Emdrive Electromagnetic Drive For Chinese Space Ships?

Discussion in 'Science' started by Fortigurn, Sep 30, 2008.

  1. Fortigurn

    Fortigurn Member

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    Source.

    Can someone explain the science behind this a little more simply? I was lost by the third paragraph. What produces the thrust, and where does it go?
     
    Last edited: Sep 30, 2008
  2. bloodbob

    bloodbob Member

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    A saw a article about this around 6 months ago however the proposal was to use super conductors for the cavity. I the way this work as the mircrowaves travel down the short end they get compressed and this acts to lower the speed of light which alter the momentum now the laws of conservation of momentum require something else to move i.e. the engine. As soon as you do any work it eats up the microwave and you loose the acceleration. They proposed this would be a good solution to keep something however up off the ground.

    But don't believe me I'm a chemistry major not a physics major.

    http://emdrive.com/theorypaper9-4.pdf

    Yeah that is pretty poor. That's what 500KW+ to equal gravity.
     
    Last edited: Sep 30, 2008
  3. bondy_112

    bondy_112 Member

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    It is only intended for propulsion in space. From what I understand it's almost comparable to the amount of thrust ion drives typically generate. However all it requires is electricity to work, no fuel needed.

    Then again, I really have no confidence that it will actually work.
     
  4. hlokk

    hlokk Member

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    To simplify the idea:

    EM radiation has momentum. If you shine a laser at a sail in space, it will move (due to photons imparting energy).

    This device produces EM radiation, then the waves are channeled to two different sized areas. The shape changes the 'group velocity' which in turn changes the momentum.

    If the two sides are applying different momentum, there will be a net force in one direction.

    This disobeys standard conservation of momentum equations but the device claims there will be a disparity due to relativistic effects (each photon has its own frame of reference).

    The 'Q' they are talking about is kind of like a thermal insulation rating. The higher, the more energy is kept inside.



    Whether it actually works as claimed is a completely different question. People in the relevant field (relativistic electrodynamics). Conservation of momentum applies for basically all the relevant areas (newtonian, relativity, quantum, EM equations and a combination of them). So either this machines numbers are correct and our theories are all wrong, or the inventors calculations involve a mistake. I know which I would be going for at this stage.
    Many a device has been invented claiming something extraordinary, when it turns out that the person doesnt understand the concepts well enough not to make mathematical mistakes. Such as a free energy device (in QLD I think, and recently) where the guy had ignored the effect of inductor power and calculated the power of the batteries too simply.


    edit: according to other links, more power can be gained with superconductor containers. Also, Ion drives only produce a couple of hundred kN for the xenon ones. The benefit is that as long as theres thrust, theres acceleration, and ion drives can provide thrust for quite a while unlike a rocket with much higher impulse. The power requirement is 'only' 10s of kW though
     
    Last edited: Sep 30, 2008
  5. SLATYE

    SLATYE SLATYE, not SLAYTE

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    The Wikipedia article on this drive is worth reading.

    I'm pretty sure that it won't work. I wouldn't like to be the person promoting it if it fails, since China will have spent a lot of money on it.
     
  6. Goth

    Goth Grumpy Member

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    The "EmDrive" created a huge amount of controversy when it was first presented in the (not quite an eminent peer reviewed journal) New Scientist a couple of years ago, because, well, it seems to be bullshit. The Wikipedia article elaborates thusly.

    Well, if you have what seems like a very small force, but you can keep that force running constantly, then you still have something that is useful for spaceflight, even if the magnitude of the acceleration is really small. That's the idea behind ion engines, for example.

    Imagine we could accelerate continuously at 1 g — what we're comfortable with on good old terra firma — to the midpoint of our voyage, and decelerate continuously at 1 g until we arrive at our destination.

    That force, the same force as gravity, would indeed require an enormous amount of power - but look at the speeds you're accomplishing with it.

    The trip to Mars takes about 2 days, Pluto, about 17 days, and Alpha Centauri - 4 years.
     
  7. bloodbob

    bloodbob Member

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    Fair enough I've looked it up and it is in the same ball park of the ion drives.
     
  8. OP
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    Fortigurn

    Fortigurn Member

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    Yep, so far so good. Like a solar sail.

    I think I almost understood that. So if the EM radiation is channeled into two different sized areas, the amount of momentum of the EM radiation in each area will be different, one will be less than the other, and there will be a net force in one direction. But how does the shape change the momentum? And how can any force be generated if the EM radiation is staying in one area?

    Ok, that made no sense to me.

    Well that makes sense to me at least.
     
  9. hlokk

    hlokk Member

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    C'mon goth, whats the error here ;)
    (assuming the dist is 4ly, so not getting nitpicky about decimal places)


    EM radiation is bouncing back and forth, so locally its moving (same way a light can bounce between mirrors at a billion km/h but not go anywhere).
    The easiest way to visualise the change in group velocity is to think of it like pipe flow, the narrower the pipe, the faster the flow for a given pressure, or the same velocity will have lower pressure. Its not completely analogous of course.


    Thats because the idea doesnt make sense :lol:
    What the inventor is claiming, is that the light will have its own reference frame (this is fine by relativity), and because the reference frames are different the net effect isnt what is 'expected'.
    Imagine if due to time dilation, one side was slower, there would be less photons per second (or less energy per photon) relative to the other side because time slows down in one reference frame. For a fixed observer the net effect is more force on one side than the other. At least thats roughly what i think he's getting at.
    The idea doesnt make a lot of sense even to experts in electromagnetic relativity. Conservation of momentum holds even in relativity, so either our equations or his working out is wrong.
     
  10. OP
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    Fortigurn

    Fortigurn Member

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    Ah, so locally moving, but not exiting the system?

    So it's not just me. :lol:

    :Paranoid:

    Time slows down in one reference frame. Right. :wired: But how does the fixed observer's relative point of view make any difference to what is actually happening? Or is it because of relativity that we can't say what's actually happening, only what appears to be happening from different relative points of view? :confused:

    I was fairly sure the whole 'conservation of momentum' thing was pretty reliable. :Pirate:
     
  11. Luke212

    Luke212 Member

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    2.2 light years is 20813760000000 metres
    it works out 24 days to make that distance at 9.8ms-2

    amazing. only problem is the speed limit. final velocity would be 20813760000000ms-1, which is too fast (300,000 is the absolute limit) :Paranoid:

    goth is right because speeding up and slowing down to speed of light only takes 17 hours each.
     
    Last edited: Sep 30, 2008
  12. SLATYE

    SLATYE SLATYE, not SLAYTE

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    He may have taken into account length contraction, which would occur at such high speeds.

    I think you've made a mistake in the working. 2 light years is 1.89 * 10^16m. Using standard (non-relativistic) equations, with a = 9.8m/s/s, the time is ~720 days (about two years).

    The final velocity would be 6.09 * 10^8 m/s. This is roughly twice the speed of light, and therefore not practical.

    The Wikipedia article links this, which seems like a good argument against the drive working.
     
  13. Luke212

    Luke212 Member

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  14. neutralizer

    neutralizer Member

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    Doesn't this violate conservation of momentum...:confused:

    EDIT: Note to self - Read the entire thread before posting
     
    Last edited: Sep 30, 2008
  15. Luke212

    Luke212 Member

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    thats weird. what did i do wrong?
    2ly = 300,000*60*60*24*365*2 = 1.89x10^13 metres :(

    edit: light speed is 300,000,000 ms fark me!
     
    Last edited: Sep 30, 2008
  16. Assasinator_2

    Assasinator_2 (Banned or Deleted)

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    I'm going to hazard a guess and say the error here is you not realising that he could well be talking about in the frame of reference of the spacecraft occupants, rather than earth or alpha centuri. ;) (Note: not crunched numbers to prove this).
     
  17. Goth

    Goth Grumpy Member

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    Assume we're in the frame of reference of the spacecraft, so we'll apply simple classical non relativistic mechanics. ;)

    We know x = 1/2at2 where acceleration is constant, and we're accelerating to the half way point and then deccelerating again, so say t = 2 (x/g)1/2.

    Plug in a distance of x = 4.365 ly for a trip to Alpha Centauri A, and you get 4.11 years.
     
  18. Whisper

    Whisper Member

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  19. Cadbury

    Cadbury Member

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    That sounds alot like how a gas laser works; light bounces back and forward between two mirrors. One mirror is less reflective than the other. At some point laser light pushes through it.
     
  20. irR4tiOn4L

    irR4tiOn4L Member

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    To say the controversy has been 'huge' is almost an understatement. This thing supposedly violates the conservation of momentum, yet comes from a fairly reputable scientist and now seems to have others who say theyve replicated the result!

    I dont know what to think. Costella says its utter rubbish, a reputable Chinese university research team say theyve replicated the result. Who is right, and what are the implications for our physical laws and modes of travel?

    This could be huge

    EDIT: Whoops! Necromancy!
    My apologies
     
    Last edited: Nov 13, 2009

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