Ciro Santilli @cirosantilli 34

If you point a light detector to any empty area of the sky, you will still get some light.

The existence of this is quite mind blowing, since "there is nothing there emitting that light".

To make sense of how it is possible to see this light, you can think of the universe as the expanding raisin bread model, but it expands faster than light (thus the existence of the cosmological event horizon), so we are still receiving light form the middle, not the borders.

CMB is basically perfectly black-body radiation at 2.725 48 K, but it has small variations with variations of the order of 200 microKelvin: cosmic microwave background anisotropy.

There is a slight variation in temperature of CMB across the sky of the order of 200 microKelvin. It is small to the ~2.7 K average temperature, but it can be measured.

If the initial conditions of the Big Bang and the laws of physics were perfectly symmetric, then we could expect the Universe to just be one perfectly uniform boring soup.

But instead some asymetry made all the fun weird things we see today happen eventually, like galaxies and life.

And the cosmic microwave background serves as a way for us to look back in time to the early conditions of the universe, as it was set in stone as soon as the universe became transparent to this light during recombination.

Or if you want to get poetic, it is the closest we can ever get to listening to the original word of God when he setup the initial conditions of the universe.

The ansiotropies of CMB is the ultimate astronomical compass we will ever have, as it is the thing with the least proper motion.

The discovery of the photon was one of the major initiators of quantum mechanics.

Light was very well known to be a wave through diffraction experiments. So how could it also be a particle???

This was a key development for people to eventually notice that the electron is also a wave.

This process "started" in 1900 with Planck's law which was based on discrete energy packets being exchanged as exposed at On the Theory of the Energy Distribution Law of the Normal Spectrum by Max Planck (1900).

This ideas was reinforced by Einstein's explanation of the photoelectric effect in 1905 in terms of photon.

In the next big development was the Bohr model in 1913, which supposed non-classical physics new quantization rules for the electron which explained the hydrogen emission spectrum. The quantization rule used made use of the Planck constant, and so served an initial link between the emerging quantized nature of light, and that of the electron.

The final phase started in 1923, when Louis de Broglie proposed that in analogy to photons, electrons might also be waves, a statement made more precise through the de Broglie relations.

This event opened the floodgates, and soon matrix mechanics was published in quantum mechanical re-interpretation of kinematic and mechanical relations by Heisenberg (1925), as the first coherent formulation of quantum mechanics.

It was followed by the Schrödinger equation in 1926, which proposed an equivalent partial differential equation formulation to matrix mechanics, a mathematical formulation that was more familiar to physicists than the matrix ideas of Heisenberg.

Inward Bound by Abraham Pais (1988) summarizes his views of the main developments of the subjectit:

Bibliography:

These are closely related to lasers, as they do a similar basic job: take a DC source as input and amplify light. Lasers just happen to use the input voltage to also generate the incoming light.

The science and engineering of light!

When dealing more specifically with individual photons, we usually call it photonics.

Initially light was though of as a wave because it experienced interference as shown by experiments such as:

But then, some key experiments also start suggesting that light is made up of discrete packets:and in the understanding of the 2020 Standard Model the photon is one of the elementary particles.

This duality is fully described mathematically by quantum electrodynamics, where the photon is modelled as a quantized excitation of the photon field.

A squeezed coherent state of light.