photon

History

As early as the 17th century, Newton suggested that light consists of tiny particles. But the real revolution happened in 1905 when Albert Einstein explained the photoelectric effect — the ejection of electrons from metal by light — precisely with the help of light quanta. Although the word 'photon' itself appeared only in 1926, coined by chemist Gilbert Lewis, the idea of a light particle became one of the pillars of quantum physics.

How it works

When an electron in an atom jumps from a high orbit to a low one, it emits excess energy in the form of a photon. This process is like a stone thrown into a pond: the impact energy spreads in circles on the water. However, the photon manages to be both a wave and a particle at the same time. It is this dual nature that allows it, for example, to interfere or reflect, but also to be registered by a detector as a point flash.

History

As early as the 17th century, Newton suggested that light consists of tiny particles. But the real revolution happened in 1905 when Albert Einstein explained the photoelectric effect — the ejection of electrons from metal by light — precisely with the help of light quanta. Although the word 'photon' itself appeared only in 1926, coined by chemist Gilbert Lewis, the idea of a light particle became one of the pillars of quantum physics.

How it works

When an electron in an atom jumps from a high orbit to a low one, it emits excess energy in the form of a photon. This process is like a stone thrown into a pond: the impact energy spreads in circles on the water. However, the photon manages to be both a wave and a particle at the same time. It is this dual nature that allows it, for example, to interfere or reflect, but also to be registered by a detector as a point flash.

History

The concept of the discrete nature of radiation originated in 1900 when Max Planck introduced energy quantization to explain the spectrum of a black body. Albert Einstein in 1905 developed the hypothesis, suggesting that light is not only emitted but also exists in the form of quanta, successfully explaining the photoelectric effect. In 1923, Arthur Compton experimentally confirmed the corpuscular nature by observing the scattering of X-ray photons on electrons. The term 'photon' was proposed by Gilbert Lewis in 1926. The subsequent development of quantum electrodynamics (Feynman, Schwinger, Tomonaga) ultimately established the photon as the exchange particle of the electromagnetic field. From the provided list, James Clerk Maxwell, whose equations describe electromagnetic waves, and Alain Aspect, who conducted experiments with photons to test Bell's inequalities, are associated with the photon.

How it works

A photon is emitted when a quantum system (e.g., an electron in an atom) transitions from a higher energy state to a lower energy state, carrying away the energy difference. Conversely, absorption of a photon excites the system to a higher energy state. The photon always propagates at speed c in a vacuum, which is a consequence of the principle of the constancy of the speed of light. In an electromagnetic wave described by Maxwell's equations, the photon represents the elementary excitation of the field modes. Its energy and momentum are given by the formulas E = hν and p = h/λ; thus, the higher the frequency (shorter the wavelength), the greater the photon's energy. According to the Heisenberg uncertainty principle, it is impossible to simultaneously measure the arrival time of a photon and its energy precisely. Photons obey Bose-Einstein statistics and can accumulate in a single quantum state, which underlies the operation of lasers. Interaction with matter leads to phenomena such as Hawking radiation near black holes and gravitational lensing — the curvature of photon trajectories in gravitational fields.

Open problems & fun facts

How exactly does a photon interfere with itself in the double-slit experiment under 'non-disturbing' measurement?; The nature of a single photon — can we obtain its complete spatio-temporal 'photograph'?; Creation of an ideal single-photon source of high purity for quantum communications.

💡 Photons do not age. The ancient light we observe as cosmic microwave background radiation has traveled to us for nearly 13.8 billion years — almost since the birth of the Universe.