Quantum Black Holes – Why Worry?

susskind_by_matthew_black for LA TimesWell, you may not have gone to the chat between K C Cole and Leonard Susskind that I mentioned a while ago at the LA Central Library downtown. I couldn’t make it either, being away at the Aspen Center for Physics. I expect it was good. Anyway, I found a little bit of a report on the conversation, done by reporter John Johnson for the LA Times. It is here. (Clickable image of Susskind to the right is by Matthew Black, for the LA Times.)

It gives you some of the simply-stated reasons as to why there was a big argument between Stephen Hawking and Leonard Susskind in the first place (and between several other physicists too… there are hosts of people working on these things, and it took hosts of people to sort it out to where we are now, not just those two, giants though they are). I recommend having a look, as it is especially for the lay-person, and will give you a good idea of what the fuss is about.

You can also see a little bit about his new book on the subject and a link to a video interview with Brian Cox (the physicist, not the actor) at the LA Times blogs here. There are also links to his Stanford continuing education course on quantum mechanics, including the online lectures you can view at your convenience. What a resource!

You might wonder why we care about all this, since currently the only way we know for sure to make black holes in the universe (astrophysical processes making stellar black holes or the supermassive ones at the cores of galaxies – search on black holes in the archives for more on all these) gives black holes that are large (relatively speaking), and since the quantum effect that serves as the key lightning rod for all the discussion – Hawking radiation, the stuff that escapes from them by using quantum physics – goes inversely with the size of the black hole, it is tiny for these black holes.

So in effect, all the black holes we know about simply don’t care about this discussion since their Hawking temperature (setting the rate at which they’d radiate due to quantum effects) is colder than the rest of the universe, and so at best they’d absorb radiation from the rest of the universe and not send it out (Susskind wisely mentions this in his conversation too). (I’m ignoring all the non-quantum ways in which black holes can do marvellously active things involving throwing out huge plumes and jets of matter and radiation from their neighbourhood. In contrast to the putative Hawking radiation, none of this stuff actually comes from inside the hole, representing loss, just near the hole.) So we either have to wait for the universe to cool down so as to be colder than black holes (that would be a very long time), or find much smaller black holes created in ways that are not to do with astrophysics. Well, we could make these tiny ones ourselves if particle physics and quantum gravity conspire in a nice way at the next level of collider experiments (see earlier discussions), (this is regarded as an unlikely possibility) or we might look for ones made in other ways (perhaps by some process or other in the early universe 13.7 billion years ago) that are radiating (explosively, so we can see it) just about now. Not much sign of either giving us clues just yet.

So instead, we’ve just been thinking hard about the problem, scribbling and arguing and calculating and so forth. (Yes, that stuff of my day to day I tell you about sometimes.) Turns out that just trying to get the physics to work on paper has been really useful. It is the issue of solving a problem that is there in principle even if you won’t face it any time soon, because the effort of solving it is a challenge on the one hand, and is likely to teach you wonderful things on the other.

The issue is all about trying to make gravity and quantum mechanics work properly together to make what we call “quantum gravity”, and we’ve learned a huge amount about the problem in recent years in string theory. Understanding about how to describe quantum aspects of black holes has been at the foundation of a great deal of string theory’s success iin recent years. It really seems to be a robust and contentful theory of quantum gravity (going beyond just talking about gravitons being in the spectrum, as used to be all that was said in the old days). This has all helped build people’s confidence that this theory might be telling us something about our universe – that it really has the right sorts of ingredients.

We don’t yet know if the quantum gravity that string theory contains is our quantum gravity – the one that our world uses – but it has been instructive all right. Marvellously so. It has sharpened the string theory toolbox a lot, and we understand it a lot better as a result. There have been satisfying results of this too. Recall that I’ve told you a lot about the promising applications of string theory to exploring the properties of new phases of matter in the lab? (See here, and for more recent ideas on possible new applications see here.) Well, those techniques that we use themselves grew out of work on understanding quantum black holes. In effect, when you turn it around, the quark-gluon plasma, that new form of matter that the experimenters at Brookhaven create and study, seems to be a type of fluid that is rather well modeled (in some aspects, so far) by mapping it to one of these versions of quantum gravity that we can control well with string theory, with quantum black holes and so forth all intimately involved. So even if turns out that it’s not our quantum gravity that strings describe (too early to tell so far), the physics of some non-quantum-gravity problems from our world might be able to be mapped to it as a means to describing them better.

So, you see, worrying about arcane problems can help sharpen your tools for solving others.


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11 Responses to Quantum Black Holes – Why Worry?

  1. Athena says:

    Speaking of information loss, I have a question, but it’s not related to black holes though. : ) I’m curious about when you are on retreat. In trying to focus on your work, do you respond to emails or do you refrain from doing so? (I also ask because I have a question that I would like to ask you offline, but would prefer to wait until later in case it would disturb you.)

  2. Jennifer West says:

    If you are wondering whether to buy the book, check out Sean Carroll’s review:
    and note that L.S. is always entertaining, agree or disagree with him as you will…

  3. Clifford says:

    – Hi Athena,

    On retreat or not, I must confess that my email S-matrix is not unitary. Too much noise in the system.

    There’s also definitely a huge redshift which means that time-in minus time-out can be large and sometimes divergent.

    I’ll do what I can.

    – Jennifer: Hi! Thanks.



  4. Amiya Sarkar says:

    Whether black holes hold-up information or encrypts it in their own dialect is a matter of controversy. Are ‘black holes ain’t so black’?

    I particularly enjoyed Clifford’s comment on Athena.

  5. Clifford says:


    Thanks. Right…. black holes are not so black, when you include quantum mechanics. That has been known since the 70s since Hawking’s observation about their radiating characteristic. The point of the post is to make it clear that the controversy (which some now regard as no longer a controversy, including apparently Hawking) about whether they destroy information as a result (contradicting standard quantum mechanics and all the laws of Nature as we know them) led workers in the field to a lot of useful insights into how to formulate quantum gravity, helping shape some of our understanding of string theory. Etc., etc.



  6. John says:

    Dear Clifford,

    I have a question concerning gravity and black holes. It’s probably a dumb question, but I’ll go ahead and ask it. If the gravitational field of a collapsing star (mostly) comes from its mass, gravity is mediated by the graviton (traveling no faster than the speed of light), and the collapse forms an event horizon (with the mass behind the horizon) how is it that there is a nonzero gravitational field outside the event horizon? In other words, how does gravity escape the event horizon to influence observers just outside the horizon if gravity is mediated by the graviton? I know the graviton is the particle associated with the perturbation, h_mu_nu, to the background metric, but the full nonlinear Einstein equations can be derived by (let’s use terribly simplistic vocabulary) “summing” the contributions of all orders of h_mu_nu.

    So, I’m a bit confused here. Thanks!

  7. John says:

    Dear Clifford,

    I just typed a long question but the submission wasn’t successful, so now I’ll have to type it up again. Basically, what I asked was that if gravity is mediated by gravitons which travel no faster than c, and if the gravity of a collapsing star comes (mostly) from its mass, and the collapse forms a black hole, why is there a nonzero gravitational field outside the event horizon? How does gravity “escape” the event horizon? I know that the graviton is the particle associated with the perturbation, h_mu_nu, to the background in the linearized theory, but the full Einstein equations can be obtained by (loosely speaking) summing the contributions of all orders of h_mu_nu. How do the gravitons “escape” the well?

    I’m a little confused by this. Thanks!

  8. Clifford says:


    Gravity is more than gravitons, and so a black hole is not a point source spraying gravitons in all directions, and so their escape velocity is not really at issue here. Gravitons should be thought of roughly as the propagating degrees of freedom of gravity on a given spacetime background. But the background itself is what you think of the main gravitational effect, and that includes the horizon.

    So sure you can study gravitons moving near a black hole, and you’d treat their behaviour near a horizon just as you would any other massless particle. But thinking of the black hole itself as a sort of “condensate of gravitons” is fun, but not so computationally useful – and runs you into the sorts of problems you mentioned… (especially since you’re (1) ignoring that gravity gravitates – it is non-linear – and (2) taking a quantum mechanical object like a graviton and only considering its classical properties (escape velocity and so forth), so you’re going to miss a lot of other physics too, I’d guess…).

    Staying classical for now, yes you can treat gravitons in a potential well such as a black hole’s – look in Hartle or Wald for a treatment, but it is not useful (nor correct) to think of the long range gravitational field of a black hole as arising from gravitons located inside the black hole. It does not really make sense, since for a start there is no localized source, and all of that way of thinking ignores the background itself (which is, if you want to think of it in terms of gravitons, a highly non-linearly interacting system that is much better treated as a curved spacetime background in which gravitons can propagate).



  9. JTankers says:

    Great question John and excellent answer Clifford. I enjoyed reading both.

    My concern is that Hawking Radiation is being relied on as a safety factor in the event that micro black holes are created that might not have velocities sufficient to escape Earth’s gravity. The prediction that the void of space may be filled with dark energy might help explain what would happen to virtual particles created outside the event horizon of a black hole. They might fall into black holes and cause growth. “Reverse Hawking Radiation” tends to predict that the smaller the black hole the faster the growth, which appears to match astronomical observation.

    But that is just speculation. What concerns me more are Dr. Rossler’s “Seven Reasons for Demanding an LHC Safety Conference”.


    The reasons concern me because they are highly credible but may go un-challenged before particle collisions begin.

  10. Clifford says:


    (1) “Highly credible” is perhaps in the eye of the beholder sometimes.

    (2) There have been several discussions and reports about LHC safety concerns. See several of the posts I pointed to above.



  11. Quasar9 says:

    Why worry? – Be happy!