Date: May 5, 2010

Title: The Continuing Copernican Revolution

Podcaster: Rob Knop

Organization: My home page: http://www.sonic.net/~rknop
MICA: http://www.mica-vw.org
MICA Public Events: http://www.mica-vw.org/wiki/index.php/MICA_Events

Description: Humanity has long been trying to understand the nature of the cosmos.

Bio: Rob Knop obtained a PhD in Physics from Caltech in 1997. He then worked with the Supernova Cosmology Project and was part of the discovery that the expansion of the Universe is accelerating. After six years as an assistant professor at Vanderbilt University, he worked in the computer industry for two years. This semester, he’s teaching physics at Belmont University in Nashville, and next fall
will join the new college Quest Unviersity in British Columbia. He gives regular astronomy talks in Second Life in association with the Meta-Institute of Computational Astronomy.

Today’s sponsor: This episode of “365 Days of Astronomy” is sponsored by Mr. Thomas — Dedicated to my students: may they fly to work with jet-packs, and may some teach math to their students in their school on the moon.

Additional sponsorship for this episode of 365 Days of Astronomy has been provided by Kylie Sturgess and the Token Skeptic podcast, a weekly show about superstition, science and why we believe – at www.tokenskeptic.org.

Transcript:

I am Dr. Rob Knop. This coming fall, I will be joining the faculty of Quest University in Squamish, British Columbia, where I will be teaching physics and related subjects.

Cosmology is the study of the Universe as a whole. Nowadays, the study of Cosmology is the study of the Big Bang and its aftermath, and is heavily steeped in Einstein’s theory of Relativity. However, humanity has attempted to understand the Universe as a whole starting long before the advent of the 20th century.

For a long time, Western Civilization had a view of the Universe given to us by the ancient Greeks. It was informed by the physics and world-view of Aristotle, who believed that everything had a natural place. Heavier things were naturally lower than lighter things, explaining why, for example, rocks sink in water. One important part of the philosophy that Aristotle adhered to was the separation of the heavens and the Earth. The Earth, the realm of humanity, was imperfect. The Celestial realm, the realm of the planets, was supposed to be perfect. The physical laws that applied on Earth, in this philosophy, were not necessarily linked to the laws that applied in the heavens.

The model of the Universe that went along with this philosophy is attributed to Ptolemy. And, indeed, Ptolemy had developed extensive mathematical tables that allowed quite accurate predictions of the positions of stars and planets in the sky based on this model. This model placed the Earth at (or at least very near) the center of everything. The planets were on “celestial spheres”, huge spherical shells that surrounded the Earth. As these spheres rotated at different rates, so we would see the planets move relative to the stars. The stars themselves were believed to be on the outermost celestial sphere, the most perfect sphere that had the simplest rotation.

In this view, there were seven planets: Mercury, Venus, Mars, Jupiter, Saturn, the Sun, and the Moon. Yes, the Sun and Moon were once considered planets! Next time you’re feeling sorry for Pluto and its status demotion, remember that the Moon itself was once considered a planet. In order to properly predict the orbits of the planets, Ptolemy couldn’t have them simply sitting on the insides of these huge spherical shells as the shells rotated. For instance, planets such as Mars or Saturn will often show what’s known as retrograde motion. They move one way relative to the stars through most of the year, but then for a while they turn around and go the other way. To account for this in his model and in order to correctly calculate the positions of the planets, Ptolemy had to add epicycles, additional smaller rotating circles on the surface of the huge celestial spheres.

In the 16th century, Polish astronomer Nicolaus Copernicus revolutionized our view of cosmology by proposing a model whereby the Sun, not the Earth, was at the center of the Universe, and all the other planets orbited around the Sun– except for the Moon, which orbited around the Earth. He showed that you could explain things like the retrograde motion of planets easily, with the planets orbiting in simple circles; there was no need for complication similar to the epicycles of Ptolemy. In this day and age, all educated people know that the Earth goes around the Sun, and it doesn’t seem very troubling philosophically. However, at the time, this represented a huge shift in humanity’s view of their place in the cosmos. The Earth had always been thought of as being at the center, and now it was moved away, placing the Sun at the center. Indeed, some decades after Copernicus’ death, the Church declared his works heretical, and among others Galileo got into trouble for providing supporting telescopic observations.

Although Copernicus had moved Earth away from the center, there was still no notion that the laws governing the motions of the celestial bodies would be the same as the laws governing physics on the Earth. In the 17th century, Isaac Newton developed his law of universal gravitation. This enormously successful theory provided a single law of gravity that explained the falling of objects on the Earth, and the orbits of the planets. In a sense, this continued the revolution of Copernicus. Previously, the Earth had been a special place, at the center of the Universe. Now, not only was the Earth not at the center, but the physical laws that applied here were understood to be the same as the physical laws that apply everywhere.

Although Copernicus– and Kepler and Newton following him– firmly established the Sun as the body that the other planets orbit around, it was only later that we came to understand that the Universe encompassed more than the Solar System. Copernicus considered the Sun not only to be the center of the Solar System, but to be the center of the Universe itself. Some ancients had proposed that stars were similar to the Sun, and in the late 16th century– after the work of Copernicus– Giordano Bruno proposed that the stars were distant Suns, perhaps with planets of their own. During the next century, astronomers came to accept that indeed the stars were other Suns. This, also, represented a continuation of the Copernican revolution. As Newton was moving towards his universal theory of gravitation, scientists came to accept that the Sun was not particularly special, but was just one star among many.

This trend of moving ourselves away from the center would continue. In the 18th century, British astronomer William Herschel made star counts in the sky, and concluded that the Sun had to be offset from the center of the Universe, for there were more stars in one direction of the sky than in the other. This view would be affirmed by Harlow Shapley’s model of the Galaxy, which is remarkably close to our modern view, whereby most of the stars in the Galaxy are in a flattened disk, with the Sun well offset from the center of this disk.

The next step in moving ourselves away from the center would be when we realized that the Universe encompassed more than the Galaxy. At the beginning of the 20th century, astronomers were torn as to whether the “spiral nebulae” as they were called, represented nebulous clouds of matter inside our own Galaxy, or if they were “island universes”, other galaxies analogous to our own. Shapley was one who believed that our Galaxy was the entire Universe, and that the spiral nebulae were in fact just nebulae. It was Edwin Hubble, in the second decade of the 20th century, who measured that these other galaxies were much farther away than the size of our own Galaxy, confirming that they were in fact island universes. Once again, our conception of the Universe as a whole had expanded, making that part of it in which we live relatively that much smaller. Not only is the Sun just one star among many, but our entire Galaxy is but one among many.

The trend of moving ourselves away from the center of all has continued on into modern cosmology. In the latter half of the 20th century, astronomers became increasingly convinced that most of the matter of galaxies and clusters of galaxies was not made up of the normal atoms that we are made out of. Rather, most of the mass of galaxies is in Dark Matter, a mysterious substance that almost never interacts with regular matter, except through gravity. The evidence for Dark Matter was very strong at the close of the 20th century, and irrefutable evidence came in the first decade of the 21st. Today, we know that there is something like five or six times as much Dark Matter in the Universe as there is normal matter, matter made out of protons and neutrons and electrons.

On top of that, at the close of the 20th century was the discovery that the expansion of our Universe is accelerating. In order to explain this, astronomers had to conclude that the Universe is filled with a substance even more mysterious than Dark Matter, a substance that today we call Dark Energy. Multiple completely independent observations have confirmed these results by now. Our current picture of the Universe is that it is about 75% Dark Energy, about 20% Dark Matter, and only about 5% normal matter. This represents just the latest step in the continuing Copernican revolution. Many previous steps involved moving ourselves further away from what we understood to be the center of the Universe; first the Earth was moved away, then the Sun was moved away, then we understood that our very Galaxy was one among many. This is a more abstract moving away from the center: we have come to understand that what we are made out of is not what most of the Universe is made out of.

The trend that Copernicus started, of moving the Earth away from a central place in our view of the Universe, is today a core principle of cosmology. The Cosmological Principle, as it is called, effectively states that “we are nowhere special.” Our Galaxy isn’t particularly special, and there are many like it out in the Universe. There are many other stars in our own Galaxy that are very similar to the Sun… and, indeed, today we know for sure that many of them do have planetary systems. Some questions remain… for instance, the relative densities of Dark Matter and Dark Energy are suspiciously close, and the simplest model for what Dark Energy can be suggests that this situation won’t last for long– that eventually Dark Energy will drive the Universe to be so spread out that we almost wouldn’t be able to observe distant enough galaxies to see the expansion of the Universe at all. Why are we then so lucky as to live at a time in the Universe where their densities are so close to each other? This is a challenge to the Cosmological Principle, for which astronomers may well find an answer in coming years.

End of podcast:

365 Days of Astronomy
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