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Comment by Vytuin | Hacker News Reader

Just a few numbers and even one equation, sorry: The heart is a magnetic dipole, and the field of a magnetic dipole decays as 1/r³. Therefore, if it can be detected at a distance of 5 cm, then at a distance of 50 kilometers it will be (5,000,000 cm / 5 cm)³ = 10¹⁸ times weaker. A quintillion times smaller. I think the discussion of signal-to-noise ratio is a bit... uh... misleading.

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I don't think it's too misleading. There have been atomic-optical magnetometers developed with sensitivity of 10s of fT/√Hz, so approaching the quantum noise limit. The issue seems to be SNR to a certain extent.

In an ideal situation, such as being isolated in a desert, detecting the heart within meters is achievable, within a kilometer plausible, within 10s of kilometers implausible.

It's also a matter of just getting plausible detection. If you get a reading that might be a heart, you can move closer. Being able to move the detector provides much better search capabilities.

How do you distinguish a heart of a human and the heart of a rat with at that distance and that noise level?

I've used https://en.wikipedia.org/wiki/Lock-in_amplifier and in ideal conditions you get a 10^6 amplification, when you send a very clean signal and measure how it bounce.

My expert opinion is that 10^18 is im-facking-possible.

> How do you distinguish a heart of a human and the heart of a rat with at that distance and that noise level?

A rat has a resting heart rate above 200bpm. The only thing similar to a human heart beat would be of a dog, pig, goat, or sheep.

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