Physics (from Greek from φυσικός (phusikos): natural, from φύσις (fysis): Nature) is the science of Nature in the broadest sense. Physicists study the behaviour and interactions of matter and radiation. Theories of physics are generally expressed as mathematical relations. Well-established theories are often referred to as physical laws or laws of physics; however, like all scientific theories, they are ultimately provisional.
Physics is very closely related to the other natural sciences, particularly chemistry, the science of molecules and the chemical compounds that they form in bulk. Chemistry draws on many fields of physics, particularly quantum mechanics, thermodynamics and electromagnetism. However, chemical phenomena are sufficiently varied and complex that chemistry is usually regarded as a separate discipline.
Below is an overview of the major subfields and concepts in physics, followed by a brief outline of the history of physics and its subfields.
In condensed matter physics, the biggest unsolved theoretical problem is the explanation for high-temperature superconductivity. Strong efforts, largely experimental, are being put into making workable spintronics and quantum computers.
In particle physics, the first pieces of experimental evidence for physics beyond the Standard Model have begun to appear. Foremost amongst this are indications that neutrinos have non-zero mass. These experimental results appear to have solved the long-standing solar neutrino problem in solar physics. The physics of massive neutrinos is currently an area of active theoretical and experimental research. In the next several years, particle accelerators will begin probing energy scales in the TeV range, in which experimentalists are hoping to find evidence for the higgs boson and supersymmetric particles.
Theoretical attempts to unify quantum mechanics and general relativity into a single theory of quantum gravity, a program ongoing for over half a century, has yet to bear fruit. The current leading candidates are M-theory and loop quantum gravity.
Many astronomical phenomena have yet to be explained, including the existence of ultra-high energy cosmic rays and the anomalous rotation rates of galaxies. Theories that have been proposed to resolve these problems include doubly-special relativity, modified Newtonian dynamics, and the existence of dark matter. In addition, the cosmological predictions of the last several decades have been contradicted by recent evidence that the expansion of the universe is accelerating.
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