existence is nuts. i was watching a sean carroll video about the scale of the universe and began thinking about extraterrestrial life. exoplanet surveys suggest that planets are extremely common around stars, and one analysis of kepler observations estimated that approximately half of sun-like stars could host rocky planets capable of supporting liquid water under suitable conditions. the milky way itself also contains something on the order of a hundred billion stars, and the observable universe contains far more galaxies than we could ever hope to examine individually…

it seems nearly impossible to believe that no life exists beyond earth, particularly if we consider the possibility of microbial organisms or life with an entirely unfamiliar biochemistry. of course, we can, unfortunately, not calculate those odds with any real confidence.

i believe that people seem to underestimate the scale of the universe and time when it comes to making contact. life existed here for billions of years before humans arrived; earth is approximately 4.5 billion years old, while the universe is about 13.8 billion years old. our ability to transmit information over interplanetary distances is a recent development. if another civilization had existed a million years before ours, it could conceivably have disappeared. two civilizations could occupy the same galaxy and never exist at the same time…... there could be a civilization forming right now around some distant star unaware that humans have already been transmitting signals into space for roughly a century. even if intelligent life is not particularly rare, the period during which any particular civilization is detectable might be extremely short compared with the age of the galaxy.

even if an intelligent civilization existed nearby, the distances involved would make communication extraordinarily difficult…… proxima centauri, the nearest star to the sun, is approximately 4.24 light-years away. light travels at about 300,000 kilometers per second, which means that even ome message sent from the nearest star system cannot arrive here in less than four years. the andromeda galaxy (our nearest neighbor) is approximately 2.5 million light-years away. when we observe andromeda, we are seeing light that began its journey long before modern humans existed. if some civilization there were looking at the milky way through some magnificent telescope, it would be receiving similarly ancient information about our galaxy. imagine two intelligent species attempting to communicate across that distance. one sends some message and the other receives it 2.5 million years later, assuming the signal remains detectable. a reply would require another 2.5 million years. i suppose this is one of the stranger implications of astronomical observation: distance is also a separation in time. voyager 1 was launched in 1977 and crossed the heliopause into interstellar space in 2012, which is a remarkable achievement. voyager 1, however, travels at roughly seventeen kilometers per second relative to the sun. at that speed, a hypothetical journey to proxima centauri would take more than seventy thousand years.

according to special relativity, an object with mass cannot be accelerated to the speed of light by supplying a finite amount of energy. there are theoretical discussions of wormholes and spacetime geometries that might permit unusual forms of travel, but these are not established engineering possibilities. the mathematical existence of a particular solution does not tell us that the required physical conditions can be produced, much less maintained around a spacecraft carrying actual people (unless you read physicists’ hyperspace theories! ha)

i do wonder how much this could change with technological development. we have spent only a relatively short period of human history building machines capable of leaving earth, and our computational abilities have advanced rapidly enough that the next few decades are difficult to predict. artificial intelligence could become extraordinarily useful in the development of propulsion systems or in designing autonomous probes that operate for years without instructions from earth, but extraordinarily capable artificial intelligence would not make the energy requirements of interstellar travel disappear. an autonomous probe does not need a human crew or the systems required to keep that crew alive, and designs involving beamed propulsion or light sails offer possibilities very different from conventional chemical rockets.

which brings me to the fermi paradox! given the age of the universe, it seems possible that other civilizations could have developed technology millions of years before we did. if some of them became capable of interstellar travel, why haven't we found any convincing evidence of their existence? it is a reasonable question, although the strength of the paradox depends on assumptions about how frequently technological civilizations emerge and whether they would have any interest in expanding across the galaxy.

perhaps interstellar travel is simply too difficult to be worth undertaking….. perhaps civilizations tend to disappear before reaching the point at which they could accomplish it. the idea of a great filter suggests that one or more stages between the formation of a potentially habitable planet and the development of a long-lived technological civilization may be exceptionally improbable. we do not know where such a difficulty would lie, assuming it exists at all, and the implications are rather different depending on whether the hardest step is behind us or still ahead.

i have also wondered whether we are making unreasonable assumptions about what an advanced civilization would want. we tend to imagine technological progress culminating in expansion, partly because human history provides so many examples of people attempting to occupy places previously inaccessible to them. another species might regard a journey lasting thousands of years as a spectacular waste of resources. its technology might produce little that we could recognize or detect from another star system. or it might be interested in exploration without being interested in us.

and it is worth remembering how little of the potential search space we have actually examined. projects devoted to the search for extraterrestrial intelligence have examined particular regions of the sky and certain types of potential signals, but the possibilities are much broader than the observations completed so far. our own ordinary radio transmissions would be extremely difficult for a civilization with equipment comparable to ours to detect from distant star systems. if another planet had been broadcasting for a century, we might simply be too far away to know.

astronomers are also looking for possible biosignatures in exoplanet atmospheres, which would allow us to investigate biological activity without requiring a civilization to send us a deliberate message. atmospheric oxygen, for example, can be associated with biological processes, although it can also be produced abiotically, so identifying a particular molecule would not necessarily amount to discovering life. the interpretation would depend on the surrounding atmospheric chemistry and the properties of the planet itself. i wonder what it would feel like to see convincing evidence that another world supports life, knowing that its inhabitants might remain permanently inaccessible to us.

we can photograph galaxies millions of light-years from earth, yet we cannot say whether there is a single living organism anywhere outside our own planet. perhaps the universe is absolutely crowded with life, each civilization watching its own tiny portion of the sky and wondering where everybody is. very strange and beautiful.