How do photons move

In physics one describes with photon (from Greek φως, phos = light) the elementary excitation (quantum) of the quantized electromagnetic field. It is one of the subjects of study of quantum electrodynamics, the oldest part of the Standard Model of particle physics. In clear terms, photons are the “building blocks” of electromagnetic radiation, something like “light particles”. However, it should not be forgotten that all moving (elementary) particles including photons also have wave properties, this is called wave-particle dualism.


The symbol γ (gamma) is generally used for the photon. In high-energy physics, however, this symbol is reserved for the high-energy photons of gamma radiation (gamma quanta) and the X-ray photons that are also relevant in this branch of physics receive the symbol X (from English: X-ray). Very often a photon is also due to the energy it contains or shown. Are in it Planck's quantum of action and the (light) frequency, in the second notation and the angular frequency.


All electromagnetic radiation, from radio waves to gamma radiation, is quantized into photons. This means that the smallest amount of electromagnetic radiation of any frequency is a photon. Photons have an infinite natural lifespan, but can be created or destroyed in a variety of physical processes. The experimentally determined and accepted upper mass limit is around 10-47 kg (source: Particle Data Group). A free photon is never at rest, but always moves at the speed of light. It follows from this that it cannot have any rest mass. In optical media, the effective speed of light is reduced compared to the speed of light in a vacuum due to the interaction of the photons with matter. Since photons have energy, they interact with gravity according to the general theory of relativity.

Generation and detection

Photons can be generated in many ways, in particular through transitions (“quantum leaps”) of electrons between different states (e.g. different atomic or molecular orbitals or energy bands in a solid). Photons can also be generated in nuclear transitions, particle-antiparticle annihilation reactions (annihilation) or any fluctuations in an electromagnetic field.

For the detection of photon currents z. B. photomultiplier, photoconductor or photodiodes can be used. CCDs, vidicons, PSDs, quadrant diodes or photo plates and films are used for the spatially resolving detection of photons. Bolometers are also used in the IR range. Photons in the gamma ray range can be individually detected by Geiger counters. Photomultipliers and avalanche photodiodes can also be used for single photon detection in the optical range, whereby photomultipliers generally have the lower dark counting rate, but avalanche photodiodes can still be used with lower photon energies up to the IR range.


The Rest mass of a photon is assumed to be zero. This results on the one hand from the type of electromagnetic potential and on the other hand from the speed of light with which photons travel in a vacuum.

  • If photons had mass, the electrical potential would not be a Coulomb potential, but a Yukawa potential. The potential of an electrical charge would therefore be weakened with an additional exponential damping term. Furthermore, a photon mass would change the behavior of magnetic fields[1]. Such deviations could not be proven experimentally so far, from which the current (status 2007) existing upper limits for the photon mass result[2].
  • The special theory of relativity not only forbids reaching the speed of light c for every object whose rest mass m0 is greater than zero, but also provides direct mathematical evidence that massless particles move at the speed of light.

The relativistic total energy (Hamilton function) of a particle is

In the Hamilton formalism, the velocity is the derivative of the Hamilton function according to the generalized momentum

where v = c was set here for photons. However, this equation is only true if m0 = 0 is.

However, a photon can be assigned a relativistic mass, since it is always for the energy of a photon

applies. About the relationship between energy and mass m = E./c² follows

This mass can be interpreted as a possibility to create massive particles. A photon with a corresponding minimum energy can e.g. B. when interacting with an atomic nucleus generate a particle-antiparticle pair via pair generation.


Photons are spin-1 particles and therefore bosons. Any number of photons can occupy the same quantum mechanical state, which is implemented in a laser, for example. Photons mediate electromagnetic interaction: They are the particles that allow other particles to interact electromagnetically with one another. Since the electromagnetic interaction is a so-called gauge theory, the photons are among the gauge bosons.

Photons in a vacuum

In a vacuum, photons move at the speed of light in a vacuum c = 299792458 ms−1. The dispersion relation, i.e. H. the dependence of energy of frequency ν (ny), is linear, and the constant of proportionality is Planck's constantH,

Numerical values, as they typically occur in optical spectra, can be determined as follows:

 ,   E. where in eV (electron volts), ω in s-1, 1 eV corresponds approximately to a ω of 1.520 · 1015 s-1
 ,   E. where in eV (electron volts), λ in μm, 1 eV corresponds to about 1.240 μm = 1240 nm

The impulse p of a photon is thus

Photons in media

In a material, photons interact with the medium surrounding them, resulting in changed properties. The photon can be absorbed, whereby its energy does not disappear, of course, but changes into elementary excitations (quasiparticles) of the medium such as phonons or excitons. It is also possible that it propagates through a medium, for example as a coupled phonon-photon pair (polariton). These elementary excitations in matter usually have no linear dispersion relation, and their speed of propagation is lower than the speed of light in a vacuum, down to only a few meters per second for special materials.

Interaction of photons with matter

Photons that hit matter trigger different processes at certain energies. In the following, the energy ranges in which they are relevant are specified for various processes:

These effects make a significant contribution to the fact that this radiation can be detected and that certain substances with certain effects can be detected using gamma spectroscopy.

Research history

Since ancient times there have been different, often contradicting, ideas about the nature of light. In the 19th century, wave and particle theories competed. While many phenomena, such as interference and polarization phenomena, spoke in favor of a wave nature of light, there were also indications of a particle character. A historically very important experiment, which pointed to the particle nature of light, was the observation of the photoelectric effect by Heinrich Hertz and Wilhelm Hallwachs in 1887.

The quantization of electromagnetic radiation goes back to Max Planck's explanation of black body radiation in 1900 (Planck's law of radiation). Planck himself did not envision the electromagnetic radiation as being quantized, but explained the quantization by saying that the oscillators in the walls of the black body resonators can only exchange discrete amounts of energy with the electromagnetic field.

In 1905, Albert Einstein described light in his publication on the photoelectric effect as consisting of light quanta with particle properties (for this work he was awarded the Nobel Prize in 1921). The formal quantum theory of light was only developed in 1925, starting with the work of Max Born, Pascual Jordan and Werner Heisenberg. The theory of electromagnetic radiation, which is still valid today and which also describes light quanta, quantum electrodynamics (QED), goes back to a work by Paul Dirac in 1927 in which he describes the interaction of quantized electromagnetic radiation with an atom. The QED was developed in the 1940s and awarded the Nobel Prize in Physics to Richard P. Feynman, Julian Schwinger and Shinichiro Tomonaga in 1965.

The term photon was coined in 1926 by the chemist Gilbert Newton Lewis, who did not understand the light quantum by it. He used the term in the context of a proposed (and generally not accepted) model of the interaction of atoms with light.


  • Chandrasekhar Roychoudhuri, Rajarshi Roy: The nature of light: What is a photon? In: Optics and Photonics News 14 (2003), No. 10 SUPPL., Pp. 49-82 [1], ISSN 1047-6938
  • Harry Paul: Photons: An Introduction to Quantum Optics. 2nd edition Stuttgart: Teubner, 1999 (Teubner-StudienbücherPhysik). - ISBN 3-519-13222-2
  • Klaus Hentschel: Einstein and the light quantum hypothesis. Naturwissenschaftliche Rundschau 58 (6), pp. 311-319 (2005). - ISSN 0028-1050
  • Liang-Cheng Tu, Jun Luo, George T. Gillies: The mass of the photon. In: Reports on Progress in Physics 68 (2005), No. 1, pp. 77-130. - doi: 10.1088 / 0034-4885 / 68/1 / R02
  • HydrogenLab 3D animations of atomic transitions: absorption and emission of photons (semiclassical)

Individual evidence

  2. Particle Data Group, Properties of the Photon PDF

Categories: Elementary Particles | Quantum physics