KAHIBARO
Table of Contents
Messenger of the electromagnetic interaction
In the Standard Model, the photon is the gauge boson of electromagnetism. It is the particle associated with the electromagnetic field, and it carries electromagnetic energy and momentum. Light is made of photons, but photons are also involved in all electromagnetic processes, not only visible light.
A useful way to think about the photon is that it is the quantum, or smallest packet, of electromagnetic radiation. Radio waves, microwaves, infrared light, visible light, ultraviolet light, X rays, and gamma rays are all made of photons. These differ by frequency, wavelength, and energy, not by being different kinds of photon.
The photon is the gauge boson of the electromagnetic interaction.
All electromagnetic radiation is made of photons.
Different parts of the electromagnetic spectrum correspond to photons with different energies and frequencies.
Basic properties
The photon has several important properties that make it special among the particles of the Standard Model.
| Property | Photon |
|---|---|
| Symbol | $\gamma$ |
| Type | Gauge boson |
| Interaction | Electromagnetic |
| Electric charge | $0$ |
| Rest mass | $0$ |
| Spin | $1$ |
| Travels in vacuum at | $c$ |
Because the photon has zero rest mass, it always moves at the speed of light in vacuum, $c$. Its lack of electric charge means that it is not directly deflected by electric or magnetic fields in the simple way charged particles are.
The photon has spin 1, so it is a boson. As a boson, it belongs to the family of force carriers in the Standard Model.
Important photon properties:
$Q = 0$, photon is electrically neutral.
$m_0 = 0$, photon has zero rest mass.
Photon spin is $1$.
A photon in vacuum moves at speed $c$.
Photon energy and momentum
Even though the photon has no rest mass, it still carries energy and momentum. This is one of the central ideas of quantum physics.
The energy of a photon is related to its frequency by
$$
E = hf
$$
where $h$ is Planck's constant and $f$ is the frequency.
Since the wave speed in vacuum is $c = f\lambda$, this can also be written as
$$
E = \frac{hc}{\lambda}
$$
where $\lambda$ is the wavelength.
The momentum of a photon is
$$
p = \frac{E}{c} = \frac{h}{\lambda}
$$
So a higher-frequency photon has higher energy and higher momentum. A shorter-wavelength photon also has higher energy.
| Quantity | Formula |
|---|---|
| Energy | $E = hf$ |
| Energy from wavelength | $E = \frac{hc}{\lambda}$ |
| Momentum | $p = \frac{E}{c} = \frac{h}{\lambda}$ |
Key photon formulas:
$$
E = hf
$$
$$
E = \frac{hc}{\lambda}
$$
$$
p = \frac{E}{c} = \frac{h}{\lambda}
$$
A photon has momentum even though its rest mass is zero.
Photon as the carrier of electromagnetic force
In the Standard Model, forces are described through exchange particles. For electromagnetism, that exchange particle is the photon.
When two electrically charged particles interact, such as two electrons repelling each other, the interaction can be described in quantum field theory as the exchange of photons. In many ordinary electromagnetic effects, these exchanged photons are not directly observed as light. They are called virtual photons.
This idea explains why the same particle, the photon, is connected both to visible light and to electric and magnetic interactions between charged particles.
There is an important distinction between real photons and virtual photons. Real photons can travel freely and be detected as radiation. Virtual photons are internal parts of the interaction process and are not observed directly as separate particles.
Photon coupling
Photons interact with particles that have electric charge. Electrons, protons, muons, and quarks all interact electromagnetically because they carry electric charge. Neutrinos, which have no electric charge, do not couple directly to photons in the simplest Standard Model picture.
The strength of the electromagnetic interaction is characterized by the fine-structure constant, usually written as $\alpha$, with approximate value
$$
\alpha \approx \frac{1}{137}
$$
This number appears in many calculations involving photons and charged particles.
Photons couple directly to electric charge.
Charged particles emit, absorb, and exchange photons.
Neutral particles with no electric charge do not directly couple to photons in the simplest Standard Model description.
Massless nature and range of the force
A very important consequence of the photon's zero rest mass is that the electromagnetic force has infinite range. This means electromagnetic effects can act over very large distances.
This is different from the weak interaction, whose carriers, the $W^\pm$ and $Z^0$ bosons, are very massive. Because of that mass, the weak force acts only over a very short range.
So the massless photon is the reason electromagnetism can reach across atoms, circuits, planets, and even astronomical distances.
Polarization states
Although the photon has spin 1, a massless spin 1 particle has only two physical polarization states in free space. These correspond to two independent transverse directions of oscillation of the electromagnetic wave.
For a light wave traveling in one direction, the electric and magnetic fields oscillate in directions perpendicular to the direction of travel. This is why electromagnetic waves are transverse waves.
This wave picture is useful, but the photon is not just a tiny classical wave. It is a quantum object that shows both wave-like and particle-like behavior.
Photon in the Standard Model framework
The photon arises from the gauge symmetry underlying electromagnetism. In the full electroweak theory, the photon is related to a mixing of gauge fields. After spontaneous symmetry breaking, one combination becomes the massless photon, while the other combinations produce the massive $W^\pm$ and $Z^0$ bosons.
For beginners, the main result to remember is simple. The photon is the surviving massless gauge boson associated with the electromagnetic interaction.
Within the electroweak theory, the observable photon is massless and mediates electromagnetism.
Its zero mass is tied to the long range of the electromagnetic force.
Emission and absorption
Charged particles can emit photons when they lose energy, and they can absorb photons when they gain energy. This appears in many physical processes, such as atomic transitions, radiation from accelerating charges, and scattering.
If an atom changes from a higher energy state to a lower one, it can emit a photon with energy
$$
E_\gamma = \Delta E
$$
where $\Delta E$ is the energy difference between the two states.
If the atom absorbs a photon, the photon energy must match the required energy difference.
This simple rule explains why atoms produce and absorb specific spectral lines.
Photon compared with other gauge bosons
The Standard Model contains several gauge bosons. Comparing them helps show what is special about the photon.
| Gauge boson | Interaction | Electric charge | Rest mass |
|---|---|---|---|
| Photon, $\gamma$ | Electromagnetic | $0$ | $0$ |
| Gluon, $g$ | Strong | $0$ | $0$ |
| $W^+$ | Weak | $+1$ | large |
| $W^-$ | Weak | $-1$ | large |
| $Z^0$ | Weak | $0$ | large |
The photon and gluon are both massless gauge bosons, but they play different roles. The photon mediates electromagnetism between electrically charged particles. The gluon mediates the strong interaction between particles with color charge. The photon does not carry electric charge, while gluons carry color charge and can interact among themselves in a more complicated way.
A simple picture of photon exchange
A simple sketch can help visualize the idea of electromagnetic interaction through photon exchange.
This is only a symbolic diagram, not a literal picture of tiny balls being thrown back and forth. It represents how quantum theory describes the interaction.
Why the photon matters
The photon is central to both modern physics and everyday life. It explains light, radio communication, lasers, atomic spectra, and the electromagnetic forces that shape matter. In the Standard Model, it is one of the clearest examples of a force carrier particle.
For beginners, the most important points are that the photon is a massless, neutral spin 1 boson, that it carries electromagnetic energy and momentum, and that it mediates the electromagnetic interaction between charged particles.
Summary of the photon:
It is the gauge boson of electromagnetism.
It has zero electric charge and zero rest mass.
It carries energy and momentum:
$$
E = hf, \qquad p = \frac{h}{\lambda}
$$
It mediates electromagnetic interactions between charged particles.
Its zero mass gives electromagnetism infinite range.