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    • CommentAuthorjoshs
    • CommentTimeApr 2nd 2014
     
    Posted By: Andrew PalfreymanSure - hence the aqueous humor. Light had to penetrate at least the shallow waters
    You are dangerously close to suggesting that whackadoos eyes are not mostly water. But if not water, what could they be made of? Candy from Candy Mountain?
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    I always thought they had stars in their eyes.
    https://www.youtube.com/watch?v=9M0PIBXkaGs
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    Posted By: joshs
    Posted By: Andrew PalfreymanI have, and it's called The Oxonian Cart, documented in the Files section of my Yahoo! group "spacedrives"
    Then I think there is something major missing in your description. Dropping the sand inside the rail car requires that the cart surrender energy accelerating the sand. When you dump that sand outside the cart the energy consumed accelerating the sand is lost from the cart. The cart therefore slows down no matter what direction it is traveling relative to the rail car.

    I am also wondering why a scheme that requires this constant addition and dumping of mass from and to the environment is part of any discussion of propellant-less drives. Isn't the idea to be able to accelerate where you can't suck in propellant and you don't want to use stored propellant?
    I doubt you've taken the trouble to read the little two-page paper, so suffice it to say that the reason I created and described that model was to demonstrate that if you can modulate mass then you can build a propellantless drive that constantly accelerates and breaks conservation laws. Now, self-evidently I don't know how to modulate mass (although that's Woodward's schtick), so I needed a mundane embodiment for demonstration purposes. That means that the sand used to temporarily increase the puck's mass has first to be accelerated up to the matching speed on its overhead conveyor, thus requiring work. Of course, the work done accelerating this helper sand is precisely the amount of woo energy that is created by a true propellantless drive. Were you to be mathematically inclined, you can find the equations of motion in that paper too.
    • CommentAuthorjoshs
    • CommentTimeApr 20th 2014
     
    Posted By: Andrew Palfreyman
    Posted By: joshs
    Posted By: Andrew PalfreymanI have, and it's called The Oxonian Cart, documented in the Files section of my Yahoo! group "spacedrives"
    Then I think there is something major missing in your description. Dropping the sand inside the rail car requires that the cart surrender energy accelerating the sand. When you dump that sand outside the cart the energy consumed accelerating the sand is lost from the cart. The cart therefore slows down no matter what direction it is traveling relative to the rail car.

    I am also wondering why a scheme that requires this constant addition and dumping of mass from and to the environment is part of any discussion of propellant-less drives. Isn't the idea to be able to accelerate where you can't suck in propellant and you don't want to use stored propellant?
    I doubt you've taken the trouble to read the little two-page paper, so suffice it to say that the reason I created and described that model was to demonstrate that if you can modulate mass then you can build a propellantless drive that constantly accelerates and breaks conservation laws. Now, self-evidently I don't know how to modulate mass (although that's Woodward's schtick), so I needed a mundane embodiment for demonstration purposes. That means that the sand used to temporarily increase the puck's mass has first to be accelerated up to the matching speed on its overhead conveyor, thus requiring work. Of course, the work done accelerating this helper sand is precisely the amount of woo energy that is created by a true propellantless drive. Were you to be mathematically inclined, you can find the equations of motion in that paper too.
    This all sounds like: "Take special pleading A, add in special pleading B, and get magical result C." I typically stop at the first special pleading.
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    You typically stop short of reading anything that requires effort in order to further a discussion. There is no pleading; I am using high school physics. Not that you're able to evince that without the small amount of effort it would take.
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    Does the curvature of spacetime change when an electric (and/or magnetic) field is present? Certainly the energy density has been changed - but how is this manifest in the geometry?
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      CommentAuthorAngus
    • CommentTimeMay 7th 2014 edited
     
    As there is energy in these fields, I believe that it does change (very slightly).
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    I agree, but if it's isotropic, there appears to be no preferred direction for the curvature. This is what has me scratching my head.
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      CommentAuthorAngus
    • CommentTimeMay 7th 2014
     
    If the field is localized somewhere then the energy is somewhere. Energy does depend on field strength after all. It curves around that somewhere as if it were a rock, I suppose.
    • CommentAuthorloreman
    • CommentTimeMay 7th 2014
     
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    Posted By: AngusIf the field is localized somewhere then the energy is somewhere. Energy does depend on field strength after all. It curves around that somewhere as if it were a rock, I suppose.
    You're talking about a field with finite gradient. I'm thinking about static fields, where each point in space can be assigned an electrostatic potential. Consider a finite (can be as small as you like) volume of empty space within which the potential is everywhere the same, and nonzero. Hmm - there needs to be a gradient methinks unless we're inside a sphere, where identically E = 0. But the potential inside a charged sphere is everywhere the same, and thus equal to the potential on its surface. (end of musing out loud).

    So considering the spacetime inside a charged sphere, where isotropy holds and E=0, what happens to the spacetime as we change the potential?

    ETA I guess it has no effect because it's E that determines the energy density, and that's everywhere zero.
    How odd.
    In that case, consider the spacetime between the plates of a capacitor. How does it bend as the field strength is increased?
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      CommentAuthorAngus
    • CommentTimeMay 7th 2014 edited
     
    Posted By: Andrew Palfreymanvolume of empty space within which the potential is everywhere the same


    Since the field is given by the gradient of its potential there is no field there to talk about.

    the spacetime between the plates of a capacitor. How does it bend as the field strength is increased?


    Since there is a uniform field between the plates and none outside (bar fringing fields) I assume it bends space approximately like a very light hockey puck.
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    And that's all I wanted to point out - that although spacetime responds to electric field, it does NOT respond to electric potential. I find this odd, because potential is a very real thing - and absolute too. It represents the work done bringing up a test charge from infinity. Obviously spacetime doesn't give a toss about the potential. Well, I hadn't actually explicitly realised that before.
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      CommentAuthoralsetalokin
    • CommentTimeMay 7th 2014 edited
     
    Posted By: Andrew PalfreymanAnd that's all I wanted to point out - that although spacetime responds to electric field, it does NOT respond to electric potential. I find this odd, because potential is a very real thing - and absolute too. It represents the work done bringing up a test charge from infinity. Obviously spacetime doesn't give a toss about the potential. Well, I hadn't actually explicitly realised that before.

    No, potential, that is, voltage, is relative and depends on position within the field. Were it not so, it would take the _same_ amount of work to bring that charge in from infinity, to any location, and we know this isn't true.

    https://www.youtube.com/watch?v=oDW3iBYya3I

    Charge is absolute and conserved, but is not the same as potential (voltage).
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    Posted By: alsetalokin
    Posted By: Andrew PalfreymanAnd that's all I wanted to point out - that although spacetime responds to electric field, it does NOT respond to electric potential. I find this odd, because potential is a very real thing - and absolute too. It represents the work done bringing up a test charge from infinity. Obviously spacetime doesn't give a toss about the potential. Well, I hadn't actually explicitly realised that before.

    No, potential, that is, voltage, is relative and depends on position within the field. Were it not so, it would take the _same_ amount of work to bring that charge in from infinity, to any location, and we know this isn't true.

    https://www.youtube.com/watch?v=oDW3iBYya3I

    Charge is absolute and conserved, but is not the same as potential (voltage).
    Nobody would disagree that potential difference is real. And indeed, spacetime notices and responds to that. But the absolute value of the potential is also real, and yet spacetime neither notices nor responds to it. Your video doesn't contradict that point.

    ETA An analogous situation with gravity. The height of an object above the surface of a massive body like Earth is described by an absolute value of potential energy (where the zero value is taken to be at infinity). Spacetime notices and responds via a particular curvature. However, in this case a nonzero field exists. To make the analogy more exact, consider the gravitational potential inside a sphere. The field is always zero in the static free space case, but the potential changes depending on how massive is the shell. Does the spacetime inside the shell notice and respond to shell mass changes?
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      CommentAuthorAngus
    • CommentTimeMay 7th 2014
     
    I don't see why that's a problem.In the rubber sheet model the size of the field-free region can correspond to the width of the dent and the weight of thesphere to its depth.
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    Yes, it works for gravity - spacetime responds to potential change even in a field-free region. Why then doesn't it work for electrostatics? Or maybe it does, but with smaller effect...

    ETA I started watching those ridiculous videos by the German Reid until I could stand it no longer. I thought that the best way to refute this quantum vacuum stuff was to find a property of electromagnetism that spacetime didn't respond to. Seems like I can't. But that's not a bulletproof way to confront his gobbledegook because (as I said) GR says nothing about virtual particles.
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      CommentAuthorAngus
    • CommentTimeMay 7th 2014 edited
     
    Why not for electrostatics? The mass now corresponds to the charge on the sphere (i.e. the stored energy).