Hacker News Board Hacker News, browsed like a classic message board
Live data from Hacker News

The Fermi Paradox

waitbutwhy.com

Post reply on HN
Print view View original

51–60 of 168 posts

Re: The Fermi Paradox

#51

Post by jcfrei »

maaku wrote: post #11
equil wrote: post #9

I've always wondered at what distance it would be impossible for current technology to detect life on earth.

A very short distance. IIRC something like 20 or so light years, but I'm having trouble finding the citation. That's for radio waves (and the number is getting smaller as the technology is getting better -- EM radiation blasted into the universe is considered waste by hardware engineers). For direct detection of life, that's still an open question. I remember a poster done by a grad student who looked at whether life…

Would be great if you could find the citation, because this seems like the best explanation so far. There are only about 54 stars which at some point had a distance of less than 17 light years (that we know of): http://en.wikipedia.org/wiki/List_of_nearest_stars_and_brown... . Assuming that some stellar source of energy is required for life we can already remove 9 stars from that list because they only consist of brown dwarfs. Doing the math with the conservative estimates in the blog post, this puts the chance of detecting signs of life at a pretty low percentage. Assuming that 22% of these stars have at least one planet with earth like conditions and a 1% chance that some form of life develops on these planets, we arrive at the low chance of 46x0.22x0.01, ie. ~10% of finding any life in our vicinity.

Re: The Fermi Paradox

#52

Post by epaulson »

ScottBurson wrote: post #17

How about this possibility: the Great Filter is now! We entered it, perhaps, when we developed sufficient collective nuclear weapons capability to wipe out most or all human life; we won't be out of it until the major threats our civilization poses to the biosphere have been resolved.

Even if we wipe ourselves out tomorrow, that still means that a civilization was able to broadcast its existence for about 80 years. If the Great Filter is now or ahead of us, that at least implies that pre-filter civilizations can be advanced enough to signal their existence.

At some point our signals will be too weak for anyone to pick up or distinguish from cosmic background noise, which probably limits the radius of the sphere of stars that can hear us, but anyone inside that sphere gets at least 80 years to be listening to potentially hear us.

So, even if civilizations only last for say 100 years on average, and you can only hear other civilizations within N light years, if N is big enough eventually someone should hear someone, and if N is really big enough, somewhere in that 100 years everyone should hear at least one other someone.

Re: The Fermi Paradox

#53

Post by venomsnake »

gpsx wrote: post #6

This article does a good job of covering a lot of different ideas on this paradox. I have a problem with the idea of "big numbers" in it though, like where it implies 500 billion billion is nearly infinite, or that one-in-a-billion is a freak occurrence. There is a saying that if enough monkeys pound on a keyboard, one will type Hamlet. This would take a lot of monkeys. If we take the odds of hitting one character to…

Depending of whether life is a fluke or normal outcome. Until we know how life on Earth began, we will have no idea for what to look for in the sky.

http://www.scientificamerican.com/article/a-new-physics-theo...

This theory could be quite important or wrong. Or both. But it gives very interesting definition of what life is.

Re: The Fermi Paradox

#54

Post by darkmighty »

maaku wrote: post #10

The "we are the first" argument seemed less convincing than it could have been. A better argument from that perspective is the observation that Type III civilizations expand at an alarming rate. Whatever the motivation for expansion[1], basic simulations and back of the envelope calculations show that even with technology we could imagine building in the not so distant future, humans could expand into the cosmos at >…

But the Milky Way is only 50,000 light years in radius (~ average distance between stars?). That's not much at all on a evolutionary scale. So your argument explains why we're not currently observing an alien civilization (i.e. they would expand so close to light speed we would likely either be conquered or observing nothing -- so observing nothing is a consistent scenario). But it does not explain why no Type III civilizations (continuing with the assumption that civilizations all quickly go on to become Type III) has risen at all in the past hundreds of millions of years.

You can close that gap that via an anthropic principle by saying that if some civilization were colonizing our solar system, life on Earth would be extinguished/not possible.

Re: The Fermi Paradox

#55

Post by sampo »

jacquesm wrote: post #36
sampo wrote: post #21

Earlier quoted context omitted.

> until all of a sudden the most distant stars go dim Why the most distant?

Because if the events move at 0.9c the light still moves at 'c', so the furthest stars (of which there are more than nearby ones) will be the first to show you that something is up. The light from the older events will reach us before the events or the light from later events will reach us. Unless of course the whole thing originates in one of the stars nearby but the chances of that are smaller than that it will hap…

Because of 3d geometry, there are a lot more distant stars/galaxies than there are nearby ones. That much I agree.

But the most distant galaxies we see, we see them 13 billion years in the past. So this type III civilization turning off starts at the edge of our observable universe would have developed in just 700 million years after the Big Bang. (If we take 13.7 billion years as the age of the universe.) That I find quite improbable.

There is a smaller amount of nearer objects than far away objects, but the far away objects are (or we see them as) younger, and the nearby objects are older. There must be some balance between the respective volumes in near and far, and the available time it probably takes for the type III civilization to have developed?

Re: The Fermi Paradox

#56

Post by danieltillett »

maaku wrote: post #10

The "we are the first" argument seemed less convincing than it could have been. A better argument from that perspective is the observation that Type III civilizations expand at an alarming rate. Whatever the motivation for expansion[1], basic simulations and back of the envelope calculations show that even with technology we could imagine building in the not so distant future, humans could expand into the cosmos at >…

I actually posted about this very topic a few days ago here.

Once you have von Neumann machines spreading they will take over each new star system pretty much as soon as they reach it. Assuming the von Neumann machines are small and are being powered by an energy source (i.e lasers) from their origin star system they will spread through the galaxy at close to the speed of light. This spread method creates an interesting effect in that we would not see the stars go out until the von Neumann machines had almost reached us if the origin star was within our galaxy - basically the von Neumann machines would be traveling only a little behind the light front from the origin star system (this would make a good science fiction story).

If the origin star system was far enough a way, then the slightly slower speed that the von Neumann machines would travel at should allow us to see them progressing though a distance galaxy. Such a galaxy would look to us like part of the galaxy had a chunk taken out of it. It would be worth looking through the galaxy classification data set [1, 2] to see if there are any galaxies like that out there that look like this. My feeling is we won't find any as I think intellegent life is near unique within the visible universe, but it is at least a testable hypothesis.

1. http://www.galaxyzoo.org

2. http://en.wikipedia.org/wiki/Atlas_of_Peculiar_Galaxies

Re: The Fermi Paradox

#57

Post by furyofantares »

jacquesm wrote: post #49
furyofantares wrote: post #45

Earlier quoted context omitted.

Wouldn't the most likely place for it to start be around halfway between us and the most distant observable stars in whatever direction it starts, then? And then we'd see the sphere of dimming stars increasing in size, while still seeing things behind, in front, or to the sides of the sphere?

The part that we can see is so much smaller than what is out there that you'd have to resort to very leaky analogies to try to get the point across. The upper limit to how much larger the universe is compared to the observable (and not the currently observed) universe is somewhere around the 10^20 to 10^25 times larger. Those are hard numbers to grasp but large enough that the chances of an event 'x' taking place out…

Ah, right, the ones that will disappear are furthest-observed, not furthest-observable... you have to already have been observed to disappear from observance!

So, it would be an already-large and rapdily growing sphere intersecting with the sphere of observed stars.

edit: Wait, actually, I thought you were saying we CAN (theoretically) observe 10^20+ times as far as we HAVE observed. But re-reading your comment, this sounds wrong:

> The upper limit to how much larger the universe is compared to the observable

Did you mean to say "the upper limit for how much larger the observable universe is compared to the observed universe"?

Or are you actually talking about what might be outside of the observable universe? I figured starting points outside the observable universe were not a consideration for this problem, since they are causally unlinked from us and could not expand into the observable universe faster than it becomes causally unlinked from us as well.

Re: The Fermi Paradox

#58

Post by wavesum »

I think the biggest fallacy in this rationalization is when it is assumed that advanced civilizations would want to expand like yeast; consuming everything, building dyson spheres... Why would they want that much energy, and why extract it from the sun? Trying to picture myself as one of these superlifeforms, I think would like to have the sun visible... For sunbathing and sh... plants to grow etc... Maybe they have built little fusion reactors wherever they need energy?

Re: The Fermi Paradox

#59

Post by gpsx »

jacquesm wrote: post #37
gpsx wrote: post #32

Earlier quoted context omitted.

That is a good point. I agree with you. And people do have suggestions as to how life can be more probable than naively expected. But this answers the question why there is life at all as opposed to why we don't see more of it. My bias is that life is highly improbable and that we will not find any other life in our universe. But I also think it is no miracle that we exist, as I will describe. Given that I am trying…

> My bias is that life is highly improbable and that we will not find any other life in our universe. It's probable enough for you to be writing this. The only options are 0, one and many. We can rule out '0'. That leaves us to decide whether the chances of us being the only one are larger than the chances that we are one of many. Obviously the second one has more chance of being true the one where there is only one.…

The point is that there are multiple universes, according to the quantum mechanics interpretation I subscribe to. So all of your options are true: 0, 1 and many. My position, which is admittedly no better than a guess, is that most universes by a far majority have 0. By virtue of me writing this, among other things, we live in a universe with at least 1. Again, my position would be that it is unlikely we have more than 1 in our universe.

What you are saying it true under the assumption there is a single universe.

Re: The Fermi Paradox

#60

Post by coldtea »

darkmighty wrote: post #54
maaku wrote: post #10

The "we are the first" argument seemed less convincing than it could have been. A better argument from that perspective is the observation that Type III civilizations expand at an alarming rate. Whatever the motivation for expansion[1], basic simulations and back of the envelope calculations show that even with technology we could imagine building in the not so distant future, humans could expand into the cosmos at >…

But the Milky Way is only 50,000 light years in radius (~ average distance between stars?). That's not much at all on a evolutionary scale. So your argument explains why we're not currently observing an alien civilization (i.e. they would expand so close to light speed we would likely either be conquered or observing nothing -- so observing nothing is a consistent scenario). But it does not explain why no Type III ci…

>But it does not explain why no Type III civilizations (continuing with the assumption that civilizations all quickly go on to become Type III) has risen at all in the past hundreds of millions of years.

I find the assumption about Type III at best. It's like expecting Moore's law to go on forever.

Post reply on HN