
How do black holes destroy information and why is that a problem? - sohkamyung
https://backreaction.blogspot.com/2019/08/how-do-black-holes-destroy-information.html
======
chasd00
man, watch the PBS YouTube channel SpaceTime. They're short, informative, and
very well done. they dedicate a handful of episodes to getting you ready for
Hawking radiation.

[https://youtu.be/qPKj0YnKANw](https://youtu.be/qPKj0YnKANw)

~~~
dharmab
I love their dedication to building up over multiple episodes instead of
dumbing things down every time.

------
hyperpallium
> The important thing is that all evolution equations that we know of are
> time-reversible.

I've heard this before, but it's so surprising to me, when one of the simplest
mathematical operations - addition - is not reversible. e.g. if _a+b=4_ , you
can't infer the values of _a_ and _b_ (beyond their linear relationship). This
non-injectivity is a kind of summarising, with less information, where more
than one state maps to one.

So "evolution equations" (reality) are not like addition; state changes never
summarise.

~~~
myaccount80
It is not reversible because you’re ereasing information. If you want to have
a reversible addition you are not allowed to destroy the information.

The same happens if you apply the standard AND operator to two bits because
the output is only one bit, therefore it is impossible to figure out the two
input bits. To have a reversible AND operator you can define it like this:
f(x1, x2, b) = (x1, x2, b XOR (x1 AND x2)). E.g: f(1,1,0) = (1,1,1). From this
output you can get back to the input. This way you can implement a NOT gate as
well, and all others gates. At the end you can implement addition using bits
and your summation becomes reversible :) You can see that with this definition
we are not ereasing any information, hence it becomes reversible.

~~~
codernyc16
You’d have to do that for all results, not just one. This is basically what an
event sourced system does: you store all the events and fold your state, so
you can “play back” to any point in time. You could theoretically reverse from
the end, but I haven’t seen it done.

~~~
Zenst
A transactional database would tick that box, events are stored and can
rollback to a point in time.

~~~
acoma
Not quite. Dropping a column or updating a row is typically irreversible.

~~~
continuational
In PostgreSQL, schema changes are transactional.

~~~
anarazel
> In PostgreSQL, schema changes are transactional.

With a few exceptions (that refuse to run in an explicit transaction): E.g.
{CREATE, DROP} DATABASE, {CREATE, DROP} TABLESPACE, {CREATE INDEX, DROP INDEX,
REINDEX} CONCURRENTLY, REINDEX {SCHEMA, SYSTEM, DATABASE}.

~~~
zcrackerz
Also, try adding a value to an enum via ALTER TYPE ... ADD VALUE

~~~
anarazel
Not anymore ;). Well, at least in the upcoming PG 12.

[https://git.postgresql.org/gitweb/?p=postgresql.git;a=commit...](https://git.postgresql.org/gitweb/?p=postgresql.git;a=commit;h=212fab9926b2f0f04b0187568e7124b70e8deee5)

    
    
      commit 212fab9926b2f0f04b0187568e7124b70e8deee5
      Author: Thomas Munro <tmunro@postgresql.org>
      Date:   2018-10-09 12:51:01 +1300
      
          Relax transactional restrictions on ALTER TYPE ... ADD VALUE (redux).
    

There's still some restrictions:

> This patch removes that restriction, and instead insists that an uncommitted
> enum value can't be referenced unless it belongs to an enum type created in
> the same transaction as the value.

------
ailideex
Do they destroy information? Last I checked they don't:
[https://physics.stackexchange.com/questions/29175/why-is-
inf...](https://physics.stackexchange.com/questions/29175/why-is-information-
indestructible)

Maybe there is some recent development I am not aware of an Susskind is wrong
though.

More:

\- [https://physics.stackexchange.com/questions/450326/why-is-
th...](https://physics.stackexchange.com/questions/450326/why-is-the-
information-paradox-restricted-to-black-holes)

~~~
ben_w
That’s why it’s a problem — the two best tested laws of physics contradict
each other.

~~~
ailideex
Which law of physics say information must be lost in a black hole?

AFAIK if information is lost in a black hole it will also violate the second
law of thermodynamics - so really you have many many laws saying it cannot be
lost - and Hawking temporarily claimed it will be lost in black holes before
recanting and saying it won't.

So ... I don't quite follow. I mean this title is as concerning to me as
something that claims the earth is flat.

~~~
sohkamyung
I believe the 'No-hair theorem' is closest to a 'law' that says Black holes
destroy information [1]. If it is true, then when you throw anything into a
black hole, information about what you threw in (whether it is a chair, a
star, a planet, etc.) is 'lost' as to the outside world they all yield the
same observable parameters of the black hole.

[1] [https://en.wikipedia.org/wiki/No-
hair_theorem](https://en.wikipedia.org/wiki/No-hair_theorem)

~~~
TeMPOraL
IANAPhysicist, but couldn't information be radiated away? When you have a
particle and antiparticle annihilating, the information about what collided
and when is carried away by the radiation from that annihilation event;
nothing remains at ground zero to be inspected.

As matter traverses the accretion disk and falls towards the event horizon,
couldn't information about it be radiated away? As an observer, you could
definitely tell whether somebody threw a planet or a planet-sized bunch of
paperclips into a black hole by observing the accretion disk as it happened.

~~~
platz
As it stands, as far as we know, Hawking radiation is completely thermal and
random, so as the article states, you can't tell the initial state from the
final state even if you collect all the radiation.

> observing the accretion disk

Well that is not part of the black hole now, is it.

~~~
ailideex
> As it stands, as far as we know, Hawking radiation is completely thermal and
> random

Is it? Can you cite?

IANAP but the best I could find on the matter is:

[https://en.wikipedia.org/wiki/Black_hole_information_paradox](https://en.wikipedia.org/wiki/Black_hole_information_paradox)

> There are various ideas about how the paradox is solved. Since the 1997
> proposal of the AdS/CFT correspondence, the predominant belief among
> physicists is that information is preserved and that Hawking radiation is
> not precisely thermal but receives quantum corrections.

Maybe that is wrong, but not sure what the basis for the claim to the contrary
is.

Another citation which speaks to the views of Hawking on the matter:

[https://iopscience.iop.org/article/10.1088/1367-2630/7/1/203...](https://iopscience.iop.org/article/10.1088/1367-2630/7/1/203#nj185922bib67a)

> Now if information is really lost down a black hole as Hawking originally
> proposed [64], and if the Hawking radiation really has negligible
> correlations between what is emitted early and late, then it might be true
> that A/4 −tr(ρ ln ρ), suitably regularized, would never decrease. But since
> this information-loss proposal has been controversial since near the
> beginning [65, 66], and since now even Hawking has given it up [67];

Direct quote from Hawking:

[https://www.newscientist.com/article/dn6193-hawking-
concedes...](https://www.newscientist.com/article/dn6193-hawking-concedes-
black-hole-bet/#ixzz5xjLC8rNa)

> “If you jump into a black hole, your mass energy will be returned to our
> Universe, but in a mangled form, which contains information about what you
> were like, but in an unrecognisable state.”

~~~
platz
The reason the black hole information paradox is a paradox is because it
presents us with an unresolved contradiction ("There are various _ideas_ about
how the paradox is solved"). So there are several theories with try to resolve
the contradiction, but they are all competing theories (i.e. physicists do not
agree). Also, some of those theories assume Anti-de Sitter space which
contradicts the positively mesaures cosmological constant.

So in the sense that before the paradox is resolved, we don't know if the
radiation is thermal or not, but the idea of the radiation being thermal was
based on the existing bog-standard theory of QM and GR. What is under debate
is how that needs to be modified, which we haven't agreed yet.

------
stochastimus
This was actually a really accessible description for a layperson. Thank you.

~~~
soulofmischief
> Think of mixing dough. You’ll never be able to unmix it in practice. But if
> only you could arrange precisely enough the position of each single atom,
> you could very well unmix the dough. The same goes for burning a piece of
> paper. Irreversible in practice. But in principle, if you only knew
> precisely enough the details of the smoke and the ashes, you could reverse
> it.

This was a nice description of entropy.

------
Ultimatt
This is literally what the holographic principle addresses...
[https://youtu.be/B2ksDczJOAs](https://youtu.be/B2ksDczJOAs) Leonard Susskind
has enough accessible vids and lectures to give a conceptual idea of how it
works out.

~~~
platz
Also see Black hole Firewalls - with Sean Carroll (speaking at 20:25) and
Jennifer Ouellette
[https://www.youtube.com/watch?v=_8bhtEgB8Mo](https://www.youtube.com/watch?v=_8bhtEgB8Mo)

which explains the paradox and several different competing theories at a high
level.

------
openasocket
The other major example of a time-irreversible operation/object is much more
mundane: the collapse of a wave function. I've never been able to find a
decent layperson's explanation of what the wave function collapse really
means, and why physicists seem to have no issue with it being time
irreversible but seem quite concerned with black holes

~~~
DiogenesKynikos
It is a problem. Quantum mechanics, as it's normally taught, basically has two
components:

1\. Schrödinger's equation, which governs how the wavefunction (or quantum
state) evolves in time. This equation is time-reversible: given a state at
time t, you can calculate what the state was at time t-T. Technically, that
means that the "time evolution operator" is invertible. All the information
about the history of how system's state is contained in the present state. No
information is ever destroyed.

2\. Observation. A quantum state looks like a_1 * psi_1 + a_2 * psi_2 + ... +
a_n * psi_n, where psi_i are all the possible states of the system and a_i are
complex numbers called "amplitudes." When you observe a state (I'm obviously
leaving out some mathematical details here, so anyone with physics knowledge
please forgive me), you observe it to be in one of the possible states, psi_i,
with i between 1 and n. The probability of observing it to be in state i is
proportional to |a_i|^2. This operation destroys information, because the
state collapses to psi_i, and all the amplitudes, a_j, j≠i, are lost. You can
no longer reconstruct the previous state of the system.

I think most physicists who "seriously" think about quantum mechanics do not
believe that step 2 above actually happens. It is a simplification of a much
more complicated process called "decoherence." In order to understand
decoherence, you have to change your perspective on what observation means. If
you treat the observer as a system governed by Schrödinger's equation, which
interacts with the system that's being measured, you find that the observer
becomes entangled with the system under observation. The observer ends up in a
superposition of states, each of which has observed a different outcome. It
appears to each state of the observer as if there has been wavefunction
collapse, but there actually is a larger quantum system containing both the
observer and thing being observed, in which no information has been lost.

The theory of decoherence and the "many-worlds interpretation" began to be
developed in the 1950s by one of Wheeler's students, Everett. Somehow, it
hasn't really made it into undergraduate physics yet, and most physicists can
get by without thinking too deeply about what observation means. You can do
most calculations assuming wavefunction collapse happens.

------
graycat
Q. At time t1 = 0, we know the _state_ of our system. Then we send a photon
into an atom. Assume the atom absorbs the photon by having an electron move to
a different energy level.

Some time later, at time t2, the electron returns to where it was and emits a
photon. As I understand, t2 - t1 is random, with exponential distribution, and
independent of everything else in the universe. So, at time t2, we cannot
recover the _state_ at time t1 or even know just when t1 was. That is, the
reversible time evolution equation of state at time t2 can't tell us the
random time t2 - t1 or t1 so cannot recover the state at time t1 -- at time
t2, we just do not know just when or how we got to the state at time t2.

So, the time evolution equation is time reversible but the event of the
emission of the photon is not time reversible.

This _thought experiment_ seems to conflict with the OP. Where am I going
wrong?

~~~
yk
You are collapsing the wave function at t1 and t2. At t1 explicitly by stating
that the state is known, and by t2 implicitly by defining that you have a
defined distance t2- t1.

The emission of an photon at an explicit time t2 is not reversible, the
emission of an photon according to the wave function of the atom leads to an
photon entangled with the atom and the total has a reversible wave function.
(For illustrative purposes, when you measure and you find the photon at a
distance (t- t2)*c from the atom, then you will also find the atom in a state
that has emitted a photon at t2, no Idea if that is especially illustrative.)

------
maffydub
(I am not a physicist.) One thing I've struggled with around black holes is
what it means for anything to fall into a black hole.

From a reference frame outside the black hole, observing an object falling
into the black hole, don't we observe time slowing down as the object
approaches the event horizon? In this reference frame, does it ever cross the
event horizon, or does it just asymptotically approach it? If it doesn't cross
the event horizon, how can anything ever fall into the black hole?

(From the reference frame of the object falling into the black hole, my
understanding was that the event horizon shrinks away from you as you approach
it.)

Please correct/inform me!

~~~
jadams5
(also not a physicist) I think you're correct that objects falling into a
black hole never cross the event horizon from our reference frame, but at the
same time the light from the object red shifts towards zero energy as it
approaches the event horizon as well.

~~~
maffydub
Thanks!

Does the fact that the light red shifts towards zero energy mean that the
information has been lost, or just that you can't observe it _at_the_moment_?

If the black hole then shrinks due to Hawking radiation, what happens to the
object that _never_quite_fell_into_ the black hole?

If it hasn't yet fallen into the black hole, presumably its light becomes
slightly-less-red-shifted once more, meaning that you can observe its
information again?

------
misja
Isn't this resolved by the holographic principle?
([https://en.wikipedia.org/wiki/Holographic_principle](https://en.wikipedia.org/wiki/Holographic_principle))

~~~
GuB-42
Yes, but it introduces other problems. And most importantly, it hasn't been
successfully tested yet.

~~~
throwaway_law
Are there even proposed experiments to test this?

My layman understanding of the Holographic Principle is an object falling into
a blackhole has it's 3D information copied onto a 2D surface/perimeter (event
horizon) and that 2D information can tell us everything about the energy that
continues to exist within the 3D volume of the blackhole.

Fractal Mathematics suggests it is possible right? In other words we can
infinity extend the perimeter of a shape while maintaining the same volume,
but what are the proposed experiments, if any, to observe this phenomena in
nature for quanta?

~~~
marcosdumay
The Holographic Principle shares with String Theory the problem that it is a
generic name for a huge number of theories, some that have been falsified
already, some practically impossible to test, and some just out of reach.

------
mherrmann
She also has a music video about Schrödinger's cat:
[https://youtu.be/I_0laAhvHKE](https://youtu.be/I_0laAhvHKE)

------
acollins1331
Simple: the laws of most physics say you can look at something now and work it
backwards. Once it falls into a black hole the previous state of what falls
into it isn't able to be derived from working backwards because it just fell
into a black hole. This isn't rocket science! (Maybe eventually though)

~~~
dchest
_... because it just fell into a black hole_

That doesn't really explain anything.

~~~
acollins1331
I said simply!

------
ricardo81
Perhaps this is why we perceive time to be going in one direction. The fact
that a black hole exists in our universe means that there is only that
direction.

------
maxheadroom
To save you the read, the summation is in the second-to-last paragraph:

> _As you have probably noticed, I didn’t say anything about information.
> That’s because really the reference to information in “black hole
> information loss” is entirely unnecessary and just causes confusion. The
> problem of black hole “information loss” really has nothing to do with just
> exactly what you mean by information. It’s just a term that loosely speaking
> says you can’t tell from the final state what was the exact initial state._

~~~
JDEW
I think the article is already quite concise. Moreover it is nicely written
and explains a current open problem in physics from first principles.

A summary like this is useful if the underlying article is bloated and
clickbait-y but in this case potentially detracts a reader from a good
article.

As featured on the front page a few days ago [1]: knowledge without
understanding is meaningless.

[1]
[https://www.theguardian.com/commentisfree/2019/aug/24/dougla...](https://www.theguardian.com/commentisfree/2019/aug/24/douglas-
adams-was-right-knowledge-without-understanding-is-meaningless)

~~~
jinushaun
It’s the opposite of well written and concise! The writing style is very much
like someone talking, full of sentences that are too long or too short.

~~~
raphaelj
That's because it's a direct transcript of the Youtube video above.

------
vectorEQ
mv universe.info /dev/null

i see no issues there

------
cellular
A black hole is not a singularity. If that were true, they would ALL emit the
same Hawking radiation pattern.

------
ropiwqefjnpoa
She must be a teacher at Professor Xavier's School for Gifted Youngsters.

