The Quiet Galaxy

Explanations · They are there and we cannot see them

Interstellar travel is impractical: the distances are so great and the costs so high that nobody ever goes anywhere, so nobody has come here

Proposed by Sebastian von Hoerner, 1962 fc left alone

The strongest version of the Fermi question is not "why do we hear nothing" but "why is nobody here". Michael Hart argued in 1975 that a civilisation able to travel between stars would have settled the whole galaxy within a few million years, so the fact that Earth has not been settled means no such civilisation exists. The reply is that the premise is wrong: the distances between stars are so vast and the energy needed to cross them so enormous that no civilisation ever does it on a scale that matters. The galaxy may be full of civilisations, each confined to its own star system, each as unable to visit us as we are to visit them.

The nearest star beyond the Sun is about four light-years away, which is roughly forty trillion kilometres. Voyager 1, the most distant spacecraft humans have built, moves at about 17 kilometres a second and would take about 75,000 years to get there. Crossing that gap in a human lifetime would need a ship moving at a good fraction of the speed of light, and the energy needed to get a ship of useful size to such a speed, and to slow it down again at the other end, is greater than the whole of humanity's present energy use for years or decades. The German astronomer Sebastian von Hoerner worked through these limits in 1962. He did not claim to have proved interstellar travel impossible and left the final judgment to the reader, but he found the requirements so extreme that he concluded, as his own view, that space travel would stay confined to our own planetary system for ever, and that the only means of communication between civilisations would be radio signals.

The argument took on a new role after 1975, when Michael Hart published the paper that turned Fermi's lunchtime question into a scientific problem. Hart argued that the absence of visitors was strong evidence that we were the first civilisation in the galaxy, because any earlier one would have had time to settle everywhere. His critics answered that settlement need not be as automatic as he assumed. William Newman and Carl Sagan, in 1981, did not argue that travel was impossible: they argued that a settled region spreads far more slowly than a single ship travels, and that there may be reasons every society holds back, such as the need to limit its own numbers. A separate reply, the one this page is about, is that the travel itself is too costly. Perhaps every civilisation looks at the cost, as we do, and decides that it is not worth it; perhaps a few try and the ships fail; perhaps the physics of life makes long voyages impossible for any living thing. If nobody travels, the absence of visitors tells us nothing about how many civilisations there are.

The bearing on the Drake Equation is nil, which is the point. The equation counts civilisations detectable by radio and says nothing about travel. This explanation answers Hart's question while leaving Drake's open: the factors can be whatever they are, and the silence in the radio bands, if it continues, still needs an explanation of its own. It sits in the "unseen" family because it says the absence of visitors is about the difficulty of visiting rather than about the number of civilisations.

The objections are strong, and most researchers think this explanation fails on its own. Travel need not be fast. A ship that takes ten thousand years to reach the next star is slow for a crew but fast against the age of the galaxy, and a civilisation that sent such ships, with frozen passengers, with generations born on the way, or with no living crew at all, would still fill the galaxy in a few tens of millions of years. Machines need no life support and can wait. And a civilisation does not need to want to settle the galaxy; it only needs to settle the next star once in a while, and the next star's settlers will do the same. The reply is that every one of those steps is a guess about technology nobody has, and that the universal failure to detect even a single slow probe may mean the guesses are wrong.

What it says about the equation

Every explanation is a claim about one or more of the seven pieces that are multiplied to give the number of detectable civilisations. This one bears on 1 piece:

The test: what would strengthen or weaken it

Would strengthen it: A physical result showing that interstellar flight even at a few percent of light speed faces an insurmountable barrier, such as damage from the thin gas and dust between the stars that cannot be shielded against; a long record of detected civilisations that have never left their home systems.

Would weaken it: A detected probe or craft from elsewhere; humanity's own successful launch of an interstellar probe; evidence of civilisations that have settled more than one star.

These two lines are what the record is read against when a new paper or discovery is logged. A result of the kind named here, published with its evidence, is what would settle it; a press claim on its own settles nothing.

The last reading

Once a week the records are read against the news, each against the test above. Open a line to see what that week held. Every reading ever made of this record is on the reading page.

  • 2026-10-16 Nothing new By hand

    No physical result showing a barrier. A 2026 preprint on shield materials carries affiliations that could not be confirmed and is not counted; the 2023 analysis found a few millimetres of shielding enough for a gram-scale probe. Breakthrough Starshot has been paused since 2024 to restructure, with no launch date. The third interstellar object was a natural comet.

How seriously it is taken

The first and most natural answer to Hart's argument, and still raised; among researchers it is widely considered insufficient on its own, because slow and uncrewed travel evades the energy objection and would still settle the galaxy in time.

The papers that bear on it

Books, films and other work that take it up

Read next

Notes

Corrected 2026-10-16, three points. (1) The body said the fastest object humans have launched would take tens of thousands of years to reach the nearest star; the fastest spacecraft, Parker Solar Probe, is faster than that at its closest pass to the Sun (about 700,000 km/h) but is not leaving the solar system, so the text now uses Voyager 1: NASA gives its speed as 17.0 km/s (https://science.nasa.gov/mission/voyager/voyager-1/), and about 4.25 light-years at that speed is about 75,000 years. (2) The body and proposed_source said Sagan answered Hart by saying settlement was hard or costly. Newman and Sagan's reply (Icarus 46, 1981, https://ntrs.nasa.gov/citations/19810061288; and the early 1978 Cornell draft, https://ntrs.nasa.gov/citations/19790011801) does not rest on travel being impossible; it argues that settlement spreads slowly and that societies have social reasons to hold back, and the wording now says so. (3) The body said von Hoerner concluded travel by living crews should be set aside as beyond any budget; he wrote that interstellar travel is not shown to be impossible, left the final judgment to the reader, and gave his own conclusion that space travel would stay within our planetary system and that radio would be the only means of contact (Science 137, 18-23 (1962), reading of the printed paper at https://wwwadd.zah.uni-heidelberg.de/publikationen/mhb/mhb008/mhb008.htm). The page range 18-23 is confirmed there.

Think this page gets the explanation wrong, or misses an objection? Say so, or read how disagreement gets folded in. The record itself is data/explanations/interstellar-travel-impractical.toml in the public repository. This page's address will never change; if the explanation is ever retired, the address will say what replaced it.