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I Energy contained in fuel in speed of light ship

  1. Mar 7, 2017 #1
    Hi, Simple question. if a ship is getting close to the speed of light its momentum (mass) will continue to increase requiring more energy to propel faster. The fuel in the ship will also gain energy and mass as it accelerates. at C it will require infinite energy because it will have infinite mass. Wont the fuel now have infinite energy? ie relative to the ship and people in the ship wont everything be relative. As the ship expels fuel the ship will proportionately get lighter so should power through the light barrier If it exists.
    I have never seen this included in discussions so were am I wrong?
    I have an engineers brain and remember when experts said traveling faster than 90 miles an hour would suck the air from your lungs, I am new to this forum so lets have some fun.
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  3. Mar 8, 2017 #2


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    Energy cannot be gained out of nowhere, it is conserved in a closed system.

    Also, relativistic mass is an archaic concept, you will not find many physicist who use it for anything but confusing laymen with popular science. I suggest reading my PF insight on the subject (see link in my signature).
  4. Mar 8, 2017 #3
    Any massive object can not be accelerated to the speed of light. It will always move slower than that. Thus, its energy will always stay finite.
  5. Mar 8, 2017 #4
    Thank you for replying I agree that relativistic mass is realy only good to use for station closed systems.
    But we don't have a stationary closed system.. No were in the natural world is there objects in space that accelerate by internal means. Gravity is the only way something natural can have constant acceleration.(correct me if I am wrong) Then along came man using accelerated mass to accelerate an object at constant rates. Momentum stays the same, ship gets less massive but energy increases with speed. On board the ship with the constant 2 g's of acceleration over many years the crew watching the relative speed approach the speed of light. Excluding the complicated effects of external gravity, what will stop there acceleration and at what speed will it become evident? Ie Motor running and rate of acceleration begins to slow? are there answers out there or just controversy. Lets think a little out side the box and explore!!!!!!!!!!!!.

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  6. Mar 8, 2017 #5
    Wrong. Example: Sun's light pressure on a dust grain gives it constant acceleration (on a distance scale much smaller that the distance to the Sun).

    What? Momentum of a rocket does not stay the same (for one, it was zero at the beginning!). Energy of any rocket decreases, does not increase (because it loses exhaust mass).

    Even with a theoretically best possible rocket engine - photon rocket, fuel exhaustion will eventually stop acceleration.
  7. Mar 8, 2017 #6
    Hmm didn't consider solar winds, But it is still an outside force like gravity. Ok when I mentioned constant momentum I meant the increase in momentum is constant. Energy, the kinetic energy is increasing. and rocket motors. The plasma drive is in its infancy Sort of like telling the wright brothers that no one will fly higher than 50 feet because the engines will never be powerful enough and one mile of range because of the amount of fuel they burn.
    Ok Ladies and Gentlemen What will happen as they approach the speed of light, don't just say it cant be done, explain why, any one up on the latest theories I would love to here.
  8. Mar 8, 2017 #7


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    Approaching the speed of light, even getting arbitrarily close to it, is in principle possible. It is not, however, possible to reach or exceed the speed of light. There's a pretty good description of what's involved here: http://math.ucr.edu/home/baez/physics/Relativity/SR/Rocket/rocket.html
  9. Mar 8, 2017 #8


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    Relativistic mass is not good to use for any system.

    Nonsense. The decay products of any unstable particle accelerate the same way that a rocket does. This happens all the time in nature. Furthermore, the energy extracted from the fuel can be far greater than we can produce using modern rocket engines. So we can examine the limits of physics.

    In all cases, any decay product with mass travels at some v<c. Regardless of how small that mass is. Regardless of how much momentum the ejected mass imparts. Regardless of how much of the initial mass energy is converted to kinetic energy.

    In all cases, any massless decay products will travel at v=c.

    Nothing will stop their acceleration. Their proper acceleration can (in principle) continue forever at the same magnitude. Their coordinate acceleration will reduce, but never stop.
    Last edited: Mar 8, 2017
  10. Mar 8, 2017 #9


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    The faster the ship is moving relative to its launchpad, the greater the kinetic energy the unburned fuel onboard has. However, the additional energy that is released by burning that fuel is unchanged, and that's what we need to further increase the speed of the ship.
    The ship does get lighter, but the energy required to further accelerate the remaining mass increases more rapidly than the ship gets lighter. You can work this out for yourself from the expression for relativistic kinetic energy, ##E_k=(\gamma-1)mc^2##: if we reduce the mass by ##\Delta{m}<m## and increase the energy by amount ##\Delta{m}c^2## (complete conversion of fuel to energy in the best case of an ideal and physically impossible reactionless drive) the value of ##\gamma## remains finite, meaning the speed is still less than ##c##. We can't get to lightspeed until the ship's mass is reduced to zero.
  11. Mar 8, 2017 #10
    You don't need the latest theories. It is a consequence of the Lorentz transformation. If an object tries to reach c than the proper acceleration (as measured in its rest frame) will always go infinite. It doesn't matter how you play with energy, mass, forces or whatever - you will never find a way to accelerate a massive object to c or even above. You would need to turn the ship into light in order to reach c.
  12. Mar 8, 2017 #11


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    You started this thread with a "I" prefix, so you'll be familiar with the relativistic velocity addition law: ##w=(u+v)/(1+uv)## (if you aren't, google for "relativistic velocity addition").

    Suppose that burning fuel for one second will accelerate the ship from speed zero to speed ##u##. That's the acceleration as described using a frame in which the ship is initially at rest, so it's the way that the people on the ship describe their situation: they were at rest while the launchpad and the rest of the universe is moving backwards; they ran the engine for one second and they accelerated to speed ##u##. But now we consider the same acceleration from the point of view of us back on earth: the ship was initially moving at speed ##v## and now it's moving at speed ##w=(u+v)/(1+uv)## which is less than ##u+v##. Play with the formula some and you'll see that no matter how many times you run the engines, and even if ##u## increases each time as the ship becomes lighter, you can't get the relative speed between ship and earth up to #c##.

    As for when this effect becomes evident? It depends on how good our measuring instruments are. In principle we could see it with a ball thrown from a passing car (speed of ball relative to ground is not equal to speed of ball relative to car plus speed of car relative to ground). In practice cars and balls move so slowly that the effect is unmeasurably small, even with our best instruments.
  13. Mar 8, 2017 #12


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    For a stationary system, relativistic mass and regular mass are identical. In other words, the concept of relativistic mass is not useful anywhere.

    Inside the spacecraft, you will never notice any effect from moving fast relative to Earth. Why would you? There is nothing special about Earth. As long as you have enough fuel, you can maintain a constant acceleration as felt by people on the spacecraft. As seen from Earth, however, the spacecraft acceleration will get slower, the relativistic velocity addition formula tells you how much exactly.
  14. Mar 9, 2017 #13
    OK lets imagine that a ship leaves earth and accelerates to near light speed than cuts the engine so is in free fall. then most of the crew enter a ship on the nose of the fist ship which is at rest in relationship to the second ship which now accelerated away from the first ship and reaches near light speed. The crew left in the first ship will look back and see the earth moving away at near light speed and forward the other ship moving away from them at near light speed. On earth both ships will appear to be moving at near light speed. Why, because that's the way the maths work Einstein was not happy with his formulas being used for objects outside of a stationary system but there you are. Also to throw another spanner in to the works who is to say that the universe was not traveling at near light speeds when the big bang happened, We have no fixed point of reference to go by.
    I look forward to interesting points of view and perhaps some facts to prove me wrong.
  15. Mar 9, 2017 #14


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  16. Mar 9, 2017 #15


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    Einstein was happy with it, and did it himself as well. There is not even a meaningful definition of a "stationary system" in an absolute sense - that is the key statement of relativity.
    That doesn't make sense. There is no "speed of the universe".
  17. Mar 9, 2017 #16


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    You should be careful not to mix invariant quantity with relative quantity. The Kinetic energy of the fuel (or the whole ship BTW) only exist in the frame of the earth and is only relevant if the ship comes crashing on you.

    In the frame of the ship the fuel "energy" is something totally different than kinetic energy, and its accelerating power is a constant.

    Einstein was probably a happy guy, but I suppose happiness is relative to.

    Universes have no speed. This is really nonsensical (speed with respect to what ?)
  18. Mar 9, 2017 #17
    That's what I said "No reference"
  19. Mar 9, 2017 #18
    Ok what ive been trying to get to is perhaps the speed of light is just that, the maximum speed light can travel and that it has no reference to mass at all even if something was traveling faster than the speed of light we would only detect it as traveling at the speed of light...................... Ugg gasp string him up, burn him at the stake. blasfema,...........

    My physics lecture put forward once that if 2 spaceships were traveling towards each other, each at 3/4 light speed. each crew would detect the other traveling towards it at the speed of light (Ignor the time for light to be seen by the other ship). An outside observer would detect their approach speed to each other at the speed of light yet if they were 1 an a half light years apart on detection it would only take 3/4 year to collide not 1 year Why because inder the current maths no mass can travel faster than the speed of light relative to anything else.yet..........................
  20. Mar 10, 2017 #19


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    It's not clear to me what you're actually claiming here. But you might like to Google Bertozzi's experiment. It's a straightforward demonstration that increasing energy gives increasing speed in line with the relativistic prediction, asymptotic to c, not infinity.

    It's more that there's literally a hundred years of tests of relativity and no failures so far.

    But a gentle reminder of the forum rules on philosophy and personal theories might not be out of place.

    I think you misunderstood something. Two ships doing 0.75c in my frame will each measure the other as doing 0.96c, not c.

    I'm not sure about your thought experiment because it seems to be rather confused. Did you mean something like "there are two marker bouys 1.5 light years apart at rest in my frame. Two ships pass them simultaneously in my frame doing 0.75c in opposite directions. What happens in my frame and in the frame of one of the ships"?

    If so, you can analyse it straightforwardly. In my frame they meet 1 year later, yes. In their frame I need pencil and paper, but it's easy enough to do.

    Edit: Label the ships A and B and their respective buoys A and B. I get that in the frame of ship A:

    Ship B passes buoy B 2.26ly from ship A.

    1.70 years later, buoy A passes ship A.

    0.66 years after that, ship B passes ship A (for a total of 2.36 years after it passed buoy B).
    Last edited: Mar 10, 2017
  21. Mar 10, 2017 #20


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    Your physics lecture obviously failed to specify in which frame this speed is measured. So everybody will suspect "earth frame".
    It is not clear that you have understood that speed is relative concept. The ships have no absolute speed. They have an infinite number of speed in an infinite number of frame of reference.

    The formula, as well as experiments proves you wrong. There are online version if you want to try

    Which "outside observer" ?. Outside ship observer frame is called earth frame. The speed is 0.75 in that frame.

    Now about clock and duration, there are more formula to relate those values. They also works pretty fine.
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