Artifical gravity

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DreadArchon
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Re: Artifical gravity

Postby DreadArchon » Sat Jun 13, 2009 2:26 am UTC

ssbookyu123 wrote:This has been bothering me for a while now and I can't think of a way that artifical gravity is feasible at all so I'm open to suggetions.
Because I don't think it's been said:

Science really doesn't understand gravity. Really. Physicists admit this. Restrictions you put on it will be made unreliable by the fact that we don't even know what gravity is doing under normal circumstances, much less under hypothetical tweaked-for-our-use circumstances.

Edit: My point being that sci-fi's "real" antigravity isn't very troubling from a scientific perspective. There are fundamental reasons that faster-than-light travel won't work with any amount of technobabble (even if you're "jumping" through some sort of "warp"), but antigravity is pretty much in the category of "if the author wrote it in, we assume they found a way, and that's fine."

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Re: Artifical gravity

Postby Synergy » Sat Jun 13, 2009 3:21 am UTC

What if gravity is just acceleration? Maybe we just aren't modeling our frame of reference properly to see it as such.

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Re: Artifical gravity

Postby PM 2Ring » Sat Jun 13, 2009 8:31 am UTC

Meteorswarm wrote:
Synergy wrote:What if gravity is just acceleration? Maybe we just aren't modeling our frame of reference properly to see it as such.

Ok, and what would that imply?

It would imply that the chair I'm sitting in is accelerating upwards at 9.8 m/s². General relativity says I'm allowed to choose such a reference frame, if I want to.

I don't give a fig about artifical gravity. Or is "artifical" an alternate spelling for "arty fecal"? :P

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Re: Artifical gravity

Postby Kow » Sat Jun 13, 2009 6:53 pm UTC

Synergy wrote:It would imply that the chair I'm sitting in is accelerating upwards at 9.8 m/s². General relativity says I'm allowed to choose such a reference frame, if I want to.

Except that when you're not in it, it doesn't go flying up as if you dropped it. You're the one accelerating.
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Re: Artifical gravity

Postby Sir_Elderberry » Sat Jun 13, 2009 7:16 pm UTC

Kow wrote:
Synergy wrote:It would imply that the chair I'm sitting in is accelerating upwards at 9.8 m/s². General relativity says I'm allowed to choose such a reference frame, if I want to.

Except that when you're not in it, it doesn't go flying up as if you dropped it. You're the one accelerating.

Or, rather, everything around you is accelerating.
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Re: Artifical gravity

Postby thoughtfully » Sun Jun 14, 2009 6:47 pm UTC

The Equivalence Principle applies only locally. Acceleration doesn't produce tidal forces, for instance.
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Re: Artifical gravity

Postby Ran4 » Wed Jun 17, 2009 1:48 am UTC

Now, I'm trying to wrap my head around how artificial gravity from a rotating spaceship works out.

Let's say you live inside a variant of the Stanford Torus (http://en.wikipedia.org/wiki/Stanford_torus), but it's completely hollow and filled with air (and so on: humans can live inside it without extreme modifications or space suits). It's radius is 0.9 km and rotates at 1 rpm, giving the same gravity as the earth.

Then you build two towers on opposing sides, going towards the center, 850 m high. If you went up an elevator in one of those tower and jumped out on top, would (artificial) gravity be low there? Would it be almost like weightlessness? Say you had a small jet engine with you to de-accelerate your rotation and prevent you from going too far off (which I suppose would increase your acceleration towards the "ground", eventually ending with you dying from the fall...), wouldn't this be an awesome type of morning gymnastics? :roll:

Or what if you had a helicopter, went straight up, then rotated in the opposite direction of the spaceships rotation (iee. "stopping rotation relative to space" == increasing the rotation relative to the rotation of the spaceship). Then you descend, would you start rotating as you go further down? If not, then when you hit the ground, wouldn't you quickly be hit by a house (or some other high structure), rotating extremely fast?

Basically, what I'm saying, is the gravity less in the center ("further "up"") than "further "down""?

Yes, I'm quite confused on how this works :D

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Re: Artifical gravity

Postby jmorgan3 » Wed Jun 17, 2009 3:38 pm UTC

Ran4 wrote:Then you build two towers on opposing sides, going towards the center, 850 m high. If you went up an elevator in one of those tower and jumped out on top, would (artificial) gravity be low there? Would it be almost like weightlessness?


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The above gif (stolen from Wikipedia's page on the Coriolis effect) shows roughly what would happen if you jumped off the tower. Neglecting air resistance (which is probably not a good idea, but I'm lazy), you would feel weightless as you fell (just like on earth) and you would travel in a straight line. An observer in the rotating reference frame of the torus would see you travel in the curved path seen in the bottom half of the above picture.
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Re: Artifical gravity

Postby Agent_Irons » Thu Jun 18, 2009 3:12 pm UTC

Ran4 wrote:Basically, what I'm saying, is the gravity less in the center ("further "up"") than "further "down""?

Simply put, yes. It is of course more complicated than that.

at the top of an 850 m tower the "gravity" would be less. If you jumped out, you wouldn't necessarily follow a "straight line" downwards (from the perspective of someone standing on the "ground", but you would eventually get there. The animated GIF above you is rather effective for this. However, you aren't moving straight down in the first picture, but instead across the disk at an angle.

You're an astronaut on a short string, and I'm god. I'm swinging you around and let go. You move in a straight line, but not necessarily directly away from me. As you move in this straight line, you can pretend the wheel is turning beneath you, like a record player. Don't think of it(the wheel) as a stationary reference frame, the coriolis effect will murder you in an alley. I wish I had a snazzy animated gif like the one above.

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Re: Artifical gravity

Postby gmalivuk » Thu Jun 18, 2009 3:36 pm UTC

Agent_Irons wrote:Don't think of it(the wheel) as a stationary reference frame, the coriolis effect will murder you in an alley.

More specifically, it will murder you by moving the alley itself in unexpected ways.
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Re: Artifical gravity

Postby EricH » Thu Jun 18, 2009 5:09 pm UTC

Mother Superior wrote:"The spin of the carousel could be stopped if necessary; when this happened, its angular momentum had to be stored in a flywheel, and switched back again when rotation was restarted."


This is a case where Clarke probably made a mistake, either in his design or his description of the system. (Die-hard Clarke fans, please don't come round my house.) As Zhatt and quintopia noted, the gyroscopic effects of the rotating carousel are not negligible, and any spacecraft that needs to change orientation is going to run into some issues. (That's not to say it can't be done, just that it's a more complicated engineering choice, and engineers would avoid it unless there were some other compelling reason.) I worked, a few years ago, on a satellite with a relatively heavy rotating antenna structure (a significant fraction of the total mass), and because we needed attitude control of the entire spacecraft, the simple solution was to include a counter-rotating flywheel--in other words, as the antenna was spun up, the flywheel was spun in the opposite direction, at just the speed necessary to counter the angular momentum of the antenna. Since the net angular momentum of the (antenna+flywheel) was zero, any gyroscopic effects from the antenna were exactly countered by gyroscopic forces from the flywheel, when we turned the whole spacecraft.
Back to the story--with a system design like that, the only way that the whole spacecraft spins is if the control system fails first, and leaves at least one of the two rotating components in motion. (In itself, that's not very good design, but we can't assume there's no such failure mode.) It's very unlikely that both the carousel and flywheel would fail at the same time, so whichever fails first would impart its angular momentum to the ship, through friction, and the ship would continue spinning until the other component failed. But the spin adds drama (perhaps that's the compelling reason?), so I can't really fault Clarke-the-author for writing it that way, only Clarke-the-scientist.
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Re: Artifical gravity

Postby Zeroignite » Thu Jun 18, 2009 9:25 pm UTC

Can't you just apply the force at 90 degrees to the direction you actually want to move?
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Re: Artifical gravity

Postby Mother Superior » Thu Jun 18, 2009 11:43 pm UTC

EricH wrote:(Die-hard Clarke fans, please don't come round my house.)

As a Die-hard Clarke fan, au contraire my friend. I'd like to think Clarke would be happy to admit his mistake.
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Re: Artifical gravity

Postby EricH » Fri Jun 19, 2009 4:52 pm UTC

Zeroignite wrote:Can't you just apply the force at 90 degrees to the direction you actually want to move?

That's what I was referring to when I said
EricH wrote:That's not to say it can't be done, just that it's a more complicated engineering choice


As long as the attitude control force is small relative to the angular momentum of the gyroscope, yes; but the gyroscopic force is not unlimited, so with a larger control force, you end up with an angle of less than 90 degrees, but more than 0. Some appropriate mathematics will let you find the correct control force to make your desired change in attitude, but that's more programming for your attitude control system...
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Re: Artifical gravity

Postby Zhatt » Fri Jun 19, 2009 6:09 pm UTC

One thing that's interesting about centrifugal artificial gravity (as I understand it) is that if you start walking against the rotation, it's going to feel like you're walking down hill. I think there's a way to explain it using vectors and whatnot, but basically you're not rotating as fast now so now you weigh less. Conversely, if you walk with the rotation, you'll weigh more and it'll feel like you're walking up hill. Concerning the running-so-fast-that-you're-weightless idea, it's like running down an ever steepening hill till you've fallen off a cliff.

What I'm wondering is that if water (or air) is constantly going to be trying to flow "down hill". I feeling is that it's not and again, I think this could be figured out using vectors, but I don't quote know where to start with that.

Strangely I think there was an usually good explanation of the vectors in centrifugal force in a video I watched in high school by Bill Nye The Science Guy. I'll have to watch that again when I get home. Ah, memories.

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Re: Artifical gravity

Postby jmorgan3 » Sat Jun 20, 2009 5:44 pm UTC

Zhatt wrote:What I'm wondering is that if water (or air) is constantly going to be trying to flow "down hill". I feeling is that it's not and again, I think this could be figured out using vectors, but I don't quote know where to start with that.


Here's where to start. If I'm right, [imath]V = \omega r \hat{e}_\theta[/imath] and a pressure gradient of [imath]\frac{dp}{dr}=-\omega^2 r[/imath] are a steady solution for incompressible flow in a rotating cylinder.
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Re: Artifical gravity

Postby MHD » Thu Jun 25, 2009 11:25 pm UTC

Artificial gravity in the purest sense can be made by centrifugal force ie. spin.
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Re: Artifical gravity

Postby gmalivuk » Thu Jun 25, 2009 11:31 pm UTC

1) That's not in "the purest sense" because it doesn't really behave like gravity.
2) Spin has already been discussed quite a bit. Care to add anything that suggests you might have read the thread?
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Re: Artifical gravity

Postby meat.paste » Fri Jun 26, 2009 4:22 pm UTC

The death star did have a lot of energy stored somehow. Maybe it was a only a few hundred kg of antimatter, but if it was something like a rapidly rotating flywheel, then the energy could contribute significantly to the mass. As I think about it, there was that weird reactor looking thing in the core of the death star2. So maybe it was antimatter with its negligible mass.

I know that gravity is a result of mass bending space-time. What I've never heard is a good explanation of why. I'm reminded of the early stages of electromagnetism when to get a magnet, you found a special kind of rock. Eventually, understanding why the rock was magnetic lead to electricity, electronics, and the internet. If we can figure out _why_ a chunk of matter bends spacetime, then maybe we can learn to reproduce gravity without mass. This would allow for anti-gravity too by simply making a counteracting mass above the main mass.
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Re: Artifical gravity

Postby gmalivuk » Fri Jun 26, 2009 4:31 pm UTC

The Death Star is a terrible example on account of being in a universe with magical physics.

Definitely more than a few hundred (billion) kg of antimatter, though, if it can blow up an entire planet...
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Re: Artifical gravity

Postby Technical Ben » Fri Jun 26, 2009 5:56 pm UTC

It would need at least 2.2E32 joules apparently.
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Re: Artifical gravity

Postby thoughtfully » Fri Jun 26, 2009 6:26 pm UTC

I got 2.243E28 J, which corresponds to 1.25 trillion kg of antimatter, assuming the target supplies the other half. This is about (what's an order of magnitude among friends?) what GM estimated. I did assume the Earth had uniform density, however.

We've already had this discussion.

EDIT: Nope, you're right. The damn Wikipedia article I got the formula from goes on to do the calculation, doh! So.. 12.5 trillion metric tons of antimatter.
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Re: Artifical gravity

Postby Technical Ben » Fri Jun 26, 2009 9:25 pm UTC

Huh? Got my result from googling the death star. So don't take it as accurate ;)
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Re: Artifical gravity

Postby thoughtfully » Fri Jun 26, 2009 9:37 pm UTC

The figure the article I linked to gives is 2.24E32, I got the order of magnitude wrong, your figure agrees with the article, only you cite it to fewer digits.
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Re: Artifical gravity

Postby Random832 » Fri Jun 26, 2009 10:38 pm UTC

The gravitational binding energy of an object consisting of loose material, held together by gravity alone, is the amount of energy required to pull all of the material apart, to infinity.
It doesn't have to go to infinity to ruin your day. Even if it re-accretes immediately to a pile of rock in vaguely the same position the planet was in, there probably won't be many survivors.

Anyway, 12.5 trillion tonnes of osmium (to choose a particularly dense mundane metal) would be a sphere only 10 km in diameter, so easily within the apparent visual size of the death star. I would assume they have some way of generating that energy from some exotic source, rather than having to reload with huge balls of antimatter though.

Maybe they're able to somehow directly convert a portion of the target planet's mass into energy.

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Re: Artifical gravity

Postby Anticitizen » Mon Jun 29, 2009 6:45 am UTC

Drax wrote:Let me get this straight... the centrifuge was the only part of the ship that was actually spinning? Wouldn't the rest of the ship already be spinning in the other direction, thanks to Newton's Third?


Easy to overcome... have a second spinning ring (or counterweights) rotating on the opposite direction on the same axis.

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Re: Artifical gravity

Postby scikidus » Mon Jun 29, 2009 4:42 pm UTC

Anticitizen wrote: Easy to overcome... have a second spinning ring (or counterweights) rotating on the opposite direction on the same axis.

Exactly. Set it up like this:

Code: Select all

--------------------------------------------------------\
|  ---------------------      -----------------------   | \
|  |                    |     |                      |  |  \
|  |                    |     |                      |  |    \
|--|      Spin one way  |-----| Spin other way       |--|     |==>
|  |                    |     |                      |  |   /
|  ---------------------      ------------------------  | /
--------------------------------------------------------/
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