essel" (hiding somewhere in cyberspace)



alistair@goforit64.fsnet.co.uk (alistair) wrote in message news:<861c1b21.0406240621.244067b7@p...google.com>... > TESSEL@TUM.BOT replied to ALISTAIR with respect to the loss of > information in black holes: > > >any observer in the interior region will > >experience three dimensional space just like we do, they will just > observe > >some pretty extreme physics like large tidal stresses, weird optical > >effects, etc. > > ALISTAIR writes: > > If I put a sheet of paper in my hands with writing on it, and > pulled it until it ripped, is this what tidal forces inside a black > hole's event horizon would do to the sheet of paper? Pulling a sheet > of paper apart with my hands > will irreversibly destroy information on the sheet so if a black > hole's tidal forces tears the sheet apart then surely this means > information is lost forever in a black hole? When people talk about information loss in a black hole, they are referring to the fact that someone outside the event horizon would have no way of knowing what is written on the piece of paper. David



"alistair" schrieb im Newsbeitrag news:861c1b21.0406240621.244067b7@posting.google.com... > ALISTAIR writes: > > If I put a sheet of paper in my hands with writing on it, and > pulled it until it ripped, is this what tidal forces inside a black > hole's event horizon would do to the sheet of paper? Pulling a sheet > of paper apart with my hands > will irreversibly destroy information on the sheet so if a black > hole's tidal forces tears the sheet apart then surely this means > information is lost forever in a black hole? No, this is not the "information loss problem" of black holes. Pulling stuff apart doesn't have anything in particular to do with black holes, obviously, and is not related to information loss (non-unitarity). The apparent problem with black holes is rather that stuff "falls inside" and after a long-long time thermal Hawking radiation escapes. There would be a problem if in this (possibly overly naive) scenario the Hawking radiation carried no signature of the original stuff that fell into the black hole, roughly.



In classical thermodynamics the final state of a system is independent of the path taken to get to that state.And nobody thinks that there is a problem with information loss. So, when black holes, made in different ways, end up with the same spin, mass and charge, why is it believed that information is lost? One can say " information disppears from the universe into a black hole" but isn't the black hole part of the universe?



alistair@goforit64.fsnet.co.uk (alistair) wrote in message news:<861c1b21.0406220844.2054bbdc@p...google.com>... > If I burned a book the information in it would be lost forever with > respect to all possible observers.This is because the burning of the > book is about as irreversible a process as can be imagined. Except that an all knowing Laplace's demon could look at the state of the universe at a later stage, then reconstruct the book using the reversibility of the laws of physics. The question is whether dropping something into a black hole is more of an irreversible process than the rest of physics. The paradox here is that although we think of the second law of thermodynamics as causing decay and information loss, at the microscopic scale the law rests on the fact that information is never truly lost - if a Maxwell's demon could lose information (say by dropping it into a black hole) then he would be able to overcome the second law. Stephen Lee www.chronon.org



When information is erased entropy increases (e.g. book burning). So if the area of the event horizon of a black hole ( which gets bigger with increasing entropy) increases when a book falls into the black hole, does this mean that the information in the book has been erased?



TESSEL@TUM.BOT said: >The "black hole information loss problem", however, concerns quantum field >theory as well as gravitation physics, in (apparently) an essential way. ALISTAIR writes: Isn't Quantum field theory invoked because of the possibility that information "leaks" out of the black hole in the hawking radiation? ALISTAIR says in addition: A blank sheet of paper would increase the area of the event horizon and the increase in area would be associated with an increase in the entropy of the black hole.If I cut away bits of a heavier sheet to write some sentences,and the cut sheet had the same mass as before, the increase in area of the event horizon would be the same as before,and would not reflect the fact that the sheet carried information. I would have to conclude that, in fact, the area of the horizon does not say anything about the information that went into a black hole and that as far as black holes are concerned, entropy and information are not linked.



> When people talk about information loss in a black hole, they are > referring to the fact that someone outside the event horizon would > have no way of knowing what is written on the piece of paper. But for the outside observer it takes an infinite amount of time for the paper to fall into the black hole. So it would seem the paradox never happens. Or am I wrong? Daniel



Urs Schreiber said: >The apparent problem >with black holes is rather that stuff "falls inside" and after a long-long >time thermal Hawking radiation escapes. There would be a problem if in this >(possibly overly naive) scenario the Hawking radiation carried no signature if a Maxwell's demon could lose information (say by >dropping it into a black hole) then he would be able to overcome the >second law. Alistair says: The demon would have to expend energy to lose the information and this would increase the entropy of the universe, so the second law would not be broken.



In article <37d84b42.0406290104.3402b89f@posting.google.com>, daniel@elit.net (Daniel Elander) wrote: > > When people talk about information loss in a black hole, they are > > referring to the fact that someone outside the event horizon would > > have no way of knowing what is written on the piece of paper. > > But for the outside observer it takes an infinite amount of time for > the paper to fall into the black hole. So it would seem the paradox > never happens. Or am I wrong? > > Daniel The statement "for the outside observer it takes an infinite amount of time" for something to fall into a black hole is true in two senses, but you have to look very carefully at what those statements are to see what they tell you about information accessibility. (1) The first sense in which "for the outside observer it takes an infinite amount of time" is purely a statement about coordinate systems. In the Schwarzschild coordinates, t, $r, \phi$ and $\theta --$ which become asymptotically close to ordinary polar coordinates in flat spacetime far from the black hole, and so in a sense might qualify as appropriate coordinates for "an outside observer" -- the spacelike hypersurfaces of constant t which begin anywhere outside the event horizon never actually cross it. Figure 9 in this article: http://gregegan.customer.netspace.ne...ound03.html#s8 shows curves of constant t in $a 2-d$ slice through the spacetime. These curves never cross the horizon $(r=2M),$ so by increasing the value of t you can get closer to the horizon, but you can never actually pass through it. OK, but I can construct coordinates that do the same absurd thing in flat spacetime, so this doesn't really tell us much about the physics of black holes. I can make up coordinates which are sensible close to me, but have the property that all the spacelike hypersurfaces that intersect my world line never cross an arbitrary barrier, say 1 metre away from me. And as Figure 9 shows, it's not hard to find another sensible coordinate, t*, such that the hypersurfaces of constant t* do cross the black hole's horizon. (2) The second sense in which "for the outside observer it takes an infinite amount of time" for something to fall across the event horizon is that if you drop a flashing beacon into a black hole, and watch from a fixed distance away from the event horizon, there is no upper bound on how long (according to your own watch) you continue to receive *some* light from the beacon. Classically at least, the light becomes dimmed and red-shifted in an exponential decay, but it never strictly hits zero. However, note that: (a) if you count the flashes of the falling beacon, there is a finite number assigned to the last flash you will ever see from it. That last flash (assuming the beacon is "on" rather than "off" as it crosses the horizon) reaches you over an unbounded time, but you still only ever get to see a finite portion of the beacon's history. (b) after you drop the beacon from a Schwarzschild coordinate of r, there is a finite time $*for you*, T(r),$ after which it becomes physically impossible for you to chase after the beacon and retrieve it (even if you can tolerate arbitrarily large accelerations), or even to illuminate it with a light source of your own. Once T(r) has elapsed, it doesn't matter what you do, and it doesn't matter that you can still see the beacon; for all practical purposes, it's lost to you. The fact that you're receiving ever-dimmer light from the beacon for all eternity just tells you that the last light from it as it crossed the horizon is reaching you in a strange, drawn-out fashion. Greg Egan Email address (remove name of animal and add standard punctuation): gregegan netspace zebra net au



"alistair" wrote in message news:861c1b21.0406240841.3f5890b0@posting.google.com... > In classical thermodynamics the final state of a system is independent > of the path taken to get to that state.And nobody thinks that there is > a problem with information loss. So, when black holes, made in > different ways, end up with the same spin, mass and charge, why is it > believed that information is lost? > One can say " information disppears from the universe into a black > hole" but isn't the black hole part of the universe? > I don't understand this. I tear out a page from Nature, and put it into my super-duper CIA designed paper shredder that shreds paper into atoms! Are you saying that I, theoretically, could reconstruct that page? -- Bob Day



The correct theory of quantum gravity is expected to show that singularities do not exist inside black holes.So how could this affect the information loss problem associated with black holes? Would the singularity become a small region of space into which a particle cannot fall? If nothing can get into this region,it would have zero entropy.Since the outer part of a black hole, the event horizon, has entropy, perhaps entropy for a black hole would increase,per unit area, as we move from the centre towards the horizon. Some particles just on the edge of the small region of space at the centre of a black hole,could not move from their position unless more particles (cosmic microwave background photons for example)replaced them to keep the entropy in that area the same as it was before (else the second law of thermodynamics would be broken).Would this mean that Hawking radiation could not be emitted from the black hole without an influx of particles from outside it? Since Hawking radiation is postulated to carry information out of the hole, this would mean that the outflow of information can only occur if there is an inflow of other information.Or can someone think of an argument against this idea?



"Bob Day" wrote in message news:... > "alistair" wrote in message > news:861c1b21.0406240841.3f5890b0@posting.google.com... > > In classical thermodynamics the final state of a system is independent > > of the path taken to get to that state.And nobody thinks that there is > > a problem with information loss. So, when black holes, made in > > different ways, end up with the same spin, mass and charge, why is it > > believed that information is lost? > > One can say " information disppears from the universe into a black > > hole" but isn't the black hole part of the universe? > > > > I don't understand this. I tear out a page from Nature, and > put it into my super-duper CIA designed paper shredder that > shreds paper into atoms! Are you saying that I, theoretically, > could reconstruct that page? > > -- Bob Day Of course. Take every atom, including yours and the shredders one, invert their momentum precisely replace matter with anti matter and the paper will reassemble. It just happens to be that these are arbitrarily unlikely initial conditions to encounter. --- frank