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Comment by ben_w | Hacker News Reader

If I had to guess, it would be gravity sensors that render submarines obsolete.

Can't hide mass.

When a sub is submerged, by definition it is buoyant and therefore has the same average density as the surrounding water. Unless your gravity sensor is so sensitive it can detect the different densities on a per-cubic meter basis (or smaller).

Subs float in the water column by having the same mass as the water they displace.

The same density _overall_ but locally the density varies. Maybe there’s some way to use this for detection but I don’t see it.

Aye. Dipole and similar gross asymmetries.

I don't know about all the potential noise sources and how well any of them could be compensated for, but the raw sensory capacity of the best systems is at least enough to be worth considering the question.

Easier to detect the ferrous metal, given the size/strength requirements mean steel.

Magnetic anomaly detectors have been used since the WWII airship days. https://en.m.wikipedia.org/wiki/Magnetic_anomaly_detector

Related fact: the first Goodyear blimp was a repurposed ex-Navy submarine hunter, that likely had a MAD device fitted. You can see the restored gondola from it in the New England Air Museum.

> given the size/strength requirements mean steel

The Soviet Alpha, Mike and Sierra classes were built from titanium. Incredibly expensive but very deep-diving and invisible to MAD.

MAD has fallen out of favor. Detection range is very short so it really only works when the aircraft is low and the submarine is shallow. There are techniques to greatly reduce a submarine's magnetic signature. The P-8A doesn't even have a MAD built in.

Submarines have roughly neutral buoyancy when submerged; how will you detect the mass?

just convert it from space and into time, duh. can't weigh time

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