Tag Archives: Hawking

The Black Hole’s Tale

[Inspiration strikes in odd ways and at strange times.  Don’t ask me why I wrote this, because I’ve no idea.  In any case I hope some of you enjoy it; and the science behind it is described here.]

Quantum Theory claims: “All tales are told!”
But gravity demurs; for Einstein’s bold
Equations show that black holes tell no tales
And keep their secrets hidden deep within.

So it remained til nineteen seventy-four
When Hawking’s striking calculation showed
That black holes aren’t exactly black: they glow!
They shrink, wither, and in a flash they die
And take their hidden secrets to their graves,
Killing Quantum Theory as they go.

And if you disagreed, and did believe
that black holes’ tales are written in their glow,
No matter; this kills Quantum Theory too,
For once inside, a story can’t come out,
And copying puts a quantum world in doubt.

Thus Hawking argued that he’d made it clear
That Quantum Theory had to be revised.
“But not so fast” cried Susskind and ‘t Hooft,
For Quantum Theory’s cleverer than you think;
T’was twenty years ago the claim was made
That black holes may be complementary:
While those who venture in do find the tales
Are written clearly in the black hole’s deep,
Those outside have a very different view.
They think the stories rest upon the edge
And later end up written in the sky.

So strange this sounds! And yet, it has been shown
That in a quantum world of certain type
The information stored within a space
Can also seem to be upon its face!

Consensus grew that Quantum Theory’s safe
And even Hawking painfully agreed
The argument was strong; nine years ago
He publicly announced his change of heart.

But still it wasn’t yet precisely clear
Just how it is that black holes disappear
Without undoing Quantum Theory’s base;
And then the AMPS collaboration found,
While trying to ensure the case was sound,
The complementary black hole in fact
Could not exist! At least not as we thought,
For when the tale’s half written in the sky
The black hole’s inside could no longer be,
And anyone who reached the edge would die.

“Firewall”, the cry rose from the crowd;
And troubling it was; such walls would flout
The principles that Einstein had set out
To underpin his theory of space and time
And gravity — the very one we used
To show black holes exist, and find them too.

So something’s wrong! But what? What must we change?
Which principle is it that we must revise?
Which equation fails, and in what guise?
Confusion spreads across the blackened skies…

Proposals have been made, but none is firm.
Among them Hawking’s recent; he suggests
A black hole’s even less black than he thought:
Not only does it faintly glow, it leaks
Like politicians, whispering its tales
In code; and thus whatever is inside
Gets out! Though in a highly scrambled form.
(So do not try to enter and return!)
These holes aren’t complementary; instead
Their inner stories are somehow released
Before the holes that store them are deceased.

But be not sure; for Hawking’s story’s vague
And many others have suggested ways
That current controversy may be stemmed.
Yet none of them seem likely soon to lift
The murky darkness that still makes us blind
And hides the truth from all of humankind.

© Matt Strassler February 5, 2014

How Black is a Black Hole? An Introduction to the Paradoxes

Following on Thursday’s post and yesterday’s about black holes, specifically about Hawking’s recent vague proposal that was so widely (but rather misleadingly) reported in the media, and about the back-story which explains why there’s so much confusion about black holes among scientists interested in quantum gravity, and why Hawking made his suggestion in the first place, I’ve been motivated to write up a new introduction to the black hole information paradox.  This should provide the basic knowledge and the context that I’m sure many of you are looking for.  Please take a look and send comments!

Learning Lessons From Black Holes

My post about what Hawking is and isn’t saying about black holes got a lot of readers, but also some criticism for having come across as too harsh on what Hawking has and hasn’t done. Looking back, I think there’s some merit in the criticism, so let me try to address it and flesh out one of the important issues.

Before I do, let me mention that I’ve almost completed a brief introduction to the “black hole information paradox”; it should be posted within the next day, so stay tuned for that IT’S DONE!  It involves a very brief explanation of how, after having learned from Hawking’s 1974 work that black holes aren’t quite black (in that they slowly radiate particles), physicists are now considering whether black holes might even be less black than that (in that they might slowly leak what’s gone inside them, in scrambled form.)

Ok. One of the points I made on Thursday is that there’s a big difference between what Hawking has written in his latest paper and a something a physicist would call a theory, like the Theory of Special Relativity or Quantum Field Theory or String Theory. A theory may or may not apply to nature; it may  or may not be validated by experiments; but it’s not a theory without some precise equations. Hawking’s paper is two pages long and contains no equations. I made a big deal about this, because I was trying to make a more general point (having nothing to do with Hawking or his proposal) about what qualifies as a theory in physics, and what doesn’t. We have very high standards in this field, higher than the public sometimes realizes.

A reasonable person could (and some did) point out that given Hawking’s extreme physical disability, a short equation-less paper is not to be judged harshly, since typing is a royal pain if you can’t even move. I accept the criticism that I was insensitive to this way of reading my post… and indeed I thereby obscured the point I was trying to make.  I should have been more deliberate in my writing, and emphasized that there are many levels of discussions about science, ranging across cocktail party conversation, wild speculation over a beer, a serious scientific proposal, and a concrete scientific theory. The way I phrased things obscured the fact that Hawking’s proposal, though short of a theory, still represents serious science.

But independent of Hawking’s necessarily terse style, it remains the case that his scientific proposal, though based on certain points that are precise and clear, is quite vague on other points… and there are no equations to back them up.  Of course that doesn’t mean the proposal is wrong!  And a vague proposal can have real scientific merit, since it can propel research in the right direction. Other vague proposals (such as Einstein’s idea that “space and time must be curved”) have sometimes led, after months or years, to concrete theories (Einstein’s equations of “General Relativity”, his theory of gravity.) But many sensible-sounding vague proposals (such as “maybe the cosmological is zero because of an unknown symmetry”) lead nowhere, or even lead us astray. And the reason we should be so sensitive to this point is that the weakness of a vague proposal has already been dramatically demonstrated in this very context.

The recent flurry of activity concerning the fundamental quantum properties of black holes (which unfortunately, unlike their astrophysical properties, are not currently measurable) arose from the so-called firewall problem. And that problem emerged, in a 2012 paper by Almheri, Marolf, Polchinski and Sully (AMPS, for short), from an attempt to put concrete equations behind a twenty-year-old proposal called “complementarity”, due mainly to Susskind, Thoracius and Uglom; see also Stephens, ‘t Hooft and Whiting.

As a black hole forms and grows, and then evaporates, where is the information about how it formed?  And is that information lost, copied, or retained? (Only if it is retained, and not lost or copied, can standard quantum theory describe a black hole.) Complementarity is the notion that the answer depends on the point of view of the observer who’s asking the question. Observers who fall into the black hole think (and measure!) that the information is deep inside. Observers who remain outside the black hole think (and measure!) that the information remains just outside, and is eventually carried off by the Hawking radiation by which the black hole evaporates.  And both are right!  Neither sees the information lost or copied, and thus quantum theory survives.

For this apparently contradictory situation to be possible, there are certain requirements that must be true. Remarkably, a number of these have been shown to be true (at least in special circumstances)! But as of 2012, some others still had not been shown. In short, the proposal, though fairly well-grounded, remained a bit vague about some details.

And that vagueness was the Achilles heel that, after 20 years, brought it down.

The firewall problem pointed out by AMPS shows that complementarity doesn’t quite work. It doesn’t work because one of its vague points turns out to have an inherent and subtle self-contradiction. [Their argument is far too complex for this post, so (at best) I’ll have to explain it another time, if I can think of a way to do so…]

By the way, if you look at the AMPS paper, you’ll see it too doesn’t contain many equations. But it contains more than zero… and they are pithy, crucial, and to the point. (Moreover, there are a lot more supporting equations than it first appears; these are relegated to the paper’s appendices, to keep the discussion from looking cluttered.)

So while I understand that Hawking isn’t going to write out long equations unless he’s working with collaborators (which he often does), even the simplest quantitative issues concerning his proposal are not yet discussed or worked out. For instance, what is (even roughly) the time scale over which information begins leaking out? How long does the apparent horizon last? It would be fine if Hawking, working this out in his head, stated the answers without proof, but we need to know the answers he has in mind if we’re to seriously judge the proposal. It’s very far from obvious that any proposal along the lines that Hawking is suggesting (and others that people with similar views have advanced) would actually solve the information paradox without creating other serious problems.

When regarding a puzzle so thorny and subtle as the black hole information paradox, which has resisted solution for forty years, physicists know they should not rely solely on words and logical reasoning, no matter how brilliant the person who originates them. Progress in this area of theoretical research has occurred, and consensus (even partial) has only emerged, when there was both a conceptual and a calculational advance. Hawking’s old papers on singularities (with Penrose) and on black hole evaporation are classic examples; so is the AMPS paper. If anyone, whether Hawking or someone else, can put equations behind Hawking’s proposal that there are no real event horizons and that information is redistributed via a process involving (non-quantum) chaos, then — great! — the proposal can be properly evaluated and its internal consistency can be checked. Until then, it’s far too early to say that Hawking’s proposal represents a scientific theory.

Did Hawking Say “There Are No Black Holes”?

Media absurdity has reached new levels of darkness with the announcement that Stephen Hawking has a new theory in which black holes do not exist after all.

No, he doesn’t.

[Note added: click here for my new introduction to the black hole information paradox.]

First, Hawking does not have a new theory… at least not one he’s presented. You can look at his paper here — two pages (pdf), a short commentary that he gave to experts in August 2013 and wrote up as a little document — and you can see it has no equations at all. That means it doesn’t qualify as a theory. “Theory”, in physics, means: a set of equations that can be used to make predictions for physical processes in a real or imaginary world. When we talk about Einstein’s theory of relativity, we’re talking about equations. Compare just the look and feel of Hawking’s recent note to Einstein’s 1905 paper on the theory of special relativity, or to Hawking’s most famous 1975 paper on black holes; you can easily see the difference without understanding the content of the papers.

The word “theory” does not mean “speculations” or “ideas”, which is all that is contained in this little article. Maybe that’s what theory means at a cocktail party, but it’s not what “theory” means in physics.

Second, what Hawking is addressing in this note is the precise level of blackness of a black hole… in short, whether the name “black hole” for the objects we call black holes is really appropriate. But simply the fact that black holes aren’t quite black isn’t new. In fact it was Hawking himself who became famous in 1974-1975 for pointing out that in a world with quantum physics, typical black holes cannot be precisely black — so it’s not true that nothing ever comes out of a black hole. Black holes must slowly radiate elementary particles, a process we call Hawking radiation.

From day 1, Hawking’s observation posed puzzles about how conflicting requirements of quantum theory and Einstein’s gravity would be resolved, with quantum theory demanding that all information that fell into the black hole be neither destroyed nor copied, and Einstein’s gravity insisting that there is no way that the information of what went into a black hole can ever come out again, even if the black hole evaporates and disappears. The assumption of the community has long been that the 1970s calculation that Hawking did, while largely correct, leaves out a small, subtle effect that resolves the puzzle. The question is: what is the nature of that subtle effect?

No one, including Hawking, has posed a satisfactory answer. And that is why we keep hearing about black holes again and again over the decades, most recently in the context of the “firewall paradox”. In his recent paper, Hawking, like many of his colleagues, is proposing another possible answer, though without demonstrating mathematically that his proposal is correct.

But did Hawking really say “There are no black holes”, or didn’t he??

Talk about taking things out of context!!! Here’s what Hawking actually said.

First he suggests that the edge of a black hole — called its “event horizon”, a very subtle concept when you get into the details — really isn’t so sharp once quantum effects are considered. Many people have suggested one version or another of this possibility, which would represent a small but critical correction to what Hawking said in the 1970s (and to what people understood about black holes even earlier).

And then Hawking writes…

“The absence of event horizons mean that there are no black holes – in the sense of regimes from which light can’t escape to infinity.”

Notice the final clause, which is omitted from the media reports, and is absolutely necessary to make sense of his remark. What he means is that black holes are very, very slightly (though importantly) less black than he said in his 1974 paper… because the things that fall into the black hole do in some sense eventually come back out as the black hole evaporates. I say “in some sense” because they come out thoroughly scrambled; you, for example, if you fell in, would not come back out, even though some of the elementary particles out of which you are made might eventually do so.

And then he says

“There are however apparent horizons which persist for a period of time.”

Translation: for an extremely long time, what we call a black hole will behave in just the way we have long thought it does. In particular, there is no change in any of the astrophysics of black holes that astronomers have been studying in recent decades. The only issue is what happens as a black hole begins to evaporate in a serious way, and when you look very, very carefully at the details of the Hawking radiation, which is very difficult to do.

“This suggests that black holes should be redefined as metastable bound states of the gravitational field.”

In short: In Hawking’s proposal, it’s not that the objects that you and I would call “black holes” don’t exist!  They are still there, just with a new name, doing what we’ve been taught they do except in some fine-grained detail. Not that this fine-grained detail is unimportant — it’s essential to resolving the quantum vs. gravity puzzle.  But an ordinary person watching or exploring near a black hole would notice no difference.

Notice also all of this is a proposal, made in words; he has not shown this with mathematics.

In short, although Hawking is, with many of his colleagues, working hard to resolve the puzzles that seem to make quantum theory conflict with Einstein’s theory of gravity in this context, he’s not questioning whether black holes exist in the sense that you and I would mean it. He’s addressing the technical issue of exactly how black they are, and how the information contained in the things that fall in comes back out again. And since he’s just got words, but not math, to back up his suggestions, he’s not convinced his colleagues.

Meanwhile, the media takes the five words “There Are No Black Holes” and creates almost pure fiction, fiction that has almost nothing to do with the reality of the science. Well done, media, well done. Sometimes you’re just like a black hole: information comes in, and after being completely scrambled beyond recognition, comes back out again through a mysterious process that makes no sense to anyone. Except that in your case, it’s very clear that information is lost, and misinformation is created.

Hey! That’s a new theory of black holes! (I’ll write a 2-page paper on that this afternoon…)