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william wu
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The Canonical Windowless Spaceship
« on: Oct 26th, 2003, 3:02am » |
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You wake up from a deep sleep and find yourself inside the large open cavity of a spaceship. It has no windows or doors. You wonder to yourself whether the ship is at rest on Earth, or if the ship is accelerating in outer space. Is there any way you can tell? Assume you have some materials nearby with which you can perform experiments.
[edit:]rephrased the problem slightly to say that you are in a large open cavity[/edit]
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| « Last Edit: Oct 26th, 2003, 1:02pm by william wu » |
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TenaliRaman
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Re: The Canonical Windowless Spaceship
« Reply #1 on: Oct 26th, 2003, 6:36am » |
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Won't i be floating in outer space ?? for that matter shouldn't anyone in outer space be floating ??
or am i to assume that since i am wondering whether i am on earth or outer space , i am not floating??
am i suffering from paranoia??
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towr
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Re: The Canonical Windowless Spaceship
« Reply #2 on: Oct 26th, 2003, 7:12am » |
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on Oct 26th, 2003, 6:36am, TenaliRaman wrote:| Won't i be floating in outer space ?? for that matter shouldn't anyone in outer space be floating ?? |
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The acceleration of the spacecraft may push us to the floor, in the same way earth-gravity would.
Just like when you have a bucket of water and swing it around on a rope till the bucket is allmost horizontal the water doesn't poor out (acceleration pushes it in)
Quote:| or am i to assume that since i am wondering whether i am on earth or outer space , i am not floating?? |
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If you are floating you may wonder wether or not the craft is falling from ahtreat height, or into a very deep hole in the earth (in which case you want to get out before you're a few kilometers ounder ground since the heat there will cook you)
Quote:| am i suffering from paranoia?? |
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Probably 
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towr
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Re: The Canonical Windowless Spaceship
« Reply #3 on: Oct 26th, 2003, 7:17am » |
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on Oct 26th, 2003, 3:02am, william wu wrote:| You wonder to yourself whether the ship is at rest on Earth, or if the ship is accelerating in outer space. Is there any way you can tell? |
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Perhaps.. You might try to measure tidal forces from the moon. Though it is no sure guarantee you're on earth, it would be likely that you're not in deep space (somewhere far between celestial bodies).
I don't think you could distinguish earth gravity from (other)acceleration, since I think it's the same according to general relativity (though I may be wrong)
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Re: The Canonical Windowless Spaceship
« Reply #4 on: Oct 26th, 2003, 11:10am » |
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Yeah I think the equivalence principle of general relativity put the two cases equal. But measuring the tidal force is a good idea, if there's a satelite nearby we'll get periodic measurement.
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TenaliRaman
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Re: The Canonical Windowless Spaceship
« Reply #5 on: Oct 26th, 2003, 11:37am » |
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*given that i am a paranoid, i would simply hope i find some sleeping tablets and go to sleep and dream about lovely wimmin*
We could measure vacuum energy that surrounds the ship provided there is a way of measuring it, wherein my idea comes to a standstill and i start searching for sleeping tablets.
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Icarus
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Re: The Canonical Windowless Spaceship
« Reply #6 on: Oct 26th, 2003, 12:13pm » |
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The Principle of Equivalence in General Relativity does indeed state that locally the two situations are equivalent, and no measurement can distinguish between them.
However, there is a loophole in that statement that could allow you to determine by measurement whether you were accelerating out in space or sitting still on the surface of a planet, even if the planet has no moon or sun close enough to cause tides.
But the measurement will require some extraordinarily sensitive equipment. The effect you need to measure is extremely faint.
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william wu
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Re: The Canonical Windowless Spaceship
« Reply #7 on: Oct 26th, 2003, 12:15pm » |
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on Oct 26th, 2003, 12:13pm, Icarus wrote:
However, there is a loophole in that statement that could allow you to determine by measurement whether you were accelerating out in space or sitting still on the surface of a planet, even if the planet has no moon or sun close enough to cause tides.
But the measurement will require some extraordinarily sensitive equipment. The effect you need to measure is extremely faint. |
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Yes, I tried to phrase the problem to exploit this loophole ... 
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BNC
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Re: The Canonical Windowless Spaceship
« Reply #8 on: Oct 26th, 2003, 12:38pm » |
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If I remember correctly, gravity operates on all the particles simultaneously, whereas acceleration don't.
But I don't know if any experiment may find the difference.
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towr
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Re: The Canonical Windowless Spaceship
« Reply #9 on: Oct 26th, 2003, 12:42pm » |
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Actually, gravity also doesn't seem to work on all particles simultaneously..
But the gravity of the universe can count as a reference I think..
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TenaliRaman
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Re: The Canonical Windowless Spaceship
« Reply #10 on: Oct 26th, 2003, 12:55pm » |
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I am up at night trying to finish my journal, i got bored, i peeped in here and saw Icarus' post and got stuck trying to find this "loophole" in the principle of equivalence.Blame my luck!!
These are some of the things that went through my mind
Me : what the hell's the problem, the system exhibits local inertia , so everything pretty perfect with it
Me : however it is undergoing a varying gravitational field . Could it be the loophole??
Me : hmm don't know !! one more thing that comes to mind is newton's gravitational law F = Gm1m2/r2. Does this pose a threat here somewhere??
Me : again don't know!!even if my above doubts were true how are we supposed to measure them??
Me : I need help!!
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Icarus
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Re: The Canonical Windowless Spaceship
« Reply #11 on: Oct 26th, 2003, 12:58pm » |
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on Oct 26th, 2003, 12:38pm, BNC wrote:| If I remember correctly, gravity operates on all the particles simultaneously, whereas acceleration don't. |
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If so, then Einstein was mistaken, and General relativity is false. The Principle of Equivalence rules out such a difference.
More to the point, in both cases what is going on is that the floor of the spacecraft is pressing upwards against the test equipment. This pressure is the same both cases. (I assume we are talking about the amazingly smooth Acme Rocket Engines, "guaranteed no more vibration or variance than you find on good old Mother Earth!")
As a hint: If William really wanted to exploit the loophole, he should of mentioned that the ship was enormous.
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BNC
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Re: The Canonical Windowless Spaceship
« Reply #12 on: Oct 26th, 2003, 1:09pm » |
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on Oct 26th, 2003, 12:58pm, Icarus wrote:
If so, then Einstein was mistaken, and General relativity is false. The Principle of Equivalence rules out such a difference.
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Heck, I know my memory doesn't really serve me. But I'm an engineer -- I don't have to know anything, just need to know where to look!
Trying my memory a second time, though, I will suggest that ::on earth, the higher you are, the lesser the gravitational pull is. So, measure the g-force "down" the ship, and way "up" the ship. If there's a difference, you're in space.::
// hidden by wwu 1:21 PM 10/26/2003
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| « Last Edit: Oct 26th, 2003, 1:21pm by william wu » |
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towr
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Re: The Canonical Windowless Spaceship
« Reply #13 on: Oct 26th, 2003, 1:14pm » |
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That sounds like it should work.. And uses Icarus' hint..
And it's much simpler than what I had in mind 
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TenaliRaman
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Re: The Canonical Windowless Spaceship
« Reply #14 on: Oct 26th, 2003, 1:55pm » |
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ok the varying field that's the answer
// hidden by wwu 3:25 PM 10/26/2003
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| « Last Edit: Oct 26th, 2003, 3:25pm by william wu » |
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SWF
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Re: The Canonical Windowless Spaceship
« Reply #15 on: Oct 26th, 2003, 4:37pm » |
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BNC's suggestion of measuring at two different 'heights' is one way, and there is another way to distinguish gravity from uniform acceleration. I think Icarus was trying to give an indirect clue to it in his last post.
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redPEPPER
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Re: The Canonical Windowless Spaceship
« Reply #16 on: Oct 27th, 2003, 3:42am » |
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A couple of ideas:
- If it's a really enormous spaceship with a perfectly flat ground (otherwise we need to build a perfectly flat reference device that is as wide as possible) then we can measure the consistency of the direction of "gravity". If it's at the same angle with the ground (or our flat reference device) everywhere in the ship, we're in space. If we're on earth, the angle will vary.
- How about trying to detect the earth rotation instead of its gravitation? A gyroscope will do the trick.
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Lightboxes
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Re: The Canonical Windowless Spaceship
« Reply #17 on: Oct 27th, 2003, 10:12am » |
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If possible...::
I would communicate with earth using time. That way if my supersensitive watch (atomic clock) is syncronized 2 hours ago but is now off, I'm in space.
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aero_guy
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Re: The Canonical Windowless Spaceship
« Reply #18 on: Oct 27th, 2003, 11:03am » |
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If you had a super wide ship with perfectly flat base you could just drop a perfectly balanced ball bearing on it. It would roll to the center on the earth.
This would measure redpepper's directionality.
You couldn't really use tidal effects, as that couls be reproduced by a ship with non-constant acceleration.
Interestingly you could determine your exact altitude above whatever object was producing the gravity field that held you to the floor if you had good enough instruments... without even knowing what the mass of the object is apriori.
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towr
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Re: The Canonical Windowless Spaceship
« Reply #19 on: Oct 27th, 2003, 11:30am » |
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on Oct 27th, 2003, 11:03am, aero_guy wrote:| You couldn't really use tidal effects, as that couls be reproduced by a ship with non-constant acceleration. |
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You can use that against any method..
Suppose you're using the varying fields methods, when you climb higher the ship can simply slow down.
And I'm pretty sure that goes for any other method.. Since it is easy to adjust acceleration in any direction. And it can spin, loop etc..
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aero_guy
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Re: The Canonical Windowless Spaceship
« Reply #20 on: Oct 27th, 2003, 1:02pm » |
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That would be true if you only had one measurement device, but if you were able to read from several simultaneously...
I guess a stronger problem against that method is that it requires a much longer period of time to decide, and during such a period of time it is possible for a spacecraft to be flying past another gravitational body and create the same effect.
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rmsgrey
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Re: The Canonical Windowless Spaceship
« Reply #21 on: Oct 27th, 2003, 1:22pm » |
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I'd use a Forward mass detector, named after its inventor: Robert L Forward (who also holds the patent on "statites" - satellites in geosynchronous "orbit" near one of the poles held in place by light-sails)
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the device consists of two dumbbell-shaped masses connected through their centres by a stiff torsional spring to make an X. Suspended, and encouraged to oscillate, the device's period of oscillation is affected by the presence of nearby masses in the plane of the X - with sufficiently dense spheres and long enough arms on the dumbbell, air resistance becomes negligible compared to the measured "tidal" effects. The situation is complicated by the device only measuring within the plane of the X, but it's not too hard to work around that - though you probably also need to know the strength of the apparent gravitational field. It's been quite a while since I last read the technical details, so I may have misremembered a few. Also, the original design was for use in apparent 0-g conditions as a navigational aid for spaceships in free-fall - multiple Forward mass detectors on different axes allow for resonably efficient location and tracking of significant local masses, among other things offering a passive alternative to radar scanning for objects on collision course.
::
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Icarus
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Re: The Canonical Windowless Spaceship
« Reply #22 on: Oct 27th, 2003, 5:22pm » |
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Both the height gradient and the center of gravitation methods are examples of the "loophole" I mentioned, which - if you haven't figured it out already - is that the equivalence between gravitational and mechanical acceleration is only local. Gravitational force generally varies with location in a different fashion than mechanical forces do.
However, I'm not sure that even our most sensitive instruments could the detect the tidal forces introduced between two objects by the differing directions of gravity when they are only separated by less than, say a kilometer. If the spaceship also allows you to go 1 Km up, then you will need to be able to measure a difference in acceleration of about 3 mm/sec2 (on Earth), which is more reasonable I think. But that is one very tall spacecraft!
The problem of a varying acceleration is easily solved. Leave a stationary detector recording acceleration. If it measures a varying field, you are either accelerating in space, or very quickly need to get there because the planet you are on is either breaking apart or being bombarded. Neither one is a good thing for long-term survival!
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Re: The Canonical Windowless Spaceship
« Reply #23 on: Oct 27th, 2003, 6:44pm » |
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Foucault
Was this hidden above? If so, then great minds think alike.
Redpepper seemed to be thinking along these lines.
Specifically, arrange a pendulum to swing back and forth, and if the arc of the pendulum tends to rotate around the point from which it is hanging, then you are on earth. Would this work on the equator? It is clockwise in the northern hemisphere, does it change downunder?
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| « Last Edit: Oct 27th, 2003, 6:48pm by Speaker » |
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SWF
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Re: The Canonical Windowless Spaceship
« Reply #24 on: Oct 27th, 2003, 7:40pm » |
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on Oct 26th, 2003, 12:58pm, Icarus wrote:
Was that one of your surreptitious hints (ie. earth attracts toward at point while uniform acceleration does not)? Sometimes it is hard to tell.
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