timo

http://www2.corepower.com:8080/~relfaq/FTL.html

While googling around for another post I found this: http://www2.corepower.com:8080/~relfaq/headlights.html If you don´t want to read the text, the short resume of it is: You cannot travel at lightspeed.

wrote a long text and while doing it my login expired so it´s gone now ... short answers: - relativistic mass M does not have any physical meaning (dependancy of coordinate system). - when talking about the mass of a particle one means the restmass because it´s independent of a coordinate system. - photons have a zero restmass. - you don´t have to divide by zero. If you take the limit(v->c) of E you´ll see that energy approaches infinity as v approaches c (differential calculus is the english name for the field of maths that deals with limits of real values, I think. Dave will certainly know that for sure). - the given equation implies E=0 for v->c if m=0. But the equation is not defined at v=c, so it does not apply to particles moving at c.

>> I hope i posed the question clearly ? sadly you didn´t. Velocity (and thus it´s magnitude speed) as I know it is defined at a point (as an tangential vector on the curve x(t), where x(t) is position at time t). I don´t really understand what you mean by being faster than light globally (of course I can be faster than light: Send a ray of light to the sun and then reflect it to a point 10m away from me ... I´ll be there faster).

sry ed84c, I didn´t read your post carefully enough so the formula I gave you was not a complete answer to your question. You have to add: M = g*m, where m is the restmass and M is the "reativistic mass" (which you plug in the famous E=Mc²). so since g->inf for v->c it follows that M->inf for v->c if m>0. I was so used to "Mass=Energy" that I forgot to explicitely mention it. What you worked out (mathematical discussion about correct treatment of the limit v->c let aside) is that a particle moving at lightspeed must have a restmass of 0 which is perfectly true (photons, as stated above).

E = g*m*c², g = 1/sqrt(1- v²/c²), where m is the restmass

Engines don´t have speeds, only powers. The rest of your post thus becomes useless due to improper definition of the problem (no offense, it´s just impossible to scientifically correctly answer the question without further assumptions). Nevertheless, any additional condition you could give (which must make sense, of course) will lead to conservation of energy (most likely due to friction of the wheels that rotate to fast or to slow on the tracks).

@ 2nd question: - kills living but leaves buildings intact: I allways thought the reason for this was that neutrons mainly kick out hydrogen atoms off their bounds which is quite unhealthy (production of radicals) for living beings (water and organic molecules have a very high portion of hydrogen) yet more or less irrelevant for a tank. But if YT2095 sais it´s your calcium that kills you that might be also correct. I´m not really an expert in killing people. - leaves almost no radiation: Afaik, the main reason why the radiation is said to vanish quite quickly is that free neutrons have a livetime ~10min which is very little compared to that of uranium and plutonium (millions of years and thousands of years if I remember that correctly). That´s of course only a rough estimation because the neutrons can be catched by atom cores resulting in a longer living radioactive isotope ... but would you really expect your military to tell you "that bomb we just invented is a really dirty one with limited military usage but great potential for making people suffer!" ? @topic: The fascination people put into "how do I build a really nasty bomb"-topics is quite a bit shocking for me. Had a seminary with 1st semester physicists last year and about 1/3 of them were chosing "how do I build an atom-/h-/neutron-bomb" as topic of their speech ...

One of the fundamental axioms used in general relativity is that nature does not care about the coordinate system (=point of view) you chose to describe it. So your assumption is right. The twins paradoxon has kind of a different origin (plus: It´s no paradoxon if you handle it correctly): Both twins take two different paths from point A to B. Their age is actually nothing more than the lenght of the path they take (measured in a hyperbolic space with an indefinite metric, but that´s not the point here). There is no reason why two different paths must have the same length so there is also no reason why they must have the same age (=lenght of the path) after having travelled from A to B. And to close this post: Since the length of the path does not depend on the coordinate system you chose (I admit that it might be hard to imagine a length of a path without a coordinate system, but why should nature need one to work ... ) the difference of the length is also independent of the coordinate system and so is their age-difference.