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Comment by krisoft | Hacker News Reader

There are two subsequent sentences in the first paragraph. One is complete bullshit, while the other has correct reasoning. It really doesn't help when skeptic articles do this.

> The very notion of dowsing — that an above-ground stick will react to the presence of something hundreds of feet below — breaks the laws of physics.

Really? It breaks the laws of physics? Tell that to my gravimeter. Oh you are saying a human with a simple wooden stick is not a gravimeter? Idk they are quite complicated. They can be many things. Maybe they can be a gravimeter too?

> Every single controlled test of the practice has failed, yielding results indistinguishable from chance.

This is the important bit. We know that "dowsing" doesn't work because we tried it many times, carefully, and so far it never worked better than chance. It has nothing to do with breaking the laws of physics.

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A human with stick (in the colloquial sense, they ones used by Dow sets) would absolutely not be able to perceive the differences in gravity with/without the presence of underground water. The signal to noise ratio would be too high to know the difference. In that sense I don’t think it’s too much of a exaggeration to say it defies the law of physics IMO

Animals can do all sorts of weird things that might have once been believed to be impossible:

"Birds Can See Earth's Magnetic Fields, And Now We Know How That's Possible" (https://www.sciencealert.com/birds-see-magnetic-fields-crypt...)

The only reason you can state with confidence that a human with stick can't detect water underground is that they've never been able to do it.

Being able to sense Earth's (extremely strong) magnetic field was never "believed to be impossible." The paper is just about resolving the specific mechanism in birds' bodies that allows them to do it.

The gravitational variation caused by water deep below the surface is infinitesimal. Its effect on a stick in your hands cannot be felt. You experience far more variation in gravity due to the moon's orbit, and you can't feel that either.

https://www.researchgate.net/publication/272887140_Subsurfac...

> The gravity variation computed from the total water storage (sum of soil moisture and ground water) using the Bouguer approximation was 12 µgal peak-to-peak (1 µgal== 1 x 10-8ms-2).

That's a 0.0000012% difference. You'd experience vastly more difference in force due to the motion of air against the stick as you moved, or the accumulation of dust on the stick over time. Or the local humidity and pressure. Or the position of the moon. Or your elevation. Or your latitude. Or...

Meanwhile, it turns out that if you're over a groundwater deposit, you can dig pretty much anywhere and hit water eventually.

plants can apparently sense water tens of feet away, through solid earth and metal pipes

> The signal to noise ratio would be too high to know the difference.

That's a very good reasoning why it is an unlikely thing to be. But the sensitivity of humans to gravitational differences is not a law of physics. This is a very important distinction. The phrase "breaks the laws of physics" is not just a rhetorical flourish.

If I point at a go-kart looking thing and say "that thing can go 0.8 Mach" that's a practical impossibility. For many reasons it is very unlikely, to be true. But does not break the laws of physics. (Who knows, maybe it has a nuke under it, which can loft it that fast for one glorious moment of go-karting fame.)

If I point at the same go-kart looking object and say "that thing can go 1.2c" that is a whole different ballpark. It might still happen, but if it happens that will make us re-evaluate nearly everything we think we know about how the universe works. This is what it means that something "breaks the laws of physics".

A gravimeter is not a stick.

https://www.nasa.gov/mission_pages/icebridge/instruments/gra...

No it is not. But can a human+stick system act like one? I'm open to the explanation how it would break the "laws of physics".

Simple: The change in gravity is so small that it is not physically perceptible by the human nervous system.

A long enough, massive enough stick could theoretically be a gravimeter. You'd just have to be so impossibly perceptive (or use an interferometer) that you'd notice the difference in gravity each half of the stick experiences.

No question dowsing is pseudoscience. But it's interesting that the dowsing concept of hanging weights off a stick is quite similar to the first scientific gravimeter, which used a pendulum in the 1670s.

Also Clarke's first and second laws in action: "LaCoste's most famous invention is the ship, and aircraft-mounted gravimeter. …this invention was considered impossible until LaCoste demonstrated it." Clarke's third law is asking the black brick in your hand "Siri, what's the local gravity here?"

That first sentence hit me the same way. We have all kinds of above ground fancy sticks that react to the presence of things hundreds of feet below. It doesn't break any laws of physics!

What matters is that, as you note, we've tested this pretty thoroughly and under controlled conditions, and have strong evidence that dowsing is bullshit.

The article is essentially repeating Adam Savage's words on the topic in his video linked in the article, re "laws of physics".

This is the same defense used by practitioners of astrology -- that humans are somehow able to unconsciously perceive and be affected by infinitesimal gravitational forces.

It's bunk.

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