What “modern” means in physics
When physicists say modern physics, they usually mean the theories developed from about 1900 onward that go beyond the classical physics of Newton and of nineteenth-century electricity and magnetism. Classical physics works extremely well for everyday objects: baseballs, bridges, planets moving at ordinary speeds. It begins to fail in three situations: when things move very fast, when they are very small, and when gravity is extremely strong or the scale is cosmic.
Three big frameworks grew out of those failures: relativity, quantum theory, and the particle physics built on top of quantum theory. None of them throws away the older physics. Each one contains it as an excellent approximation in the right conditions.
Special relativity
In 1905, Albert Einstein published the theory now called special relativity. It starts from two ideas: the laws of physics are the same for observers moving at constant velocity relative to one another, and the speed of light in empty space is the same for all of those observers.
Those assumptions lead to surprising but well-tested consequences:
- Time dilation. A moving clock ticks more slowly as measured by an observer who is not moving with it. The effect is tiny at everyday speeds and noticeable only near the speed of light.
- Length contraction. Lengths along the direction of motion are measured as shorter by an observer for whom the object is moving.
- Mass and energy are related. The famous equation E = mc² says that a body’s rest mass corresponds to an amount of energy. It helps explain how nuclear reactions release energy.
A related consequence is that no object with mass can be accelerated all the way to the speed of light, which is why that speed acts as a universal limit for how fast information and matter can travel.
General relativity
Special relativity left out gravity. In 1915, Einstein completed general relativity, which describes gravity not as a pull between objects but as the curving of space and time by mass and energy. Objects in free fall follow the straightest possible paths through that curved geometry.
General relativity has passed many tests. It accounts for a small shift in the orbit of Mercury, the bending of starlight by the Sun (first measured during a 1919 eclipse), and the slowing of clocks in stronger gravity. It also predicts gravitational waves, ripples in spacetime, which were first detected directly in 2015. Engineers even need both kinds of relativity to keep GPS navigation accurate, because the satellite clocks run at slightly different rates than clocks on the ground. The theory also predicts black holes, regions where gravity is so strong that nothing can escape, and it underlies the modern account of the Big Bang and the expansion of the universe.
Quantum theory
Quantum theory began with a puzzle about the glow of hot objects. In 1900, Max Planck found that the observed spectrum made sense if energy was exchanged in discrete packets rather than continuously. In 1905, Einstein used a similar idea to explain the photoelectric effect, in which light knocks electrons out of a metal surface. Over the next two decades, Niels Bohr, Werner Heisenberg, Erwin Schrödinger and others developed a full theory.
Its central ideas include:
- Quantization. Some quantities, such as the energy of an electron bound in an atom, can take only certain allowed values.
- Wave-particle duality. Light and matter show wave-like behavior in some experiments and particle-like behavior in others.
- Probability. The theory generally predicts the odds of different measurement outcomes, not a single certain result.
- Uncertainty. Certain pairs of properties, such as position and momentum, cannot both be known precisely at once.
Quantum theory is among the most precisely tested theories in science. It explains why atoms are stable, how chemical bonds form, and how transistors, lasers and solar panels work. How to interpret what the mathematics says about reality is still debated among physicists, even though the predictions themselves are not.
Particles and forces
By the mid-twentieth century, experiments had revealed a large number of particles. The Standard Model of particle physics organizes them. It describes:
- Matter particles: quarks, which combine to make protons and neutrons, and leptons, which include the electron and the nearly massless neutrinos.
- Force carriers: the photon for electromagnetism, the gluon for the strong force that binds quarks, and the W and Z bosons for the weak force involved in some kinds of radioactive decay.
- The Higgs field: associated with the Higgs boson, discovered at CERN in 2012, and connected to how some fundamental particles acquire mass.
This is the world of subatomic particles. The Standard Model has been extraordinarily successful, but it does not include gravity, and it does not explain everything we observe.
What modern physics has not settled
Several large questions remain open:
- Quantum gravity. General relativity and quantum theory work well separately, but physicists do not yet have a widely accepted theory that unites them. This matters most for the deepest interiors of such objects and the earliest moments of the universe.
- Dark matter and dark energy. Observations of galaxies and cosmic expansion suggest that most of the universe’s content is not ordinary matter. What dark matter actually is remains unknown.
- Why the Standard Model has the particles it does, and why they have the masses they have.
- The meaning of quantum measurement, which is still interpreted in several competing ways.
How the pieces fit together
A useful way to remember the division of labor is by scale and speed. Newton’s laws handle ordinary objects at ordinary speeds. Special relativity matters when speeds approach that of light. General relativity matters when gravity is strong or distances are cosmic. Quantum theory matters for atoms and smaller things. Particle physics applies quantum theory and relativity together to the smallest known constituents of matter.
Reading further on any one of these topics is easier with that map in mind, and honest accounts of each will usually mention where current knowledge ends.