Momentum and Shield Generators - help needed

[higbvuyb]

Im currently in a debate in another forum. I would appreciate it if someone could give me good reasons or a good scientific explanation for why momentum from a projectile being stopped by starship shields in general sci-fi would be transferred directly to the shield generator.

The person I'm arguing against says that its wrong according to physics for a shield generator to have an effect on something outside the hull without also having an effect on the rest of the ship, as he says it, field effects are 'line-of-sight'. Because of this, he says, the bit of hull directly between the projextile and the shiels generator is actually the part of the ship that as taking the momentum, and from there, it spreads to the rest of the ship. He says that momentum cant be transferred without direct contact, for some reason. He also says that there must be shield emitters on the hull because of the beforementioned problem.
Good rebuttals against his argument would also be good.

Thanks.

[Paolo]

If momentum is not transferred to the shield generator when the projectile is stopped, then it is not conserved. Should be as simple as that.

[Luke Starkiller]

Since he is saying that "field effects are LOS only" you could ask why you can use magnets to manipulate objects through a table.

[Darth Wong]

Sounds like he's just another high-school kiddie. What kind of idiotic dumbshit would believe that forcefields magically make momentum disappear, or that momentum is some kind of tangible substance that gets stopped by the armour?

[Connor MacLeod]

This is l33telboi isnt it? in that Borg adaptation thread on SB?

[Academia Nut]

While I too was thinking about the magnetic example, you can expand it to other forces as well. Stick a point charge somewhere, and unless you can shield it with an insulator, in which case there is no point to having a point charge, if you apply a force via another charge, then the point charge will have to experience a reaction force, no matter what is between it. In order for fields to interact, it can't matter what is between them or the whole thing simply won't work. It's simply the laws of physics at work.

[Crazedwraith]

The laws of physics also say we can't go Faster Than Light and yet most science fiction has people doing just that. The standard laws don't have to apply at all if the setting doesn't want it to. It can just technobabble it away.

Still the specific arguments your opponents seems to be using sounds like nonsense regardless.

[Darkevilme]

I'd wonder how 'stop railgun slug with shield, hull plating between shield generator and slug gains slug momentum' or 'stop railgun slug with shield, external shield generator now has slugs momentum' helps him any. I mean its not just wrong it seems kind of useless even if he did prove it.

[Academia Nut]

Well, whenever you invoke a new bit of fantastic technology in sci fi, you typically have to demonstrate a mechanism for why it can go against the laws of physics as we know them. So while yes, there are things that can damp or even nullify inertia and momentum, typically they aren't infinite in strength, and its not like you're just ignoring momentum, you're just coming up with a solution to the problem.

[higbvuyb]

He's finally managed to fully explain his argument to the point where I can understand it properly.
Basically, he says that a shield emitter inside a ship is impossible, because there isn't any force that can repel a wide variety of attacks that doesn't also repel the actual ship itself. He says that this means that you must have shield emitters on the hull of the ship; the shield generator inside the ship doesn't project the shield directly, it projects the shield through these emitters on the hull of the ship.

So, he's saying that it makes more sense to assume that these shield emitters on the hull exist, even though there's no evidence for this, than to say that the shield is projected directly from the internal generator, and having some rationalisation for why the shield doesn't also repel the ship itself.

[higbvuyb]

Apparently, his reason for the momentum not being transferred directly to the shield generator is that it is transferred to these shield emitters on the hull instead, which then transfer the momentum to the rest of the ship. Never mind that this sort of system would mean that sections of the hull would be torn off their mountings by sufficiently powerful projectiles, as opposed to just an internal shield generator.

[Junghalli]

I don't see how having shield emitters on your hull would be more of a liability than having just one inside the ship. Either way, if momentum is transferred directly to the emitter something is going spontaneously to turn into a bullet inside your ship if it gets hit hard enough. It's a question of having pieces ripping off your hull vs. having the generator punch its way out of your ship like a chestbuster. Personally, the latter if anything sounds worse to me.

It all depends on your assumptions, but the basics generally still apply. If an enemy is firing a slug powerful enough to rip the generators off your hull, then his gun, and by extension, his ship must have felt that same force. Thus, unless you have an extreme technological mismatch between the fighting ships, this shouldn't be much of a concern.

Not necessarily. If you have two ships heading toward each other at, say, a combined velocity of 2000 km/s and one of them fires a slug at 6 km/s the firing ship will feel 6 km/s worth of recoil, while the target ship will get 2006 km/s worth of kinetic energy. You'd probably see a lot of situations something like this in space combat (granted rarely in visual SF, where the ships usually seem to moving around each other at the relative speeds of WWII prop fighter or slower for some reason).

[Darth Wong]

If the shield emitters are a large number of independent devices on the hull, then why does the shield bubble not always conform to the shape of the ship? Why do the shields not collapse when the hull starts to take damage? Why aren't there a large number of overlapping shields instead of just one? If shield generators can't project force through obstacles, then why does the outside of the hull look like smooth metal? Why do repairs to the shield generator not require people working on the outside of the hull to replace damaged emitters?

This guy didn't "clarify" his argument; he's simply weaseling around trying to save it. I'm sure he initially believed that it is simply impossible to project force through obstacles, completely forgetting the examples of gravity and electromagnetism.

[Lord of the Abyss]

If momentum is not transferred to the shield generator when the projectile is stopped, then it is not conserved. Should be as simple as that.

Depending on what sort of shield we are talking about, the momentum of the impact could also be spread across the whole hull and not just the generators, transferred to the ship as a unit, or radiated out of the shields on the other side of the ship as fast particles, for example.

[Darth Raptor]

At the risk of putting my ignorance on public display, wouldn't an "energy" shield that can withstand kinetic attacks actually be a *material* shield composed of some kind of exotic, ephemeral and for whatever reason translucent matter? One with actual mass (either real or virtual, whatever that means)? In that case, wouldn't the momentum be transferred into the shield itself?

In so much science fiction, we see that shields can be damaged (depleted), which doesn't make sense if it's some kind of straight-up force shield. You'd think that either an attack would overwhelm it or it wouldn't. You wouldn't expect shots to "chip away" at it until it's weak enough to expose the hull. Unless the shield really is just a layer of see-through armor you can switch on and off. Why switch it off? Well you'd definitely want to shed all that mass for a burn, and if it requires constant energy input to make it real, than you wouldn't want it on all the time.

[Winston Blake]

He's finally managed to fully explain his argument to the point where I can understand it properly.
Basically, he says that a shield emitter inside a ship is impossible, because there isn't any force that can repel a wide variety of attacks that doesn't also repel the actual ship itself. He says that this means that you must have shield emitters on the hull of the ship; the shield generator inside the ship doesn't project the shield directly, it projects the shield through these emitters on the hull of the ship.

He seems to be trying to shoehorn the characteristics of real force fields into the bizarre phenomenon of techno-magic shields. Mainly the fact that real fields generally get weaker with distance from the poles.

All you need to do is find an example of something existing between the active shield and the ship surface without it being 'repelled'. For example, shuttles or people walking on the hull. Then it's clear that the shield strength must be negligible over some distance interval and suddenly spikes at the shield surface. It's not unreasonable to extend this interval through the boundaries of the ship and have an internal shield emitter.

We're talking about Star Trek aren't we? It's clear that since objects hit the shields like a wall, they can't possibly be an ordinary inverse square or cubic force field. Why argue that 'no force can repel stuff with the same kind of stuff in the way' when the field strength already does something as bizarre as suddenly dropping off like a cliff. It should be obvious that shield strength varies over distance like no real force field.

I don't see how having shield emitters on your hull would be more of a liability than having just one inside the ship. Either way, if momentum is transferred directly to the emitter something is going spontaneously to turn into a bullet inside your ship if it gets hit hard enough. It's a question of having pieces ripping off your hull vs. having the generator punch its way out of your ship like a chestbuster. Personally, the latter if anything sounds worse to me.

If the ship has a load bearing internal frame or spine, then placing the shield emitters there would be stronger than distributing them over the outer shell.

With line-of-sight shields, the pieces being ripped off the hull in your first scenario are almost certainly going to end up smashing through the rest of the ship. An internal one can only burst out through half a ship, and may take some energy with it. A surface emitter could tear through the whole ship thickness (all other things equal, etc).

[Darkevilme]

It all depends on your assumptions, but the basics generally still apply. If an enemy is firing a slug powerful enough to rip the generators off your hull, then his gun, and by extension, his ship must have felt that same force. Thus, unless you have an extreme technological mismatch between the fighting ships, this shouldn't be much of a concern.

Though all things being equal all the enemy has to do is brace the railgun turret while mister hull-covered-in-emmitters-like-christmas-lights has to brace every emmitter to withstand the same force as opposed to just one.

[Atlan]

In so much science fiction, we see that shields can be damaged (depleted), which doesn't make sense if it's some kind of straight-up force shield. You'd think that either an attack would overwhelm it or it wouldn't. You wouldn't expect shots to "chip away" at it until it's weak enough to expose the hull. Unless the shield really is just a layer of see-through armor you can switch on and off. Why switch it off? Well you'd definitely want to shed all that mass for a burn, and if it requires constant energy input to make it real, than you wouldn't want it on all the time.

Makes sense if you run the shields off capacitors because your normal energy source can't keep up with the sudden demands of a shield recieving impacts.
The powersource (let's say... matter/antimatter reacting in a dilithium matrix) can't exceed a certain output, so you use it to charge capacitors and run your shields of those. They can give your shield much more oomph, but unless you have sufficient time to recharge the capacitors, they could run out of power, "depleting" the shield, since they're a part of the shield array.

Just my two cents.

[PainRack]

This guy didn't "clarify" his argument; he's simply weaseling around trying to save it. I'm sure he initially believed that it is simply impossible to project force through obstacles, completely forgetting the examples of gravity and electromagnetism.

He's still arguing that. His point of contention is that this isn't the case in ST, because we saw a case where the ship rolled with the impact from weapons fire.

It isn't wrong, but that doesn't make what hig saying wrong either. There simply isn't a "jump" through matter.

[Lord of the Abyss]

At the risk of putting my ignorance on public display, wouldn't an "energy" shield that can withstand kinetic attacks actually be a *material* shield composed of some kind of exotic, ephemeral and for whatever reason translucent matter? One with actual mass (either real or virtual, whatever that means)? In that case, wouldn't the momentum be transferred into the shield itself?

Gravity and magnetic fields can deflect matter from it's course without being made of matter. On the other hand, even something as exotic as a black hole still has momentum; a star that becomes a black hole doesn't suddenly stop dead and fall towards the galaxy's center of mass.

Where the momentum would go would depend on the exotic technology being used to generate the shield.

In so much science fiction, we see that shields can be damaged (depleted), which doesn't make sense if it's some kind of straight-up force shield. You'd think that either an attack would overwhelm it or it wouldn't. You wouldn't expect shots to "chip away" at it until it's weak enough to expose the hull.

I'm no physicist either, but I recall some types of shields as being an . . . accumulation of energy, for lack of a better word. Like a standing wave of matter-deflecting force that you pour more power into to make more intense. Or a variant on a magnetic field. In the former case, I'd think there would be only limited coupling between the shield and the ship; the impact energy would go somewhere, but the shield wouldn't transmit it to the ship. It might even just move the shield, and not the ship - in which case the ship would have to push it back, and you would have, in effect, the force of the blow being placed on them. Or, it might be emitted out the other side as particles, or transmitted to some medium we can't detect - which would look like the negation of momentum to our eyes, but wouldn't be.

[Darth Wong]

If a shield were a massive object, you still need to exert force in order to hold it in its position relative to the ship. What exerts that force? Whatever it is, there must be reaction force, as per the law of action/reaction.

This is a very simple concept that people are having trouble grasping. Unfortunately, good ol' Newton is still beyond the grasp of most of the population. Anyone ever seen that old Newton desktop ornament, with the series of steel balls where the force of impact is transmitted through the series?

PS. I suppose the idea could work if the shield is a free-floating object and very massive (much more so than you could reasonably expect for something that is invisible and vanishes instantly when the projecting mechanism is shut off), so that it is allowed to move freely and soak up momentum. In that case, it would act like a huge barrier wall, but it would also need structural rigidity to work this way. If it's a cloud of particles, why would it have structural rigidity? One could always argue that something in the ship makes it rigid, but in that case, the reaction forces still end up going into the projector system.

[Winston Blake]

It all depends on your assumptions, but the basics generally still apply. If an enemy is firing a slug powerful enough to rip the generators off your hull, then his gun, and by extension, his ship must have felt that same force. Thus, unless you have an extreme technological mismatch between the fighting ships, this shouldn't be much of a concern.

I don't think it's as simple as that. The firing and defending ships both feel the same impulse, not force. Generally whether something is ripped off depends on the maximum stress on its supports. That'll depend greatly on how they and their mounts are constructed. Force can be reduced if the railgun recoils a long distance, or if the shield 'flexes' a lot. The stress is reduced if the railgun and shield emitter mounts spread their force over a large area. We can't know if emitters breaking off would be a problem without a lot more detail.

[Paolo]

I don't think it's as simple as that. The firing and defending ships both feel the same impulse, not force.

Forces and torques are just the first time derivatives of linear and angular momentum, therefore in a conservative system they must sum to zero.

[Chardok]

I've always thought that shields absorbing kinetic impactors should act as a sort of shock absorber, but still allow the shield to "Shift" in the direction of the impact (towards the hull) all the while distributing the force of the impact over a wide area before it finally contacts the hull. Thus saving the generator from destruction and the ship as well....did that make sense?

[Ted C]

The person I'm arguing against says that its wrong according to physics for a shield generator to have an effect on something outside the hull without also having an effect on the rest of the ship, as he says it, field effects are 'line-of-sight'. Because of this, he says, the bit of hull directly between the projextile and the shiels generator is actually the part of the ship that as taking the momentum, and from there, it spreads to the rest of the ship. He says that momentum cant be transferred without direct contact, for some reason. He also says that there must be shield emitters on the hull because of the beforementioned problem.

I wonder if anyone else has pointed out that field effects are not line of sight. Put a strong magnet inside an opaque cardboard box and its magnetic field will still exert force outside the box. Also, if you know where in the box the north end of the magnet lies, you can approach it with the north end of another magnet from the outside, and the repelling force will be exerted against the interior magnet, not the cardboard that lies between the magnets.