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The Edge of Nowhere

Week of October 27, 1997
family portrait of the solar system After many weeks, our tour of the solar system has finally come to an end. As we pass through the Kuiper Belt and Oort cloud, we have left the realm of the observable solar system. Is there anything else that lies beyond the frigid shell of comets? No one knows. There have been theories that the Sun has an invisible companion, named Nemesis, on a 26 million year orbit. It was proposed to explain periodic mass extinctions seen in the fossil record. As the theory goes, the eccentric orbit of Nemesis dips it into the Oort cloud every 26 million years, sending a shower of comets to rain down on the Earth, wiping out the majority of life on the planet. Is this theory correct? Well, no one has ever seen Nemesis, nor is there any evidence of it. Even the periodicity of mass extinctions in the fossil record is suspect.

So is this vast interstellar space empty? Not really. The Milky Way is more than just a vast collection of stars. It is also composed of gas and dust, which is strewn clumpily throughout the disk of the Galaxy. The average density of particles is about one per cubic centimeter, which is a better vacuum than has ever been achieved in a lab here on Earth. Not only that, but photons, the particles of light, are abundant in space, as well as more esoteric particles such as neutrinos. Maybe gravity itself is communicated via particles, which theorists call gravitons, and these too would be flying at the speed of light through space. What we think of as the emptiness between stars is hardly that. It is filled with things we cannot see, things we cannot touch, but which affect us profoundly just the same.

And the story continues. Theories about the way space itself behaves posit this crowded vacuum boils and froths with tremendous energies, but on a scale so small that comparing the scale to a proton is like comparing a proton to the Sun. These energies may be contained in the smallest imaginable volume, but the scope of them is tremendous. Some theories even say that these energies can somehow escape our own Universe and become tiny pocket universes on their own, separated completely from ours. Who knows?

As we turn around on our journey outwards and see the Sun, trillions of kilometers away, as just another star, and the planets themselves can barely be seen, how can we escape the concept of the strange vastness of the Universe? What awaits us out there as we take our first tentative steps at looking around us? We have sent probes out to explore our solar system, and some of them are flying fast enough to leave the Sun's influence and wander about the Galaxy. What will they see, eons from now?

Maybe they'll see us, waiting for them when they arrive.

The image above is a mosaic of Voyager images taken when the probe was commanded to turn around and take pictures of the solar system from a great distance. This "family portrait" shows just what we look like from a great height. For more information, try taking a look at The NSSDC site which has details about the picture, and a full sized image.

©2008 Phil Plait. All Rights Reserved.

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