>Hi Hilmar,
>
>One more question about which I've wondered. Can matter be accelerated to near the speed of light in the vacuum of space? Assume for proposes of the question, that in a vacuum there would be no resistance to the matter or any other thing that acted upon the matter like gravity, other than the thrust of the rocket engine. Common sense would indicate that a steadily applied force, like in a rocket's thrusters, would over time accelerate the matter to near light speeds or beyond.

One more thing about common sense - this time related to the speed of light. This time, I am talking about the Theory of Relativity, not about classical physics.

If a car moves at 60 km/h, and you run after it at 10 km/h, you would expect the car to recede from you at a speed of 60-10 = 50 km/h. If you run in opposing directions (along a straight line, for simplicity), your distance from the car would increase (or decrease) by 60+10 = 70 km/h. And this is indeed what happens - at low speeds. These speeds would be called the "relative speed" - the speed as measured by a moving observer.

Now at greater speeds: a beam of light moves at ca. 300,000 km/sec. Let's say this is with relation to Earth. Assume you go with a speed of 100,000 km/sec. (1/3 the speed of light); then you would expect the ray of light to have a relative speed of 400,000 km/sec (or of 200,000 km/sec).

Strangely enough, all sorts of experiments have been done, with ever-increasing accuracy, and no deviation has ever been found in the speed of light! Despite the fact that the Earth rotates, and moves around the Sun, for instance.

(This is the negative result of the Michelson-Morley experiment I mentioned before.)

It is this strange fact, of the constancy of the speed of light, which the Special Theory of Relativity takes as one of its guiding principles. Another guiding principle is the principle of relativity: the claim that laws of Nature don't change, whether you are moving, or at rest (meaning that you can't specify an "absolute velocity").