Photon
In 1905, Albert Einstein published a paper proposing that light consists of discrete energy quanta. This idea challenged the dominant wave theory of James Clerk Maxwell from 1865. Max Planck had previously suggested energy came in integer multiples called energy elements during his study of black-body radiation. Einstein took this concept further by applying it directly to electromagnetic waves themselves. He argued that many phenomena like the photoelectric effect required light to be localized into point-like particles. These particles moved independently even if the wave spread continuously over space. Most physicists rejected this view for decades because they trusted Maxwell's equations too much. Arthur Compton later provided experimental proof with scattering experiments in 1922. His work showed photons carried momentum proportional to their wave number. This evidence finally convinced the scientific community that light possessed particle properties alongside its wave nature.
Photons possess no electric charge and generally have zero rest mass according to current physical theories. Their speed remains constant at the speed of light measured in vacuum regardless of frequency or energy level. Experimental upper limits on photon mass reach values as small as 10^-53 grams. If photons had any measurable mass, lower-energy red photons would travel slightly slower than higher-energy blue ones. Such a variation would alter Coulomb's law and create an extra degree of freedom within the electromagnetic field. Scientists tested these possibilities using galactic vector potentials and hollow conductors subjected to external fields. No such effects appeared in precision tests setting strict bounds on potential mass. A photon traveling through space maintains two possible polarization states representing circular polarization directions. These states correspond to spin angular momentum values of plus one-half or minus one-half. Collections of photons may mix these values to form linearly polarized beams acting like equal numbers of both pure states.
When detected by instruments, photons register as single particulate units rather than continuous waves. Yet probability calculations for their location follow equations describing wave behavior. This combination defines wave-particle duality observed throughout quantum mechanics experiments. Thomas Young demonstrated interference patterns with water waves supporting a wave model over Newton's particle theory. Later double-slit experiments showed individual photons creating interference patterns despite arriving at screens as discrete points. Energy does not spread out during propagation nor divide when encountering beam splitters. Instead received photons act like point-like particles absorbed whole by systems smaller than their wavelength. An atomic nucleus roughly 10^-15 meters across can absorb entire photons despite being much smaller than typical wavelengths. Max Born later introduced probabilistic interpretations inspired by Einstein's search for more complete theories involving ghost fields guiding point-like photons. Werner Heisenberg developed matrix mechanics partly due to failures in earlier semiclassical models attempting to preserve Maxwellian continuity.
Within the Standard Model of particle physics photons serve as gauge bosons resulting from symmetry requirements at every spacetime point. Electromagnetic field strength arises from Abelian U(1) symmetry allowing phase variation without affecting observable functions. Quanta of such fields must be massless uncharged bosons unless symmetry breaks via mechanisms like the Higgs process. Sheldon Glashow Abdus Salam and Steven Weinberg unified photons with W plus W minus and Z zero bosons in electroweak interactions. These other three gauge bosons possess mass unlike photons owing to broken SU(2) symmetry. Their work earned them the 1979 Nobel Prize in physics. Virtual photons mediate static electric and magnetic interactions while real photons carry energy between distant points. Transient virtual states may adopt unphysical polarization depending on chosen gauges contributing measurably to event probabilities. Renormalization techniques correct infinite contributions arising from second-order perturbation calculations involving these transient intermediates. Modern notation describes electromagnetic field quantum states using Fock states representing photon numbers within specific modes.
Gilbert N. Lewis popularized the term photon for light quanta in a letter published in Nature on the 18th of December 1926. Before this moment scientists used words like quanta or energy elements derived from Latin roots meaning how much. Max Planck first suggested energy came in finite equal parts during his black-body radiation studies around 1900. Albert Einstein called these units light quanta in German as ein Lichtquant when publishing his photoelectric effect paper. Leonard T. Troland had previously used the word photon in 1916 regarding retinal illumination units but not for physics itself. Arthur Compton cited Lewis at the 1927 Solvay conference proceedings acknowledging him as suggesting the name photon. Einstein never adopted the terminology himself despite his foundational role in establishing particle concepts. The symbol gamma often denotes photons deriving from gamma rays discovered by Paul Villard in 1903 and named by Ernest Rutherford three years later. Chemistry and optical engineering fields typically use h nu to represent photon energy combining Planck constants with frequency variables.
Modern technology exploits individual photon detection through photomultiplier tubes knocking free electrons upon striking metal plates. Semiconductor charge-coupled device chips generate charges on microscopic capacitors detectable after incident photon absorption. Geiger counters utilize ionization of gas molecules causing conductivity changes within devices. Two-photon excitation microscopy achieves higher resolution because samples absorb energy only where overlapping beams intersect significantly. This method causes less damage since lower-energy photons suffice compared to single-beam approaches. Fluorescence resonance energy transfer allows nearby systems to steal absorbed energy and re-emit different frequencies useful for molecular biology studies. Hardware random number generators send photons to beam splitters producing equal probability outcomes determining binary sequences. Researchers at Massachusetts Institute of Technology announced bound photon triplets in 2018 potentially involving polaritons. Quantum cryptography relies heavily on entangled photons enabling secure communication channels resistant to eavesdropping attempts. Scientists continue exploring nonlinear optical processes including spontaneous parametric down conversion used to produce single-photon states essential for quantum computing architectures.
Common questions
When did Albert Einstein publish his paper proposing that light consists of discrete energy quanta?
Albert Einstein published a paper proposing that light consists of discrete energy quanta in 1905. This idea challenged the dominant wave theory of James Clerk Maxwell from 1865.
Who popularized the term photon for light quanta and when was it first used?
Gilbert N. Lewis popularized the term photon for light quanta in a letter published in Nature on the 18th of December 1926. Before this moment scientists used words like quanta or energy elements derived from Latin roots meaning how much.
What is the rest mass of photons according to current physical theories?
Photons possess no electric charge and generally have zero rest mass according to current physical theories. Experimental upper limits on photon mass reach values as small as 10^-53 grams.
Which physicists unified photons with W plus W minus and Z zero bosons in electroweak interactions?
Sheldon Glashow Abdus Salam and Steven Weinberg unified photons with W plus W minus and Z zero bosons in electroweak interactions. Their work earned them the 1979 Nobel Prize in physics.
When did Arthur Compton provide experimental proof that photons carry momentum proportional to their wave number?
Arthur Compton provided experimental proof with scattering experiments in 1922. His work showed photons carried momentum proportional to their wave number.