The Photon

The concept of electromagnetic waves was introduced by Maxwell, the father of electromagnetic theory, and all electromagnetic radiation can exhibit wavelike behaviour.

It also exhibits the behaviour of particles, and Einstein first proposed the idea that light (and indeed all other electromagnetic radiation) is formed of elementary particles. These particles have no mass and travel at the speed of light, and carry the smallest discrete quantum of energy. This quantum particle was later called the photon because its discovery was part of the scientific investigation into light.

It is now generally accepted that light and other radiation is composed of elementary particles called photons which exhibit both particle and wavelike behaviour. This discovery led to the principle of wave-particle duality which underpins modern atomic physics, and is a cornerstone of quantum theory.

Photons are produced when an electron orbiting an atom falls from one orbital energy state to a lower one. The photon has a frequency, or colour, that exactly matches the distance the electron falls.

For example, a sodium vapour light energizes sodium atoms to generate photons. A sodium atom has 11 electrons, and because of the way they’re stacked in orbitals one of those electrons is most likely to accept and emit energy. The energy packets that this electron is most likely to emit fall right around a wavelength of 590 nanometres. This wavelength corresponds to yellow light. If you run sodium light through a prism, you don’t see a rainbow — you see a pair of yellow lines, and sodium vapour streetlights appear yellow to the human eye.

Photons make it possible for human beings to see the world around us. In total darkness, our eyes are actually able to sense single photons, but generally what we see in our daily lives comes to us in the form of zillions of photons produced by light sources and reflected off objects. If you look around you right now, there is probably a light source in the room producing photons, and objects in the room that reflect those photons. Your eyes absorb some of the photons flowing through the room, and that’s how you see.

Probably the most common way to energize atoms to emit photons is with heat, and this is the basis of incandescence. If you heat up a metal it will get red-hot, and if you increase the heat it gets white hot. Red is the lowest-energy visible light, so in a red-hot object the atoms are just getting enough energy to begin emitting photons of lower energy states to create the red-light that we can see. Once enough heat is applied to cause white light, you are energizing so many different electrons in so many different ways that all of the colours are being generated — they all mix together to look white.

Today the existence of photons and their strange wave-particle duality is accepted. What is still debated is the tricky question of where light came from in the first place. To answer this question, physicists turned their attention to the big bang and the few moments that followed.

It is now generally believed that about 15 billion years ago, all matter and energy were bottled up in an infinitesimally small place known as a singularity. In an instant, this single point of super-dense material began to expand at an incredibly rapid rate. As the newborn universe expanded, it began to cool down and become less dense. This allowed more stable particles and photons to form.

A prevailing theory is:

  1. Immediately after the big bang, electromagnetism didn’t exist as an independent force. Instead, it was joined to the weak nuclear force.
  2. Particles known as B and W bosons (elementary particles which transmit force) also existed at this time.
  3. When the universe was just a tiny fraction of a second old, it had cooled enough for electromagnetism to break from the weak nuclear force and for the B and W bosons to combine into photons. The photons mingled freely with quarks, the smallest building blocks of matter.
  4. When the universe was ten millionths of a second old, quarks combined to form protons and neutrons.
  5. When the universe was 0.01 seconds old, protons and neutrons began to organize into atoms.
  6. Finally, when the universe was the tender age of 380,000 years old, photons broke free, and light streamed across the darkness of space.

This light eventually dimmed and reddened until, finally, the nuclear furnaces in stars kicked in and began generating new light.

Our sun turned on about 4.6 billion years ago, showering the solar system and planet earth with photons released from its vast cauldron of nuclear activity and giving us light ever since.