25
$\begingroup$

You see this in quite a bunch of movies / books (think avatar). What could be a reasoned physical explanation of such phenomenon? Would this explanation impact other things on a planet?

Like most things in nature, I'm assuming it could come from a combination of many factors. Magnetic repel (suggested by @smithkm), hydrogen retaining organisms, high density air, low density rock (like volcanic porous projections)... what else could pitch in so that it would be enough?

Low gravity was suggested but it would imply that air itself would provide less lift to objects, thus not helping with our rocks to fly.

$\endgroup$
  • $\begingroup$ Do you need real rocks or just a floating rock-like platform to stage scenes on? $\endgroup$ – TechZen Oct 17 '14 at 21:36
  • $\begingroup$ Any sort of floating ground that could harbor life was the first desire. $\endgroup$ – Sheraff Oct 18 '14 at 0:15
13
$\begingroup$

Using currently known physics it's essentially impossible.

To move into more speculative realms though if you posited a material that repelled itself similar to opposed magnetic fields you could generate "floating rocks". The material would need to not repel when held together, but chunks of it would act to repel each other.

Now seed sections of that material through conventional rock and wherever the concentration were high enough it would cause them to move apart. This would be unstable though and the rocks would tend to move and tumble and eventually fall to the ground if they found an area not seeded with the material, so this doesn't give us what we want.

That has a number of problems so lets look at a different approach that avoids the repulsion problems. Lets go one step further and say that the material generates a force that "locks" sections of it together. So for example there is a volcano that erupts containing this material, it flows out and cools. Magnetic locking is a real phenomenon that happens with super-conducting materials and magnetic force lines although that does still allow the super-conductor to move along the force lines. Lets say this hypothetical force goes a bit further though.

As it cools the force lines lock together and all the chunks of this hypothetical material lock in position with each other.

Now erosion, animals, vegetation happens and the rock starts to be worn away. But the material holds its position. (Clearly it's harder than most rock so it lasts longer).

Eventually you end up with isolated floating islands with vegetation growing on them all held in position and stationary.

$\endgroup$
8
$\begingroup$

Near surface geostationary orbit

Perfectly still or slowly rotating rocks suspended above one point on a planet's surface can be achieved, and might be found in nature, although rare. However, since this explanation only works on a planet without an atmosphere (or else only works for rocks above the atmosphere) it may not be useful for world building if an atmosphere is an integral part of the world. However, it might still be of interest for building a world on a rocky planet devoid of atmosphere with a few enclosed habitats dotted around where life resides. You could also imagine a world where there is only atmosphere in canyons, and above the canyons formations of rock can be seen suspended.

The principle of geostationary orbit is used to place communications satellites at a fixed point in the sky. A rock in geostationary orbit can be fixed above a point on the planet's surface, and can be either always facing the same side to the planet, or slowly rotating relative to the planet. The restriction is that these rocks can only be directly above the equator, at a very specific orbital radius.

Surface behaviour

A planet spinning fast enough to allow geostationary orbit near the surface would result in odd side effects. Any object at rest on the equator would be moving at speed near to orbital speed. It would have weight but much less than similar objects at the poles. A planet that formed spinning that fast would be flattened, with the equator at higher altitude. A planet spun up to that speed after solidifying as a sphere would result in any object just North or South of the equator experiencing a force towards the equator, resulting in a drift of loose rocks towards the equator. If the height of geostationary orbit was only just above the ground level, this could result in rocks finding their way into orbit simply by drifting towards the equator and then piling up.

Requirements

The derivation of geostationary altitude results in

$$r=\sqrt[3]{\frac{GM}{\omega^{2}}}$$

In order to decrease the orbital radius (and hence the altitude), either the mass of the planet must be decreased, or the angular speed must be increased. So to find near surface geostationary orbiting rocks you would need to look at small, fast spinning planets (or moons).

Much larger rocks

Two similar sized rocky planets orbiting each other would tend to slow in their rotation due to tidal forces, until they become tidally locked, facing each other. Each would then have a huge rock suspended directly above a point on its surface. I suspect this isn't what you meant though... If you allow that then there is a similar example close to home. Earth's moon spins at the same rate as it orbits, presenting (nearly) the same side to the Earth. It's not quite perfect due to libration effects, so if you stand on the moon and look up at the sky the Earth is not fixed in place, but its motion is restricted to a small region of sky. On the moon, there is a huge rock floating in the sky but it doesn't quite stay still...

$\endgroup$
  • 1
    $\begingroup$ This is such a cool answer. Absolutely not what I was expecting. I'm going to open a questions about this: worldbuilding.stackexchange.com/q/339/90 $\endgroup$ – Sheraff Sep 20 '14 at 6:02
  • 1
    $\begingroup$ Thank you! I love that new question, and it's fascinating to see answers going into much more detail than I have here. $\endgroup$ – trichoplax Sep 21 '14 at 6:33
  • 2
    $\begingroup$ If you like the tidally locked idea there is a book called Flight of The Dragonfly / Rocheworld a large part of which is set on a pair of tidally locked planets that share an atmosphere. $\endgroup$ – Tim B Sep 22 '14 at 13:24
4
$\begingroup$

This is a trope that I have used before, and probably will again, and in every single instance, I have never tried to explain it as anything other than magic, given that it is so unlikely to occur in the real scientifically explicable universe that it can be nothing other than magic.

With a sufficiently powerful magnetic field and a large amount of superconducting material at its core, rock may be made to float, but we are talking about enough magnetism to grab the watch off your wrist, and it would be a very unusual environment to produce that much room-temperature superconductor.

Another possibility is that what appears to be rock is actually a very thin layer over a life form that is able to produce hydrogen in sufficient quantities that it can float using hydrogen-filled gas cells. Hydrogen is far more likely than Helium, as Hydrogen is quite reactive and hence more easily manipulated by a life form than Helium which is inert. It has the added advantage of being more buoyant.

$\endgroup$
  • $\begingroup$ What if it is (like most things in nature) a combination of many factors? Magnetic repel (suggested by @smithkm), helium retaining organisms, what else could pitch in? $\endgroup$ – Sheraff Sep 17 '14 at 7:49
4
$\begingroup$

Science-based, you need upward forces to be equal to downward forces.

Downward forces are easy: they can be resumed as gravity. Making them smaller points to low density materials and low gravity worlds. Low density not only involves low density mineral but also incrusted gas in hollows.

Upward forces need a lot of help to reach gravity's level, even a reduced one as explained above. For these, you can use:

  • Aerostatic lift, bigger the denser air is.
  • Magnetic repel. This has the problem that magnetism is bipolar: every boulder would have north pole and south pole, and if it is floating due to repulsion but somehow it turns upwards down, it will be attracted to the floor with extra force. And these turndowns are quite common, almost unavoidable.
  • Upwards air current. These are actually able to sustain heavier-than-air things like hang-gliders, but most of these need movement to work.
  • Electrostatic repel. Similar to magnetic repel, but in this case boulders can be charged on one sign. If this is the same as ground, they'll levitate. But it will cause tons of secondary effects, from hair rising to electric discharges (rays).
$\endgroup$
  • 1
    $\begingroup$ I'll be happy to update the list of upward forces with any contribution $\endgroup$ – Envite Sep 18 '14 at 9:39
  • $\begingroup$ maybe you can add the low density materials in your "aerostatic lift" section, and the "quantum superconductors locked in a magnetic field" in the list. Secondary effects (mentioned for the "electrostatic repel") are a pretty awesome thing to have there too imho. $\endgroup$ – Sheraff Sep 18 '14 at 9:47
  • 1
    $\begingroup$ There's also the possibility of some sort of thrust generate by the stone itself. This could be some sort of system of tubes through the rock that heat the air in it and cause it to be expelled through nozzle-like structures on the underside, with intake openings on the top or sides. Or it could be organisms that break down substances in the rock (sodium bicarbonate, for example), and the gasses get ejected through downward-facing vents/nozzles. $\endgroup$ – Earl Jenkins Oct 9 '14 at 15:51
3
$\begingroup$

The only thing I can think of that remotely comes close is the combination of a superconductor and a powerful magnet, but even if there were gigantic lumps of superconductor and immensely powerful permanent magnets occurring naturally. You couldn't get them to levitate very high if you could get it to work at all.

Aerostatic lift from a low density gas or pockets of vacuum would not work. Hydrogen doesn't really lift things, it just doesn't get pulled down as hard as the air it displaces (it's less dense) and that heavier air being pulled down is what really produces aerostatic lift. You'd need the "rock" to weigh the same as an equal volume of air, but rock is much, much, much denser than air. You'd need to make the "rock" a bubble around the lift gas or vacuum, with walls so thin they probably shatter instantly. Low gravity would not help as it would also reduce the weight of the air by the same amount leading to no change in buoyancy.

$\endgroup$
  • 1
    $\begingroup$ It's not really part of the answer, but flying rocks are pretty much pure magic, and really, that's the point. The magic may be dressed up in scientific window dressing, but it's still magic, and the point of it is to be magical. $\endgroup$ – smithkm Sep 17 '14 at 7:33
  • $\begingroup$ What about (i'm just thinking about this now) plants or bacteria, some living thing pervasively embedded in the ground and emitting (or gathering) very light gas (helium)? Would that solve it? $\endgroup$ – Sheraff Sep 17 '14 at 7:35
  • $\begingroup$ @FlorianPellet, not unless the world's gravity was very low... $\endgroup$ – Monty Wild Sep 17 '14 at 7:43
  • $\begingroup$ How low then? Is there math do know this? How much helium per cubic meter of rock do you need for levitation? $\endgroup$ – Sheraff Sep 17 '14 at 7:46
  • 3
    $\begingroup$ Aerostatic lift is proportional to gravity. It's really just the weight of the air. So less gravity means the air weighs less and it provides less left for the objects in it. Also, any organism capable of producing helium would need to have a metabolism based on nuclear fusion. If you want to revise your question, edit the question rather than replying in comments. $\endgroup$ – smithkm Sep 17 '14 at 7:57
3
$\begingroup$

Any sort of floating ground that could harbor life was the first desire.

That's easier than rocks.

Mars size world orbiting a gas giant. To terraform it, mass off the gas giants atmosphere are blown off, turned to plasma and dumped on the moon. Atmosphere is mostly helium and large amounts of fluorocarbon to create a greenhouse effect and maintain atmosphere for few hundred thousand years. Atmospheric density is 2-3 times that of earth but mostly inert save for the trace nitrogen, CO2 and oxygen, which at those pressures is easily breathable in small quanties

Problems are encounter as the planet warms because vast amounts of CO2 and ice are part of the structural geology on a planet that has never thawed. As it warms, the ground heaves, erupts and slides randomly and catastrophically in pyroclastic flows flows that crush and smother every living thing. The same effect causes sudden vast sink hole to suddenly collapse. Lower areas can be flooded with asphyxiating CO2 flowing from high lands with no warning.

Ground winds are very dangerous. The winds pack 2-3 times the kinetic punch as the same velocity wind on earth. Worse, the low gravity makes it easiy to pick up dust and even pebbles. Fortunately, the thick atmosphere transfer heat around efficiently so such storms tend to be localized.

The surface is to dangerous to live on permanently.

The 1/3 earth gravity and 2-3 times atmosphere makes flight much easier. The solution for colonist is to engineer plants-like organism that grow up into free floating balloons. Sometimes tether, other times floating. Their buoyancy comes from having pure helium inside which they heat with solar radiation. They don't gain a lot of altitude just enough to say about the chaotic landscape.

Although plant like, they have some animal attributes in needing to sense the environment and move accordingly

They have a long tail of lightweight tendrils they use to anchor when needed but keeps them stable the rest. At the top they are disk shape with gas cells radiating out like large flower petals. The petal provide compact photosynthesis by employing the entire exterior membrane as the chromoplast. The flat shape provides active support on the dense atmosphere and in the wind provides lift. There top center forms either a convex dome shape or concave bowl shape.

They look somewhat like a sunflower with very large, rather thick translucent green leaves sticking out. They can reach the size of major sports stadium with a dome area 300 meters in diameter and 600 meters of leaf shaped lift cell radiating outward.

On the dome or bowl shape, is a lichen that serves as a thin but effective soil/anchor for various plant species. Humans live and farm on the "islands".

Flight in the thick atmosphere is very easy. Humans can almost flap of the ground with large wings (but not quite.) Airships need gas bags only a third of that on earth. Wings as well are small. Steam engines provide enough energy for flight. Glider work very well, can cover long distance and carry fairly significant pay loads.

The ground is treacherous and every shifting, with howling winds. Most human stay up in the sky, farming the islands and traveling by air. Coming down to the surface only to mine and build heavy industry. People who tough it out on the ground are nomadic and increasingly rely on raiding as they can't stay in one place long enough to plant or built anything.

There you go, floating island that would feel like rock when you stood on them. Stream powered airships, chaotic shifting earth below, mad barbarians lurking.

$\endgroup$
1
$\begingroup$

There is no GOOD way to do this with the physics we know:

1) low density rocks: very difficult as a solid is much heavier than a gas in all cases. The best exceptions are trapping voids or lighter gasses in the rocks. Rocks though are made of silicas which are too dense and porous. A tiny amount of success might be possible by taking liberties with geopolymer chemistry and pretend it is really like a polymer.

2) superconducting materials: are rare, low temperature, and would require incredibly strong fields to induce levitation

3) updraft: not really what you wanted was it? would require incredibly strong winds in exactly one area to prevent it from moving in any particular direction

4) sonic levation; awesome but would require the formation of very consitent vibrations in exactly the right way and be very consistent

5) antigravity; for fantasy/scify setting this is ususally the best bet as it facilitates new technologies. A natural fictional material could be described which plays on out lack of understanding of quantum gravity to either be opaque to gravitational fields (First men in the moon by wells), ignore gravity under certain conditions (I think Edge Chronicles did this when the rocks were hot), or a misuse of the term antimatter acting as if it were a material which repels matter.

$\endgroup$
1
$\begingroup$

Chemical reaction in some short ways.

Let me explain: If the small Stones are of some light kind (volcanic or likewise) and of some chemical active matter it could be possible for them to lift up for some little time when they react to some other lequid or chemical substance. There could be a burning that boost them up or the builded heat got catched in the stone (still talking of volcano stone with sponge artic strucktures) and lift them like little hot air baloons.

in Example: On the surface of the Stone a moss carpet stops the air form cumming out of the stone. In and of it self the stone weights close to some grams, the moss also. Than the bottom of the stone begins to react to something and heaten up. Hot gas comes in the Stone. Than the stone could lift for some seconds bevor it´s heavier top rotates around and it drops back on the ground.

So you got some slightly levitation. Very speculativ but also more likly than magic ;) (if you find very light chemical rocks that moss likes to rest on)

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.