It also completely ignores the concept of Von Neumann probes, and robot aliens spreading our across the galaxy over a time period of a million years or so.
I'm inclined to agree with Scaramanga on this point, particularly the possibility of long-duration "missions".
When originally expressing his paradox, Enrico Fermi seemed to be thinking in terms of sub-light-speed travel, and physical 'visits'.
The Fermi paradox is usually interpreted nowadays as the conflict between the assumption that extra-terrestrial intelligence is widespread, and the fact that we haven't observed any evidence of its existence. It was mooted by Fermi in conversation with fellow scientists Edward Teller, Emil Konopinski and Herbert York.
In 1984, Doctor Eric M. Jones of Los Alamos National Laboratory, in correspondence with Teller, Konopinski and York, attempted to recreate the conversation that led to Fermi formulating his paradox and the subsequent discussion
(Jones, E.M., March 1985 "Where Is Everybody?" An Account of Fermi's Question
" Los Alamos: Los Alamos National Laboratory)
-Courtesy of Internet Archive Wayback Machine:
Teller relates Fermi asking him about the chances of detecting a material object travelling >C
, Teller effectively discounts the possibility, Fermi thinks 10% (although I don't know how serious he was being in the given context) . Teller continues that the men then discussed whether "...flying saucers might be due to extraterrestrial people", their conclusions were "...purely negative".
It was after
this conversation- in which Teller had ruled out superluminal travel, and the men had agreed that 'flying saucers' are not from ETI's- that Fermi asked "Where is everybody?".
York recounts that after posing the question, Fermi did some calculations to estimate the possibility of ETI arising (see text, 11th image above, marked p.10). If York is correct, Fermi's musings anticipated the thematically identical Drake equation by a decade.
"He [Fermi] concluded on the basis of such calculations that we ought to have been visited
long ago and many times over."
It seems that Fermi thought that it was paradoxical that we had not been visited
even in the context of subluminal travel.
If Fermi's paradox had been dependent on the assumption that ETI travel >C,
it wouldn't be
a paradox for Teller, who had just made his scepticism of FTL travel clear.
the general argument goes like this:
a) the aliens are constrained by the rules of physics
b) these rules don't allow them to be here
c) therefore, they're not
Point (a), whatever level of technology is postulated, must be correct. But I don't think point (b) follows from this. We know material objects from interstellar space visit the solar system ('Oumuamua). AFAIK there is no absolute reason that an alien artefact couldn't traverse interstellar distances given enough time.
An interstellar "message in a bottle" vehicle, able to convey that its designers exist(ed) and perhaps deliver some information, needs to be able to retain functionality at approx. -270 Celsius and have protection or redundancy from effects of cosmic rays for prolonged periods. I don't think this is physically impossible. A hypothetical messenger could remain dormant for most of its voyage, reactivating when stellar radiation provides a useful power source.
I'm more sceptical of self-replicating probes. Unless ETI have mastered transmutation of elements, which would seem to require overcoming the strong nuclear force, a probe 'scavenging' asteroids etc. would presumably have to identify and visit several different bodies to acquire diverse elements. A rocky planet (like Earth) might have everything necessary, but then you have to build another substantial launch vehicle. Maybe a versatile nanotechnology would enable construction of functionally complex 'daughter' probes from common materials.
If such a messenger has visited our solar system, it either failed or we didn't notice (we've only been using radio for some 127 years). Personally, I think "the big silence", and the absence of evidence, might well be evidence of absence (and is more convincingly so as time passes and our instruments probe further). But if technological ETI exists, a message-in-a-bottle is not impossible, and isn't too big a conceptual (or engineering) leap from Pioneers 10 and 11, Voyagers 1 and 2.
Secondly, any alien culture is subject to the same rules of evolution as we understand them. If there is an age to the universe and it took a set amount of time for the various factors needed to have our species evolve, then a similar length of time is needed for other alien species to evolve. Go back to many hundreds of millions of years, and the universe hasn't settled into a state where planets that can evolve intelligence. If our, and the aliens evolution is constrained by the age of the universe, its unlikely they have evolved intelligence greatly beyond us
This only holds true, as an argument against ETI existing and possessing superior technology to us, if we accept it as axiomatic that Earth is the youngest world (or exactly the same age as the youngest worlds) on which life could evolve- a form of Earth exceptionalism.
There are no observations that support this.
It also requires biological evolution to somehow follow a schedule over the same timescale in different environments. But evolution is not deterministic or prescriptive like that- it isn't comparable to a chemical reaction or radionuclide decay.
There are stable Sun-like main-sequence stars more than 3 billion years older than the Sun, HD 197027 in Capricornus, 255 ly away is very similar- with very close metallicities- and is 2.32 to 3.4 (upper estimate) billion years older.
There is no scientific reason to doubt that in our galaxy there are millions of main-sequence solar analogues older than the Sun.
This might be of interest (sorry, it's Wikipedia, but it is a good outline of the topic), "Solar analog"
https://en.wikipedia.org/wiki/Solar_analog#:~:text=These stars are photometrically similar to the Sun,,less), because such a companion stimulates stellar activity
You would have to go back billions of years- not many hundreds of millions- to have an environment which might be less amenable to life (early generations of stars- and their planets, if any- had lower metallicity; the complex chemistry required by life might not have been found).
There are too many unknown variables in our own natural history to realistically state that it would take the same length of time- within a thousand years ?!- for a technological culture to evolve elsewhere, even if Earth were one of the first planets to form which could support life. -I use "within a thousand years" because (hopefully) our own technology in 1000 years time will be significantly superior to what it is now.
Life on Earth may have appeared 3.7 billion years ago or earlier. But it took perhaps 2.9 to 2.95 billion years for true multicellular life to evolve from unicellular organisms
(Erwin, D.H., Early metazoan life: divergence, environment and ecology, 19 December 2015, Philosophical Transactions of the Royal Society B
, Royal Society Publishing, London)
- If it took 2.95 billion years, are we really saying that it has to take exactly
that long, or longer, on any world with unicellular life?
If we posit an alien world exactly the same age as Earth, where multicellular life arose from unicellular life in 99% of the time it took on Earth, it gives the alien biosphere a mere 29.5 million years head-start.
We don't know how often life arises, how often it makes the leap to multicellular organisms, how often this leads to a technological culture- it might have only happened once. But if life can arise elsewhere, I don't think that there are any scientific objections to there being biospheres older than our own.
Edited by me, 04:56: Removed a long chunk where I make a ridiculously optimistic assumption about the time required to cross 10 ly based on my own flawed arithmetic.