# Famous argument against alien life

1. ### jaxonbridge

2
Hello, a couple decades ago, an argument was introduced in a notable physics journal of some kind, but I cannot recall the name of this argument, or the journal. I believe it might have been associated with Cambridge, but can't remember.

The argument is a logical theory as to why the likelihood of ET life is very close to zero.

I am wondering if anyone can help me remember the author/title/journal of this argument. This is as much a summary of the argument as I recall -- and I am seeking more details provided in the original article:

The idea is that if *intelligent* life were possible elsewhere in the galaxy, then we'd have been aware of it a long time ago. That intelligent life would really only have to succeed on one other occasion to propagate throughout the galaxy by this present point in time. That the speed with which our own human life sprung up was so quick, that if it were easily developed elsewhere in the universe, it would have had many opportunities in the distant past to do so, of which at least one other species would have succeeded by now if it were possible. That it has not, or that we are not aware of any, suggests that either it has never happened, or has happened in a way in which we will never be aware. And that therefore our understanding of ET life is not likely to ever change.

It is a relatively well-known argument against alien life, well-known primarily among well-read physicists and serious academics. If anyone can recall the source of this argument, I'd like to know and read the original article. Thanks.

### Staff: Mentor

The name of the argument is the Fermi paradox, and it has been discussed many times at this site and elsewhere. It wasn't an article. It was a lunch conversation between Fermi and some of his colleagues that spawned the name.

3. ### Ivan Seeking

12,529
Staff Emeritus
Of course the Fermi paradox assumes that we have never been visited. Circular logic.

4. ### jaxonbridge

2
Thanks. I knew that what I was looking for was an article in a British journal about 20 years ago, and you pointed me in the direction, and I found some Royal Astronomical Society articles from late 70's and early 80's on this topic. Not sure which one it was, but Hart, Brin, and Wesson have all written on this Fermi idea.

5. ### negitron

842
Why must this be an assumption? Why can it not be a conclusion based on the existing evidence of visitation--which is none?

6. ### jreelawg

450
Isn't there a good reason, something to do with how we broadcast communications in a way which could be heard els where in the galaxy for a very short time before switching to more efficient broadcasting which would not be heard. Maybe communications become more efficient and harder to detect.

7. ### jreelawg

450
How many planets outside our galaxy have we visited? I guess we don't exist? The Fermi "Paradox" is pretty dumb.

8. ### jreelawg

450
The idea is that intelligent life would take 5-50 million years only to colonize the galaxy. But the question is asked, why would they colonize the galaxy, and how would they, and what are the chances they could pull it off? There is a big chance that they wouldn't survive this effort so long had there planets been wrecked.

Firstly, space is filled with radiation, there are solar flares, that could wipe you out. Secondly, in space where biological entities are bombarded with radiation, things mutate much faster, and diseases evolve much faster. It is very likely that they would be wiped out by disease. Also it is possible that advanced civilizations can live on a single planet for millions of years. There is the possibility that the disaster that wrecks there planet comes unexpected leaving not enough time to design a method of colonizing space.

And of coarse the possibility that we have detected Alien intelligence and kept it secret.

9. ### ideasrule

2,322
The Fermi paradox argument makes way too many assumptions. Besides the ones others have mentioned, there's the assumption that interstellar travel is feasible, which we don't know for sure. We're also assuming the aliens want to make themselves known, but if they're trying to study the evolution of civilizations other than their own, they'd probably try not to interfere with what they're studying. Finally, if alien life is extremely common, there may be a lot of civilizations going through rapid technological development. Earth may be relatively uninteresting compared to the millions of other worlds they've monitored.

10. ### negitron

842
Was it really that difficult to follow the portion of the discussion I was addressing? It was not concerned with whether there really is life someplace out there or not, only with whether it's been here.

11. ### mgb_phys

8,952
Because if they were suficently advanced we wouldn't know they had visited?

Assuming that their visit was a scientific/survey mission that didn't want to disturb the subject - rather than to just eat our brains.

12. ### jreelawg

450
I disagree with both your assumption and Fermi's.

To address your point, Man kind has been around on earth how long? How long have we been recording history? What are the chances that this very narrow band of history we live in happens to be the time when we get visited. Then what are the chances that their visit would become known to you.

There are some obstacle, for one, if a craft that seams strange visits us, we cannot be sure it isn't a classified man made craft, maybe Russian, Chinese, whatever. If footage of an alien life form was played, there would be the possibility of a hoax. And it is doubtful that a Nation if contacted would disclose it to the rest of the world, and that means the public as well.

13. ### mgb_phys

8,952
That assumes a single craft with a crew to 'take to your leader'

A von Neumann machine ( a machine which can build copies of itself which then virus-like build more copies) could fill a galaxy relatively quickly. Then the brief time we have been around isn't a problem - sometime in the last 3Bn years a von Neumann machine probe would have noticed we are a life capable planet and left lots of copies in orbit.

14. ### redargon

348
I just did a quick read up about von Neumann machines, as you mentioned them, and I guess we are kind of like von Neumann machines, using surrounding resources to replicate ourselves...

15. ### CEL

639
Earth is 4.5 billion years old (add or take a few hundred million years). Mankind is here for 100000 years, or $$2.2x10^{-5}$$ of the planet's existence.
Our technological civilization has 100 years, or $$2.2x10^{-8}$$ of the planet's existence.
We have not had time to develop interstellar travel, if it is possible. And we have more than once in the last 100 years had the opportunity to destroy civilization.
If alien civilizations exist, how old are they? Are aliens more wise than us and allow their civilizations endure long enough to discover interstellar travel and spread through the galaxy?
I certainly don't know. Do you?

16. ### Emreth

101
Or there is a possibility that intelligence is inherently unstable and always tends to destroy itself before it has time to get out of its solar system. (we dont care about simple life incapable of leaving its host). Or we are in the center of the universe (as recently suggested to explain the redshifts) and the first intelligent beings, which i dont find that implausible.

17. ### Ivan Seeking

12,529
Staff Emeritus
We can only say that we have no recognized scientific evidence for a visitation - that is not proof that it has never occurred. We have tons of anecdotal evidence [claims] that could be argued to go back thousands of years. Some claims are completely unimpressive, but others can cause one to take pause - esp some of the military reports. Beyond that, if the proof for ET was sitting right under our noses, we may never even know it. We really have no idea what to look for beyond a flying saucer landing at the White House.

We have no frame of reference on which to base our expectations. We can only guess at the absolute limits of physics and odds that highly advanced races grow old enough, and are smart enough, to discover the required physics for interstellar travel [assuming that it exists beyond the physics we know]; whether they have the motive to travel the cosmos, and what that motive may be.

It is fair to say that based on what we know of physics, visitations seem very unlikely, but that is also based on the assumption that there will be no new grand discoveries tomorrow, or in a million years, that will change everything. Are we approaching the end of the road for fundamental physics, or are we just babes in the woods? Who knows?

Last edited: Sep 1, 2009
18. ### Blenton

193
It's also based on the assumption that aliens would just come crashing down to the planet making themselves aware to everyone. We don't know what the motive is of extra-terrestrial life, and we can assume if they have the technology to travel here from a distant area in space, they have the technology to hide themselves from our current detection.

19. ### Chronos

9,975
Why would aliens 'hide' from us? Any critters smart enough to 'fly' here from another star system would not feel threatened. The 'Prime Directive'? I think not. I dont know of any biologists frightened, or morally bound to inconvenience for the sake of tadpoles.

20. ### ideasrule

2,322
There are good reasons for something like a "Prime Directive". For one thing, perhaps the aliens are conducting a scientific mission and don't want to disturb the civilization they're studying. For another, maybe introducing advanced technology too soon has proven disastrous in the past. It's also possible that intelligent life is extremely common in the galaxy, and that Earth is uninteresting compared to the other worlds.