How do massive starships levitate?

Question

This scene is commonly seen: A huge starship levitates over the surface of a planet or a moon, staying effortlessly in one place, or moving slowly around at the fixed height. But how?

Required properties:

• Doesn't make noise or much of the noticeable influence below, except by making a huge shadow. It can make some effect measurable by sensitive instruments, but humans usually won't feel a thing.
• Doesn't rely on the atmosphere, i.e. it can levitate equally well over the planets and moons with no atmosphere.
• Can levitate above any solid surface, i.e. doesn't require special infrastructure or alloys below.

Desired/optional properties:

• Doesn't expend (much) energy in order to stay in place.
• It behaves as if some fixed height is a point of equilibrium (like with floating objects).
• Can levitate above liquid surface.

With the minimum possible amount of hand-waving, how can a massive starship achieve this?

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EDIT: To make things more clear, I'm thinking about starships like those from Independence Day and District 9, Star Destroyer over Jedha city in Rogue One and other floating starships and vechiles from SW and so on.

And it doesn't need to be possible with the currently known physics, but I'd like an answer that uses only Occam's-razor-like minimal required amount of new physics (e.g. if one approach needs a fifth force and another needs space wizards, I'll prefer the former). Imagine that a physicist sees such a starship and tries to explain it.

EDIT2: Typical height above the ground should be comparable to the ship's (horizontal) size.

Answer 1

Most planets in the solar system have their own magnetic fields^(ref.), so you could use those to levitate. Just grab a huge chunk of superconductive material, and cool it below its critical temperature just as you're at the right altitude.

Regarding your requirements:

• doesn't make much noise or noticeable influence: check. It might squeeze our magnetosphere a little, but humans won't be able to tell. The birds and the bees might feel something is off, though, I'm not sure about that.
• doesn't rely on the atmosphere: check. The atmoshpere won't help you with that.
• doesn't require any special infrastructure: It does rely on the magnetosphere, though. If you want to levitate over the barren surface of Mars or over the hellish environment of Venus with its acidic atmosphere, you're out of luck, but all other planets in the Sol system are fine to go to. It's a bit of hit or miss across the galaxy, but it's more of a hit than a miss, and the misses are going to be inhospitable hellholes bombarded with radiation in any case. More superconductor means you can levitate over places with less magnetosphere.

• doesn't expend much energy to stay in place: check. You just need enough cooling. Room temperature superconductors will help, but high temperature superconductors should suffice. They don't even heat up by themselves, so with enough insulation your energy costs could be dominated by other subsystems.
• it behaves as if a fixed altitude is its equilibrium - check. Less superconductor means more bobbing, but your thrusters should be able to cancel the oscillation.
• can levitate over liquids - check. Even Jupiter is a fine place to hover, and it doesn't have any real surface to speak of - just tons of increasingly dense gas that slowly turns into liquid which then slowly turns into solid. You can hover very far or extremely close depending on when you turn on the superconductivity.

Other benefits:
Superconductors are great in electronics. If the five-volt wire goes the distance of your entire ship, it better be superconductive, or else it won't be five-volt anymore at its other end. If you don't just go for an optical cable instead, you know.
You have a cheap source of superconductors, so let's wrap your entire ship with it. It lets you hover over bodies with little magnetic field, and a metallic chassis at very low temperature has a second benefit if you're into stealth. A superconductive undercarriage of should be sufficient to hold your craft above Luna (Sol d1), but I'm just guessing here.
It's an old and reliable system understood even by humans in the early 21st century (not with the kind of superconductors that would let you hover on Earth's magnetic field alone, mind you, but the idea was there). Even if your power gives out while you're hovering, you won't plummet to the surface. A well designed hover system should give you plenty of time to put on seatbelts before Earth's atmosphere starts ruining your attitude control. Impact itself might be unpleasant depending on how well you can arrest your rotation in an unpowered spacecraft, but your pilots will walk away unscathed.

Answer 2

Most science fiction starships (especially those in TV or film) have some kind of artificial gravity. For one thing, it simplifies filming when you don't need to hang your actors from the ceiling in every scene. This actually makes sense, however, since a species designed to work well in a planetary gravity well could experience negative health affects of living in a weightless environment over time. Since our starships spend a long time away from home, artificial gravity is important for customer satisfaction.

Here's where things get interesting, however. Large space vessels are vulnerable to tidal effects while in orbit, and if large enough, may even have their own gravitational effects to worry about. Thus, any massive vessel designed to come into close contact with a planet will need something to compensate for those effects. And the same technology that gives you artificial gravity can solve these problems, too.

Thanks to general relativity, we can describe gravity as a distortion of spacetime. The Physics Stack Exchange talks about this some. It would IMHO not be an unreasonable stretch, then, to suggest that anyone who could flatten out space time would be able to eliminate the effects of gravity in that region.

Whatever variety of applied phlebotinum you choose to justify artificial gravity, the implications are the same: if you can arbitrarily create gravity to hold things down, you can create it in the opposite direction to hold things up. The math is more complex when you want to cancel out tidal effects (see Robert Forward's Dragon's Egg for details), but hovering, IMHO, is easy.

Answer 3

Starships only park over cities. Ever notice that in Star Wars? Why? Because: Transparent stilts. Some absurdly super-strong metal/acrylic with the same refraction index of air. The stilts need a hard surface to stand on. Speeder bikes are lighter and can park on sand. First deployed as a kickstand for bicycles, now scaled up to starships. Why? Because it looks cool. And because not parking directly on the ground makes it harder for rebels to sabatoge the ship. And it makes take off easier if you can just do a full thrust forward, let the kickstand flip up, and not worry about blasting the ground.

There is no substance that comes anywhere close to these properties, but introducing such seemed like the minimal change to reality. Fifth force has HUGE ramifications for physics, so I discounted that. The magnetic field idea someone else explored, but a sufficiently strong field seemed likely to interfere with life and/or computer tech. So, I ended up with just the mechanical solution.

Answer 4

Spaceships only look massive

Accelerating mass costs a fortune in fuel. Perhaps weight has been cut down to the point that your spaceships weigh less than the people in them, and minimal thrust is needed to maintain height.

If you want FTL, why not kill two birds with -1 stones? Perhaps FTL works by using negative mass to stabilise wormholes. If you can acquire negative mass, then it may make economic sense to reduce fuel costs by balancing the positive masses on your ship with negative masses. Being able to loom menacingly over cities is only a side bonus.

Answer 5

The first thing that comes to mind is to keep it in a geostationary orbit, at which point the centripetal force will balance out gravity, causing the ship to orbit at a constant altitude. This is basic physics, so it requires no handwaving at all, and is completely independent of the atmosphere. It wouldn't even require much energy to stay in position - just enough to stop at the required altitude.

However, that required altitude might be a bit far up. For Earth, the required altitude for geostationary orbit is about 35,786 km. This counts as fairly high orbit. But on more faster rotating and lighter bodies, it can be lower. To levitate at lower altitudes, you would have to move faster than the planet's rotation.

Answer 6

Having a thruster with exhaust composed of particles having extremely low cross section for every interaction would mean a good solution. Neutrino is the obvious candidate, but many other "invisible" particles were theorized.

A ship could levitate anywhere with this beam, independently of magnetic field, atmosphere, surface composition, preconstructed infrastructure, or rotation period. It won't have any destructing effect on the surface, since the neutrinos would pass through the planet without interaction.

But there are problems:

• Since neutrinos are almost massless and almost travel at c, their energy is roughly c times their impulse. This means, that to levitate one kilogramm (with a 100% efficiency neutrino drive) you would need about 3*10**8*g watt of power. (Where g is the surface gravity of the given celestial body.) On reasonable sized planets this is orders of magnitude more than the power to weight ratio of the lightest nuclear reactors, not to mention all the other systems of the spaceship. You would probably need antimatter reactor (and use up a lot of fuel), or a handwavium energy source to achieve levitation.

• Because of the low cross section, we don't really know any method to make a directed stream of "invisible" particles. (There are no mirrors for neutrinos, nor can they be controlled by electromagnetic fields.) Particle accelerators are capable of producing neutrino beams, but all the efficient solutions (nuclear reactions and annihilation) emit neutrinos in every directions.

Using heavier particles, the energy efficiency can be improved by many orders of magnitude. The standard modell doesn't contain any really suitable particles, but for example, if Neutralinos would exist (required by supersymmetry theory: https://en.wikipedia.org/wiki/Neutralino) the lightest of them would be an excellent solution. (With a handwavium way to create it in large quantities.)

This engine could also serve as the main engine of the spacecraft, thus sparing weight for other systems.

Answer 7

Perhaps a negative tractor beam, that pushes against the ground, rather than pulling.

If a tractor beam can pull objects towards a ship, then it makes sense that a vessel could also push them away. If a vessel is equipped with a tractor beam powerful enough, it can push against the planet itself, and suspend the vessel in the air.

The noise it creates depends on the design of the ship itself. If its power generators can run silently, the surely so can the tractor beam.

What effect the beam has on people and other objects intersecting the beam itself is completely up to you. Cities can have designated surfaces and no fly zones specifically for this purpose. Or the beam may not be an actual beam, but anchor itself to an arbitrary point through some stable warp bubble, or whatever the tractor beam uses as a basis for its technology.

Answer 8

Your ship deploys a blimp when you enter an atmosphere. Outside the atmosphere, just orbit instead.

A blimp is a large hydrogen filled balloon. It will keep the ship in the air via buoyancy.