I was having a conversation with a colleague at school when the topic of light came up, and how our use and manipulation of it has allowed us to reach a level of knowledge and understanding of our universe that is really remarkable. Understanding the properties of light and being able to detect light has allowed us to explore the world of the big and small, over a remarkable range of size scales (if you have never seen it, do check out the 'powers of 10' site...very cool!!), so we are at a point in human knowledge where we can not only begin to wonder about how the universe began and how it is put together (man has always wondered about these things), but actually test ideas and learn some of the truths about these questions.
By light I refer to not only the obvious visible light our own eyes detect, but rather all the forms of electromagnetic radiation within the larger spectrum. Radio and microwaves, infrared (i.e. radiant heat) and ultraviolet, x-rays and gamma radiation are all exactly like visible light, only with different wavelengths and frequencies. When it comes to exploring the solar system and beyond, the only information we have comes from these forms of energy. This is the only way to gather any data at all...we detect the tiny amounts of energy in the form of little packets (both a particle and wave) called photons that make it to the earth over countless numbers of miles of space. We cannot physically make it to the places we look in the universe, but in some cases we literally rely on handfuls of photons from the most distant objects we know about and can gather an enormous amount of information from that ancient light.
With light we can determine what stars and galaxies are made of. Putting light through a prism fortunately breaks apart into the rainbow, and by carefully looking at the rainbow of the light from heated elements we see unique patterns appear for each chemical element. For instance, light from hydrogen will break up into four visible lines, one that is red, one that is a blue-green color, a bark blue and violet color. No other type of atom will make this pattern of colors, so when we look at an object that is a billion light-years away and see that same pattern, we know it is hydrogen. Even looking at light from the sun tells us that stars are the producers of the heavier elements we are made of. Carbon, oxygen, nitrogen, all the way up to iron, are made in the nuclear furnaces of stars. Elements above iron are produced when stars explode, and these elements then fly out into space, available to form other objects.
Not only can light allow us to know what distant objects are made of, but we can tell how objects that are seemingly at rest are actually movng at great speeds. The great distances between objects in the universe gives the illusion of everything remaining fixed in a static universe, for it takes far longer than a human lifetime for a distant star to change position to our naked eyes. This effect even fooled Einstein originally, when he included a cosmological constant in his general relativity equations. However, Hubble discovered, through an observation of star light and a common wave phenomenon called the Doppler effect, that stars and galaxies are really moving at high speeds away from each other. Just about everything we can see with our telescopes outside our own Milky Way galaxy has a 'red shift,' meaning they are moving away from us and causing light waves to stretch a bit to become more reddish in color. This is no different than a police car with its siren on moving away from us and hearing a decrease in the siren's pitch, as the sound waves are stretched out because of the relative motion between the siren and us. This observation has led us to Big Bang models of nothing less than the creation of the universe...remarkable! Since Hubble's breaktrough observations dating back to the 1920's and 30's, we have added the capabilities to look not just at the visible portion of the spectrum, but the entire range of light. Detecting radio and microwaves from around the universe has allowed us to test predictions of Big Bang theories (such as the cosmic microwave background radiation distributions) with ever better precision. Looking at frequencies above visible light, in the x-ray and gamma regions, has allowed us to search for the most violent and powerful objects we can imagine, black holes. By looking at light to determine the motion of stars within galaxies, we can compare that motion with our known laws of gravity to conclude that there needs to be more matter than we can detect with light, so now scientists speak in terms of dark matter and dark energy. Again, remarkable that we can even begin to ponder these concepts by looking at the few photons that happen to make it to earth! Only one of our senses can be used to explore space, and by employing a bit of technology to help our sense of sight we can talk somewhat intelligently about how the universe came to be.
While light has helped us observe the macrocosmic heavens, we also use light to develop an understanding of microcosmic world of the basic constituents of matter. The development of quantum mechanics came directly from a few scientists' attempts to understand a basic feature of light. When objects are heated enough, they begin to glow. When one looks at the light emitted by heated objects, we quickly find a particular distribution of the brightness of colors (i.e. blackbody radiation). The only theoretical way to explain this required the introduction of a concept where light had to come in packets (Max Planck, 1900), rather than being a continuous wave, and quantum theory was born. Einstein used his genius to develop relativity and the theory of photons, beginning in 1905. The other key use of light to develop what eventually evolved into quantum mechanics was Niels Bohr's theory to explain the characteristic patterns of light from each element mentioned above. The only explanation for such patterns requires electrons to orbit a nucleus with a particular, finite set of energies. Whereas objects orbiting the sun can have a continuum of values of radius and energy to chose from, electrons orbiting nuclei are restricted to very specific values; those values are quantized. Quantum mechanics continues to be one of the areas of study in physics, and its effects and consequences have moved into the worlds of chemistry and biology, as well as engineering and technology. All of this has been possible by a few observations of light. As with space, light is our only sense that is relevant in the study of the microcosm, whether it is loking at the nature of atoms with spectroscpes or by using microscopes to discover new things about cells.
Perhaps in the future we will have the technologies to add to our observational arsenal. Perhaps we will one day open new astronomical fields of neutrino astronomy or gravity wave astronomy (check out, for example, the LIGO experiment). Perhaps nanotechnology will develop nanomachines that will allow us to extend our sense of touch to the world of the small, so we can add to ur sense of sight in this realm. Time will tell, but it is nice to step back for a moment and reflect (no pun intended) on how relatively simple and basic observations of light have brought us to where we are in our understanding of the universe.