Los Alamos report that collects the recollections of Emil Konopinski, Edward Teller and Herbert York about the lunch at which Fermi asked his question, and dates it very probably to the summer of 1950.
“Fermi did ask the question, and perhaps not surprisingly, issues still debated today were part of the discussion.” source
Text transcription hosted on a University of Nevada, Las Vegas physics course page. The original report is listed at OSTI (osti.gov/biblio/5746675), which refused connections when checked.
The SETI Institute's explanation of the equation Frank Drake devised in 1961 as the agenda for the first SETI meeting at Green Bank: seven terms, of which only the star formation rate was well known at the time.
“The Drake Equation has been called the second most important equation in science.” source
Opened via seti.org/drake-equation-index, which redirects here.
NASA's explainer on the search for life. Names the Fermi paradox, says exoplanet surveys have filled in some Drake terms but most remain blank, and states there is no evidence of life beyond Earth.
“But so far, we have no evidence of life beyond Earth.” source
Date is the page's last update.
CLASSIFIEDDeclassifiedFILE NO. 025SHEET 2 OF 2
Analysis
Our own reading of the file, written from the sources on the other sheets. Opinion is marked as such.
What is documented: the question was asked. Jones's Los Alamos report reproduces the recollections of all three surviving companions, gathered some thirty-five years later. They disagree on the exact wording and on what Fermi said next, but not on the question. Fermi did ask the question, as Jones puts it.
What is not documented is an answer. NASA states plainly that we have no evidence of life beyond Earth, and that most terms of the Drake equation are still blank. Explanations range from Hart's (there is no one) to the ones York recalled Fermi himself sketching that day: interstellar flight is impossible, not worth the effort, or civilisations do not last long enough. None has been tested, because the evidence that would decide between them is exactly what is missing.
Our label is Unexplained, applied to the checkable claim: no search has found other civilisations, and no explanation for that is accepted. It would change only with a detection, or with a consensus that there was never anything to explain, which is what the strongest critics already argue.
What it shows
That Fermi asked the question in 1950, according to three participants whose accounts Los Alamos published in 1985.
That, according to NASA, there is so far no evidence of life beyond Earth.
That the Drake equation was written in 1961 as a discussion agenda, and that most of its terms remain unknown.
What it doesn't prove
That we are alone. An absence of evidence after a limited search is not proof of absence.
That anyone is out there. The Drake equation organises the unknowns; it does not measure them.
That Fermi himself believed in a paradox. He never published on it, and Gray argues that the claim that their absence here means they do not exist was first published by Hart.
The other side
The strongest case against calling the silence unexplained is that there may be nothing to explain. Sandberg, Drexler and Ord argue that the paradox comes from plugging single best guesses into the Drake equation. When each uncertain term is given its realistic range, spanning many orders of magnitude, there is a substantial probability that we are alone in the observable universe, and an empty sky is unsurprising. On this reading no exotic explanation is needed.
The paper that turned Fermi's question into an argument: interstellar travel is feasible for an advanced civilisation, migration would fill the galaxy in a few million years, so their absence here means there are none.
Peer-reviewed. Scanned PDF from NASA ADS without a text layer; the argument is summarised as described in Jones (1985) and Gray (2015).
OFFICIALRobert H. Gray, Astrobiology 15(3) · 2015-03
Argues that Fermi never published on the topic, that the claim that absent visitors means no extraterrestrials was first published by Michael Hart, and that it is not a valid paradox.
“The Fermi paradox appears to originate in Hart's argument, not Fermi's question.” source
Peer-reviewed (Astrobiology, March 2015, 15(3):195–199); arXiv copy posted April 2016.
COMMENTARYAnders Sandberg, Eric Drexler, Toby Ord · 2018-06-06
Replaces point estimates in the Drake equation with realistic uncertainty ranges and finds a substantial prior probability that no other intelligent life exists in the observable universe.
The original LA-10311-MS report at OSTI (osti.gov/biblio/5746675) and its exact date (listed elsewhere as March 1985): OSTI refused connections when checked; the UNLV transcription was used.
Hart 1975: the ADS scan has no text layer, so the paper's own wording was not read; the argument is cited as summarised by Jones and Gray.
Whether Sandberg, Drexler and Ord's paper has appeared in a peer-reviewed journal.
Found, but not linked to a source we trust. Not on the sheets until they are.