Confusion about Comets

Confusion About Comets

Some confusion about comets continues to our own time. In 1957, Carl Sagan was a graduate student at the University of Chicago’s Yerkes Observatory. Alone in the observatory late one night, he heard the telephone ring persistently. When he answered, a voice betraying a well-advanced state of inebriation, said: Lemme talk to a shtrominer”. “Can I halp you?” “Well, see, we are having this garden party out here in Wilmette, and there is something in the sky. 

The funny part is, though, if you look straight at it goes away. But if you don’t look at it, there ii is”. The most sensitive part of the retina is not in the center of the field of view. You can see faint stars and other objects by averting your vision slightly. Sagan knew that, barely visible in the sky at this time, was a newly discovered comet called Arend-Roland. So he told the man that he was probably looking at a comet. There was a long pause, followed by a query: “Wash’a comet? “A comet, Carl Sagan replied, is a snowball one mile across”. There was a long pause after which the caller requested : Lemme talk to a real shtronomer. 

When Halley’s comet reappears in 2062, we wonder what political leaders will fear the apparition, what other silliness will then be upon us. While the planets move in elliptical orbits around the Sun, their orbits are not very elliptical. At first glance they are, by and large, indistinguishable from circles. It is the comets, especially the long period comets, that have dramatically elliptical orbits. The planets are the old-timers in the inner solar system; the comets are the newcomers. Why are the planetary orbits nearly circular and neatly separated one from the other? Because if planets had very elliptical orbits, so that their paths intersected, sooner or later there would be a collision. In the early history of the solar system.

Halley’s comet. Once around the Sun is a long time if you live in the outer reaches of the solar system. Image A Light Blue Pond by © Megan Jorgensen

Those with elliptical crossing orbits tended to collide and destroy themselves. Those with circular orbits tended to grow and survive. The orbits of the present planets are the orbits of the survivors of this collisional natural selection, the stable middle age of a solar system dominated by early catastrophic impacts.

 In the outermost solar system, in the gloom for beyond the planets, there is a vast spherical cloud of a trillion cometary nuclei, orbiting the Sun no faster than a racing car at the Indianapolis 500. (The Earth is r=1 astronomical unit, 150,000,000 kilometers from the Sun. Its roughly circular orbit then has a circumference of 2 пr = 10(9) km. Our planet circulates once along the path every year. One year = 3 x 10(7) seconds. So the Earth’s orbital speed is 10 (9) km/3 x 10(7) sec. = 30 km/sec. Now consider the spherical shell of orbiting comets that many astronomers believe surrounds the solar system at a distance = 100,000 astronomical units, almost halfway to the nearest star. 

From Kepler’s third law it immediately follows that the orbital period about the Sun of any one of them is about (10(5))3/2 = 10 (7.5) = 3×10(7) or 30 million years. Once around the Sun is a long time if you live in the outer reaches of the solar system. The cometary orbit is 2пa= 2п x 10 (5) x 1.5 x 10(8) km = 10(14) km around, and its speed is therefore only 10(14)km/10(15) sec – 0,1 km/sec = 220 miles per hour. Halley's comet

Local Group of Galaxies

Local Group of Galaxies

Several million light-years across, the Local Group of galaxies is composed of some twenty constituent galaxies. It is a sparse and obscure and unpretentious cluster. One of the galaxies is M31, seen from the Earth in the constellation Andromeda. Like other spiral galaxies, it is a huge pinwheel of stars, dust and gas. M31 has two small satellites, dwarf elliptical galaxies bound to it by gravity, by the identical law of physics that tends to keep us in our chairs. The laws of nature are the same throughout the Cosmos, even if we are two million light-years from home.

Beyond M31 is another, very similar galaxy, our own, its spiral arms turning slowly, once every quarter billion years.

We, the Earthlings, live in the remote outskirts of the Galaxy, in an obscure locale near the edge of a distant spiral arm. And we find ourselves falling toward the massive center of the Milky Way.

Cluster of Hercules. Local group of galaxies © Megan Jorgensen (Elena)

We Call Them Galaxies

We Call Them Galaxies

The early universe was filled with radiation and a plenum of matter, originally hydrogen and helium, formed from elementary particles in the dense primeval fireball. There was very little to see, if there had been anyone around to do the seeing. Then little pockets of gas, small nouniformities, began to grow. Tendrils of vast gossamer gas clouds formed, colonies of great lumbering, slowly spinning things, steadily brightening, each a kind of beast eventually structures in the universe had formed. We see them today. We ourselves inhabit some lost corner of one. We call them galaxies.

About a million years after the Big Bang, the distribution of matter in the universe had become a little lumpy, perhaps because the Big Bang itself had not been perfectly uniform. Matter was more densely compacted in these lumps than elsewhere. Their gravity drew to them substantial quantities of nearby gas, growing clouds of hydrogen and helium that were destined to become clusters of galaxies. A very small initial non-uniformity suffices to produce substantial condensations of matter later on.

Galaxy. The primordial galaxies spun increasingly faster, because of the conservation of angular momentum. Image: © Elena

As the gravitational collapse continued, the primordial galaxies spun increasingly faster, because of the conservation of angular momentum Some flattened, squashing themselves along the axis of rotation where gravity is not balanced by centrifugal force. These became the first spiral galaxies, great rotating pinwheels of matter in open space. Other protogalaxies with weaker gravity or less initial rotation flattened very little and became the first elliptical galaxies. There are similar galaxies, as if stamped from the same mold, all over the Cosmos because these simple laws of nature – gravity and the conservation of angular momentum – are the same all over the universe. The physics that works for falling bodies and pirouetting ice skaters down here in the microcosm of the Earth makes galaxies up there in the macrocosm of the universe.

Black Holes and an Infinite Number of Universes

Black Holes and an Infinite Number of Universes

In the 1920’s, in a direction opposite to M31 galaxy, observers found a distant pair of spiral galaxies. Was it possible, they wondered, that they were seeing the Milky was and M31 from the other direction – like seeing the back of your head with light that has circumnavigated the universe?

We know now that the universe is much larger than these scientists imagined in the 1920’s. It would take more than the age of the universe for light to circumnavigate it. And the galaxies are younger than the universe. But if the Cosmos is closed and light cannot escape from it, then it may be perfectly correct to describe the universe a black hole.

There exists a possibility of wormholes to get from one place in the universe to another without covering the intervening distance – through a black hole. We can imagine these wormholes as tubes running through a fourth physical dimension. In fact, we do not know if such wormholes exist, but if they do, must they always hook up with another place in our universe? Or is it just possible that wormholes connect with other universes, places that would otherwise be forever inaccessible to us.

If you wish to know what it is like inside a black hole, look around you (quotations from Megan Jorgensen). Image: © M. Jorgensen (Elena)

For all we know, there may be many other universes and perhaps they are, in some sense, nested within one another.

The Hindu cosmology is the only religious idea we know that surpasses the endless number of infinitely old cycling universes. What would these unverses be like? Would they be built on different laws of physics? Would they have stars and galaxies and worlds, or something quite different? Might they be compatible with some unimaginably different form of life? To enter them, we would somehow have to penetrate a fourth physical dimension – not an easy undertaking, surely, but perhaps a black hole would provide a way. There may be small black holes in the solar neighborhood. Posed at the edge of forever, we would jump off…

Einstein and a Wave of Light

Einstein and a Wave of Light

If you have walked through the pleasant Tuscan countryside in the 1890’s, you might have come upon a somewhat long-haired teenage high school dropout on the road to Pavia. His teachers in Germany told him that he would never amount to anything, that his questions destroyed classroom discipline, that he would be better off out of school.

Se the young man left and wandered, delighting in the freedom of Northen Italy, where he could ruminate on matters remote from the subjects he had been force-fed in his highly disciplined Prussian schoolroom. His name was Albert Einstein and his ruminations changed the world.

Einstein had been fascinated by Bernstein’s People’s Book of Natural Science, a popularisation of science that described on its very first page the astonishing speed of electricity through wires and light through space. He wondered what the world would look like if you could travel on a wave of light.

To travel at the speed of light? What an engaging and magical thought (quotations from Megan Jorgensen). Image : © Meg Jorgensen (Elena)

To travel at the speed of light? What an engaging and magical thought for a boy on the road in a countryside dappled and rippling in sunlight.

You could not tell you were on a light wave if you traveled with it. If you started a wave crest, you would stay on the crest and lose all notion of it being a wave.