U.S. CitiesCity-Data Forum Index
Go Back   City-Data Forum > General Forums > Science and Technology > Space
 [Register]
Please register to participate in our discussions with 2 million other members - it's free and quick! Some forums can only be seen by registered members. After you create your account, you'll be able to customize options and access all our 15,000 new posts/day with fewer ads.
View detailed profile (Advanced) or search
site with Google Custom Search

Search Forums  (Advanced)
Reply Start New Thread
 
Old 07-11-2014, 05:03 PM
 
1,871 posts, read 2,876,050 times
Reputation: 1611

Advertisements

Fermi paradox:
The Fermi paradox (or Fermi's paradox) is the apparent contradiction between high estimates of the probability of the existence of extraterrestrial civilization and humanity's lack of contact with, or evidence for, such civilizations.

Before I start, here's a really good blog-article on the topic:
The Fermi Paradox

We've been listening to space for a while now, and outside of the "WOW!" signal anomaly back in 1977 (The Wow! Signal-Wikipedia), there has been little evidence of any intelligent life out there. This seems to fly in the face of estimates that as many as 100,000 civilizations could exist in our Milky Way galaxy alone. Where is everyone?

There are a number of possibilities. Despite the fact that documentaries will tell you that space is full of stuff, the reality is that space is big and the vast majority of it is empty by conventional standards (key word, "conventional"). Take for example the impending collision of the Milky Way and Andromeda galaxies; there is so much empty space between stars that it is highly unlikely that any of them will even touch each other when the galaxies collide in about 4 billion years. With that much space to look at and listen to, we have only begun to scratch the surface of the observable galaxy.

Another possibility is that we have grossly overestimated the number of civilizations out there. This leads into "The Great Filter", which is a conceptual idea that states that life (of any kind) generally hits a wall of evolution that only a tiny percentage ever seem to get past. "The Great Filter" could be ahead of us signaling that we are a doomed species or behind us signaling that we are a fluke and although we still might not be alone, we would be one of only a handful of species in the universe that got past it. Either possibility is not a fun thought.

Then of course there is the hot-button third possibility, that we have already contacted or received contact from alien civilizations and the government is covering it up. I won't go into this too much because that's more of a conspiracies topic rather than science but a whole lot of people seem to buy into it. I leave it to everyone to draw their own conclusions.

For more information, I highly recommend that you read the article posted at the top of this post and feel free to post your own thoughts!
Rate this post positively Reply With Quote Quick reply to this message

 
Old 07-11-2014, 05:25 PM
 
17,411 posts, read 23,458,234 times
Reputation: 19180
Because they are NOT there?
And any archeological findings that do not fit into common perception of human histiry are NOT from anunaki or other aliens? But from highly developed previous civilizations on EARTH?
Always remember Okham razor principle.
Rate this post positively Reply With Quote Quick reply to this message
 
Old 07-11-2014, 11:25 PM
 
Location: Caverns measureless to man...
7,589 posts, read 6,025,215 times
Reputation: 17939
It could be that a technologically advance civilization only emits electromagnetic radiation at detectable levels for a very brief period of their evolution before developing different means of communication, like quantum entanglement for example. Perhaps an advanced civilization only has a 100-year window or thereabouts during which we have to be pointing our instruments in just the right direction. If a species is hundreds of thousands (or even millions) of years old, the chance that we'd catch them at just the right moment could be quite slim.
Rate this post positively Reply With Quote Quick reply to this message
 
Old 07-11-2014, 11:46 PM
 
281 posts, read 341,136 times
Reputation: 429
Here I go- stop me if I make a fool of myself. The Milky Way is about 100,000 light years in diameter. We, if I understand correctly, are out on one arm. If a civilization was on a planet only half-way across the galaxy, we would be receiving their transmissions originating 50,000 years ago. Wouldn't any signal have degraded in that time? Also, if they were intelligent enough to find and/or actually come to Earth, would they recognize any difference between earthworms and us? It wouldn't make sense to contact us- they should go for the dominant species- cockroaches or ants.
Rate this post positively Reply With Quote Quick reply to this message
 
Old 07-12-2014, 03:22 PM
 
Location: Wasilla, Alaska
17,824 posts, read 22,250,232 times
Reputation: 6527
Quote:
Originally Posted by Adric View Post
Fermi paradox:
The Fermi paradox (or Fermi's paradox) is the apparent contradiction between high estimates of the probability of the existence of extraterrestrial civilization and humanity's lack of contact with, or evidence for, such civilizations.

Before I start, here's a really good blog-article on the topic:
The Fermi Paradox

We've been listening to space for a while now, and outside of the "WOW!" signal anomaly back in 1977 (The Wow! Signal-Wikipedia), there has been little evidence of any intelligent life out there. This seems to fly in the face of estimates that as many as 100,000 civilizations could exist in our Milky Way galaxy alone. Where is everyone?

There are a number of possibilities. Despite the fact that documentaries will tell you that space is full of stuff, the reality is that space is big and the vast majority of it is empty by conventional standards (key word, "conventional"). Take for example the impending collision of the Milky Way and Andromeda galaxies; there is so much empty space between stars that it is highly unlikely that any of them will even touch each other when the galaxies collide in about 4 billion years. With that much space to look at and listen to, we have only begun to scratch the surface of the observable galaxy.

Another possibility is that we have grossly overestimated the number of civilizations out there. This leads into "The Great Filter", which is a conceptual idea that states that life (of any kind) generally hits a wall of evolution that only a tiny percentage ever seem to get past. "The Great Filter" could be ahead of us signaling that we are a doomed species or behind us signaling that we are a fluke and although we still might not be alone, we would be one of only a handful of species in the universe that got past it. Either possibility is not a fun thought.

Then of course there is the hot-button third possibility, that we have already contacted or received contact from alien civilizations and the government is covering it up. I won't go into this too much because that's more of a conspiracies topic rather than science but a whole lot of people seem to buy into it. I leave it to everyone to draw their own conclusions.

For more information, I highly recommend that you read the article posted at the top of this post and feel free to post your own thoughts!
You were right, it was a good blog on the topic.

However, I have a serious problem with their estimates. The article estimates that there are 1% Earth-like planets within the Milky Way galaxy, and 0.01% have civilizations.

First, lets start with the star. The vast majority of the stars in the Milky Way galaxy (~80%) are of the spectral type M. Between 2% and 8% of the stars are spectral types F, G, and K stars. All other spectral types (O, B, and A) are less 1% of the stars in the Milky Way.

In order for life, and therefore a civilization, to form on any given planet it must be in orbit around a star that will be alive long enough to support civilization. The only model we have to go by is Earth, where simple life began ~3.8 billion years ago and complex life began ~540 million years ago, and civilization began less than 5,000 years ago. A civilization is also not going to arise if their star is approaching the end of its life. Or it may arise, only to be destroyed shortly thereafter. Therefore, the star must have a lifespan of at least 6 billion years or more to support a long-lasting civilization. That puts a severe limitation on the mass of the star. Stars with more than 1.25 M☉ will have a lifespan of less than 6 billion years. Which pretty much eliminates any star with a spectral type O, B, A, and about half the spectral type F stars.

Just as there is a maximum limit to a star's mass for a civilization to arise and progress, there is also a minimum mass limit. While a low mass star may have extraordinarily long lifespans, the smaller the mass a star has, the closer its "habitable zone" must be to the star. If a planet is too close to its star (an orbit of less than 90 to 100 days), then it will become tidally locked with its parent star. Meaning, one side of the planet will continually point in the direction of the star, while the other side of the planet will be in perpetual darkness. Which is not conducive to life, much less life that is capable of producing a civilization. Therefore, the minimum mass of the star should be greater than ~0.75 M☉, which would put the inner most "habitable zone" radius at about ~0.2 AU. This would eliminate the spectral type M stars and the vast majority of the spectral type K stars. There are still a few spectral type K stars that have more than 0.75 M☉, but very few. Out of the ~12.1% of the spectral type K stars in the Milky Way, maybe between 1% or 2% will have a mass greater than 0.75 M☉.

That leaves spectral type K (~1.5% of all stars in the Milky Way with a mass greater than 0.75 M☉), G (~7.6% of all stars in the Milky Way), and half of the spectral type F (~1.5% of all the stars in the Milky Way) stars. Which is approximately a total of 10.6 billion stars in the Milky Way that fits between the 0.75 to 1.25 M☉ range.

The best estimates claim that 22% ± 8% of all stars in the Milky Way have planets in the habitable zones of their stars. Which drops the number of stars in the 0.75 to 1.25 M☉ range with one or more planets in the habitable zone of the star to 2.332 billion ± 848 million stars.

Now here is were we get into a really fuzzy area: What constitutes an "Earth-like" planet?

Granted, the planet must have an atmosphere with high enough pressure to keep any surface water from sublimating away into space. But does the planet need to have an oxygen atmosphere? What about the density and gravity of the planet? Earth's density is 5.52 g/cm^3 with a surface gravity of 9.8 m/s^2. How much or how little density and gravity can a planet have before it is not considered "Earth-like?" A planet with an extremely low density may have the same gravity as Earth if it is large enough, but be a water world. While a water world would certainly be conducive for life, it would not spawn a civilization. A planet could also be in the habitable zone of its star and not have an atmosphere. Or be like Venus or Mars instead of like Earth. Before better estimates can be provided there needs to be a better definition of what constitutes an "Earth-like" planet.

In order to achieve that 1 billion Earth-like planet estimate in the Milky Way galaxy from the article, 42.9% of all planets within the habitable zone of their stars must be "Earth-like." That seems to be an extraordinarily high percentage, in my opinion, but once again that depends on the definition of "Earth-like."

Sources:
The Milky Way Tomography with SDSS. I. Stellar Number Density Distribution - The Astrophysical Journal, 673:864, February 1, 2008 [PDF]
The Classification of Stars
Stellar classification - Wikipedia, the free encyclopedia
One in Five Stars Has Earth-sized Planet in Habitable Zone | W. M. Keck Observatory

Last edited by Glitch; 07-12-2014 at 03:52 PM..
Rate this post positively Reply With Quote Quick reply to this message
 
Old 07-12-2014, 04:05 PM
 
1,203 posts, read 788,285 times
Reputation: 2181
Quote:
Originally Posted by Adric View Post
Fermi paradox:
The Fermi paradox (or Fermi's paradox) is the apparent contradiction between high estimates of the probability of the existence of extraterrestrial civilization and humanity's lack of contact with, or evidence for, such civilizations.

Before I start, here's a really good blog-article on the topic:
The Fermi Paradox

We've been listening to space for a while now, and outside of the "WOW!" signal anomaly back in 1977 (The Wow! Signal-Wikipedia), there has been little evidence of any intelligent life out there. This seems to fly in the face of estimates that as many as 100,000 civilizations could exist in our Milky Way galaxy alone. Where is everyone?

There are a number of possibilities. Despite the fact that documentaries will tell you that space is full of stuff, the reality is that space is big and the vast majority of it is empty by conventional standards (key word, "conventional"). Take for example the impending collision of the Milky Way and Andromeda galaxies; there is so much empty space between stars that it is highly unlikely that any of them will even touch each other when the galaxies collide in about 4 billion years. With that much space to look at and listen to, we have only begun to scratch the surface of the observable galaxy.

Another possibility is that we have grossly overestimated the number of civilizations out there. This leads into "The Great Filter", which is a conceptual idea that states that life (of any kind) generally hits a wall of evolution that only a tiny percentage ever seem to get past. "The Great Filter" could be ahead of us signaling that we are a doomed species or behind us signaling that we are a fluke and although we still might not be alone, we would be one of only a handful of species in the universe that got past it. Either possibility is not a fun thought.

Then of course there is the hot-button third possibility, that we have already contacted or received contact from alien civilizations and the government is covering it up. I won't go into this too much because that's more of a conspiracies topic rather than science but a whole lot of people seem to buy into it. I leave it to everyone to draw their own conclusions.

For more information, I highly recommend that you read the article posted at the top of this post and feel free to post your own thoughts!

The reason no signal has been detected is due to the kind of signal the current SETI program is searching for. We are not going to detect common radio communications or the sort of relatively low frequency intrastellar communications. The sort of signal they are searching for is a very strong, booming, deliberate attempt at interstellar message sending.
I personally have always believed that particular sort of signal is highly unlikely. Any species with the requisite astronomical curiosity to build such a communication device will chose physical travel instead if possible, even if only sub light speed to nearest locations. And if some sort of physical travel is possible, a civilization only a few centuries more advanced than we are likely to have discovered how. A "conversation" btw creatures separated by several light years is not one in which anything useful or worthwhile can be exchanged and a progressive creature is likely to assume that waiting for future physical travel breakthroughs is a better idea for those kinds of distances. This is why I dont expect SETI to ever have success. Although I hope I am proven wrong because it would be the only chance to experience intelligent ET life in my lifetime.

As far as physical visitation, I think intelligence is rare but has nonetheless occurred frequently enough that multiple sapient animals currently exist in the Milky Way. Subluminal travel to the outer reaches of a solar system or to nearby systems is something that we may be capable of within the next 100 years ourselves, but I suspect any sort of sci-fi type FTL or "warp" technology would be of a species considerably more advanced and highly unlikely to park its ship on the White House lawn.

We have currently detected hundreds of worlds orbiting other stars. Over the next 10-20 years highly advance space telescopes capable of detecting earthlike worlds and reading their chemical compositions will be deployed and we will know in our lifetimes where many of the living worlds in our neighborhood are. Its probably a good idea, given the age of the universe to strongly suspect that our planets location has been detected by someone else and our atmosphere identified as potentially life bearing. But they likely cant travel here nor have interest in sending a useless mega signal.......or they are extremely advanced, have already been here but for ethical reasons avoid contact.

Last edited by TheArchitect; 07-12-2014 at 04:14 PM..
Rate this post positively Reply With Quote Quick reply to this message
 
Old 07-12-2014, 04:19 PM
 
7,803 posts, read 4,037,588 times
Reputation: 9418
Quote:
Originally Posted by Albert_The_Crocodile View Post
It could be that a technologically advance civilization only emits electromagnetic radiation at detectable levels for a very brief period of their evolution before developing different means of communication, like quantum entanglement for example. Perhaps an advanced civilization only has a 100-year window or thereabouts during which we have to be pointing our instruments in just the right direction. If a species is hundreds of thousands (or even millions) of years old, the chance that we'd catch them at just the right moment could be quite slim.
Or maybe all civilizations for one reason or another self-destruct at this level of development.
Rate this post positively Reply With Quote Quick reply to this message
 
Old 07-21-2014, 03:13 PM
 
Location: The High Desert of the American Southwest
214 posts, read 213,895 times
Reputation: 359
If you're not already familiar with it, Google "the Drake Equation" for an idea of how much life is probably out there.

True, the equation is based purely on laws of statistical probability and not exo-biological science or cosmological nuances, but it still basically offers that if only one out a billion planets were candidates for life and then one out a billion of those had even bacterial life, and then one out of a billion of those had intelligent life, there would still be trillions of planets likely to harbor civilizations at least as advanced as us.
I am of the opinion that the universe is indeed teaming with life and the primary reason we have not contacted any yet is simply because of the vast distances involved. Remember: the closest star to us--except of course for the Sun--is over four light-years away. with our current propulsion technology it would take 10,000 yrs. to get there. and that's just our closest neighbor. Our galaxy alone is 100,000 light-years across, and it is only one out of hundreds of billions other galaxies.
As Sagan said: "If there is no life out there it sure is a tremendous waste of space."
Rate this post positively Reply With Quote Quick reply to this message
 
Old 07-21-2014, 07:28 PM
 
Location: Wasilla, Alaska
17,824 posts, read 22,250,232 times
Reputation: 6527
Quote:
Originally Posted by Hombre_Corriendo View Post
If you're not already familiar with it, Google "the Drake Equation" for an idea of how much life is probably out there.

True, the equation is based purely on laws of statistical probability and not exo-biological science or cosmological nuances, but it still basically offers that if only one out a billion planets were candidates for life and then one out a billion of those had even bacterial life, and then one out of a billion of those had intelligent life, there would still be trillions of planets likely to harbor civilizations at least as advanced as us.
I am of the opinion that the universe is indeed teaming with life and the primary reason we have not contacted any yet is simply because of the vast distances involved. Remember: the closest star to us--except of course for the Sun--is over four light-years away. with our current propulsion technology it would take 10,000 yrs. to get there. and that's just our closest neighbor. Our galaxy alone is 100,000 light-years across, and it is only one out of hundreds of billions other galaxies.
As Sagan said: "If there is no life out there it sure is a tremendous waste of space."
The Drake equation is not based on statistical probability at all. It is made up entirely of wishful thinking with absolutely no basis in fact or statistics whatsoever. You can literally plug in whatever numbers you like and obtain any value you desire from billions to none.
Rate this post positively Reply With Quote Quick reply to this message
 
Old 07-22-2014, 10:19 AM
 
Location: The High Desert of the American Southwest
214 posts, read 213,895 times
Reputation: 359
Quote:
Originally Posted by Glitch View Post
The Drake equation is not based on statistical probability at all. It is made up entirely of wishful thinking with absolutely no basis in fact or statistics whatsoever. You can literally plug in whatever numbers you like and obtain any value you desire from billions to none.

You're being overly harsh on the Equation by saying that it could be rendered useless by "plugging in" unsubstantiated or even intentionally-inaccurate numbers.

Well, sure you could, but That's like saying you could plug "whatever numbers you like" for the "E" and the "M" (since the "C" speed-of-light is a constant) into E=MC2 and obtain "any value you desire." Under the GIGO rule most formulas could be sullied.

BUT-- If cosmologists begin the equation with an educated guess of possible candidate planets (no wishful thinking involved!) then the Equation can serve to show us that there should really be no reason that our planet is special, and that even if we are one out a billion, life very likely could be rampant in the Universe.
And there is total absence at this time of any scientific data hinting that we ARE special. Our sun is very average; our solar system is unremarkable;and our MIlky Way Galaxy appears to be simply one out of billions of similar elliptical-types. Planets orbiting in "the Goldilocks Zone" are very likely numerous, even if only one out of a 9-planet solar system, like us, exist. (Or even one out of a 60-planet solar system!)
Over the course of the past several years, some cosmologists HAVE added a couple more conditions they believe may be needed in a solar system for one planet to harbor intelligent life. For example, some say you need a Jupiter-like gas giant on the periphery so as to gravitationally deflect or sway any potentially harmful (or fatal!) space debris like meteors or asteroids or comets from impacting your planet.
But I don't believe that alters the odds of life out there much at all, since solar systems with gas giants on the outside of the smaller, denser planets should be the norm, given the laws of motion and gravity and Newtonian physics.

Last edited by Hombre_Corriendo; 07-22-2014 at 11:28 AM..
Rate this post positively Reply With Quote Quick reply to this message
Please register to post and access all features of our very popular forum. It is free and quick. Over $68,000 in prizes has already been given out to active posters on our forum. Additional giveaways are planned.

Detailed information about all U.S. cities, counties, and zip codes on our site: City-data.com.


Reply
Please update this thread with any new information or opinions. This open thread is still read by thousands of people, so we encourage all additional points of view.

Quick Reply
Message:

Over $104,000 in prizes was already given out to active posters on our forum and additional giveaways are planned!

Go Back   City-Data Forum > General Forums > Science and Technology > Space

All times are GMT -6. The time now is 01:50 PM.

© 2005-2022, Advameg, Inc. · Please obey Forum Rules · Terms of Use and Privacy Policy · Bug Bounty

City-Data.com - Contact Us - Archive 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37 - Top