Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

The speed of light as an absolute max

  1. May 14, 2008 #1
    I realize that this has been experimentally confirmed in any number of ways, based on the external application of energy to a particle. I have read articles on this subject, ranging from the magazine variety to texts by famous physicists, over the last 40 years.

    My question relates to the possible acceleration of an object using energy supplied by the accelerated object. Say you are in a space ship or the famous elevator car with, for lack of a better example, a drive based on the total annihilation of mater. As speed increases, mass increases, but the particles being ejected by the rocket motor would also have increased in mass equally. From the view of the pilot, in relative space, is his acceleration that of the ship starting from rest. In short, he is moving or not, it is only relative to other reference points. If he were in deep space, essentially out of any significant gravitation effect of another body, wouldn’t he be able to accelerate continuously, with no speed limit.

    I guess I keep thinking of that example of an elevator car in space with the person inside only aware of acceleration not speed. If the accelerating medium is entirely contained within the elevator, what would limit his speed assuming he was feeling a continuous acceleration?

    As an astronomy buff, I have a hard time believing that there is not a way of visiting other stars in less than light speed time, which is not to say that when we get back, anyone will be alive that we knew when we departed. Said another way, I do not believe that God plays dice with us.

    Thank you:
  2. jcsd
  3. May 14, 2008 #2
    That is incorrect.

    Acceleration does not cause an increase in mass. It is true that measuring the mass of an object in relative motion is higher than the mass of an identical object at rest but this is due to relativistic effects. However the proper mass of an object does not increase when it accelerates.
  4. May 14, 2008 #3
    and even if your ship were to burn all its mass down to where there was only one electron left, it would still require an infinite amount of energy to finally accelerate it to C. and yes, you could essentially accellerate for an infnite amount of time and still never quite get to C.

    however, your question seems to be really more along the lines of "how can we travel to other places in the universe in less time than it (apparently) takes light to travel that same distance?"

    there is an answer to that question, and it relates back to first order relativity. for example, if you want to travel to a star that is 10 LY distant - if you were able to devise a ship which could go 0.9999999C, you would find that you could get to the star in about 1 year according to your ship's clock (while about 11 years would have passed on earth). the closer you get to C in your ship, the less time it takes to arrive at the star, and as V approaches C, time to destination approaches 0. we do not need FTL speeds to travel across the universe. of course, this begs the question - just exactly how far away is that star, really???
  5. May 15, 2008 #4


    User Avatar
    Science Advisor

    That same idea crossed my mind when I was much younger. I merely assumed that the increased yield is never enough to supply the increased requirement, but only enough to reduce the difference between the two.

    But then I saw this;
    Does this mean that the energy yielded by the annihilation of this matter would not increase? I can certainly see that would be a case from the vehicles frame of reference (in which the mass has also not increased). But, from an external frame of reference (in which the measured mass does increase), does the energy that can be extracted from that matter also increase?
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook