How does the sunlight travel to earth

Dr Irengbam Mohendra Singh *

Summer Time Sun Set progression :: May 2009

Against the backdrop of continued reverence to our Sun for millennia, one can't help being surprised at that the light (photons) that strikes our pupils at any moment, has taken tens of thousands of years since the Sun emitted. It's because light takes 20,000 years to reach the surface of the Sun from its core, but just 8 minutes and 20 seconds from its exterior to reach us on Earth. Photons (see below) spend a lot of time colliding with the Sun's atoms on their way out. Once there, they have a very few hindrances.

By sunlight, I mean light and heat that we experience on any day when there is no cloud or rain. Sunshine is a luxury in the UK. Britons spend a lot of money to go to 'Sunny Spain' to get sunshine and suntan (prestige signature). Nuclear fusion in the core of the Sun, creates heat, and light (photons). A photon is a quantity of light, which behaves like particles, rather than waves (cf. author's Elementary Quantum mechanics, Points to Ponder, pp 339-342).

The Universe is full of light. Billions and billions of stars radiate light from their internal nuclear reactions, but the light we see is only a tiny fraction of it. As the Sun is nearest to us we get more light from it.

Sunlight is a portion of the electromagnetic (EM) radiation (vide infra) given off by the Sun, such as X-rays, ultraviolet, visible light, infrared, and even radio waves, as a result of nuclear process in its core. The direct evidence is derived from the emission of neutrinos – the only subatomic particle, which is recently proven to have mass.

Light from the Sun can travel fast through space without losing much energy as it has no medium. It's unlike eg sea waves caused by the wind, which can travel only through a medium of water. Photons are like a stream of tiny particles travelling in different frequencies.

At school, we used to remember the seven official colours of visible light, as the rainbow by the acronym 'vibgyor' (violet, indigo, blue, green, yellow, orange and red). In the UK, my wife remembered it by the mnemonic, 'Richard Of York Gave Battle In Vain'. Newton displayed them by breaking light, using a prism.

The first light in the Universe came from 'microwave background radiation' (MBR) ie the leftover light originated after 400,000 years after the Big Bang explosion. There were also radio waves or radio photons that were once visible to the naked eye. According to new physics, the static crackling sounds you hear while tuning a radio are this stretched light that has travelled from the beginning of time ie from the time of the Big Bang.

MBR light made the universe visible. At that time, the Universe was much smaller and much hotter – about 273 million degrees Celsius – hotter than the centre of a present star. The Universe was a fiery ball of electrically charged 'subatomic' nuclei of hydrogen and helium, and electrons, known as 'plasma'. As it expanded and cooled down to about 3,000 degrees Celsius, during the evolution of the Universe, ie about 400,000 years after the Big bang, the hydrogen and helium nuclei could combine with electrons to form 'atoms'.

As the Universe became much cooler and more diffuse, the ancient light was free to travel through space, and the universe became visible. As it further evolved, the space as we know now, has become vastly stretched. And so has the light that became so stretched that the whole spectrum of light is now no more visible to our naked eye. It has moved beyond the infrared and is now visible only in the microwave and radio parts of the spectrum.

The microwave is of long wavelength, ranging from one metre and can be mapped with very sensitive instruments, as a faint light that gives a cosmic glow. It was discovered in 1964 by Arno Penzias and Robert Wilson, helping to confirm the occurrence of the Big Bang.

Our Sun like all stars, shines ie it radiates light that we can see. Very soon after the Big Bang, the elementary particles interacted and organised themselves to form atoms, along with the emergence of 'four fundamental forces and four stable elementary particle'. Countless stars were formed around 100 million years later, filling the space. Since then, the stars have been continually radiating light from further afield in space.

In particle physics, all matter around us is made of elementary particles – the building blocks of matter. A particle by definition is "not a compound of other particles". The structure of a particle is unknown. The evidence of such "particles" is the burst of light or heat given off.

Particles are mostly unstable, about 500, with only four stable charged particles known as elements: (1) electron; (2) proton; (3) positron; and (4) antipositron. Particle physics that we learnt in school, began in 1930s with the discovery that atoms are all composed of protons, neutrons and electrons. In 1964, there has been a new discovery that, protons and neutrons are found to contain "quarks" – up quarks and down quarks. These are now considered elementary particles (cf. Author's The creation of the universe, matter & sub atomic particles).

There are four fundamental physical forces: (1) gravitation; (2) electromagnetism; (3) strong force; and (4) weak force. Gravitation hold you down; electromagnetism holds the atoms; strong force holds the atoms together; and weak force is responsible for radioactive decay. Without these forces, the matter in the universe, including us, will fall apart. Gravity and Electromagnetism are just two of the forces that we can observe every day. Gravity prevents us from floating away. Electromagnetism is responsible eg for generating light that allows us to read the computer monitor screen.

Light travels fast as photons at a constant speed, but not infinitely so. James Clark Maxwell, a Scot in mid-nineteenth century (1873), before Einstein (German), but after Michael Faraday (English) and Andre-Marie Ampere (French), described light as an electro-magnetic wave ie an oscillating electric and magnetic fields, spilling back and forth, propelling each other through space. Maxwell discovered that electricity and magnetism are really different manifestations of a single phenomenon, known as electromagnetism.

A 'space' can be conceived as a continuous area that is free and available. A 'field' or the idea of a field in physics, can be explained by a room that is heated. If you go around the room measuring the temperature, you will find that the temperature varies in different parts of the room. This is called a temperature field. So, light as an aspect of electromagnetism, travels as a wave through the space that convey electric and magnetic fields.

Our life simply functions as chemistry, and all chemistry involves electromagnetism (EM), which is rearrangements of electrons in their orbits around atomic nuclei. In an electrical force, unlike gravity, like charges repulse and opposite charges attract. This quality of the electrical force is helpful to stars like the Sun for emitting light.

EM radiation is created when an atomic particle, such as an electron, is accelerated by an electric field, causing it to move. The movement produces oscillating electric and magnetic fields that travel at right angles to each other, in a bundle of light energy called a photon.

Light thus carries away energy as photon that is constantly produced by the stars and travels great distances such as to the Earth. If stars like the Sun could not radiate and spend the energy they produce, they would simply explode.

Sunlight travels in different frequencies unhindered by the empty space until, it hits the Earth's atmosphere, when the photons begin to collide with gas molecules. Light photons with long waves, red, orange and yellow can travel right through the gas molecules, while those photons with shorter wavelengths – green, blue and violet – are easily absorbed by the gas molecules, though only for an instant. Then, they shoot out again in random directions, the blue colour scattering all over the atmosphere. That's why the sky is blue.

Many of these scattered photons, also travel towards the earth, making the sky appear to glow at sunset, as the photons from the setting Sun have to travel through a larger layer of atmosphere, when more of the higher frequency photons are absorbed, leaving only beams of red, orange and yellow to enter our eyes. The Sun itself looks white as photographs by astronauts show.

The Sun emits light because it is very hot, and hot things shine ie emit light. This is sunlight. A red hot iron rod will glow and radiate heat. In the 19th century, a widely debated topic was – wherefrom does the Sun get its immense energy. In the early 20th century, new physics discovered the source of the Sun's energy as 'nuclear reactions' in its core, the amount of which is closer to a million times those of chemical reactions produced in burning coal fire.

It's because the size of the Sun is estimated to be such that, an average passenger jet like the jumbo jet that flies from London to New Delhi in eight hours, would take six months to fly around it.

This is how the sunlight reaches us.