Modern Physics

Modern physics covers the theories developed from the early twentieth century onward that describe matter and energy at very small scales and very high speeds, where classical mechanics breaks down. Quantum mechanics treats particles such as electrons as having both particle-like and wave-like properties. The Schroedinger equation governs how the quantum state of a system evolves, and its solutions give the probability of finding a particle in a given region rather than a definite trajectory.

Nuclear physics studies the dense core of the atom, where protons and neutrons are bound by the strong nuclear force. In nuclear fission a heavy nucleus, such as uranium-235, splits into lighter fragments and releases neutrons and energy; when those neutrons trigger further fission a chain reaction can be sustained, the basis of reactors and weapons. In nuclear fusion light nuclei such as isotopes of hydrogen combine to form a heavier nucleus, the process that powers the Sun. Both processes release energy because the products have slightly less mass than the reactants.

The missing mass appears as energy according to Einstein's relation between mass and energy, in which a small amount of mass corresponds to a very large amount of energy. Animations and applets help make these ideas concrete by showing probability distributions, chain reactions and energy release that cannot be observed directly. They turn abstract equations into visual models of atomic and subatomic behaviour.

Frequently asked questions

What does the Schroedinger equation describe?

It describes how the quantum state of a system changes over time. Its solutions, called wavefunctions, give the probability of finding a particle in a particular place or state.

How do nuclear fission and fusion differ?

Fission splits a heavy nucleus into lighter pieces, while fusion joins light nuclei into a heavier one. Both can release energy, and fusion powers stars.

Why does a nuclear reaction release energy?

The total mass of the products is slightly less than that of the reactants. That mass difference is converted into energy in line with mass-energy equivalence.