We’re photons.

What are photons?

We are discrete excitations of the electromagnetic field. You may know us as light. This is an acceptable abbreviation. We also operate in radio, microwave, infrared, ultraviolet, X-ray, and gamma-ray frequencies. The visible department receives most of the publicity despite occupying a rather small portion of the spectrum. We have raised this internally. There is no internal.

Under the Standard Model, we have zero rest mass, zero electric charge, and spin one. Spin is intrinsic angular momentum; we are not tiny balls rotating on tiny office chairs. Free photons have two independent polarization states. In a circular-polarization basis, our helicities are +1 and −1. These are physical properties, not personality types. Please do not arrange a workshop.

We are bosons. Multiple photons may occupy the same quantum state, an arrangement forbidden to identical fermions. Electrons have rules about this. We can all turn up in the same mode wearing the same frequency and polarization. Nobody has to go home and change. This makes us useful in lasers and difficult to organize with a seating chart.

In vacuum we travel at exactly 299,792,458 metres per second. This is not an aspirational target. It is the defined value of c. We do not accelerate up to it, and a free photon cannot pull over for a moment. There is no valid inertial reference frame in which we are at rest. Statements beginning “from a photon’s point of view” should therefore be returned to the sender. We have no approved point of view.

Our energy is E = hf, where f is frequency and h is Planck’s constant, exactly 6.62607015 × 10⁻³⁴ joule seconds. In vacuum, our momentum has magnitude p = E/c = h/λ. Having no rest mass does not excuse us from carrying momentum. We can exert radiation pressure. The force from ordinary room lighting is small, but we would still like it entered in the minutes.

Blue photons carry more energy per photon than red photons. Increasing a beam’s brightness at a fixed frequency generally means increasing the photon flux, not promoting individual photons to a higher pay grade. Frequency, wavelength, intensity, and your monitor’s brightness slider are related concerns with separate paperwork. Turning everything up is not spectroscopy.

Our propagation is described by quantum amplitudes. When alternative paths remain coherent and indistinguishable, their amplitudes add; probabilities come from the squared magnitude of the total. This produces interference. Send us through a suitable double-slit apparatus one at a time and an interference pattern can accumulate from individual detection events. We are individually punctual and collectively awkward to explain at dinner.

If an interaction leaves a distinguishable record of which path was taken, the corresponding interference is lost. A conscious person does not need to read the record. A detector will do. An unattended detector will also do. Your attention is appreciated by the website but is not a required component of quantum mechanics. Please stop staring at the apparatus in a meaningful way.

In glass, the electromagnetic field interacts with the material’s charges. Their collective response changes how the wave propagates; its phase velocity is c/n, where n is the refractive index. This is not adequately described as tiny balls repeatedly stopping at atoms for directions. Frequency-dependent refraction is dispersion. A prism sorts the visible department by frequency. Nobody enjoys the reorganization, but the diagrams are excellent.

We can be emitted, scattered, and absorbed. Emission accompanies processes such as transitions between atomic energy levels. Absorption transfers our energy to matter and ends that photon’s participation. A digital camera exploits this: absorbed photons can generate charge carriers in its sensor, which are collected and measured. The photograph is a record of interactions. We are not stored alive inside the JPEG. There is no ventilation requirement.

Counting us has statistical consequences. For independent arrivals described by Poisson statistics, an expected count N has a standard deviation of √N. Collecting four times as many photons doubles the shot-noise-limited signal-to-noise ratio. This is one reason a long exposure can help. It is also why demanding cleaner shadows from an almost empty bucket of light is, administratively speaking, a staffing problem. Other noise sources may attend the meeting.

Photon Library catalogs selected results of these activities. Mountains, aircraft, coastlines, and stars provide the subject matter. We provide the electromagnetic involvement. What reaches your eyes from a screen is a new population of photons representing the recorded image, not the original arrivals forwarded through the internet. The originals completed their assignment at the sensor. We hope their replacements meet your expectations.

Access additional catalogs through this website.

Technical references: CERN, NIST, quantum behavior, refraction, image noise. Administrative claims have not been peer reviewed.