Gravimetric Radar: Gravity-Based Detection of a Point-Mass Moving in Static Back | Hacker News Reader

Gravimetric Radar: Gravity-Based Detection of a Point-Mass Moving in Static Back

arxiv.org

39 pointsgodelmachine19 comments

I worked on torsion balances, gravity measurements, and gravity gradiometers for my PhD. Getting just a factor-of-two increase in sensitivity is significant--getting 4-5 orders of magnitude better would be incredible and also next to impossible, given the other factors to account for.

The author considers environmental gravitational noise, but you have to also consider that any movement of the instrument in a non-uniform gravitational field will create a signal. If you consider Earth as a perfect sphere, the gravitational signal of a B-2 that the author considers is equivalent to the change in gravity from a vertical displacement of 0.2nm. This is about the size of a water molecule. Vertical stability at that level is unachievable.

Yeah, I sort of stopped there as well. Next up, "If we could reduce the mass of construction materials of spacecraft 4 or 5 orders of magnitude without compromising their other physical properties such as strength, toughness, and hardness, we could build single stage to orbit vehicles with off the shelf technology!"

Okay, to be fair, cost-effective SSTOs would be possible with the 50x strength/weight ratio increase you get from molecular nanotechnology. That particular example doesn't require 4 to 5 orders.

https://en.wikipedia.org/wiki/The_Hunt_for_Red_October_(film...

The film caused a minor sensation in the black projects submarine warfare technology community.[13][14] In one scene, where USS Dallas is chasing Red October through the submarine canyon, the crew can be heard calling out that they have various "milligal anomalies". This essentially revealed the use of gravimetry as a method of silent navigation in US submarines. Thought to be a billion dollar black project, the development of a full-tensor gravity gradiometer by Bell Aerospace was a classified technology at the time. It was thought to be deployed on only a few Ohio-class submarines after it was first developed in 1973.

4-5 orders of magnitude more sensitive?

Good luck, mate.

Real world geophysical gravity measurements are already noise limited by vertical differences of about 1cm (moving w.r.t. the center of mass of the Earth and measuring the mass of the whole planet).

Gradiometry is slightly better, but still. I say build such an instrument first, _then_ try to detect airplanes.

Exactly. I don't understand what's new in this paper. We've known for decades that since gravity is a force with unlimited range and information cannot be destroyed, you could work out the positions, masses, and motions of the entire earth and everything on it by monitoring a single object with enough precision and isolation.

It's non-unique - there are an infinite number of configurations of mass that could have cause that single object to move in exactly the way it was measured.

And then you'd get a Total Perspective Vortex.

See

https://en.wikipedia.org/wiki/Gravity_gradiometry

It sounds like some of the technologies made it into prototypes, and could be feasible in some applications. e.g. ship detection is more feasible than aircraft detection

It's a perfectly good technology for finding the edges of continental plates, the heavy underground structures that trap oil, etc.

A nuclear submarine can navigate "port to port" entirely on inertial guidance and gravity gradiometry

What other applications would a 4-5 order of magnitude more sensitive detector have? There are obvious applications to geology mentioned in the paper. How would that sensitivity compare to LIGO?

“this approach will be feasible if it is possible to design gravimetric devices that are four to five order of magnitude more sensitive than current devices.“

Catch that?

How could a craft camouflage itself against this kind of detection?

Depends on the spatial resolution I suppose, but being neutrally buoyant (like an airship or hot air balloon) would seem to be much harder to to detect, since you are the same average mass as the air you are displacing.

By opening up a business nearby that causes lots of noise, so something like a truck marshalling yard or truck stop would be perfect as you have large moving objects that cause lots of vibrations, making it nearly impossible to filter the noise out well enough.

Failing that, you would want to have your craft look like a heavy truck as much as possible (vibrations should be close to the same period as a common truck suspension, etc) so their denoising algorithms discard you as yet another truck.

Any object the same density as the surrounding medium should be much harder to detect via this method. Submarines and blimps should be safe for a while.

phase shift, be in another dimension /s

They should use spin ice.

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