[suilusargaino]

So while we are talking about inertia:

I will define a "warp drive" as any mechanism that accelerates a body by warping space/time around it, rather than by applying a force to the body and an equal and opposite force to some other bodies (which will accellerate in the opposite direction).

If such a drive is possible in principle it seems to me that the Law of Conservation of Momentum goes out of the window, at least for the bits of space/time where the warp drive has been working.

As evidence, consider a warp driven spacecraft operating somewhere near Earth. It fires up its warp drive and 2 hours later is say 1.8 light hours from Earth (we'll leave faster than light travel out of the picture for now). The centre of mass of the Earth/spacecraft system has now moved, since part of that system has moved 2 light hours, without a balancing movement of any other mass in the opposite direction.

How does this fit in with conservation of momentum?

Or doesn't it?

[Kokomoxcvcv]

I know nothing of such things but the way I thought of it is if space is warped to transport the ship from A to B then there was no actual movement and thus no momentum to conserve. The ship is simply transferred from A to B or if there is any movement it isn't 1.8 light years, it would be the length of the ship as it passes through whatever mechanism provides the warping...

[suilusargaino]

I know nothing of such things but the way I thought of it is if space is warped to transport the ship from A to B then there was no actual movement and thus no momentum to conserve. The ship is simply transferred from A to B or if there is any movement it isn't 1.8 light years, it would be the length of the ship as it passes through whatever mechanism provides the warping...

OK, but from the point of view of an observer outside the spacecraft the centre of mass of the system has moved (or changed from its state of uniform motion in a straight line), so momentum has not been conserved, has it?

[rootoronpunty]

OK, but from the point of view of an observer outside the spacecraft the centre of mass of the system has moved (or changed from its state of uniform motion in a straight line), so momentum has not been conserved, has it?

are you saying that a straight line is recognized by the particles?

[rootoronpunty]

as I understand it, any line is represented to the particle by the space around it

[suilusargaino]

are you saying that a straight line is recognized by the particles?

No, I'm not saying that.

I'm not saying that isn't true either, but it's not the way I would put it.

[rootoronpunty]

No, I'm not saying that.

I'm not saying that isn't true either, but it's not the way I would put it.

I wasn't putting words in your mouth. Was just the way I thought it out.

[Kokomoxcvcv]

If you take a look at a simple system - A ship orbiting around a planet:

The centre of gravity is closer to the planet and then the ship travels through a "Warp" and now the ship is much further away but still in the same overall system. The centre of gravity of that system is further from the planet but still closer to the planet than to the ship. So, for the ship to come to a complete rest it needs to put on a greater amount of reverse thust than it would have had to do if it only moved a short distance from the planet. Isn't this extra thrust where the "missing" momentum goes?

[suilusargaino]

If you take a look at a simple system - A ship orbiting around a planet:

The centre of gravity is closer to the planet and then the ship travels through a "Warp" and now the ship is much further away but still in the same overall system. The centre of gravity of that system is further from the planet but still closer to the planet than to the ship. So, for the ship to come to a complete rest it needs to put on a greater amount of reverse thust than it would have had to do if it only moved a short distance from the planet. Isn't this extra thrust where the "missing" momentum goes?

Why would it need a reverse thrust? It just warps space in the opposite direction.

[Kokomoxcvcv]

Why would it need a reverse thrust? It just warps space in the opposite direction.

Because when it reaches its destination and comes out of the warp the gravity that was holding it in place is no longer there and so whatever momentum it had whilst orbiting the planet needs to be counteracted to stop the ship from moving...

[suilusargaino]

Because when it reaches its destination and comes out of the warp the gravity that was holding it in place is no longer there and so whatever momentum it had whilst orbiting the planet needs to be counteracted to stop the ship from moving...

But that's a tiny proportion of the total change in momentum (as viewed from an outside observer). In any case, suppose it returns to its original velocity, relative to some FoR, to keep it simple.

[Kokomoxcvcv]

But that's a tiny proportion of the total change in momentum (as viewed from an outside observer).

Like I said, I don't know much about such things - where is the other momentum? You have the minute amount of gravity from the ship acting on the planet and the large amount of gravity from the planet acting on the ship. If the ship disappeared the small shift of the planet would be un-noticeable. If the planet disappeared the shift of the ship will be large and some form of thrust would need to be employed for it to remain where it was before the planet disappeared...

[Ndptbudd]

So while we are talking about inertia:

The centre of mass of the Earth/spacecraft system has now moved, since part of that system has moved 2 light hours, without a balancing movement of any other mass in the opposite direction.



Or doesn't it?

How do you select the control boundaries for your system?

[suilusargaino]

How do you select the control boundaries for your system?

Doesn't matter. Just make them arbitrarily far away so that everything that might be affected by the movement of the warp driven ship is included.

[suilusargaino]

Furious - it's difficult keeping track of the momentum of each component, which is why I'm focussing on the position of the centre of mass of the whole system. It seems clear that the center of mass of the system has moved, with no external forces applied, so momentum hasn't been conserved.

[Alliopeti]

I imagine any space warping process would involve a suitable transfer of momentum to balance things up. Sort of analogous to the way momentum is transferred via curved space-time when a planet is revolving around a star. The planet's momentum changes from moment to moment and the star's momentum correspondingly changes from moment to moment and somehow the momentum adjustments must happen via the curved space-time between them.

Presumably an at-least-vaguely similar thing would happen via the warping of space.

If this seems vague, that's because it is. :-)

As I sit here typing my momentum must be changing in minute amounts due to my revolving around the Galactic centre. And the momentum of everything else in the Galaxy must be adjusting to take me into account.

[Ndptbudd]

Doesn't matter. Just make them arbitrarily far away so that everything that might be affected by the movement of the warp driven ship is included.

I was thinking along the lines that you could select a control boundary such that the centre of mass of the system did not move.

[Kokomoxcvcv]

with no external forces applied

Isn't that where the stabilising thrusters come into it?

[kneexyFreedly]

Doesn't matter. Just make them arbitrarily far away so that everything that might be affected by the movement of the warp driven ship is included.

But it isn't the space ship that's moving. It's a bubble of space/time surrounding the ship, that moves through space/time on the whole or larger scale, due to a warpage of the space/time around the ship, caused by an application of energy.

Likewise we see large scale expansion of space/time, with smaller regions "decoupled" by the overall effects of gravity [space/time warpage]

[suilusargaino]

But it isn't the space ship that's moving. It's a bubble of space/time surrounding the ship, that moves through space/time on the whole or larger scale, due to a warpage of the space/time around the ship, caused by an application of energy.
Nonetheless, it (the space ship) does move.