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RoboSpy
2004-Jul-20, 12:02 AM
I was thinking recently that some day, all conventional modes of detection used by the military will someday be obsolete. We already have stealth aircraft, and lord knows that technology is only going to improve. Active camoflauge will make seeing objects, either with naked eye or telescopes or what have you, very difficult, if not impossible.

But I'm a science fiction writer, and I can't just stand by and write about a future where nobody can detect anyone else because of flawless stealth systems. That's boring! So I was thinking, do you all think it would be possible to make accelerometers so sensitive that they could pick up the tiny gravitational pull of a spacecraft, even from many thousands of kilometers away? I wonder also if even this "gravar" could be thwarted by orbitting very close to planet, which would act as a massive gravity mask.

Comments?

daver
2004-Jul-20, 12:29 AM
I'm pretty sure that invisibility, as for instance in the most recent Bond film, isn't possible. Camouflage is, and could presumably be fairly effective against a bland background.

As far as space ships are concerned, you can make your spaceship radar absorbant and non-reflective, but it's still going to generate heat, and you'll have to dump that heat somehow. Maybe, for extremely short periods of time, you can fill storage locker 3 with ice and dump it there, but eventually all that ice is going to melt and you'll have to vent it outside.

If you are playing with science as we know it and your space ship is equipped with thrusters, a thruster firing is going to be pretty obvious no matter how black you paint your spaceship.

Robert Forward invented a mass detector (he called it the rotating cruciform gravity gradiometer, other people call it the Forward Mass Detector). I don't know the theoretical sensitivity of it.

Trinity
2004-Jul-20, 12:43 AM
I'm pretty sure that invisibility, as for instance in the most recent Bond film, isn't possible.

Actually, they have working prototypes of that technology. Obviously not NEARLY as good as the car in the movie, but it functions the same basic way, they're looking into that technolgy for camo suits. It functions better than current camo, but nothing like invisibility yet. I wish I had a link... sorry :(

tofu
2004-Jul-20, 01:12 AM
Some experiments were performed in WWII with the idea of using lights to camouflage tanks. If a tank is sitting on the crest of a hill, it is silhouetted against a bright sky. The only reason you can see the tank is because it's darker than the sky. Turn on the lights and poof, it disappears.

I only mention this because I recently bought a car with daytime running lights. I'm pretty sure that in some situations those lights are a hazard.

Anyway, to the question of gravity detection, you can't use an accelerometer to detect gravitational pull, because gravity acts on every atom in the accelerometer. Does that make sense? I bet the reason you thought of this was because of the movie Starship Troopers. Right? They were sitting on the bridge and noticed a cup of tea tipping toward the front of the ship. They looked at it and said, “gravity well!” hehe. Sorry but it doesn't work that way. You just don't feel acceleration due to gravity.

daver
2004-Jul-20, 01:12 AM
Actually, they have working prototypes of that technology. Obviously not NEARLY as good as the car in the movie, but it functions the same basic way, they're looking into that technolgy for camo suits.

I can't prove that it's impossible, but consider a spherical car (probably the easiest shape to manage) against a checkerboard. Each point on that sphere will have to radiate to all angles a different pattern (it's not that the point at 0 degrees N 0 degrees W is black, it has to be black when radiating due east but white when radiating 10 degrees up from due east, etc). The number of sensors and the number of pixels quickly become ridiculous.

The situation is a bit easier if the car is psychic, and knows where the cameras are.

Low res stuff is quite a bit easier.

daver
2004-Jul-20, 01:17 AM
You just don't feel acceleration due to gravity.
Which is why you need a rotating cruciform gravity gradiometer--gravity decays as inverse square, tidal forces go as the inverse cube. There is going to be a bigger pull at the nose of your ship than at its tail; if you have something swinging in a circle and have a strain gauge on it, you'll see the readings oscillate as the strain gauge rotates if you're close to a gravitational mass.

Morrolan
2004-Jul-20, 01:55 AM
they seemed to use gravity detection equipment in the movie Starship Troopers: when the girl pilot is out on her first trip with a starship and the meteoroid that will destroy Buenos Aires is coming towards them, alarms ring when the instruments detect the gravity pull of the object. you then see a screen depicting what looks like a grid (or net) with an unseen heavy object in it deforming the grid... :o

of course this is where the Bad Astronomy kicks in, because those systems apparently are not very sensitive: the pilots detect the gravitational pull before the instruments do, when the coffee in a mug is being pulled towards the object... =D> :roll:

MrObvious
2004-Jul-20, 03:26 AM
So I was thinking, do you all think it would be possible to make accelerometers so sensitive that they could pick up the tiny gravitational pull of a spacecraft, even from many thousands of kilometers away?

Even if you could the system would be inherently flawed since the gravitational pull of small nearby objects would swamp the signal you were looking for with noise. We could use the same techniques as other technologies to determine direction and distance but this would require processing the relative delay's from multiple sensors to determine a spatial map. Here the problem is not only sensitivity but extreamely high speed response. Getting an accelerometer with a given mass to respond at a fast enough rate is the difficult part.

If the sensitivity and speed issues are solved then what we would observe is moving objects only. If an object became stationary it would dissapear since this would become part of the systems steady state input which must be cancelled in order to make it work in the first place.

Resolution would also be a major stumbling block. A small object would be masked by a larger one. Two close small objects would appear as one larger one etc. In principle it would be interesting, but paracticaly I just can't see it working well.

RoboSpy
2004-Jul-20, 04:41 AM
Oooooohhhhhh......of course an accelerometer wouldn't work!


Anyway, to the question of gravity detection, you can't use an accelerometer to detect gravitational pull, because gravity acts on every atom in the accelerometer.

Yeah, you're absolutely right. That seems so obvious now. But what is this "rotating cruciform gravity gradiometer" that daver spoke of, and does anyone have a link that I can go to about it? Or, if anyone knows how it works and can explain it to me, that'd be great too. It sounds like this is the type of device I'm looking for.

tlbs101
2004-Jul-20, 05:03 PM
Oil Exploration companies employ very sensitive gravimiters to "log" oil wells. Using these detectors geologists can discover how the "layers" of Earth are oriented, underground.

I worked with Arco back in the early '80s on one such well in Wyoming. Arco "borrowed" our electrical cable for our well logging tools. Their tool was dropped into the newly drilled hole and pulled up from the bottom over the course of 12 hours (very very slowly). In that time you could see the effects of the tides 1000's of km away, the Sun and the Moon. Those effects showed up as a slow "bias" to the signals the geologists were interested in.

IIRC, on top of the bias, you could even "see" when a large truck pulled up to the drilling site. I was totally amazed at this technology.

That was 20 years ago, and I am sure the instruments they have today are improved (there can always be incremental improvements made due to new materials, and techniques).