# What is Light?

## Understanding the Nature of Light

“**Light**” is a particle (photon), that acts like a wave, and is understood as **a localized vibration in the electromagnetic field**. Or, more specifically, light is **a wave function of probable locations of excited states in the electromagnetic field**.^{[1]}

So light AKA electromagnetic energy AKA photons is a wave, that is a charged quantum field, in which we can measure particle like behavior, and thus light is “a particle and a wave”.

Light has a dualistic particle wave like-nature (like all “quanta” in the standard model), it is pure electromagnetic energy (“photons”), and it is one of four forces (AKA quantum interactions) in the physical universe. Light exists as a charged energy field, and it exists in a quantum state (it is a “quantum particle,” with quantum behavior, just like the other quanta studied in quantum physics).

Light has a charge, but has no mass, yet it can interact with other objects and add to their relative mass by adding to an objects total energy.

Light is both electricity and magnetism in constant oscillation.

Different wave lengths can be seen as different colors in the “electromagnetic spectrum“, can carry information as radio waves, or carry information as wifi. Light can cook an egg on the sidewalk by speeding up the molecules in an egg, reflecting off the sidewalk based on the laws of quantum probability, reflection, and refraction. Light can be trapped in a crystal, and when anything emits heat or light (like fire), it is emitting photons AKA electromagnetic energy AKA light we call “electromagnetic radiation”.

Light behaves like a transverse wave, cresting, canceling other light waves, amplifying, its charged field gaining or losing energy based on its interactions. Yet, light will travel in a single direction forever if unimpeded. Many photons can exist in the same exact spot creating “wave packets”, or just one photon can exist alone in its lowest energy state (still exhibiting a quantum wave-particle nature).

Light, it has lots of properties, and is in many respects the basis of our universe. We call it by many names, but it is always the same core thing, “photons”, a charged part of the electromagnetic field.

These simple(-ish) sentences pertaining to the dual wave-particle quantum nature of light have deep meaning, and we’ve only just scratched the surface.

We explain more about light and its mind-bending properties below.

What Is Light? Or, a less complicated question, “what isn’t light?”

**TIP**: Below we define light according to quantum field theory, the standard model, and the Copenhagen interpretation. These aren’t the only theories (models) for understanding light and other quanta, but these are the most widely accepted theories.

### The Photon, the Carrier Particle of Electromagnetic Energy

- The photon is the carrier particle of all electromagnetic energy, and one of the four fundamental forces of the standard model of particle physics (the quantum physics model that describes how the building blocks of the universe, the elementary particles, interact with each other).
- The photon is massless and is one of only two massless particles (the other being the gluon). These particles have momentum, but no rest-mass (it is worth noting here specifically that photons always travel light speed unimpeded and never truly rest; they are in a sense “pure motion”).
- The term photon has been around for decades, and it stuck due to simplicity. Even though this terminology is correct, photons aren’t “just particles.” They are a measurable excited state, or localized vibration, in the electromagnetic field that travels in a wave-like motion (quantum field theory).
- Each wavelength/particle typically consists of a packet of waves forming an aggregate wavelength, at its core, a single wavelength in a packet can still be described as a photon, but can also described as a one dimensional vibrating string (superstring theory). Newton called a single “light” particle a corpuscle.

Light waves, visible and invisible – Lucianne Walkowicz.

Light Is Waves: Crash Course Physics #39.

### Light Speed and the Momentum of a Photon

- The wave-like pattern it travels in aside, the photon only travels in a single direction only. It has “unidirectional” linear momentum.
- The photon travels at a single and constant speed called light speed (in a perfect vacuum). Light speed is one of the universal constants and is one of the only non-relative things in the universe. As Einstein noted, because light speed is constant time and space are relative (but spacetime, a composite measure of space and time, isn’t).
^{[2]}^{[3]} - The photon can travel over an infinite distance unless impeded.
- Light doesn’t “slow down” under normal conditions (it doesn’t change linear momentum in free space), but it can change energy content (slowing its frequency of vibration and cooling down, or increasing frequency and heating up), bounce off objects, and be generally manipulated in strange ways within these criteria (see here and here).
- The photon also has spin (angular momentum). It, like other force carrier particles, has a spin of “1”. Matter particles (fermions) have a spin of “+ or – 1/2”.
- Since a photon has a spin of “1” it doesn’t have to adhere to the Pauli Exclusion principle and thus more than one photon can be in the same place and the same time. Lasers are made from condensing many photons into one spot, this increases the energy content of the packet of photons, and results in a really “hot” laser.

**FACT**: Photons can’t interact with each other directly (outside of forming wave packets and absorbing and emitting other photons) and have no rest-mass when traveling through empty space, yet in special circumstances they can exist as **Photonic matter**. Photonic matter phenomenon where photons interacting with a gas develop apparent mass, and can interact with each other, even forming photonic “molecules.”

The Speed of Light is NOT About Light | Space Time | PBS Digital Studios.

### Electromagnetic Radiation and the Electromagnetic Spectrum

- A photon can emit other photons and can absorb other photons (within limits), this changes the photon’s energy content and frequency.
- Other quantum particles can also absorb and emit photons; this is what happens when electrons change energy states.
- Electromagnetic radiation describes emitted photons, like the ones that we call visible light (although not every emitted photon is visible).
- Visible light is a specific range of frequencies in the electromagnetic spectrum (precise frequencies of electromagnetic radiation that the human eye can see).
- The electromagnetic spectrum describes all the possible frequencies of vibration in the electromagnetic field, the visible spectrum describes the wavelengths we can see with the naked eye.
- Different “size” waves have different properties, and each color of light has a unique range of vibrations.
- White light describes multiple colors of light and can be separated into different colors of light (like Newton did with a prism and wrote about in his book
*Opticks*). - The energy content of a photon is determined by its frequency times Max Planck’s constant (which represents the universe’s smallest unit). The equation is simply
*E*=*h**f*. - Despite the Planck representing the smallest unit, the minimum wavelength of a photon is zero and there is no maximum. Despite the quantizing nature of quanta, there is, in one sense, no minimum or maximum energy content of a photon. The rule is that it must be a positive integer greater than zero (it must have a frequency greater than zero).
- Frequency is a way to measure waves and vibrations by examining their patterns. The wavelength is the distance between successive crests of a wave. The higher the frequency, the shorter the distance between crests.
- Only certain frequencies of electromagnetic radiation produce visible wavelengths. Short waves are toward blue tones, including ultraviolet, which bees can see, and X-rays. Long waves are toward red and include infrared, which the pit viper can see, and radio waves.

NASA – Tour of the Electromagnetic Spectrum

### Light, Both a Particle and a Wave

- The photon’s wave and particle qualities are two observable aspects of a single phenomenon. In 2015 the long-standing theory that light is both a particle and a wave was confirmed and published in
*Nature Communications*. - The photon doesn’t just travel in “a wave,” it travels as a transverse wave specifically. Transverse describes the direction the wave moves in (wave vibrating at right angles to the direction of its propagation).
- Electromagnetic energy is a quantum wave, and it doesn’t need to travel through a medium. It is not a mechanical wave like gravity or sound (that travels through a medium).
- All waves transfer energy; electromagnetic waves are no different.
- Electromagnetic energy oscillates in a “near field” of electricity and magnetism. The term “far field” describes the potentially infinite field in which the photon travels via linear momentum (forward movement).

The Quantum Experiment that Broke Reality (a video about the double slit and wave-particle duality) | Space Time | PBS Digital Studios. Light, it really is a particle and a wave… but that isn’t the weird part. It is the quantizing, elusive, and probable nature of light that is really fascinating. See our page on the observer effect to dive into one of the unknown aspects of light.

### The Quantum Nature of Light, Quantum Fields, and the Photon

- As noted above, the photon is a quantum particle or a quanta. “Quantum” describes the fact that photons jump to discrete states in the electromagnetic field. Instead of moving in a continuous wave, it “quantizes” to probable locations.
- All quantum particles, including the photon, exist as localized vibrating states in their respective field and move as quantized transverse waves. This behavior is explained by Quantum Field theory (QFT) and this behavior is where quantum physics gets its name.
- The electromagnetic field is a single field that covers all of space and time, like a container for electromagnetic energy. Each particle of the standard model of particle physics has its own field.
- When the electromagnetic field contains a localized vibration, we call it a photon.
- Each particle has a separate field. Particle interactions occur when charged excited states in fields overlap.

Quantum Field Theory by Fermilab.

### Probability, Uncertainty, and Reflection

- The quantum behavior of light is described by laws of probability, uncertainty (of position and speed simultaneously), and quantum mechanics.
- When photons reflect off a surface, they reflect based on probability. For example, instead of one out of every four photons bouncing off a surface, 25% of all photons that hit a surface bounce off. There is no way to tell what a single photon will do. Reflection is a matter or odds, not a certainty. Most quantum behavior works this way.
- We can’t “perfectly” localize a photon as it has no mass (and only momentum), and thus it can’t exist without moving (one reason why its better to generally describe light as a wave function of probabilities). We can predict a range of places in which a photon will travel, but we can’t determine it with certainty or localize it perfectly. Or rather, the uncertainty principle says “the more precisely the position of some particle is determined, the less precisely its momentum can be known, and vice versa,” this is due to its quantum nature. It only gets more complex when we consider the bizarre effects of the double slit experiment and quantum entanglement.

What is the Heisenberg Uncertainty Principle? – Chad Orzel by Fermilab.

### Heisenberg, Feynman, Maxwell, Planck, and Einstein

- Heisenberg’s uncertainty principle describes the uncertainty of quantum mechanics noted above.
- Maxwell’s equations describe light using mathematics. Einstein showed us that kinetic energy was equivalent to a body’s mass and the constant speed of light. We can use Max Planck’s work to see that kinetic energy is also equal to Planck’s constant multiplied by frequency.
- Later, Richard Feynman broadened the theory of Quantum ElectroDynamics (QED), which describes the nature of light, sometimes using shockingly simple Feynman diagrams.

QED: Photons — Corpuscles of Light — Richard Feynman (1/4). An amazing talk on light.

### The Big Picture, Light and the Other Forces

- Electromagnetic energy describes all energy that isn’t dark energy, gravitational energy, or (weak or strong) nuclear force. This includes all kinetic energy such as heat and even the energy stored in calories.
- We can consider most of the energy bound to particles as charge or motion to be photons. When particles interact they exchange virtual bosons (the name for all the carrier particles of the four forces).
- When we think of all four forces together – electromagnetic, gravity, and both nuclear forces, and their respective “bosons” – then we can say
**energy**is simply the kinetic and bound potential energy of a system in any form. We can describe this in one word, mass-energy.^{[4]} - We can sum this up by saying, “All elementary particles, including light, exhibit properties of mass-energy, and are understood as wave-like quantum fields, that interact with other quantum fields, in excited quantized states called particles.”
- You are starting to understand quantum physics if you can picture the physical universe as composed of massless vibrating energy field interactions in which everything is relative to their nature. Constants derived from the behavior of these fields provide our only reliable measure of what is real.

Quantum Theory – Full Documentary HD

- What is Light?
- Can somebody explain how time is relative in layman’s terms?
- What Is Spacetime, Really?
- Photon kinetic energy

"**What is Light?**" is tagged with: Albert Einstein, Energy, Isaac Newton, Light, Mass, Probability and Odds, Quantum Mechanics, Theoretical Physics