Photon
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In physics, the photon (from Greek φοτος, meaning light) is a quantum of the electromagnetic field, for instance light. Photons were originally called "energy quanta".
In some respects a photon acts as a particle, for instance when registered by the light sensitive device in a camera. In other respects, a photon acts like a wave, as when passing the camera optics. According to the so-called wave-particle duality in quantum physics, it is natural for the photon to display either aspect of its nature, according to the circumstances. Normally, light is formed from a large number of photons. At low intensity, it requires very sensitive instruments, used in astronomy, for instance, to detect the individual photons.
The photon is one of the elementary particles, along with the electron. Together with the particles that make up nuclei, their interactions account for a great many of the features of matter, such as the existence and stability of atoms, molecules, and solids. These interactions are studied in quantum electrodynamics (QED), which is the oldest part of the Standard Model of particle physics.
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Symbol
A photon is usually given the symbol <math>\gamma<math> (gamma), although in high-energy physics this refers to a high-energy photon (a gamma ray; a photon of the immediately lower energy range is denoted <math>X<math>, an X-ray).
Properties
Photons are commonly associated with visible light, but this is actually only a very limited part of the electromagnetic spectrum. All electromagnetic radiation, from radio waves to gamma rays, is quantised as photons: that is, the smallest amount of electromagnetic radiation that can exist is one photon, whatever its wavelength, frequency, energy, or momentum. Photons are fundamental particles. Their lifetime is essentially infinite, although they can be created and destroyed. Unlike most particles, photons have essentially zero mass, which can be asserted to a high degree of accuracy, and accounts for some of their unique properties. Nevertheless, because they have energy, the theory of general relativity states that they are affected by gravity, and this is confirmed by observation.
Creation
Photons can be produced in a variety of ways, including emission from electrons as they are accelerated, or change energy state in atoms or molecules. Radio waves are emitted by accelerating electrons in the transmitter antenna, and, in a reverse manner, they get absorbed in the receiver's antenna. Sunlight, emitted by the hot plasma in the Sun's outer atmosphere (the photosphere) is absorbed in green plants by a sequence of processes known as photosynthesis, thus providing energy for the plant. Such molecular quantum processes involve individual photons.
Special devices like masers and lasers can create coherent low energy photon radiation.
More energetic photons can be created by nuclear transitions, particle-antiparticle annihilation, and in high-energy particle collisions.
Spin
Photons have spin 1 and they are therefore classified as bosons. Photons mediate the electromagnetic field. That is, they are the particles that enable other particles to interact with each other electromagnetically and with the electromagnetic field, so they are so-called gauge bosons. In general, a boson with spin 1 should be observable with three distinct spin projections (−1, 0 and 1). However, the zero projection would require a frame where the photon is at rest. Because the (rest) mass is zero, such a frame does not exist, according to the theory of relativity. So photons in empty space always travel at the nominal speed of light, and show only two spin projections, corresponding to two opposite circular polarizations. On account of the zero mass, photons are therefore always transversely polarized, in the same way as electromagnetic waves are, in empty space.
Quantum state
Visible light, from the Sun, or a lamp, is commonly a mixture of many photons of different wave-lengths. One sees this in the frequency spectrum, for instance by passing the light through a prism. In so-called "mixed states", which these sources tend to produce, light can consist of photons in thermal equilibrium (so-called black-body radiation). Here they in many ways resemble a gas of particles. For example, they exert pressure, known as radiation pressure, which (in part) accounts for the appearance of comets as they travel close by the Sun.
On the other hand, an assembly of photons can also exist in much more well-organized states. For instance, in so-called coherent states, describing coherent light such as emitted by an ideal laser. The high degree of precision obtained with laser instruments is due to this organization.
The quantum state of a photon assembly, like that of other quantum particles, is the so-called Fock state denoted <math>|n\rangle<math>, meaning <math>n<math> photons in one of the distinct "modes" of the electromagnetic field. If the field is multimode (involves several different wavelength photons), its quantum state is a tensor product of photon states, for example:
- <math>|n_{k_0}\rangle\otimes|n_{k_1}\rangle\otimes\dots\otimes|n_{k_n}\rangle\dots<math>
Here <math>k_i<math> denote the possible modes, and <math>n_{k_i}<math> the number of photons in each mode.
Molecular absorption
A typical molecule, <math>M<math>, has many different energy levels. When a molecule absorbs a photon, its energy is increased by an amount equal to the energy of the photon. The molecule then enters an excited state, <math>M^* \,<math>.
- <math>M + \gamma \to M^* \,<math>
Photons in vacuo
In a vacuum, empty space, all photons move at the nominal speed of light, c, defined as equal to 299,792,458 metres per second, or approximately 3×108 m s−1. The metre is defined as the distance travelled by light in a vacuum in 1/299,792,458 of a second, so the speed of light does not suffer any experimental uncertainty, unlike the metre or the second, which rely on the second being defined by means of a very accurate clock.
When photons pass through matter, such as a prism, different wavelengths travel at different speeds. This is what causes the color dispersion, where different photons exit at different angles.
The associated dispersion relation for photons is a relation between frequency, f, and wavelength, λ. Or, equivalently, between their energy, E, and momentum, p. It is simple in vacuum, the speed of the wave, v, is given by
- <math>v=\lambda f = c\,<math>
The photon quantum relations are:
- <math>E=hf \,<math> and <math>p=h/\lambda \,<math>
Here h is Planck's constant. So one can also write the dispersion relation as
- <math>E=pc \,<math>
which is characteristic of a zero-mass particle. One sees how remarkably Planck's constant relates the wave and particle aspects.
Photons in media
In a material, photons couple to the excitations of the medium and behave differently. These excitations can often be described as quasi-particles (such as phonons and excitons); that is, as quantized wave- or particle-like entities propagating though the matter. "Coupling" means here that photons can transform into these excitations (that is, the photon gets absorbed and medium excited, involving the creation of a quasi-particle) and vice versa (the quasi-particle transforms back into a photon, or the medium relaxes by re-emitting the energy as a photon). However, as these transformations are only possibilities, they are not bound to happen and what actually propagates through the medium is a polariton; that is, a quantum-mechanical superposition of the energy quantum being a photon and of it being one of the quasi-particle matter excitations.
According to the rules of quantum mechanics, a measurement (here: just observing what happens to the polariton) breaks this superposition; that is, the quantum either gets absorbed in the medium and stays there (likely to happen in opaque media) or it re-emerges as photon from the surface into space (likely to happen in transparent media).
Matter excitations have a non-linear dispersion relation; that is, their momentum is not proportional to their energy. Hence, these particles propagate slower than the vacuum speed of light. (The propagation speed is the derivative of the dispersion relation with respect to momentum.) This is the formal reason why light is slower in media (such as glass) than in vacuum. (The reason for diffraction can be deduced from this by Huygens' principle.) Another way of phrasing it is to say that the photon, by being blended with the matter excitation to form a polariton, acquires an effective mass, which means that it cannot travel at c, the speed of light in a vacuum.
See also
- Particle physics
- Photonics
- Optics
- Spectroscopy
- Photoelectric effect
- Compton scattering
- Bremsstrahlung
- Cerenkov radiation
- List of particles
Derived meanings
- For the Japanese anime video, see Photon (anime).
External links
- James Schombert's "Photon self-identity problems (http://zebu.uoregon.edu/%7Ejs/glossary/wave_particle.gif)" scan (humor)
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| Fermions : Quarks | Leptons | |
| Gauge bosons : Photon | W+, W- and Z0 bosons | Gluons | |
| Not yet observed: | |
| Higgs boson | Graviton | |
| Supersymmetric Partners : Neutralinos | Charginos | Gravitino | Gluinos | Squarks | Sleptons | |
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