The philosophy of physics encompasses the underlying ontology and epistemology of physics, which is the study of the nature and relationship of matter and energy. Philosophy, in general, is the analysis of the fundamental questions regarding reality, comprehension, interpretation and logic. The philosophy of physics addresses these aspects in their relevance to modern physics: classical mechanics, thermodynamics, quantum mechanics and physical cosmology.
The philosophy of physics can be divided into three branches. The first is the analytical study of major concepts such as spatial, temporal and causal relationships. The second is the empirical study of the methodologies and models used in physics, including theorization, experimentation, assessment and amendment. Finally the philosophy of physics can simply be a thoughtful examination of the fundamental principles of physics using examples (e.g. thought experiments).
According to Nobel Prize recipient Niels Bohr (1885-1962), it is a mistake "to think that the task of physics to find out how nature is. Physics concerns what we can say about nature." Distinguished for his structural theory of the atom, Bohr arrived at the conclusion that despite our best efforts to describe nature, the reality of nature remains unknown and, at best, our theories can only describe how nature appears to us. Our observations, on which the study of any science is completely dependent, are subjective, inescapably influenced by human perspective and bias.
By definition, the laws of physics must apply universally: gravity, time, conservation of matter and energy, thermodynamics, etc. As a corollary, the laws of physics must be the same regardless of perspective or place. In some ways, the speed of a moving train is dependent on the perception of the observer, whether he is sitting inside the train or whether he is standing on a platform as it passes by. The principle of relativity expounds on this by stating "the laws of physics are the same in all inertial reference frames." This concept has been acknowledged by physicists since Galileo.
The definition of inertial and non-inertial reference frames leads to the debate of absolutism and relationalism. An observer on a platform is considered non-inertial with respect to what? The train; the earth; the sun? At the beginning of the 20th century, physicists seem to naturally assume that an absolute frame of reference existed, without actually defining one. The definition requires the establishment of space and time as either substantive, possessing its own properties independent of objects or events, or as relational, requiring more than an object or event to define it. This debate took place between Gottfried Leibniz and Sir Isaac Newton as early as the early 18th century and has developed into two schools of thought. A third view is one of conventionalism which insists on a coordinative definition that is established by convention.
In quantum mechanics, the philosophy of physics branched even further. Quantum theory attempts to provide a mathematical resolution to the principle of complementarity, the observation of the dual, particle-wave nature of light, matter and energy. It is based on the mathematical construct of the wave function, which calculates the probability of a system found in one of these dual states at a point in time. These principles led to the introduction of Heisenberg's Uncertainty Principle and the Bohr-Einstein debates on indeterminism. These concepts struck at the core of the philosophical concepts of determinism and absolute natural law, allowing for the possibility of general universal rules without necessarily having a predetermined future.
Albert Einstein acknowledged the connection between physics and philosophy in a letter to nuclear physicist Robert L. Thornton, "So many people today -- and even professional scientists -- seem to me like somebody who has seen thousands of trees but has never seen a forest. A knowledge of the historic and philosophical background gives that kind of independence from prejudices of his generation from which most scientists are suffering. This independence created by philosophical insight is -- in my opinion -- the mark of distinction between a mere artisan or specialist and a real seeker after truth." In certain regards, it can be restated that there can be no philosophy of physics independent of the physical theory. The universalism of the concepts enables investigation and argumentation that transcends any singular epoch in the history of the science itself.