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Mass speed of light help

  1. Sep 13, 2011 #1
    Hi All,

    I can not get my head around this.

    If nothing can go near the speed of light then why can the below happen?

    electrons travel near c

    proton beams around the LHC travel at near c

    as far as i know they both have mass and therefore should not be able to do this??

    also if they do travel at such speeds, why would we not use them as space propulsion? (ie some way of ejecting electrons into space from the ship, or shooting a proton beam out the back from a particle accelorator)??

    please help
  2. jcsd
  3. Sep 14, 2011 #2
    near is the key word. You can get VERY close to the speed of light, but you can never actually reach it.

    And a similar concept to the space propulsion that you are talking about exists. It's called an ion thruster.
  4. Sep 14, 2011 #3
    Hi Thanx for your reply, ok so with the ion thruster, would this not accelerate a ship close to the speed of light (or near as we say)?

    obv there is thrust to weight ratios but surely after things being thrown out the back at near c the ship would also get to that speed?
  5. Sep 15, 2011 #4


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    Regardless of exhaust speed a space ship can never reach c.

    At the LHC, the energy required to accelerate particals to speeds aproaching c is very much greater then that computed using Newton's laws. It is, however, extactly as predicted by Einstein.
  6. Sep 15, 2011 #5
    I understand that we wont be able to reach c but if we can get things close (ie the particles at LHC) then shouldnt we be able to do the same with a ship? (obv using a LARGE amount of energy)
  7. Sep 15, 2011 #6
    You use imprecise words like near and close, and I'm not sure what they mean to you here - the speed of light is the limiting speed for a particle in an accelerator and also for a massive object like a spaceship. You can come as close as you like if you are willing to expend the energy necessary to get there.
  8. Sep 15, 2011 #7
    With some rather distant future technology, spaceships could get close to the speed of light.
    You can get very close to the speed of light, but you cannot reach it. Only particles without mass, like the photons, can travel with the speed of light.

    However if a ship gets close to the speed of light, the travel time to its destination will be shortened greatly due to relativistic length contraction.
    Shortened for the occupants of the ship, as only they will feel the relativistic effects.
  9. Sep 16, 2011 #8
    Hi All,

    I obv have not been clear enough. im not asking about reaching c. I understand it is the physical limit and to reach it the energy required becomes infinate as the mass would become infinate.

    Im asking that if we can accelerate particles upto around 90% c , then why can we not do that on a ship using things at those speeds? (thinking like a electron thrust system or proton thrust (like LHC but on a ship and out the back not contained))

    i understand that energy would be needed, and alot of it, but would this not be a way to get to 90% c in a ship providing energy supply was not an issue?
  10. Sep 16, 2011 #9
    Maybe we could, maybe we couldn't. I'm sure other ways of propulsion can be imagined.
    We just need to wait 100-200 years for technology to evolve enough and give us an answer. Of course we'll all be dead before we get that answer.
  11. Sep 16, 2011 #10


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    Contrary to popular belief, the mass of an object does not increase as it approaches c. In the frame of the moving object is it always exactly the same mass.

    The reason we cannot make a ship reach near c currently is because it takes a LOT of energy, and hence fuel, to reach that. To even get to a tenth of c would require something around 100 parts fuel and 1 part ship using current technology. All of this also has to be launched from the ground, requiring the launch vehicle to also have about 100 parts fuel to 1 part ship/ship fuel.

    Currently there is no propulsion system that does not use the principle of ejecting mass out one side to provide thrust. The way they accelerate ions and electrons in the LHC is NOT a feasible way of providing thrust. Accelerating these particles takes energy, and hence fuel, and due to inefficiencies and sheer size of the device it would never work.

    Similar principles happen in an Ion drive, but the energies of the ions are nowhere near what they are in the LHC.
  12. Sep 16, 2011 #11
    Ignoring solar sails and similar concepts, rockets work by chucking mass overboard at some velocity and enjoying an impulsive increase in ship velocity. In space the increase in ship velocity follows the rocket equation (http://en.wikipedia.org/wiki/Rocket_equation), and basically is proportional to the exhaust velocity times the ratio of the mass being chucked to the total mass of the ship+mass. Throwing a proton overboard at a significant fraction of c is good in terms of exhaust velocity, but really, really bad in terms of mass change - now factor in the cost, complexity, weight and size involved with accelerating and firing a proton overboard at a significant fraction of c, and you can imagine why an LHC-powered spacecraft is not in anyone's crystal ball.
  13. Sep 18, 2011 #12
    thanx all for replying, so basically yes it would work but not feasable since power requirements are insane?

    lets hope we can get new power generating technologies that are more efficent than at the moment. :)
  14. Sep 18, 2011 #13
    Hey maybe Im the one thats wrong but I dont think youre quite getting the picture, the main thing involved in accleration in a spacecraft and any flying/jet propelled craft really, is thrust, or more simply force. F = ma, so you can have alot of mass going relatively slow, little mass going fast, or the best, alot of mass going really fast. Im pretty exhaust velocity doesnt really limit speed, as youre still applying force. For example, if you throw a 1 kg ball bearing out the back window at 2m/s^2, or a 2 kg ball bearing out the back at 1m/s^2, you will end up with the same amount of acceleration on the craft. Then again I dont know if it will still work like this if relativistic effects will play into the thrust.

    The way I understand it is, if you want short term large acceleration, go with a liquid oxygen/liquid hydrogen rocket. If you want long term fuel "mileage" and slower steadier acceleration, go with something such as a ion drive. Ion drives and the like provide a higher specific impulse, meaning how much thrust you get for a given amount of fuel. So, if youre looking to travel a long way, it would be better as you may not accelerate as quickly but you end up with a higher velocity for a given amount of propellant.

    Lol but if you want to actually have fun and experience some serious G`s dude, go with a rocket! Haha, but seriously, people think top fuel dragsters are the fastest accelerating wheeled vehicles in a quarter mile.......no sir. The late great Kitty O`Neil had a H2O2 rocket built for her car with which SHE SET A 1/4 MILE RECORD OF 3.22 SECONDS @ 396MPH!!! Tony Schumacher came close in his 496ci hemi running nitromethane making around 8000HP with a 4.42 @ ~330mph, at over a second and 60mph slower...
  15. Sep 19, 2011 #14
    Yeah that's basically it. Power requirements don't have to be insane, as you don't have to use LHC energies, but the lower you go the less your final speed will be.

    IF you did want to go close to the speed of light, the power levels would indeed be quite insane. So insane that no matter how efficient your powerplant would be, simply carrying this energy on your spaceship would take up most of the ship's mass due to e=mc^2

    And that's not even considering the reaction mass.
  16. Sep 22, 2011 #15
    unless we find some new ways to make power?

    an example , and before you go off at me i know this is tv show and not really science but in stargate (and atlantis etc) there is a ZPM device which pumps out more power than the US grid. surely we havent discovered all ways of making power?
  17. Sep 22, 2011 #16


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    There isn't even a theoretical power source that comes close to what is portrayed in most shows and movies. Not one that is actually expected to work eventually.
  18. Sep 22, 2011 #17
    US grid? Pfft...impessive as you make is sound, the Saturn V Rocket isn't all that much weaker. That's 60s technology.

    What we're discussing here goes orders of magnitude beyond that. We're talking the most energy dense substance available to theoretical physics. Energy packed so densely, that it turns into solid mass.

    Even if you could stuff THAT into a spaceship, you would still be hard pressed to get anywhere close to the speed of light. Although, granted, you could probably get to some reasonable fraction of it.
    Last edited: Sep 22, 2011
  19. Sep 22, 2011 #18


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    There is a reason they call it Science Fiction. Hypothetically a perfect reactor would convert mass directly to energy. If we had one of these then one kilogram of matter (e.g a litre of water) could power the whole world for ~1 hour 40 minutes.

    However there is no science outlining how this could be done and nothing on the table to suggest how this could be used in propulsion. The issues here are totally non-trivial and are hugely varied.
    Last edited: Sep 22, 2011
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