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We've been able to fly or emit radio signals only for a bit more than 100 years.

If there exist any other civilisations and we assume they start independently, there should be ones that started billions of years ago. Which is enough to visit entire galaxy even at very slow speeds.

The paradox is that we are not seeing any signs of any other intelligent life.

Only if they are in our own Solar system. But you can easily imagine a supercivilization visiting lots of solar systems, but not establishing permanent colonies everywhere. As I said, what if 5% of the solar systems are teaming with life? That would still be tens of millions of solar systems in our galaxy. The diversity of life could be something to make Star Wars look unimaginative. And we'd have no idea they are out there.

What kind of argument is this "only if they are in our own solar system"?

Even at the speed of Voyager 2, (15km/s), in a billion years it would cover about 4 * 10^17 km or about 40 thousand light years. I am pretty sure people who have been developing for hundreds of millions of years can do better than that, by orders of magnitude.

Also you put a totally strange assumption that all civilisations will only restrict themselves to visiting planets. If there is a lot of civilisation it is more likely they will have different motivations.

If the dark forest hypothesis is true, you can bet any civilization advanced enough to detect us will go out of their way to avoid being detected by us.

For all we know, there could be a gigantic tungsten rod slowly and purposefully accelerating toward us that would destroy all life on earth in ten thousand years, and we'd have no way of knowing.

Why a tungsten rod in particular?

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

Presumably because it's high density and thus higher destructiveness vs detectability?

Although I guess they could also just paint their weapon black.

A different matter how they hide the engine's infrared signature. Perhaps by just not running it when approaching, except for the final course corrections.

I am not sure about usefulness of kinetic impactors at near light speeds over long distances.

Imagine you are in impactor frame of reference -- any particle of dust it meets will cause tremendous damage to it regardless of what the impactor is built of (assuming our existing technology).

It is in my opinion very likely that over long distances the impactor will just get converted to a shower of high energy particles over very large area.

Stars move relative to each other. You won't need much higher speeds than what we can already achieve, as long as you're willing to wait for another star to move nearby.

Stars don't have same velocities:

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

Gliese 710 is probably going to cross Oort cloud in 1.29M years. Perhaps we should wait and hop to this star?

https://en.wikipedia.org/wiki/Gliese_710#Computing_and_detai...

If they visited us a billion years ago, would we know?

Time is as big as space. If you’re positing that aliens must exist and can explore, they might have just done that and lost interest a million years ago, or they do sweeps every hundred thousand years.

Plot twist: dinosaurs were really an alien species. After they were wiped out by an asteroid slung at the earth by an evil prince, the earth was written off as a memorial site.

>The only question is not why we didn't "see" any aliens so far. It's why we wasted so much money on SETI. Was Carl Sagan not able to do the back of the envelope calculation I just did here?

Sagan was perfectly capable of doing these calculations. However, SETI was a nice way to divert funds to radioastronomy.

Not the first time Sagan was 'creative' - per his Nuclear Winter model, if sometimes lights a match in the poles the entire planet will burn.

Well said. The "Drake Equation" is complete and utter garbage. There is no such thing as a "probability" of any of those terms, in the way we normally understand probability, i.e. dice or card games.

That said, there probably are intelligent civilizations somewhere in the universe, but we have that pesky problem of distance and the speed of light, as you said. People like to imagine that eventually civilizations find a way around it, but in all likelihood they don't.

> There is no such thing as a "probability" of any of those terms...

> That said, there probably are intelligent civilizations somewhere in the universe

...pick one :-)

I gather you're getting all triumphant there: "Hah! A contradiction! So there."

Not really, sorry.

"Probability" as in "something you can assign an exact number between 0 and 1 to."

"Probably" as in "I dunno, maybe."

No, he was rightly pointing out that your "there probably is" statement was bullshit, not supportable from the evidence.

> bullshit, not supportable from the evidence

Two different things. You can't prove there aren't, and I can't prove there are.

"Bullshit", as I use the term, is misrepresentation of the state of your knowledge by confident assertions you can't back up with evidence. The statement is precisely that, bullshit.

oh, lighten up.

No one is getting all oracular here. I have my opinion, you have yours. "Probably" is not a "confident assertion" at all.

"Probably" is a quantitative assertion. If you want to be properly vague, use "possibly".

I think AlbertCory was saying that to assign specific probabilities, as the Drake equation does, is BS.

To say "there's probably life out there", in a vague, speculative sense of probably, is a different sort of thing.

Thanks, right.

It's like saying "the probability of a market crash is 0.01."

There is no number you can put on it.

Even then, he is putting bounds on probabilities. Specifically, that the chance that life arises around a random star is >> 1/(number of stars in visible universe).

The argument is really just trying to elevate vague and incorrect intuition beyond anywhere it has any right to be. It comes from not understanding just how serious a bias can be introduced by observer selection.

First, I was gently poking fun, not seriously criticizing. But since this is apparently that sort of conversation: no.

> There is no such thing as a "probability" of any of those terms...

There is a probability for each term in the Drake equation, but I assume you mean that we don't have a way of judging what it is. I agree.

> That said, there probably are intelligent civilizations somewhere in the universe

"there probably are" does not equal "I dunno, maybe" -- I'd argue it's a direct summarization of the nonsense that is the Drake equation, but just semantically it at least means greater than 0.5 probability, which we clearly can't establish.

>There is no such thing as a "probability" of any of those terms, in the way we normally understand probability, i.e. dice or card games.

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

If you mean the world is divided into frequentists and Bayesians, you're right. I think most people who throw around the Drake Equation are frequentists.

On this one, I'm neither. There is no way to put a number on it.

It seems to have been pretty successful at its intended purpose:

The equation was formulated in 1961 not for purposes of quantifying the number of civilizations, but as a way to stimulate scientific dialogue at the first scientific meeting on the search for extraterrestrial intelligence (SETI)

Even today, it’s not the end all be all tool but there are still serious papers being published that refer to it.

> there probably are intelligent civilizations somewhere in the universe

How can you even say that?

I type and the characters come out.

It's an opinion. You can have your own.

>Well said. The "Drake Equation" is complete and utter garbage. There is no such thing as a "probability" of any of those terms, in the way we normally understand probability, i.e. dice or card games.

I'd say that any result you try to pull from the Drake Equation is "complete and utter garbage."

The value in the Drake Equation isn't in any calculation we might do with it. Rather, it's a way to categorize both the variables necessary for the development of technological civilizations, and our ignorance as to both the value of such variables and variables we don't even know exist (which can be added as we learn more).

In fact, the Drake equation[0] was never intended to be useful as an equation:

   The equation was formulated in 1961 by Frank Drake, not for purposes of 
   quantifying the number of civilizations, but as a way to stimulate scientific 
   dialogue at the first scientific meeting on the search for extraterrestrial 
   intelligence (SETI)

[0] https://en.wikipedia.org/wiki/Drake_equation

Your calculation is likely wrong. See, you can build a laser in a spectrum not generally emitted by stars, and point directly to another star system. You dont need much power, yes, perhaps some terawatts, but it can be pulsed, total energy would be low.

And getting a low power signal doesnt means its impossible to detect, but that the bandwidth is short. Instead of 200 bits per second, you would get 1 bit per hour. But that´s enough.

That's really not much of a rebuttal.

The simple truth is, if a clone of the human race existed on proxima centauri, long enough for signals to propagate, we'd have no idea and that's an extremely optimistic scenario.

Why would a race be casually pointing terawatts of laser at every star around them. Tremendous waste of resources for essentially no gain within human scale lifetimes.

All beams will broaden and distort though. No matter the wavelength or pulsing, anything made by not-a-pulsar is likely to get garbled and shifted beyond comprehension. You can fix this with some really really big fancy lenses, maaaaaybe. But you'd have to know the distance of the point you're trying to image the laser onto first. Collimated light over those distances just isn't ever going to stay that way.

OP is saying how would we detect a civilisation unknown to us, and for who we are also unknown. A system such as you describe would only be practical for a civilisation trying to communicate with a known counter-party. Otherwise which stars do you target your laser on, and for how long?

Let’s say the Centaurans developed advanced technology 100 million years ago, would they beam this signal at our star for all 50 million years until we happened to evolve? Why our star system, why not any of millions of others? As this system is so directional each detector could only monitor one star at a time. How many do we build, and which stars do we point them at, and for how long?

For direct communication with an established target this would be fine, but as a broadcast system it’s way too target specific to work at galactic scale.

Just want to point out the currently known composition of the universe:

~5% matter. The stuff you and I are made of. Most of this is H and He though, bit of Li in there too. And it's all mostly burning in a star. Like, 99.99999% of it.

~25% dark matter. We know little about it. We do know that it falls down though and doesn't like to touch other stuff. But that's kinda it.

~70% dark energy. We know pretty much nothing about it. It falls ... up (?!).

Considering that the stuff we're all made of is the 5%, and even then most of that is down in a deep gravity well and on fire, I think we may need to look at the other 95% of the stuff out there too if we want to find friends.

Dark matter appears to be a very weakly interacting fluid, completely unsuitable for developing any kind of structure at the local level. Dark energy is, well, energy. Also incapable of local structure. I’m afraid conventional matter it is.

> it's just some aliens that are not pointing an antenna straight at us

Even worse, don’t modern digital signals look similar to noise, are usually encrypted, use much higher frequencies, and have less gap between bands? Good luck trying to spot a cellphone signal from Proxima Centauri (my phone has trouble getting signal at home!)

Many analogue broadcast signals have been replaced by digital, and broadcast as a technology is in its twilight years (although my parents do still get TV and listen to the radio!)

So signals might only be detectable in nascient technology civilisations - they then move on to undetectable signals?

Yup, information theory says that Gaussian signalling is optimal in term of bandwidth utilization in many cases. And we've spent decades trying to achieve this by various techniques in coding and modulation. And yes encryption/secrecy adds another layer of randomization. I believe a radio signal eavesdropped from a US military base is not much different than some random noise.

That being said, the question is to tell if a signal is man-made or not, which is much easier than to discover what information it conveys. Based on modulation carrier frequency and the transmit (on/off) pattern you can still very much tell if that signal is man-made or not.

The paradox is that the our solar system should have been visited. But so we haven't been able to detect anything.

It's much closer than the centauri system.

>The paradox is that the our solar system should have been visited. But so we haven't been able to detect anything.

>It's much closer than the centauri system.

That assumes there have been other(s) technological civilizations in our galaxy. Which isn't necessarily the case. We may well (or not) be one of the first space faring (for some values of that term) civilizations in our (part of) the galaxy.

A recent discussion about this can be found here[0] (PBS Space Time episode from 9 November 2022).

[0] https://www.youtube.com/watch?v=uTrFAY3LUNw

That is a possible solution for the paradox. The comment I replied to was talking about a different star system.

Flip the point of view for a moment - what's the absolute best way we could try to say "hey, we're here" to the universe in the hope of detection from far away civilizations?

I like the idea of Contact's Prime Number signal but how would you do this on a large scale? I was thinking Dyson Sphere a star as a huge Flag Semaphore to send the message, but then what rate do you flip bits to make it optimal for the receiving end far far away?

Now flip it back... would the current SETI pick up this signal if this were happening somewhere close in the Milky Way? If not, then yeah maybe SETI should be wound down :(

And what if we broadcast and whomever hears us wants us dead? Maybe it’s so quiet because to be loud results in death of the entire civilization.

If you are an advanced civ that fears new competition, you don't wait for hearing random radio transmissions.

You build replicating factories to build relativistic kill missiles and preemptively destroy all the planets in the galaxy.

Given that planets currently exist in our galaxy, either no advanced civilizations exist that fear competition or... apparently not?

Yep, I think it makes little sense for them to fear competition (if they exist). I suspect that intergalactic warfare is impractical and there is nothing to gain.

Reasons being: There is no stealth in space, there is no FTL, resources are abundant everywhere, destroying planets/megastructures is rather easy.

Hmm maybe. If anything, I don’t think it would be killing for competition. Killing a planet is “easy” if you can expend the energy. The risk of accidentally offending another civilization is much easier, offending them to the point of them wanting to wipe you out isn’t that much harder (see Earth History). Thus, it almost makes sense to just wipe out any civilization preemptively.

If an advanced civilisation was so advanced that it could hop around the galaxy on a whim, why would they come here to destroy us? It's not like the would possibly need any of our resources

It's not difficult to think of realistic (if not necessarily sensible) motivations if we try.

Maybe they're religious fanatics and there's no we can satisfy their tenants so.. Or maybe the local alien boss doesn't want to deal with their EPA equivalent and decides to deal with the problem before anyone notices. Or the Grand Poobah just acts whimsically and everyone must do as it says.

>Now flip it back... would the current SETI pick up this signal if this were happening somewhere close in the Milky Way? If not, then yeah maybe SETI should be wound down :(

The problem is the other way around. An unintentional or broad signal is extremely unlikely to be picked up. However, it would be trivial to send out an intentional signal to a candidate planet that can be picked up without the locals having an equivalent of SETI, and there's no rational* reason not to do it if you decided to send. SETI's useless either way.

* The moment you get to irrational aliens, is the moment you need to consider the option they exterminate anyone who replies, so SETI for listening to irrational signals is also a bad idea.

We might be able to see their drive plumes if they use some of the theoretical high performance reactive drives we know of on big crafy. Eq. various fusion plasma drives, Orion, Nuclear Salt Water Rocket release so ludicrous ammounts of energy that they might be visible over interstellar distances, especially if pointed in our general direction when running.

Massive solar sails as well would be very very big and very reflective.

> release so ludicrous ammounts of energy that they might be visible over interstellar distances

Ludicrous at human scales, but utterly insignificant compared to the output of even the smallest stars. Bear in mind that small stars are hard to detect more than a few dozens of light years away. Well you can detect them, but they are hard to range. It’s difficult to pick out nearby stars in the dense stellar background as being nearby. Take a look at the deep field images to get an idea of the clutter. A drive plume would just be utterly lost in all that background noise.

People imagine the sky as being inky black with a few bright objects to look for, and that even a drive plume light years away would stand out against the blackness, but that’s not how it is at all. The sky is full of distant clutter that makes picking out particular objects extremely difficult. And if your plume is in a system anywhere near a star, forget it. You’d have to know exactly what you’re looking for and make extremely precise long term observations to have any chance whatsoever.

Space is large. If you don't look in some direction, you don't see what's there. Big, or not.

Let's take the Orion engine. It keeps blasting away thermonuclear bombs. Are we looking straight at it? Why? Ah, maybe because they are in the neighborhood of a star. But then we are also looking at the star, which is trillions or quadrillions times brighter. Same for fusion plasma drives or anything else.

If the Orion engine is traveling between stars, we'd have a chance, but then how would we know where to look ?

If they're approaching us, they might be using the Orion engine to decelerate on long final.

Here's an amusing thing. Even if they were within Jupiter's orbit and decelerating and pointing straight at us, we wouldn't be able to detect them.

Nuclear bombs produce a fireball here on Earth. In the first microsecond when the explosion actually happens, they produce gamma rays and X rays. The atmosphere is opaque to those rays, which is to say, it absorbs them. Their energy is enormous, so the part of the atmosphere that absorbs them becomes superheated. That's what the fireball is. That's what produces the bright flash.

In space it goes without saying that there's no air. The fireball doesn't form. The explosion itself will produce gamma rays and x-rays (the blackbody radiation at 100 million degrees).

They will go in all directions, including towards Earth. But once they get to Earth, they'll be absorbed by the upper layers of the atmosphere.

As seen from Earth a nuclear explosion happening somewhere in the vacuum of the Solar system will be invisible.

But we do have gamma ray and X-ray telescopes. On satellites in space. Only not that many of them. If they don't look in the right direction exactly when the explosion happens, they see nothing. Another difference between a nuke detonated in space and one on Earth is that on Earth you can see things (like the mushroom cloud) for hours after the explosion. In space, the explosion happens, and one second later nothing. There is absolutely nothing happening one second after the nuke goes boom. So our telescope needs to point exactly in the right direction, exactly at the right time to have a chance to see something.

Rocket propulsion benefits from directional discharge.

Project Orion lead to the investigation prospects for nuclear shaped charges, in which the blast is highly directional. The concept was developed further in the colourfully-named "Casaba-Howitzer":

<https://en.wikipedia.org/wiki/Casaba-Howitzer>

Odds are reasonably good that any space-faring civilisation would create similarly directional reaction devices for space travel if they were utilising explosive nuclear fission.

... Though, thinking this through, the shaped charge would probably be directed at rather than away from the pusher-plate in an Orion design. There might of course be other directed reaction-engine designs. They'd be generating similar-magnitude energy signatures, however, and again, in a deceleration phase, would be directed toward us.

And whilst naked eye observations would of course be difficult, that's really not what we're talking about, but instead sensors which could pick up specific radiation. And I strongly suspect that sudden-onset x-ray and gamma-ray sources within the Solar System would in fact be detectable.

(It was devices that were looking for such bursts on and near Earth from Earth orbit via the Vela satellites as part of nuclear war early-warning systems which first detected astronomical gamma-ray bursts.)

Given that small meteor impacts on the Moon are quite detectable from Earth, nukes near Jupiter should be readily detectable. A 2013 lunar meteor impact from what's thought to have been a 40kg mass traveling at 25 km/s was visible to the naked eye as a 4th magnitude event. That's about 5 tons TNT equivalent.

<https://science.nasa.gov/science-news/science-at-nasa/2013/1...>

The Orion concept was based on 0.15 kt (150 ton TNT) charges, or 30x larger than the meteor impact discussed here, exploding repeated at a rate of 1 Hz. That would be a pretty significant signature.

Cool link about the Casaba howitzer. Thank you.

A relatively recent discovery of mine, and ... interesting to contemplate.

Nowadays there are satellites with omnidirectional gamma-ray detectors to detect gamma-ray bursts, so that optical and other telescopes can be turned on them.

Its a question of if these would be sensitive enough, but I think we can only expect the detection satellites to improve over time.

I'm not convinced we would be able to view any of those. The only exoplanets we've been able to directly image are huge gas giants that emit a ton of infrared light. A massive solar sail will likely be much smaller and emit a lot less light.

See here for a list with images to understand the level of detail we're getting: https://en.wikipedia.org/wiki/List_of_directly_imaged_exopla...

I'd think you can easily have galaxy-hopping civilizations that would still be orders of magnitude less noticeable than these planets.

And most exoplanets are observed indirectly: doppler effects on the central star's light, occlusions of the central star's light measured through variances in the observed light from that star, and in a few cases absorption spectra of the exoplanet's atmosphere, again, by occlusion of the central star.

All occlusion observations require orbital planes which are aligned such that they're viewable from our solar system, further reducing observable planets.

And yes, there are a very few exoplanets which have been directly observed, but those are very much the exception.

Detecting any signals or emissions of a technological alien race would be exceedingly difficult even at near range.

Though mention of possible rocket systems and the fact that these are directional suggests that one of the more likely signals to be detected might be a decelerating ship, or ships, approaching the Solar System directly.

>How would we know they are there?

How about pointing an optical telescope at them to take a look?

Direct Multipixel Imaging and Spectroscopy of an Exoplanet with a Solar Gravity Lens Mission

https://www.youtube.com/watch?v=NQFqDKRAROI

https://arxiv.org/abs/1802.08421

Which is absolutely amazing and I desperately hope it happens in my lifetime.

But we haven't done it yet, so it's hardly relevant to the "why haven't we seen any?" part of the "Fermi" "Paradox".

How does the interstellar medium interact with communication signals like radio waves?

There are other methods of detection and signatures that, assuming similar civilizations to our own also follow similar energy use growth patterns, should be detectable at very far distances in other ranges of wavelength.