Citation for the 2022 (the 10th) Nishina Asia Award
Dr. Suvrat Raju
Professor, International Centre for Theoretical Sciences, Tata Institute of Fundamental
Research, Bangalore, India
For his original and influential insights into the resolution of the black hole information
paradox and the principle of holography in quantum gravity.
The black hole information paradox has been a longstanding problem in quantum gravity since
the discovery of Hawking radiation and black hole evaporation in the 1970s. Although some of
the important questions on a quantum black hole have been clarified in the light of string
theory, the information paradox which jeopardizes quantum unitarity remains a critical issue.
Dr. Raju made an original and essential contribution to the subject by pointing out thecharacteristic feature of the quantum Hilbert space of a black hole with Hawking radiation,
which can describe the states inside and outside of the black hole horizon at the same time.
His proposal has significantly influenced the recent research on the black hole information
paradox, such as on the island proposal, and has drawn attention to the importance of the nonsplit property of the black hole Hilbert space.
Detailed descriptionUnderstanding of quantum gravity is one of the most fundamental and difficult problems in
high-energy physics. While string theory has provided an elegant solution to the well-known
difficulty of uncontrollable ultraviolet divergence encountered in quantizing gravity, quantum
gravity comprises yet much deeper conundrums of a serious nature.
This started long ago from the recognition that surprisingly general relativity exhibits thestructure of thermodynamics and in particular, as argued by J. Bekenstein, a black hole is athermodynamic object with (in general) finite temperature and carries the entropy
proportional to the area of its horizon (not to the volume of a black hole as one wouldexpect). Moreover, S. Hawking found that a black hole, when the quantum effect is taken
into account in a semi-classical approximation, can emit quanta the spectrum of which isexactly that of radiation emitted by a black body.
These findings implied that quantum black holes must have truly enigmatic and troublesome
properties: (i) There appears to be a violation of unitarity in the time evolution from an
initial pure state that collapses to a black hole and ends up in a mixed state which emits
thermal radiation. (ii) This would in particular imply that the information thrown into a
black hole will be eternally lost, in acute contradiction with the principle of quantum1mechanics. (iii) As the entropy measures the logarithm of the number of possible
microscopic quantum states under a specified macroscopic condition, how can it beproportional to the area while the black hole is a three-dimensional object?
The last question prompted such people as G. ’t Hooft and L. Susskind to put forward a boldidea that all the information is stored on the horizon just as in holography. Further, J.
Maldacena made this idea much more precise with concrete examples and proposed the“bulk/boundary duality”, where a gravitational theory in the bulk of spacetime can beequivalent to a suitable non-gravitational theory defined on the boundary. If this conjecture
is fully substantiated, the serious problems listed above may be solved provided that theboundary theory respects unitarity. Namely, the corresponding bulk theory with theemergent gravitational degrees of freedom will also be unitary, and if treated exactly theapparent problem of information loss by a black hole should disappear.
However, upon close examination, it appears that a naive application of this promising idea
gives rise to another serious problem. S. Mathur and J. Polchinski and his collaborators
independently pointed out that if one assumes the unitarity of the boundary theory, a region
with highly excited quanta, dubbed “firewall”, would form in the neighborhood of the blackhole horizon, and the celebrated equivalence principle of general relativity would
breakdown. Then spacetime would end at the horizon and the interior of a black hole
together with the information stored within cannot be described by the boundary theory.
In this context, Dr. Raju, together with K. Papadodimas, presented a novel ingenious idea
to resolve this impasse. They used the thermo-field double description of field theory at
finite temperature, which uses a highly entangled system of two copies of the original
system under consideration. In the limit of large N for the typical boundary theory with SU(N)gauge symmetry, they demonstrated that one can construct an effective operator, in termsof a non-trivial combination of operators of the first copy describing the system outside thehorizon, which when acted on the thermo-field double state can create a state in the second
system in a characteristic state-dependent way. They further argued that this second copy
can be interpreted as the Hilbert space describing the interior of a black hole. Thus, their idea
materialized the notion of so-called “black hole complementarity” and asserts that theinformation inside a black hole can already be fully encoded in the system outside.
Although their work aroused a substantial amount of interest, the due influence of its
essential importance has started to become apparent rather recently. In the past couple ofyears, substantial progress has been made by the recognition that the failure of unitarity
inferred from the original computation of Hawking is due to the omission of the important
contribution from a semi-classical saddle, called an “island”, roughly inside the black holehorizon. Although this has been established in the path-integral formulation, it appears
quite significant that the degrees of freedom of the island are made up of complicated2state-dependent combinations of Hawking radiation modes. This of course is highly
reminiscent of Papadodimas-Raju construction.
The deep influence of the work by Dr. Raju, including those after the Papadonimas- Raju
paper, can also be seen in the recent significant work by S. Leutheusser and Liu tries tocreate the emergent “time” in the interior of a black hole. It was duly emphasized that ofintrinsic importance is the so-called “non-split property” of the Hilbert space, namely thatthe Hilbert space of a black hole cannot be factored as the tensor product Hinside ⊗ Houtside.
Their work in turn prompted Witten and his collaborators to study a change in theproperties of the von Neumann algebra of observables when the 1/N gravitationalcorrections from the large N limit are taken into account in the holographic framework. It
should be recognized that these promising developments can be said to have their roots inthe work of Dr. Raju and K. Papadodimas.
We must mention also that more recently Dr. Raju has produced further influential
contributions to the holographic principle in quantum gravity and the black hole paradox byanalyzing these problems for more realistic situations. One such work is the study ofholography in four-dimensional flat spacetime, performed with his collaborators. They
showed that, at least for massless excitations, all the information present at future null
infinity is also present near its past boundary. This original proposal for flat space
holography is a significant step toward understanding quantum gravity in a realistic
spacetime.
Another recent work of Dr. Raju and his collaborators is a critical assessment of the “islandproposal” for the recovery of unitarity mentioned above. They pointed out that such a
proposal is applicable only in the presence of a non-gravitational bath and massivegravitons. Also as a more robust approach to understanding the unitarity of black holeevaporation, it was emphasized that the aforementioned non-split property of the Hilbert
space and the associated structure of the von Neumann algebra of the observables must be
duly taken into account in the presence of gravity.
With his outstanding scientific achievements described above and his strong leadershipestablished in India, it is clear that Dr. Raju is irrefutably qualified for receiving theNishina Asia Award.
References
1. Alok Laddha, Siddharth Prabhu, Suvrat Raju, and Pushkar Shrivastava, “The Holographic
Nature of Null Infinity”, SciPost Phys. 10 (2021), 041.
2. K. Papadodimas and S. Raju, “Remarks on the necessity and implications of state3

dependence in the black hole interior”, Phys. Rev. D., 93, 084049 (2016).
3. K. Papadodimas and S. Raju, “An Infalling Observer in AdS/CFT”, JHEP 10 (2013) 212.

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