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A simple quantum test for smooth horizons, with Kyriakos and Pushkal.

https://arxiv.org/abs/1910.02992

We use the short distance behaviour of the two-point function to set out some very general necessary conditions for a quantum state of the fields to be smooth at any null surface. In the case of black holes with both an inner and an outer horizon, where we apply our test, these conditions require that the degrees of freedom near both horizons be occupied at just the right temperature and also be entangled with partners just behind the horizons.

There is an interesting story here. If you talked to me a couple of months ago, I would have explained to you (as I did explain to several people!) that this meant that quantum mechanical effects always imposed strong cosmic censorship regardless of all the recent classical discussions on the subject. Note how tricky it is for all the conditions above to be satisfied. The modes near the outer horizon have to be automatically “heated” by the geometry to attain the temperature of the inner horizon when they reach there. Moreover, in attempting to cross the inner horizon, we have the problem of the monogamy of entanglement: since the modes between the inner and outer horizon are entangled with partners outside the outer horizon they cannot be entangled with fresh modes behind the inner horizon. So, surely, one would be unable to extend the field into that region?

This intuition is right in higher dimensions. But for the BTZ black hole, this naive conclusion is just wrong! In that case (and, as far as we know, only in that case) the geometry does magically heat the field by just the right amount as they reach the inner horizon; and the existing degrees of freedom can then be cleverly juggled to extend the field behind the inner horizon.

Does this mean that strong cosmic censorship is violated for the BTZ black hole as Dias, Reall and Santos argued a few months back? What would this mean for AdS/CFT? We think that quantum effects could still have a few tricks up their sleeve at very late times, but I now think this is a very interesting question.

Many of us have been disturbed by what is happening in Kashmir, and have felt helpless at our inability to do anything about it.

Some of us felt that it would be very useful to have a statement from members of the academic and scientific community registering our opposition to the government’s policies. Our objective was to (a) at least marginally modify the public narrative by emphasizing that not everyone in India agrees with the Indian government’s policies (b) extend solidarity to our own Kashmiri colleagues and friends, who are obviously going through very difficult circumstances.

We were able to gather more than 500 signatures in about 24 hours.

This is the statement that we issued https://docs.google.com/document/d/1aGJ8jmPJcsTnrsx9FCqQQEKuhn4o2pUeEW0gUg0JQLk/edit?usp=sharing

The statement has had some broader impact and today, several media outlets have reported on it. Here are a few reports. The BBC newshour also recorded a brief story on the issue (update: link below)

https://www.thehindu.com/news/national/detaining-opposition-leaders-in-jk-deeply-undemocratic/article29478999.ece

https://www.telegraphindia.com/india/500-scientists-and-academics-condemn-communication-block-in-kashmir/cid/1706556

https://scroll.in/latest/938047/jammu-and-kashmir-over-500-academics-scientists-issue-statement-calling-for-end-of-curfew

https://thewire.in/rights/appalled-by-restrictions-504-academics-scientists-express-concern-over-kashmir

https://www.thenewsminute.com/article/restore-communication-kashmir-indian-academics-write-letter-indian-govt-109269

BBC Newshour: https://www.bbc.co.uk/sounds/play/w172wq4whqzlbmv (from 14:00)

This is an op-ed I wrote for the Hindu on the U.S.-Iran conflict. https://www.thehindu.com/opinion/op-ed/talking-sanctions-endangering-peace/article28287389.ece?homepage=true

My views on what is happening in Iran (for the blog and next issue of Aspects of India’s Economy). This piece is a little long — 6500 words — but I’ll try and write a shorter summary soon. (Update: see Hindu op-ed above.) https://rupeindia.wordpress.com/2019/06/27/imperial-overreach-in-iran/

New essay asking “Is Holography Implicit in Canonical Gravity?” The precise conjecture is that if two solutions of the Wheeler-deWitt equation coincide near the boundary of anti-de Sitter space at one point of time, then they must be equal. (i.e. they cannot differ even in the bulk.)

https://arxiv.org/abs/1903.11073

Of course, such a claim cannot hold in the classical theory—where it is easy to construct metrics and matter configurations that differ in the bulk but coincide asymptotically—but the essay lays out some evidence that the quantum theory does have this property.

This property might not sound surprising to string theorists but in talking to people who work in canonical gravity, I often get the impression that they think of holography as something mysterious that emerges from the ultraviolet properties of string theory. The suggestion here is that this is not the case, and holography is implicit even in the low-energy canonical formulation of quantum gravity.

If you have comments, please send them to me, especially if you find the argument provocative/wrong/obviously right!

(This post comes with the usual apologies to my non-physics friends if Facebook puts this on your timeline.)

After the chess puzzle in my previous post, there was a lot of discussion about chess programs, but no one seems to have attempted the puzzle itself :-) Its a difficult puzzle, but the solution is also really beautiful and it helps to explain why engines don’t find it. So I thought I would devote another post to it. If you want to try the puzzle yourself, please stop reading here.

To recap, the initial position is the one on the left. With white to play, the correct continuation is 1.Qc8 Kg8 2. Bc7!

This is the move that engines cannot find, since it involves a immediate huge loss of material and the payback is only because of a positional novelty that manifests itself several moves down the line. So presumably, most algorithms just prune this move. After this move is played, engines can find the continuation, which is: 2. …Qxc8 3.gxf7+ Kh8 4.Be5 Qc5 5.Bb2 Nc7 6.Ba1

This brings us to the position in the second figure below. Now Black is not, strictly speaking, in zugzwang. But Black will be in zugzwang after 4 moves! Play continues 6. … a4 7. Bb2 a3 8.Ba1 a2 9. Bb2 a1=Q 10. Bxa1 Qe5+ 11. Bxe5 Nd5+ 12. Ke6+ Nf6 13. Bxf6#

At my son’s children’s chess tournaments, I sometimes meet a gentleman who rails against the recent advances in artificial intelligence. “Machines lie”, he roars. “Even in chess, they have no positional understanding.” Usually, I just listen quietly but one day I decided to floor him with a gotcha! “So, show me a position where Stockfish makes a demonstrable mistake. Not just one in which your positional judgment disagrees with Stockfish.” And to my surprise, he immediately produced the first position below.

This is actually a mate in 13, but Stockfish misses the winning line even after hours of analysis. It is true that specialized mate-finding programs will find the solution, but can you find the mate yourself? Feel free to post solutions in the comments. I’ll post the right answer a little later :-)

A few weeks later, I had another conversation with him. This time, he showed me the second position below (with the multiple white bishops). Stockfish tells me that the evaluation is +20.7. Of course, its obvious to a child that the correct evaluation is 0.

I’m not sure what to make of these examples. Do they show something deep about the lack of versatility of machines? Or are they just curiousities with no broader implications? I’m curious to hear what people think. [Fine print: I suppose the interesting question is whether machine-learning programs do better. I tried both positions with Lc0. There are so many possible parameters and possible weight files that I cannot make a general statement. But with the weight file I used, Lc0 couldn’t find the first mate. But I’m glad to say that it correctly evaluated the second position, although if I understand its algorithm correctly, this was almost guaranteed.]

Following the last post, I got a number of additional questions about the paper presenting a toy model for the information paradox. This is a talk I gave at a workshop in Princeton some time ago. The organizers at the Institute for Advanced Study did a really nice job (as usual!) in recording the audio clearly and combining it with the slides. If someone would just like to understand the basic idea, and some parts of the calculation in the paper seem a little technical, then this may be a useful resource. (This is a specialized seminar, so this post comes with advance apologies to my non-physics friends if Facebook shows this on your news feed!) https://www.youtube.com/watch?v=1QDQp7JbV8E