Is a gravity scanner possible?

[monstah]

Had the field be even at all site, there'd be no acceleration to any preferred direction... Look up field theory.

Erhm. You're thinking of the gravity potential field, not the gravitational field. Gravitational field is a vector, it points in a direction (the same as the acceleration) at every point in space. Near the surface, it can be approximated as constant (i.e., "even"), and pointing down.

Gravity potential, on the other hand, is a scalar field which varies in the same direction the gravitational field points.

It's pretty simple : take the Rosetta spacecraft as an example. Nobody knows precisely how heavy (uh, massy ?) 67P/C-G was back then. But accelerometers do exist (or even, the mission mentions about precise tracking), and you can tell, as the spacecraft moves around the comet's influence, which direction it's headed to and what's the magnitude of that acceleration. Distance to comet can be deduced by radar (or, worst case, angular separation), then you get the mass.

The last part (about using accelerometers) is unfortunately also wrong. Like K^2 said (well, actually, Einstein before him):

The Equivalence Principle states that you cannot distinguish between gravity and acceleration locally.

That means your acceleration due to gravity is not felt by the accelerometer: that's what "free fall" is all about. Just use your accelerometer in KSP! It's zero while in orbit, unless you're firing your thrusters.

We CAN, however, measure Rosetta's acceleration by looking carefully at it from Earth. Direct observations, radar doppler shifts and all that.

Please no-one ninja me?

- - - Updated - - -

Take perfectly spherical planet. Field is stronger as you get closer. This causes a tidal "stretching" force in the radial direction. What's interesting, it also "squishes" things in the two other directions. These sorts of things can actually be measured.

This quote is also interesting. The OP seems to be asking wether we can directly measure gravity in any way - as in, put a sensor in a probe and it just tells you the number, like we could do with temperature, pressure, acceleration, magnetic field... As said during the topic, we have several techniques for measuring gravity, but none of them direct in this sense. The gavitational gradient (as in, tidal force), however, can be directly measured!

So, change the flavour text in the Part Description and Science Reports, and Negative Gravioli Detector suddenly obeys the laws of physics.

-edit again: it adds as updated on my previous post! Oh, cool.

Edited January 30, 2015 by monstah
Appearently I can't post twice in a row. Good to know. Also rhymes with... Dow?

[airelibre]

Erhm. You're thinking of the gravity potential field, not the gravitational field. Gravitational field is a vector, it points in a direction (the same as the acceleration) at every point in space. Near the surface, it can be approximated as constant (i.e., "even"), and pointing down.

Gravity potential, on the other hand, is a scalar field which varies in the same direction the gravitational field points.

The last part (about using accelerometers) is unfortunately also wrong. Like K^2 said (well, actually, Einstein before him):

That means your acceleration due to gravity is not felt by the accelerometer: that's what "free fall" is all about. Just use your accelerometer in KSP! It's zero while in orbit, unless you're firing your thrusters.

We CAN, however, measure Rosetta's acceleration by looking carefully at it from Earth. Direct observations, radar doppler shifts and all that.

Please no-one ninja me?

- - - Updated - - -

This quote is also interesting. The OP seems to be asking wether we can directly measure gravity in any way - as in, put a sensor in a probe and it just tells you the number, like we could do with temperature, pressure, acceleration, magnetic field... As said during the topic, we have several techniques for measuring gravity, but none of them direct in this sense. The gavitational gradient (as in, tidal force), however, can be directly measured!

So, change the flavour text in the Part Description and Science Reports, and Negative Gravioli Detector suddenly obeys the laws of physics.

-edit again: it adds as updated on my previous post! Oh, cool.

Thanks for the answer, it clears up a lot in relation to my question.

One more question, if there were a way to detect gravitons, would it be possible to detect gravity directly? I know next to nothing about particle physics, so humour me!

[-Velocity-]

Thanks for the answer, it clears up a lot in relation to my question.

One more question, if there were a way to detect gravitons, would it be possible to detect gravity directly? I know next to nothing about particle physics, so humour me!

I believe that gravitons have exceptionally low energies. If they're anything like photons- and there is a lot of analogous behavior between electromagnetism and gravity- then their energy is dependent on their wavelength/frequency, with shorter and shorter wavelengths resulting in higher and higher energies. To a certain extent, the shorter the wavelength and higher the energy of a photon, the more it begins to act like a particle- certainly, it becomes easier to detect as a quantized particle. Most gravitational forces are static or move and accelerate very slowly, so gravitons should be essentially undetectable, just as you cannot detect photons from static or slow varying electric and magnetic fields. The photons emitted by very slow varying or static fields remain "virtual" particles, undetectable as real particles, and by analogy gravitons ought to be virtual too in all but incredibly extreme cases that possibly only existed after the Big Bang, if ever at all.

Just my take on it.

Edited January 30, 2015 by |Velocity|

[meve12]

Take perfectly spherical planet. Field is stronger as you get closer. This causes a tidal "stretching" force in the radial direction. What's interesting, it also "squishes" things in the two other directions. These sorts of things can actually be measured.

I seem to recall that Foward mass detectors do precisely that.

[KSK]

So where exactly i was not precisely right?

The bit when you said you couldn't measure gravity directly even in a perfectly circular orbit about a perfectly homogenous sphere. Whereas even under those circumstances, the field will decrease radially, so you can measure the difference in field strength between the two ends of your probe.

[Iskierka]

With a sensitive enough load cell detecting the differential force on distant parts of a satellite, most likely you could measure gravity gradient, and then knowing distance to the surface would give you a lot of information about local g.

However, I suspect there's a reason the gravity satellites so far have used lasers, so that's probably the better method.

[YNM]

Erhm. You're thinking of the gravity potential field, not the gravitational field. Gravitational field is a vector, it points in a direction (the same as the acceleration) at every point in space. Near the surface, it can be approximated as constant (i.e., "even"), and pointing down.

Gravity potential, on the other hand, is a scalar field which varies in the same direction the gravitational field points.

--------

The last part (about using accelerometers) is unfortunately also wrong. Like K^2 said (well, actually, Einstein before him):

That means your acceleration due to gravity is not felt by the accelerometer: that's what "free fall" is all about. Just use your accelerometer in KSP! It's zero while in orbit, unless you're firing your thrusters.

We CAN, however, measure Rosetta's acceleration by looking carefully at it from Earth. Direct observations, radar doppler shifts and all that.

Sorry for that then... well kudos for the Rosetta team, very precise things there. Regarding why I mentioned fields, it's due to the fact that differences in potential give rise to acceleration, no ?

[monstah]

Regarding why I mentioned fields, it's due to the fact that differences in potential give rise to acceleration, no ?

Yup, that's it!

[PB666]

Considering there is no net force on a ship travelling in a stable circular orbit, how would a gravity scanner work in space?

It uses two space ships in a near perfect orbit, it measures the distance between the two in light travel time as the ships slow and speed up in earths gravitational field.

[YNM]

Regarding why I mentioned fields, it's due to the fact that differences in potential give rise to acceleration, no ?

Yup, that's it!

And differences in acceleration over extended bodies causes tidal "force" (shouldn't be force right)... Ah, messed up the field theory.

It uses two space ships in a near perfect orbit, it measures the distance between the two in light travel time as the ships slow and speed up in earths gravitational field.

OP was asking whether a device akin to GravMax Negative Gravioli Detector is a possibility IRL. So one ship only.

[monstah]

It uses two space ships in a near perfect orbit, it measures the distance between the two in light travel time as the ships slow and speed up in earths gravitational field.

OP was asking whether a device akin to GravMax Negative Gravioli Detector is a possibility IRL. So one ship only.

It makes you wonda, tho, doesn't it?

Here's what I'm picturing, based on the GRACE and GRAIL experiments PB666 mentions:

A container.

(dramatic pause)

...much like the Goo Experiment. In it, a magnetic field constrains the positions of free-floating specks of metal. Around their stable equilibrium points, the magnetic force on the specks is (vanishingly close to) zero, so they're in ballistic trajectories as long as they are in a fixed position relative to the container/craft. However since the specks of metal and the craft all have different centers of gravity, their trajectories should deviate the slightest. The magnetic field starts kicking in and pushing the specks back to their place, but you can calculate how that force affects them and subtract it. Measure their deviations, do heavy math sorcery, you get gravity?

Of course, the actual experiments involved spacecraft separated by a few hundred km, measuring their relative distance with a microwave-based radar; IRL you (probably) can't (so far) measure the positions of floating specks of metal with the needed accuracy, but hell, this is Kerbal World, those little green "men" can do it!

[PB666]

http://www.bbc.com/news/science-environment-34298363

[insert_name]

a. no orbit is circular, all are at least slightly eliptical

b. there is net force pulling you down to the planet, otherwise you would be moving in a straight line, not an orbit