Blog

With Mv Ramana in The Hindu today on the eleventh anniversary of the Fukushima disaster.

https://www.thehindu.com/opinion/op-ed/shutdown-this-misguided-energy-policy/article65215060.ece

As many of you know, some time back Elsevier, Wiley and the American Chemical Society filed a suit against Sci-Hub in the Delhi High Court. A group of scientists, of which I am a part, filed an intervention petition in the case. We requested the court to examine the public interest involved in the issue. (See here for more details: https://indianexpress.com/article/cities/delhi/delhi-hc-to-hear-academicians-in-case-against-open-access-sites-7136229/)

This article provides some perspective and an update on the case. I tried to mention the Scipost model and briefly point out how — even though the commercial publication model makes no sense and journals add little to no value to a paper — the model persists because a subset of the academic community benefits from the current system.

https://restofworld.org/2022/sci-hub-trial-india/

Another talk on the split property and the information paradox at the Institute for Advanced Study workshop on Quantum Information and Spacetime. A little shorter than previous talks, so perhaps easier to follow.

https://www.youtube.com/watch?v=tQYsb5QF2nU

A colloquium I gave at the Perimeter Institute a few days ago. The idea was to target a broad theoretical physics audience and so much of it is relatively non-technical. The abstract below is hopefully self explanatory.

https://pirsa.org/21110026

In an ordinary quantum field theory, the “split property” implies that the state of the system can be specified independently on a bounded subregion of a Cauchy slice and its complement. This property does not hold for theories of gravity, where observables near the boundary of the Cauchy slice uniquely fix the state on the entire slice. The original formulation of the information paradox explicitly assumed the split property and we follow this assumption to isolate the precise error in Hawking’s argument. A similar assumption also underpins the monogamy paradox of Mathur and AMPS. Finally the same assumption is used to support the common idea that the entanglement entropy of the region outside a black hole should follow a Page curve. It is for this reason that computations of the Page curve have been performed only in nonstandard theories of gravity, which include a non-gravitational bath and massive gravitons. The fine-grained entropy at future null infinity does not obey a Page curve for an evaporating black hole in standard theories of gravity but we discuss possibilities for coarse graining that might lead to a Page curve in such cases.

The CQG journal is bringing out a special issue on the recent developments on the Page curve and the editors invited me to contribute something. So I took the excuse to write a “perspective essay” on the topic that appears on the arXiv today: https://arxiv.org/abs/2110.05470

The essay focuses on an incorrect assumption that plagues many discussions of black-hole information. The assumption, loosely speaking, is that the Hilbert space in gravity should factorize the way it does in QFT and so one can specify the state of a system independently inside and outside a bounded region.

Of course, it is not hard to see why this is appealing. The assumption holds in nongravitational field theories. It even has a nice formal name: “the split property”. And, in general relativity, one can find solutions that differ inside a bounded region and coincide outside. But this assumption fails when both quantum mechanical and gravitational effects are important. Instead theories of quantum gravity obey a “principle of holography of information” … once all observables outside a bounded region have been specified, the state inside the bounded region is completely fixed.

One immediate objection is: “we live in a world where gravity is presumably quantized, but we make local operations all the time.” Of course, even in gravity one can construct approximately local operators if one neglects effects suppressed by (typical-energy scale/Planck scale). This ratio is negligible in everyday life, and so our experience is well described by such approximately local operators. But if one starts asking fine-grained questions about the entanglement entropy in a gravitational theory then the fact that these operators are only approximately local and not exactly local becomes important.

This issue is definitely important for questions of black-hole information since the typical energy scale is the Hawking temperature and (Hawking temperature/planck scale) is just the inverse of the black-hole entropy. Therefore, if one tries to take the Planck scale to infinity to get rid of these unusual gravitational effects this throws out the baby with the bath water: the entropy diverges and there is no meaningful question left to ask about black hole information.

For this reason, it is impossible to ignore the unusual localization of quantum information in quantum gravity for questions of black-hole information. If one insists on taking intuition from local QFT/classical GR seriously, this often leads to a paradox.

People sometimes ask: “where is the mistake in Hawking’s argument for information loss?” The mistake is that Hawking assumed factorization of the Hilbert space up to the constraints of the no-hair theorem. The assumption is quite explicit in Hawking’s paper, and I even included a quote from the specific paragraph in the original paper to point out precisely where the error was made.

The same incorrect assumption of factorization/locality led to the monogamy paradox of Mathur, which was later revived by AMPS.

And the same incorrect assumption underlies the idea that the entropy of black-hole radiation should follow a Page curve.

In fact, it is because this assumption fails in standard gravity that, in the recent literature, the Page curve has only been computed in theories with a nongravitational bath and massive gravitons that do not obey the Gauss law. These computations are nice, but these models are very different from standard theories of gravity and it is not clear what these computations teach us about realistic black holes.

So the bottom line of the essay is: “the reason for studying the black-hole information paradox is that it teaches us about new physical effects in gravity. As such, one of the lessons that the paradox teaches us is that gravity localizes information unusually. This is a striking effect that persists in the low-energy theory.

A computation of the Page curve is not necessary to resolve the information paradox and, historically, this idea was based on a flawed physical analysis. Although it is possible to modify the system of an evaporating black hole in a standard theory of gravity so as to force a Page curve upon it, this tends to obscure the interesting physics that we learn from the paradox.”

Our statement, as part of the Coalition for Nuclear Disarmament and Peace (CNDP), on AUKUS and the Quad dialogue. The full text is available here:

https://docs.google.com/document/d/1Br3K8xp2fviF-eh64RKAjZZI_gnNWM_u/edit#

If you are around today at 7 pm, please come for this discussion hosted by BSS Karnataka. We will discuss the public discourse around mathematical models that have been used for the pandemic in India. The talk will be livestreamed at:

https://www.facebook.com/bss.karnataka

(unfortunately, there is no public Zoom link).

Here is the abstract.

The past year has seen the emergence of numerous mathematical models for the COVID-19 pandemic whose results have been publicized as scientific predictions for the future. In several cases, models with obvious flaws have received wide publicity, and may have influenced policy. I will discuss, among other examples, early model-based claims that a “49 day lockdown” was necessary in India, and subsequent claims that the Indian lockdown “averted thousands of deaths”. I will also discuss the incorrect conclusion, supported by modelling, that India would not see a second wave at all. The program of attempting to make detailed predictions about the pandemic using simple mathematical equations provides a textbook example of poor science. But it also shows how governments and others with an agenda can use scientific-sounding claims to mislead people.

In a paper today, with Chandramouli Chowdhury, Victor Godet and Olga Papadoulaki, we establish a link between two famous but seemingly different approaches to quantum gravity. On one hand, we have the Wheeler-DeWitt (WDW) equation that was first written down more than 50 years ago. On the other hand, we have now known for almost 25 years that gravitational theories in anti-de Sitter space (AdS) are holographic, and this has been an extraordinarily fruitful idea.

In today’s paper, we show that a version of holography can be derived from the WDW equation. More precisely we show, within perturbation theory, that two wavefunctionals that solve the WDW equation in AdS, and coincide at the boundary of AdS for an infinitesimal time interval, must coincide everywhere in the bulk.

Contrast this with ordinary quantum field theories (QFTs) where it is obvious that one can prepare states that coincide outside a bounded region but differ inside. This is why, in ordinary QFTs, information is stored locally. The result today shows that the WDW equation prevents the existence of such “split states” in quantum gravity and that information in gravitational wavefunctionals can be always be found near the boundary of a Cauchy slice.

Our analysis is perturbative but several separate arguments that we have given over the last couple of years, for the “principle of holography of information” in gravity, suggest that the final result should extend beyond perturbation theory.

Read more at: https://arxiv.org/pdf/2107.14802.pdf