dlg@philabs.UUCP (Deryl Gaier) (10/06/83)
All of this recent talk about traveling at or near the speed of light has reminded me of a question of mine that was never answered. I have been away from the study of relativity long enough that I don't remember a lot of the details, so maybe someone out there who is more familiar with them can help me out. My question has to do with traveling at or near the speed of light. I have heard the argument that as your speed approaches that of light, your mass also approaches an infinite value, thus you need an infinite amount of energy to accelerate it. I can agree with this if the source of energy is some type of chemical reaction such as is used to power the present day spacecraft, since the energy released is primarily from atomic bonds. Now to the point of my whole question (after I have bored half of you silly!): Does this case still apply if the energy source for the spacecraft is a mass to energy reaction (such as nuclear fusion)? I would think that since the amount of energy released is proportional to the mass that is "lost" in the reaction, that as the ship's speed goes to infinity, the amount of mass "lost" in the reaction (and hence the energy available to propel the ship) should also approach infinity. Since these two quantities seem to be staying in the same proportion to each other (thus giving the infinite energy required to continue accelerating), why is there a limit to the speed which can be attained? Have I missed something in my line of reasoning, and if so what is it? As I have said, I have been away from the subject for a while, so a medium level discussion of this question whould be most helpful.
Shinbrot.WBST@PARC-MAXC.ARPA@sri-unix.UUCP (10/10/83)
Afraid you've missed something. The mass of a fast-moving object does increase with its speed, but not due to changes in chemical bonding or similar properties. Furthermore, from the vantage point of sthe fast-moving object, NOTHING has changed. This is fundamental to the theory of relativity. There is a limit to the speed of a spaceship because its inertia (mass) increases as its speed approaches c. Therefore the force needed to accelerate it an amount a, is increasingly large (since F = ma). Inside the spaceship, again, this is not observable. Newtonian laws still seem to be obeyed, the mass of the spaceship seems unchanged, the mass of the fuel seems unchanged. The speed of the ship at any time, seems to be zero (if you ignore the stars whizzing by) once the acceleration is stopped. - Troy PS. As I've recently prosletized, there's a great book that makes all of this tractable, "Einstein for Beginners."
gwyn@brl-vld@sri-unix.UUCP (10/14/83)
From: Doug Gwyn (VLD/VMB) <gwyn@brl-vld> This business about increased mass has nothing to do with how the object got to be moving. The principle is, If an object would appear to have mass m0 to an observer at rest with respect to the object, then the object appears to have mass m(v) = m0 * c / sqrt( c^2 - v^2 ) to an observer moving with velocity v with respect to the object. You can turn the description around and say that the object is moving with velocity v with respect to the observer, if you wish. "mass" as used here is not an absolute property, since it involves the state of motion of the observer. Some of us would prefer to reserve the word "mass" for m0, usually known as the "rest mass" of the object. The reason the concept is used this way is because this definition of "mass" obeys many of the laws of Newtonian physics.