Thought experiments have been the bread and butter of modern physics since its inception. Galileo used a thought experiment to argue that all objects fall at the same rate regardless of mass, Isaac Newton used a thought experiment to argue for the absolute nature of space, Maxwell used a thought experiment to argue the nature
My theme is thought experiment in natural science, and its relation to real experiment. I shall defend the thesis that thought experiments that do not lead to theorizing and to a real experiment are generally of much less value that those that do so. To illustrate this thesis I refer to three examples, from three very different periods, and with three very different kinds of status. The first is the classic thought experiment in which Galileo imagined that he had, by pure thought, demolished Aristoteles' dogma that heavier bodies fall more quickly than light ones. I will show that he was mistaken. The second is the Einstein-Podolsky-Rosen paper purporting to show that quantum mechanics must be incomplete in its domain of application. This thought experiment is a very good one, not because its conclusions are correct, but precisely because it was fruitful, leading to theory and, above all, to a real experiment. Finally I discuss the modern string theory of everything, which, while it is regarded as a physical theory by its instigators, shares some properties of the least successful sort of thought experiment.
The author describes several thought experiments that are important in modern physics, by which he mean the physical theory and practice that developed explosively from the late 19th century onwards. Some thought experiments provide the basis for rigorous arguments with clear conclusions; others seem to work simply by drawing attention to an important question that otherwise might not have been apparent. While the thought experiments to be discussed here are key turning points in the history of modern physical theory, in several cases their full implications remain to be plumbed. It is an extraordinary fact that most of the decisive thought experiments in 20th-century physics were born from the fertile imagination of one person, Albert Einstein. This forces to ponder the importance of individual creativity in the advancement of science. The Einstein-Podolsky-Rosen (EPR) thought experiment dominates investigations of the foundations of quantum mechanics and plays a defining role in quantum information theory.
Those defile the purity of mathematical and philosophical truthy who confound real quantities with their relations and sensible measures. —ISAAC NEWTON, Principia Before we say anything about quantum mechanics, let us take a quick look at the Newtonian (or ‘classical’) view of the physical universe. According to that view, nature consists of an enormous number of particles. When Newtonian physics is combined with the theory of the electromagnetic field, it becomes convenient to think of these particles as dimensionless (even if there is a kind of conceptual strain involved in trying to think of something as having a mass but not any size ), and as possessing electrical properties – negative charge, or positive charge, or neutrality. This leads to the well-known ‘solar system’ view of the atom – with the electrons whirling around the nucleons (neutrons and protons) just as the planets whirl around the sun. Out of atoms are built molecules; out of molecules, macroscopic objects, scaling from dust motes to whole planets and stars. These latter also fall into larger groupings – solar systems and galaxies – but these larger structures differ from the ones previously mentioned in being held together exclusively by gravitational forces. At every level, however, one has trajectories (ultimately that means the possibility of continuously tracing the movements of the elementary particles) and one has causality (ultimately that means the possibility of extrapolating from the history of the universe up to a given time to its whole sequence of future states).
Quantum physics has long occupied a space between imagination and reality, captivating scientists and philosophers with its strange, counterintuitive principles: superposition, entanglement, uncertainty, and nonlocality. What once appeared as hypothetical mathematics now manifests in real technologies—lasers, semiconductors, superconductors, atomic clocks, MRI machines, and emerging quantum computers. This mini-review traces the conceptual journey of quantum physics from an abstract theory born in early twentieth-century thought experiments to a practical framework shaping modern engineering, computation, communication, and materials science. It highlights key developments in interpretation, experimental validation, and application, while reflecting on how quantum physics reshapes our understanding of nature, reality, and information. Although quantum mechanics remains philosophically perplexing, it has become a powerful and indispensable foundation for technological innovation. By examining how imaginary constructs transitioned into concrete implementations, this article illustrates the remarkable interplay between theory and experiment that continues to redefine what is possible.
Abstract A scientist in the late nineteenth century could be forgiven for thinking that the major elements of physics were built on unshakeable foundations and effectively established for all time. The inspirational work of Galileo Galilei and Isaac Newton in the seventeenth century had been shaped by a further 200 years of theoretical and, in particular, experimental science into a marvellous construction which we now call classical physics. This physics appeared to explain almost every aspect of the physical world: the dynamics of moving objects, thermodynamics, optics, electricity, magnetism, gravitation, etc. So closely did theory agree with and explain experimental observations of the everyday world that there could be no doubt about its basic correctness- its essential ‘truth’. Admittedly, there were a few remaining problems but these seemed to be trivial compared with the fundamentals - a matter of dotting a few is and crossing some ts.