Light is a major instrument in our interactions with the world around us. Without it, we would be condemned to live in perpetual isolation, and we would have evolved in a dismal darkness that would be as stifling as any self-centered existence. For it is light that weaves the distant corners of the universe into a cosmic wholeness and that informs us of the presence of people and things beyond ourselves. Light reveals shapes and sizes and beauty. It speaks to us of unreachable celestial bodies, of their nature and structure; it tells us if far-flung nebulae are approaching us or receding away. It is thus the source of all knowledge. It guides us and it enlarges our horizons.
But life is not simply knowledge and information. The life experience includes enjoyment, too. Here again light serves us well. There is more to light than brightness. Light is vibration not only of varying intensities, but of varying durations, which causes colors. Color adds splendor to the world. Were it not for colors, the world would be a drab gray of changing shades.
Yet color is not intrinsic to light; it is a result of interactions between vibrations and our optical systems. It is the human brain that transforms undulations into magnificent hues. Our optical system is wondrously complex, and miraculous in what it accomplishes. This arrangement of the retinal screen and nerves turns vibrations into visual reality. All the beauty of forms and splendor of colors, the shimmer and shade and brightness, arise because of whatever the retina and the brain do. The cosmos in all its complexity would be plunged into invisibility if no eye responded to a narrow band in the electromagnetic spectrum. No glorious sunset nor twinkling stars, no glitter or sparkle, if the ethereal vibrations palpitated unnoticed, unrecognized as light and color. How was the universe during those eons before life emerged? It was certainly not resplendent in this grand glory. Even when roses bloomed and leaves changed to their autumnal splendor, it was all bleak and insipid when there were no eyes with rods and cones. Those multicolored creatures in aquariums, the majestic rainbow, the yellow sunflower and the purple violets—all such have been there for eons before the human retina evolved, but never recognized as such. Let us not underestimate our role on this planetary speck in a grand universe. Our eyes add substantially to it all. The human brain shapes an uninteresting and uninspiring surrounding into something spectacular.
There would be no electromagnetic waves without electric charges. There would be no light without the human optical system. Our world of experience is a world of perceived reality indeed.
The properties of light enhance the charms of the world. The changing colors of the diamond beetle, for example, arise not from pigmentation, but from what we call the diffraction of light. The glory of the rainbow and the colors of the icicle result from the simple properties of reflection and refraction. Diamonds would be as inconsequential as a speck of charcoal were it not for light. Rubies, emeralds, and sapphires would be dark as the depths of hell, if there was no light.
Light is also a life-sustaining principle—for it is sunlight that cleverly collaborates with the green of the earth to produce the food that sustains and nourishes life on our planet. If we look for miracles, this is where we find one. Light spans every region of the physical universe, and it has been there since the first big bang of cosmic creation. Nothing we know is as omnipresent or eternal as light.
We are accustomed to associate white with purity, and for generations, it was imagined that white light was the purest of all. But this is not so. White light arises from the mingling of every colored light there is. This surprising root of perceived reality can be discovered with a prism. White light, the most colorless we can imagine, turns out to embody every color from violet to red that spans the rainbow.
How do objects acquire their colors? Why does the leaf appear green and the apple red? This is because the atoms and molecules of most materials absorb some of the visible wavelengths that fall on them. Which particular ones they absorb depends on their structure and constitution. Atoms and molecules have their characteristic tastes for waves, as it were. If a material sucks in every component of white light, it gives us back nothing, and appears dark, as with the charcoal, for example.
Next consider the speed of light: In this age of bullet trains and supersonic jets, speeds of a few hundred miles an hour don’t impress us any more. Yet, we may be shocked to know about speeds in the physical world: The sun and the stars, air molecules and electrons in atoms, move with inconceivably greater speeds, of the order of hundreds of thousands of kilometers an hour. But when we come to the speed of light, it is altogether unimaginable. Light covers 300 million meters a second. We can hardly conceptualize a speed of this magnitude. Yet, this is the most common speed in the universe, for more than anything else, electromagnetic waves pervade every nook and corner of the physical universe.
More impressive is the mind that measures this. By complex and ingenious means, we have come to know quite precisely how fast light travels in empty space and in other media. And this is the fastest speed allowed in the universe. No physical body, massive as stars or minute as electrons, can ever reach a speed equal to that of light. We may, in theory, picture particles zooming with speeds very nearly equal to that of light, but never, never equal to it. Light speed is the ceiling that naught can break through.
There is another aspect of the velocity of light that is even more remarkable. This speed does not depend on the motion of the observer relative to the source of light. Imagine you are riding your bike at 15 km/h toward a car. If the car is approaching you at 60 km/h, it will be appearing to be coming at 75 km/h since every hour this is the amount by which the distance between the car and yourself is diminishing. Likewise, for an observer moving forward in the same direction as the car, the car’s relative speed will be 35 km/h. Only if you are stationary will the car seem to be coming at 60 km/h.
This commonsense calculation won’t work with light! Replace the car with a light wave, and the cyclists with fast moving rockets, and everyone will find light to be traveling with the same speed (relative to oneself). The speed of light is a universal constant, as one says in physics, absolutely independent of the state of motion of the observer.
Strange as it may sound, light itself is invisible: We can never see a ray of light passing somewhere in space. The effulgent beam of light spouting out from a luminous source that a movie company displays as its logo cannot be seen if that light is splashed into empty space, nor the laser beams of science fiction movies. Only when it strikes our retina do we become aware of light. When we stare at the night sky, there is ample sunlight there. But it is only when some of it bounces back from the moon or a planet do we see it, and in the process, become aware of the moon and the planets.
This brings us to another property arising from matter-light interaction. Ever since human beings turned their gaze upward, they have admired the soothing azure tint of the sky. But soon after sunset, all the blueness vanishes. Even when the moon is full and bright, the sky at night is never blue.
Since ancient times, poets and painters have taken note of this, but it is only in the 19th century that we came to understand why it is so. The phenomenon is related to a property of waves called scattering. When light waves fall on a smooth surface, they are reflected back. However, when they encounter small particles, they bounce back every which way: Light is scattered in all directions. Then again, not all wavelengths of light are scattered to the same degree. This depends on the structure and size of the scattering center. When a beam of white light hits a molecule of oxygen or nitrogen, the blue component is scattered away while the red and orange ones go right through. This has a dramatic effect on the perceived world. When sunlight enters the atmosphere, its bluish components are scattered, and when they reach our eyes, the sky looks blue. If our atmosphere were made up of some other (life-supporting) gases that had the property of scattering the green component primarily, we would be enjoying a green sky.
Much of the light that makes normal living possible arises from this property. The light in the room when the windows are open and the light in the shade under a tree is there because light is scattered by the air. Take away the air, and sunbeams will illumine only the patch on which they fall.
So now we come to this realization: Objects are visible to us not simply because of the light that falls on them, but equally because of the air around! Take away the light, and nothing can be seen. Take away the air, and not everything can be seen. Things will not be as visible in a lunar room even in broad daylight because there is no air there.
We light the candle or the log, we flip the switch in the room or press the button on the flashlight, and light appears. Or else we have the sun which, at every rising, floods our surroundings with light also. But, like the city kid who thought that the source of milk was the carton or the bottle, we would be mistaken to think that the source of light is the candle or the log, the light bulb, or the sun.
Light is created in the physical world by complex processes at the heart of matter. In the core of stars, there is the perennial transformation of matter into energy as per the Einstein equation. There, substantial matter is transformed into insubstantial energy: electromagnetic waves of many different wavelengths. So, like aroma from brewing coffee, light emerges from the depths of stars as nuclear brewing goes on.
But when ordinary matter is rendered hot, processes arise that cause the emission of light. The atoms of substances have several levels of energy. Normally, electrons are circulating at the lowest levels, whirling around in the atom, but if enough energy is imparted to any of them, they jump to a higher level, and promptly fall back to a lower one. In the process, they release the absorbed energy as electromagnetic waves. If the wavelength is appropriate, this is visible light. So we say that light results from electronic transitions within atoms. Whether it be light from an electric lamp, or from a candle, the waves result from electronic jumps, that is all. In the one case, we make the electrons jump to a higher orbit by giving them thermal energy through a match stick; in the other case, electrical forces accomplish this. In other words, light arises from the core of matter from atomic transitions involving dancing electrons galloping from orbit to orbit. If we yearn for wonders, this is something we may reflect upon.
Light from any source, when analyzed through a suitable optical device, gives a pattern of colors, be it of discrete lines or broader bands or continuous patches. This pattern is the source’s spectrum. The spectrum revealed by light from a source is characteristic of its chemical composition. It is a sort of fingerprint of the chemical elements present in the source of light. The spectrum can also tell us about the temperature of the source, and even about its motion or rest relative to us.
These discoveries opened up a whole new world for physicists. All we can get from the sun and the stars is light, and light can tell us about the constitution of matter. Just analyze the light from a distant source, and like a letter from a friend, we can know a good deal about the state and substance of the source.
In 1868, Janssen was in India to observe a total solar eclipse. He studied the solar prominences that are particularly visible during eclipses. Here he was puzzled by a strange line in its spectrum. This was like finding the fingerprint of an individual that is not in the police records. He sent this to Joseph Lockyer, an expert on solar spectra. After careful study of the line, Lockyer concluded that this must be a hitherto unknown element. He called it helium in honor of the sun. If detective stories are fascinating, this one can beat any. Consider the tortuous route: A lens-grinder recognizes the spectra of elements in the 1820s, an astronomer discovers a new element in the sun in the 1860s. Light unraveled the existence of an element that is out there, 93 million years away.
So we see how physicists have a way of finding out what the sun is made up of, and Polaris and Betelgeuse. It gradually became clear that stars and planets, high and mighty as they seem, are made up of the same sort of stuff as makes up this our modest planet. Aristotle wasn’t quite right when he preached that celestial bodies consisted of incorruptible matter while earthly ones degenerate and decay. No, all bodies are created equal, though not all posses equal amounts of every kind of matter. Analysis and reductionism can give lots and lots of interesting information about perceived reality.
Whether we like it or not, our eyes are sensitive to—that is to say, we can see—only a narrow band of the electromagnetic spectrum. But this is not to say that other regions of the spectrum are insignificant or that we cannot know about them. One of the goals of science is to unravel every aspect of physical reality that can be perceived, whether directly or indirectly.
Beyond red light, there are electromagnetic waves of longer wavelengths: the so-called infrared (IR) or heat radiations. Beyond this are the microwaves.
In 1932, while he was trying to eliminate the hissing noise associated with radio reception, Karl Jansky discovered that the Earth is being showered from outer space by electromagnetic waves of wavelengths longer than the infrared. Let us reflect on this for a moment. When we look at the sky and see the sun or the stars, clearly light reaches us from the heavens. But who would have thought that we are also being inundated constantly with imperceptible microwaves? And if light can tell us so much about the sun and the stars, could microwaves also tell us other things? Thus began radio astronomy: the exploration of the universe, not with the aid of visible light, but with the microwaves that are continuously pouring into our terrestrial world from many niches in the universe. In mindless physical reality, waves are mere carriers of energy. In the world of perceived reality, they carry information, too.
Radio telescopes have expanded our vision of the universe. They have put into evidence a great many sources of microwaves: those arising from supernova eruptions, others originating from electrons going amuck in interstellar magnetic fields, and yet others coming from transitions in the atoms of hydrogen spread all over space. Radio astronomers have detected carbon-containing molecules like cyanogens and formaldehyde, suggesting possibilities of organic structures elsewhere in the universe. They have discovered that elliptical galaxies emit considerably more radio waves than do most others. Thanks to microwaves, we have come to know of thousands of incredibly powerful extra-galactic radio sources that are among the most fantastic objects in the outskirts of our universe— mammoth star-like agglomerations spewing out incredible amounts of energy as they rush away at delirious speeds: respectable fractions of the speed of light. These have been named quasi-stellar objects or quasars. Radio astronomy is another window into the universe through which we recognize more wonders of perceived reality.
Many astronomers are convinced that somewhere out there among the countless billions there must be more mind-endowed entities, perhaps not our replicas, but maybe life forms more evolved than ourselves, thinking and feeling, probing and inventing. If they are intelligent beings, they must have their radio astronomers, too—sending “knock-knock” signals and expecting “who’s there?” responses. So we are on the lookout for communications from extra-solar intelligence, not in hard-copy postage, but in cryptic coding of microwaves. Eager cosmic eavesdroppers from among us have been spending countless hours and tidy sums to see if one of their recordings is a patterned signal of sorts. Whether we succeed in this quest for a cosmic connection is not as important as the fact that human ingenuity has come up with tangible ways of confirming whether or not there is interstellar intelligence. Like prayer, irrespective of whether it reaches a target, the effort itself enhances the human spirit.
In the 1940s, microwaves were used to determine the location and motion of distant objects. The invention that accomplished this is the radar. Like all inventions, it has evolved to considerable complexity, used not only to take planes through thick and opaque clouds, but also to help controllers spot and guide them. Microwaves are the principal carriers of television signals and they also serve in long-distance telephony. They penetrate right through the reflecting layers of the upper atmosphere and are thus useful in communicating with astronauts in space. They have come to serve computers and medicine, and yes, they also help us heat foods quickly. Science is no longer natural philosophy: love of knowledge of nature. Rather, it has become a tool for application and power, an instrument to exploit and control nature. This ceaseless obsession to make life more comfortable, enjoyable, and materially fulfilling often loses sight of the grander goal of science, which is to probe into and interpret the wonders of the world, to uncover the subtle cogs and wheels of atoms and laws that make the world tick, and to marvel at the roots of perceived reality.
According to an ancient Chinese legend, a supreme architect called P’an Ku was born of the Cosmic Egg. Working hard for 18,000 years, he built this grand universe of ours. The ripples of this momentous event have not quite died out: P’an Ku’s breath and sighs are still present as winds and rising clouds, the roaring majesty of his voice still resounds in thunder, his flesh congealed into our earth, where his lush hair has become green grass and tall trees. Subterranean metals and minerals are vestiges of his bones, while the abundant sweat of his lasting labors still drip down as rain. Yes, he also had lots of lice infecting his body, and they may still be seen as swarms of humans populating the earth.
This picturesque tale underscores the idea that what we perceive today are consequences of a distant event, a majestic primordial event of immense complexity that ultimately grew into the forms and patterns of today.
But wonder of wonders, an echo of the big bang can be perceived by Homo sapiens these long eons after it occurred. For in 1966, radio astronomers reported the discovery of a microwave radiation that is all pervasive and isotropic. It is interpreted as a remnant of the world-generating big bang. In other words, the enterprise of radio astronomy has put into evidence what may well be described as the first shriek of Baby Cosmos.
At the lower end of invisible electromagnetic waves, we have the X-rays of radiography, and gamma rays whose wavelengths are so short that atomic dimensions are large compared with them. Their frequencies are mind-boggling: of the order of 1022 Hz. Even as visible light emerges from electronic jumps in atoms, gamma rays arise when nuclei shiver, as it were—an electromagnetic outpouring of nuclear agitation. Gamma rays are like super-penetrating bullets. When they encounter a living cell, they simply shatter it.
Gamma rays from unknown sources were detected in space in the 1970s. The Compton Gamma Ray Observatory was launched as a satellite to study them. Now we know that they are produced in abundance in distant galaxies.
Ordinarily, light consists of many waves that move along different directions, their phases not quite in step, somewhat like crowds walking out of the sports stadium. This is only to be expected if we recall that light emerges every time an electron jumps to a higher orbit and promptly falls back to a lower one. Since atoms are distributed at random and the lifting-up energy reaches them in a random manner also, the ensuing waves arise independently without any coherence. But it is possible to create perfectly coherent light—i.e., light made up of identical waves in perfect step. Since this is the result of light amplification by simulated emission of radiation, it has received the acronym of laser. In our crowd analogy, a laser is like the same crowd walking out in perfectly synchronized step along a single path.
The contrivance to produce a narrow pencil of bright red light moving along a straight line like an arrow started out as a toy. One could use it in a lecture to point to a diagram on a screen. But very soon, the invention found the most unexpected applications. Today, lasers are used in compact discs and in check-out counters. They are used to clean up paintings, to treat detached retinas, in communication systems, and in detecting continental drifts. They are integral to our computers and serve in precise measurements. Thanks to lasers, we have come to know the distance of the moon with an error of just one foot. It is remarkable that human ingenuity has created a kind of light that, as far as we are aware, never existed before. Human beings are knowers and doers also.
Thus far, I have reflected on light as a wave that wings its way from point to point as electromagnetic vibrations. But light also behaves as if it is a volley of infinitesimally small specks of pure vibrations, carrying tiny bits of energy, while spinning furiously on an axis. Light (any electromagnetic wave, for that matter) behaves as if it is made up of innumerable little bundles of energy zooming with the same speed of 300 million meters per second in empty space.
The particle aspect of electromagnetic waves is called the quantum or the photon. The energy carried by a photon depends on the frequency of the associated wave; the higher the frequency, the more the energy borne by the photon. But then, particle and energy are fundamentally different: One is localized, and the other diffused. How can the same thing be both? But it is, or so it seems from all experiments. It is this sort of thing that made Niels Bohr say that if you are not jolted by it, you haven’t understood quantum theory.
But then, one may wonder, how can a logical and calculating physicist burst into tears when faced with a tragedy? Yes, she can, because that is part of being fully human.
Similarly, light behaves as particle or wave depending on the circumstance. Is not a coin both head and tail? Throw it, and only one side will show up when it falls on the floor. It is like that with light also: It is both wave and particle; do an experiment, and only one aspect will become apparent.
These are among the fascinating features of light, both visible and invisible. In the spiritual poetic vision, it was there when God asked it to be.