[jchallis]

This is the most obvious practical application of my PhD topic. A Bose Einstein Condensate is an extremely sensitive detector of gravity- a nuclear submarine could use it to make an ultra-accurate map of the world based on variations in the gravitational constant g. This would remove one of the primary reasons subs need to surface , in order to GPS lock.

It’s a naval chart that would not require surfacing.

A French postdoc in my lab swore this was the moneymaker for our entire subfield, and it seems he was on to something …

[jedc]

Former submariner here with a quibble on this:

> one of the primary reasons subs need to surface , in order to GPS lock

While getting a GPS position is helpful, the primary reason a submarine goes to periscope depth regularly is for communications. The Navy needs to send information to submarines and know that they'll get it and take action within a certain timeframe. That's by far the primary driver.

[eyko]

From layman to former submariner, a very silly question I've always wondered about and that I now have the rare chance to ask someone with some expertise: Do submarines mostly roam about or do they tend to stay relatively quiet/idle? I always imagined submarines constantly moving about but at the same time it feels like a waste of fuel.

[jedc]

Ohio-class submarines (the ones that carry missiles): when they're "on station" they're just tooling around staying as quiet as they can. There's a relevant phrase for them: "Three knots to nowhere"

Los Angeles and Virginia-class submarines are always doing something: doing exercises, transiting from one location to another, etc. And typically multiple things at once. While the boat is transiting from an exercise area to homeport, the team is doing engineering drills, or other kinds of training. Or the forward part of the boat is doing exercises with a carrier battle group while the engineering team is doing engineering drills. (There's ALWAYS engineering drills or maintenance happening.)

Fuel isn't a primary concern: a nuclear reactor is fueled for the life of the boat, so 30-ish years. That said, effective life of a reactor is something the Navy tracks closely, and depending on the life of the boat, the life left in the reactor, some boats are decommissioned as they get close to the end of their fuel life, and others get re-fueled. (And in the case of the USS San Francisco, who had recently been refueled before it hit an underwater mountain, they cut off the front half of the submarine and welded the front half of a recently-decommissioned submarine on, because the reactor and fuel was too valuable to go to waste)

[ateng]

What about diesel boats? They are exclusively attack subs but fuel is relatively limited.

[jedc]

The US doesn't have diesel boats anymore. Though other countries (like Australia) do.

[jwithington]

French (& American and Royal Navy) submarines are nuclear powered. No fuel necessary.

They do have measures of "nuclear fuel" remaining, but it lasts about 30 years (at least in the American boats) so generally doesn't impact day-to-day considerations.

[gresrun]

While I'm 100% positive the details of operational concerns like this are classified, there are 2 distinct types of submarines today with 2 different objectives:

1) Attack Submarines (e.g. Los Angeles-class & Virginia-class for USN) which usually roam within a designated operations area, surveilling, tracking, and generally keeping tabs on other nations' surface & sub-surface fleet dispositions. These subs typically have multi-week sorties and may intermittently surface for surveillance & comms.

2) Ballistic Missile Submarines aka "Boomers" (e.g. Ohio-class for USN) which are given a strategic area in which to operate and their objective is to remain silent & undetected, waiting for the hopefully-never-coming order to launch their SLBMs. These subs usually have multi-month sorties and often don't surface until the end of their patrol.

[ThinkBeat]

Clearly the Ballistic Missile Submarines surfaces intermittently surface for comms as well? If not, they won't know when to set off their missiles making then not very useful as a deterrent

I have often wondered how close to the surface they need to get.

I would presume retractable antennas could be extended from a sub from a non-trivial depth. Or cable attached to buoys Or something much smarter that I have not thought about yet.

[yencabulator]

There's a couple of different "wake up" signals that can reach deeper into water. Their biggest limitation is very low bandwidth, so an attack sub will emerge (/send up a buoy on a tether) to get an updated tactical map.

https://hackaday.com/2020/07/15/the-many-methods-of-communic...

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

[ethbr0]

Also highlighting the E-6B Mercury (and the upcoming EC-130J), which among other communication options has a 5-mile (!) VLF antenna it deploys vertically in midair (!!) to establish limited-bandwidth communications with submarines.

https://m.youtube.com/watch?v=en-qekZX4ws

But I believe that's only good down to ~60 ft. Anything deeper requires the really long land-based ELF arrays.

[oynqr]

Layman here as well, but there are probably no fuel concerns because the reactors are constantly running anyway.

[thatguy0900]

I wonder what the fuel difference is between fighting the currents and generating electricity while staying in the same area VS just moving around

[jedc]

The main thing is that you need to be moving at least a few knots in order for your control surfaces (rudder, forward planes, stern planes) to work.

[timthelion]

Do military subs have antenna buoys on cables that they can raise up without having to surface?

[nradov]

Yes, modern subs typically carry comms buoys. Some are tethered and allow for two-way communication. Others are transmit only and float up to the surface independently.

[jedc]

antenna buoys on cables? No.

But there are VLF antennas that Ohio-class submarines have to receive low-data-rate comms while submerged.

Any reasonable communications have to be made while at periscope depth. (Which is subtly different than "having to surface"... at PD just the thin mast is out of the water.)

[otikik]

A submarine itself is quite massive, and has big moving parts (e.g. the rotor). The crew has less mass, but it would be very close, and also in constant movement. Wouldn’t this interfere with those measurements? Would they need to stop the engine and play “frozen TikTok” while the measurement device is active?

[samus]

Surely the influence of some of these sources of noise can be modeled and filtered out. A University of Vienna currently conducts experiments to measure the gravitational force at very small sales, and they already have to do this with things like crowds moving about the city during rush hours.

[westurner]

Sources of Noise?!

Doesn't that mean that the gravitational field is an information storage and transmission medium; a signal channel?

Is it lossy?

Is there a routable signal path to block harassing calls?

Is it slower or cheaper than nonlocal entanglement; like quantum radar?

What is the minimum energy necessary to cause a propagating thresholdable-as-binary pertubation of a gravitational field?

[omginternets]

I think you may be engaged in circular reasoning, here. Noise, by definition, is what can’t be modeled. The idea would be to have some model of the signal produced by the submarine’s machinery, and use that to reduce the noise (increase SNR) to the point that this nav system becomes useful.

[samus]

Of course we would always filter out noise completely if it was that easy to model. My point was that some components of that noise (for example the moving parts of the engines) can be modeled very accurately.

[omginternets]

I think we’re saying the same thing?

[samus]

Not a formal definition, but noise is simply the undesirable parts of the output of any information processing device, be it a sensor, an amplifier, or a transmission medium. But on Earth it's simply not practical as such. Gravitational waves observatories are used to detect events such as neutron star or black hole mergers that are otherwise unobservable, but they require an insane level of precision to be detected.

[westurner]

Are there yet any instruments better than LIGO at a lagrangian point outside Earth's atmosphere and the Van Allen radiation belt?

[_Microft]

No, there’s nothing yet. I think you‘re thinking about (e)LISA btw.

[adonovan]

It sure is a medium! One Denver (IIRC) gravitational experiment could measure the field distortion caused by 100,000 people and cars at a local sports arena. [citation needed]

[westurner]

Is there a butterfly effect -like minimum pertubation for gravitational wave propagation through matter or massful things, at least?

Is there something that's low-enough power to not EM-burn a brainstem in a crytographically-keyed medical device with key revocation?

Does a fishing lure bobber on the water produce gravitational waves as part of the n-body gravitational wave fluid field, and how separable are the source wave components with e.g. Quantum Fourier Transform/or and other methods?

Butterfly effect: https://en.wikipedia.org/wiki/Butterfly_effect

Chaos theory: https://en.wikipedia.org/wiki/Chaos_theory

Quantum chaos: https://en.wikipedia.org/wiki/Quantum_chaos

Superfluid Quantum Gravity:

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

[JumpCrisscross]

> Wouldn’t this interfere with those measurements

Towed sensors and multiple passes over known calibration zones?

[kryogen1c]

> big moving parts (e.g. the rotor)

The shaft and screw are confined to the end of the sub. A navigation instrument like this would not be in the engineroom.

[otikik]

It is still way closer to the instrument than the deep end of the sea, most of the time. Square of distance is a very potent thing.

I am reminded of this:

https://www.theguardian.com/science/2015/may/05/microwave-ov...

[RobotToaster]

Imagine spending 17 years to find the source of interference, and it's just Kyle with his chicken tenders.

[RobotToaster]

Mount it on a UUV?

[rtkwe]

It'd be interesting to see if they're sensitive enough to do the opposite detect the gravity anomaly that is the sub. There's a couple near future submarine warfare books where that's one of the devices used to attempt to find subs.

[Karliss]

Gravity force is proportional to mass and the average density of submarine is close to water density so (possibly slightly less as positive bouyancy is preferable for safety reasons) so gravity force produced by submarine should be very close to the equivalent volume of water. So instead of detecting big chunk of metal (which is still relatively small for the scale of gravity), sensor would have to be sensitive enough to detect mass distribution within submarine or the minor density difference due to slight positive bouyancy.

[djyaz1200]

I suspect that may be what these satellites are doing... https://en.wikipedia.org/wiki/GRACE_and_GRACE-FO#:~:text=As%....

[earthscienceman]

I love when HN commenters get out of water on topics. The GRACE retreivals to reconstruct mass changes for "mascons" that are several hundred kilometers in width\height take a month of static overpasses and incredible mathematics and signal processing. There is absolutely no way that their data could be used to locate a submarines, ever.

[refulgentis]

Lolll it’s my fave, especially when it’s after a sibling comment that explains “uh no” using ~7th grade science

[gitfan86]

Interesting, seems hard to zero out the clouds and air density, and bodies of water

[QQ00]

Would love to see a list of such books.

[rtkwe]

The main one I've read is Joe Buff's Deep Sound Channel series, found it after a recommendation from someone else on HN even so glad to continue the chain. Fell off it a bit in book 4 but that might have been me burning out on them, I ripped through the first 3 and focused too hard on them.

[QQ00]

Thank you so much, I love when people recommend good books to me.

[1nf_]

The majority of INS platform drift is from a thing called "tilt error" - where the IRS initially misjudges the exact direction of the gravitational vector during alignment. All this will do is improve the accuracy of the accelerometers but will still have the drift caused in the INS itself. How will this make such a large improvement over what we have already?

[beambot]

Let's use a magnetometer as an example. Inertial navigation system (INS) is just using the magnetometer as a compass, so errors in bearing accumulate over time. Instead, if you built a map of magnetic field strength, the slight spatial variation of field strength would let you precisely localize on the map.

In robotics parlance, this is the difference between dead reckoning versus SLAM (simultaneous localization and mapping).

[mc32]

Then it looks like underwater landslides, volcanoes and earthquakes could throw things off from time to time.

[Retric]

Traditional dead reckoning is reasonably accurate over moderate timescales in the mostly empty ocean as subs are big/stable and relatively slow. Thus the existing approach of only occasionally surfacing for GPS. This is therefore more a supplement as being able to regularly recenter even a few times a day is good enough.

[fho]

I would assume that you'd use that data in conjunction with others (sensor fusion / kalman filter).

Wenn you know you were at location X 10 minutes ago and now one sensor tells you you are at Y ... You can reasonably assume that that sensor is wrong.

[ta988]

Doesn't matter just take into account what didn't move. you don't have to recalibrate at each instant.

[bluGill]

The earths magnetic field is not constant. I don't know how much it changes, but I know magnetic north drifts a bit every year. And every once in a while the field reverses (IIRC we are like 10k years overdue for a reversal if we read the history of them right - a lot of guesses go into that of cousre)

[AndrewKemendo]

There’s nothing saying that you can’t do a re-localization or remapping path with basically infinite frequency, or any frequency that we know that there is variance around. At a minimum then, it becomes a better standard for which other things can bear on.

[bequanna]

Right, but it seems it is constant enough in the near term for practical purposes.

[bluGill]

I didn't mean to imply that it wasn't useful for practical purposes. That is what the article is about. I just meant to point out that there are limits and disadvantages to work around. I'm expecting to make this useful they will have to have a team to constantly remap the earth, and send those updates to whoever needs the information.

[underdeserver]

The accuracy of the reading determines the error. If today's error is so large that you have to resurface for GPS several times a day to reset it, maybe this can lower that error so you only have to resurface several times a month.

Huge game changer if true.

[pc86]

Especially if it means you can go into enemy territory for days or even weeks before exiting to friendly or international waters to surface and reset.

[throwawaylinux]

It's not dead reckoning, it's continually fixing to a map.

[elbigbad]

Can you talk more about this source of error? When I google this, your post is the main thing that comes up for the term…

[bumby]

A long, long time ago I used to work as a technician in a shop that aligned INS for aircraft (although I didn't work on them myself). I think what the OP is referring to is that during the alignment process, the assumption is that the acceleration due to gravity is always perpendicular to the INS gimbal frame. So if there are any errors in the INS leveling during alignment, they can cause errors in the INS calibration. I assume this is what the OP means by "tilt error" (although this is the first time hearing of the term). These errors then get compounded during use. You can look up INS alignment processes for more information.

[sampo]

> It’s a naval chart that would not require surfacing.

Other sources say this is about inertial navigation, not based on charts.

"Accelerometers measure how an object’s velocity changes over time. With this, and the starting point of the object, the new position can be calculated."

https://www.imperial.ac.uk/news/188973/quantum-compass-could...

[George83728]

What's the difference between measuring accelerations and measuring gravity?

I think with any INS system, if you want to be very accurate you need charts of gravitational anomalies. So these seem like one and the same to me.

[rcxdude]

There isn't any inherent difference (indeed, you can do some coarse gravity surveys by basically watching the drift of an IMU relative to GPS), but there's a pretty big difference in terms of the parameters of your sensor. Most gravimeters don't make for good accelerometers and vice-versa. One of the biggest reasons being dynamic range: most gravimeters are only able to sense a very small range around 1g, which means they quickly saturate when there's any significant motion. I don't know if that's something which these quantum sensors can work around, however. (Also, you need a gyro good enough to go along with those accelerometers, which may still be a challenge)

[rogers18445]

What would you do with the g measurement to get your location? Can g deltas be matched to sea floor contour maps? Otherwise it seems like subs would have to be following g-mapped routes.

[etrautmann]

With a highly detailed map, it becomes a statistical fitting problem, to locate yourself based on an estimate of position history using intertidal measurements and your time history of gravitational measurements, starting from an initial position estimate. I would imagine this could be quite accurate.

[quickthrower2]

Dead reckoning with gravitational adjustments?

[tehlike]

Pretty much a slam application.

[etrautmann]

exactly - presumably a subset of the full SLAM challenge if a detailed map already exists and you're just localizing within it

[etrautmann]

Oops fat fingers - s/intertidal/inertial

[unhammer]

what a strangely fitting typo though :-)

[jchallis]

A typical sub drifts by only a few tens of meters per day - obviously this builds up . The point of the BEC gravity map is to reduce this inaccuracy an order of magnitude, thereby extending the subsurface range of the subs.

[rogers18445]

But how would you do this? Sensor fusion with inertial sensors is well understood, how do you fuse g into the equation to improve things?

[themeiguoren]

By having a base map of the gravity of the ocean floor. Picture a contour map, but for values of g.

Same as if you were navigating based on a physical contour map of the ocean floor, except that getting an accurate depth like that in deep waters isn’t possible, and in shallow waters requires sonar.

[wongarsu]

I'd assume features below the sea floor have a much bigger impact than surface features. Large iron deposits, volcanic activity, natural gas deposits, whatever the LLSVPs are, etc. But you could use ships to correlate gravity measurements with GPS locations and make an accurate map that way. You don't need to map the entire ocean, just enough locations to allow subs to occasionally recalibrate their position.

[closewith]

>LLSVP

Large low-shear-velocity provinces, for anyone else wondering: https://en.wikipedia.org/wiki/Large_low-shear-velocity_provi...

[crispyambulance]

I don't understand how this would work.

Do we somehow already have that super-accurate "map of g"? Wouldn't one already have to exist to be able to use quantum navigation? Moreover, would it not also require integration with a gyro and a lot of history to be able to know, for instance, which place it's at of many in any given ocean where g = 9.8100023?

Does g really vary that much that you can figure out where you are with it with sufficient accuracy to do military stuff?

[andromeduck]

Yes, and yes. We've had the gravity maps for about a decade now, and the tech for recovering the location given such data was mastered in the late 80s, but no way to make accurate enough measurements while at sea until now.

https://news.ycombinator.com/item?id=36223373

[themodelplumber]

Fascinating. Q for you since you are so clued in.

I am a ham radio operator and was thinking about novel forms of signal propagation.

I landed on the idea of gravitons. Could gravitonic signal propagation be done somehow? While researching, I keep coming across familiar terms I know from radio land.

Just curious if the fields here could be harnessed for communication. Thanks.

[jchallis]

The big big downside of gravity waves is that they have no dipole radiators - with electric charge you have + and - and that allows very efficient propagation. For gravity there is only one charge ,and this allows only quadrupole radiation - making it extremely awful for transmission.

[yodon]

Yes, but there is no dispersion relation or absorption by the interstellar medium (not directly related to the quadrupole nature, but that which makes a radiation hard to detect also makes it hard to interfere with).

[_joel]

The detectors are such precision instruments it took over 10 years of calibration until they even had the first detection. Maybe some far off comms network could use them, but I doubt anytime soon. I wonder if you could use erasure codes with them too :)

[comboy]

Accelerating star-like masses to 99% speed of light in a few seconds also seems somewhat less efficient than your typical FM transceiver.

[conradev]

The Three-Body Problem science fiction series involves gravitational waves. I’m not going to spoil it, but the series is a great read :)

Not quite possible today, though, as others have pointed out

[JumpCrisscross]

Aren’t neutrinos holy grail for signals?

[HyperSane]

Gravity waves are way too hard to detect and generate to be used for communication. The waves we can just barely detect too 8 solar masses worth of energy to generate.

[johndunne]

Remember that that 8 solar mass gravity wave source is potentially 100’s millions of light years away. A sci-fi author could conjure up a ‘portable box’ containing a couple of orbiting micro-scopic black holes in a box to generate gravity wave signals someone somewhere in the solar system could pick up with a portable LIGO device. Consider this comment a patent on the idea :-)

[multjoy]

And people complained about next door's aerial farm!

[L_226]

Yeah, and if you put one in some kind of "spy balloon" you could also use it to map secret underground structures that may or may not exist in certain parts of the US ;)

[andromeduck]

Yup and it's called is called Terrain Contour Matching!

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

[smilespray]

Isn't this the old radar terrain mapping system codnamed FINGERPRINT?

It's conceptually similar, but they didn't use quantum gravity mapping in the 50s.

[andromeduck]

Think so? I don't find anything on Google. I thinkOme documentary said it was called SLAM on the tomahawk?

[comboy]

> This would remove one of the primary reasons subs need to surface , in order to GPS lock.

Huh? Wouldn't it be enough to send some small thing connected by thin wires or wirelessly to the submarine (lasers, ultrasound, whatever) that could aquire location and communicate it? Given military budgets it could probably even be disposable.

Leaving beacons in areas of interest also seems like a good idea. With good encryption their signal could be indistinguishable from noise happening around it.

[dmos62]

Both these things are probably already being done to some extent. Submarine broadcast stations for navigation were definitely talked about a lot. Note that military submarines is a secretive field, so you ever know only so much.

[vl]

They raise beacons which communicate via laser with satellites, thus avoiding giving away position from radio or audio.

They use audio for short-range underwater communication, but this has side effect of giving away position.

US also built giant ELF (extra low frequency) transmitter to send orders to subs which operated till 2004.

Fun fact: subs have all necessary codes to launch nuclear strike on board since there is no way to transmit these code to them if they are submerged.

[monocasa]

You don't want to send out beacons like that from your nuclear sub. Your adversaries can detect those, which negates the purpose of having a nuclear sub in the first place.

Additionally if they communicate through radio, you're back to the same problem because it's the thousand feet of water that blocks most radio waves. If they communicate via ultrasound you've just tied your sub to a nearby active sonar source. If they communicate via laser you have to be very close otherwise the power of the laser creates sonar detectable variations in the water near the sub.

[George83728]

> Wouldn't it be enough to send some small thing connected by thin wires or wirelessly to the submarine (lasers, ultrasound, whatever) that could aquire location and communicate it?

They do, using either tethered buoys or 'buoyant cable antennas' (the cable is the antenna and is itself buoyant) They can use these without coming to periscope depth, or even slowing down.

[paulmd]

> A Bose Einstein Condensate is an extremely sensitive detector of gravity- a nuclear submarine could use it to make an ultra-accurate map of the world based on variations in the gravitational constant g

it would seem the Total Perspective Vortex has obvious military applications

https://www.urbandictionary.com/define.php?term=Total%20Pers...

[fho]

May I ask: how sensitive?

Sensitive enough to, say, detect the movement of something moving at a known frequency on the other side of the planet?

Are we on the edge of gravitation based communication channels?

[hwillis]

Local surface gravity varies by up to a ten-thousandth, about .001 m/s: https://en.wikipedia.org/wiki/Gal_(unit)

Sensitive detectors can measure differences in hundreds of millionths (10^-8 parts) or around .1 um/s^2.

A 1 million tonne mass on the other side of the earth will create a TOTAL acceleration of 4*10^-16. Changes in that will not be detectable.

[fho]

Ah yes ... That's some orders of magnitude too much :-)

[white_dragon88]

I would imagine that there is is an insane amount of gravitational noise from things like the planet’s asymmetry and events occurring all the way from the core to the crust, the ocean… to cut through that noise you need one big thumper.

[martopix]

The most obvious source of noise seems to me the fact that the submarine is moving and has therefore nonzero accelerations all the time

[andromeduck]

Most obvious would be the phase of the moon.

[fho]

Thing is, you don't have to cut through the noise these days. Low energy/low bandwidth radio communication routinely sends messages below the noise floor.

Depends on you application, but if you only send some bits of information ("stocks just crashed") having a big thumper with a very known impact profile is sufficient.

[Rygian]

I would wonder how massive the thing on the other side of the planet has to be, in order to be detectable.

The more mass necessary, the more energy you'll need to send messages.

Unless the detection is extremely accurate, you would be better off spending that energy in electromagnetic radiation that bounces off the atmosphere.

[failbuffer]

For the most part yes, but for market arbitrage, the milliseconds saved by communicating thru the planet might be worth figuring out how to oscillate an extremely large weight very rapidly.

[roywiggins]

Maybe easier to use neutrinos? It has, technically, already been done at short range:

https://physicsworld.com/a/neutrino-based-communication-is-a...

[georgeecollins]

It's apparently extremely sensitive. Check this out:

https://www.jpl.nasa.gov/missions/cold-atom-laboratory-cal

[julienfr112]

Is the gravity really constant ? I mean, big chunk of lava moving with different concentration of iron would make it change, wouldn't it?

[mytailorisrich]

No, as the parent said it isn't. This is why you can create a map of the variations like this one: https://svs.gsfc.nasa.gov/11234

[unyttigfjelltol]

Undersea mountains have gravity signatures very similar to overland mountains, even though the latter are significantly closer to space?

[igufogkgg]

"Closeness to space" doesn't affect the gravitational force due to a particular piece of matter. Now, if water was as dense as rock, then undersea mountains wouldn't make such a difference to the gravitational field.

I'm trying to understand why you think "closeness to space" would be a factor. Maybe you are thinking that the seabed is going to be further from a detector than the terrain would be?

It is true that the further from the Earth the detector is, the smaller the anomaly will be.

Note that even hills are large enough to deflect the gravitational vector at the surface: https://en.wikipedia.org/wiki/Vertical_deflection

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

[DaiPlusPlus]

> big chunk of lava moving with different concentration of iron would make it change, wouldn't it

Why do you think the concentration of iron in magma would not be evenly distributed throughout the mantle?

[samus]

The magma is not exactly a liquid. Neither is it homogeneous - the roots of the components of continental plates extend down hundreds of kilometers into the mantle. It is also possible to seismologically trace the remains of ancient continental plates which have otherwise all but disappeared from the crust.

[varjag]

Gravity is not magnetism, it does not care if it's iron or anything else.

[jh00ker]

His point was that lava density could have high-variability.

[thinkingemote]

There might be some confusion in these replies related to the gp's mixture of two terms. g = gravity of the earth which varies and the gravitational constant G which is a Constant and the same across the universe.

[beerpls]

Um how do you figure?

Gravity cares about distance and mass, and as large parts of the planet move their density distribution shifts affecting both of those factors

[beerpls]

Lol this point is true but it’s downvoted.

I love how the IQ of this place keeps spiraling downward.

[abhayhegde]

As a fellow PhD student having a bit of idea on quantum metrology and sensing, I'd love to know more on your thesis! Kudos, by the way.

[5ersi]

Doesn't magnetic pole shift due to fluid dynamics in Earth core? Wouldn't that also affect gravity field?

[rtkwe]

Yes it does but we also have satellites mapping those shifts so it could be accounted for. The big question is are those fine grained enough to resolve the errors in modern INSs.

[RationPhantoms]

Wouldn't that then require communication with those satellites to receive the shift/drift info? Doesn't that eradicate the point of a navigational system without reliance on GPS/satellites?

Or is the drift is so minor that this quantum navigation system can operate for up to 6 months without sufficient core drift alignment calculation?

[rtkwe]

No idea but there are low bitrate coms available to submarines that could update that data maybe. The changes should be pretty slow at the bulk scale.

[narenkeshav]

Would it be possible to replicate that on mobile phones? Or a special device is required? 1. trying to figure out the feasibility?

[thesz]

https://en.wikipedia.org/wiki/Bose%E2%80%93Einstein_condensa...

"n condensed matter physics, a Bose–Einstein condensate (BEC) is a state of matter that is typically formed when a gas of bosons at very low densities is cooled to temperatures very close to absolute zero (−273.15 °C or −459.67 °F)..."

I guess, a special device is needed. The size of such device will be in thousands of mobile phones just in volume needed.

Yet, it is feasible. ;)

[wongarsu]

There wasn't much money in miniaturizing them so far. Cooling down tiny amounts of matter is much easier than cooling down larger amounts, so maybe there's hope for a microchip sized BEC in 20 years.

[thesz]

Let us take a look at what boson is.

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

"multiple identical composite bosons (in this context sometimes known as 'bose particles') behave at high densities or low temperatures in a characteristic manner described by Bose–Einstein statistics: for example a gas of helium-4 atoms becomes a superfluid at temperatures close to absolute zero."

Helium leaks as nothing else, even more than hydrogen.

Here's some leak rates of heilum: https://www.researchgate.net/figure/Comparison-of-the-helium...

Let's say we have a um^3 of helium in the device proposed. Let's assume leak rate of 8*10^-9 of cm^3/sec. (1e-6/1e-2)^3/(8e-9) is equal to 1.2499999999999998e-4 - all helium will leak in 1/8 of microsecond in the device proposed.

I hope this helps.

[ClumsyPilot]

whats the smalled device to contain a gradient of 300 degrees C?

this is a but solly, like saying you will eventually have a fridge the size if a m9bile phone - us it usefull?

[jubjubbird]

The MEMS accelerometers in our phones are in theory sensitive to gravity changes, but they're not nearly sensitive enough. This group is trying to improve the MEMS sensing technology, but it's doubtful it will be as good as classical gravity meters (falling mass, or spring) or quantum (BEC) ones:

https://www.gla.ac.uk/research/beacons/nanoquantum/wee-gglas...

One important distinction is between measuring gravity (generally, acceleration in the vertical direction) and the gravity gradient. The latter is generally easier and more useful for navigation.

[alexpotato]

> a nuclear submarine could use it to make an ultra-accurate map of the world based on variations in the gravitational constant g

I believe in the book version of the Hunt for Red October by Tom Clancy, they mention that the Soviets did something similar but using a magnetometer.

Super cool to see the newest versions of this are going from sci-fi to fact.

[awesometech]

Submarines do not have to surface to get a GPS lock.

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

"On the surface or at periscope depth, submarines have used these methods to fix their position"...GPS being one of them.

A submarine can also navigate by the stars without surfacing.

https://www.usni.org/magazines/proceedings/2021/october/navi...

"This is precisely the information required by the traditional method of celestial navigation, and it is accomplished without the need to surface or use a sextant!"

[awesometech]

Why was I downvoted please? I provided facts to my counterpoint.

[wolverine876]

Never attribute to malice what can be attributed to ignorance.

Also, look up the HN guidelines on debating your downvotes.

Welcome to HN!

[jofer]

It's somewhat unlikely (but not impossible) that this method is based on navigating by matching patterns of the shape of the geoid (aka variations in g).

We need accurate maps of the geoid for a lot of different reasons. (Military as well as civilian. Potential fields geophysics is super useful for all kinds of different geologic use cases, and regardless, if you want to target an ICBM, you need an accurate geoid.)

However, a super precise gravimeter doesn't help much. We have more precision than we can use already. Rather ancient spring-based instruments from 100 years ago can actually still give more precision than we can use in many cases. Modern ship-borne gravity instruments work on different principles, but the signal is very noisy for the same external reasons.

The biggest issue is that you also need to know absolute elevation very precisely to use the measurement of g that you get. A few millimeters of error in elevation significantly changes the anomaly measurement you make. Sure, submarines can get accurate hydrostatic measurements of depth, but those assume a lot of things and critically aren't absolute. The ocean has currents - that's another way of saying that the surface of the ocean isn't "sea level". Those vary through time and would require satellite information to correct for. However, once you get down in the weeds, it gets tougher still.

Remember that we're dealing with an inverse square distance relationship. Things close by matter quite a lot.

People nearby standing in different positions? That actually does affect things. Easy enough to mount the instrument away from people, though. Different distributions of mass in the submarine? Also affects the measurement. You can correct for all of these in various ways, though, so long as you have information on it. It's just more complexity and another source of noise.

In the end, the "free air anomaly" measurement you'd be correcting things to is an bathymetry map, to the first order. If there's a landslide, that affects things quite a bit, and those happen all the time.

Finally, you'd be matching a "fingerprint" time series measurement as you travel to a pre made map. That's a non unique relationship. You'd have heading/etc information to help the non uniqueness part significantly, but when things don't vary much (i.e flat topography and not a ton varying geologically), you don't have much of a unique signal to match to.

At any rate, it's a very useful tool for many other things, but I'm skeptical it could be turned into a precise navigational aid. In combination with traditional gyroscopic/etc measurements of heading and distance, it could help constrain uncertainty, but it's not an independent measurement and it's relatively noisy.

Now that I think about it, though, a fully passive "seamount proximity sensor" is rather useful, and that's something you'd get even with a noisy signal...

[krisoft]

> The biggest issue is that you also need to know absolute elevation very precisely to use the measurement of g that you get.

I don't think so. What you are saying is true if for some reason you want to work with absolute values, but nobody would do that. You measure many data points as your submarine flies over the landscape for a time, and then you match the measured curve with predicted curves from bathymetric maps. This is an optimisation problem where you try to find the best match. Precise elevation is an output from this process not an input requirement.

> but when things don't vary much (i.e flat topography and not a ton varying geologically), you don't have much of a unique signal to match to

100%. This is also true for cruise missiles which fly by TERCOM[1]. And there is an interesting consequence to it. Submarines and cruise missiles don't just bumble around randomly. The navigators also know this limitation so they set trajectories which plays to their strength. In the case of the cruise missile planners they have tools to evaluate the navigational quality of a terrain contour matching algorithm over a proposed trajectory with Monte Carlo methods. Probably submariners have the same.

This means in practice you can know that the submarines are more likely to take certain routes. They will prefer approaching from hilly terrain over flat, but also over extended flat areas they will prefer to overfly butes to regain navigational accuracy. This of course won't tell you precisely where the submarine is, but can help an adversary more economically allocate their ASW assets.

> At any rate, it's a very useful tool for many other things, but I'm skeptical it could be turned into a precise navigational aid

Yeah. I mean I heard that people proposed to make measurements of stars to find your location. The fools. Haven't they heard of clouds? Sometimes you can't see as far as your own nose for days.

Every navigational system ever devised have limitations and peculiarities. If you are comparing gravitational tercom with the ease and quality and simplicity of GPS then of course it will look crude and cumbersome. But of course GPS adds other complications and dangers to the life of a submariner. Used well, and in the right circumstances it can be potentially very valuable technique.

1: https://en.wikipedia.org/wiki/TERCOM

[jofer]

> What you are saying is true if for some reason you want to work with absolute values, but nobody would do that. You measure many data points as your submarine flies over the landscape for a time, and then you match the measured curve with predicted curves from bathymetric maps.

The variations due to changes in elevation are orders of magnitude larger than the variations you're relying on using for navigation. Distance from the center of the Earth is the primary signal you're measuring, for better or worse. It's not just relative vs absolute. You can probably assume the sub isn't changing depth rapidly and ocean currents are long wavelength, which would allow relative measurements to be somewhat feasible.

However, nothing about this requires a new quantum sensor. Ship-borne gravity anomaly measurements have been around for half a century. The previous methods are more than precise enough. In fact, they were done on submarines first before ships - it's easier to measure without waves.

What's triggering this now? Something doesn't add up... If it were purely based on using gravity anomaly along track measurements as a "fingerprint", it wouldn't need a new sensor. There's likely another mechanism they're using or they're using it for other reasons in addition to navigation.

[krisoft]

> However, nothing about this requires a new quantum sensor.

Sure. The news can be many thing. Maybe they got a marginally better gyroscope. Maybe it is not better in terms of better error characteristic, but cheaper, or smaller, or lower maintenance.

Or maybe they got a new software package which brings them benefits, but the way they could sell it to the leaders is by telling them that the sensor they are using is "quantum".

Or maybe none of the above and they are just trying to fake out an adversary to trigger a costly mistake.

[RobotToaster]

Couldn't it also be used to detect submarines as well then? Given they're quite big.

[phkahler]

They have the same (average) density as water.

[stealthcorp]

Very interesting! What year did you complete your PhD? Care to share a link to your thesis?

[punnerud]

Can you also use it to detect other submarines? If they affect the gravitation

[dumbo-octopus]

I wonder how close you'd need to be for any small-scale buoyancy differences to be detectable, given the entire object would clearly be neutrally-buoyant.

[EVa5I7bHFq9mnYK]

Can it also be used for missile guidance, in case of GPS denial?

[paulsutter]

Does this detect gravity as distinct from acceleration?

[digging]

Gravity isn't distinct from acceleration though - how would it?

[paulsutter]

Thats why I asked. And they are different.

If you throw a baseball in an accelerating enormous elevator in a vacuum, the baseball will follow a parabolic arc.

If you throw a baseball on the vacuum surface of a planet, it will almost (but not quite) follow a parabolic arc. The force of gravity is lower at the peak, unlike in the elevator where acceleration is constant everywhere in the elevator.

Therefore, you can distinguish gravity from acceleration

[snovv_crash]

Isn't this the method used by the sub in Hunt for Red October? Or did they also use a gyroscopic and look for changes in the gravitational vector, not just its magnitude?

[DaiPlusPlus]

I haven't read the book, but in the film the problem was the Red October was using the caterpillar-drive https://en.wikipedia.org/wiki/Magnetohydrodynamic_drive (and not a propeller/impeller) but a chance detection by Dallas' sonar along with the ingenuity of Dallas' sonar operator, who figured out how to detect and track Sean Connery's boat. Nothing to do with gravity or gyroscopes at all.

[snovv_crash]

Yes, but Red October had some super accurate underwater gravity map of some kind that allowed them to navigate at speed through areas Dallas couldn't follow IIRC.

[jchallis]

Red October used the usual gyroscopes approach that gets more and more inaccurate as it sails. I think a key plot point is that they need to surface to take a latitude / longitude , precipitating much suspense.

[1letterunixname]

Passive sonar in the SOFAR channel.

[hackernewds]

what a sad outcome it is that research such as this is funded for, and classified by military interests such that it is barred from public collaboration and societal progress