Quantum mechanics has not only revolutionised the world of science but also our understanding of the very structure of matter at the fundamental level. It is undoubtedly one of the two most successful theories ever formulated in human history, the other being the Theory of General Relativity. None of the experiments carried out in the past hundred years have called these two theories into question. Not only is Quantum Mechanics along with General Relativity a triumph of 20th-century science but of the human mind itself.
Quantum mechanics is the underlying principle behind the working of the microscopic world of atoms and molecules. Modern gadgets and appliances that have become integral to our lives, like the mobile phone, transistors, LEDs, lasers, and computers are based on quantum mechanics. Today one cannot think of life without this sophisticated technology. But Quantum theory right from its evolution has been bizarre and counter-inituitive. Some philosophers are yet to come to terms with the probabilistic Quantum Mechanical description of reality and are stuck right where they were 100 years before. This probabilistic description of reality is so compelling and pressing that even physicists like Einstein and Schrodinger, who played a fundamental role in its formulation, had a tough time reconciling with it. Unlike a deterministic version of the universe, Quantum physicists like Bohr and Heisenberg argued for an observer-dependent view of reality.
The Theory of General Relativity, formulated almost at the same time as Quantum Mechanics, is described by many as being the most elegant and beautiful among all theories of physics. It describes the large-scale structure of the universe and redefines the Newtonian Law of Gravitation. It is an extension of Einstein’s own theory of Special Relativity. It explains gravity in terms of distortion, curvature or warping of space time due to the presence of massive objects. From the discovery of black holes to the explanation of the planet Mercury’s orbit, from the detection of gravitational waves to gravitational red shift, the theory of general relativity has stood the tests of time. Like Quantum Mechanics it also has met the observational scrutiny of the past hundred years. But unlike Quantum Mechanics, General Relativity is highly intuitive.
On larger scales of length above the Planck Scale, the two theories describe beautifully different aspects of physical reality. For lengths smaller than Planck Scale, however, both fail, leaving us ignorant of structures and events in this realm of scale. The smooth geometry that is so essential and central to General Relativity gets distorted or disrupted due to Quantum fluctuations which arise as a result of Heisenberg’s Uncertainty Principle. So, to deal with time intervals smaller than Planck time, we require a theory that unifies General Relativity and Quantum Mechanics. No such theory is yet adequate for such a purpose. Over the years many theories with beautiful mathematics have been formulated to resolve this contradiction, but none of them have as yet succeeded at it.
It is also yet to be seen whether Quantum Mechanics and General Relativity really work at all scales. One of the theories which has received wide acceptance and has emerged as a strong candidate for such unification is the String Theory. But String Theory still lacks experimental evidence. David Richardson, an Australian philosopher, in his book ‘String Theory and Scientific Method’ argues in defence of the theory by invoking Unexpected Explanatory Coherence Meta Inductive argument and No Alternative argument. Though the book has been received well by Nobel Laureates like David Gross, but such a view has also faced stiff and sharp opposition both from physicists like Paul Davies as well as philosophers of science, who accuse David Richardson of abandoning the centuries-old tradition of science as empirical in nature. Physicist Sabbine Hosenfielder in her book ‘Lost in Math: How Mathematics leads Physics astray’ writes that Mathematics has been used as a tool in Physics and not as a guide, hinting at String theorists who are stuck with the beautiful mathematics of the theory. Whatever be the case, the next 30 to 40 years will be crucial for String theorists. If this theory meets expectations, which seems unlikely right now, it will be the ultimate triumph of the human mind in history.