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Comment by gadilif | Hacker News Reader

So, going from a very narrow frequency band (up to 1000Hz) to a much wider range, which can theoretically encode information (e.g. frequency modulation)... Hmm, I'm wondering if comms over gravity is something a sufficiently advanced civilization might consider using, and should we be looking for that 'Hello, world' in some 'natural frequency' like we're doing for EM radiation?

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I'm not sure I see what the advantages of communicating this way would be. The amount of energy required per bit transmitted would be astounding.

I feel like, while theoretically possible, it's pretty much all downsides and no upsides. At least for communication purposes.

However, your comment reminded me of an interesting PBS Space Time episode discussing the possibility of finding alien civilizations via the gravitational waves produced by their massive ships accelerating to near light speed.

https://www.pbs.org/video/could-ligo-find-massive-alien-spac...

Thanks for the PBS reference, interesting! As to the energy required, well, yes, I guess manipulating huge masses will be costly, but, if there is an efficient way to do this, then gravity waves are a parallel plane of communication. The analogy I heard once is about tribes communicating with smoke signals, while the air around them is filled with radio waves. Maybe we can't hear anyone out there because we're not listening to the right thing...

The medium is the message. If we detected a modulated signal in gravitational waves, it would be like an Iron Age tribe receiving a 100 foot tall perfectly polished stainless steel statue with ornate inscriptions and pictograms. It would be recognizable and within our conception, but it would also be a demonstration of development and access to resources beyond our imagination. That's the upside. In a galaxy of sparse and sparingly advanced civilizations, the message might be "fear us and stay away" in a way that EM would not convey.

>it would also be a demonstration of development and access to resources beyond our imagination. That's the upside [...] In a galaxy of sparse and sparingly advanced civilizations, the message might be "fear us and stay away" in a way that EM would not convey.

I think you've hit the crux the question. If there are only a few civilizations, I agree, that'd be an awe-inspiring deterrent.

However, if you don't know how many civilizations there are that are similarly advanced to your own, sending out a big "we're here!" message may be quite risky.

In terms of game theory, it's a sequential and incomplete information game. I think the smartest decision is to remain quiet.

I think the UN should force nations to remain quiet. Srop the signals that can be stopped, especially the ones designed to go far.

https://en.m.wikipedia.org/wiki/Dark_forest_hypothesis

Maybe that is the filter. If your society hasn't figured out the tech to do it efficiently, the rest of the galaxy doesn't care about what you have to say.

I've always been a bit confused about the idea that an advanced civilization would be uninterested in us because we've "only" reached the ability to communicate via radio waves. We're either a threat or an ally to these other civilizations, and if you were them and you detected a society that was clearly on course to eventually catch up, it would be in your best interest to treat them like one of those things ASAP so you can have a hand in their development.

To me it feels most likely that our signals just have not had enough time to get to them.

I think the "dark forest hypothesis" can apply here.

The advanced alien civilization may indeed be interested in us, but still not consider it their best interest to act as soon as possible.

If they decide to act, other civilizations (perhaps even more advanced) may decide to intervene in some way. A civilization that decides to reach out (in a friendly or hostile way) also reveals their own location in the universe.

The most risk adverse choice is probably to remain quiet, especially if they are millennia ahead of us technologically.

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

You can detect gravitational waves based on the strain (amplitude) directly, rather than relying on intensity like electromagnetism.

Because the amplitude is inversely proportional to the distance, but intensity is inversely proportional to the distance squared, this could allow for communication over longer distances.

The upside is that space is quite transparent for gravitational waves.

What would a gravitational wave generator look like? A machine to "wiggle" an asteroid, or say a moon? What if you made a huge array of small machines that "wiggle", say, a bowling ball, in perfect sync.

In principle, almost anything: any system of masses will emit gravitational waves with an intensity proportional to its mass quadrupole moment (which may be 0, as it is for rotationally symmetric systems). But the proportionality constant is extremely small. Realistically you're looking at stellar-mass objects, if not larger.

Hopefully we build a small transmitter and experiment with it, now that we have a receiver

Nah, asteroid is not enough. you'd need to wiggle a couple or more large black holes in super close proximity. But who knows, may there be alternatives we are not aware of? Is it Higgs boson that gravitational field carrier particle , similar to electrons for EM field? Maybe there is a way to mass-produce those and modulate gravity waves that way, eh ?

>I'm not sure I see what the advantages of communicating this way would be.

Gravitational waves might be the best way to communicate between our world and the dark matter world/dark sector?

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

...or maybe there is a lower noise floor for gravitational wave comms?

>The amount of energy required per bit transmitted would be astounding.

Has someone calculated this out? Or is it more of a "well we need an exceedingly sensitive instrument to detect some of the most energetic events in the universe from half-a galaxy away" gut-feel? Any reason something like a phased-array for directional comms / beam forming wouldn't work with gravitational waves?

>Has someone calculated this out? Or is it more of a "well we need an exceedingly sensitive instrument to detect some of the most energetic events in the universe from half-a galaxy away" gut-feel?

I was thinking about the energy required to transmit the gravitational waves, not receive them. Being able to move objects massive enough (stellar mass or more) to create detectable gravitational waves in a quick and precise enough manner to allow for communication would require mind-boggling amounts of energy.

Right, sorry I wasn't clear. If you were more interested in "local" communications, and less interested in broadcasting to the rest of the cosmos, do you still need gigantic amounts of energy at the transmitter? And how much power would you need, even if the energy is relatively high? For low power transmissions, what is the limiting factor on the receiving end? Or why can't you detect really low power transmissions? Can't get your receiver close enough to absolute zero, so thermal fluctuations kill receiver sensitivity? Background gravitation noise floor is too high across the band? Quantum fluctuations are a limiting factor? Can't make an X-ray/gamma-ray interferometer? "Antenna" size scales with length rather than area? Other?

I suppose the ratio of Coulomb's constant to the gravitation constant (or something similar) govern the relative difficulty in using gravitational vs. EM? But that's not obvious to me that it would make gravitational wave communications inefficient in absolute terms.

Oh, yeah sorry, I was thinking more along the lines of inter-galaxy communications!

I definitely do not know enough about the topic to approach answering your questions, but I'd certainly be interested in knowing the answers. I really hadn't thought about it in that context.

Maybe you don’t need to generate the waves, if you can collect them and redirect them into the shape you want.

Since gravitational waves would still be restricted to the speed of light what would the benefit be?

Would "obstacles" be circumvented? I would think interference would still be possible but instead of line of sight it would be large gravitational distortions (black hole, stars).

Humility clause: I don't know what I'm talking about.

Lots of naysayers but I can think of one very valid reason this might be the case. Our species, and many species on our planet, can see a very narrow band of the radiation spectrum. It’s possible an advanced civilization never developed that but was instead far more intimate with gravitational energy instead. We are talking about the universe after all.

I can't really imagine how such a transmitter works?

Somehow moving planet sized objects around to create gravity waves?

Of course, the cop-out "using their advanced tech we don't have a clue about" answer could actually be correct.

Create tiny black holes and throw them into each other.

This idea was featured in The Three Body Problem trilogy.

The problem is modulating the signal. The only way is to move large masses quickly.

I'm thinking you could also create gravity waves by annihilating matter and then creating matter over and over. If you could get a bit of that wave to reflect back to you and then sync your creation/annihilation, you could build a huge wave over time

If we are going all in on imaginary civilizations, even a very narrow (well, not that narrow) frequency band could be used to encode information, couldn't they?

Yes, but not efficiently (it will be very slow) and will be hard to detect because of noise (the article mentions the 'cacophony' of gravity noise). This is similar to EM radiation, so, you do frequency modulation (or amplitude, but that seems harder with gravity - you'll need to modify mass...), so you have a base high frequency on top of which you add lower frequency. The substraction later gives you a clean signal (or, Gravity Radio).

Will you distinguish it from noise? Surely those will be compressed and encrypted.

Some of it, definitely. But if the purpose is to seek others and announce yourself (like we did with Voyager 1 and 2), then you'll want it to be plain. Also - Public radio transmissions are not encrypted (as your purpose is to have as many listeners as possible... You monetize on ads).

I'm not sure "slow" matters that much. If (and that's a big if) there is advantage to announce yourself to other civilizations using gravity waves, instead of other means, then it probably does not matter too much if transmitting a reasonable amount of information takes a decade or so. You'd put it on repeat of course, and hope that interested civilizations don't have too short of a lifespan to still make sense of it.

"Zorlax the mighty would like to connect on LinkedIn"

https://xkcd.com/1642/

Did you read Death's End by Cixin Liu?

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