What is the purpose of light? How does light work? Why does Light exist? There had been a controversy about whether light was particles or waves. Albert Einstein showed that light must come in discrete packets of energy which today we call photons, because that’s the only way to explain the photoelectric effect.
Light has characteristics of both waves and particles, but it is neither. It’s a quantum object which obeys the Schrodinger equation, which has a wave function in it. This function shows that quantum objects and their quantum states such as position, momentum, spin etc. are in flux, similar to the way that the amplitude of a classical wave is in flux. So, if anything, a photon is a wave of probability, the probability of finding it at any particular location if we look there.
But about 50 years before quantum mechanics, James Clerk Maxwell came up with his laws of electromagnetism, which revealed that light was an electromagnetic wave. And that its speed could be determined precisely by two constants which are part of electromagnetic theory – the dielectric constant and magnetic constant, or the permittivity and permeability of free space. This fact that the speed of light is determined by the innate property of empty space led Albert Einstein to hypothesize that the speed of light in space is the same for every reference frame. And this led to the theory of Relativity. So light played an important role in development of both quantum mechanics and Relativity – which together is the best understanding we have today of how the universe works.
But light is an essential currency that our universe uses to transfer energy from one thing to another. For example, the sun shines because in its core, Hydrogen atoms combine to form a helium nucleus through fusion. Much of this energy released is in the form of gamma rays, which are high energy photons.
Almost all the artificial light created on earth is the result of electrons collapsing to lower orbits in an atom, after they have been excited by some source of energy into a higher energy state.
Light is also important for what we can feel. Quantum Electrodynamics developed in the 1940s by Richard Feynman and others, revealed that the electromagnetic force is mediated by photons. So a significant number of forces you feel would not be possible without photons.
Without light, you wouldn’t feel anything, because touch is a result of electromagnetic interactions between your skin and the surface you are touching. And since the atoms in your body are held together due to the electromagnetic interactions between the positively charged nucleus and negatively charged electrons, atoms would not exist without light either.
No light means no atoms, no cars, no houses, no you and me. and no universe as we know it. Sound could still exist, but you wouldn’t be there to hear it.
But the universe could still exist if we took away electromagnetic force, but it would not resemble the universe we know. We would still have gravity, the strong force and the weak force, so we could still form nuclei and have certain nuclear decays. Very large nuclei could probably form in such a universe since there would be no electrostatic repulsion between protons preventing very large nuclei from forming.
Other than the universe looking the way it does today, light plays a vital role in almost all processes where energy is exchanged. Whenever, energy is either expended or absorbed, photons are almost always involved. From chemical reactions, to thermal radiation, to friction, even to many nuclear processes, photons are involved.
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The speed of light is present in Einstein’s mass/energy equivalence equation E=mc^2. What this means is that if all the mass of an object were converted to energy, it would be in the form of photons traveling at the speed of light. Photons would be the objects carrying this energy of what had been the mass.
Every object in the universe radiates photons. This is called black body radiation. And it’s a property of any object above absolute zero. In the process of energy exchange via photons, information is also exchanged. Light is the fundamental currency used to exchange energy and information from one part of the universe to another.
