## Speed of light

###### About the speed of light
• The speed of light, 299,792,458 meters per second, is the maximum speed at which information can be transmitted through space.
• The fact that the speed of light is invariant for all observers, regardless of their relative motion, has important implications for our understanding of the behaviour of matter and energy in the universe.
• The first consequence of E=mc² is that if the speed of light (c) never changes then it is also true that m=E/c² so matter and energy are not distinct and separate entities but are instead different manifestations of the same underlying substance.
• The fact that the speed of light is invariant forms a key part of Einstein’s theory of relativity, which describes how the behaviour of space, time, and matter are interrelated.
• The invariance of the speed of light means that the relationship between mass and energy, as described by Einstein’s famous equation E=mc², is a universal relationship that holds true for all observers, regardless of their relative motion.
• The fact that the speed of light is invariant also forms a key part of Einstein’s theory of relativity, which describes how the behaviour of space, time, and matter are interrelated.
• The square of the speed of light (c2) appears in the equation because it relates the energy of an object to its mass and shows that a small amount of mass can be converted into a large amount of energy, and vice versa.
• If the speed of light were different, then the (c2) factor in the equation would be different as well. So the value of (c2) in the equation is a consequence of the nature of energy and mass, and the fact that the speed of light is a fundamental constant in the universe.
• The square of the speed of light (c2) serves as a conversion factor that relates the mass of an object to its energy content and shows the enormous amount of energy that can be released when a small amount of mass is converted into energy, as in nuclear reactions.
• The speed of light multiplied by itself = 299,792,458^2 = 89,875,517,873,681,764

## Speed of light & Planck scale

###### About the speed of light and Planck scale
• The Planck scale is the scale at which the effects of gravity are expected to become comparable to the other fundamental forces of nature, and it is defined by the Planck length, Planck time, and Planck energy. These values are incredibly small and represent the smallest possible length, time, and energy that can exist in the universe according to our current understanding of physics.
• Interestingly, the speed of light squared is related to the Planck energy through the famous equation E=mc^2. This equation shows that energy (E) is equal to mass (m) times the speed of light squared (c^2). At the Planck energy scale, which is around 1.22 × 10^19 GeV (gigaelectronvolts), the energy required to create a black hole is reached.
• Moreover, the Planck length can be derived from fundamental constants including the speed of light (c), the gravitational constant (G), and the reduced Planck constant (ħ), and is approximately equal to 1.616 x 10^-35 meters. Therefore, the speed of light squared also plays a role in the definition of the Planck length as it is one of the fundamental constants used to derive it.
• In summary, the speed of light squared is related to the Planck scale through its connection to energy, as well as its role in the definition of the Planck length.

## Speed of light waves & photons

##### Speed of light waves
• The speed of a light wave is a measurement of how far it travels in a certain time.
• The speed of light is usually measured in metres per second (m/s).
• Light travels through a vacuum at a bit less than 300,000 kilometres per second.
• The exact speed at which light travels through a vacuum is 299,792,458 metres per second.
• Light travels through other media at lower speeds.
• A vacuum is a region of space that contains no matter.
• Matter is anything that has mass and occupies space by having volume.
• When discussing electromagnetic radiation the term medium (plural media) is used to refer to anything through which light propagates including empty space and any material that occupies space such as a solid, liquid or gas.
• In other contexts, empty space is not considered to be a medium because it does not contain matter.
• When light is described in terms of photons rather than waves the following points are important:
##### Speed of photons
• Light exhibits wave-particle duality, meaning it can be described as both a wave and a particle (photon).
• Photons are massless particles that travel at the speed of light.
• Photons carry energy and momentum in quantized discrete units.
• “Quantized discrete units” refers to the way energy and momentum are carried by photons.
• In quantum mechanics, certain physical properties, such as energy and momentum, are quantized, meaning they can only take specific discrete values rather than a continuous range of values.
• For photons, this means that their energy and momentum come in distinct, non-continuous packets or “units.”