Physics is often defined as the study of Matter in Motion. This is part of the reason that physics relates to the other sciences in the way it does: all other sciences observe that which is made of matter, and in motion.
The phrase 'matter in motion', requires us to define matter and motion. Presently, motion is considered the trajectory of an object through Space-Time.
While the definitions of spacetime have changed throughout the years, the current definition, is that of [[General Relativity],] which is geometric in nature.
Here, we will analyze the best description of matter: Quantum Mechanics.
Before we can do this, we must address the conflict between these two descriptions. General Relativity is unambiguous, and precise, and is highly tested. There is little, if any, debate over how to interpret it. With Quantum theory, there is no consensus about how it is to be interpreted. Quantum theory has also been rigorously tested, as well as giving birth to modern industry, but it itself, is arguably not even a theory, but rather a recipe.
There is also not much to be said about what the theory claims about the physical world.
One of the major issues with the recipe, as stated by John Bell, is that the theory invokes observers, and measurement. Einstein as well, was uncomfortable with the idea that measurement of a physical system could effect the physical system.
Part of the issue is that the theory, or recipe, offers no precise mathematical definition of an observer.
Considering the double slit experiment shows that an electron can have the effect of an observer, one is forced to find mathematics that are symmetrically descriptive of both an electron and a scientist.
While the theory allows for excellent predictions of the results of a certain class of experiments, it does not provide a univocal account of the physical world.
One example of this is that the collapse of the wave function, as well as the wave function itself, has a mathematical existence, but it is as of yet unsolved whether or not it has a physical existence.
A lot of these issues have been avoided with the use of language about the predictive ability of quantum mechanics, which allows physicists to avoid trying to settle on an interepretation of the theory.