Trying to Accelerate an Infinite Mass

Short entry today; an easily digestible bit of physics.

A number of times in physics books and articles, I’ve come across people stating that “as an object approaches the speed of light, its relativistic mass increases, so it’s harder and harder to accelerate the steadily increasing mass, thus, you can’t speed up.”

I’d like to call bullshit on this argument. As your ship’s mass increases, so does the mass of your propellant in equal parts. If your mass doubles, so does the propellant mass being ejected out of your rockets, and you’re still obeying the law that for every action there’s an opposite and equal reaction. It’s just as easy to accelerate as it was before. You still can’t measurably reach the speed of light, but that’s for another entry I’m working on.

See? Easy peasy.

There’s a more sensible way to get the same results. For the outside observer watching the accelerating spacecraft, as it approaches relativistic speeds, time appears to slow down. For the guy ON the spaceship, he/she has the same amount of propellant per second going out his exhaust as before. For the outside observer, since the spaceship’s time is slowed down, the observer sees less propellant-per-second being used by the ship. As the ship gets closer and closer to the speed of light, it’s using less and less propellant. It accelerates slower and slower. It can never reach light speed.

To the person on the ship, his acceleration is the same as always; if he felt 1 gee at the start of the journey, he still feels 1 gee in his reference frame.

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