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Near speed of light question

  1. May 21, 2012 #1
    If I were to watch a ship reach the speed of light from earth, I would eventually see the ships engine slowing down as well as the crew and the ship itself, but with the ships engine slowing down from my perspective how would the ship continue to accelerate to near the speed of light? Since the engines are slowing down from my perspective?

    If you need me to clarify more, i'll try.

    Thank you.
  2. jcsd
  3. May 21, 2012 #2
    Exactly. They still produce less and less thrust (and the spaceship appears to have more and more mass), but this is one of the reasons nothing can accelerate to the speed of light. Your hypothetical spaceship would approach c, but never actually reach c (from the Earth's FoR, of course.)
  4. May 21, 2012 #3
    For example, a car that goes up to 100mph will need to increase its engines speed faster and faster to reach 100mph, Same with the ship that wants to reach 88% the speed of light

    But from the planetary perspective, the ship will reach something like 50% speed of light and the the ship would begin to "slow down" due to time dilation, thus the engines would slow down as well, but yet it will continue to accelerate?

    Thats like slowing down your car engine, but yet the car continues to accelerate forward
  5. May 21, 2012 #4
    Not exactly. Due to friction, car engines apply enough force to maintain the car at the given speed. Due to the lack of friction in space, engines will pretty much keep something accelerating without needing to increase their strength. Also, from the ship's point of view, they're at rest for this whole experiment, while accelerating. Very confusing.

    Yes, but the engines won't appear as powerful.

    As I pointed out before, car engines apply enough force to maintain a car at a given velocity, canceling out the friction, basically, while spaceship engines just accelerate the spaceship. To go faster, the engines just need to be going, you don't need to turn them up.
  6. May 21, 2012 #5


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    You are confusing what YOU see with what is actually happening. What is seen from the planetary perspective, as you put it, is irrelevant to the ship itself, which just keeps on accelerating asymptotically towards c.
  7. May 21, 2012 #6
    Good question, time dilated acceleration. Is one g observed, actually 1g for the traveler?

    I'd guess it is.

    Is acceleration invariant?
  8. May 21, 2012 #7
    So since from the planetary point of view (assuming its a "Ramjet/scoopjet design, using Hydrogen fusion to propel the ship forward, bursts of explosions) the explosions will slow down the closer it gets to the speed of light, meaning the force of the explosion won't push the jet anymore because its expanding at an insanely slow rate, thus the spacecraft will receive no more "thrust" no? Yet the spacecraft will still continue to accelerate

    But yet from the spacecrafts point of view, the explosions will still be detonating and propelling the spacecraft forward, no?
  9. May 21, 2012 #8
  10. May 21, 2012 #9
    Then how does the spacecraft propel forward if the explosions do not provide anymore thrust? Because from the spacecraft view he will be accelerating, but from our view he can't accelerate because the explosions have slowed down due to the time dilation
  11. May 21, 2012 #10


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    You insist on basing what happens on the ship on what we see. You really need to get over that. What we see is IRRELEVANT to what happens on the ship.
  12. May 21, 2012 #11
    Could you elaborate?
  13. May 21, 2012 #12


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    Take a look at this:

    www.phinds.com/time dilation [Broken]

    The people on the ship see, in their reference frame, a passage of time that is exactly what the planet-bound people see in THEIR reference frame. It is the RELATIVE view of what each sees of the other that you are confusing with what they each actually experience.
    Last edited by a moderator: May 6, 2017
  14. May 21, 2012 #13
    Hi You seem to have an idea that there is a threshold for necessary thrust to maintain acceleration. In reality any force/thrust will result in acceleration no matter how small.
    So you are correct that earth frame observers will see a diminishing thrust (as for instance the trail of ejected mass) and a decreasing coordinate acceleration but the observed thrust and the coordinate acceleration will never reach zero just as the velocity will never reach c.

    Edit: there is a certain negligible threshold due to loss from heat conversion and other factors but that is not rally important regarding your basic question here.
    Last edited: May 21, 2012
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