Solve Fermi's Non-Paradox: Astronomers Weigh In

[sbrothy]

TL;DR Summary

Wasn't Fermi's Paradox already resolved in 2008?

I post this here because most of the papers dealing with this (sorta speculative) stuff seems to be written by astronomers. Feel free to move it.

I was under the impression that Fermi's Paradox had been resolved (dissolved) by this paper already in 2008:

Dissolving the fermi paradox :
https://arxiv.org/abs/1806.02404

Nevertheless, papers as late as 2020, like e.g:

1105.2497
2003.0428
and others

still treat it as a mystery, Great Filter and all. Is this just because the paradox attracts attention or is the first paper there somehow flawed.?

Regards,
Søren

 

[hutchphd]

The paper you reference https://arxiv.org/abs/1806.02404 is dated 2018...I'm confused.

 

[sbrothy]

Oops my bad. I'm pretty heavily nearsighted. My argument stand though. Not as vividly but still. People still write about the paradox even in 2020. I could find more examples but due to the dissolvement (is that a word?) fleshed out in 1806.02404 I thought it was grave enough.

Still, given the first paper I mention there, which to my - admittely - limited knowlegde, drives a stake through the paradox idea, why do people still think Fermi's Paradox is a thing?

Regards.

 

[hutchphd]

I haven't read the paper carefully but I can propose two possibilities:

1. The Drake equation is a product of not very well circumscribed factors
2. Fermi was a pretty bright guy

So definitively dismissing the paradox will require some heavy lifting...I will try to read the paper.

 

[sbrothy]

...

1. The Drake equation is a product of not very well circumscribed factors
2. Fermi was a pretty bright guy

...

1,) Yes and using proper statistical distributions (and monte carlo sims? It's been a little while since I read it but) I seem to remeber that the number they arrived at was somwthing like a 50 to 90% probability that we are in fact alone in the observable universe. Bad news for space opera scifi writers bit good news for thise of us who'd rather not meet another spacefaring aggressive apex-predator out there.. :)

2.) Yes, but it's been awhile and smarrness in and of itself is no guarantee your ideas persist. I can see how a minor inferiourity comolex would prevent some stepping up to say: "This genius was wrong am I'm right. I have an Etch&Sketch here with my math.". :)

 

[PeroK]

1,) Yes and using proper statistical distributions (and monte carlo sims? It's been a little while since I read it but) I seem to remeber that the number they arrived at was somwthing like a 50 to 90% probability that we are in fact alone in the observable universe. Bad news for space opera scifi writers bit good news for thise of us who'd rather not meet another spacefaring aggressive apex-predator out there.. :)

Probability theory can only effectively be used where you have a good understanding of all the data involved. Any conclusions about the amount of life in the observable universe must be based on a lot of guesswork. No one has run the experiments to determine the likelihood of the various factors and how necessary each factor is.

It doesn't matter how clever you are: 50-90% is a wild guess.

Also, there is also a confidence level on any estimate like this. That's a measure of how confident you are of your results. The confidence level for an answer like that must be close to zero. You could test this hypothetically as follows:

Let's infer that if that result is correct there are very few civilisations in the observable universe and none in our galaxy (apart from us). I.e. the probability of there being another civilisation in our galaxy is millions to one against. Otherwise, we can't possibly be alone in the observable universe of however many billions of galaxies.

Now we have odds of millions to one against there being another civilisation in our galaxy. But, wait a minute. There are two ways there can be another civilisation in our galaxy:

a) If the report is correct, then a very small probabilty.

b) If the report is wrong, then potentially a large probability . (Note: This probability dominates)

Now, what are the odds the report is wrong? Millions to one against? Hardly. The probability in b) dominates. This is always the problem with quoting a very low probability unless you know you have complete information: the probability you are wrong dominates the tiny probability you calculate (assuming you are correct).

This is what happens when someone wins the lottery twice and the newspapers give some calculation that esimates the odds at a trilllion to one against. It's only a trillion to one against if the calculation in the newspaper is correct. The probability that the newspaper calculation is wrong may be 90% or 99.999%. In fact, it's almost certain that someone somewhere will win the lottery twice.

In conclusion, the probability there is no other civilisation in our galaxy cannot be millions to one against. Unless you have a calculation that itself cannot possibly be wrong. And given how little data we have, we cannot have unimpeachable calculations of such probabilities.

 

[Buzz Bloom]

In conclusion, the probability there is no other civilisation in our galaxy cannot be millions to one against.

Hi PeroK:

Unless I misunderstood the rest of your post, I think the quote above was intended to say the opposite.

The probability there is another civilisation in our galaxy cannot be millions to one against.​

Regards,
Buzz

 

[PeroK]

Hi Perok:

Unless I misunderstood the rest of your post, I think the quote above was intended to say the opposite.

The probability there is another civilisation in our galaxy cannot be millions to one against.​

Regards,
Buzz

The abstract from the paper says:

"we find a substantial probability of there being no other intelligent life in our observable universe"

If there is probably none in the universe, there isn't much hope of anything close to home!

 

[hutchphd]

This is the same type of issue as medical tests for a disease whose incidence is rare. A test which delivers a false positive result at even a small rate can grossly misreport the true incidence of disease.

 

[PeroK]

This is the same type of issue as medical tests for a disease whose incidence is rare. A test which delivers a false positive result at even a small rate can grossly misreport the true incidence of disease.

The point is that to make any reliable estimate of the number of civilisations in the universe you would need, as an absolute minimum, extensive data from across at least one galaxy.

 

[Buzz Bloom]

The abstract from the paper says:
"we find a substantial probability of there being no other intelligent life in our observable universe"

Hi PeroK:

I downloaded the article

Dissolving the Fermi Paradox​

Anders Sandberg, Eric Drexler and Toby Ord​

Future of Humanity Institute, Oxford University​

June 8, 2018​

in PDF format and searched for

"we find a substantial probability" .​

There was no such text in the abstract, but I found in the body (Section 6) the following.

When we update this prior in light of the Fermi observation, we find a​

substantial probability that we are alone in our galaxy, and perhaps even in our​

observable universe (53%–99.6% and 39%–85% respectively).​

I apologize for misunderstanding what you intended.

Regards,
Buzz

 

[BillTre]

A few months ago, a paper was published with the conclusion that:

Summary:: A alternative to the Drake Equation for thinking about how likely intelligent life on other planets has been recently published.

predicted high and low densities of intelligent communicating civilizations in the galaxy. The densities predicts a lower limit of 36 (+175, -32) civilizations (or between 4 and 211 civilizations, I guess). They use these densities to predict an average distance between civilizations of 17,000 (+33,600, -10,000) light years (between 50,600 and 7,000 light years), which makes communication between communication capable civilization unlikely within the limits of light speed.

A PF thread on this is here.

My understanding of the Fermi Paradox is based on his presumed quote of "where is everyone?".
This seems to be taken as "if life (having developed into observable (probably space traveling) civilizations) is widespread in the universe, why is it not obvious enough that we would at this time not notice it?".
I don't think it is a particularly well defined question.

If we take Earth as the only case of life origination and development of an (almost) space faring civilization, which sounds like the kind of "people (technologically advanced societies)". Even wonderful Earth (our only example) has not yet reached this advanced stage of technological development.
Fermi's musing ("where is everybody?"), sounds like he was expecting them to be seen zipping around the solar system.
This would be like what we have on earth, but more advanced, since we are not really space faring yet.
So, there are no real examples of this in the known universe).

Beyond that:
The only known example of life originated from geochemical processes on a planet containing a wide variety of elements, making possible a complex mix of different chemicals, which are needed for chemically sustained biological functioning. Earthly life depends on this for its functioning.
The elements require at least a second generation star to generate the require diveristy of atoms for the more complex chemistry.

This provides a time limit of how fast early life could be generated.
Less time to spread everywhere at sub-light-speed velocities and get everywhere, in order to be observed.

 

[Buzz Bloom]

I am hopeful someone can correct the understanding I arrived at by my scanning (not a careful thorough read) through the article I cited in post #11 which I will refer to here as the SDO article. Basically I find their conclusion agreeing with my own guesses, but I find their method troubling.

The authors substituted a range of probability values (rather than a single probability) for each of the seven variables in the Drake equation.

N = R_* f_p n_e f_l f_i f_c L​

Then they performed a series of Monte Carlo trials randomly drawing a probability value for each of the seven ranges. There are references of papers from which they may have used for the ranges. One possibility is:

Nicolas Glade, Pascal Ballet, and Olivier Bastien. A stochastic process approach of the Drake equation parameters. International Journal of Astrobiology, 11(02):103–108, 2012.​

Unfortunately I do not have access to this article, and I would very much like to see what ranges were used in this article.

I failed to find in the SDO article any detailed astronomical explanation for any rational for the chosen probability ranges. This is what troubles me. Some of the ranges might have reasonable values based on current astronomical evidence, but others seem to be based entirely on personal guesses. The f_l variable (the fraction of Earth-like or otherwise habitable planets that have life) seems to be in no way based on any reasonable astonomical information. As far I as have been able to understand this issue, it will require some clear astronomical detection of oxygen gas in the atmosphere of a candidate planet. At the present time it is not possible to make any reasonable guess regarding the likelihood of such an in-the-future discovery. My own feeling is that it is plausibly likely that astronomical technology will within the next 20 years will be able to detect atmospheric oxygen gas in the atmosphere of a distant planet that does have oxygen in its atmosphere, (such a planet within a distance that has a fair number of candidate planets).

 

[thereaverofdarkness]

The simplest and most direct answer is that most people have not heard of that paper. But I would like to dive into a slightly deeper analysis and say that the paper itself has a wrong result. Without even reading the contents, I can already debunk the abstract. It claims that re-working the equation can yield a very substantial chance of there being not one single interplanetary civilization in our galaxy, and without even invoking a "yet".

Now as this paper, the Fermi Paradox, the Drake Equation, and many other sources have correctly surmised, calculating the probabilities are fraught with too many unknowns to yield a reasonable result. But what they all seem to miss is that the probabilities we can best rule out are the most extreme ones. We can narrow down the possible ranges to fields many orders of magnitudes across, yet still far narrower than a complete guess. Doing so can actually rule out some guesses entirely.

As one example, we don't know if life can exist in very many circumstances outside our own, but there is a range of circumstances in which we know that life can exist, stable enough to last for billions of years yet unstable enough to provoke gradual evolution. Furthermore, we can speculate significantly on the availability of such environments within our galaxy. We know that the primary factors leading to our home and its stability are:
1.) third-generation star with high metallicity (with accretion disk containing substantial iron fraction) of spectral class K, G, or F
2.) circular orbit within "Goldilocks" zone, with large moon at small distance
3.) local galactic habitability (not being in a sterilization zone such as with strong gamma radiation present)
possibly 4.) shepherd planet like Jupiter orbiting outside the "Goldilocks" zone

All other major factors are known to have a high likelihood or near-certainty of happening, such as stable planetary magnetic field (assuming it's an iron-core planet), presence of surface water and wide variety of lighter elements, abiogenesis within bio-stability timeframe, evolution due to natural selection, planet failing to be sterilized in major extinction events due to large variety of life across different types of habitats.

We know that a prospective star ~5 billion years ago (or ~10 billion years after the Big Bang) within the Milky Way galaxy has a pretty high likelihood of being the right spectral class and with high enough metallicity. While the vast majority of stars in the size range have low metallicity at this age of the universe, there are nonetheless at minimum millions of stars within this range just in our own galaxy, forming ~10 BYA after the Big Bang or earlier. Planetary systems around stars of high metallicity should have an extremely high likelihood of containing large amounts of iron, due to iron's position at the fusion peak--thus formation of planetary elements within a supernova will always produce substantial amounts of iron. We know many examples already of planets within the right mass range and within the "Goldilocks" zone--this is a highly likely occurrence. Estimates for galactic habitability are generally high as well; given how many times our own planet had to orbit our galaxy, it is extremely unlikely that greater than half of the galaxy lies within uninhabitable regions. So the only remaining factor is chance to obtain a moon like Earth's. This last item could be very rare, but planetary models eliminate it being more than a few orders of magnitude down. So you still have at absolute minimum hundreds of planets within our galaxy forming long-term life 5 BYA or earlier, and having pressured that life into evolving up until today (or occasionally, until its host star grew too bright and sanitized the planet).

While we have little to explain the true likelihood of life achieving the ability to exit its home planet (within its stability timeframe and prior to the current day) under these conditions, we can point to perhaps 1-2 dozen major evolutionary breakthroughs in our own evolution which led us down this path. Examining every single one, we consistently find that life took a path of fairly low resistance--while it may have taken trillions of chances to get the mutation that started the next wing of development, it always bridged the mutation gaps in bridges the size of one single mutation. What can appear to us at a glance to require three or more simultaneous and cohabiting mutations will, virtually unfailingly, happen anyway one mutation at a time. Maybe it's just the universe we live in, or maybe the apparent extreme unlikeliness hides a plethora of other extremely unlikely options of which anyone could have pushed us forward. But one way or another, no matter how many branches reach an evolutionary dead-end, there are always those that don't, and they always dominate the next niche. Our evolution toward stepping into the stars may have been nearly guaranteed, and at minimum we can cast some VERY strong doubt on its likelihood being extremely low.

So given a lower bound of perhaps hundreds of worlds like ours beginning greater than 5 MYA (more reasonably thousands to tens of thousands), and given an ever-increasing rate of these worlds forming as the universe and the galaxy age, and given the relative ease with which a planet like ours can produce people like us within the available time frame, we can cast strong doubt on the lower bound estimate of ~50% chance we're alone in the galaxy, and virtually eliminate any figures pushing higher than 90%, regardless of their methodology. They can't be right. Here I have demonstrated that Enrico Fermi was right in proposing that our galaxy contains many spacefaring civilizations; we can be relatively certain that it indeed does. But perhaps more importantly, we can dispel the Fermi Paradox easily by looking forward rather than backward: there are two assumptions which it relies upon, both of which are extremely unreasonable to the point of invalidating the proposition outright and suggesting that in fact, regardless of how many interstellar civilizations there are in our galaxy (even millions!) and how old they are (billions of years!), we should actually fully expect to NOT see any of them at our current tech level.

The first assumption is that we will expand into our galaxy on a timescale too short to be significant relative to the age of the universe. But as faster than light travel has been absolutely thoroughly debunked and as we gain a greater understanding of the needs of habitation in space, we see more and more that it doesn't matter whether we use robots, engineered life, or any other advanced technology, there simply isn't a way to maintain the expansionist mentality itself during space expansion. Ecology trumps expansion to such an enormous degree that no civilization can thrive in space (due to physics, regardless of genetics) without fully embracing an ecologist mentality. Each new position we occupy must focus predominantly on its own maintenance, and will only support a robust spaceship industry after it has been developing for a very long time. Spaceships will sluggishly drift through the stars on not-so-insignificant timescales. And we will be forced to come to terms with these limitations in order to thrive out there. Will we expand on timescales in the thousands of years? No. It's not physically possible. Will we expand on timescales in the millions of years? Again, no. This time it would be physically possible, but it would require devoting all resources toward expansion while simultaneously fully understanding and embracing the futility of that mindset. No race of people intelligent enough to expand into space won't abandon expansionism when in space. So we will expand across our host galaxy on timescales of billions of years--and given how long it took for people like us to step into the stars in the first place (~10+ billion years), it's entirely expected that our galaxy is less colonized than not.

The second assumption is that we could see them if they weren't very far away (~1-1000 light years) yet the brightness of a beacon capable of signalling us at those distances is so enormous as to be completely absurd to expect anyone to build such a thing. What would it be for? To signal to young civilizations that they aren't alone? If they're dying to know, they'll figure that out probably within a few thousand years when they start making solar-system sized collector arrays (already within our tech level), and can detect signals SEVERAL orders of magnitude fainter than what SETI can detect today with a planet-sized collector array. Since these civilizations take billions of years to crawl across the galaxy, they have no reason to make enormously bright beacons just to say hi to us when we can't even respond. No, they will communicate with lasers, direct to target. If they wanted to say hi to us, they would do it with a laser, and they would leave it running for bursts of decades or centuries at a time. SETI already failed to find such a communication within its first major sweep, so there's little reason to continue looking at this point, to be honest. I say that not with high likelihood, not with extremely high likelihood, but definitively. It can't not be true. Extreme projects of extreme stupidity in design don't happen at all, anywhere, because of the way intelligence works. We can be as certain of this as we can be of the Boltzmann Brain hypothesis being wrong.

 

[stefan r]

...the number they arrived at was somwthing like a 50 to 90% probability that we are in fact alone in the observable universe. Bad news for space opera scifi writers bit good news for thise of us who'd rather not meet another spacefaring aggressive apex-predator out there.. :)...

10% to 50% chance that something will happen is huge. Sci-fi writers have it extremely easy. Compare to romance novels. The odds of getting laid are much lower than that.

 

[Buzz Bloom]

2.) circular orbit within "Goldilocks" zone, with large moon at small distance

Hi reaver:
This item seems to me to require an additional statistic. What fraction of planets in a "Goldilocks" zone actually have a large moon like the Earth's moon? I would guess that the efforts to deduce how our moon came to be would be helpful in making such an estimate, but I have never been able to find such a published estimate. Do you know of one?

I recall reading about several ideas for the moon's origin which have changed over time due to new information about comparing the stuff the moon is made of to the stuff that Earth is made of. Since the actual material of the moon is not relevant to its role in the origin of life on Earth, a best estimate of f_l would include the sum of the probabilities of all the various possible ways a moon the approximate size of our moon could have come to exist.

Also, here is another observation regarding f_i (of those planets with life, what fraction develop intelligent life). On Earth it seems that a very rare fluke caused intelligent life to evolve.

https://en.wikipedia.org/wiki/Chicxulub_crater​

The Chicxulub Crater lends support to the theory postulated by the late physicist Luis Alvarez and his son, geologist Walter Alvarez, that the extinction of numerous animal and plant groups, including non-avian dinosaurs, may have resulted from a bolide impact (the Cretaceous–Paleogene extinction event).​

If there had not been a very large asteroid hitting the Earth and thereby destroying all the dinosaurs, the mammals would probably not have come to evolve into humans during the period since then.

Regards,
Buzz

 

[PeroK]

If there had not been a very large asteroid hitting the Earth and thereby destroying all the dinosaurs, the mammals would probably not have come about to evolve into humans during the period since then.

You have to distinguish between the probability of a specific event and the much greater probability of any such event. The dinosaurs may have died out eventually for other reasons.

If you go though the history of the Earth and ascribe a probability of each key event happening when it did and assuming each is necessary for a civilisation eventually to have evolved, then it's not hard to get a probability of trillions to one against this whole sequence of events. But that is decidedly not the probability of an advanced civilisation evolving an an arbitrary planet.

The key point is that we do not know which assumptions can be relied on. The data from one solar system is not sufficient to know which assumptions are reliable.

 

[Buzz Bloom]

The key point is that we do not know which assumptions can be relied on. The data from one solar system is not sufficient to know which assumptions are reliable.

Hi PeroK:

I agree completely. I interpret this as implying that, at our present level of technology, estimating a value (or a range) for many of the seven Drake equation factors is at best a rational speculation. At worse, it is just wild guesses.

Regards,
Buzz

 

[thereaverofdarkness]

Hi reaver:
This item seems to me to require an additional statistic. What fraction of planets in a "Goldilocks" zone actually have a large moon like the Earth's moon? I would guess that the efforts to deduce how our moon came to be would be helpful in making such an estimate, but I have never been able to find such a published estimate. Do you know of one?

I didn't put a lot of energy into studying this one because the high fraction of moons observed within our solar system strongly suggests that even if it is a bit of a fluke, it's no extreme fluke. It would have to be an extreme fluke in order to invalidate that we should expect to find multiple or many other long-term homes for life elsewhere within the Milky Way. I think a guesstimate as low as only ten is as likely as a guesstimate as high as a million.

The going models suggest an oblique impact between two protoplanets is more or less necessary (likely to be the most common source), so it's really just a matter of how many of those get kicked out of their natural circular orbits to the point of hitting another--and given a protoplanetary disk virtually always forms too many planets to live with each other (assuming its metal-rich enough, which is already needed for other parts of the "equation"), there will nearly always be some protoplanets kicked into weird orbits, so it's really just a matter of whether they get ejected or hit a planet first. I'm fairly certain the likelihood of the lesser is no more than an order of magnitude from the greater. Every way you look at it, we're operating within the realm of normalcy. There's nothing truly strange about our past.

- - - -

(Regardless of the way it happened), if the KT Extinction event had not happened, probably dinosaurs would not have spawned a spacefaring race by today. The mammals did and that makes sense. Though it's not particularly outside of the realm of possibility for dinosaurs to have done it, either. But if the KT extinction hadn't happened, something else would have happened to push our planet along that path. There's a bit of a fluke in having a species that relies on inventive tool-use, because at earlier stages of mental development on that path, having an instinct for specific modes of tool operation is generally the superior method. But look around, we have dozens of separate genetic lines which, entirely through convergent evolution, developed tool use. Several of them share no common ancestry in their ability to use tools. The next most inventive tool user line on Earth (after primates) is octopuses. With this many tool-using lines, it was only a matter of time before one of them became powerful enough to dominate their planet's environment. Such dominance automatically leads to expansionism, which causes them to fill their planet's "population capacity" and think about moving outward; a species without a shortage of territory won't waste energy creating new ones.

And that is one of perhaps a few dozen specific events in which our way forward toward stepping into the stars may have been a fluke, yet nonetheless a given for it to happen eventually. That is why I believe that simply giving a biome enough time will eventually yield spacefarers. That time was around four billion years for us, and I strongly suspect we're on the short end of that timeframe because of all of the extinction events propelling us forward in almost a clockwork fashion. I'm still collecting data on this (and there is a LONG way to go) but it seems to me that for every environment as gently hostile as ours, there must be tens of others that are either too passive to push evolution forward or too hostile to keep their biome alive and healthy, and perhaps even larger numbers of biomes so fringe they simply never had room to produce spacefarers (such as eyeball planets or sub-crust biomes). Thus, it is not outside of reasonable speculation to think that maybe life usually never gets enough time. If it takes five billion years and requires the planet to remain in the goldilocks zone the whole time and without tidally locking to its star, then it could happen on a substantial range of stellar classes from K though F main sequence. But if the time needed is ten billion years, it'll be exclusive to upper K or lower G class main sequence (our own Sun is not within that range), and if it needs fifteen billion years then there aren't any stars that can host it. Therefore, as the average time needed tends upward, the likelihood of each individual (probably common) gently-hostile biome lasting long enough to spawn spacefarers becomes more and more of a fluke itself. So after eliminating the vast majority of the equation as typical, I come to the conclusion that the largest factors in speculating on the preponderance of spacefarers at any given age of the universe is:
A.) you need some (likely several) billions of years
and
B.) the high time requirement might also further reduce the likelihood of success by a significant amount

 

[Buzz Bloom]

I didn't put a lot of energy into studying this one because the high fraction of moons observed within our solar system strongly suggests that even if it is a bit of a fluke, it's no extreme fluke.

Hi reaver:

I read somewhere, (I apologize that I cannot find the reference) that the Earth's moon is the only moon in the solar system that has an angular momentum (including its orbit) larger than the spin-axis rotational angular momentum of the planet which the moon revolves around. This characteristic is, as I understand it, relevant to the role our moon played to the origin of life on Earth.

ADDED

For moon's angular momentum (L) in orbit around the earth:
L = R × M × V

https://en.wikipedia.org/wiki/Angular_momentum​

R = 3.844 × 10⁸ m (semi-major axis)
M = 7.342 × 10²² kg (mass)
V = 1.022 × 10³ m/s (average orbital speed)

https://en.wikipedia.org/wiki/Moon​

L = 2.884 × 10³⁴ kg m²/s^-1

For Earth's angular momentum (L) rotating around its axis:

https://socratic.org/questions/what-is-the-angular-momentum-of-earth​

L = 7.2 × 10³³ kg m²/s^-1

For comparison purposes, consider the moon Oberon (the solar system's largest moon) and it's planet Uranus.

https://largest.org/geography/moons-solar-system/​

Oberon
R = 5.8352 × 10⁸ m (semi-major axis)
M =3.076 × 10²¹ kg (mass)
V = 3.15 × 10³ m/s (average orbital speed)
L = 3.046 × 10³⁴ kg m²/s^-1

Uranus

https://en.wikipedia.org/wiki/Uranus​

M = 8.681 × 10²⁵ kg (mass)
R = 2.5372 × 10⁷ m (mean radius)
T = 17 h 14 m 24 s = 6.2064 × 10⁴ s (sidereal rotational period)

https://farside.ph.utexas.edu/teaching/301/lectures/node122.html​

L = I × ω
I = 0.4 × M × R²
ω = 2π / T
L = (0. 8π × 8.681 × 10²⁵ × (2.5372)² × 10¹⁴ / 6.2064 × 10⁴) kg m²/s^-1

= 2.283 × 10³⁸ kg m²/s^-1​

​

Ratios
L_Moon: L_Earth = 4.0
L_Oberon: L_Uranus = 0.000133

Sorry: I misread the list of moons. I should have used Jupiter's Ganymede.

Ganymede

https://en.wikipedia.org/wiki/Ganymede_(moon)​

R = 1.07041 × 10⁹ m
M = 1.4819 × 10²³ kg
V = 1.088 × 10⁴ m/s
L = 1.726 × 10³⁶ kg m²/s^-1

Jupiter

https://en.wikipedia.org/wiki/Jupiter​

M = 1.8982 × 10²⁷ kg
R = 6.9911 × 10⁷ m
T = 9.925 h = 3.573 × 10⁴ s

L = (0. 8π
× 1.8982 × 10²⁷
× (6.9911)² × 10¹⁴
/ 3.573 × 10⁴)
kg m²/s^-1

= 2.597 × 10³⁷ kg m²/s^-1​

Ratio
L_Ganymede: L_Jupiter = 0.0664

Regards,
Buzz

 

[sbrothy]

I seem to remember reading about a computer simulation of the creation of our solar system which didn't make sense before adding a gas giant getting slung out. I can't find the reference right now (I'm mildly inebriated and it's an uncanny sunny afternoon here in Denmark.). Anyone know the source?

[uncanilly?]

 

[sbrothy]

The simplest and most direct answer is that most people have not heard of that paper. But I would like to dive into a slightly deeper analysis and say that the paper itself has a wrong result. Without even reading the contents, I can already debunk the abstract. It claims that re-working the equation can yield a very substantial chance of there being not one single interplanetary civilization in our galaxy, and without even invoking a "yet".

Now as this paper, the Fermi Paradox, the Drake Equation, and many other sources have correctly surmised, calculating the probabilities are fraught with too many unknowns to yield a reasonable result. But what they all seem to miss is that the probabilities we can best rule out are the most extreme ones. We can narrow down the possible ranges to fields many orders of magnitudes across, yet still far narrower than a complete guess. Doing so can actually rule out some guesses entirely.

As one example, we don't know if life can exist in very many circumstances outside our own, but there is a range of circumstances in which we know that life can exist, stable enough to last for billions of years yet unstable enough to provoke gradual evolution. Furthermore, we can speculate significantly on the availability of such environments within our galaxy. We know that the primary factors leading to our home and its stability are:
1.) third-generation star with high metallicity (with accretion disk containing substantial iron fraction) of spectral class K, G, or F
2.) circular orbit within "Goldilocks" zone, with large moon at small distance
3.) local galactic habitability (not being in a sterilization zone such as with strong gamma radiation present)
possibly 4.) shepherd planet like Jupiter orbiting outside the "Goldilocks" zone

All other major factors are known to have a high likelihood or near-certainty of happening, such as stable planetary magnetic field (assuming it's an iron-core planet), presence of surface water and wide variety of lighter elements, abiogenesis within bio-stability timeframe, evolution due to natural selection, planet failing to be sterilized in major extinction events due to large variety of life across different types of habitats.

We know that a prospective star ~5 billion years ago (or ~10 billion years after the Big Bang) within the Milky Way galaxy has a pretty high likelihood of being the right spectral class and with high enough metallicity. While the vast majority of stars in the size range have low metallicity at this age of the universe, there are nonetheless at minimum millions of stars within this range just in our own galaxy, forming ~10 BYA after the Big Bang or earlier. Planetary systems around stars of high metallicity should have an extremely high likelihood of containing large amounts of iron, due to iron's position at the fusion peak--thus formation of planetary elements within a supernova will always produce substantial amounts of iron. We know many examples already of planets within the right mass range and within the "Goldilocks" zone--this is a highly likely occurrence. Estimates for galactic habitability are generally high as well; given how many times our own planet had to orbit our galaxy, it is extremely unlikely that greater than half of the galaxy lies within uninhabitable regions. So the only remaining factor is chance to obtain a moon like Earth's. This last item could be very rare, but planetary models eliminate it being more than a few orders of magnitude down. So you still have at absolute minimum hundreds of planets within our galaxy forming long-term life 5 BYA or earlier, and having pressured that life into evolving up until today (or occasionally, until its host star grew too bright and sanitized the planet).

While we have little to explain the true likelihood of life achieving the ability to exit its home planet (within its stability timeframe and prior to the current day) under these conditions, we can point to perhaps 1-2 dozen major evolutionary breakthroughs in our own evolution which led us down this path. Examining every single one, we consistently find that life took a path of fairly low resistance--while it may have taken trillions of chances to get the mutation that started the next wing of development, it always bridged the mutation gaps in bridges the size of one single mutation. What can appear to us at a glance to require three or more simultaneous and cohabiting mutations will, virtually unfailingly, happen anyway one mutation at a time. Maybe it's just the universe we live in, or maybe the apparent extreme unlikeliness hides a plethora of other extremely unlikely options of which anyone could have pushed us forward. But one way or another, no matter how many branches reach an evolutionary dead-end, there are always those that don't, and they always dominate the next niche. Our evolution toward stepping into the stars may have been nearly guaranteed, and at minimum we can cast some VERY strong doubt on its likelihood being extremely low.

So given a lower bound of perhaps hundreds of worlds like ours beginning greater than 5 MYA (more reasonably thousands to tens of thousands), and given an ever-increasing rate of these worlds forming as the universe and the galaxy age, and given the relative ease with which a planet like ours can produce people like us within the available time frame, we can cast strong doubt on the lower bound estimate of ~50% chance we're alone in the galaxy, and virtually eliminate any figures pushing higher than 90%, regardless of their methodology. They can't be right. Here I have demonstrated that Enrico Fermi was right in proposing that our galaxy contains many spacefaring civilizations; we can be relatively certain that it indeed does. But perhaps more importantly, we can dispel the Fermi Paradox easily by looking forward rather than backward: there are two assumptions which it relies upon, both of which are extremely unreasonable to the point of invalidating the proposition outright and suggesting that in fact, regardless of how many interstellar civilizations there are in our galaxy (even millions!) and how old they are (billions of years!), we should actually fully expect to NOT see any of them at our current tech level.

The first assumption is that we will expand into our galaxy on a timescale too short to be significant relative to the age of the universe. But as faster than light travel has been absolutely thoroughly debunked and as we gain a greater understanding of the needs of habitation in space, we see more and more that it doesn't matter whether we use robots, engineered life, or any other advanced technology, there simply isn't a way to maintain the expansionist mentality itself during space expansion. Ecology trumps expansion to such an enormous degree that no civilization can thrive in space (due to physics, regardless of genetics) without fully embracing an ecologist mentality. Each new position we occupy must focus predominantly on its own maintenance, and will only support a robust spaceship industry after it has been developing for a very long time. Spaceships will sluggishly drift through the stars on not-so-insignificant timescales. And we will be forced to come to terms with these limitations in order to thrive out there. Will we expand on timescales in the thousands of years? No. It's not physically possible. Will we expand on timescales in the millions of years? Again, no. This time it would be physically possible, but it would require devoting all resources toward expansion while simultaneously fully understanding and embracing the futility of that mindset. No race of people intelligent enough to expand into space won't abandon expansionism when in space. So we will expand across our host galaxy on timescales of billions of years--and given how long it took for people like us to step into the stars in the first place (~10+ billion years), it's entirely expected that our galaxy is less colonized than not.

The second assumption is that we could see them if they weren't very far away (~1-1000 light years) yet the brightness of a beacon capable of signalling us at those distances is so enormous as to be completely absurd to expect anyone to build such a thing. What would it be for? To signal to young civilizations that they aren't alone? If they're dying to know, they'll figure that out probably within a few thousand years when they start making solar-system sized collector arrays (already within our tech level), and can detect signals SEVERAL orders of magnitude fainter than what SETI can detect today with a planet-sized collector array. Since these civilizations take billions of years to crawl across the galaxy, they have no reason to make enormously bright beacons just to say hi to us when we can't even respond. No, they will communicate with lasers, direct to target. If they wanted to say hi to us, they would do it with a laser, and they would leave it running for bursts of decades or centuries at a time. SETI already failed to find such a communication within its first major sweep, so there's little reason to continue looking at this point, to be honest. I say that not with high likelihood, not with extremely high likelihood, but definitively. It can't not be true. Extreme projects of extreme stupidity in design don't happen at all, anywhere, because of the way intelligence works. We can be as certain of this as we can be of the Boltzmann Brain hypothesis being wrong.

Wow. You put some thought into this. Respect. I think the operative word is "yet". As always in these discussions (sadly?) we have only one data set.

 

[BillTre]

This characteristic is, as I understand it, relevant to the role our moon played to the origin of life on Earth.

What is the logic that the moon is important to the origin of life on earth?

 

[Algr]

What is the logic that the moon is important to the origin of life on earth?

https://www.iop.org/explore-physics/moon//how-does-moon-affect-earth

Apparently the moon stabilizes the Earth's rotation on its axis. Without it, seasons would range from non-existent, to extreme from one era to another. But the average temperature of Earth would not be affected. Personally I am not at all convinced that this would prevent life from existing on Earth - the Earth we know has undergone wild variations in climate in the past, even with a moon:



If a large moon is a necessary predecessor for life, it would simply reduce the value of ne in the drake equation. There would be no need to add a new variable for it. We've always known that the range of plausible values for ne is wide, but that uncertainty works in both directions. For example, if liquid methane can replace water, then every star would have a second habitable zone for that kind of life. That would enlarge ne massively.

 

[Buzz Bloom]

What is the logic that the moon is important to the origin of life on earth?

Hi Bill:

There was some discussion about this in another thread, post #10.

https://www.physicsforums.com/threads/a-competitor-to-the-drake-equation.990441/#post-6356940​

That post also referred to an article about this concept.

https://www.newscientist.com/articl...on-earth-suggests-theory/?ignored=irrelevant#​

Posts #12 and #14 in that thread might also be of interest to you.

Regards,
Buzz

 

[stefan r]

Hi reaver:

I read somewhere, (I apologize that I cannot find the reference) that the Earth's moon is the only moon in the solar system that has an angular momentum (including its orbit) larger than the spin-axis rotational angular momentum of the planet which the moon revolves around. This characteristic is, as I understand it, relevant to the role our moon played to the origin of life on Earth.

...

Charon!

Regardless of what label we use it still says something about the odds of double planets occurring. Is Earth a planet with a big moon or a double planet with a small secondary?

All 4 gas and ice giants have more orbital angular momentum than the Sun's rotational momentum. Not sure if that is relevant.

 

[Buzz Bloom]

Regardless of what label we use it still says something about the odds of double planets occurring.

I agree it is plausible that some double planet configurations may have properties relevant to life originating there, but my wild guess is that this is not likely. In order to have a better feeling regarding this possibility I would guess that one must examine all (or at least most) of the various plausible specific reasons that a large moon has a role in life forming on a planet in order to determine if the same reasons would apply to a double planet configuration.

Wikipedia says

https://en.wikipedia.org/wiki/Double_planet​

that the use of the term "double planet" is not uniformly agreed to by scientists in the relevant field. Thus in exploring the above plausible possibility, one should provide a clear definition including
(a) what is the required mass for a body to be a planet, and
(b) what is the acceptable lower limit for the ratio of the smaller planet mass to the larger planet mass.

Regarding the definition of a planet, Wikipedia says

https://en.wikipedia.org/wiki/IAU_definition_of_planet​

the following three conditions must be satisfied: it

1. is in orbit around the Sun,
2. has sufficient mass to assume hydrostatic equilibrium (a nearly round shape), and
3. has "cleared the neighborhood" around its orbit.

I am not sure what (2) exactly implies regarding the mass.

Regards,
Buzz

 

[Dragrath]

Interesting conversation here discussing the problem with arguments that claim to "solve" the "Fermi paradox" or that the latter is even a thing given the sheer quantities of unknowns. As far as I am aware the only forms of this "paradox" with any validity given the largely unconstrainedsample size is that there is no evidence for galaxy spanning Dyson bulding civilizations in the nearby universe which is dubious to interpret meaning from as it is dependent on a large number of assumptions.

*snip*
All other major factors are known to have a high likelihood or near-certainty of happening, such as stable planetary magnetic field (assuming it's an iron-core planet), presence of surface water and wide variety of lighter elements, abiogenesis within bio-stability timeframe, evolution due to natural selection, planet failing to be sterilized in major extinction events due to large variety of life across different types of habitats.
*snip*

Fairly well spoken argument in discussion but the highlighted paragraph seems a bit problematic in assumptions to me since we don't really know enough to make some of these statements.

For example we still don't understand why Venus lacks a magnetosphere we have guesses but they remain speculation as we lack actual data or even a complete understanding of the extremely chaotic dynamics of (12 degrees of freedom for any given element in the simplest model for Magnetohydrodynamics(MHD)
These systems are path dependent i.e. the state of the MHD is dependent on what happened in the past thus there are far too many unknowns to call this a near certianty. Plus from observations in our own solar systemit is likely that a stable magnetic field isn't enough you need a field thathas a large enough dipole moment to
direct the stellar wind around the planet entirely otherwise the field might just boost the rate of atmospheric erosion.While on this subject I have a number of problems with the typical drake equation due to how simplistic some of the terms are relative to what we can at least glean from the fossil record on Earth for all that it is a single data-point. For example the first isotopic and chemical evidence for aerobic photosynthesis appears over 3.5 Ga with the innovation to trade out dihydrogen sulfide for dihydrogen monoxide in photosynthetic reactions but oxygen didn't build up until the Great Oxygenation Event (GOE) around 2.4 Ga suggesting some other innovation or major development was needed but the fermi paradox assumes all the steps are inevitable. This is particularly relevant since there really only appears to be a single development of aerobic respiration compared to the far larger ensemble of sulfur, carbon dioxide based or metal ion(Fe(III) as the most dominant metabolism among anaerobic life probably due to both its initial preGOE abundance based respiration pathways. This is hard to directly interpret but the studies of the Long Term E. Coli Evolution Experiment clearly demonstrates that not only are mutations random but the rate of accumulation of a number of characteristics are typically needed to evolve new and novel traits they can't just purely emerge on their own but rather come from repurposing what is already there particularly benign neutral mutations.

It seems rather explicit that the evolutionary development of a system is as much up to chance as it is to past events. This seems to be a consistent property of chaotic systems in nature from the Navier Stokes equations, Magnetohydrodynamics to orbital dynamics. As the properties of the drake equation depend on these highly nonlinear chaotic systems for their inputs. Suffice to say using the Drake equation to estimate the number of intelligent civilizations even for parameters we can place empirical results to fill in is fraught with issues.

 

[bob012345]

I wonder exactly what Fermi would have taken as clear evidence intelligent life did exist outside of humanity? Was he was expecting a 'flying saucer' to land on the White House lawn in front of TV cameras (The Day the Earth Stood Still scenario)? Or something else?

 

[DrStupid]

I wonder exactly what Fermi would have taken as clear evidence intelligent life did exist outside of humanity?

First we should ask what he would have taken as intelligent life. If it includes something like humans it could easily be recognised by the resulting environmental degradation. We have such a strong impact on the environment that we produced our own geological epoch - the Anthropocene. Space faring civilizations (or swarms or however they might be organized) could be even worse. The very existence of our biosphere is an indication that Earth had no visitors like us before.

 

[bob012345]

First we should ask what he would have taken as intelligent life...

Fermi's original view was that if they existed at all the galaxy should be utterly filled with them. So the ability to colonize would be the sign of intelligence. But there are still a lot of assumptions built in that. Perhaps they are particular as to what worlds are suitable. Perhaps they don't need worlds at all. Perhaps they have a non-interference policy. Perhaps they examined Earth during the Jurassic period and decided it was too dangerous. But even if they were in our stellar neighborhood, we are only now getting the technology to see hints such as the chemical composition of exoplanet atmospheres. Perhaps they are here but only discreetly and occasionally visit from bases hidden from us observing, waiting but for what?

We know more about the universe since 1950 when Fermi pondered this question. We now know about theoretical possibilities such as the Alcubierre drives and wormholes. It's impossible for us now to use these but what about an technologically advanced society perhaps a million year further along?

https://www.seti.org/seti-institute/project/fermi-paradox

 

[sbrothy]

“Non-interference policy”

And perhaps they’re a bunch of peaceful dirty pacifiist hippies with 30 arms so they can play several guitars while they welcome their new overlords (us) and entertain us while we pillage and burn their planet looking for our McGuffin. :)

 

[DrStupid]

Perhaps they don't need worlds at all.

I would expact that from an interstellar space faring species. However, they do need resources.

Perhaps they have a non-interference policy.

It is very unlikely if not even impossible that there are global policies all over the galaxy. We don't even manage that on Earth even though we have realtime communication. A message from one end of the Milkyway to the other takes 100 thousand years. That's time enough for a lot of civilisations to come and go. That would result in a fragmentation of the galactic population into countless fractions which not only differ in their policies. They wouldn't even realize that that they originate from the same species.

Perhaps they examined Earth during the Jurassic period and decided it was too dangerous.

Maybe many visitors would decide not to land on Earth because it is too dangerous for the dinosaurs. But it is sufficient if only one of them decides to mine Earth for resources. The Chicxulub impact would have been a day at the beach in comparison.

Perhaps they are here but only discreetly and occasionally visit from bases hidden from us observing, waiting but for what?

It wouldn't be sufficient to hide and wait. They would need to protect us from others for many millions of years.

 

[bob012345]

If an advanced civilization did develop Alcubierre drives or learn to use wormholes, messaging would be quick as would travel allowing for a cohesive civilization over galactic distances. That might be the motivation to become a galactic civilization rather than remain a provincial backwater merely interstellar civilization.

 

[BWV]

The basic FP expansionist logic just requires sub-light travel. Self-replicating Von Neumann probes traveling at 0.1C could still have completely overrun the galaxy by now. Megastructures built by K1+ level civilizations would be another indicator that does not require space-opera stellar empires. Additionally, it is rational to assume any technological civilization would become concerned with its eventual extinction as its home star ages