Investigating the paradoxes of black holes

The year 2015 marks the centenary of Einstein’s general relativity, the remarkable theory he came up with in 1915 to merge relativity and gravity. Over the last 100 years, general relativity has been tested with many experiments, and it has passed every test convincingly. Nevertheless, one area remains unproven: black holes, those mysterious objects that have event horizons -- surfaces through which things can fall in, but from inside which nothing, not even light, can escape. Black holes are a direct prediction of Einstein's theory of general relativity, but they are so strange and bizarre that Einstein himself doubted their existence. Yet, astronomers have found countless objects in the universe that they believe to be black holes. But believing is not the same as proving, and it is very difficult -- nearly impossible -- to prove that a candidate object is truly a black hole. Dr. Ramesh Narayan, Thomas Dudley Cabot Professor of the Natural Sciences at Harvard University, uses astrophysical and computational techniques to study this and other fundamental questions surrounding the paradox of black holes. Just as any basic science research can potentially find practical applications down the road, the understanding that Dr. Narayan brings to black hole astrophysics may be foundational to how we perceive the world in the future.

Dr. Narayan is a theoretical astrophysicist who has worked in many areas of research within astronomy, astrophysics, and cosmology. He has worked on topics as diverse as astronomical image processing, gamma-ray bursts, gravitational lensing, pulsars, quasars, and X-ray binaries. For the last two decades, however, he has focused almost exclusively on black holes, with much of his work done in collaboration with Dr. Jeffrey E. McClintock, a Senior Astrophysicist at the Smithsonian Astrophysical Observatory. As a leader in the astronomical community, Dr. Narayan regularly produces high-quality astrophysical results on black holes. He is also highly respected in the physics community, especially by scientists working in gravitational physics and general relativity.

Dr. Narayan’s research on black holes follows two tracks:

  • Developing Models: Dr. Narayan seeks to develop models to explain the wealth of observational data that is available on astronomical black holes. In particular, he studies the process of gas accretion on black holes which causes huge amounts of radiation to be emitted by the accreting gas. This radiation is seen in many bands: radio, infrared, optical, ultraviolet, X-rays, and gamma-rays. Dr. Narayan models the accretion process using both theoretical methods and numerical computations with high performance national supercomputers, and he attempts to understand how the observed radiation is produced.
  • Understanding Black Holes: Dr. Narayan seeks to use his theories, coupled with observations, to answer some of the deepest questions on black holes.
    • Are the objects that scientists study in astronomy really black holes? In order for an object to qualify as a black hole, it must possess something called an “Event Horizon,” a surface of no return, from which not even a light ray can emerge. But proving that an object has an event horizon is extremely difficult since the horizon produces no signals. Dr. Narayan looks for evidence of event horizons by taking various indirect approaches.
    • Even though no signals can come out of a black hole, is it still possible for energy to flow out? In 1969, Dr. Roger Penrose at Oxford University argued that a spinning black hole is a potential source of energy which is available to the outside world. Various astrophysicists have since proposed mechanisms whereby this energy might produce observed phenomena like “Relativistic Jets.” Dr. Narayan studies the viability of these ideas using computer simulations. He also searches observational data for direct evidence of energy extraction from spinning black holes.
    • Is it true that black holes are the simplest objects in classical physics? According to general relativity, each black hole is completely described with just a few numbers: mass, spin, and, possibly, electric charge. Dr. Narayan looks for tests of this remarkable theoretical prediction.

Dr. Narayan’s work in the first track above, “Developing Models,” lies within mainstream astrophysics and is viewed favorably by astronomy funding agencies. However, his work in the second track, “Understanding Black Holes,” is viewed by these same agencies as physics research and therefore not appropriate for their support. Meanwhile, physics funding agencies think the work is astronomy-related and not suitable for physics funding either! Private funding is therefore invaluable to continue this kind of interdisciplinary black hole research, which is central to our understanding of gravity and the universe.

Bio

Dr. Ramesh Narayan has always enjoyed solving puzzles. He found out early that he has an aptitude for mathematics and science, so a research career was natural. He feels extremely lucky to be able to spend his life thinking about the biggest mysteries in the universe.

Over the course of his career, Dr. Narayan has experienced two transitions. At Bangalore, India, he finished his formal education in physics, receiving a Ph.D. in the field of condensed matter physics. Soon after though, he became interested in astrophysics, a field he knew nothing about when he first started. He describes this transition as “a great thing that happened to [him],” when, after completing his Ph.D., he joined the Raman Research Institute, Bangalore, and started talking with astrophysics colleagues there and attending astrophysics talks. He was so inspired by all the new ideas he was exposed to that he decided to pursue astrophysics as a career. He studied astrophysics in India for a year or two, and then experienced a second major turning point when he went to Caltech, as a postdoctoral fellow. He was struck immediately by the extraordinary drive and intensity of the US research culture, the deep desire of the science community here literally to shoot for the stars and never to settle for second-best. This was far different from the more relaxed atmosphere he had experienced back in India. In that sense, he went through not only a cultural transition between two different nations, but also a cultural transition between two different science environments. He took to his new environment like a duck to water, and has never looked back! He truly appreciates the kind of research he gets to do. Getting up every morning to go to work feels really special to him.

Outside of research, Dr. Narayan is very interested in listening to music -- both Indian classical and Western classical music.

For more information, visit https://www.cfa.harvard.edu/~narayan/CV.pdf

In the News

See That Black Hole?

This press report was triggered by a paper published by Dr. Narayan and colleagues in 1997, where they presented evidence that stellar-mass black hole candidates in X-ray binaries do have Event Horizons. This was a landmark study. Prior to this work, nobody had bothered to ask whether the many objects that astronomers had discovered, and that they had identified as black holes, were truly black holes. One of the defining characteristics of a black hole is the existence of an Event Horizon, a one-way membrane through which stuff can fall in, but from inside which nothing, not even light, can escape. The cited study showed for the first time that at least one class of objects that was thought to be black holes truly are black holes.

Spinning Black Hole Pushes the Limit

This press report was on the discovery by Dr. Narayan and colleagues in 2006 that the stellar-mass black hole in the famous X-ray binary GRS 1915+105 is spinning extremely rapidly.

Scientists Find Black Hole's 'Point of No Return'

The story behind this 2006 press report began in 2002, when Dr. Narayan and Dr. Heyl first showed that there is a way to distinguish black holes, which by definition have Event Horizons, from other objects that do not, by looking for so-called "Type 1 X-ray bursts" (which are astrophysical explosions, not dissimilar to hydrogen bombs on Earth). In 2006, Dr. Narayan and colleagues carefully analyzed X-ray data on a number of systems and verified that objects previously suspected to be black holes do have Event Horizons and therefore are indeed black holes.

Reaching for the stars

This article appeared in 2006 in the national newspaper "The Hindu" in India soon after Dr. Narayan was elected as a Fellow of the Royal Society (London).

Black Hole Jets

This report was on a 2012 study in which Dr. Narayan and Dr. McClintock presented preliminary evidence that there is a correlation between the measured spins of black holes and the power of relativistic jets ejected by these systems. It had been speculated for decades that such a connection (which has deep connections to black hole theory) should exist, but until this work there was no confirming evidence.

Distinguishing Black Holes from Naked Singularities

This report was on a 2014 paper from Dr. Narayan’s group in which they showed how one might distinguish a black hole from a so-called "naked singularity". The latter is an object that possesses a singularity, just like a black hole, but does not have an Event Horizon. Naked singularities are allowed, in principle, by relativity theory, though many scientists believe that they do not actually exist in the universe.

Awards

Presidential Young Investigator Award, National Science Foundation, 1989

This Award was the vehicle by which the NSF identified and rewarded talented scientists early in their faculty careers. The award guaranteed stable grant support for five years.

George Darwin Lecturer, Royal Astronomical Society (London), 2002

This Lecture “is given annually by a distinguished and eloquent speaker on a suitable topic in astronomy.” It is one of the main awards given by the Royal Astronomical Society. Past Lecturers include a Nobel Laureate and numerous giants in the field of astronomy.

Elected Fellow of the Royal Society (London), 2006

Founded in 1660, the Royal Society is the oldest active academic association in the world. Its elected Fellows include the greatest scientists of the British Empire, e.g., Isaac Newton served as President of the Society in the early 1700s. In modern times, the greatest honor that a scientist from the British Commonwealth (of which India is a member) can aspire to is being elected a Fellow of the Royal Society.

Elected Member of the U.S. National Academy of Sciences, 2013

Founded by President Abraham Lincoln in 1863, the National Academy of Sciences is the US version of the Royal Society. Being elected to the National Academy is among the highest honors in science available to a US citizen. Dr. Narayan was elected soon after he took up US citizenship in 2010.

Robert M. Walker Distinguished Lecturer, Washington University, St. Louis, 2014

This lecture series is organized annually by the McDonnell Center for the Space Sciences in Washington University, and is named for its first Director, the late Robert M. Walker. The Lecturer is invited to give a technical Colloquium at the McDonnell Center, followed by a non-technical Public Talk.