Book & Author
Professor Leonard Susskind: The Black Hole War — My Battle with Stephen Hawking to Make the World Safe for Quantum Mechanics
By Dr Ahmed S. Khan
Chicago, IL
“I, a universe of atoms, an atom in the universe.”
― Richard P. Feynman, Theoretical Physicist
As the 21st century progresses, new discoveries at quantum and cosmic scales are changing the understanding of science and technology. In the world of physics, determinism is losing ground to quantum mechanics. In attempts to comprehend new discoveries physicists are engaged in thought experiments, in the process coming up with new ideas, which in fact pose more questions than provide answers in clarifying the new thinking. In this expanding cosmos, the key questions have become these: What is the size of the cosmos? What is knowledge? What is the scope of human knowledge?
In the early twentieth century, brilliant individual work of physicists like Bohr, Dirac, Heisenberg, and Schrodinger led to a revolution in the form of quantum mechanics, which turned traditional notions of classical mechanics on their heads. The domain of quantum physics is full of fuzziness and uncertainty. While in classical mechanics objects exist in a specific place at a specific time, in quantum mechanics objects exist in a cloud of probability; that is, they have a certain chance of being at a certain location. Quantum mechanics also describes entities at the smallest scales of the energy levels of atoms and subatomic particles, where matter and energy have a dual wave-particle nature. The complex nature of quantum mechanics led Einstein to state that “God does not play dice.” For his part, Richard Feynman added: “I think I can safely say that no one understands quantum mechanics.”
Einstein, via his theory of relativity, proposed the existence of black holes and gravitational waves. Black holes, he theorized, are curvatures in the space-time domain, where gravity is so powerful that the space-time fabric, consisting of three dimensions of space plus the fourth dimension of time, is bent so far that it becomes a hole. Gravitational waves are vibrations in the space-time domain generated by the acceleration of mass, and the gravity of a black hole is so strong that even light cannot escape it. Furthermore, near a black hole the slowing of time becomes so extreme that time virtually stops. Here, the lines of a T.S. Eliot poem ring true:
Time present and time past
Are both perhaps present in time future
And time future contained in time past.
And, Rumi, the great Sufi poet, expounding on the mystery of time has observed: “Fling me across the fabric of time and the seas of space. Make me nothing and from nothing - everything.”
The collision of black holes can also generate a powerful and far reaching gravitational wave. In 2015, the Laser Interferometer Gravitational-Wave Observatory (LIGO) detected a cosmic chirp, a gravitational wave, generated by the collision of two black holes around 1.3 billion years ago. This discovery confirmed the existence of gravitational waves, and led to the 2017 Nobel Prize in physics.
Addressing such issues, The Black Hole War: My Battle With Stephen Hawking to Make the World Safe for Quantum Mechanics narrates the story of an intellectual combat: how Leonard Susskind, a professor of theoretical physics at Stanford, and Dutch Nobel Laureate Gerard 't Hooft disproved Hawking’s claim about bits of information vanishing in a black hole. Susskind, who is renowned as a particle theorist and an early pioneer of string theory, has also been called “the bad boy of physics” and “the man who proved Stephen Hawking wrong.” The disputing parties also provide contrasting backgrounds — Hawking was the son of Oxford graduates and Susskind is the son of a plumber from a working-class family in New York.
The author sets the stage for a black hole war by educating the reader in a simple manner about the relevant concepts of quantum mechanics, general relativity, and a further array of concepts, from string theory to the holographic principle. Susskind states: “Today it is widely believed that gravity and quantum mechanics will play equally important roles in determining the laws of elementary particles. But the size of nature’s basic building blocks is so inconceivably small that no one should be surprised if a radical rewiring will be needed to understand them. The new wiring, whatever it is, will be called quantum gravity, but even without knowing its detailed form, we can safely say that the new paradigm will involve very unfamiliar concepts of time and space. The objective reality of points of space and instants of time is on its way out, going the way of simultaneity, determinism, and the dodo. Quantum gravity describes a much more subjective reality than we ever imagined.”
Discussing the modern challenges of physics and Hawking’s paradox, the author observes: “Theoretical physicists are struggling to gain a foothold in a strange land. As in the past, thought experiments have brought to light paradoxes and conflicts between fundamental principles. This book is about an intellectual battle over a single thought experiment. In 1976 Stephen Hawking imagined throwing a bit of information—a book, a computer, even an elementary particle—into a black hole. Black holes, Hawking believed, were the ultimate traps, and the bit of information would be irretrievably lost to the outside world. This apparently innocent observation was hardly as innocent as it sounds; it threatened to undermine and topple the entire edifice of modern physics. Something was terribly out of whack; the most basic law of nature—the conservation of information—was seriously at risk. To those who paid attention, either Hawking was wrong or the three-hundred-year old center of physics wasn’t holding.”
At the basis of Hawking’s paradox is the question of how much information, measured in the 1s and 0s of the binary code, can fit into a black hole. The amount of information depends on the horizon of the black hole rather than its volume. (An event horizon is a shell of "points of no return,” that is, the boundary where the gravitational pull of a massive object becomes so strong as to make escape impossible). Susskind explains that every time a bit falls into a black hole, its opening expands by one square Planck length (an area billions and billions of times smaller than a proton). As in a hologram, the three dimensions are contained within two.
While Stephen Hawking is considered one of the great physicists of the modern era due to his work in astronomy, his theories were also controversial. As Susskind observes, “Physicists incessantly question whether Hawking belongs among the greatest physicists of all time and where he ranks in the hierarchy. In response to those who doubt Hawking’s greatness, I will only suggest that they go back and read his 1975 paper ‘Particle Production by Black Holes’.… Yet no matter how great he may be, on at least one occasion, Stephen Hawking lost track of his bits, and that’s what started the Black Hole War.”
The author invigorates his narrative with a wealth of anecdotes. In the 1970s, many physicists wondered whether black holes could somehow decay into other particles. Unlike many physicists, Susskind had the pleasure of discussing this issue with the prominent physicist Richard Feynman. Susskind recalls his 1994 visit to the University of Cambridge, to inform Hawking of the error of his insights, but Hawking was not available due to his illness, and Susskind ended up in a local pub discussing black holes with strangers.
Discussing the debate on the present state of physics, the author states: “Predictivity or determinism had to go, and failed classical rules of logic had to be replaced by quantum logic. Uncertainty and complementarity were expressed in terms of abstract, infinite, dimensional Hilbert spaces, mathematical commutation relations, and other bizarre inventions of mind….The new views of the physical world that evolved over a little more than a decade involve a new kind of relativity and a new kind of quantum complementarity.” The latter term refers to a concept of quantum mechanics, which states that objects have certain pairs of complementary properties that cannot all be observed or measured simultaneously.
Susskind summarizes the state and status of physics in this nutshell: “Confusion and disorientation reign; cause and effect break down; certainty evaporates; all the old rules fail. That’s what happens when the dominant paradigm breaks down. But then new patterns emerge. They make no sense at first; quantify and codify them in mathematics, even new laws of logic, if necessary. Replace the old wiring with new and become familiar with it. Familiarity breeds contempt, or at least acceptance. Very likely, we are still confused beginners with very wrong mental pictures, and ultimate reality remains far beyond our grasp. The old cartographer’s term terra incognita comes to mind. The more we discover, the less we seem to know. That’s physics in a nutshell.”
So, what was the outcome of the black hole war? It was a Susskind victory. In 2004, Hawking finally admitted defeat, accepting that information is not lost in a black hole. But has the black hole war finally ended? Perhaps not, as many quizzical minds continue to wonder if there is any objective evidence from astrophysical observations that black holes follow the principles of quantum mechanics? If not, what permits both Hawking and Susskind to start from the fundamental hypothesis that the physics of black holes can be described in a quantum mechanics framework dominated by locality and unitarity?
Unitarity and locality serve as the foundation of quantum field theory. Unitarity represents a concept that the sum of all probabilities describing every potential outcome of any quantum event is always equal to one. Locality means that an object is influenced by its immediate surroundings. But there is a major problem: when gravity is added to quantum theory, for some cases, locality and unitarity break down and do not work.
“Humility,” the last chapter of the book, starts with a Hawking quote: “We are just an advanced breed of monkeys on a minor planet of a very average star. But we can understand the Universe. That makes us something very special.” Yet, what is the status of our understanding of the universe? Do we adequately understand the great complexity and magnitude of the scheme of things in the universe? Based on present knowledge, the cosmos consists of about 4% "visible or normal" matter, 23% dark (invisible) matter in an unknown form, and as much as 73% of the even more mysterious dark energy. What is this dark energy? We simply don't know yet, and it reveals the limits of human knowledge!
What is the quantized size of human knowledge in comparison with the size of the known cosmos? The estimated total of human knowledge accumulated during the past 5,000 years, quantized in digital form (trillions of bits) can be transmitted anywhere in the world over state of the art optical fiber networks at speeds of Tera (trillion) bits per second in less than half an hour! Modern day physicists, before making paradoxical claims, ought to ponder the words of Confucius: “To know what you know, and know what you do not know – that is knowledge.”
The Black Hole War is an interesting read for technical and non-technical quizzical minds to enhance their understanding of the present state of knowledge in the domain of physics.
(Dr Ahmed S. Khan - dr.a.s.khan@ieee.org - is a Fulbright Specialist Scholar - 2017-2022).
