Anti-gravity or gravity shielding - hover cars

An off-topic post, but this sprang fully formed into my head from a combination of movies and SF books. Watch or read about a futuristic setting and one thing that tends to appear are hover cars. Our hero jumps into one, takes off and chases after the baddie. If it's a slightly harder SF setting you might get a word about how this works thanks to anti-gravity engines or gravity shielding and that's about it. I mean who cares they both mean being able to fly so they're both really just the same aren't they?

It is now the future and you're the test pilot for the new hover cars being developed one an anti-gravity model (AG) with the other using gravity shielding (shield). Hop in and take off.


Before you do though a quick and simple recap of gravity. Masses attract and the particle doing the work is called the graviton. In our AG vehicle we're producing an anti-graviton that causes repulsion between masses rather than attraction; in our shielded car we're stopping the gravitons having an effect. For both our vehicles we assume there's a method to increase and decrease this effect.

Into our AG car and we turn on the engine and start producing our anti-gravitons from the underside. Repulsed from the ground we start to rise. To keep things simple our net result is an acceleration of 1m/s/s so after 3 seconds we have a speed of 3m/s/s and are six metres in the air. If we reduce our AG we start to slow down as normal gravity takes hold. To hit our goal height at that instant we need to unmoving and have both upwards and downwards forces balanced. We are now hovering.

To land we need to do the reverse. Reduce out AG to fall and as we approach the ground increase it to reduce our acceleration and speed so we don't crash. Again the goal is to balance the forces so that we're not moving when we reach out landing height.

Into our shielded car. We hop in and power up the shield and nothing happens. That's because although we're no longer attracted to the ground there's nothing above us pulling us up (yes technically there may be other aircraft, planets, stars etc. but net effect - zero) In order to move upwards we need a propellant - something aimed downwards to push us upwards. One simple squeeze of the trigger and a short burst of air downwards sees us heading upwards. As we approach our height goal we just decrease our shielding to slow us down then increase it to balance our forces. With no movement and shields at maximum we are now hovering.

To land we don't need any upwards facing jet we can just reduce the shielding and let gravity pull us down. As we approach landing height we increase shielding, but that doesn't slow us down it just reduces our acceleration we need to fire our propulsion system to counter until we reach landing height.

So here we have our first difference. An AG system can rise and fall using only the AG; a shielded system requires an additional propulsion system. In both cases the degrees necessary to balance out the forces means that ascent and descent would fall under computer control. Rather than pull back on a joystick a consumer would be more likely to pick a height or press the "Land" button and let the computer do all the calculations.

Back into the air and hovering it's time to move about. For this both our cars need the same system of propulsion - to move forwards - a backwards facing jet; to stop - a forward facing jet; and to move from side to side jets on either side. Why not use the AG or shielding systems here? What are you repulsing against or being attracted to? For our shielded car we already need a propulsion system so we can just tie the two together.

At this point I think it's worth pointing out that while either car travels vertically or horizontally it does so while maintaining its own level. The front half and the back half are equally being repulsed or shielded, even firing the rear jets will just see the car moving forward at a level. Pulling back on the joystick doesn't take you into a nose high position; pushing forward doesn't create a nose-dive; these are gravity based systems not jets relying on air-pressure differentials.

Congratulations you have now taken off, landed, and moved around. However that's not the end of the test-run.

So far you've just been roaming around this flat test field - time to go up a hill. For an AG car this is simple. As you approach the hill the front half of the car will be repulsed and the car will tilt backwards. The distance between the ground and the car will remain the same. With the shielded car you'll drive into the hill - your height is not dependent on ground level. You need to raise the car up in steps or just raise it to a height to drive straight over the hill.

At first glance it seems the AG system is better in this terrain as it raises or lowers itself automatically; however this presents a problem. Now you're at an angle gravity is pulling the rear (or front) of your car downwards you have to use your jets to get up the hill or stop yourself from crashing at the bottom. Another problem this 'benefit' produces depends on your height.

With the propulsion used by aircraft or rockets a force is thrown out the back which makes the vehicle move forward; however an AG system doesn't do that, nothing is being moved you are simply being repelled. It's less like rockets and more like magnets. The question for an AG system is - once you gain height what are you being repelled by?

Well there's the air underneath you, but that's a fluid (it moves) so if you repel that it'll just squirm away from under you; as you constantly repel the air no more can enter and a vacuum is created. Maintain it long enough and they'll be a vacuum between you and the ground and a massive downflow of air being pushed around the edges as air moves in and is forced downwards by the edge of the AG. Now pass over a pedestrian. A gust of wind and then they're flattened to the ground by the full weight of your vehicle (force downwards has to balance your weight). All AG vehicles would be confined to roads and all at one height which makes them no different to normal road vehicles.

Shielded cars make no difference to anything passing under or over them. An object beneath simply isn't attracted to the mass of the vehicle above it; air just passes along as if the vehicle wasn't there. They can travel anywhere and at any height.

Our decision is easy we take the shielded car and advise the AG vehicle be scrapped with the technology being confined to tubes used to shoot cargo and passengers up into orbit.

With all that it's easy to see why SF films and books just state they work and leave them at that, but next time you watch something like the Star Wars prequels and that Anakin speeder chase scene ask yourself what system is fitted to allow the vehicle to behave the way it does.