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I think the basis of these theories is the size of the Universe. I mean, there is nothing bigger than Space/Universe, correct?
We are all confident on how big this place is. I think the analogy they used to give was, in the largest NFL football stadium in the US, Earth would be a sand particle or maybe even a dust particle.
That analogy actually may make Earth bigger compared to the Universe than it actually is.
So if that is the case, then what is going on in the rest of the Universe?
The Universe encompasses all there is from the infinitesimally small to galactic superclusters and filaments. As it stands, the observable universe, which has a radius of approx 46.5 billion light years, is all we can see. Most scientists however believe it to be only a small portion of the whole universe. According to Alan Guth who penned the theory of cosmic inflation, it's supposed to be at the very least 250 times the size of the observable part and could be much larger.
As to your second question, let's do the math:
Volume of the Earth: 1.1 * 10^21m3
Volume of the Universe: 4 * 10^80m3
Since the Earth is an approximate sphere, it does not fill up all the empty space, so we need to apply a waste factor of 0.524. That's the ratio between the volume of a sphere and that of a cube provided they have equal dimensions.
Volume of the Universe / Volume of the Earth * 0.524 = 1.91 * 10^59
Obviously, this involves the volume of the observable universe since what lies beyond is entirely speculative.
Thing is, all those kinds of estimates are based on unprovable assumptions piled on top of other unprovable assumptions.
Just as an example, what is a reasonable estimate for the fraction of planets that could support life? Is it 1 in 10 like in our solar system, or 1 in 100, or 1 in 1,000,000? On what basis are you going to pick a number?
Now consider the number of such assumptions that need to be made, NONE of which is actually subject to actual data, and each one of those fractions pulled out of thin air gets multiplied by the rest, and you've got estimates that vary by hundreds of orders of magnitude.
And once again, that's where I make a comment about angels dancing on the point of a pin and go to the bar for another drink.
The problem is that we're extrapolating from a single positive data point - Earth.
The bolded part you wrote is quite right. What are the odds that life develops around an average star (ie, around any star, taking into account that some stars are going to be more suitable than others)? The answer, of course, is that no one knows. Is is greater or less than 1-in-200,000,000,000? I use that number because two hundred billion appears to be the median estimate of the number of stars in the Milky Way. No one knows.
To call astrobiology a data poor field is a massive understatement.
Quote:
Originally Posted by Diesel23
You’re exactly right. The assumptions are just that, semi educated guesswork.
Are we alone? Almost certainly not. The Milky Way galaxy is VAST, let alone the Local Group of galaxies. 200 to 400 billion stars in just our galaxy. Many, probably a large fraction of those billions of stars have planets. The odds say we share the galaxy.
Your second paragraph contradicts your first.
The argument you are making is based on variables that you do not know. That no one knows.
You are impressed by the number 200,000,000,000 (roughly, the stellar population of the Milky Way) because it is big. But the number of intelligent, technologically-advanced civilizations we can expect to find in the galaxy is a function of that variable (call it X) times the odds of an average star hosting a technologically-advanced civilization at any given time (call it Y). And we don't know those odds. We have no idea as to the variable Y, except that it must be greater than zero (because of the single known positive data point of Earth).
To put it simply, are those odds greater or less than 0.000000000005? That number is the inverse of the generally accepted number of stars in the Milky Way. You do not know if it is greater or less. I do not know if it is greater or less. That is why neither of us can say whether the one data point we have is about what we'd expect to see, or much less than what we can expect, or unusually high.
After all, maybe the odds are not 0.000000000005 but 0.00000000005 (one less zero). In that case we can expect 10 such civilizations in the galaxy. Or maybe it's 0.0000000005 (less another zero). Then, we can expect 100 of the aforementioned civilizations in the Milky Way. It that fraction is smaller still, the number would be even higher.
On the other hand, maybe the odds aren't 0.000000000005 but are instead 0.00000000000005 (two more zeroes). In that event, we can expect that only one galaxy as large as ours (and ours is pretty big - it's the 2nd-largest galaxy in the Local Group, which comprises at least 80 galaxies) in one hundred will possess such a civilization. And it could be that the odds are much longer than that.
The problem is that we're extrapolating from a single positive data point - Earth.
The bolded part you wrote is quite right. What are the odds that life develops around an average star (ie, around any star, taking into account that some stars are going to be more suitable than others)? The answer, of course, is that no one knows. Is is greater or less than 1-in-200,000,000,000? I use that number because two hundred billion appears to be the median estimate of the number of stars in the Milky Way. No one knows.
To call astrobiology a data poor field is a massive understatement.
Your second paragraph contradicts your first.
The argument you are making is based on variables that you do not know. That no one knows.
You are impressed by the number 200,000,000,000 (roughly, the stellar population of the Milky Way) because it is big. But the number of intelligent, technologically-advanced civilizations we can expect to find in the galaxy is a function of that variable (call it X) times the odds of an average star hosting a technologically-advanced civilization at any given time (call it Y). And we don't know those odds. We have no idea as to the variable Y, except that it must be greater than zero (because of the single known positive data point of Earth).
To put it simply, are those odds greater or less than 0.000000000005? That number is the inverse of the generally accepted number of stars in the Milky Way. You do not know if it is greater or less. I do not know if it is greater or less. That is why neither of us can say whether the one data point we have is about what we'd expect to see, or much less than what we can expect, or unusually high.
After all, maybe the odds are not 0.000000000005 but 0.00000000005 (one less zero). In that case we can expect 10 such civilizations in the galaxy. Or maybe it's 0.0000000005 (less another zero). Then, we can expect 100 of the aforementioned civilizations in the Milky Way. It that fraction is smaller still, the number would be even higher.
On the other hand, maybe the odds aren't 0.000000000005 but are instead 0.00000000000005 (two more zeroes). In that event, we can expect that only one galaxy as large as ours (and ours is pretty big - it's the 2nd-largest galaxy in the Local Group, which comprises at least 80 galaxies) in one hundred will possess such a civilization. And it could be that the odds are much longer than that.
We.
Have.
No.
Idea.
I don't think we could ever truly estimate the likelihood of alien life. If we find conclusive evidence of life beyond Earth, then are data point automatically jumps to beyond 1, but it still raises the question of how common life actually is. What of we just got really lucky and stumbled upon the only other life in the Milky Way? What if they are nearly nonexistent within the Local Group, but rather common in the Virgo Cluster?
According to research, there are certain locations that are viewed as unfavorable for the development of life.
That would be: Red Dwarfs: Habitable zone is far too close to the star, leading to tidally locked planets. High flare activity. O-type stars: Too short lived. The biggest stars only live for a few million years before going supernova. Globular Clusters: Far too dense with a lot of star perturbation as a result.
It's worth pointing out, that all of this presumes that life is carbon based, whereas we don't know whether that is a prerequisite or not.
It's worth pointing out, that all of this presumes that life is carbon based, whereas we don't know whether that is a prerequisite or not.
Well we do know that carbon is the fourth most abundant atom in the Universe, so it's reasonable to believe that most potentially life-generating chemistry will involve that atom.
Well we do know that carbon is the fourth most abundant atom in the Universe, so it's reasonable to believe that most potentially life-generating chemistry will involve that atom.
Well we do know that carbon is the fourth most abundant atom in the Universe, so it's reasonable to believe that most potentially life-generating chemistry will involve that atom.
Quote:
Originally Posted by Milky Way Resident
Still an assumption until proven otherwise.
The words reasonable and most make it perfectly clear that, yes, it is an assumption.
Further, given the scope of the universe, it's not even a remotely possible thing to prove*. That would require a thorough exploration of all the billions of galaxies which, while this could theoretically be accomplished, being that said galaxies are separated by billions of light-years, the resulting knowledge of such an exploration would always be billions of years out of date from any given point of view.
So any notions of 'until proven' are nonsensical.
*The threshold of proof is one of the misunderstood and misused scientific terms to be found, especially by non-scientists.
There are a LOT of assumptions in any such analysis. And the good part, for those who do the estimates, is that is absolutely no way to prove them wrong - or right.
I would like to make some assumptions too. Earth, the sun, and other planets in our solar system are round shaped due to the spinning motion.
I, therefore, assume that other stars and planets in the universe are round shaped also.
If biological technological life exists elsewhere in the universe, it would look similar to humans, I assume. Why? Because on earth no matter how smart the elephants, birds, dolphins, monkeys, and octopus are, they can't build machines that enable explorations beyond earth. And this is the result of billions of years of evolution.
As for the question of whether technologically intelligent life exists elsewhere in the universe, I would love for that to be true but we need proof.
If biological technological life exists elsewhere in the universe, it would look similar to humans, I assume. Why? Because on earth no matter how smart the elephants, birds, dolphins, monkeys, and octopus are, they can't build machines that enable explorations beyond earth. And this is the result of billions of years of evolution.
If a biological extraterrestrial entity with technological abilities looked similar to humans, that would be convenient, but I don't think it would necessarily be essential to resemble humans. They'd need a brain capable of thought and imagination, and digits (or tentacles) to create and manipulate things. They'd also likely have to be land dwellers, at least originally, since they'd need to make use of fire, which you couldn't very well do underwater.
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