I was an undergraduate astronomy major studying the physics of the “Schwarzschild Black Holes” when I decided to study the laws of the quantum field theory in a black hole’s curved spacetime to round up my education. I really like that it is trying to reconcile quantum theory with gravity which is one of the greatest scientific challenges of our time. These have always been my two favorite subjects. They each have their own regimes of applicability and have issues when we try to push them too far. According to eminent mathematical physicist Roger Penrose, these issues arise in spacetime singularities such as the big bang, where curvatures seemed to go infinity and crushed at a point of infinite density.
Spacetime singularities also occur in black holes and they have become a fascinating research area not only because they are a very important constituent of the universe but they also provide a challenging arena in which to probe singularities and explore the challenges posed by the marriage of general relativity and quantum mechanics—the Holy Grail of the Modern Physics. Black holes crop up all over physics and they figured prominently not only in Christopher Nolan’s Interstellar, one of the most scientifically-accurate sci-fi films of all time, but in the era from the mid-1960s to the mid-1970s, during the Golden Age of theoretical black-hole research. In the vanguard of this theoretical black-hole research was a young physicist of growing renown who has been in the forefront not only in the pursuit of unified physics but also one of the most dedicated boosters in the search of the elusive gravitational waves from astrophysical sources such as black holes. His name: Kip Thorne, the Feynman Professor of Theoretical Physics, Emeritus at the California Institute of Technology who is more popularly known in public as the scientific advisor and executive producer of Interstellar. Thorne shared the 2017 Nobel Prize in Physics last week with fellow Caltech physicist Barry Barish and Rainer Weiss of MIT for their contributions to the LIGO detector and for their leading roles in the discovery of gravitational waves, tiny fluctuations or undulations in space-time that are detectable only when the universe’s densest objects – black holes or neutron stars – smash together cataclysmically.
And here’s one of the reasons why LIGO’s successful detection of gravitational waves creates such a buzz and excitement in the scientific community: the possibility of reconciling quantum mechanics and general relativity. LIGO’s detection not only presages the future in gravitational-wave detectors but it also caps off a century of speculations, doubts and hard work. Future gravitational-wave observatories with even greater sensitivity may open a new window on the universe and could potentially test ideas about quantum gravity and, maybe, detect primordial gravitational signals from the Big Bang itself, the most eagerly sought-after gravitational waves (not all gravitational waves are alike.) These predictions may open new vistas for science and would tell us very much not only how the universe started but will also unveil the laws of quantum gravity in all their intimate details.
The direct detection of the gravitational waves brought to bear the amazing insights and theoretical predictions of a multitude of scientific luminaries from the front ranks of twentieth-century physics such as Richard Feynman, Stephen Hawking, Herman Bondi, Roger Penrose, John Wheeler, Kip Thorne, among others, but let me begin with an earlier era – with Albert Einstein, who gave us the profound theory of general relativity. Only a few months after he published his general theory of relativity, the German astrophysicist Karl Schwarzschild discovered the mathematical solution to Einstein’s field equations—which describes the spacetime geometry of a single, nonrotating, electrically neutral lump of matter—the same year Einstein predicted gravitational waves. Subsequent physicists realized that his formula seemed to describe an object that has “cut itself off” from the rest of the universe, an object to which John Wheeler would give the name black hole (he also coined the term “wormhole”). The solution contained the first general relativistic description of a black hole, and nonrotating, electrically neutral black holes are now known as Schwarzschild black holes. (Meanwhile, world renowned cosmologist Stephen Hawking found a way to apply sophisticated mathematical equations of the general theory of relativity and quantum mechanics to describe charged, rotating black holes.)
Black holes and gravitational waves are just some of Einstein’s incredible predictions decades ago are not only dominating the headlines today but they are also winning Nobel Prizes for other scientists. Case in point: The 1993 Nobel Prize, for example, went to physicists Russell A. Hulse and Joseph H. Taylor for their 1974 discovery of a unique binary system, thereby confirming the existence of gravitational waves indirectly by analyzing the motion of double neutrons. Also, the 2001 Nobel Prize went to three physicists who confirmed the existence of Bose-Einstein condensates, a new state of matter existing near absolute zero that Einstein predicted in 1924 with the collaboration of Indian physicist Satyendra Nath Bose. In 2011, the Nobel Prize went to three physicists who discovered the accelerating expansion the universe—possibly because of a small positive cosmological constant in Einstein’s equation or could involve some mysterious “dark energy,” an invisible energy field pervading all of space.
Einstein’s Quest for Unification
Albert Einstein was one of the greatest scientists of all time, and certainly the greatest physicist of the twentieth century and one of the greatest of all time, a towering figure who ranks alongside Galileo, Newton, Maxwell and Darwin. Einstein was the definitive genius of our age. More than a decade ago, he had topped the Time magazine list of the 100 most influential people of the 20th century and was voted the “Person of the Century” beating the likes of Mahatma Gandhi, Franklin D. Roosevelt, Muhammad Ali, the Beatles, and Bill Gates. Not only did his theories of relativity reveal the esoterica and abstruse bone-jarring concepts of space and time but his unparalleled scientific insights also triggered a cascade of scientific breakthroughs that gave us smartphones, solar power, nuclear-powered plants, computer, laptops, lasers, GPS, fiberoptic communications, CT, MRI and PET scans, Blu-ray players, etc.
And here’s another proof of his unparalleled genius from the not-so-distant past: In 1905, a year christened as Einstein’s Annus Mirabilis or Year of Miracles, Einstein published four groundbreaking Nobel-worthy papers as he introduced the idea that light comes in discrete bundles of energy, and used it to understand photoelectric effect; he explained that the zigzag “Brownian” motion of small particles suspended in in fluids is a consequence of collision with fluid molecules, providing evidence for the atomic nature of matter; he invented the Special Theory of Relativity; and he inferred from it the relation between mass and energy in the now-famous equation E = mc^2.
It is not surprising that, after his successful scientific triumphs in relativity at the turn of the twentieth century, he would turn his focus and attention to the search for a complete “theory of everything.” When Einstein’s unified field theory bursts upon the world, some of the most brilliant minds of our century have sought to decipher its complexities and mysteries decades later. The idea has been to devise a single equation that would explain all the behavior of all the known particles and forces of physics. It bothered the iconoclastic physicist that the two fundamental forces guiding the behavior of the universe—gravity and electromagnetism—appear to play by different rules. He wanted to demonstrate that all types of matter and energy are governed by the same logic. His quest for unified physics isolated him from the mainstream of physics, which, understandably, was far more excited about delving into the newly emerging framework of quantum mechanics. In what I consider as one of his most intellectual witticisms, Einstein wrote an oft-cited letter to a friend, “I want to know how God created this world. I want to know his thoughts. The rest are details.”
Of course, Albert Einstein never fulfilled his dream of unifying the laws of nature—his exotic quest to reconcile his general relativity with Maxwell’s law of electromagnetism because the deck was stacked against him. His search was thwarted on two fronts. First, there were only two known forces of nature known to mankind at the time: gravity and electromagnetism; there are now four fundamental forces in nature—gravity, electromagnetism, and two purely quantum forces that operate inside the nuclei of atoms, called “weak” and “strong” nuclear forces. Moreover, Einstein abhorred quantum mechanics although he was instrumental to its birth and was seeking a purely classical theory—an approach we know to be incomplete or flawed. He did not know that the most important unification is with quantum mechanics.
The Holy Grail of Modern Physics
Over the course of time, the greatest advances in the history of physics have been marked by unifications: events or discovery of theories that gave explanation of disparate phenomena that had previously seemed unrelated. In the quest for a unified, all-encompassing, and coherent description of all nature – a “theory of everything” – physicists have developed a detailed set of mathematical models of the fundamental forces in the universe and unraveled the mysteries linking perplexing phenomena. And from its earliest days as a science, physics has sought for unity in nature.
In the 17th century, Isaac Newton launched a scientific revolution by unifying celestial and terrestrial physics with the law of universal gravitation, showing that the force that makes objects fall is also the force that keeps the Moon and planets in their orbits—what we now call gravity. In the 19th century, James Clerk Maxwell not only worked out the inconsistencies on the equations formulated by Michael Faraday, Henry Cavendish, Charles-Augustin de Coulomb, and Andre-Marie Ampere but he also constructed a consistent set of equations that link two seemingly distinct phenomena—electricity and magnetism—and also predict the existence of electromagnetic waves. In the 20th century, physicists Steven Weinberg, Abdus Salam and Sheldon Glashow had unified electromagnetic and weak nuclear forces to build the so-called “electroweak unification”—the prediction of which have since been verified in particle accelerators and had earned them the coveted Nobel Prize in Physics. And today, all known elementary particles plug neatly into a mathematical structure called the Standard Model of Particle Physics.
However, the paradigm-shifting search for unification was triggered in the early 20th century when an iconoclastic young physicist transformed our understanding of the universe. Before Einstein, the greatest unifiers of physics were Newton and Maxwell. With his special and general theories of relativity, Albert Einstein overturned the solid certainty that was Newton’s clockwork and replaced it with a picture that defied common sense. Newtonian mechanics was replaced by relativistic mechanics, and older ideas of absolute time were abandoned. Mass and energy were shown to be interchangeable. His work brought us the famous equation E = mc^2, defined light as the cosmic speed limit, wove space and time into a single fabric called spacetime, showed how apples and planets fall along the fabric’s curves, and ushered in the idea that the universe began in a hot, dense fireball we now call Big Bang. These insights also gave rise to one of the most difficult concepts in modern physics: black hole.
In retrospect, Einstein was simply ahead of his time. More than half a century later, his dream of a unified theory has become the holy grail of modern physics and the task of unification has fallen to subsequent generations of physicists. One scientist who has been in the thick of the quest of unified physics is Stephen Hawking. In his seminal work A Brief History of Time, he argued that such a unified description of the forces is within the bounds of possibility. As he pointed out in his closing paragraph, deducing the correct theory of everything would enable us to “know the mind of God.” Most of Hawking’s research is based on the Einstein’s second great theory—the general theory of relativity—and he is most famed for his mathematical proof that, as he puts it, “black holes ain’t so black.”
General Relativity + Quantum Mechanics = Troubled Marriage in Short Distances
One of the greatest scientific challenges today is the unification of gravity and mechanics. Einstein’s baby or prima donna, his general theory of relativity, described the laws of the cosmos and the birth of the universe, the orbits of the heavenly bodies and the fall of Newton’s apples. Quantum mechanics described atoms and molecules, subatomic particles such as protons, electrons, quarks, etc. Yet in the places where both theories should apply—where both gravity and quantum effects are so strong, such as the spacetime singularities in the big bang and black holes—they also seem incompatible. Despite decades of work by physicists, including a dozen or so Nobel laureates, a quantum theory of gravity remains elusive…
(To be continued here)
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