Fermi's Paradox II: What's Blocking Galactic Civilization? Or Are We Just Blind To It?

This puzzle, known as the Fermi Paradox, has burned up a lot of cerebrum cycles when scientists tried to reconcile the lack of company with the expectation that there are many advanced alien societies.

One possible explanation is that interstellar travel is just too costly. Consider how expensive it would be for us to populate another star system. Imagine sending a small rocket to Alpha Centauri, one that’s the size of the Mayflower (180 tons, with 102 pilgrims on board). Your intention is to get this modest interstellar ark to our nearest stellar neighbor in 50 years, which requires about 150 billion billion joules of energy.

No one’s sure what aliens pay for energy, but here on Earth the going rate is about ten cents a kilowatt-hour. So the transportation bill per pilgrim would be $40 billion. That’s a lot of moolah, a lot more than it takes to buy each emigrant a few thousand six-bedroom palaces and set him up for life. The fact that the trip is costly, in whatever currency, is reason enough to deter any alien society from trying to settle distant real estate. With far less expenditure, the extraterrestrials could pursue the good life at home.

Of course, if energy costs can be brought way down, for example with fusion or matter-antimatter technology, or by capturing more of the radiation spewed into space by the home star, this explanation might not hold water.

But even if the aliens can afford colonization, maybe they haven’t got the stamina to see it through. Subduing the Galaxy takes more than sending a ship full of restless nomads to the next star. The nomads have to settle that star, and then spawn pilgrims of their own. And those émigrés have to produce yet more settlers. And so on. If each and every colony eventually founds two daughter settlements (a pretty decent accomplishment), then 38 generations of colonists are required to bring the entire Galaxy under control. Even the Polynesians, who swept across the western Pacific domesticating one island after another, didn’t manage this. Maybe the aliens can’t do it either.

On the other hand, if a few of them remain committed to expansion, their project might still succeed – just more slowly.

Some researchers suggest that the Galaxy is colonized, but we just don’t notice. Arthur C. Clarke pointed out that truly advanced engineering projects would be indistinguishable from magic. Perhaps the evidence of alien presence is so beyond us that we simply don’t recognize it (somewhat like mice in The Louvre checking out the Mona Lisa). Another thought is that the aliens find Earth an interesting nature park, and have arranged matters so that, while they can observe us, we can’t observe them. The idea that we may be some aliens’ high-tech ecological exhibit is called the "zoo hypothesis."

These explanations, and a bushel-basket more, have been proffered to deal with the Fermi Paradox. Any of them might be true. Nonetheless, some scientists find them too contrived, too unlikely to work in every case. Will all the aliens find colonization too costly? Will they all run out of empirical steam? Are we so special that someone has really gone to the trouble to put us behind invisible bars?

Next time, we’ll consider some of the more obvious – if more disquieting – resolutions of the Fermi Paradox.

Hasn't found anything so far, you mean. But we've only just started to look; if we'd found something by now, it would surely mean that extraterrestrial civilizations are overwhelmingly abundant throughout the galaxy.

A planet like Earth we'd be able to see clear across the galaxy, if we happened to look in the right direction.

If you are a physicist then how powerful a signal on how big an antenna would it take for us to hear the signal from the nearest star. Why do you think we would be able to see an earth sized planet near a Sol sized sun halfway across the galaxy? We are only just now able to see jupiter sized planets and I think those are only a few hundred light years away.

Nice post. I understood most of it.

Great! Explain it to the rest of us; the last three paragraphs made zero sense to me.

I have a question for you. Has someone already come up with the theorem for time differential (I think that is the proper word) on a Galactic scale?

What I mean is the different states of time (mass(+/-)+/-speed(+/-)=time) in relationship to different regions and locations in the Milky Way Galaxy.

There is. It's called gravitational time dilation. Time moves more slowly by a very tiny fraction near the center of the galaxy than it does in the outskirts.

What you are looking for, however, is a big difference in the flow of time, but we can rule that out. For one thing, orbits that pass through such different regions would be unstable, and this is not observed. For another thing, a large difference in time rates (as measured by a difference in the relative speed of light from one region to the other) would result in a large index of refraction, and this also is not observed.

We do see a small index of refraction around gravitating bodies, caused by the gravitational time dilation I mentioned. This was first measured by Eddington, when he measured the displacements of star positions during an eclipse in 1919. We also observe it at cosmological distances, with the phenomenon of gravitational lensing.

Physicist I think your post #82 also answered biblewonk. To whit, if you stationed observatories out further than Pluto you can use our star Sol as a gravitational lense and create a really, really powerful telescope.

What is really needed for a civilization to move to the stars are barely sub-light ships and a way to absorb the momentum from your starship at each end of your trip so that it can be re-used. A large ring around a solar system like a particle accelerator that can shoot your ships to another system and at that other system have a ring ready to catch your ships.

A group called "The Living Universe Foundation" got started from a novel called "The Millenium Project" or something like that by a guy named Savage. In his book he has an 8 step program to seeding the galaxy with humans.

If you are a physicist then how powerful a signal on how big an antenna would it take for us to hear the signal from the nearest star.

There are a number of unspecified parameters there, such as the frequency, bandwidth and isotropy of the signal, but according to my calculation, Arecibo would be able to hear a 100 megawatt radio signal on Alpha Centauri (if it could point in that direction, which it can't). The Earth pumps out much more than 100 megawatts of RF.

Why do you think we would be able to see an earth sized planet near a Sol sized sun halfway across the galaxy? We are only just now able to see jupiter sized planets and I think those are only a few hundred light years away.

There are lots of ways to "see" across the galaxy. We were talking about radio communications, not about optical imaging.

That lens would be far too weak, I'm afraid. It's big, sure, but its power is poor. Remote galaxies are a much better bet, but they're no help looking at galactic sources.

One way to image objects clear across the galaxy would be with an x-ray interferometer such as MAXIM. From their website:

X-ray interferometry has the potential to resolve the event horizon of a supermassive black hole in the nucleus of a nearby galaxy and at the center of our galaxy. This is equivalent to

resolving a feature the size of a dinner plate on the surface of the sun,
observing a 100 km emission knot on the surface of Alpha Centauri,
imaging the disk of a star in the Magellanic CLouds,
mapping in detail the accretion disk at the center of the Milky Way,
directly measuring the parallax of a star in the Virgo Cluster of galaxies, or
resolving one-tenth of a light year at the far extent of the visible Universe.

The only way out is to have a ship that does not carry its own fuel, such as a Bussard ramjet. When we come up with a proof-of-principle for such a device, we can start to talk about it.

You've made me think. It's possible that such vessels can only travel in regions where there is a sufficient density of ions or whatever to provide them with fuel. It is also possible that there are great "voids" where there is insufficient fuel, and thus no interstellar travel. Perhaps ramjets can only chug around near the central disk of the galaxy, and out here in the arms it's impossible to get around. So ... if we live in such a void, then of course we see no sign of alien signals in our neighhborhood. Perhaps you've solved the Fermi Paradox for me.

Does the earth emit X-rays?

I remember reading about a nifty deep space probe that would travel 25-50 billion miles from earth. They were going to use a 1 meter parabolic mirror with a laser at the focus to communicate with the earth. I wonder what the numbers regarding wattage per distance at a given data rate show. I wonder how much better this is than radio frequency.

Does the earth emit X-rays?

More precisely, it reflects them. I don't know what the Earth's albedo is in that frequency range, however, so I can't tell you how far away we'd be able to see an Earthlike planet with MAXIM. But what it shows is that with interferometry, you don't need a gigantic telescope to get awesome resolving power.

Yes, the Kecks were set up for just that purpose. Considering what kind of intruments it takes just to detect an entire earth sized planet, why do you suppose people think that we could aim some crummy little 100 meter radio scope at such a planet and hope to hear their infant monitors and TV communications, or even a 100 meter radio telescope aimed at us tranmitting 10,000 watts?

Considering what kind of intruments it takes just to detect an entire earth sized planet, why do you suppose people think that we could aim some crummy little 100 meter radio scope at such a planet and hope to hear their infant monitors and TV communications, or even a 100 meter radio telescope aimed at us tranmitting 10,000 watts?

If we used several as an interferometer and listened in the right bands, yes. We won't be able to disentagle the cacophony, of course, but we'll see that it's there.

From where I'm sitting in my office, I can hear a crowd cheering at Franklin Field here at Penn, when there's a game going on. That's true even though I can't possibly hear somebody whispering there from two seats away.

Detecting a signal, and resolving a signal are two different issues.

The ability to resolve a signal, such as imaging an extrasolar planet, is proportional to the effective linear dimension (e.g., diameter) of the collecting device. (Do a web search on "Dawes limit" for more details.)

Weak signal detection, on the other hand, depends on the sensitivity of the detector and how much signal you can feed into it. The quantity of signal you can collect is a function of the effective surface area of the "collector," which varies with the SQUARE of the collector's linear dimension, e.g., diameter.

Thus, our ability to detect weak signals improves much faster than our ability to resolve weak signals, as the size of our instruments increase.

That's why it is easier to "detect" a signal from a distant star or planet than it is to "see" distant stars or planets. It is far less demanding.

As always, I defer to the resident FR physics/astronomy factotums if I've mucked up any of the details in my simplified explanantion.

Thus, our ability to detect weak signals improves much faster than our ability to resolve weak signals, as the size of our instruments increase.

I'm an ATM so I know the difference between resolution and light gathering ability thanks.

So do the math and figure out what the requirements for LGM to send a signal that we could detect give that 8 watts is detectable from a 10ish foot dish from 5 billion miles away by a 300 foot dish. Detectable in the sense that we can actually decode data, though at a low data rate. We need more than just a carrier since every single object out there is transmitting a carrier. We need intelligence modulated onto it.

I'm going to defer this one to "RadioAstronomer" since it is right up his alley. He will know what the limit on detector sensitivity is, etc. much better than I do.

I think he will disagree with your premise about having to to detect more than a carrier.

What's an ATM? I assume you don't dispense cash or switch packets. ;-)

The goal of SETI is not to decode the signals, it's just to see whether they are there.

There simply aren't any natural narrowband radio sources in the relevant channels. In the bands that we use for communications, Earth is at least 1000 times brighter than any natural source in the galaxy. You can't hear the sun with a transistor radio, even though it subtends an area far larger with respect to your radio than any antenna you're likely to listen to.

What's an ATM? I assume you don't dispense cash or switch packets. ;-)

Amature Telescope Maker, but I quit before it was complete.

The goal of SETI is not to decode the signals, it's just to see whether they are there.

In order for it to be a genuine LGM signal it has to be more than just a carrier, it has to have some non natural occuring signal. Carl Sagan's favorite LGM beacon was a series of prime numbers of pulses which could be AM'd, FM'd or PSK'd or even on off modulated onto a carrier.

So you believe that at 107.9 MHZ, from 50 light years away, the earth, or sources on the earth, emits more signal than the sun? No Way.

So you believe that at 107.9 MHZ, from 50 light years away, the earth, or sources on the earth, emits more signal than the sun? No Way.

I can't say whether that's a good frequency to use, but I'm certain there are frequencies where the Earth would be brighter at 50 light years than the sun at 1 A.U. But even if there weren't, you could still pick out the Earth.

The two-sided coin is that you have to pick the right frequencies to see the signal in the first place. That introduces the difficulty of looking at millions of channels simultaneously, but it also allows you to rule out natural sources. A natural source will cover a very broad band, so many contiguous channels will be active simultaneously. The sun will look nothing like the Earth.

Finding such a signal is not like trying to find a needle in a haystack; it's more like trying to find a needle in a swimming pool full of pudding. The odds may be small that you find it in any given mouthful, but in the right mouthful its presence will be unmistakable.

This is where you are completely wrong. The scintillation of the interstellar medium will pretty much "chew" up any modulation (other than on/off), so SETI is doing just that. Looking foe an extremely narrowband CW signal (no information or modulation needed). And when I say narrow, I mean in the .8 Hz range. Just the fact a .8 Hz narrowband signal exists, denotes an artificially generated signal.

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