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Author Topic: Explaining an infinite free fall with physics?  (Read 3592 times)

Offline Justa Ghost

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Explaining an infinite free fall with physics?
« on: 25/12/2014 10:59:13 »
For a couple of years, I've been tossing around this idea in my head of a reality with only sky, and I'm trying to think of a valid way to explain the nature of this reality without the use of magic and fantasy. In a nutshell, there would be magnetic beings living on land masses that are in constant free fall and they could 'fly' from island to island by controlling their fall though body positioning, technology, or whatever; thats not important. My main question has to do with the free fall part.

After doing some research, I've come up with this idea of a hollow planet filled with breathable air, that also has an orbiting moon, but I don't know if this would work. Could there be a theoretical situation like this where the pull from the orbiting moon causes objects inside the shell to move in a circular motion, creating an infinite free fall effect? If this shell was massive enough, would the air be uniformly distributed or would it condense at the center possessing a gravitational pull of its own? Would the moon pull the gasses inside the shell along with any other objects virtually eliminating resistance from the air? If this wouldn't work, is there anything else that could cause an infinite free fall effect?

I'm interested in using this as a setting for a story, and If I could explain it using real world physics, then that would make it so much more interesting.


 

Offline Bill S

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Re: Explaining an infinite free fall with physics?
« Reply #1 on: 25/12/2014 17:27:31 »
Hi Justa, welcome.

I’m having some difficulty sorting out my mental picture of your set-up.

Why “magnetic beings”?
Are your “land masses” spheres?
Relative to what are they in “constant free fall”?
Your beings “ 'fly' from island to island”, so I assume there is breathable air out there, but later you have “breathable air” inside your planer.  Is it in both places?

I think your idea of basing SF on real physics is a good one.  I wish you luck with it.

 

Offline PmbPhy

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Re: Explaining an infinite free fall with physics?
« Reply #2 on: 25/12/2014 17:41:52 »
For a couple of years, I've been tossing around this idea in my head of a reality with only sky, and I'm trying to think of a valid way to explain the nature of this reality without the use of magic and fantasy. In a nutshell, there would be magnetic beings living on land masses that are in constant free fall and they could 'fly' from island to island by controlling their fall though body positioning, technology, or whatever; thats not important. My main question has to do with the free fall part.

After doing some research, I've come up with this idea of a hollow planet filled with breathable air, that also has an orbiting moon, but I don't know if this would work. Could there be a theoretical situation like this where the pull from the orbiting moon causes objects inside the shell to move in a circular motion, creating an infinite free fall effect? If this shell was massive enough, would the air be uniformly distributed or would it condense at the center possessing a gravitational pull of its own? Would the moon pull the gasses inside the shell along with any other objects virtually eliminating resistance from the air? If this wouldn't work, is there anything else that could cause an infinite free fall effect?

I'm interested in using this as a setting for a story, and If I could explain it using real world physics, then that would make it so much more interesting.
What you're describing is a situation where there is a total absence of gravity. So why don't you simply use a system which doesn't have a gravitational field?
 

Offline David Cooper

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Re: Explaining an infinite free fall with physics?
« Reply #3 on: 25/12/2014 19:27:01 »
You might want to consider a gas cloud in space containing asteroids near the centre which strongly repel each other at close range but which are still pulled close together by gravity - you would need a repulsion force that decays faster over distance than gravity's attraction does. With this situation, you can jump off an asteroid and fall through the gas cloud until such time as you meet another asteroid, and a bit of flapping of wings could help you home in on them. The relative speeds of travel of these asteroids would also be slow, so there would be no problems with high-speed collisions when you leave one and try to land on another. The asteroids could glow and emit some heat due to some kind of nuclear process, but it would not harm the beings that live there, and the heat they put out would be sufficient to keep the gas as a gas - it would be thin, but may be sufficient for these beings to use for breathing. They might live in slow motion from our point of view, but they won't know that, so everything seems to run at normal speed to them. If you use this idea, cut me in for 1% of the earnings.
 

Offline evan_au

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Re: Explaining an infinite free fall with physics?
« Reply #4 on: 25/12/2014 20:13:52 »
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a hollow planet filled with breathable air
This would work
  • inside a hollow sphere is a zero-gravity environment (or more accurately, a microgravity environment).
  • You could imagine this hollow sphere filled with air.
  • There is no real problems with a moon orbiting the planet, which would impose a slow, periodic tidal movement on the matter inside the sphere.

There are a few technical problems with this scenario:
  • There are immense forces involved...
  • From the air pushing outwards
  • And the mass of the skin pulling inwards
  • Tidal stresses from the orbiting moon
  • And occasional impacts from meteorites.
  • Having a source of energy to power life
  • For life-as-we know it, solar power is needed to support a high metabolic rate.

In science fiction, you don't need to explain everything about your imagined environment, but some of these problems could be resolved by imagining the skin of your hollow planet to be made of some strong and transparent material (like diamond?). But the plants would tend to gather around the edges, leaving the bulk of the interior dark, like the deep ocean on Earth.

Quote
living on land masses that are in constant free fall
There have been several science-fiction novels that imagined something like the ice rings around Saturn, or the dust bands seen around young stars. A few "shepherd moons" provide the tidal forces to keep material and breathable atmosphere in a ring around a star, at a similar distance to Earth's orbit. So energy is is provided by sunlight, life abounds on the very large surface area of asteroids, and there is breathable air which supports a human-scale story-line. Each asteroid has a very low surface gravity, birds (or aeroplanes) could fly between asteroids, and the volume of habitable space is truly mind-boggling.

In imagining this donut-like environment, the authors also glossed over a few details, like:
  • the pressure of an atmosphere at "room temperature" is quite high
  • while the restoring tidal force of shepherd moons is quite small.
  • Multiple asteroids or shepherd moons are likely to clump into a single massive planet, over time. 

But that's why we call it "science fiction" you suspend belief in some areas, so you can explore the implications in other areas.

Have a look at: http://en.wikipedia.org/wiki/The_Smoke_Ring_%28novel%29
 

Offline Justa Ghost

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Re: Explaining an infinite free fall with physics?
« Reply #5 on: 25/12/2014 22:45:38 »
First of all, thanks to those who have offered interesting alternatives to this idea, but it seems that some aren't picturing it as I am. Here's the original idea before the physics explanation: theres a world of endless sky where all things are in an infinite free fall, people, land masses big and small, basically any object. I wanted this world to be falling through air so that there was a constant air resistance on things. This way, windmills set up at the edge of falling islands would be able to harness the power of the wind rushing upwards. Also, with air resistance, people could skydive from island to island by nosediving faster than the island, or spreading their bodies out or use a parachute to fall slower than the island, allowing vertical travel. I also wanted 'gravity' on these islands so that when you landed on them you could stand and walk on them. If the people and the islands were both in free fall this would normally not happen, thats why I want to use magnetic beings that can be pulled towards the islands at close proximity, perhaps because the islands contain a high level of iron or whatever.

With physics, I thought that maybe the only viable way to have this perpetual free fall would be to be in some kind of large orbit. The objects in this orbit would have the illusion of falling in a straight line, and it would have the added effect of a loop where if you fell far enough you would end up where you started. With the hollow planet idea, I could contain the air easily without massive quantities like a gas giant, and the air pressure could be minimized. The orbiting moon would create the orbital motion of objects within the planet. My question is whether I'm considering everything or if I'm missing something that might not allow this to work. Ignore the sunlight problem, and possible asteroid collisions with the planet. Would the air inside the hollow planet condense and be more pressurized at the center? Maybe this would create a 'goldilocks zone' of livable air pressure within the planet. Would the air inside also move in an orbital motion at the same speed as the other objects? If this was the case, then there wouldn't be air resistance like I'm imagining it.

What do you guys think?
 

Offline jeffreyH

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Re: Explaining an infinite free fall with physics?
« Reply #6 on: 26/12/2014 00:36:38 »
What you need is a hollow planet with technology in orbit to focus gravity. If an equally spaced set of satellites sent angled beams of focused gravity into the hollow planet this would cause a gravity flow much like a river flow. This would produce a constant free fall which would tend toward the inner surface. This could be counter balanced by a device in the centre that sent gravity outwards. This would tend to keep the flow centralized.
 

Offline evan_au

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Re: Explaining an infinite free fall with physics?
« Reply #7 on: 26/12/2014 03:08:59 »
Quote from: Justa Ghost
...falling through air so that there was a constant air resistance on things. This way, windmills...
Any time you have wind resistance (and windmills), you tend to reduce the difference in speed between the air and the objects -  unless you have a steady input of power.

I guess a rotating moon could represent a continuous input of power...
 

Offline David Cooper

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Re: Explaining an infinite free fall with physics?
« Reply #8 on: 26/12/2014 20:04:09 »
You need a tidal effect on air, but we don't see much of that happening in our atmosphere because it's too small to notice easily and is masked by other effects (http://en.wikipedia.org/wiki/Atmospheric_tide). The effect of a pull from the moon would be to compress the air inside your planet at the side closest to the moon while the part of the shell of the planet at that side is held back by the rest of the shell further out, so there could be some movement of air from this relative to the rotating shell of the planet, but it would be small. Of course, the planet would be of very low mass, so the moon would have to be of even lower mass... well, it might be better for your planet to be a hollow moon going round a gas giant so that you can get more pull into the system to make the air move about a bit more. (The hollow moon/planet would eventually get tidally locked such that the same side would point at the gas giant all the time, but that would take a good while.)

...theres a world of endless sky where all things are in an infinite free fall, people, land masses big and small, basically any object.

All these things will accumulate at the centre (even though there is zero gravity at the centre) unless you introduce some special kind of material for your landmasses which allows itself to stick together at close range while repelling at a distance - that's necessary for the land masses to remain separate from each other while still allowing each land mass to hold together as a clump of stuff.

Quote
I wanted this world to be falling through air so that there was a constant air resistance on things. This way, windmills set up at the edge of falling islands would be able to harness the power of the wind rushing upwards.


All this stuff will end up being blown along with whatever wind is generated by tidal forces, so windmills will feel no air passing them. You'd need a mechanism to create more irregular wind flows so that the land masses are always playing catch-up and never succeeding.

Quote
Also, with air resistance, people could skydive from island to island by nosediving faster than the island, or spreading their bodies out or use a parachute to fall slower than the island, allowing vertical travel.

That's really the key idea to the whole thing - it's a stroke of creative genious and everything else has to be set up to enable it, or something as close to it as possible. If you have holes in the shell of the planet, you could have chaotic effects from external heating driving irregular air currents which flow through to the inside of the planet, and that means you'll always have the potential for air to flow past land masses and for people to detatch from the low gravity of those land masses to fly/sail to other ones, but unfortunately you don't actually have a gravitational acceleration in the system to enable your beings to accelerate and decelerate by changing their shape in the way that skydivers do. A being would be accelerated by the air flow for a while, but he/she would soon end up going at the speed of the air with no ability to control it, so they'd really need to power themselves with flapping wings. This could enable reasonable downwind travel from one land mass to another, but going upwind would be harder unless they use powered flight. However, the wind direction could change from hour to hour and the land masses would move around a bit, so return journeys would be possible - it's just a matter of waiting for the right conditions to make the trip to where you want to go.

Quote
Would the air inside the hollow planet condense and be more pressurized at the center?

You might have a ball of liquid nitrogen at the centre and high pressure air around it, but the action of the story doesn't need to visit the centre. You just need some special force to stop the land masses merging, and that could keep them away from the centre and from the outer shell of the planet too.
« Last Edit: 26/12/2014 20:07:23 by David Cooper »
 

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Re: Explaining an infinite free fall with physics?
« Reply #8 on: 26/12/2014 20:04:09 »

 

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