But is it? We can detect things about the chemical makeup of the atmospheres of extra solar planets. Our own atmosphere is broadcasting the likely presence of life, plus near certainty of industrial civilization. We could detect that up to some distance with our technology. What could advanced aliens detect? What if they used gravitational lensing to magnify the signal?

We can’t rule it out.

Our signal for oxygen and other bio-signatures is so much stronger than the signals we have been sending recently and it's been going out for...quite some time.

Maybe they found Earth already and they just find it boring: "yet another planet likely filled with idiots"

I mean, the Great Oxygenation Event is ~2 billion years in the past, and the Cambrian Explosion is only half a billion years ago. If intelligent, space-faring aliens checked it out, there's a 75% chance they arrived while the most interesting thing on Earth was, well, I can't think of anything that wasn't "technically before the Cambrian but like, only by 50 million years". Fungi?

We should look for oxygen-rich atmospheres, but we need another signal before we jump in a rocketship and go looking for someone to talk to or eat.

I think it can be done cheaper than that, and with technology we have access to.

Detonate a handful of large hydrogen bombs in deep space, set apart by <prime number> minutes. Say hello to our fellow Aliens.

How far away do you think such a thing could be detected? Say someone set off a 100 megaton nuke in space within the Centauri star system. Could we detect it?

My intuition says no. I don't think we'll be detecting exoplanet asteroid strikes anytime soon for instance.

Well, someone running a nearby Dyson Swarm might as well run a couple thousand telescopes when they are at it, even to just make really sure no habitats are ending up in collisions with comments and extrasystem asteroids. In spare time such system could be used to look for interesting stuff in other systems, possibly like a big synthetic telescope of some sort.

its inverse square falloff for radio energy dissipation so I'd say it goes away really really quickly. also you have the problem of overcoming Sun's 'blaring' background signal unless you took your signal generator and placed it outside of oort cloud or further.

Yes, why not the oort cloud! It might take a decade or two to get them out there, but completely within our current technical capabilities.

oort cloud is just a stand-in for generating enough parallax in the signal from sun. further out you go better it would be. in fact it just occurred to me that you could just launch an emitter perpendicular to galactic plane with enough velocity that it bobbles up and down say 3x over what a normal star would and it would serve as a galactic lighthouse for intelligent civilization.

Another idea is to put a mylar parabolic mirror behind the bombs. A very short burst of light is going to get focused in a beam before the mylar is vapourised. Maybe someone smart here could calculate exactly how short the beam would be.

That short burst could be in itself a significant signature. What, in nature, could produce such a short concentrated energy burst?

problem with such a tight-beam is you have to be in direct line of sight for it to be seen. which means a very small number of receivers civilization can ever know, that is exact opposite of the goals of a 'lighthouse'. perhaps something that sweeps around at regular intervals with some precession thrown in for coverage, that makes it closer to GP's comment about neutron star emitting prime numbers.

Luckily, the aliens are wearing Swiss wristwatches, they know how long a minute is, and so are able to notice that the intervals are a prime number of minutes :-)

If the intervals are relatively prime, it wouldn't take much to work out what the intended time unit was.

OK, that makes a bit of sense. But I don't know what it means for two real quantities to be relatively prime. Primality is about factoring, right? And you can only factor integers. So to observe primality, you'd have to partition interval measurements into an integral number time-units.

Can two reals be relatively prime no matter how you partition them into integers?

The ratios of several smallish prime integral time steps would bear out.

You're looking at the sky and you see a bright and otherwise unexpected signal. Then you see it again, some time later. You write that time down as T_1. You don't know of any astronomical phenomena with period T_1, so you wonder whether this could be a weird rare event. Then you see it again, spaced by T_2 this time, and you start to wonder whether it could be a purposeful signal. Then T_3 etc. How could they be linked? They're getting farther and father apart, so maybe it is a sequence with some significance. Maybe it's the fibonacci sequence for example. So you try dividing all the intervals by T_1. And you get 1,1.503,2.496,3.500,5.506... huh, lots of nearly-half-integers in there, maybe T_1 was supposed to be 2 units actually?

> so maybe it is a sequence

At that point, you introduce the assumption that the interval is quantized. Up until then, you're dealing with an arbitrary period of time, which is a real, and can't be made into sequences. Once you have sequences you have integers, and so you can have primes. But you can't just switch from reals to countable numbers without explanation, and then say that two reals can be relatively prime.

So begin with a sequence of 1 1 1 1 1 1 1 1 1 1. That establishes a baseline cadence of minutes, or hours or whatever. A clock signal if you will.

Then switch to primes. I think anyone looking at that kind of message will figure it out eventually. It can't be mistaken for a natural occurence.

> So begin with a sequence

That is the trick that I was calling out. The intervals are reals, and cannot be made into a sequence. Beginning with a sequence is even worse; what is in evidence (for the aliens) is a set of real intervals. That's what the aliens have to begin with.

Now explain how they get to primes.

If the precision between the intervals is good, how can you not get to primes? Of course the intervals would be real, but there would be a very discernible pattern.

It would stare you in the face, or what am I missing?

I disagree. Sequences of reals are still sequences. So maybe hypothesis one has nothing to do with integers, maybe it's that the intervals are increasing geometrically. You're still going to take ratios and and start having suggestive numbers fall out. "can't just switch ... without explanation". The point is that this putative alien observer would be looking for the explanation. Maybe the first few explanations they come up with are the wrong ones, but they are trivial to check. It's not like it's a research project to reject the Fibonacci hypothesis.So you would still have the right answer on day one.

Agreed. I read somewhere that if you pick a random direction and fly through space, you have a 0% chance to ever hit anything.

That is how big and empty the universe is.

Well, you're almost certain to hit a hydrogen atom within a meter of travel in deep space. Larger objects will be less common, no doubt.

> That is how big and empty the universe is.

Indeed. The corrollary is that there's an awful lot of space to travel through in search of something to hit.

Regarding that 0%: I don't think it's possible for you to travel through space fast enough to catch up with the expansion of the Universe. It follows that your search for something to hit goes on forever, and the chance of eventually hitting something must be 100%.

Whoah! Not so fast, Denton! As you are travelling looking for something to hit, the Universe is expanding, and the distance between galaxies is growing. So the longer you travel, the less likely you are to find something to crash into.

I don't know how to work out the sums. I don't know how to multiply a 0% chance by a 100% chance. Or rather, I do; it just gives an answer that I don't like intuitively. It should be a 0% chance; I guess it probably is.

The ultimate flex from a type 3 civ, love it!