There is a unique relationship between electrons and photons. During the Big Bang
the higher frequency photon radiation created electrons and a plasma of quarks and gluons.
Then after some 380,000 years the universe went dark when all the original photon radiation was used up. The Cosmic Microwave Background (CMB)* shows the remnants of this era.
The universe cooled down so that electrons were able to attach to quarks and create protons and neutrons. After more cooling stable atoms formed.
During the next million years atoms of hydrogen and helium formed into huge clouds by the force of gravity.
When enough matter collected together gravity started a nuclear fusion reaction and a star was born. This was the first new light that the universe saw.
The big bang process involved photons** creating electrons. Interesting that the original photons created electrons and now electrons create photons. Electrons are the only way that light is made.
When a photon is absorbed by an electron, it is completely destroyed. All its energy is imparted to the electron, which instantly jumps to a new energy level. The photon itself ceases to be. In the equations which govern this interaction, one side of the equation (for the initial state) has terms for both the electron and the photon, while the other side (representing the final state) has only one term: for the electron.
The opposite happens when an electron emits a photon. The photon is not selected from a "well" of photons living in the atom; it is created instantaneously out of the vacuum. The electron in the high energy level is instantly converted into a lower energy-level electron and a photon. There is no in-between state where the photon is being constructed. It instantly pops into existance.
So the question is: where does the photon come from?
Strangely, it doesn't seem to come from anywhere. The universe must put the extra energy somewhere, and because electrons in atoms are electromagnetic phenomena, a photon is born with the required energy. In a weak-force interaction, say the decay of a neutron, that energy goes into a neutrino particle which is also instantaneously created. Each force has its own carrier particles, and knows how to make them.
* Cosmic Microwave Background corresponds to a temperature today of 2.7 Kelvin: 2.7º C above absolute zero, or -455º F.
Today, we see this light as the cosmic microwave background. Because recombination happened everywhere in the universe, we see CMB light coming from all directions. The CMB provides the best data we have on the early universe, and the structure of the cosmos on the largest scales.
The cosmic microwave background is a snapshot of the oldest light in our universe, from when the cosmos was just 380,000 years old. The colors of the map represent small temperature fluctuations that ultimately resulted in the galaxies we see today. Credit: ESA and the Planck Collaboration
The CMB looks almost exactly the same, no matter what part of the sky we look at. The term for that in cosmology is “isotropic”, and the small deviations from perfect sameness are called anisotropies. Measuring the larger-sized anisotropies reveals how much dark energy, dark matter, and ordinary matter are contained in the universe.
The smaller anisotropies reveal the tiny fluctuations in density that gave rise to the pattern of galaxies and galaxy clusters we see today, which astronomers call the large-scale structure of the universe. Without those small irregularities, there wouldn’t be any galaxies, and we wouldn’t be here to observe them. Likewise, larger anisotropies wouldn’t produce the universe we see.
The overwhelming sameness of the CMB also tells us something about the early universe. Two points on the CMB on opposite sides of the sky shouldn’t have almost exactly the same temperature, since they weren’t close together at recombination. The most popular explanation for this is “inflation
”: a tiny fraction of a second after the Big Bang, quantum fluctuations caused the universe to expand at an extreme rate. Points that were far apart at recombination today were neighbors before inflation, so they have nearly the same temperature.
According to theory, inflation left its mark on the CMB in the form of the twisting of light known as polarization. Astronomers use modern telescopes to look for that polarization, in hopes of seeing the behavior of the universe when it was only a fraction of a second old.
** Photon is a particle of light defined as a discrete bundle (or quantum
) of electromagnetic (or light) energy. Photons are always in motion and, in a vacuum (a completely empty space), have a constant speed of light to all observers. Photons travel at the vacuum speed of light
(more commonly just called the speed of light) of c
= 2.998 x 108 m/s.
Basic Properties of Photons
According to the photon theory of light, photons:
History of Photons
- behave like a particle and a wave, simultaneously
- move at a constant velocity, c = 2.9979 x 108 m/s (i.e. "the speed of light"), in empty space
- have zero mass and rest energy
- carry energy and momentum, which are also related to the frequency (nu) and wavelength (lamdba) of the electromagnetic wave, as expressed by the equation E = h nu and p = h / lambda.
- can be destroyed/created when radiation is absorbed/emitted.
- can have particle-like interactions (i.e. collisions) with electrons and other particles, such as in the Compton effect in which particles of light collide with atoms, causing the release of electrons.
The term photon was coined by Gilbert Lewis
in 1926, though the concept of light in the form of discrete particles had been around for centuries and had been formalized in Newton's construction of the science of optics.
In the 1800s, however, the wave properties
of light (by which is meant electromagnetic radiation
in general) became glaringly obvious and scientists had essentially thrown the particle theory of light out the window. It wasn't until Albert Einstein
explained the photoelectric effect
and realized that light energy had to be quantized that the particle theory returned.
Wave-Particle Duality in Brief
As mentioned above, light has properties of both a wave and a particle. This was an astounding discovery and is certainly outside the realm of how we normally perceive things. Billiard balls act as particles, while oceans act as waves. Photons act as both a wave and a particle all the time (even though it's common but basically incorrect, to say that it's "sometimes a wave and sometimes a particle" depending upon which features are more obvious at a given time).
Just one of the effects of this wave-particle duality
(or particle-wave duality
) is that photons, though treated as particles, can be calculated to have frequency, wavelength, amplitude, and other properties inherent in wave mechanics.
Fun Photon Facts
The photon is an elementary particle
, despite the fact that it has no mass. It cannot decay on its own, although the energy of the photon can transfer (or be created) upon interaction with other particles. Photons are electrically neutral and are one of the rare particles that are identical to their antiparticle, the antiphoton.
Photons are spin-1 particles (making them bosons), with a spin axis that is parallel to the direction of travel (either forward or backward, depending on whether it's a "left-hand" or "right-hand" photon). This feature is what allows for polarization of light.
There are many interpretations of what this and other phenomena in quantum mechanics mean on a deeper level. But my personal philosophy approximates that of the famous physicist Richard Feynman, inventor of the Feynman diagrams, who said to have uttered when queried: "Shut up and calculate."