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The shocks and perils of falling into black holes as seen by a distinguished astrophysicist

Updated: Dec 19, 2020

In today’s blog we talk to Dr. Sandip Chakrabarti, PhD from the University of Chicago, and founder, Director and Distinguished Professor at the Indian Centre for Space Physics in Kolkata, India, about his contribution to the foundations of black hole astrophysics.


David: Hi Sandip. Understanding how matter falls into black holes is fundamental to black hole astrophysics. You have shown that shocks may form in low angular momentum flows around black holes. Others have explored similar flows onto black holes and found no shock solutions. Can you describe the generality of your assumptions compared to the work of others, and the difficulty in finding solutions?


Sandip: Any low angular momentum matter with low viscosity falling rapidly will face the centrifugal barrier, slow down and when the centrifugal barrier is strong enough, will form a shock. I showed that a significant part of the parameter space allows for shock formation. In the extreme case when angular momentum is zero, the solutions are the famous Bondi flow. Later, with students and collaborators, we showed that this parameter space shrinks when viscosity is increased. In other works, we also showed that the parameter space which does not allow for steady shocks, allows for oscillatory shocks in the presence of dynamical cooling (i.e., escaping of matter through time varying outflows), or radiative cooling (when the cooling rate roughly agrees with the compressional heating rate during in-fall), and causes quasi-periodic oscillations (QPOs).


These important solutions were missed by westerners because of their approach to the problem. They chose to integrate from the outer edge of the flow, which is flawed, and leads to missing shocks where the entropy jumps. It’s technical but their algorithm cannot find this solution since the post-shock flow does not connect the black hole horizon to infinity. Every flow entering a black hole is transonic and therefore integration must be done from the sonic points. Once at a conference one of these experts said: “Sandip, sadly, we do not find shocks”. My answer was: “I have not seen a dog telling lies, but a dog does not talk in the first place. The self-similar flow, does not have any sonic points or any other length scale. It has a constant Mach number. How can you find shocks?”


Between 1997 and 1999, Abramowicz, Novikov, Lu, Yuan and others, followed my method in their MNRAS/ApJ papers and many westerners after that at places like George Mason University, and Montana State University, among others, found my shock solution. This solution is the most stable since it is the higher entropy component of the available solutions. Not all energy and angular momentum values for the flow generate shock solutions or even steady shocks. But the non-steady shocks are more common than steady shocks since it is difficult to satisfy the Rankine-Hugoniot conditions. It is a subject of study all in itself and people have to explore the details of my earlier papers dating back to the late 1980s as well as in the 1990s to understand this.


I made no assumption other than restricting the flow to low angular momentum values (less than the marginally bound value). It is the most general solution having viscosity parameter less than a critical value (I defined a notion of criticality in 1990). In fact, nature does not like Keplerian flows, but many start there because it is easier. Keplerian particle dynamics is one thing, but fluid dynamics is another. If standard Keplerian disks are so common, all black holes would be in soft states all the time with no energetic jets.


It is also important to point out that our solutions are not difficult to obtain but some effort is required. Einstein's equation is difficult to solve but that’s what you have to do if you want to get at the way nature behaves. You can’t assume the Universe to be self-similar and solve the Einstein equation under that simplification, and then give it a name that attempts to generalize what you did and give the false impression that it captures nature. The simplification might actually violate basic energy conditions. What happened in black hole accretion is actually worse. When people did not find the solutions by quick push-button methods, they simply declared mine to be wrong and advertised the easiest, self-similar solutions of my equations as the correct solutions, and started applying them to various systems. Money and prestige were enough for others to follow them blindly such that the entire subject has been misdirected for the last 25 years.


In a sense this was a blessing in disguise, since it allowed me to complete the subject quietly. Today, every observation of the spectral and timing properties can be explained with my solution using only four physical parameters. All other approximations/simplifications to my solution used by others requires at least 10 parameters. Our parameters are physical (accretion rates, shock location and shock strength), while others use non-intuitive parameters such as optical depth, temperature, flux, geometric factor, enhancement factor and more.


As mentioned above, others assumed that the advective flow connects to a Keplerian flow and integrated from the outer edge, totally ignoring the fact that my equations had two physical solutions and both are used by the flow through shock formation (pre-shock and post-shock). Others developed the impression that 'shocks' are a generic term whose locations one cannot determine exactly, so why bother. They considered shocks as something you add in in an ad hoc way. They did not recognize that shocks are actual features of the solution. Technically, they are the eigenvalues of the problem. The relaxation method for solving the equations pre-supposes shock-free solutions. I did not bring shocks into the solution. I simply allowed them to emerge by not restricting or simplifying the equations. The Europeans and the Americans don’t have any new solutions; they created new names for the solutions which gave this false impression.


Look at the Bondi flow. By construction it does not release energy and thus enters the black hole with its initial energy and without viscosity. It is hot and non-radiating. Now generalize this with some angular momentum and some viscosity. You will have a shockless Bondi-type flow which people have named ADAF/RIAF/JDAF/ZDAF etc. All these solutions are special cases of my transonic flow solution, which also produces jets self-consistently. The jets automatically disappear in soft states. Should I be surprised that a disk-jet connection would be there? It has to be there! Where else would the jets come from? Should I be surprised that supermassive black holes also accrete these flows or have quasi-periodic oscillations with scaled up time periods? No, as all these features are coming from the same solution.


The bottom line is that the difference between their solution and mine is their incorrect assumption of self-similarity (which, as I emphasized, is wrong in black hole accretion). People who accept those solutions without checking are grounding their work in faith. They should check whether their foundations are solid. They are not. You can get a sense of how unaware people are of their flawed foundations. Ask any speaker after his/her talk: "What is an ADAF?" Nobody will be able to answer that in detail. They wave their hands for a while and then quote some 1994 self-similar paper. Ask them to write an equation. Surprisingly many people do not think it is necessary to solve any equations to believe in an ADAF. But if they provide you with an equation, it will be mine.


David: Radio quasars, some neutron star X-ray binaries, and possibly even stellar mass black hole binaries, produce jets in soft states. The simple disk-jet connection of your solutions does not capture that.


Sandip: I think you are missing some crucial points. (a) You can have super-Eddington accretion which will generate enough radiation to blow away matter. This is the other extreme case from the ion-pressure dominated flows we consider. (b) If you have a strong open field embedded in a Keplerian disk you will have steady breeze all over the disk area and the total amount of matter could be considerable, but the speed may be small. (c) In supermassive black holes, the disk and the jet you see in the same time, were not really there at the same time. The tip of the jet may have left a hundred to a thousand or more years before. So how do you guarantee that the soft disk produced the same jet that you see? This mistake itself produced an additional model in the literature, eager to explain anything that we see, without properly taking care of the retarded time factor. In neutron stars, there are two boundary layers. One is the normal boundary layer or NBOL, and the other is the same as the post-shock region of a black hole accretion flow, which is the centrifugal pressure supported boundary layer or CENBOL. So the states are softer. But the anchoring of magnetic field is also strong which can smuggle out matter without waiting for thermal pressure. So those are extra issues.


David: I don’t see how this explains differences in radio morphology and especially redshift distribution of active galaxies. This is a major problem that, I think, the community has failed to properly address. What I want to understand better here, is this claim that your solutions are being hidden. How does one accomplish that? They are in the literature for all to check.


Sandip: But there are N number of ways to ensure people don’t read them. For example, I have been emphasizing since 1987 with Dave Arnett that black hole accretion will have significant nucleosynthesis. Then I see things like an ADAF paper stating that “contrary to what Sandip says, we do not see nucleosynthesis in ADAFs”. Now, this misrepresents my view. I am not talking about zero-accretion rate ADAFs as in the paper in which that statement appeared. Why do they deliberately try to confuse people? In another example, I wrote several papers since 1993 that extreme mass black hole binaries would be interesting for multi-messenger astronomy (see my Phys Rev D 1996 paper). Shortly thereafter, an ADAF paper comes out concluding “We do not see any effect on gravity wave signals in ADAFs.” But I was not talking about ADAFs. Do you know that in October 1995, at the 3rd Huntsville Symposium on 'Gamma ray Bursts', I presented a paper M-P17 with the title 'Are Gamma Ray Bursts Birth-Cry of Black Holes?' Today, this is a common phrase, but nobody cited the fact that I coined the phrase 'birth-cry of black holes', 25 years back.


David: At La Sapienza in Rome when I was a student in the mid 1990’s, there was a rumor about a professor claiming he had coined the term ‘black hole’. Wheeler is given credit even though he explicitly pointed out that he got the term from someone else. Maybe it was Ambartsumian.


Sandip: If you ask me, I would point you to the British newpapers of 1756 June, where the phrase "BLACKHOLE" of Calcutta' appeared for the first time after referring to the deaths of a few British soldiers at a Calcutta prison in a hot summer night of that year. All maps of antique British maps of Calcutta clearly write 'Site of Black Hole event'. I am told that John Wheeler, 210 years after this event first exclaimed, while looking at the description of the behaviour of matter around black holes: "Boy! Its like a black hole of Calcutta!". I consider this story to be more believable not because I am from Calcutta and all the black hole exponents, such as Kip Thorne, Rashid Sunyaev, and the like, rush to see this spot in their first Kolkata trip where the prison existed, but because it makes sense as it was a well known phrase. You can Google it to believe.


Misguiding has surpassed all boundaries for money and fame. Explain the following. While showing the black hole image of M87, why did people have to show GRMHD results? The 3GM/c^2 photon sphere has nothing to do with GRMHD. People are simply justifying the money they obtained from their funding agencies by pointing out ‘Hey look how productively I spent your money! Our results are all over the press’. This entire subject is closed group driven.


David: My understanding is that they needed some guidepost for the likelihood analysis. But it is like a club. That’s the way humans behave. By and large, people are aware of this. It’s just difficult to change things. That’s why these conversations need to take place.


Sandip: They marginalize the most important solutions and do not cite the very papers which gave them the ideas in order to uphold their cause. Fortunately, truth is not decided by vote, ballot and majority.


David: You are cited over 10,000 times.


Sandip: If I were sitting in any western school, the citations would have been ten times more. I would be sitting at the center of a network designed to echo my work.


David: Are there 1 or two references that you can single out for scholars to focus on?


Sandip: I think my 1996 Physics Reports paper would be a good starting point. The review articles written recently would have better perspectives on the whole subject.


David: Your publication record shows no signs of slowing down. You seem to be as prolific as ever.


Sandip: Thank you. When you find there is a conscious effort to suppress your work, to pressure editors out of publishing your ideas, you work harder to ensure that despite efforts to silence me, I am still heard.


David: Do large collaborations stifle creativity and originality?


Sandip: Fundamental ideas do not come through pack hunting. See how Landau, Einstein, Chandrasekhar, Zeldovich or Paczynski worked. They had one or two collaborators at a time. Very large groups consisting of theoreticians and data analysts are mostly pack hunters. They may write twenty papers enough for jobs, promotions, recognitions etc. for each member of the team and rotate their authorship to keep everybody happy, but the signal to noise ratio is very low and the results may be put in two or three well written papers by a single sharp person. Instead, their citations are twenty times higher because they spread things out by repetition. Of course, large observational projects like EHT, ALMA, LIGO etc. require a huge team effort and money and I have no reason to challenge that. What I desire is that large teams have competent theoreticians to rigorously interpret data. Instead, I see quick and shallow interpretations, emphasizing directions that are less relevant, driven by peer pressure.


David: The peak of the Bell curve may not be in a good place, but the community is made up of many others. Otherwise, we would not be having this conversation. Many are trying to make our world a better place.


Sandip: Ideally, a good scientist must be a good person also, so that human factors do not interfere in his/her scientific judgments. Scientific papers must be judged with an open mind independent of caste, race, creed or sex.


David: Individuals are biased in many ways. I don’t know that we can overcome that. But the scientific process is meant to minimize the global effect of that bias over space and time. It’s possible, of course, to undermine that process.


Sandip: As you know, in every country, the politicians choose those in the scientific advisory committees who lobby the most, not necessarily those who are the best, and these people can abuse their power. That’s how the peak of the Bell curve gets offset. The project PI then chooses people whom he knows best, and not necessarily the best people. That’s how you lose U(1) symmetry. The members of the advisory committee first pour in national and international accolades in rotation till every member of the committee gets some award or two while the committee lasts. Ultimately the country suffers. India is a very good example. I am sure this happens everywhere.


David: When I was in graduate school, a professor complained that we live in an era of awards; everyone gets one. Why not write a paper that outlines these connections between your solutions and these models?


Sandip: I am accused of writing too many. I have not only completed theoretical work and wrote about a hundred papers on this, I have written another 50 to prove their soundness with 10 different numerical codes. I showed that the solutions are both sound and stable. Then with my students, I have used the same solutions to fit data with minimum number of parameters. Will write only if it adds substantial knowledge.


David: It’s about time you wrote a review article for The Astronomy & Astrophysics Review with all the people that have contributed to this over the years.


Sandip: I am doing better. I am finishing a book. It is going to have the main results and more.


David: All features of black hole accretion flows in Nature will be reflected in future numerical simulations. Do you agree with that?


Sandip: Absolutely. As I said, with a diverse set of codes, I have myself shown them to be correct solutions. That is because I knew that my country and place of work will not inspire others to take up the job.


David: You have worked at top research institutions in the USA, Italy, and India. You clearly see major tension between science and culture. Some might argue that although the human component might dominate, stifle, perhaps silence ideas, for a generation, they will eventually emerge in the long term. Is that reasonable?


Sandip: In India we have a Sanskrit phrase "Satyameva Jayate". Truth will prevail. Ideally we should not be working to seek glory in this life as it is transient. As a natural scientist, I would be satisfied if my work added even a single sentence to the subject a hundred years from now and take humanity from a lower truth to a higher truth. When we talked about shocks and slowing down of matter, say, at 10 Schwarzschild radii (about two hundred miles away from a typical stellar mass black hole), people were saying 'Are you a fool? Why would matter slow down and almost stop next to a black hole, after coming all that way?' They forget that the very ISCO they use is the place where centrifugal force balances gravity. So gravity is not the only force that one should be worried about.


David: That’s if you look at things from the moving frame perspective.


Sandip: Yes. In India we grew up with many ideas from our childhood. I also love to learn other languages. I read many scriptures in original Sanskrit. A good one in this context is 'Tat Briddhi Pronipateno, poriproshneno Sevayah'. One's knowledge will increase only sitting at the feet (of a guru), and asking questions politely and even serving him so that he may bless you with trade secrets. Such attitude is missing from most of the referees I encountered. They think that because papers are not from some specific groups, the work must be wrong, and thus editors must reject it. I have received editor replies such as 'The referee is not convinced'. Just like that!


David: It’s not uncommon for referees to try to dismiss papers without understanding them. Especially if your ideas are not mainstream.


Sandip: I will add something more. Insisting on the originality of ideas that are not original to you in the west comes from the social (funding agency) pressure, very similar to the time when the US panicked at the launch of Sputnik and responded by creating NASA. Originally, the US did not have any accretion flow solution named after an American. Let’s look at the names: Bondi flow (British), thick disks (Polish), thin disks (Russians), generalized advective flows (Indian) contributed the most. There was a desperation to get an American solution. So something was done and a new name was coined.


David: How about the mechanism for angular momentum transport in thin disks in the early 1990’s? It was discovered by Chandrasekhar some 35 years earlier.


Sandip: The mechanism of angular momentum transport has not been understood properly. You need a considerable amount of viscosity to keep a disk angular momentum Keplerian with a significant amount of matter accretion. Numerical simulations of Hawley-Balbus also showed that the generated viscosity by magnetic instability is very tiny. You require the so-called alpha parameter around 0.1, but you get around 0.01.


Here’s an example of how society influences science. Back in 1999, it was made clear to me that I either toe the party line or get lost. After centuries of Mughal and British rule in India, our spirit was broken, and we experienced a collective feeling that our self-worth in endeavors like science could only be validated by the west. The legacy of this mentality can be seen today in the fact that we must publish in western journals to get promotions. In any case, I did not bow to this pressure, quite the contrary. I founded the Indian Centre for Space Physics (ICSP). A whole new set of youths joined me to play around with various ideas I had in astrochemistry, ionospheric science and balloon borne experiments, and of course to explore data analysis with my solutions. Out of the 45 or so PhDs I have supervised, only about 70 percent are in black hole astrophysics.


David: That seems like a lot to me.


Sandip: While I pursued theoretical works in my regular place, the ICSP was my weekend workplace for tinkering with ideas. Every weekend for the last twenty years I would come to ICSP about 20 km away to interact with a new set of students. Would you believe that when I and my wife asked in Astronomy and Astrophysics in 2000 'Can DNA molecules form in interstellar clouds?', John Bahcall from Princeton wrote 'God bless you!' in an email to me. This was the first time anyone computed the biomolecule formation rate in space, though I did not wait to have an accurate reaction cross-section. I simply used the code of nucleosynthesis around black holes and converted to chemical evolution in collapsing clouds. Since 2000, this new subject of estimating the abundance of pre-biotic molecules and their pre-cursors, has got underway. Even though I am considered a theoretician, I also designed balloon payloads and sent 115 balloon borne payloads into the stratosphere. I participated in each mission from launching to landing, chasing the parachutes.


David: How did you create ICSP?


Sandip: ICSP was created ironically only because of the wrong policies of some people. The funding agency in Delhi chose to support astrophysics only in two Institutes, one in South India, the other in North India. Hence, Kolkata would not be able to do astrophysics and space science. In one interview for a directorship of a national institute, the chairman of the committee, a well-placed scientist close to the prime minister asked me: ‘You want to bring space science to Kolkata? My God! Space science should be done in Bangalore. Kolkata is for nuclear science!’ The committee laughed, but I laughed last. In the recent Stanford University Survey, I was found to occupy the topmost position in India and to be in the top 1.41% in the world list of astrophysicists. No one from those two Institutes on which the government spent millions of dollars, appears on that list. I persevered.


Please remember that great philosophers of India contributed to fundamental ideas in mathematics and all branches of science, but you will not see anyone's name in those scriptures. In Indian philosophy, commercialization of thought is not a priority. In the West, even a petty thought is patented. In Europe, our old ideas arrived late. America is like a toddler, having little in terms of tradition. The older the civilization, the greater the tolerance and respect for the work of others. Westerners have appropriated both material as well as intellectual property time and again with impunity. How is it possible that if intellect is democratically distributed, all the international accolades go to westerners? Do you require money to be intelligent? During British rule, British scholars protected some Indian interests and thus scientists like J.C. Bose, M.N. Saha, S.N. Bose, and C.V. Raman, flourished. If they were born today, all of their intellectual properties would have been copied away or they would have migrated to the west to get some recognition. In any case, I stay away from the rat races and am not a member of the back-patting society. That helps me to concentrate on my work.


David: Thank you Professor!



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