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I’ve never physically been in the U.S. on the day of a presidential election, but at least I got to be there virtually yesterday at the Black Hole Initiative at Harvard!

BHI colloquium recording

Here are some talks I gave at Stanford and at Perimeter in the past couple of weeks on the “Holography of Information from semiclassical gravity.” The talks cover somewhat similar ground, but the emphasis is slightly different because the Perimeter talk was aimed at a non-string theory audience, whereas the Stanford talk was for a string-theory audience.

Perimeter: http://pirsa.org/displayFlash.php?id=20090001

Stanford: https://stanford.zoom.us/rec/play/UewZOtxyYd1v0s-xOErSUZehU9oFB54uEzFYFeWdA_Rf5mhwo-DIeGn5dSHbccuguLFvs1WDOjJ5OqMy.XiAGe6h-M6DELocW

P.S: And I cannot help adding that the time-difference between North America and India is as bad as possible. Yet, all I had to do was start at 8:30 am one morning, and end at 10:30 pm one evening. Its probably a sign that I am getting older, but I can’t honestly say that I miss the jet-lag or that we should all go back to it when this ends.

Tomorrow (4 Sept), I’m going to speak at a webinar titled “Lets eradicate nuclear weapons before they eradicate us.” This is jointly organized by Rotary clubs in Cochin, Karachi and Michigan. I think there are a number of very interesting talks. Please attend if you can and spread the word. One needs to register in advance here for the Zoom link: https://docs.google.com/forms/d/e/1FAIpQLSc5use3IADdls9g34hHtTjcbl6tdSjHkrlbSJAuBvaN9AXm7g/viewform

I’m going to describe some of our recent work in the QASTM seminar hosted by Sayantan tomorrow. I’ll try and keep the discussion as simple as possible, so please come and ask questions. (update: recorded video here https://www.youtube.com/watch?v=wYgCFKlwJ3E)

QASTM seminar recording

The recorded video for the Chai and Why discussion today on “Asking questions about mathematical models” is available here: https://youtu.be/sWTU0KsJj3U

Thanks very much to Chai and Why? and to March For Science Mumbai for organizing the event, and for giving me the opportunity to discuss these issues.

This event marked National Scientific Temper Day which is observed on 20 August to commemorate Dr. Narendra Dabholkar.

There were many questions after the talk, but if there are more questions or comments, please send them over.

I’m going to lead a virtual discussion tomorrow (Sunday, 16 Aug, 11 AM) as part of the TIFR Chai and Why series titled: “Asking questions about mathematical models”. I promise to keep the discussion very simple. So if you have been reading (or writing) about these models, it may be of interest. Here is a more detailed abstract. Please come if you can and spread the word to others who might be interested.

The past few months have seen the emergence of numerous mathematical models, accompanied by claims from the models’ originators to have uncovered key insights about the COVID-19 pandemic. But in several cases, models with obvious flaws have received wide publicity. I will discuss, among other examples, early model-based claims that a “49 day lockdown” was necessary in India, and later claims that the Indian lockdown “averted thousands of deaths”. These are textbook examples of poor science, but they also show how governments and others with an agenda can use scientific-sounding claims to mislead people. I will conclude by arguing that mathematical models can be useful tools, but they cannot be treated as black boxes: we must carefully examine the assumptions and methodology of each individual model before we take its conclusions seriously.

Asking Questions about Mathematical Models — Chai and Why

With Chandramouli and Olga: “A physical protocol for observers near the boundary to obtain bulk information in quantum gravity”.

https://arxiv.org/abs/2008.01740

Think of observers who live near the boundary of global AdS and can only explore the spacetime on a single Cauchy slice at large values of the radial coordinate. We put the theory in a low-energy state, and ask these observers to glean as much information they can about the state, without ever directly entering the bulk. How much information can they get?

In a local quantum field theory, the answer is obviously “nothing”. The observers are spatially separated from the region in the middle of AdS, and so operators there commute with all observables accessible to the observers.

It turns out that when gravity is turned on, the surprising answer is “everything”! We develop a protocol, which involves only simple manipulations of the state, and simple measurements from the near-boundary region, and allows the observers to completely determine the state of the bulk.

This is a perturbative check of the idea that signatures of holography are already implicit in semiclassical gravity. It is also closely related to one of the lessons that we learn from a study of black-hole evaporation: information about the interior is also available in a scrambled form in the exterior. This has been discussed for several years — most recently in all the work on “islands” — and so what we have here is a controlled example, where a very similar effect can be studied precisely, and directly in a Lorentzian setting.

Another implication is for quantum information measures. If the von Neumann entropy is understood physically as a measure of lack-of-information, then our result suggests that the von Neumann entropy of an “annular region” (Fig 1b) between r = r0 and r = \infty is zero in a theory of gravity. This is very different from the local QFT result, which would be proportional to r0^(d-1). (Remember, this region is not the entanglement wedge of any subregion on the boundary.)

There has been a lot of recent discussion of the entanglement entropy of regions in theories of gravity, but I think adequate thought isn’t being given to what this quantity means. The idea seems to be do a calculation where gravity switches off exactly at some point, and then just claim that the same result should hold in the presence of dynamical gravity because the corrections must be “small” when gravity is weak. But the fine-grained entropy is not a perturbative quantity, and so the idea that “weak gravity = no gravity” is just wrong in some cases, as this example shows!

If you have questions, comments or, especially, criticism, please email.

I will be part of some panel discussions tomorrow (6 August).

1. The first is organized by the All India Peace and Solidarity Organization (MP). The topic is “75वें हिरोशिमा दिवस पर वेब-चर्चा” and I will speak about the “Iran nuclear conflict and U.S. imperialism”. Achin Vanaik will speak about the situation in South Asia. This event is from 3 pm – 5 pm at https://meet.google.com/rgo-bytk-key. This meeting will largely be in Hindi.

2. The second is organized by the India Peace Centre. Achin will be a panelist here as well, and we will also be joined by Vaishali Patil. The title is “No Nukes: No More Hiroshima”. This event is from 5 pm – 7 pm at https://zoom.us/j/7122556952?pwd=QjYvbFZVT3U3d3ZCOHRac1FlQUpSQT09 I think the language will be a mix of English and Hindi. I am planning to speak about “Challenges and opportunities for the peace movement.” [update: recorded video at: https://www.facebook.com/indiapeacecentre.nagpur/videos/3181889788559884/]

3. The third is organized by India March for Science (Karnataka). Here the discussion is about issues of nuclear energy and its relevance (or lack thereof?) in India. The other panelist is George Joseph, and the event will be moderated by Anil Jagalur. This event is from 7 pm – 8:30 pm. You can join at https://www.facebook.com/watch/?v=1007791319677011 The language will be English.

Please join if you can and spread the word to others who might be interested.