The Way of Science

UNIT 4

Cosmology and Relativity

VIII. Einstein's Theory of Relativity

For example, Einstein's two theories of relativity gave us not only warped spacetime, but even stranger phenomena. (Most of Einstein's work has now been thoroughly verified, so don't even suggest that these are "just theories.")

For example, the speed of light in a vacuum (c) is as close to an absolute as we get in the sciences. What's so weird about that? Consider two cars approaching each other, each one moving at 60 mph relative to the highway. Their speed relative to each other is, thus, 120 mph. Now imagine the same scenario with two spaceships, each moving at 60% c, according to an outside observer. Each ship has its headlights on. You might think that either beam of light would approach the other ship at over 120% of c, but that is not so. The actual speed of light remains 186,000 mps, as seen from either ship.

This "cosmic speed limit" leads to even more counter-intuitive phenomena. Why can't a ship, or any mass, go at or beyond the speed of light in a vacuum? To increase speed, any ship (or subatomic particle) must add energy. But this energy, at significant fractions of c, is converted to mass! That has been well-verified experimentally. The ship, or particle, thus becomes more massive, which in turn requires more energy to accelerate, which produces more mass, and so on. At c, the ship or particle would have infinite mass, which is impossible. Something else very odd would be seen by an outside observer, but not by the ship's crew. The shape of the ship changes. Be sure to read p, 165 in Trefil and Hazen to find out about the change.

Another strange phenomenon predicted by Einstein is time dilation. There is no absolute time; it will vary depending on speed and nearby mass. See your text for details, but in the meantime, consider the twin paradox.

In the far and hypothetical future, suppose a pair of identical twins is born. Soon after birth, the parent take one twin with them on a sightseeing trip to our nearest star, Alpha Centauri (4.3 ly distant). The other twin remains behind on Earth. The ship travels at a significant fraction of c. When the round-trip is complete, something odd has happened: the twin on Earth is many years older than the twin on the ship. Atomic clocks on the ship show much less elapsed time than sister clocks on Earth. The greater the speed, the slower the time runs. Which one is the "real" time? Answer: turnips. (It's a nonsense question. Both are real, since there is no absolute time.)

Let's see where we are in our quick trip through the major theories and models of science. All of these models covered so far continue to be rich sources of new knowledge about the Universe. Cosmology is poised on the edge of a major leap, with new technology able to look more accurately into very distant realms. In biology, the "DNA revolution" continues like a juggernaut. One of its great beneficiaries is the study of Evolution. Plate tectonics continues to be mined for insights into large-scale geological processes; it's likely that we will have a solid picture, quite soon, of how the Earth generates and reverses its magnetic field.

One highly important model that is still missing this semester is quantum mechanics, which applies primarily to the subatomic world. One great problem in modern physics is reconciling Einsteinian gravity (which is a continuous phenomenon) with the quantum view of everything , which is discrete. Do glance at Trefil and Hazen, for a brief and understandable coverage of quantum mechanics. One very interesting application of the quantum view is in answering one of the great philosophical questions, which is now a great physical question also. Where did everything come from? How can one get something from nothing?

Quantum mechanics may eventually supply at least a partial answer. Take the idea of 'empty space," i.e., the vacuum. QM says that complete emptiness cannot be, since virtual particles and their antiparticles constantly appear (from nothing!), briefly exist, and then cancel each other. This phenomenon has been experimentally verified. With a little fooling about, it may be possible to get the "Big Bang" from quantum nothingness. On the other hand, some major theoreticians have proposed an endless (and beginningless) series of "bubble" universes, budding off from each other, each with different laws of physics, and unfortunately completely isolated from each other. How does one falsify such concepts? That's a really good question!

All of us who teach this course hope that at least a few of you will develop a continuing interest in this, or any other, major scientific endeavor covered this semester. To those of you who have that kind of intellectual curiosity, we wish a heartfelt "bon voyage" on your search for knowledge.

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