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Speed of light, propellent incease in mass?

  1. Apr 17, 2009 #1
    So i just came across some information that states that if you were in a spaceship trying to reach the speed of light, since the reacting force is being produced on the spaceship and all mass traveling towards the speed of light increase. This means that fuel mass increases and so does the thrust produced due in part to the increase in fuel mass. So does this mean that as you move towards the speed of light then your energy output would increase proportionally to your velocity?
  2. jcsd
  3. Apr 17, 2009 #2


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    As far as the spaceship is concerned there is no increase in mass. The relativistic effect is seen by outside observers going at a much different speed.
  4. Apr 17, 2009 #3


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    Remember that the laws of physics do not change with speed. The fuel will work the same, irregardless of your speed with respect to any reference point.
  5. Apr 17, 2009 #4
    So then as you reach the speed of light what is it that limits you from reaching c and or over coming it...doesn't your mass increase to infinity?
  6. Apr 17, 2009 #5
    If the observer is on the spacescraft, the spacecraft will probably have to lose mass for acceleration. then the only propellants you have available are a) light photons, created using only energy, or b) throwing mass out the back, i.e., using energy to accelerate ions, or chemical propellants
    to produce thrust.
    If you cannot afford to lose mass, then maybe a laser beam will have to do.

    If you want to use a nuclear reactor, then maybe Project Rover should be revived. If you want to use nuclear explosions, then ???
  7. Apr 17, 2009 #6
    so then its more a matter of the amount of fuel needed to actually get there, increase...
  8. Apr 17, 2009 #7


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    Your rest mass never changes at all, and physicists these days usually prefer to avoid the concept of "relativistic mass" and just talk about energy instead. In the frame of reference where you're approaching the speed of light, your energy does approach infinity as your approach c, so you will never reach c as long as only a finite amount of energy is being used to do work on you and increase your kinetic energy.
  9. Apr 17, 2009 #8
    I kind of get it, now the actual constant of the speed of light, do physicists know what gives us this universal constant?
  10. Apr 17, 2009 #9
    sry to correct, i guess its not a universal constant
  11. Apr 17, 2009 #10
    What you can do is exploit time dilation. If you have a fuel factory on Earth that has some fixed output per unit time, then you can take some of the fuel, travel close to the speed of light and then when you return, you'll have a huge amount of fuel available, allowing you to travel even closer to light and with the larger time dilation you will have even more fuel available when you return.
  12. Apr 18, 2009 #11
    Post 10 is just plain silly....
  13. Apr 18, 2009 #12

    Not silly at all. Suppose you have an antimatter factory that works on solar power that is in orbit around the Sun. You have a photon rocket that is powered by antimatter. Your goal is to travel to some distant location in the least possible proper time.

    Mass of photon rocket without fuel is M

    Define tau to be the time it takes for the factory to produce an amount of M of matter/antimatter fuel for the rocket.

    We want to travel to a location that is at distance L = lambda c tau.

    What is the optimal strategy that will allow the traveler to arrive at the location in the least proper time?
  14. Apr 18, 2009 #13


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    If you use a inertial guidance system not programmed to deal with relativity (it senses acceleration and multiplies by time to get velocity), you'll think you have accelerated beyond C. But if you look in your rearview mirror, Earth will be closer than your navigation system says it should be.
  15. Apr 18, 2009 #14

    That is wrong. The term "relativistic mass" has been died out. Because just multiplying a constant "c2" we get the energy E= mc2/(1-u2/c2). It can be called " relativistic energy" .
    If the mass increases then conservation law will be broken. Mass is conserved. Energy is also conserved but your system's energy can increase because external force works on it. But your system itself does not increase energy.

    So, in every closed system, the total relativistic energy and momentum are conserved.

    But rest mass is not conserved. For example think of a nuclear bomb where mass is transformed to energy following E=mc2
    Last edited: Apr 18, 2009
  16. Apr 20, 2009 #15
    Post #12 is unrelated to post #10...
  17. Apr 20, 2009 #16


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    I really like the idea of the spaceship travelling close to the speed of light trying to pick up a huge stockpile of antimatter fuel at rest. Some of the resulting little bits of spaceship should definitely get even closer to the speed of light.

    If the mission could be done by the year 2109 do you think we will still have the "Darwin Awards"? It seems like a winner to me.
  18. Apr 20, 2009 #17

    Just solve the problem explained in post 12. What does the solution look like in the limit of large lambda?

    You assume that you start with a photon rocket without any fuel.
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