Communication by means of modulated Johnson noise | Hacker News Reader

Communication by means of modulated Johnson noise

pnas.org

66 pointsaustinallegro35 comments

I’m away from computer so only skimmed the article until I noticed they use the ADG90x RF switch family.

IIRC that family has a single positive supply rail, and they make a nagative rail internally. The advantage is easy of implementation but you also inject some noise depending on switch position. Is this dealt with in the pub?

They do conduct an experiment to control for both switching noise, or other feed-through transmissions. They put two 50 ohm terminators on as the load, and find the receiver gets nothing. They also try cooling down one of the loads in liquid hydrogen, and the transmission resumes.

This is extremely strange. Shouldn't this be impossible by the second law of thermodynamics?

Receiving more energy when a resistor in thermal equilibrium is connected to an antenna seemingly implies that energy gets transferred from an antenna with a resistor to a an antenna without a resistor, even at thermal equilibrium.

Maybe we'll later find it's an experimental subtlety, like the faster than light neutrinos.

AI will explain it, but it's against HN rules for me to post the explanation.

Basically they are misleading when they say "thermal equilibrium", same physical temperature does not imply thermal equilibrium.

FWIW, the "Significance" blurb at the very top says as much:

> An interesting feature of the system is that all components of the system are at the same physical temperature, but it functions because they have different noise temperatures.

(I assume it's not AI-generated.)

But it should. If you can get energy to flow one way between things at the same physical temperature that seems to violate thermodynamics and allow for perpetual motion.

only for passive systems.

what's described in this paper is not passive, it has a powered amplifier in there which lowers the "noise temperature"

same way a heat pump can move heat between things at the same temperature - inside and outside of house when you start it

I'll pretend you don't mean "sustained" to allow me to type something. Does this qualify?

Put a bowl of water down at room temperature, wait for one molecule to "evaporate", and I think you did it? Unfortunately they are now no longer at the same temperature.

Temperature is an average. You can still take advantage of the fact that there will be much lower and higher temperatures present. Also, what about Peltier effect? Same temperature when it starts.

Yes. If you try to design any device to capture all the molecules like that and extract their energy, you'll find that just as often your device dislodges a water molecule the other way, back into the pool and losing energy. Thus although You aren't the first to invent the Brownian ratchet.

The device shown in the OP appears to allow for a sustained thermally-driven energy transfer one way or the other depending on the bit value ou are sending.

Now consider the alignments of red blood cells.

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I'm not sure, as this paper is very confusing in its style, but I think they try to modulate the load resistance connected to an antenna on the TX side. Since the open has a large real resistance (25k in their case), it's poorly impedance matched to the antenna and most of the noise powwr cannot be transferred to the antenna at 1.4 GHz they are using. 50 ohm load is well matched, hence the max noise power transfer to the antenna. This is all at TX.

I don't see a thermodynamic problem here. Noise modulation with switching is well known in electronics. Switched resistors and their noise is already analyzed. It is often in the context of noise parr of SNR though. They are using this noise modulation to transmit signals, which isn't too novel. I guess the transmit power is very low but this would be horrible for Shannon spectral efficiency because of the need for noise power detection, which cannot be instantenous.

I suspect GP's question is answered by the fact the system is not in thermal equilibrium. The view the antenna has is approx half of the sky whose temperature is far lower than room temp. Even indoors, the roof of your house is sufficiently transparent in RF that you are still seeing the sky behind.

If the same experiment was done in metal box with everything of constant temperature, I don't think it would be possible to transmit any bits.

26 bps at 1.5 m: And what exactly do we expect to carry with this communication channel? If you add horned antennas it becomes a ridiculous design just to gain a few more bytes. If you add antenna arrays then it's no longer cheap. Your transmitter's output power is basically hard limited by Boltzmann constant, so just increasing range means you will have a bad time decoding it. Digital tricks, cryogenics... maybe a few more bytes, but is it enough?

I'm really struggling to see if this can be useful for anything. Dicke radiometer reference is cool, though.

Magic, got it.

(I am floored that this works. This is amazing.)

A cacophony of meaty slaps in morse-code.

Yeah are we gonna sit here and not have a chuckle about Johnson noise article hosted by pnas.org!?!

I tip my hat to you sir, I had missed that

oh well caught!

> the maximum achievable throughput is 26 bps at 1.5 m and goes down to 22 bps at 4.5 m.

And when there are more than one transmitters in real world conditions instead of anechoic chamber?

It's cool and all and if this work is for it's own sake, for the sake of research, no issues. But otherwise, I struggle to think of practical use cases. I grant that my imagination may be deficient.

> I struggle to think of practical use cases.

I guess your not on Twitter. It's not a coincidence this research paper from 2022 got posted today.

What happened today ?

I think someone from OpenAI said something like "air gapping AI can't (hypothetically) work because it will figure out a way to basically morse code across the airgap". (I think this was in response to people calling them out for not airgapping)

> "air gapping AI can't (hypothetically) work because it will figure out a way to basically morse code across the airgap"

All things well-known for good two decades and written about by x-risk/LessWrong crowd, but to date continued to be dismissed as lunacy.

On one hand it's great to see it demonstrated in practice in concrete terms, on the other hand it's sad we have to basically make the mistakes ourselves, with live and close-to-dangerous systems, because we can't believe the obvious warnings.

You're likely not familiar with TEMPEST/EMSEC but this stuff was known about and discussed when designing air gaps going back to the 60s..

https://www.nsa.gov/portals/75/documents/news-features/decla...

the x-risk/LessWrong crowd aren't adding anything to the conversation that seriously people haven't already considered.

I didn't mean they first invented or described the methods, but that they've been consistently presenting a solid argument that these and other methods make airgapping and sandboxing AGI a dangerously flawed idea from the start. They are the serious people who considered this, and to date nobody listens.

Well... they aren't the serious people... which is probably a good thing. You don't really want to know the serious people.

you really don't

We need a Wildfire base like in the Andromeda Strain

Less wrong gives off a give of wanker doomsday cult in general. Like they just enjoy being scared or something, but are not actually serious about anything. Like the true crime YouTube series enjoyers. That's purely vibes based of course

Yet they were much better at predicting Covid or AI than the "serious people" which dismissed these subjects until it smacked them in the face.

I'm pretty sure this is from the plot of the show Continuum.

There needs to be something on the other side to receive that morse code. Unless it's like Metamorphosis of Prime Intellect and it figures out how to rearrange matter just by modulating some electronic signal.

If you think this is a realistic possibility, you might be experiencing AI psychosis.

They were thinking of Van Eyck Phreaking

Maybe it's more of theoretical interest, e.g. the interplay between thermodynamics and information theory, etc.

Figure 9C in the paper shows testing in a wooded area. Figure 9F shows transmitted data collected in a residential building, so not a test chamber.

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