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Gravity in space

Posted by: Stefan Sigwarth - Sat Aug 21, 2004 11:30 am
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Gravity in space 
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Post    Posted on: Sun Aug 22, 2004 5:11 pm
TerraMrs wrote:
just want to point out that this was a course in inertia, not general relativity :P


Actually, it is relativity: the realization that inertia and gravity were indistinguishable was first made by Einstein as part of his work on the General Theory of Relativity (which, oddly enough, provided one of exceedingly few constants in our Universe: c, or the speed of light).

Ekkehard: good job. And the only problems with using a rotation station are tidal effects (being pulled in multiple directions at once, because the body is undergoing too fast a rotation too close to the centerpoint) and the Coriolis effect (when the spinning of the torus causes acceleration opposite to the direction of rotation -- this is best seen in pictures of the clouds on Venus: they make huge parabolic arcs due to the Coriolis force), both of which are reduced well within acceptable limits when the station is built large enough.

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Post    Posted on: Sun Aug 22, 2004 9:56 pm
spacecowboy wrote:
TerraMrs wrote:
just want to point out that this was a course in inertia, not general relativity :P


Actually, it is relativity: the realization that inertia and gravity were indistinguishable was first made by Einstein as part of his work on the General Theory of Relativity (which, oddly enough, provided one of exceedingly few constants in our Universe: c, or the speed of light).


true, but that doesn't change the fact that the "gravity" caused by a rotating station has very little (i don't want to say nothing) to do with relativity.

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Post    Posted on: Mon Aug 23, 2004 6:40 am
Why? Einstein worked out the effects of gravity on time by considerations of what would happen to a clock based on beams of light onboard an constantly accelerating spaceship and later it has been proven that his results are valid under real gravity. So the by real acceleration simulated gravity onboard a space station should have identical consequences.

Where I am wrong?



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Post    Posted on: Mon Aug 23, 2004 12:34 pm
The whole rotation idea causes a problem for me.

If you've got this rotating torus, how do you get into it? You either have to get your docking spacecraft in sync with the torus - which could be nigh on impossible, or somehow stop the torus spinning so that you can climb on, at which point you're back in your weightless freefall scenario.

Anyone got any ideas about how to get onto a rotating torus? :)

/Dan


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Post    Posted on: Mon Aug 23, 2004 12:50 pm
Hello, dolby_uk,

there seem to be at least twi conventional possibilities:

First possible way: The spacecraft may be docked to the center and from there the passengers may "go" to the torus.

Second possible way: As known from the Apollo flights a spacecraft has no problem to correct directions, to turn or to change velocity. So there will be no problem to get docked to the torus itself I think.



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Post    Posted on: Mon Aug 23, 2004 1:07 pm
Try playing any one of a number of video games (starting with the original Elite), and you will find that docking with the hub of a torus is probably the best way. Its easier to rotate a spacecraft along its axis than to try and match the outer velocity of the torus, and its rotation.

I cannot think of a single SF book for example (exept Ringworld for obvious reasons) that tries to dock ships with the outer edge rather than the centre of the hub.

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Post    Posted on: Mon Aug 23, 2004 2:01 pm
If I'm right in Star Trek spacecrafts are docking to Deep Space Nine at the outer edge.

But I'm doubting wether SF is a good source of ideas for reality without research as NIAC does. A few years ago NIAC has been reported to look after the ideas of SF-authors and to research for their realistic contents.

...



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Post    Posted on: Mon Aug 23, 2004 3:21 pm
Ekkehard Augustin wrote:
If I'm right in Star Trek spacecrafts are docking to Deep Space Nine at the outer edge.


Right, but DS9 didn't rotate for gravity.


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Post    Posted on: Mon Aug 23, 2004 3:28 pm
in a spinning spacecraft, the coriolis force will be a lot more noticeable than on Earth, due to the smaller radius of the rotation, imposed by hardware constraints(unless you tether two ships far apart and spin them from the center of mass), which will require a much faster rotational velocity.
basically, on earth coriolis force affects only winds, whereas on a spinning ship it will affect everything that moves, and I believe it will play havoc with your inner ear.

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Post    Posted on: Mon Aug 23, 2004 3:57 pm
Docking to the center of the torus doesn't require rotation of the spacecraft. In the ecnter of the torus can be something like a not rotating axis as an inner part with a rotating outer part around it. This outer part would be connected to th torus. If the outer part won't be rotating too fast an easy transit from the inner part would be possible I think.



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Post    Posted on: Mon Aug 23, 2004 5:22 pm
Ekkehard Augustin wrote:
Why? Einstein worked out the effects of gravity on time by considerations of what would happen to a clock based on beams of light onboard an constantly accelerating spaceship and later it has been proven that his results are valid under real gravity. So the by real acceleration simulated gravity onboard a space station should have identical consequences.

Where I am wrong?


... i don't think you are, though i'm not entirely sure what you're saying. einstein proved that gravity does in fact affect light, despite the fact that it acts as a wave. i don't know what the effects of gravity on time were, you may be thinking of what happens when you approach c. centrifugal force and gravity aren't distinguishable as far as holding you to something is concerned, but inertia won't simulate the more esoteric properties of gravity as far as i know.

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Post    Posted on: Mon Aug 23, 2004 6:38 pm
I' ll try to remeber the explanation given in a book I left at my parents' home.

Einstein considered gravity's effect on time similar to this: First magaine a spacecraft not moving, You are not at the front wall and not at the back wall but at one of the sides of the craft and on the opposite side there is a clock every second a light beam sending to you. The distance from the clock to you is one meter so the light beam has to move one meter to indicate each second to you. The velocity of the beam is known and always the same what situation however. Outside the craft there is another clock showing the seconds too for comparison to the clock inside the craft.

The second clock in this consideration will never be moved and comparisons to the clock onboard the craft won't be effected by motions of th craft.

Now the craft will be accelerated constantly. The clock will continue to indicate each second like before. But because of th accelration of the craft it has not to go vertically to your side but diagonally - different to the situation with the not-moving craft you are not one meter away from that point in space the beam has been sent from. By the motion of the craft you have been moved a liitle distance away. So the beam will be sent diagonally to that point you will recieve the beam - but this means that the beam has to go a longer way too than one meter to arrive at your eyes and because the velocity of the beam is the same as if there were no motion it takes longer to reach you if time is measured by the non-moving clock outside the craft. Because this is valid for each beam indicating the next second sent by the clock onboard the craft as long as it is accelerated constantly each second onboard the accelerated craft is longer than a second at the place of the non-moving clock outside the craft.

Now acceleration is equivalent to gravity - from this follows that the results of the considerations above are valid under gravity too: At the place of the non-moving clock gravity is zero whereas at the place of the moving clock is different from zero.

A few decades ago there have been comparisons from extremly exact going atomic clocks on earth to extremly exact going clocks onboard satellites - and it was found that there was a significant difference between th times measured.

Onboard the torus everything is accelerated permanantly and constantly because of permanent changes of direction. Because of the equivalency it's not distinguishable from gravity. So clocks onboard the torus will measure less seconds than non-moving clocks outside the torus I suppose. Since there is no rotating torus in space yet noone can do an experiment to find out what's right. But if all this is right this dicussion included geral relativity from its beginning - but I may be wrong, I#m no physicist and may have done unremarked errors considering all this and it may be I#M remembering the book wrong.



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Post    Posted on: Mon Aug 23, 2004 7:49 pm
well, i don't know. that affects your perception of time, but it doesn't actually have any effect on time itself. i'll leave it to a greater physicist than i to decide.

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Post    Posted on: Mon Aug 23, 2004 10:36 pm
Ekkehard Augustin wrote:
<snip>


You got it backwards. Time dilation is caused by speed, not gravity. As your speed approaches the speed of light, the outside world seems to slow down (please note that to the outside world, you appear to slow down).

TerraMrs: granted. It doesn't really have anything to do with relativity. But it's always fun to invoke the Power of the Theory and see what it'll cause.

109Ace: I thought that the Coriolis force could be reduced to within acceptable limits in a reasonably-sized station, but don't quote me on that.

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Post    Posted on: Tue Aug 24, 2004 1:00 am
spacecowboy wrote:
You got it backwards. Time dilation is caused by speed, not gravity. As your speed approaches the speed of light, the outside world seems to slow down (please note that to the outside world, you appear to slow down).

TerraMrs: granted. It doesn't really have anything to do with relativity. But it's always fun to invoke the Power of the Theory and see what it'll cause.

109Ace: I thought that the Coriolis force could be reduced to within acceptable limits in a reasonably-sized station, but don't quote me on that.


i can't dispute that, lol. everyone likes invoking the Power.

i agree with you on the coriolis thing. the bigger the station, the smaller the tidal force in the spokes, since you're rotating at the same speed no matter where you are, and the larger it is, the slower you go (i am right, right?). hence also why gravity decreases as you move closer to the center.

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