We look back to the last century and look forward to the next in a journey with quantum physics.
BY DEBDUTTA PAUL
A fundamental theory of nature is thriving a century after its birth. The theory of quantum mechanics has not only spread its wings, but it has now given rise to many directions of enquiry.
Physicists grappled with the questions that emerged at the end of the twentieth century, from how to explain the radiation we receive from the Sun to the stability of the atom. Physicists have made progress over the century, gaining fascinating insights about nature and ending up overthrowing deeply held ideas. But, many questions remain. ICTS-TIFR organised a special program, ‘A Hundred Years of Quantum Mechanics’ to celebrate 2025 as the International Year of Quantum Science and Technology. Speakers included luminaries like Michael Berry, Ashoke Sen, Peter Sarnak, Nathaniel Craig, Urbasi Sinha, G. Baskaran, Subir Sachdev, Mandar Deshmukh, Umesh Waghmare, Ashwin Nayak, Pedram Roushan, Arindam Ghosh, Juan Maldacena, Philip Kim, Sankar Das Sarma, Senthil Todadri, Vedika Khemani, Peter Zoller, R Vijayaraghavan, David Di Vincenzo, Immanuel Bloch, Barbara Terhal, Rajaram Nityananda, Roger Blandford, and Nikita Nekrasov. It gave us an overview of the directions of investigation that steer research in the field and an idea of what research in the next century would look like.
According to classical electromagnetic theory, atoms would radiate energy and collapse within themselves. Thus, they wouldn’t be stable. Moreover, scientists could not explain the photoelectric effect using the wave theory of light, the theory of electromagnetic waves.
Max Planck’s work on blackbody radiation, Albert Einstein’s work on the photoelectric effect, and Niels Bohr’s work on the hydrogen atom’s spectrum opened up a new line of argument. It significantly deviated from the earlier ideas in physics. According to it, the world consists of matter and radiation that come in discrete packets. These quanta’s interactions explained the long-standing experimental observations.
But, fundamental challenges arose. How about the fact that experiments after experiments showed that light was indeed electromagnetic waves? A concrete theory was in order.
The early success of quantum theory resulted in two decades of hectic mathematical developments and active philosophical debates involving physicists like Max Planck, Albert Einstein, Niels Bohr, Satyendranath Bose, Marie Curie, Werner Heisenberg, Erwin Schrödinger, Paul Dirac, Max Born, Louis de Broglie, and many eventual Nobel laureates who contributed massively to solving the jigsaw puzzles, finally culminating in a concrete mathematical theory of nature — today called quantum mechanics — in 1925. The experiments were explained.
But, not many liked the solutions. Some physicists, like Einstein and Schrödinger, raised questions about the ‘interpretation’ of the new theory. Stalwarts clashed, and debates ensued. A breakthrough put quantum mechanics at odds with local hidden variable theories that proposed that quantum mechanics was incomplete and explaining the probabilities in quantum mechanics needed more variables, till then hidden by nature in a more concrete theory: John Stewart Bells’s proposal in 1964, called the ‘Bell inequalities’. According to this inequality, such theories, if they existed, would not be local. That is, information about quantum states can travel across regions at speeds faster than the speed of light. Although such theories are yet to be validated, scientists have validated the Bell inequalities, and quantum mechanics come out with flying colours each time.
Many ideas are still pursued as to what quantum mechanics means.
However, as the discussions at ICTS reminded us, there hasn’t been a single experimental deviation from quantum mechanics in the last century. Not only has the theory had tremendous success explaining various natural phenomena, from atoms to the inside of stars, but it has also given rise to much of the technology that drives modern developments.
The successful coming together of quantum mechanics with special relativity gave rise to quantum field theories, explaining high energy physics that culminated in the discovery of the Higgs particle at CERN, lending credibility to the standard model of particle physics. Condensed matter physics, the study of many particles that each behave quantum mechanically, and statistical physics, the study of many particles in general, are rooted in quantum mechanics. Importantly, the physics of many interacting quantum objects shows that the collective behaviour of many is not the same as the added behaviour of each. As a consequence, how the world around us emerges is not obvious.
Moreover, even though quantum mechanics can now explain three forces of nature — the electromagnetic force, the strong nuclear force, and the weak nuclear force — a crucial barrier remains. It cannot fully explain gravitation, which, according to the general theory of relativity, is the curvature of spacetime. Quantum mechanics operates on flat spacetime. Quantum gravity is a hard problem, but theoretical physicists are chipping away at its various aspects. The ICTS program gathered luminaries in string theory, a quantum theory of gravity, and cosmology, the study of the universe at its largest scales — which again requires quantum mechanics. After all, the universe started very small.
One of the highlights of the program was quantum information and quantum computing. With the recent advent of quantum computers, research along these lines has been in the limelight. However, the line of enquiry goes back a long way. It ties to how quantum mechanics describes things around us — states. These mathematical quantities interact and evolve in ways set by the rules of quantum mechanics, but the implications of these rules for many quantum states interacting with each other without any outside interference remain largely outside the domain of experimental science.
With its many open questions and challenges, the future of quantum mechanics is bright.