Force Fields?

I have seen the following news item the last few days:

Do you think the answer to the question about force fields might need some revisiting? Also, I have seen sound waves used to levitate Styrofoam. should that also be considered in fairness?

See “Plama window”

The big problem is the sheer energy needed to generate one. Let’s say I hit the force field with my plasma photon energy ray or whatever futuristic weapon I have. This force field must be able to put out an equivalent amount of force in the opposite direction. Not only that, but it must do so over a wide area since I could reasonably aim my weapon from any direction. That’s a lot of energy.

It might be possible to use something like a confined plasma that is so hot and dense that it could destroy whatever physical object that is attacking whatever you like to protect, but again, the energy requirement would be tremendous. Nor, would it be able to protect against weapons that use non-physical forces (such as an x-ray laser).

There would be better ways of protecting yourself that use much less energy.

You know I tried working out a shield via plasma magnetic confinement. It was just a thought experiment, but one question that popped up was what slows down the projectiles?

Here’s what I mean. My thought experiment didn’t have futuristic weapons, just a couple of fighter jets duking it out. Bullet hits high energy plasma and is vaporized. Well that’s nice but doesn’t the bullet cloud still have all the kinetic energy of the bullet? Is it better to be hit with a bullet, or a billion high temperature little bullets?

Also another problem is Newton’s 3rd law. Say one of those dog fighting jets successfully nails the other with a missile, and the shield successfully blocks the explosion from damaging the aircraft. Newton’s third law says equal and opposite reaction. In other words something is going to have to be accelerated with the whole kinetic energy of the missile blast. You might say the plasma, but now the shield generator has to forcefully hold explosion accelerated plasma with high kinetic energy in place. It’s still going to have deal with that kinetic energy.

Is the shield generator fastened to the aircraft securely enough to absorb the energy of something that would rip the aircraft to shreds? Either way the aircraft’s hull is going to have to deal with the explosion.

Real-life plasma windows (essentially plasma suspended in a magnetic field) are viscous enough to deflect about 9 atmospheres of pressure. So assuming you scale something like that up, it would deflect a certain amount of energy from balistic projectiles.

But as qazwart said, you would probably be better off using all that energy for propulsion to allow you to have more armor or avoid attacks altogether.

Also, it is better to be hit by a billion little projectiles over a larger area than one single projectile of equivalent mass and velocity over a small area.

Not necessarily. If you can capture and then redirect the kinetic energy, then the hull doesn’t have to absorb anything. Of course, there’s a significant difference current systems designed to absorb blast energy from man portable anti-tank weapons in urban warfare, but this is already quite a speculative discussion.

The question is how do you redirect it, and into what? You’d either need to put the energy into something that could withstand the forces of a missile blast, convert it to another kind of energy…

Hmm, I guess the system could be a on a fly wheel. It doesn’t need to be fastened to the hull, and immobile. That way it could be accelerated, without being damaged, or damaging the hull. For bonus points put it on a dynamo, or a generator, and use weapon impacts as an energy source for the shield. So I guess you’re right. It could be harmlessly controlled.

This is only converting the energy, don’t forget that you must also conserve momentum. So the system will still be subject to massive acceleration if there is a blast nearby. The problem with momentum is that it only comes in one form. So you can’t convert it.

Hadn’t noticed it before, but the Staff Report says that Warp 9 is nine times the speed of light. Not so. Initially, in the original Star Trek series, warp factors were the cube of the number itself, with Warp 1 being the speed of light, so that Warp 9 was 9x9x9, or 729 times the speed of light. By the time of Star Trek: The Next Generation and the later series, the warp scale had been recalibrated, but Warp 9 was never a mere nine times the speed of light, IIRC.

That was my objection too. However if you put the shield mountings on one side of a flywheel, and some counter weights on the other side for balance. Then as a result accelerating the shield generator will just spin the flywheel. Magnetic breaking can be used to convert the fly wheel’s momentum to electrical energy, and slow it down. The electrical energy could be applied to the shields, saving on the aircraft’s energy needs, and providing a useful output for the energy. Another use for that energy would be as a weapon. Imagine fighting an aircraft that the hard you nail it with a missile, the more lightening bolts it throws at you.

Linear momentum will still have to be conserved. If a projectile slams into your vessel and your shields bring it to a stop, then your vessel will be propelled with that same momentum. Even if the projectile hits a wheel, this would make the wheel push against its axis which pushes against your spaceship.

So what your saying is it’d spin like a helicopter with broken tail rotator? Seems like a counter fly wheel of equal weight and mass distribution being accelerated with half the momentum from the explosion could fix that.

The analysis needs to stand back and apply first principles.

You have a spaceship sitting, minding its own business, in space. It gets hit by a weapon that blasts it with very high energy particles. (Could be anything, bullets, big rock, sci-fi accelerator weapons, very high energy photons, whatever.) The spaceship has a shield system that surrounds the ship and absorbs the energy and momentum in a form that is not dangerous to the ship.

OK, the blast - energy, mass, momentum, can be modelled as a single entity - since it is channelled by the shield system to the generator. Thus we can model it as a moving mass that impacts on the generator. And the ship can be modelled as a single system, it has a total linear, and total rotational momentum, with a given total mass. Thus we can apply simple conservation laws to the system comprised of the equivalent blast mass, and the ship. The total momentum (of both kinds) is preserved. It doesn’t matter what sort of amazing mechanical wizardry is inside the ship, you can simply lump it all together and observe it as a system externally. And the moments are preserved.

For instance, say the blast was perfectly symmetric about the ship - i.e. concentrated on the ships exact centre. If you had the shield generator in this position, and mounted on a flywheel, the axle for the flywheel must be off centre in the ship. As the flywheel receives the impulse of the blast it will begin to rotate, but this rotation only occurs because there is an opposite force from the ship against the axle (otherwise the generator would simply vanish out the back of the ship) and thus the axle imparts a rotational torque onto the ship. And hey, look at that - the total angular momentum is preserved. Not only that, but the impulse also transfers the linear momentum to the total system via the axle. Which is where we came in. You can’t convert between rotational and linear momentum in a closed system. And here the blast and ship together constitute the closed system. You can add another flywheel, and apply a torque to it, so the ship as a whole stops rotating, But the system still contains the total angular momentum, And you can’t get rid of it in a closed system.

The clue about how to get past this is the closed part. If you are able to accelerate mass (i.e. rocket engine) you can eject fast moving mass, and you are no longer closed. So you can dump momentum. Angular stabilisation of many satellites is done in just this manner. Reaction wheels are motor driven to take the momentum from the satellite. However eventually they saturate (i.e. the mechanics of the system can’t cope with making them spin any faster) so they are desaturated. Usually you brake the wheel - which transfers the momentum back to the satellite, and by using a tiny bit of fuel through an attitude control motor you torque the satellite to compensate. It is the mass leaving that allows this to work. The HST can’t use rocket motors - due to the risk of contamination - so it uses electromagnets to hold itself against the Earth’s magnetic field whilst desaturating, and thus it transfers its excess angular momentum to the Earth.

This neglects the question of where the energy went. One assumes the equivalent of an inelastic collision.

So, the overall point remains. If the blast is so powerful that it delivers an impulse that will tear the shield generator out of its mountings and damage the ship, there is nothing that mounting it on a wheel will do to help. The momentum will be conserved, in all forms.

It’s like the concept of tractor beams. In most science fiction, somehow, one ship is able to pull another towards it without that first ship moving. If a tractor beam is like a rope which is being pulled in, both ships will feel the same amount of energy.

Or, in Star Trek when the captain says “full stop”. Full stop compared to what? The entire galaxy is rotating while various stars in it are moving hither and thither in the galaxy. Everything is in motion compared to other objects. There is no such thing as absolutely no motion.

Or, when Superman picks up a bus, he doesn’t seem to have an issue that the bus weighs a lot more than he does. It isn’t that Superman might not have the strength to pick up a bus, it’s that Superman’s center of gravity would be affected by the bus, and unless Superman anchors himself to the ground, he’s more likely to lift himself off the ground than the bus.

Then again, if we didn’t ignore these piddling physical laws, we really wouldn’t have much of a show.

I was talking about linear (as opposed to angular) momentum, so I wasn’t saying anything about how things will spin. At any rate, Francis Vaughan’s post does a good job explaining the issue.

In short: draw a box around the whole ship/shield/shield-generator system. That system’s total momentum will change at a rate given by the sum of the external forces. Nothing you do inside the box can change that.

Of course, if you’re ejecting mass from the box into the outside world, that’s a different story.

Hmm so I guess the moral of the thread is a shield is only as good as it’s mountings.

This is my favorite bit of crazy comic book physics(*). But at least Superman wouldn’t be likely to be harmed if he experienced the “equal and opposite reaction”. Cyclops of the X-Men would probably break his neck every time he fired his optic blast.

(*) Runner up: Every time someone falls from a plane/cliff/tall building the hero catches them about a foot before they hit the ground. Too bad suddenly taking away all their downward momentum is still going to kill them, whether it’s the hero or the ground that does it. Yet it never happens that way in comic books. Well, except for Gwen Stacey, I guess.

Well, the problem isn’t just that the shield generator could be ripped out of you. Even if it holds in place, the momentum of the impact gets transferred to your ship as a whole. If you’ve got an Imperial Star Destroyer, maybe you don’t really care, since it’d take a pretty hard hit to give a ship that big a noticeable jerk. But if you’re flying around in a little TIE fighter and something explodes right outside your window, it’s not going to feel too great for the pilot, deflector shields or not. (Do TIE fighters even have deflector shields? If not, replace with X-Wing or something.)

Well Superman’s unbalanced lifting ability could be explained by his powerful reactionless flight ability. If he’s accelerating hard into the ground, harder then gravity accelerates the whole bus, then he functionally does weigh more, and can keep his center of gravity over his feet while he picks it up.