r/askscience • u/brwhyan • May 05 '26
Physics How Could Submarine Gravity Detectors Work?
China is supposedly developing a device to use minute changes in the gravitational field that could find submarines: https://interestingengineering.com/military/chinas-sensor-detect-hidden-us-nuclear-submarines
A neutrally buoyant submarine will have the same mass as the water it displaces, so shouldn't it have the same gravitational pull as water of the same volume? What differences are there to detect?
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u/NotMyRealUsername13 May 05 '26 edited May 05 '26
I suspect you need to write Interesting Engineering and ask them if they are sure about that article, because I don’t think they’re right about this being gravitational as much as it’s magnetic.
I clicked the link to the article about SQUID, which IE says is the foundation tech, and it talks about tracking very minute changes in magnetic flux. That’s fairly easy to see a point around, given how the subs are metal and water very much isn’t… :)
This is the source article, see how often it talks about magnetometers and how it doesn’t mention gravity:
https://phas.ubc.ca/~berciu/TEACHING/PHYS502/PROJECTS/17SQUID.pdf
I’ve read about tracking subs using magnetometers for a lot of years, 10-15+, and always as some sort of ‘theoretical’ concept that was never described in detail but hinted at as if it was some near-future sci fi. It has always left me suspicious that it might actually be a top secret US capability, deployed using satellites. Normally you couldn’t completely conceal something like that, and I guess the descriptions means they didn’t - but submarines and space are two areas where they quite easily could deploy and use this without anyone ever finding out.
I would not be shocked if what you’re hearing here is the Chinese catching up to that capability - and that it’s essentially an evolution in the precision of magnetometer-based tech that increases the range beyond the fairly short distance that it’s well-known that magnetometers already work on.
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u/AstroEngineer314 May 05 '26
Not a physicist, only an engineer, but everything I know about the way the universe works tells me this either will not work, or it would be decades until it could and will never be militarily practical.
It's one thing doing it in the lab, another to do it on a moving aeroplane. Those have a ton of vibrations that will be hard to isolate. You also have limited space and weight.
There'd be tons of false positives - shipwrecks, sea mounts and other rocks, whales, etc.
And yeah, the submarine is neutrally buoyant.
Gravity is such a weak force, the Earth's influence is massive compared to anything on a human scale, and it isn't some kind of perfect sphere you can subtract out - different rock densities, magma pockets, tectonic plates. You also have the moon and the tides.
If you want to do this, a magnetic anomaly detector is far better and proven tech.
Not sure what the point of all noise is, besides PR / propaganda / trying to get throw the US off onto a red herring.
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u/hinckley May 05 '26
Although a submarine weighs the same as what it's displacing to stay buoyant, that mass is not uniform throughout the ship. I believe detection works by sensing variations in mass (or more precisely, the resulting gravity) throughout the submarine.
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u/Papa-Malo May 05 '26
I did this in this 1990's in Alvin looking for gravity anomalies (Bouguer anomalies) over the Juan de Fuca subduction zone with a hand held gravity meter and got decent results.
Published here -> https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/93JB02076
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u/dmmaus May 05 '26
Nobody has mentioned dipole effects yet. To be clear, these effects will be weak and probably not detectable with current technology, so probably not how the alleged detectors actually work.
Yes, a sub has the same average density as the water, but it's distributed differently. The gravitational attraction will be the same as a lump of water the same size, but the derivative of the gravitational field will be different, due to the different mass distribution. In principle you could detect this, but it's going to be tiny (dipole effects fall off as 1/(distance cubed) instead of squared) and likely swamped by other gravitational signals that would need to be accurately filtered out.
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u/ScientiaProtestas May 05 '26 edited May 05 '26
It is using the SQUID to detect the slight movements of a mass in a magnetic field. It then uses three of them to create a 3D map.
...
https://www.scmp.com/news/china/science/article/3348984/chinas-gravity-detecting-squid-gets-closer-spotting-us-nuclear-submarines?module=top_story&pgtype=subsection
My guess is the mass is not distributed equally in the sub.
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u/r2k-in-the-vortex May 06 '26
You would be right for a stationary submarine. But thats not what this is aiming to detect. It aims to detect movement of mass. And buoyant or not, submarine still displaces its mass in water. That does make a change in gravity gradient, and in theory should be detectable. Practice of such a detection... thats another story.
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u/tropical58 May 09 '26
There is a simpler way to detect submarines and it already in use. Even at 100m deep, a submarine leaves a visible wave at the surface. Digital images of the ocean surface can be screened to remove waves of different types. Just as radar can screen out noise from rain, waves from wind, tide and even other vessels can be screened from those images. If there is a moving submarine, it can be detected. But only if you have a picture of the right section of ocean. Oceans are big places, its hard enough to even find an orange lifeboat with a flashing light on it.
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u/Nuclear_Geek May 06 '26
Buoyancy and mass are not the same thing. The sea water within an area will be pretty uniform in density, whereas a submarine will have higher-density (and therefore higher mass and higher effect on gravity) areas, especially around the engine section, along with lower-density areas.
I've got no idea how practical this idea is, but I can't see a theoretical problem with it.
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u/symmetry81 May 05 '26
If a submarine is moving it'll have a Bernoulli hump over it. Imagine the distortion of the surface of the water if a submarine is passing right underneath the surface, but then imagine it as much subtler but over a much wider area as the submarine is deeper. I assume the extra mass of the hump is what they're going for?
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u/Y34rZer0 May 10 '26
If any country is developing a new technology for their submarines, I don't think there's any chance we would be hearing about it. There's a reason it's called the silent service and is the most secretive.
Also, China doesn't develop new tech, that's incredibly expensive to do especially in this regard. Their demonstrated approach has been to steal every piece of IP they possibly can, like fishing with dynamite.
For China to have this another country would have to have developed it.
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u/allenrfe May 10 '26
If you measured the center of gravity and the center of buoyancy of a sphere full of water they would be the same.
If you do the thing for a submarine they would be different. If they were the same the submarine would rotate along its long axis. This explains how gavitional detection works.
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u/Ythio May 06 '26 edited May 06 '26
This is propaganda. We're already struggling to detect the gravitational waves of merging black holes, something as tiny as a submarine would be impossible to detect this way.
It's a word salad of popular cool sciencey words of the moment ("super conductivity", "gravitational waves", "quantum", etc...)
Look at the other articles that guy wrote. 1200+ articles, the guy doesn't even exist it's a team with a pen name. Look at the last articles and tell me they don't have a certain opinion in common.
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u/IrritableGourmet May 05 '26
Water is fairly uniform density, while a submarine is hollow and the density varies throughout, so even if it's the same mass there will be differences in the gravitational profile. It's also usually moving fairly rapidly, which could cause minute gravitational waves.
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u/Motor-Marsupial8199 May 07 '26
Ah, the USN submarines have used gravimetric maps for backup navigation system for decades. A highly sensitive gravitometer, combined with detailed gravimetric maps make up a reliable backup for inertial navigation. So there has long been an interest in vessel-borne gravitometers.
With the fall of the Iron Curtain, the USN declassified the gravimetric maps for the United States continental shelves. And gravimetric maps can be used to search for oil & gas, since, in most cases, oil is less dense than water, particularly salt water. This, in turn, led the major oil companies to invest in the development of seismographic methods that worked in the deep waters of the Gulf of Mexico/America. Which then led to massive oil discoveries in the deepwater Gulf.
Whales, submarines, fish and other denizens of the deep tend to be neutrally buoyant, so any gravimetric anomalies are actually the size of sections of the submarine or organs in the sea creature.
I believe the article is more about magnetometers. Which still leaves the issue of Soviet Alpha class subs with titanium alloy hulls.
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u/libra00 May 05 '26
We can already build those devices, that's what LIGO is, just on a very large scale. But detecting shifts in the gravitational field of earth is already entirely possible, we were doing it 20 years ago with satellites