Vol.32 (Dec) 2022 | Article no.40-2 2022
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 regarding quantum black holes have been clarified from the perspective 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 a characteristic 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 recent research on the black hole information paradox, such as the island proposal, and has drawn attention to the importance of the non-split property of the black hole Hilbert space.
Understanding 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 still poses many deep questions that have remained unanswered.
The complexity of quantum gravity perhaps started with the recognition that, surprisingly, general relativity exhibits the structure of thermodynamics. As argued by J. Bekenstein, a black hole is a thermodynamic object with (in general) finite temperature and carries an entropy proportional to the area of its horizon (not to the volume of a black hole as one would expect). Moreover, S. Hawking found that a black hole, when the quantum effect is understood in a semi-classical approximation, can emit quanta, the spectrum of which is exactly 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, which ends up in a mixed state that emits thermal radiation. (ii) This would imply that the information thrown into a black hole will be eternally lost, in acute contradiction with the principles of quantum mechanics. (iii) As the entropy measures the logarithm of the number of possible microscopic quantum states under a specified macroscopic condition, how can it be proportional to the area when the black hole is a three-dimensional object?
The last question prompted G. ’t Hooft and L. Susskind, among others, to put forward a bold idea 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 be equivalent 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 the boundary theory respects unitarity. Namely, the corresponding bulk theory with the emergent gravitational degrees of freedom will also be unitary and, if treated exactly, the apparent 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 a “firewall”, would form in the neighborhood of the black hole horizon, and the celebrated equivalence principle of general relativity would break down. Spacetime would then end at the horizon and the interior of a black hole together with the information stored within would not be described by the boundary theory.
It is in this context that Dr. Raju, together with K. Papadodimas, presented a novel idea that would resolve this impasse. They made use of the thermo-field double description of field theory at finite temperature, which uses a highly entangled system consisting of two copies of the original system. 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 terms of a non-trivial combination of operators of the first copy describing the system outside the horizon, 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 the information inside a black hole can already be fully encoded in the system outside.
Although their work aroused a substantial amount of interest, its essential importance started to become apparent only rather recently. In the past couple of years, 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 an important contribution from a semi-classical saddle, called an “island,” roughly inside the black hole horizon. Although this has been established in the path-integral formulation, it appears quite significant that the degrees of freedom of the island is made up of complicated state-dependent combinations of Hawking radiation modes. This, of course, is highly reminiscent of the 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, which tries to create the emergent “time” in the interior of a black hole. It was duly emphasized that of intrinsic importance is the so-called non-split property of the Hilbert space, namely that the 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 the properties of the von Neumann algebra of observables when the 1/N gravitational corrections 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 in the 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 by analyzing these problems for more realistic situations. One such work is the study of holography 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 “island proposal” 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 massive gravitons. In addition, as a more robust approach to understanding the unitarity of black hole evaporation, 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.
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G. M. prepare the manuscript. The author(s) read approved the final manuscript.
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Bobomurat Ahmedov wondered why we have an even number. Yokoyama answered that 20 is the possible largest number and there are other members with a voting right in the council, such as the ex officio.
Michael Francis Ian Vega II was trying to understand the background of the proposal and the broader rationale of the change. He was curious about whether there was a sentiment that the current number of council members represents a severe underrepresentation among member societies. He stated that the increase in the number of AAPPS member societies does not necessarily imply underrepresentation. Yokoyama responded that the reason for the proposal is to appropriately represent member societies whose number has been increasing. Currently, some member societies are sending more than one council member.
Tou Teck Yong expressed his concern that it would become financially more difficult for a small society to host a council meeting and invite all of the council members, as the Malaysian Institute of Physics did on the occasion of APPC14. He imagines that a small society with a limited budget would never be able to host council meetings unless the council members pay themselves to travel to the venue. Although he appreciates that APPCs are successfully organized mostly by financially stronger, active, and supportive member societies, he is not sure whether the AAPPS council assures to help smaller societies. Tou pointed out that we might have a situation where only four countries or regions occupy the whole council.
Tou stated that he does not think it practical to have more council members than currently. Tou wondered why five more council members are needed for only one meeting, where a vote occurs only every 3 years. Yokoyama clarified that this situation describes an OGM, where official representatives gather once every 3 years, whereas the council itself has many more meetings.
Vandana Nanal focused on two points. First, due to travel restrictions and other difficulties for council members, hybrid meetings ensure wider representation. Second, regarding the size of the council, the representation of member societies in the AAPPS council appears to be not very balanced. The reason why member societies want to be a part of the council is that they want to be more interactive regarding the policies discussed in the meetings with major societies in the Asia Pacific region. She was unsure if this could be achieved simply by increasing the number of council members. As mentioned earlier (See agenda item 4), at least through the scheme of associate membership, we will be able to keep the whole council balanced without expanding its size. She suggested having associate members from all member societies. Yokoyama agreed that better representation could be realized by continuing hybrid or online meetings with the inclusion of associate council members.
Kirrily Rule pointed out that increasing the number of council members to 20 would not automatically give better representation to the smaller societies. She stated that the whole point of increasing the number is to give fair representation; consequently, the constitution would need to be changed even further to state that at least one member from each member society should be represented at council meetings. Tou confirmed that presently, not all the member societies that pay their respective membership fee send a council member.
Nanal commented that there was a discussion in the Indian Physics Association when they paid the membership fee. They want to be more associated with the Asia Pacific regional activities, and they are a member. However, the specific benefits of AAPPS membership are very difficult to explain. This will go back to the point that there is no fair representation in the council. Yokoyama welcomed the restoration of India to AAPPS with payment of all remaining membership fees. He explained that we did not receive any nomination from the Indian Physics Association although we sent the call and reminders. Yokoyama stated that active participation in the election is highly appreciated. Nanal responded that it would be now possible with an online format. She stated that there might have been some communication problems, but the community people will feel more involved if the Indian Physics Association would play a major role in AAPPS activities.
Wang stated that he has trouble understanding the reason for increasing the number of council members. He always had the impression that the current scheme of the council is working quite well and he finds no rationale to increase the number of members. He added that we might fail to have 20 candidates, considering that only 18 candidates were nominated for 15 seats this time. Yokoyama responded that on the other hand, all those who are interested in joining the council will be accepted if we increase the number to 20. Wang agreed that that is also true.
Rawat understands the point that the council should include representatives from as many societies as possible. He stated that we might probably need to qualify our tradition that both societies, which have paid or unpaid fees, can send a council member. He expressed his thoughts on the reason why a few major societies represent a high total percentage of the council. Those societies are actively participating and showing their presentations, by paying not only the membership fees but also through other contributions, as will be explained in the financial report. Namely, these five leading societies contribute strongly to the functioning of AAPPS, and their large representation in the council could be considered as real recognition of their respective societies’ contributions to AAPPS. As the Constitution was formulated 30 years ago, we need to think about its updates. One of the past drawbacks of the operation of AAPPS was that council meetings were held only on-site and participation from smaller member societies was not easy. A positive change is the possibility for online meetings, where we could be more inclusive. He summarizes that we will have to look into not only the number of council members but also several factors in the Constitution itself.
Yokoyama stated that, as pointed out by Prof. Tou, a council consisting of members of only four societies is in principle possible but would never happen because an equal voting right is offered to each member society at the OGM. This is indeed observed in today’s election result (see item 4), where some member societies had sent more than one nomination and they were not fully elected. The current scheme is working well and the council concluded that just increasing the numbers should work.
Yokoyama has been serving as the president of AAPPS since January 2020. The start of the term was just after the outbreak of Covid-19. Therefore, AAPPS activities were mostly restricted to online ones. So far, council meetings were held nine times (from the 43rd to the 50th Council Meetings, including the 44th: Parts I and II). Previously, the council meetings were held on-site and only once a year as it cost too much for council members to directly meet more frequently. Thanks to the development of the online platform, many more council meetings were held this term than previously.
At the beginning of Yokoyama’s term, there were 18 member societies. There are now 20 member societies, as Uzbekistan and Pakistan have now joined AAPPS. The Physics Society of Iran also expressed interest to join and will deliver a presentation at a future council meeting for approval of membership. In comparison, IUPAP has 60 member societies and AAPPS has a significant percentage of the physicists in the world. Yokoyama showed the list of officers, current council members, and their member societies and explained that five societies are providing extra support in addition to the membership fees.
The council organized the Asia Pacific Physical Societies’ Forum in November 2021, at which 14 member societies and the Council of Uzbekistan Physicists delivered presentations. India, Nepal, and the Philippines have also rejoined the activities of AAPPS.
The headquarters of AAPPS is hosted by and located at APCTP in Pohang, Korea. Yokoyama acknowledged support from APCTP to AAPPS and stated that this meeting is attended by Yunkyu Bang, the president of APCTP, and Jae-Hyung Jeon, the executive director of APCTP. The memorandum of understanding between APCTP and AAPPS was first signed in 2011 and has been automatically renewed every 5 years.
The main activities of AAPPS consist of publication of the AAPPS Bulletin, organization of APPCs and Asia Europe Physics Summit (ASEPS) meetings, selection and awarding of the CN Yang Award, and division activities.
The next APPC (APPC16) is planned to take place in Beijing in 2025. Yokoyama discussed with the European Physical Society (EPS) to resume a face-to-face meeting of ASEPS, which had been suspended due to the Covid-19 pandemic. The next ASEPS will be hosted in Europe.
In 2017, the previous council determined the rules on sponsorship, co-sponsorship, and endorsement of AAPPS activities, under the former President Long. Part of the rule will soon be promoted to the Code of Conduct, whose draft was approved at the 50th Council Meeting. AAPPS has provided support to the conferences held in Malaysia in 2021, Nepal in 2021, and Thailand in 2022. Some speakers were sent to these conferences from the AAPPS council and member societies. The main scope of the conference in Thailand was applied physics, and two plenary speakers were recommended by JSAP, which has been providing extra financial support to AAPPS activities. Yokoyama has so far attended 10 meetings as the president.
The CN Yang Awards are currently disbursed in partnership between AAPPS and APCTP every year. Yokoyama acknowledged APCTP for supporting the prize money for three awardees every year and holding ceremonies in the years between the APPCs. It has been pointed out that in order to attract a greater audience to the award ceremony, it may be better to have the ceremony at an annual meeting of the member society to which the awardee belongs. Yokoyama requested the official representatives to consider the feasibility of this change. He also stated that he welcomes opinions on how we should incorporate gender, geographical, and subject balances.
Sparked by Yokoyama’s participation at a meeting of the Physical Society located in Taipei, the AAPPS-XPS Award was proposed as a joint award for talented young researchers, where X represents the initial letter of the member society’s name. The joint award of the Physical Society located in Taipei launched as a pilot program and the first-year prizes were already provided to three researchers from the Physical Society located in Taipei. The winners received gold medal plates from AAPPS, which were donated by the AAPPS president, and monetary prizes from the Physical Society located in Taipei. At the 49th Council Meeting, the guidelines for establishing such an award were formulated and approved (the related material has been sent to the presidents of member societies by e-mail). JPS is also planning to establish a joint award in a different format. Yokoyama stated that these new joint awards will enhance the visibility of AAPPS, similarly to the IUPAP’s Early Career Scientist Prize (formally the Young Scientist Prize), which has spread the visibility of IUPAP among young researchers.
There were three divisions when Yokoyama’s term began. The Division of Plasma Physics (DPP) was the first division created under AAPPS and has its own review journal. The Division of Astrophysics, Cosmology and Gravitation (DACG) organizes annual conferences and schools. In the Division of Nuclear Physics (DNP), the Asian Nuclear Physics Association (ANPhA) acts as the division and conducts many activities. The Division of Condensed Matter Physics (DCMP) is the newest division, established in January 2021, which covers all the fields of condensed matter physics. AAPPS has a similar division structure as the 12 divisions of the EPS, but has not reached a stationary state yet. The foundation of the Division of Particles and Fields is underway and that of the Division of Computational Physics is under discussion.
Women-in-Physics activities are important in AAPPS. On Tuesday August 23, 2022, Women-in-Physics sessions will be held at APPC15. In some countries in Asia, such as Malaysia and Myanmar, the majority of physicists are women. Some of the members of DNP are organizing education programs to support physicists in Myanmar, who have difficulties under military rule.
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