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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?


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.

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  • $\begingroup$ ?With minimum handwaving, closest you have is a helicopter... $\endgroup$ – Mołot Mar 4 '17 at 13:42
  • $\begingroup$ @Mołot, helicopters make wind and noise. They also don't operate in vacuum. $\endgroup$ – Danijel Mar 4 '17 at 13:45
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    $\begingroup$ Thousands of hippies surround them and concentrate really hard. $\endgroup$ – Spencer Mar 4 '17 at 17:21
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    $\begingroup$ anti-gravitons. $\endgroup$ – Michael Mar 5 '17 at 3:20
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    $\begingroup$ Since you didn't specify a height, it seems anything in geosynchronous orbit would fulfill your requirements. $\endgroup$ – Kevin Mar 6 '17 at 7:22
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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.

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    $\begingroup$ Do you have a reference for this general hovering property of superconductors over magnetic fields? I think it only works if the field, the superconductor and gravity have a specific orientation. $\endgroup$ – Paŭlo Ebermann Mar 4 '17 at 19:20
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    $\begingroup$ @PaŭloEbermann It's called flux pinning - the superconductor locks the magnetic field inside of it when it becomes superconductive. I believe that works in any direction, although I'm not sure to which extent the magnetic force is aligned with the gravity when you're in an oblique longitude. Good point. But even if you do want to hover at an oblique altitude, this should help you immensely. $\endgroup$ – John Dvorak Mar 4 '17 at 19:31
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    $\begingroup$ One problem I'm having with this answer: it's not explicitly stated in the question, but it seems to me that not only the human bodies below the ship should be undisturbed, but also that humans below it should be able to keep going after their daily routine as before. An electromagnetic field of this magnitude will probably collapse the electrical grid, destroy a lot of electronics irreparably, etc. $\endgroup$ – rumtscho Mar 5 '17 at 22:50
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    $\begingroup$ I don't expect nearly as much effect on the magnetic field as you suggest. Not to be the best way to ruin your electronics for a while is a rapidly oscillating one. Don't forget every computer has a couple of neodymium magnets inside, at least they did until the advent of SSDs. What do you suggest the magnitude of the ship’s effect would be? $\endgroup$ – John Dvorak Mar 5 '17 at 23:03
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    $\begingroup$ If you take a chunk of superconductor it doesn't levitate in the Earth's magnetic field alone, therefore even if you built your entire ship out of superconductor it would not levitate. Planetary magnetic fields are just too weak. $\endgroup$ – alessandro Mar 6 '17 at 12:03
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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.

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  • $\begingroup$ Curvature of the spacetime doesn't correspond to the gravitational field, it corresponds to the inhomogeneity of gravitational field. If you remove the curvature (flatten the spacetime), the gravitational field won't go away. You can't even measure the local gravitational field due to the equivalence principle (EP) - one of the core tenets of the general relativity (GR). So this approach might work only with EP-violating (and therefore GR-violating) laws of physics. $\endgroup$ – Danijel Mar 5 '17 at 1:18
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    $\begingroup$ @Danijel Given a typical definition of "curvature", that's not true. A flat spacetime does not have any gravity. $\endgroup$ – David Z Mar 5 '17 at 11:53
  • $\begingroup$ @DavidZ A spacetime which is everywhere flat (Riemann tensor=0) does indeed have no gravity. However, a spacetime might be locally flat in some region and still have nonzero gravitational acceleration (w.r.t. stationary coordinate system) in that region. That would be the case if you "flatten" the spacetime just locally around your ship. $\endgroup$ – Danijel Mar 5 '17 at 17:52
  • $\begingroup$ I'd pick the first paragraph as the typical correct answer to this question, but the second two paragraphs strike me as "um really?" $\endgroup$ – Dronz Mar 5 '17 at 22:28
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    $\begingroup$ I don't think it's worth arguing about spacetime distortions in the context of this question, especially since we have people like @Danijel who clearly understand this better than I. To that end, I have struck the third paragraph of my answer to focus on the key element -- artificial gravity isn't just for floors. $\endgroup$ – papidave Mar 6 '17 at 17:44
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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.

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  • $\begingroup$ The first part is one of the most funniest posts I've ever read here :-) For the second: fifth force is considered on many place in the physics, some of them: 1) interaction between the Higgs-field and the other particles can be considered as "fifth force" 2) there is a minority, but still strongly researched possibility, that the dark matter doesn't exist, instead a fifth force modifies the gravitational behavior in large (around galaxy-size) distances. | So, fifth force is not so unimaginable thing. $\endgroup$ – Gray Sheep Mar 4 '17 at 16:57
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    $\begingroup$ And with enough air traffic control, you might crash no more than 10, maybe 20 commercial airline jets a month! $\endgroup$ – Jack M Mar 4 '17 at 23:28
  • $\begingroup$ @JackM Airline jets? Those guys have personal hovercars! :D $\endgroup$ – Luaan Mar 6 '17 at 12:00
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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.

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  • $\begingroup$ All you need is some medieval phlogiston. That has negative mass... $\endgroup$ – Dronz Mar 5 '17 at 22:32
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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.

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    $\begingroup$ Also, it only works directly over the equator, not over higher latitude positions. $\endgroup$ – Paŭlo Ebermann Mar 4 '17 at 19:17
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    $\begingroup$ @b.Lorenz: I think it might be interesting to put a "scale" reminder here. The Earth diameter 12,742 km, so this is about 3 times the diameter. Needless to say, it would have to be fairly massive to be observable from the ground without instruments. $\endgroup$ – Matthieu M. Mar 4 '17 at 19:52
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    $\begingroup$ Another point to scale: the Moon's orbit is 385,000 kilometers. If your ship is shorter than 9.3% the size of Earth's moon, it won't even be able to block out the sun. Although, if you do block out the sun for one spot on Earth, you'll need little extra to block it out for the entire planet at once - something you'd need a much bigger craft if it were to stay in low ---orbit--- hover. $\endgroup$ – John Dvorak Mar 5 '17 at 5:57
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    $\begingroup$ You just need to hang the space ships from a string to a counterweight above geostationary orbit. Still have the equator problem though. $\endgroup$ – Stig Hemmer Mar 6 '17 at 8:36
  • $\begingroup$ Everything orbit other than geostationary orbit isn't stationary and now I've specified that the height above the ground should be comparable to the ship's size (haven't noticed lack of that detail before). Nice try, though. :) $\endgroup$ – Danijel Mar 6 '17 at 12:33
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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.

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  • $\begingroup$ Neutrino thrusters came to my mind and problems with directing the thrust might be dealt with using the femtotechnology, i.e. neutrino thrusters could be made out of somehow stabilized nuclear matter. However, energy requirements would be absurd, regardless of the implementation, which would make it impractical and inelegant. $\endgroup$ – Danijel Mar 5 '17 at 1:29
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    $\begingroup$ You may have just invented the WIMP drive. $\endgroup$ – James K Mar 5 '17 at 8:17
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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.

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    $\begingroup$ Hello and welcome to WorldBuilding.SE! Could you write a bit more about your idea? It sounds like this could be a good answer, but we like more elaborate answers around here that go into detail quite a bit. For example how do you handle the requirement: "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."? Please edit your answer accordingly. $\endgroup$ – Sec SE - clear Monica's name Mar 6 '17 at 12:05
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    $\begingroup$ Welcome to worldbuilding. I agree with @Secespitus, this answer is a bit too short to provide an adequate answer to the question since you do not really explain how it would work. If you do not edit to expand your answer, then there is a risk that it will be tagged as low quality and potentially deleted. There are three links at the bottom of your answer saying "share edit flag", use the edit link there to open up the editor where you can improve your answer. $\endgroup$ – Mrkvička Mar 6 '17 at 12:16
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    $\begingroup$ What is the conceptual difference between a negative tractor beam and a thruster? $\endgroup$ – Danijel Mar 6 '17 at 12:34
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    $\begingroup$ 'Repulsor beam' $\endgroup$ – Pete Kirkham Mar 6 '17 at 13:49
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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.

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  • $\begingroup$ This does not look like an answer to me. Please provide a more detailed view of your idea as it is currently very short. For example how do you handle the requirement: "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."? Please edit your answer accordingly. Plus I don't really get what you mean with "deploy a blimp". Please elaborate on this. $\endgroup$ – Sec SE - clear Monica's name Mar 6 '17 at 13:49
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    $\begingroup$ @Secespitus Blimps don't make noise. They are completely passive levitation devices. $\endgroup$ – PyRulez Mar 6 '17 at 14:06
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    $\begingroup$ This violates the "doesn't rely on an atmosphere" requirement $\endgroup$ – Josh King Mar 6 '17 at 15:06
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    $\begingroup$ This violates condition #2: "Doesn't rely on the atmosphere." A blimp always relies on the atmosphere. $\endgroup$ – Frostfyre Mar 6 '17 at 15:32

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