A Physics Digest—Part 8: Relativity and Astrophysics

This is the final entry of my physics digest and is appropriate as a conclusion: turning to look outwards at our magnificent and unimaginably large universe. When we attempt to comprehend the vastness of the universe we can’t help but see how tiny, alone, and precious our little planet is.

Albert Einstein founded the concept of relativity a little over a hundred years ago, by conducting brilliant thought experiments. (His experiments had to be conducted in his head, since he had no way to check them out in a lab. Years later technological developments allowed others to confirm his theories.)

Einstein realized that space and time are linked; whereas Newtonian mechanics assumes they are independent. When we move through space, for example, our sense of time can be altered. Motion, in fact, is relative; how it appears to someone at rest differs from someone who is moving. If I drive alongside a car on the interstate, it appears to hardly be moving; while someone standing by the roadside sees us both zipping past.

The first postulate of relativity is that all of nature’s laws are the same in any uniformly moving (constant speed) reference frame. If I toss up a ball while riding in that car, it will fall back into my hand, just as it will if I’m standing at roadside.

The second postulate of relativity is a little trickier: the speed of light is absolute; it's the same, regardless of one’s frame of reference. While things are relative at slower speeds (as when I’m driving in my car), the speed of light never changes (it would be measured the same for me as for the person standing by the roadside). How can light be absolute? Einstein realized that at near-light speeds both time and space contract, and they do so in a manner that, no matter how fast one goes, the speed of light is observed as constant. This happens because time slows down and objects mysteriously become squashed. It only happens for subatomic particles moving near the speed of light, not for plodding objects like people.

These results are all from the “special theory of relativity.” It’s special because it describes those objects moving at a constant speed. Einstein’s general theory—dealing with accelerating objects—took him another ten years to decode. He saw that an accelerating object behaves the same as if were under the influence of gravity. Release an apple (gravity acting upon it) and it will accelerate towards the ground. If you’re in an elevator that’s starting upward (accelerating), you feel heavier, as if suddenly under a stronger gravitational field. When the elevator starts down, you feel lighter, as if momentarily under less gravity. So motion and gravity are also linked.

Einstein realized that gravitational effects were still true for light waves—even though they have no mass. How can this be? It’s because huge bodies, like the sun, literally warp space around them. Light waves simply follow bent space. That’s another concept that physicists are still trying to wrap their minds around.

Finally, astrophysics: from the minute to the immense. The study of astrophysics often parallels the passage of time: how our universe began and is unfolding. It all apparently got underway with the Big Bang, about 14 billion years ago. An unimaginably tiny hot spot blew up and expanded into an unimaginably big universe. That’s the current best guess. Physicists continue to struggle with the mathematical description of that beginning.

The early universe—as it expanded outward—was composed almost entirely of hydrogen, with a dash of helium thrown in. That’s all. No carbon, oxygen, iron, lead. The first stars got formed when clouds of hydrogen collapsed, at which time the high pressure and temperature set off a nuclear fusion process. Those early stars burned hot and fast—lasting but a few million years. As they burned out they collapsed yet a little more, bringing crushing pressures inside, which created even more fusion into other elements. They then blew up in a super nova, spraying all those new elements into space.

Later forming stars (like our sun, born five billion years ago) were created when the new debris collapsed. But now there was only 99% hydrogen. Most of the other elements became fashioned into planets. Our sun is currently at its mid life. In another five billion years it will begin to die. It will do so at first by expanding, consuming, and frying the inner planets. What further evolutionary developments will alter life on our little planet in that upcoming five billion years? No one knows; we’ve just begun.

New tools have recently allowed astronomers to observe planets orbiting other stars. It gives us our first proof that Earth and her sister planets are not alone. We also recently have found that life is far more robust than we once thought, and that conditions exist elsewhere (on some of Saturn’s moons, for example) that likely are conducive to these tough forms of life. Will we find that our planet is not alone in harboring life in this vast universe? No one knows. It’s all speculation for now. But I’m doubtful that extraterrestrial life—if it’s out there—will be bipedal and speak English, as Star Trek would have us believe.