What Is Physics? Research Areas, Funding, and Career Paths

Physics is the natural science concerned with matter, energy, force, motion, space, and time — and with the fundamental laws that govern how these behave at every scale, from subatomic particles to the observable universe. It asks the most basic possible questions about the physical world (what is matter made of? what makes objects move the way they do? what is energy, and how is it conserved?) and answers them with mathematical models that are tested, refined, and sometimes overturned by experiment. Physics is often described as the most foundational of the natural sciences, since chemistry, materials science, much of engineering, and parts of biology and earth science ultimately rest on physical laws — but it is also a large, highly specialized modern research field in its own right, with distinct subfields, instrumentation, and funding pathways covered in depth below.

What physics actually studies

At its core, physics studies four closely related things:

• Matter and its structure — what things are made of, from atoms and subatomic particles up through bulk materials, and how that structure determines physical properties.
• Forces and interactions — the four fundamental forces recognized in modern physics (gravity, electromagnetism, the strong nuclear force, and the weak nuclear force) and how they govern everything from planetary orbits to radioactive decay.
• Energy and its transformations — how energy moves between forms (kinetic, potential, thermal, electromagnetic, nuclear) and the conservation laws that constrain those transformations.
• Space, time, and motion — how objects move, and, since Einstein’s relativity, how space and time themselves behave, particularly at very high speeds or in strong gravitational fields.

Physics is fundamentally an empirical science: a physical theory is only as good as its ability to predict the outcome of an experiment or observation that hasn’t been run yet, and a theory that makes a testable prediction which fails is discarded or revised regardless of how elegant its mathematics is. This interplay between mathematical theory and experimental test — build a model, derive a testable prediction, test it, revise the model — is the field’s defining method, and it’s what separates physics from purely descriptive natural history or from mathematics for its own sake. Physicists also lean heavily on symmetry and conservation laws (conservation of energy, momentum, and electric charge, for example) as organizing principles: a deep result in 20th-century theoretical physics, Noether’s theorem, shows that every such conservation law corresponds to an underlying symmetry of the physical system, which is part of why symmetry arguments are so central to how physicists actually think about problems.

A brief history of how physics developed

Physics as a distinct, mathematically formal discipline traces to the Scientific Revolution: Galileo Galilei’s experimental work on motion and falling bodies in the early 1600s, and Isaac Newton’s 1687 Philosophiæ Naturalis Principia Mathematica, which unified terrestrial and celestial motion under a single set of laws of motion and universal gravitation. This “classical” framework — mechanics, later joined by thermodynamics (the study of heat and energy, formalized through the 19th century) and by James Clerk Maxwell’s 1860s unification of electricity, magnetism, and light into a single electromagnetic theory — dominated physics until the early 20th century.

Two revolutions then reshaped the field. Albert Einstein’s special theory of relativity (1905) and general theory of relativity (1915) showed that space, time, and gravity behave very differently from the classical picture at high speeds and strong gravitational fields. Around the same period, work by Max Planck, Niels Bohr, Werner Heisenberg, Erwin Schrödinger, and others established quantum mechanics (roughly 1900–1926), which showed that matter and energy behave in fundamentally probabilistic, non-classical ways at atomic and subatomic scales. Relativity and quantum mechanics together are usually called “modern physics,” and nearly all of physics research since has built on one or both frameworks.

The mid-to-late 20th century saw the development of the Standard Model of particle physics — the current best theory of the fundamental particles and three of the four fundamental forces — built up through the 1960s and 1970s and tested at particle accelerators ever since; its last major predicted particle, the Higgs boson, was confirmed experimentally in 2012 by the ATLAS and CMS collaborations at CERN’s Large Hadron Collider. The 21st century has added new experimental capability rather than a new overarching framework so far: the first direct detection of gravitational waves in 2015 by the LIGO collaboration opened an entirely new way of observing the universe, and precision cosmological measurements have made dark matter and dark energy — phenomena current physics can measure but not yet fully explain — central open problems in the field.

Core open questions and how physicists approach them

Physics is not a finished body of knowledge; several major questions remain genuinely unresolved and drive much of current research funding and effort:

• Reconciling quantum mechanics and gravity — general relativity and quantum mechanics are both extremely well-tested individually, but no complete, experimentally confirmed theory unifies them (string theory and loop quantum gravity are two prominent theoretical approaches, neither yet experimentally confirmed).
• The nature of dark matter and dark energy — together believed to make up roughly 95% of the universe’s total mass-energy content based on cosmological and astrophysical observations, but not directly detected or explained by the Standard Model.
• Why matter dominates over antimatter in the observable universe, when the Standard Model’s known processes don’t fully account for the imbalance.
• Extending or completing the Standard Model — for example, understanding neutrino mass (not originally predicted by the Standard Model but experimentally confirmed) and searching for physics beyond it at high-energy colliders.

Physicists approach these questions through the same basic cycle across every subfield: theoretical modeling to generate testable predictions, experiment or observation to test them, and — especially since the mid-20th century — large-scale computation to simulate systems too complex to solve analytically. Which of these three modes dominates varies a great deal by subfield, which is part of why physics splits into the specialized areas described next.

Major branches and sub-disciplines of physics

Physics is commonly organized into a set of major subfields. Most are not fully separate — a single research problem (say, a neutron star merger, or a novel superconductor) routinely draws on several at once — but each has developed its own methods, instrumentation, terminology, journals, and funding pathways:

• Classical mechanics — the physics of motion and forces for macroscopic objects; the oldest branch, still foundational to engineering and to more advanced physics.
• Electromagnetism — the study of electric and magnetic fields and their unification, underlying essentially all of modern electronics, optics, and telecommunications.
• Thermodynamics and statistical mechanics — the physics of heat, energy, and how the collective statistical behavior of huge numbers of particles produces macroscopic properties like temperature and pressure.
• Quantum mechanics — the physics of matter and energy at atomic and subatomic scales, and the mathematical framework underlying most of modern physics.
• Particle physics (high-energy physics) — the study of the most fundamental constituents of matter and the forces between them, typically studied using large accelerator facilities.
• Nuclear physics — the structure, behavior, and reactions of atomic nuclei, distinct from particle physics in scale and typical instrumentation, though closely related.
• Condensed matter and materials physics — the physics of solids and liquids, including phenomena like superconductivity and magnetism; the largest subfield by number of working physicists in many countries, and closely tied to materials science and nanotechnology.
• Atomic, molecular, and optical (AMO) physics — the physics of atoms, molecules, and light, including laser physics and the atomic-scale techniques (like laser cooling and trapping) that underpin quantum computing and precision measurement.
• Plasma physics — the physics of ionized gases, central to fusion-energy research and to understanding stars and the interstellar medium.
• Biophysics — the application of physical methods and models to biological systems, from the mechanics of individual proteins and membranes to neural signaling.
• Medical physics — the application of physics to medicine, particularly radiation therapy, diagnostic imaging (MRI, CT, ultrasound), and radiation safety.
• Computational and mathematical physics — developing and applying numerical methods and mathematical formalism across the other subfields, increasingly a distinct specialty as simulation has become central to the field.
• Geophysics — physics applied to the structure and dynamics of the Earth (seismology, geomagnetism, geodesy); often housed administratively in earth-science departments rather than physics departments.

Astrophysics and cosmology — physics applied to stars, galaxies, and the universe as a whole — are close enough to being their own established discipline, with their own instrumentation (telescopes and observatories) and largely separate funding programs, that CASRAI covers them in a dedicated companion guide: What Is Astrophysics? Research Areas, Funding, and Career Paths.

How physics relates to neighboring disciplines

Physics sits at the foundation of several other fields, and modern research increasingly happens at the boundaries between them:

• Chemistry — physical chemistry and chemical physics apply quantum mechanics and thermodynamics directly to molecular structure and reactions; CASRAI’s companion guide on what chemistry is covers that neighboring discipline in full.
• Astronomy and astrophysics — as noted above, essentially the application of physics to celestial objects and the universe as a whole.
• Materials science and engineering — draws directly on condensed matter physics to design and characterize new materials.
• Structural and molecular biology — techniques developed in physics (X-ray crystallography, nuclear magnetic resonance, and cryo-electron microscopy) are now core tools for determining the 3D structure of proteins and other biomolecules; see CASRAI’s guides on what biochemistry is and what molecular biology is for how those fields use physics-derived methods.
• Computer science — computational physics has long driven high-performance computing methods, and quantum computing research sits directly at the intersection of AMO physics, condensed matter physics, and computer science; see CASRAI’s companion guide on what computer science is.
• Biology — biophysics applies physical models to living systems, from single-molecule mechanics to whole-organism physiology; see CASRAI’s guide on what biology is.

Physics is one of the disciplines mapped in CASRAI’s overview guide to the branches of science, which shows how physics relates to the full landscape of scientific disciplines rather than just its closest neighbors.

Who funds physics research

Physics research — particularly experimental physics — is often capital-intensive: particle accelerators, national laboratories, large detector facilities, and specialized instrumentation are expensive to build and operate, so the field’s funding landscape is dominated by a relatively small number of national agencies, supplemented by a set of major private foundations. In the United States, the core federal funders are:

• The National Science Foundation (NSF), primarily through its Physics Division within the Directorate for Mathematical and Physical Sciences (MPS), which funds fundamental physics research at universities across essentially every subfield described above, alongside MPS’s other divisions covering astronomy, chemistry, materials research, and mathematics.
• The Department of Energy (DOE), primarily through its Office of Science — the United States’ largest single federal sponsor of basic physical-science research — which funds physics through several dedicated program offices: the Office of High Energy Physics (particle physics), the Office of Nuclear Physics, the Office of Basic Energy Sciences (which includes condensed matter and materials physics, among other areas), and the Office of Fusion Energy Sciences (plasma physics for fusion energy). DOE also operates and funds the national laboratory system (Fermilab, Brookhaven, SLAC, and others) where much large-facility experimental physics is actually carried out.
• NASA, mainly for the overlap between physics and space science — heliophysics, space physics, and fundamental-physics experiments conducted in space or using space-based instruments.
• Defense agencies — the Department of Defense, through basic-research offices such as the Air Force Office of Scientific Research (AFOSR), the Army Research Office (ARO), and the Office of Naval Research (ONR) — fund substantial fundamental physics research, particularly in areas with long-term dual-use relevance such as quantum information science, materials, and optics; see CASRAI’s guide to the DOD research funding landscape and comparison of early-career DOD and DOE award mechanisms.
• The National Institute of Standards and Technology (NIST) conducts and funds physics research directly relevant to measurement science and standards, including major programs in quantum information and precision metrology.

Outside the US, national funders play the equivalent role — the UK’s Science and Technology Facilities Council (STFC) funds UK particle physics, nuclear physics, and the country’s share of large international facilities, while the broader Engineering and Physical Sciences Research Council (EPSRC) funds other areas of UK physical-sciences research. A number of major private foundations are also genuinely active in physics specifically, typically funding basic research, named institutes, or instrumentation rather than running government-style grant competitions: the Simons Foundation supports mathematics and the physical sciences broadly, including theoretical physics, and the Kavli Foundation funds a network of named research institutes, several focused on physics-adjacent fields. Because eligibility rules, program names, and funding priorities change from year to year, always confirm current solicitation details directly with the funding agency or foundation before relying on them for a specific proposal. Early-career researchers should also compare available postdoctoral mechanisms directly — see CASRAI’s comparison of NSF postdoctoral fellowships by directorate.

Research methods, tools, and equipment

Physics research methods vary substantially by subfield, but generally fall into three broad modes that most physicists combine to different degrees:

• Experiment — direct measurement using instrumentation ranging from tabletop optics and cryogenic setups to enormous shared facilities like particle accelerators, neutron sources, and gravitational-wave observatories. Large experimental physics increasingly happens through international megacollaborations with hundreds or thousands of contributing scientists, which raises distinctive research-administration questions around authorship and credit that CASRAI covers directly in its guide to authorship practices at ATLAS, CMS, and LIGO.
• Theory — developing and refining mathematical models, from pencil-and-paper derivations to formal proofs, that make testable predictions or explain existing experimental results.
• Computation and simulation — an increasingly central third mode, using high-performance computing to simulate systems (from molecular dynamics to cosmological structure formation) too complex to solve analytically, and to process the very large datasets modern experiments produce.

Data infrastructure is a genuine methodological concern in its own right in physics, particularly in high-energy and nuclear physics, where individual experiments can produce enormous datasets that outlive any single funding cycle and are reused for years by researchers who weren’t part of the original collaboration; CASRAI’s guide to HEPData, the particle-physics research data repository, covers one well-established example of that infrastructure. Physics was also the origin of arXiv, the preprint server that reshaped scholarly communication norms well beyond physics itself — see CASRAI’s guide to arXiv preprints for how that system works and how it’s used today.

Careers and training in physics

Professional physics research is an advanced-degree field. The typical training path is a bachelor’s degree in physics (or a closely related field, with a substantial physics component), followed by a PhD — commonly around 5-6 years in the US graduate-school model — involving coursework, qualifying examinations, and original research culminating in a dissertation. Many physics PhD holders then complete one or more postdoctoral research positions, often multi-year fixed-term appointments at universities or national laboratories, before moving into a permanent faculty, national-laboratory, or industry research position. Because permanent academic and national-laboratory research positions are limited relative to the number of PhDs trained, a large share of physics PhD holders build careers in adjacent fields where the discipline’s quantitative, computational, and problem-solving training transfers well — including data science, software engineering, finance, semiconductor and photonics industry research, and science policy.

Physicists in the US are supported by long-established professional societies: the American Physical Society (APS), the principal professional society for physicists, publishes the Physical Review family of journals and organizes the field’s major annual meetings (the March Meeting and April Meeting), and the American Institute of Physics (AIP) is a federation of physical-science societies that publishes Physics Today and conducts workforce and education research across the discipline. Internationally, national physical societies and, for specific subfields, international bodies such as CERN’s member-state governance structure play an equivalent coordinating role. As with astrophysics, large physics collaborations have also developed their own formal authorship and credit policies to fairly attribute genuinely collective research efforts — the CASRAI guide on megacollaboration authorship linked above covers this directly.

Frequently asked questions

What is the simplest definition of physics?

Physics is the science that studies matter, energy, forces, motion, space, and time, and the mathematical laws that describe how they behave — from subatomic particles to the largest structures in the universe.

What is the difference between physics and chemistry?

Physics studies the most fundamental laws governing matter and energy at every scale; chemistry focuses specifically on how atoms combine into molecules and how those molecules react. The two overlap substantially in physical chemistry and chemical physics, which apply physics’ quantum-mechanical and thermodynamic tools directly to chemical systems.

See CASRAI’s companion guide, what is chemistry, for the full picture of that neighboring discipline.

What are the main branches of physics?

The major branches are classical mechanics, electromagnetism, thermodynamics and statistical mechanics, quantum mechanics, particle (high-energy) physics, nuclear physics, condensed matter and materials physics, atomic/molecular/optical physics, plasma physics, biophysics, medical physics, computational physics, and geophysics — with astrophysics and cosmology close enough to a separate discipline that CASRAI covers them in their own guide.

Is physics a good career?

Physics offers deep, quantitative training with strong transferable skills, but it is a competitive field with a long training path — typically a PhD plus one or more postdoctoral positions — and a limited number of permanent academic and national-laboratory research positions relative to the number of people trained. Many physics PhD holders build successful careers in adjacent quantitative fields (data science, software engineering, finance, semiconductor and photonics research) rather than in academic physics research specifically.

Who funds physics research in the US?

The core federal funders are the National Science Foundation (mainly through its Physics Division), the Department of Energy’s Office of Science (through its High Energy Physics, Nuclear Physics, Basic Energy Sciences, and Fusion Energy Sciences program offices), NASA for space- and physics-related work, and defense agencies such as AFOSR, ARO, and ONR for fundamental research with long-term dual-use relevance. A smaller number of major private foundations, including the Simons Foundation and the Kavli Foundation, also fund physics research, typically through named institutes and instrumentation rather than broad grant competitions.

How long does it take to get a physics PhD?

In the typical US model, around 5-6 years after a bachelor’s degree, including coursework, qualifying examinations, and original dissertation research — though this varies by country, program, and subfield, and many researchers go on to complete one or more postdoctoral positions afterward before a permanent role.

Related CASRAI resources

Physics is one of many major scientific disciplines covered in CASRAI’s overview guide to the branches of science, which maps how physics relates to neighboring fields across the natural, formal, and life sciences. For a dedicated look at physics applied to the universe as a whole, see CASRAI’s guide to what astrophysics is. On the research-administration side, see CASRAI’s guides to authorship practices on major physics collaborations and the HEPData particle-physics data repository, and its comparisons of DOD and DOE early-career award mechanisms and NSF postdoctoral fellowships by directorate. On the funding side specifically, see CASRAI’s guides to how STFC funds UK physics, the Kavli Foundation’s research institute model, and Simons Foundation funding for mathematics and the physical sciences. Readers researching other scientific disciplines may also be interested in CASRAI’s companion guides on chemistry, biology, and computer science, part of the same discipline-guide series.