Careful experiments with radiation, molecules, atoms and subatomic systems have convinced physicists over the years that the laws governing them (embodied in quantum mechanics) are quite different from those governing familiar objects of everyday experience (embodied in classical mechanics and electrodynamics). Quantum mechanics has turned out to be a very accurate and reliable theory though, even after more than seventy years of its birth, its interpretation continues to intrigue physicists and philosophers alike. Thanks to enormous technological advances over the last couple of decades, it has now become possible actually to perform some of the gedanken experiments that the pioneers had thought of to highlight the counterintuitive and bizarre consequences of quantum theory. So far, quantum mechanics has emerged unscathed in every case, and continues to defy all attempts at falsification. The spectacular success of its working rules has spurred physicists in recent times to grapple seriously with its foundational problems, leading to new theoretical and technological advances.

On the other hand, general relativity, the paradigm of classical field theory, continues to remain as accurate and reliable as quantum mechanics in its own domain of validity, namely, the large-scale universe. To wit, the agreement between the predictions of general relativity and observation of the energy loss due to gravitational waves emitted by binary pulsars is just as impressive as the agreement between the prediction of quantum electrodynamics and the measured value of the Lamb shift in atoms.