Rough probability that aliens would visit the Earth?

• B
According to a survey in 2017, nearly half of the Americans believe, that aliens visit earth.

However, I think that a lot of conditions have to be met, so that aliens would visit earth.

First of all, they have to exist.

Meaning
• their home planet has to be a habitable planet (but only a very low proportion of planets is habitable at all)
• biological life must be created out of dead chemical elements (like from the elements C, H, N and O on earth)
• their home planet must stay habitable over millions of years

Second, they must find earth
Meaning
• Among a lot electromagnetic backround noise they must pick up one of our electrmagnetic signals
• And they have to search a very very huge universe for us
• And if they find a signal from us, it would be maybe already millions of years old, and we may not exist any more

Third, they must travel to us
Meaning
• They must be capable to travel somewhat light-years to us
• They must be willing to do such exertion journey

Every bullet point mentioned above is already pretty unprobable. When assuming a probability of 1% for each of the eight bullet points above, that would compute a total probability of

1% ^ 8

= 0.01 ^8

= 1 E -16

= 1 / 10^16

Do you agree, roughly?

Last edited:

Answers and Replies

For a race of beings capable of traveling light years, I'm certain they would have the technology to make them selves unknown to us (regardless of our attempts). I also question why they would want to visit or even observe an inferior race of beings. Michio Kaku said: “Imagine walking down a country road, and meeting an ant hill. Do we go down to the ants and say, ‘I bring you trinkets. I bring you beads. I give you nuclear energy and biotechnology. Take me to your leader?’ Or we have the urge to step on a few of them??” which adds further to the question of "why".

My gut reaction, for largely physics based reasons is that for the five bullet points from the second and third categories, the likelihood of any of them being viable would be infinitesimal or less. This leaves as undefined the likelihood of a suitably intelligent race of aliens.

Our radio/EM signature would be indistinguishable from noise, not too far out, no matter how you calculate it.

Such a search would have to have something identifiable worth detecting....... This would likely necessitate live scouts or at least robot ships. See above for why remote searching is unlikely.

Physics, as we understand it, likely rules out actual FTL travel. The issues and difficulties (and relative costs, even if possible) with a sub-C journey (as has been time and again discussed near-exhaustively on PF) largely preclude even a 'shortish' range, highly directed exploratory mission, even if there was someone to make it.

&c, &c...

diogenesNY

given that there has been plenty of time for space-faring aliens to have colonized the entire galaxy travelling at sub-light speeds, the question is why have we not seen any sign of extraterrestrial life? (the Fermi Paradox)

Ball park probability - between 0 and 1. There are so many unknowns.
why have we not seen any sign of extraterrestrial life? (the Fermi Paradox)

I can only conclude that living in space is a lot harder than we think. And don't forget that many of the worlds that could harbour life have stronger and deeper gravitaional fields, as well as thicker and deeper atmospheres, which might effectively rule out ever leaving their planet.
If Earth was much smaller it could not have hosted complex life at all, so we might be in a very sweet spot amongst all the variables.

Ball park probability - between 0 and 1. There are so many unknowns.

I can only conclude that living in space is a lot harder than we think. And don't forget that many of the worlds that could harbour life have stronger and deeper gravitaional fields, as well as thicker and deeper atmospheres, which might effectively rule out ever leaving their planet.
If Earth was much smaller it could not have hosted complex life at all, so we might be in a very sweet spot amongst all the variables.

Earth rockets are somewhere between 85 and 92% fuel, if Earth were a little larger we couldn't leave with a chemical rocket.

Cheers

CalcNerd
"Earth rockets are somewhere between 85 and 92% fuel, if Earth were a little larger we couldn't leave with a chemical rocket."

More stages ?
Or air-breathing jet / rocket combo such as the Sabre that 'Reaction Engines Ltd' have designed ?
If your atmosphere is significantly denser, perhaps a 'Rockoon' option opens...

phinds
Science Advisor
Gold Member
2021 Award
Do you agree, roughly?
No. I think you vastly overestimate the odds of some of those things happening and leave out other significant ones and end up with a probability that is way too high.

> their home planet has to be a habitable planet (but only a very low proportion of planets is habitable at all)

Why do you think so? We now know that planetary systems are common. So the "very low" above might be actually something like "one in 50 stars has a habitable planet".

> biological life must be created out of dead chemical elements (like from the elements C, H, N and O on earth)

Happened on Earth pretty much "immediately" after it cooled down enough.

> their home planet must stay habitable over millions of years

It is quite usual for planets to have surface conditions stable.

> Among a lot electromagnetic backround noise they must pick up one of our electrmagnetic signals
> And they have to search a very very huge universe for us
> And if they find a signal from us, it would be maybe already millions of years old, and we may not exist any more

Yes, the detection is hard. However, aliens may simply visit _every_ solar system as their civilization expands.

> They must be capable to travel somewhat light-years to us
> They must be willing to do such exertion journey

Do you think that if we would have the means to organize an expedition to Alpha Centauri, we would hesitate to do so??

> Every bullet point mentioned above is already pretty unprobable.

No, not all of them. Some are rather high probability, see above.
One step which might be very improbable which you missed is the appearance of *intelligent* species, not just life, on the planet of the aliens.

stefan r
This is a variation of the Drake equation:
https://en.m.wikipedia.org/wiki/Drake_equation
Indeed, it is a variation of the Drake equation. However, the Drake equation ends up with the total number of possible intelligent llife forms in the universe. But the total number - let's just assume 5 here to show the problem - of possible intelligent life forms in the very huge universe, can be easily misinterpreted as - 5 - chances that we would meet them, which would the opposit of the truth.

> Every bullet point mentioned above is already pretty unprobable.

No, not all of them. Some are rather high probability, see above.
One step which might be very improbable which you missed is the appearance of *intelligent* species, not just life, on the planet of the aliens.

No. I think you vastly overestimate the odds of some of those things happening and leave out other significant ones and end up with a probability that is way too high.

I guess, the very low proportion of habitable planets is the main probability driver, here. Even if there would be intelligent life somewhere in the very very huge universe, they would be so many light years away, that they could not detect us and neither could they travel to us.

To visualize this widely accepted assumption, one should calculate an average distance of possible alien live, given the very low proportion of habitable planets (from all planets) and the density of planets in the universe.

Consuli

This is all a wel trod area of speculation, but several possibilities have not been mentioned yet. One pessimistic possibility is that intelligent species tend to destroy themselves through warfare or environmental destruction so none survive to launch interstellar probes (again thin distances really don’t matter because a starfaring civilization could colonize the whole galaxy either in person or with Von Neumann probes in a few million years and the galaxy is old enough for this to have happened)

The Wikipedia article is a good summary

https://en.m.wikipedia.org/wiki/Fermi_paradox

Even if there would be intelligent life somewhere in the very very huge universe, they would be so many light years away, that they could not detect us and neither could they travel to us.

This is likely to be NOT the limiting factor in "meeting the aliens" - it's probability is not low.

Expansionism is a common trait of life in general - it is evolutionary selected. Expansionist species are less likely to go extinct.
Similarly, expansionist societal "ideology" is similarly selected - it is more likely to survive than a secluded sedentary society.

Aliens don't even need to be expansionist as a whole - it's enough for just one branch of their first few interstellar colonies to adopt a worldview where expanding deemed important. If they do this, their civilization is likely to start expanding at close to their maximum interstellar travel speed.

It's not a given (I'm not saying probability of this happening to interstellar-capable civilization is ~1), but it's likely: probability is somewhere in 10%-90% territory.

And unless they have *ridiculously awesome* interstellar drives, large interstellar "hops" would be still hard for them. It's likely they will prefer smaller ones. This means that they will not just travel to the selected "best" systems hundreds of light-years distant from one another - they will hop to nearest ones. This means almost every stellar system will be visited within the expanding "ball" of colonized space. This means they will visit Earth as soon as this expanding "ball" reaches Earth. No search is necessary.

Last edited:
russ_watters
Mentor
Indeed, it is a variation of the Drake equation. However, the Drake equation ends up with the total number of possible intelligent llife forms in the universe. But the total number - let's just assume 5 here to show the problem - of possible intelligent life forms in the very huge universe, can be easily misinterpreted as - 5 - chances that we would meet them, which would the opposit of the truth.
Right, so what one would need to do is add or modify the terms to incorporate the desired constraints.

russ_watters
Mentor
given that there has been plenty of time for space-faring aliens to have colonized the entire galaxy travelling at sub-light speeds, the question is why have we not seen any sign of extraterrestrial life? (the Fermi Paradox)
I've never been very impressed by the Fermi Paradox. It has strained premises and slipped and stretched steps of logic.

E.G., the mere fact that we could traverse the galaxy in a million years at 10% of the speed of light is just math unconnected to reality. Perhaps Fermi assumed that such a trip would be both easy and desirable? There's no good reason to believe either would necessarily be true.

IMO, the Drake equation and its variants are much more informative, useful and connected to reality.

Last edited:
I've never been very impressed by the Fermi Paradox. It has strained premises and slipped and stretched steps of logic.

E.G., the mere fact that we could traverse the galaxy in a million years at 10% of the speed of light is just math unconnected to reality. Perhaps Fermi assumed that such a trip would be both easy and desirable? There's no good reason to believe either would necessarily be true.

IMO, the Drake equation and its variants are much more informative, useful and connected to reality.

But there are no such assumptions - it’s a framework around the proposition that conservative inputs to the Drake equation give a large number of potential intelligent alien civilizations and the galaxy is old enough for these to have spread throughout the galaxy. IF interstellar travel was feasible (again does not have to be in person, could be self-replicating Von Neumann probes or some other automated method) then we should have some sign of being visited OR be able to see some sign of these cultures outside our solar system (or galaxy even)

... OR be able to see some sign of these cultures outside our solar system (or galaxy even)

What signs would you expect to be visible if Andromeda would be undergoing large-scale colonization?

russ_watters
Mentor
But there are no such assumptions - it’s a framework around the proposition that conservative inputs...
Inputs=assumptions.

And maybe I'm being overly harsh in having the benefit of an additional 50+ years of history and technological development, but:
... IF interstellar travel was feasible..
...calling it an "if" doesn't soften the fact that when you insert it into an equation or line of logic you have assumed it to be true. And not for nothing, but my read of the history is that they really did believe it at the time. And this is why I say maybe I'm being overly harsh: the idea was developed from about 1950-1975, which probably not coincidentally overlaps the Space Race. I think there really were people who really did believe that after 1972 (the last moon landing), the next stop was Mars and after that, the stars. But the reality of the next 45 years is we have done nothing but go backwards, both for manned and unmanned spaceflight.

Inputs=assumptions.

And maybe I'm being overly harsh in having the benefit of an additional 50+ years of history and technological development, but:

...calling it an "if" doesn't soften the fact that when you insert it into an equation or line of logic you have assumed it to be true. And not for nothing, but my read of the history is that they really did believe it at the time. And this is why I say maybe I'm being overly harsh: the idea was developed from about 1950-1975, which probably not coincidentally overlaps the Space Race. I think there really were people who really did believe that after 1972 (the last moon landing), the next stop was Mars and after that, the stars. But the reality of the next 45 years is we have done nothing but go backwards, both for manned and unmanned spaceflight.

you have just posited one possible solution to the paradox, but it still remains the framework for this problem. Interstellar travel does not seem to me much of a leap for a K level 2 civilization as they would have all the energy and materials of their solar system at their disposal for the attempt - for example they could produce as much antimatter as they needed for fuel or build large enough lasers to propel craft with light sails.

you have just posited one possible solution to the paradox, but it still remains the framework for this problem. Interstellar travel does not seem to me much of a leap for a K level 2 civilization as they would have all the energy and materials of their solar system.

Just because you have a name for a type of imagined civilization, it does not mean such civilizations are definitely possible.

russ_watters
Just because you have a name for a type of imagined civilization, it does not mean such civilizations are definitely possible.

the phrase ‘definitely possible’ is without meaning as we are all talking about possibilities and their logical implications if true

Are you saying that a K2 civilization is ‘definitely impossible’ ? - as that is the only antonym to the phrase and implies a level of certainty that no one possesses. Plenty of scientists have thought that K2 or K3 level civilizations are possible, but we will never know for certain until we either find one or become one

it does not seem that great of leap to think that barring some catastrophe, humans could reach that level

Are you saying that a K2 civilization is ‘definitely impossible’?

I am saying that it might be impossible.

Moreover, I don't think that merely having access to the full extent of star system's resources/energy necessarily leads to creation of immense devices processing/emitting all this energy at one installation.

For example, cumulative US electrical power output is ~1000 GW, but we did not find any use for even a "measly" one gigawatt continuous wave laser. If anything, recent advances of our technology lie mostly not in larger and more powerful devices, but in more efficient, more precise and/or smaller devices.

IOW: "K2 civilization" may be unobservable on intergalactic distances, even if it cumulatively does process a lot of energy.

russ_watters
E.G., the mere fact that we could traverse the galaxy in a million years at 10% of the speed of light is just math unconnected to reality.

Exactly. A collision at 10% light speed with a small grain of sand would cause an (nuclear) explosion, roughly equal big than the one from the asteroid, that had wiped out the dinosaurs from earth.

Thus, travelling light-year ranges will be a very tough one, even if the evolution of those aliens would be ten times faster than ours (which would be another too optimistic assumption).

Consuli

russ_watters
russ_watters
Mentor
you have just posited one possible solution to the paradox, but it still remains the framework for this problem.
No.

I know it isn't your fault (you didn't develop it), but by calling it a paradox in need of a solution - the framework for this problem - we assume it is a properly formed line of logic, with accurate inputs, that points to an actual problem with reality (opposite purpose from the Twins Paradox). Instead, it is much more likely to simply be just an improperly formed line of logic. So rather than treating it as an already proven valid conclusion:

We should see aliens here already.

...it should be formulated as a question:

Should we see aliens here already?

That way it doesn't seem like as big of a deal to refine it: I'm not looking for a "resolution" to a known paradox but instead simply refining a known faulty assumption or line of logic. And this is why the presentation of the Drake equation is better IMO: It's readily acknowledged that none of the inputs are certain and are undergoing constant refinement. It's a question, not a claim. There is no established conclusion to be the basis of a "paradox".

So no, I don't agree that we should consider the Fermi Paradox to be a useful "framework for this problem". The "framework" the OP provided is much better, and it's his thread, so we should follow his lead.
Interstellar travel does not seem to me much of a leap for a K level 2...
No.

@nikkkom put it succinctly, but I will expand:

I think you are using that backwards (opposite of the logic issue discussed above). I don't think it was intended to be a statement that Kardashev believes such civilizations were realistically possible (I haven't seen the orginal background of his work), but rather positing if they were possible, asking what would they look like. So it can't be used as an input to the Fermi Paradox/Drake Equation: given our current knowledge it is not reasonable to assume such things are realistically possible for the purpose of this discussion.

[mod hat]
Also, in general, please, let's keep this thread confined to known reality and realistic/reasonable speculation. It's a problem in the astronomy forum we are working to correct. And what is reasonable speculation? Per the discussion in the thread: It is reasonable to speculate we could land people on Mars in a decade or two. It is not reasonable to assume that we will develop a method for traveling at 10% of the speed of light, cheaply, in the next decade or two (or fifty).

The assumption "interstellar travel is realistically possible" is not supportable by current knowledge of science. It may not be used as a basis for a line of logic about reality here. Further discussion of it may be deleted. [/mod hat]

Bandersnatch
russ_watters
Mentor
Plenty of scientists have thought that K2 or K3 level civilizations are possible...
Caveat to my previous post: if you can provide a reference to a respectable scientist who has said they believe interstellar travel is realistically possible - not just theoretically possible - then it may be used as part of this discussion (as in anput to the problem/equation).

Exactly. A collision at 10% light speed with a small grain of sand would cause an (nuclear) explosion, roughly equal big than the one from the asteroid, that had wiped out the dinosaurs from earth.

Wrong. Grain of sand would be a rock up to ~2mm in size, or ~0.025gram in mass. At 10% light speed its energy would be ~3 tons of TNT (edit: was 2 tons, but 3 seems to be closer).

Caveat to my previous post: if you can provide a reference to a respectable scientist who has said they believe interstellar travel is realistically possible - not just theoretically possible - then it may be used as part of this discussion (as in anput to the problem/equation).

if that is the standard then you have been remiss in your mod responsibilities by letting this thread continue. The OP question was the probability of aliens visiting earth. We do not currently know of any realistic method of interstellar travel - if that is the criteria then there is no discussion - we might as well be talking about time travel and you should close the thread.

However the question implies what a technological civilization that has existed for thousands or millions of years might be able to accomplish - no one has a clue about this, just what our current knowledge of science allows or disallows as theoretically possible. The Fermi Paradox is the established framework for thinking about these possibilities

"Earth rockets are somewhere between 85 and 92% fuel, if Earth were a little larger we couldn't leave with a chemical rocket."

More stages ?
Or air-breathing jet / rocket combo such as the Sabre that 'Reaction Engines Ltd' have designed ?
If your atmosphere is significantly denser, perhaps a 'Rockoon' option opens...

The basic rocket equations show that there is not enough energy in chemical fuels to lift a payload greater than around 15% of the total mass of the rocket, regardless of how you go about it. Separate stages are used because the engines of the later stages have nozzle designs that are a better match for the higher atmosphere increasing the efficiency. There are nozzle designs called aero spike engines which work in all atmospheric conditions alleviating the need for some stages.

You could build something in orbit I guess, or as you say some combination of other lift mechanisms to gain altitude. Imagine the size of a ballon or something required to lift something like the final stages of the Saturn 5 in order to get to the moon, possible I suppose but difficult.

Cheers

Wrong. Grain of sand would be a rock up to ~2mm in size, or ~0.025gram in mass. At 10% light speed its energy would be ~3 tons of TNT (edit: was 2 tons, but 3 seems to be closer).

Sorry, I have taken some zeros to much, over the thumb. However, 3 tons of TNT will be a pretty big damage for a space ship, right?

However the question implies what a technological civilization that has existed for thousands or millions of years might be able to accomplish - no one has a clue about this, just what our current knowledge of science allows or disallows as theoretically possible. The Fermi Paradox is the established framework for thinking about these possibilities

However, the assumption that such an million years old and technologically almighty civilization would exist at all, is very optimistic. Especially it is all based on the szenario, that aliens would have significantly better conditions than we have.

But on earth the Chicxulub-asteriod, that wiped out the dinosaurs, had exactly the perfect size. If it would have been smaller, the dominant species on earth would be dinosaurs and birds, now (not humans). If it would have been bigger, the whole atmosphere would have collapsed. Thus, the size of the Chicxulub-asteriod had been a perfect lucky event, to speed up the evolution on earth.

So, it is pretty probable, we on earth had the perfect conditions and alien life on other planets has much more worse conditions!

And still the maximum capacity of the planet earth for human living will be reached soon, and we have no clue at all, how we can get to next habitable planet!!

It is much more probable, our civilization will end in a big war one day, when the ressources of earth are going to end. (Maybe this depressing prospect is the reason, why we like to think about an almighty alien species, that will save us one day, so much. )

Consuli