Where Does Visible Light Come From? | Universe Today

Written by Fraser Cain

It wasn't too long ago (13.7 billion years by some accounts) that a rather significant cosmological event occured. We speak of course, of the Big Bang. Cosmologists tell us that at one time there was no universe as we know it. Whatever existed before that time was null and void – beyond all conception. Why? Well there are a couple answers to that question – the philosophic answer for instance: Because before the universe took form there was nothing to conceive of, with, or even about. But there's also a scientific answer and that answer comes down to this: Before the Big Bang there was no space-time continuum – the immaterial medium through which all things energy and matter move.

Once the space-time continuum popped into existence, one of the most moving of things to take form were the units of light physicists call "photons". The scientific notion of photons begins with the fact that these elementary particles of energy display two seemingly contradictory behaviors: One behavior has to do with how they act as members of a group (in a wavefront) and the other relates to how they behave in isolation (as discrete particles). An individual photon may be thought of as a packet of waves cork-screwing rapidly through space. Each packet is an oscillation along two perpendicular axes of force – the electrical and the magnetic. Because light is an oscillation, wave-particles interact with each other. One way of understanding the dual-nature of light is to realize that wave after wave of photons affect our telescopes – but individual photons are absorbed by the neurons in our eyes.

The very first photons travelling through the space-time continuum were extremely powerful. As a group, they were incredibly intense. As individuals, each vibrated at an extraordinary rate. The light of these primordial photons quickly illuminated the rapidly expanding limits of the youthful universe. Light was everywhere – but matter was yet to be seen.

As the universe expanded, primordial light lost in both frequency and intensity. This occured as the original photons spread themselves thinner and thinner across an ever-expanding space. Today, the first light of creation still echos around the cosmos. This is seen as cosmic background radiation. And that particular type radiation is no more visible to the eye as the waves within a microwave oven.

Primordial light is NOT the radiation we see today. Primordial radiation has red-shifted to the very low end of the electromagnetic spectrum. This occured as the universe expanded from what may have originally been no larger than a single atom to the point where our grandest instruments have yet to find any limit whatsoever. Knowing that primordial light is now so ternuous makes it necessary to look elsewhere to account for the kind of light visible to our eyes and optical telescopes.

Stars (such as our Sun) exist because space-time does more than simply transmit light as waves. Somehow – still unexplained^-1 – space-time causes matter too. And one thing distinguishing light from matter is that matter has "mass" while light has none.

Because of mass, matter displays two main properties: Inertia and gravity. Inertia may be thought of as resistence to change. Basically matter is "lazy" and just keeps doing whatever it's been doing – unless acted upon something outside itself. Early in the formation of the universe, the main thing overcoming matter's lazyness was light. Under the influence of radiation pressure, primordial matter (mostly hydrogen gas) got "organized".

Following light's prodding, something inside matter took over – that subtle behavior we call "gravity". Gravitation has been described as a "distortion of the space-time continuum". Such distortions occur wherever mass is found. Because matter has mass, space curves. It is this curve that causes matter and light to move in ways elucidated early on in the twentieth century by Albert Einstein. Each and every little atom of matter causes a tiny "micro-distortion" in space-time^-2. And when enough micro-distortions come together things can happen in a big way.

And what happened was the formation of the first stars. No ordinary stars these – but super-massive giants living very fast lives and coming to very, very spectacular ends. At those ends, these stars collapsed in on themselves (under the weight of all that mass) generating tremendous shock waves of such intensity as to fuse entirely new elements out of older ones. As a result, space-time became suffused with all the many types of matter (atoms) making up the universe today.

Today, two types of atomic matter now exists: Primordial and something we might call "star-stuff". Whether primordial or stellar in origin, atomic matter makes up all things touched and seen. Atoms have properties and behaviors: Inertia, gravity, extension in space, and density. They can also have electrical charge (if ionized) and participate in chemical reactions (to form molecules of tremendous sophistication and complexity). All matter we do see is based on a fundamental pattern established long-ago by those primordial atoms mysteriously created after the Big Bang. This pattern is founded on two fundamental units of electrical charge: The proton and the electron – each having mass and capable of doing those things mass is liable to.

But not all matter follows the hydrogen prototype exactly. One difference is that newer generation atoms have electrically-balanced neutrons as well as positively-charged protons in their nuclei. But even stranger is a type of matter (dark matter) that doesn't interact with light at all. And furthermore (just to keep things symmetric), there may be a type of energy (vacuum energy) that doesn't take the form of photons – acting more like a "gentle pressure" causing the universe to expand with a momentum not orignally supplied by the Big-Bang.

But let's get back to the stuff we can see…

In relationship to light, matter can be opaque or transparent – it can absorb or refract light. Light can pass into matter, through matter, reflect off matter, or be absorbed by matter. When light passes into matter, light slows – while its frequency increases. When light reflects, the path it takes changes. When light is absorbed, electrons are stimulated potentially leading to new molecular combinations. But even more significantly, when light passes through matter – even without absorption – atoms and molecules vibrate the space-time continuum and because of this, light can be stepped down in frequency. We see, because something called "light" interacts with something called "matter" in something called "the space-time continuum".

In addition to describing the gravitational effects of matter on space-time, Einstein performed an extremely elegant investigation into the influence of light associated with the photo-electric effect. Before Einstein, physicists believed lights' capacity to affect matter was based primarily on "intensity". But the photo-electric effect showed that light effected electrons on the basis of frequency as well. Thus red light – regardless of intensity – fails to dislodge electrons in metals, while even very low levels of violet light stimulate measurable electrical currents. Clearly the rate at which light vibrates has a power all its own.

Einstein's investigation into the photo-electric effect contributed mightily to what later became known as quantum mechanics. For physicists soon learned that atoms are selective about what frequencies of light they will absorb. Meanwhile it was also discovered that electrons were the key to all quantum absorption – a key related to properties such as one electrons relationships to others and with the nucleus of the atom.

So now we come to our second point: Selective absorption and emission of photons by electrons does not explain the continuous spread of frequencies seen when examining light through our instruments^-3.

What can explain it then?

One answer: The "stepping-down" principle associated with the refraction and absorption of light.

Common glass – such as in the windows of our homes – is transparent to visible light. Glass however reflects most infrared light and absorbs ultraviolet. When visible light enters a room, it is absorbed by furniture, rugs etc. These items convert part of the light to heat – or infrared radiation. This infrared radiation is trapped by the glass and the room heats up. Meanwhile glass itself is opaque to ultraviolet. Light emitted by the Sun in the ultraviolet is mostly absorbed by the atmosphere – but some non-ionizing ultraviolet manages to get through. Ultraviolet light is converted to heat by glass in the same way furnishings absorb and re-radiate visible light.

How does all this relate to the presence of visible light in the Universe?

Within the Sun, high energy photons (invisible light from the perimeter of the solar core) irradiate the solar mantle beneath the photosphere. The mantle converts these rays to "heat" by absorption – but this particular "heat" is of a frequency well beyond our capacity to see. The mantle then sets up convective currents carrying heat outward toward the photosphere while also emitting lesser-energized – but still invisible – photons. The resulting "heat" and "light" passes to the solar photosphere. In the photosphere ("the sphere of visible light") atoms are "heated" by convection and stimulated through refraction to vibrate at a rate slow enough to give off visible light. And it is this principle that accounts for the visible light emitted by stars which are – by far – the most significant source of light seen throughout the cosmos.

So – from a certain perspective, we can say that the "refractive index" of the Sun's photosphere is the means by which invisible light is converted into visible light. In this case however, we invoke the idea that the refractive index of the photosphere is so high that high energy rays are bent to the point of absorption. When this occurs lower frequency waves are spawned radiating as a form of heat peceptible to the eye and not simply warm to the touch…

And with all this understanding beneath our intellectual feet, we can now answer our question: The light we see today is the primordial light of creation. But it is light that materialized some few hundreds of thousands of years after the Big Bang. Later that materialized light came together under the influence of gravity as great condensed orbs. These orbs then developed powerful alchemical furnaces de-materializing matter into light invisible. Later – through refraction and absorption – light invisible was rendered visible to the eye by rite of passage through those great "lenses of luminosity" we call the stars…

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^-1 How all things cosmological transpired in detail is probably the major area of astronomical research today and will take physicists – with their "atom-smashers", astronomers – with their telescopes, mathematicians – with their number-crunching super-computers (and pencils!) and cosmologists – with their subtle understanding of the early years of the universe – to puzzle the whole thing through.
^-2
In a sense matter may simply be a distortion of the space-time continuum – but we are a long way from understanding that continuum in all its properties and behaviors.

^-3 The Sun and all luminous sources of light do display dark absorption and bright emission bands of very narrow frequencies. These of course, are the various Fraunhofer lines related to quantum mechanical properties associated with transition states of electrons associated with specific atoms and molecules.

About The Author:Inspired by the early 1900's masterpiece: "The Sky Through Three, Four, and Five Inch Telescopes", Jeff Barbour got a start in astronomy and space science at the age of seven. Currently Jeff devotes much of his time observing the heavens and maintaining the website Astro.Geekjoy.

Filed under: Astronomy

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