So far I have kept the discussion well within a basic understanding of high school physics. Even the most abstract and difficult advances in modern science are based on those foundations, as well as those I will continue to discuss here and in the following articles in this series. Our goal is to understand the reasoning behind these advances, even if the latest doctoral thesis is riddled with jargon; therefore, for the time being we must temporarily feign ignorance of much that we currently understand in order to understand the universe as the earliest pioneers did.
In the previous article we briefly reviewed the early history of astrophysics and the unification of earthly laws and heavenly laws with the publication of Isaac Newton’s Principia. Shortly before Newton, Robert Boyle had published his Sceptical Chemyst (1661) making him the founder of modern chemistry. It's important to note that without Newton’s Principia, much in the field of chemistry might still have developed, but virtually no one would’ve believed it possible to extrapolate those findings as having anything to do with the heavens. Newton’s unification of the laws of motion and Universal Law of Gravitation opened the sky to inquiry by way of earthbound experimentation.
As a result, an important clarification was made that does not occur in chemistry - mass and weight basically mean the same thing to an earthbound observer, but in space objects can have mass without weight. This was understood long before the first astronaut. In physics, the mass of an object is defined by its gravitational influence on other mass-bearing objects. A hairpin exerts gravity on the earth and vice-versa, but the overwhelming difference in mass between them moves the hairpin, not the earth.
Still, celestial objects remain remote and inaccessible to direct examination. As more observers gradually began taking seriously the notion that the earth was indeed revolving around the sun further evidence of this and a greater understanding of the universe would occur on two fronts: Geometry and Optics.
The Universe in 3D
From as early as the 6th century BCE, Thales of Miletus reasoned that the distance of a ship at the horizon from any port could be determined by marking the difference in its position as seen from two points on the shoreline. This difference in position can easily be demonstrated by extending your thumb out in front of your eyes and then alternately opening and closing each eye. This is known a parallax. In fact, most creatures have two eyes producing stereoscopic vision or 3D. Parallax requires only that we understand some basic principles about triangles and that given certain known values (lengths and angles) all other values for a triangle can be determined (note the illustration below).
If indeed the earth revolves around the sun, then the distance between the earth and the sun is its orbital radius (93 million miles) and its diameter is about 186 million miles. This means that every six months the earth is 186 million miles from its previous position. This makes it possible to determine the distances to relatively nearby stars insofar as our instruments are highly precise. The more distant an object is, given the orbital diameter to serve as the base of a triangle, the smaller the angular difference in parallax.
The Final Proof of Copernicus
Given a circle of 360 degrees, each degree equals 60 minutes and each minute equals 60 seconds; therefore, one arc second is 1/3,600 degrees. That's the shift our telescopes detect between that 186 million mile distance for a star that is one Parsec (parallax second) or 3.26 light years. It was this method that ultimately ended all doubt in the sun-centered model of Copernicus, because all of the previously known "fixed" stars in our stellar neighborhood were found to move in perfect synchronicity along tiny circles. This only amplified the absurdity in Ptolemy's theory of epicycles. Imagine sitting in a car parked in the rain. As the car accelerates all the rain drops gradually change from a vertical descent to a horizontal descent against the windshield in perfect synchronicity. Do we attribute this to the motion of the rain or to the motion of the car? What child has not marveled in our time the snowflakes that rush up to the windshield of a car?
A New Vision
This method works for stars within several hundred light years, but what of stars and other objects still further? The field of optics is known to extend as far back as the tenth century BCE but made no significant advances until the 11th century when Greek ideas were resurrected among Muslim scholars. Alhazan, in particular, wrote his Book of Optics over a ten year period from 1011 to 1021 CE in which he proposed the current understanding of vision. Previously, leading scholarship believed that the eyes actually produce the light that made the universe visible.
As strange as that may seem, it would make perfect common sense in the absence of careful thought. Virtually no one doubted this notion for hundreds of years because while we now know that information enters the eyes in the form of light we also now know that meaning is tied to observation and it is the observer who projects meaning. What we typically refer to as optical illusions are most often cognitive illusions associated with how the mind organizes visual information. Even today there continues to be a great deal of confusion in popular culture between what we see and what we think we see. Alhazan had merely questioned a long held assumption concerning our perception of the universe, just as Copernicus would do nearly 500 years later.
But the first telescopes were not used in astronomy until Galileo in the early 17th century. Galileo observed the imperfect surface of the moon and far from accepting it as proof that heavenly objects were much like earthly objects, many accused him of sorcery. That shows how much human understanding has had to overcome over the centuries. Those who ridicule the ignorance of our ancestors only reveal their own ignorance that we have their conflicts to credit for our slow and gradual progress.
Shedding a Little Light on the Matter
Newton and others had improved on the design of the optical telescope, but the most significant advances in optics occurred with the development of spectroscopy as Newton and others began to realize that light is emitted, absorbed, reflected, refracted and diffracted by matter, and how light interacts with matter depends on the kind of matter. This would make it possible to observe the spectral signature of various chemical elements in light coming from the stars.
This is important because the recent discovery of dark matter suggests that there is a kind of matter that does not interact with light at all, kind of like those movies of an invisible man whose only means of detection are his footprints in the snow - matter that only interacts with other matter. The problem is that no such matter exists in our stellar neighborhood, despite the overwhelming statistical probability that it should if 95% of the universe is comprised of this kind of matter.
Still further, enormous advances in chemistry and physics over the past several centuries seem to suggest that no such matter could possibly exist, even if it does make for entertaining movies. It was the work of Michael Faraday, followed by James Clerk Maxwell in the early 19th century that unified our understanding of magnetism, electricity and light into the electromagnetic spectrum. Maxwell's equations revealed that light was a phenomenon not limited to the perceptible colors of the rainbow, but extended into a vast range of wavelengths and amplitudes beyond our physical perception. Given the invisible man analogy we would simply aim our infrared camera in the direction of the footprints and - Voila! - the invisible man becomes visible.
Phenomena that is beyond our physical perception is therefore not beyond scientific detection. Yet it would seem that Dark Matter is and that is the dilemma that may well require us to revise centuries of science on the matter (pun intended). In the next article in this series we will dig deeper still into
advances in 20th century astrophysics and detail the discovery of dark matter and dark energy.
Sources:
- Goldsmith, Donald Ed. (1991) The Astronomers: Companion to the PBS television series, St. Martin's Press, NY.
- Hawking, Stephen Ed. (2002) On the Shoulders of Giants: the great works of physics and astronomy, Running Press
- Ferris, Timothy Ed. (1991) The World Treasury of Physics, Astronomy and Mathematics, Little Brown and Company.
