[nogridbag]

Please excuse my complete ignorance - this is something that continues to perplex me no matter how many times I hear it explained and I may sound like a complete idiot here. Can someone explain how this would work in layman's terms?

Let's say that life on another planet 150 million billion miles away wanted to send us a message. So that's 25,000 light years away. They send us one single 1 minute duration message at t=0 and that message travels at the speed of light as an example. And let's just say t=25000 happens to be today. Does that mean that we have a one minute window to intercept part of the message otherwise it's lost?

If so, let's say the life on that planet was persistent and continued to send messages continuously directly to us and no other planets for 500 years. Then I would assume we have a 500 year period to play with. If they started sending signals let's say 10,000 years ago, it seems we have absolutely no chance to receive the message unless this program is operational for the next 15,000 years or so. But our odds of receiving an intelligent signal is vastly higher simply due to the number of planets in our galaxy. Is my understanding remotely correct?

[jessriedel]

Traditionally, SETI is not specialized to messages intentionally crafted for us (or, in fact, necessarily crafted for anyone). It's just a search for radio transmissions that are unlikely to have been created by natural sources using various statistical techniques.

Even for intentional messages, it's certainly conceivable that another civilization with sufficient power simply transmits the message in all direction continuously.

[scrumper]

Even for intentional messages, it's certainly conceivable that another civilization with sufficient power simply transmits the message in all direction continuously.

I think that scenario would imply a relative scarcity of intelligent life: after all, if a space-faring civilization needs to transmit powerful, omnidirectional signals in the blind, it must be pretty lonely. Maybe, just maybe, the fact that we're not seeing much is actually indicative of a fairly rich sea of life in the galaxy, communicating point-to-point with each other. Not so much to randomly overhear in that world.

[RALaBarge]

Personally, I am willing to bet that all intelligent life will end up being quite lonely. Unless we find ways to bend space, there is little chance of us ever getting out of our local solar system, let alone galaxy or the local galactic cluster.

I am a big fan of science and science fiction, do not get me wrong. I do feel that technology evolves and will surprise even the most tech savvy, but to violate faster than light travel seems more far fetched than any theory out there to me.

[bhc3]

I'm expecting somewhere way down the road, scientists will unlock the secrets to prolonging our lives for nearly forever. They'll stop the telomeres from degrading, come up with ways to address cancer quickly and effectively, etc.

At the same time, space travel technology will also have been advanced, allowing for self-sustaining spacecraft that offer protection against the harsh environment of outer space.

At that point, you can imagine that a hearty band of explorers will undertake a mission of leaving our solar system and exploring way beyond our world. Who knows how far they could get.

These are the sorts of advances I can imagine becoming real many generations from now. I wouldn't underestimate our capacity to make it happen.

[S4M]

If you want to speculate, how about the possibility of uploading people's mind to a robot? Let's say that we are able to send outer space a bunch of self replicating robots (they can travel to various planets and build a copy of themselves using the raw materials they can find there). After some time, we have lots of robots at many points in space, then you make a copy of your mind and send it at the speed of light to those robots. Because of the self replicating nature of the thing, we can have lots of robots so anybody who wants to can upload his/her mind to one of them. That way we have a non risky an relatively cheap way to explore the galaxy - it would just take couple of thousands of years to have everything set up.

[XorNot]

Still a relatively slow way to get around. We have some co-factor here for "median time between alien contacts" which might be thousands of years. In which case they came, they saw, and the people who saw them remembered it as folk tales.

[specialist]

> stop the telomeres from degrading

Both our bodies and minds need to hardened to endure deep space.

10,000 years between stars is a long time to be playing solitaire. The voyagers may need to sleep, hibernate, slow down their metabolism, alter their perception of time, or something, to make the journey bearable.

[chongli]

10,000 years between stars is a long time to be playing solitaire.

That's assuming our space ships resemble a Saturn V or the Space Shuttle. Why limit ourselves to that? How about a colony ship the size of New York City, built in orbit, housing millions of people? At that point we don't even need to live thousands of years; we'd have a generational ship.

[aswanson]

I'm skeptical as well...but...something at the beginning of the universe caused it to expand far faster than the speed of light...something, some unknown force is causing it's continuous expansion. There are many deep, fundamental questions we don't know about the universe. FTL travel may be possible, but I wouldn't bet on it.

[RALaBarge]

I present to you the answer that I most prefer for that question: http://www.multivax.com/last_question.html

[arglebarnacle]

I'm hoping that even though we don't seem to be so lucky, there are other solar systems out there with multiple worlds where intelligent life developed roughly in parallel.

At least someone in the universe would be living out my space opera fantasies!

[eridius]

SETI isn't just looking for spacefaring civilizations. It's looking for any form of intelligent life. Take us, for example. We've been broadcasting radio signals into space for many years now (basically as a side-effect of broadcasting them to ourselves). Given sufficient time for the radio waves to travel, an alien equivalent of SETI would be expected to recognize them as non-natural.

[0xffff2]

That's not really true (for better or worse). Radio signals attenuate with distance. In the case of omnidirectional signals expanding in a sphere, they attenuate at a rate of 1/r^3. As a result, virtually all of our radio signals intended for terrestrial consumption are basically noise before they even leave the solar system.

[eridius]

The wikipedia page on Path Loss[1] suggests that radio and antenna engineers typically model path loss (in decibels) using the formula

  L = 20 * log10(4 * pi * d / λ)

(where λ is the wavelength and d is the distance between the transmitter and receiver expressed in the same units as the wavelength)

If I'm doing the math right[2], this says that the path loss over 1 lightyear for an 80MHz transmission (which seems to be in the analog TV range?) is something like 330 decibels. Now I'm not sure how strong TV transmissions are, but I'm guessing it's not that strong.

Darn. Seems Futurama and every other piece of mass media that claims that aliens are watching our TV from 80 years ago is wrong.

[1]: https://en.wikipedia.org/wiki/Path_loss

[2] Which is to say, if I'm typing it into Wolfram Alpha correctly.

[joe_the_user]

Maybe, just maybe, the fact that we're not seeing much is actually indicative of a fairly rich sea of life in the galaxy, communicating point-to-point with each other.

Except for the small question of how those many civilizations would have found each other in the first place.

[scrumper]

True. But nearer the galactic core where stars are so much closer together? SETI might have a much higher hit rate there. We are in kind of a backwater out here. It's easy to forget about the big city when you're out in the sticks.

It's not very likely though. Occam's Razor alone suggests that the reason for loneliness is that there aren't a plethora of civilizations all talking to each other over narrowband channels. And we'd be very unlikely to pick up the omnidirectional beacon of a lonely civilization way out here anyway.

[adamtj]

Or, perhaps planets able to support civilization are relatively rare, and the few civilizations dumb enough to loudly broadcast their presence soon get exterminated when their planets are colonized.

[MattConfluence]

Sounds about right. Frank Drake created his famous equation [1] to try to quantify the probability of detecting ETI, but you do need to make guesses at some of the inputs.

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

[nsajko]

Relevant xkcd: http://www.xkcd.com/384/

[oh_sigh]

Due to the expansion of spacetime during those 25,000 years, we would have slightly longer than 1 minute to receive the signal, but it would be so insignificant that you might as well call it a minute.