WHAT IS A PHOTON?

kieronconway
Apr 15
6 min read

Updated: 3 days ago

A Brief History

The classical wave theory of electromagnetic (EM) radiation indicates that white light can be thought of as rays of coloured light that create white when added together.

Classical light waves consist of electric and magnetic oscillations at right angles to each other in the EM field, propagating through space at a right angle to the oscillations.

However, in the early part of the twentieth century, classical wave theory could not explain the photoelectric effect.

So, Albert Einstein (1879 – 1955) came up with the idea of a particle-like quantum-packet of EM energy to explain the effect where light can “kick” electrons out of atoms to be gathered up in electric currents.

Later someone else christened Einstein's quantum of energy as the photon.

The Wave-Particle Duality of light

The fundamental outcome of 'is light a wave or a particle'? became known as wave-particle duality, one of the central ideas of quantum physics.

So, scientists were faced with two separate descriptions.

Light as a Wave

Classical light waves explain;

Interference (two light waves superimpose, creating alternating patterns of light and dark fringes as in the famous double slit experiment).
Reflection (light waves bouncing off surfaces)
Refraction (bending of light traveling from one medium to another).
Diffraction (the spreading of light when passing through a gap).
Rainbows!

Classical light is clearly a wave in the electromagnetic field, with a frequency, wavelength and constant velocity in space, where c = fλ, described by the equations of James Clerk Maxwell (1831 – 1879).

Light as a particle

Quantum theory explains;

Light transfers energy to electrons and can give them sufficient energy to free them from atomic orbitals in the photoelectric effect.
Einstein proposed that light energy comes in discrete quanta, each with energy E = hf, where “h” is Planck's constant.
Frequency of light must be above a threshold frequency before electrons are ejected from atoms.
Photons are “particle-like” rather than classical “wave-like” entities.

The outcome

Light is both a wave and a particle OR it's neither, which confused the general layperson enormously as well as many scientists.

The basic fact is that EM excitations in the EM field cannot be explained in normal every-day terms.

So, How Should We Think About Light?

In certain situations, it's best to think about light as waves. Some examples are;

Light as Waves

Optics is a whole branch of physics that allows us to develop and explain things like microscopes and telescopes, using refraction and reflection.
How rainbows are formed and how white light is split into its component colours when passed through a prism are best explained using optics.
Waves best describe the polarisation of light.
Large scale EM waves explain radio and TV broadcasting and receiving as well as radar systems.
The design of radio/radar antennae is best achieved using waves.

Light as Photons

In some circumstances, it's best to think of light being made up of streams of photons.

How electrons are boosted out of atomic orbitals is associated with how photons transfer energy to electrons using quantum field theory (QFT).
How light is produced when electrons drop from one energy level to another in atoms is explained by the production of photons.
Photons explain how hot objects emit light.
Photons explain how light is scattered by electrons.
Photons mediate the EM force.
Lasers are explained by the stimulated emission of photons.

In modern physics, a photon is the quantum excitation of the EM field and the carrier of the EM force

Using Both Waves and Photons

The best example of this is in explaining the double slit experiment.

1) Wave physics is used to describe the interference pattern created by shining light on two narrow slits separated by a short distance. This experiment proved that light was a wave in the early 1800s.

2) Photon physics is used to describe how sending single photons through the two slits, one at a time, with a delay between each, still produces an interference pattern but as a cumulative effect over a period of time. This was first achieved in 1909 using extremely low intensity light.

The dual nature of electromagnetic waves allows phenomena to be explained using waves or photons as appropriate.

Is a photon a particle?

Not exactly!

A photon is the quantum of the electromagnetic field, the smallest, discrete excitation of the field.

It's neither a particle, nor a wave.

1) When you measure a photon's energy or momentum, it acts like a particle.

2) When a photon is travelling, it behaves like a component of a composite wave.

Each photon is associated with a particular frequency, though in many real situations, light is made up of a spread of frequencies resulting in a mixture of many photons each of a slightly different energy.

The image at the beginning of this article attempts to show how light from a torch can be visualised as a steady stream of wave-fronts, or a steady stream of discrete photons.

1) In wave theory, a colour is produced by a wave of fixed frequency and wavelength, where c = f λ.

2) In quantum theory, a photon has an energy E = hf and travels at c as a localised wave-packet.

Does a Photon have an Electric and a Magnetic Field?

An individual photon is the smallest quantised excitation in the EM field, just like the excitation in a particle field.

A single photon does not correspond to a classical, oscillating, electric and magnetic field. Instead, classical EM waves emerge from the average behaviour of photons.

This is analogous to the creation of sound waves made up from the average behaviour of individual vibrating air molecules.

The Wave Packet Description of a Photon

In classical physics, a light ray of a single frequency and therefore colour, extends across all of space and propagates for ever until it interacts with something.

A photon is a localised effect that can be considered as a short duration burst of EM energy moving through space until it interacts with something.

A photon does not produce classical electric and magnetic field oscillations but its motion through space does.

Like a particle, a photon has energy, momentum and angular momentum (spin), but unlike a particle, it has no mass.

How can single Photons Produce Interference

In the double slit experiment, firing single photons at the two slits still provides an interference pattern after a large number of individual photons have passed through the slits, with a clear interval in time between photons.

A photon has a quantum wave-function, which determines the probability of detection and effectively describes it as passing through both slits.

Each photon can be said to interfere with itself through its probability amplitude.

The Probability wave-function of a Photon

Photons are described with probability wave-functions as depicted in the following diagram.

This probability wave-function is a mathematical entity that tells us where the photon is most likely to be.

When a photon's position is measured, the mathematical wave-function can be said to collapse and a measured position results.

The square of the amplitude at any point gives the probability of finding the photon at that point.

Unlike electrons, photons cannot be described by a simple position wave-function.

Because they always move at the speed of light and are excitations of the EM field, their behaviour is described using quantum field theory rather than a standard particle wave-function.

Here is a depiction of a single photon in motion where the waves represent the oscillating EM fields and the bulges represent the probability distribution.

The two images above, represent visual interpretations of a photon's probability wave-function at a point in space and how moving photons re-create the field components of classical wave theory.

Virtual Photons

In quantum field theory, the electromagnetic field is never completely quiet. Even in empty space it exhibits small fluctuations due to the Heisenberg uncertainty principle.

These fluctuations can be described mathematically using “virtual photons”, which appear in calculations of electromagnetic interactions.

Virtual photons are not directly observable particles but rather bookkeeping tools that help physicists describe how charged particles interact.

The Big Picture of the EM field

From its backround behaviour, to the passage of classical light waves or quantum photons:

Background Behaviour of EM field.

The spontaneous, low-level, short-lived excitations of the EM field that we call virtual photons and constitute quantum foam, are derived from borrowed vacuum energy.

Light as Classical Waves

Light waves moving through the EM field at constant velocity c, consist of electric and magnetic field components, oscillating at right angles to each other and the direction of propagation.

Light as Quantum Photons

Discrete, localised quanta of energy, described by mathematical wave-functions indicating the probability of finding a photon.

Only when in motion, at “c” does the photon recreate the electric and magnetic components of EM waves.

The Final Takeaway

When there are a large number of light photons moving through the EM field, their probabilities and phases combine and this collective behaviour makes them appear like traditional light waves.

Photons are among the strangest entities in physics.

Photons behave like particles when detected, like waves when propagating and yet in modern physics they are best understood as tiny, quantised excitations of the EM field filling all of space.

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