2 SETI and Temporal Copernicanism
9
Finally, and more relevant for SETI, when considering the long-term future history
of the universe, the problem of intentional influence from advanced technological
civilizations (from local effects such as the atmospheric composition of planets,
up to mega-engineering on planetary or galactic scale) cannot be excluded: this
introduces a completely new kind of uncertainty, that can possibly dominate the
values of parameters we wish to average over.
Despite the fact that temporal averages cannot be well-defined in the standard
cosmological model, temporal Copernicanism has often been applied to infer general
conclusions based on the present state of the universe. One egregious example is
the “Doomsday Argument” [13], which attempts to estimate the future duration of
humanity from an explicit Copernican premise. A more recent application, relevant
to astrobiology and SETI, is the proposal that a dangerous early universe increases
the number of habitable sites in the universe today [18]. If temporal Copernicanism
cannot be justified, such arguments are clearly not well-grounded.
2.3 SETI and Temporal Copernicanism
A further example, directly relevant for astrobiology and SETI, is the temporal distribution of habitable planets in the Milky Way. This was first calculated in a pioneering
study [16] and subsequently elaborated more precisely by various authors [5, 17, 20].
Such studies established that the formation of Earth-like planets started somewhat
more than 9 Gyr ago, and has a temporal distribution of ages reaching a maximum
(at some billion years ago), then declining to the present-day value. This distribution
will conceivably extend for an indefinite amount of time into the future, although
with an ever-decreasing rate. Also, as we already pointed out, various studies have
addressed the problem of the overall habitability of the universe in time [10, 15].
This kind of investigation is still in its infancy and its conclusions are very uncertain. However, there are no compelling reasons to expect that the probability for the
appearance of life in the universe is uniform in time, and in fact there are quite more
arguments to the contrary. This very fact clashes with the presumption that we are
observing a typical epoch of cosmic history.
The unwarranted assumption of uniformity in time has a direct impact on the
chances of success of SETI (or, more broadly, the search for signatures of technological species, or “technosignatures”). In fact, most past studies in the field adopted,
explicitly or not, an underlying presumption of stationarity for the appearance of
life over cosmic history: one notable example of this approach is the use of the
Drake equation [11] to estimate the number of communicating species present in the
universe, in which any time dependence of the relevant astrophysical or biological
factors is neglected [8]. The shortcomings of such approach are particularly evident
when we consider the strict requirement that any electromagnetic signal (or, more
generally, any physical interaction) has to satisfy in order for it to be observed by
us today. Because of the short crossing-time of our galaxy (∼ 10
5 years), the time
interval we can directly observe is very small, compared to the age of the universe,
9
Finally, and more relevant for SETI, when considering the long-term future history
of the universe, the problem of intentional influence from advanced technological
civilizations (from local effects such as the atmospheric composition of planets,
up to mega-engineering on planetary or galactic scale) cannot be excluded: this
introduces a completely new kind of uncertainty, that can possibly dominate the
values of parameters we wish to average over.
Despite the fact that temporal averages cannot be well-defined in the standard
cosmological model, temporal Copernicanism has often been applied to infer general
conclusions based on the present state of the universe. One egregious example is
the “Doomsday Argument” [13], which attempts to estimate the future duration of
humanity from an explicit Copernican premise. A more recent application, relevant
to astrobiology and SETI, is the proposal that a dangerous early universe increases
the number of habitable sites in the universe today [18]. If temporal Copernicanism
cannot be justified, such arguments are clearly not well-grounded.
2.3 SETI and Temporal Copernicanism
A further example, directly relevant for astrobiology and SETI, is the temporal distribution of habitable planets in the Milky Way. This was first calculated in a pioneering
study [16] and subsequently elaborated more precisely by various authors [5, 17, 20].
Such studies established that the formation of Earth-like planets started somewhat
more than 9 Gyr ago, and has a temporal distribution of ages reaching a maximum
(at some billion years ago), then declining to the present-day value. This distribution
will conceivably extend for an indefinite amount of time into the future, although
with an ever-decreasing rate. Also, as we already pointed out, various studies have
addressed the problem of the overall habitability of the universe in time [10, 15].
This kind of investigation is still in its infancy and its conclusions are very uncertain. However, there are no compelling reasons to expect that the probability for the
appearance of life in the universe is uniform in time, and in fact there are quite more
arguments to the contrary. This very fact clashes with the presumption that we are
observing a typical epoch of cosmic history.
The unwarranted assumption of uniformity in time has a direct impact on the
chances of success of SETI (or, more broadly, the search for signatures of technological species, or “technosignatures”). In fact, most past studies in the field adopted,
explicitly or not, an underlying presumption of stationarity for the appearance of
life over cosmic history: one notable example of this approach is the use of the
Drake equation [11] to estimate the number of communicating species present in the
universe, in which any time dependence of the relevant astrophysical or biological
factors is neglected [8]. The shortcomings of such approach are particularly evident
when we consider the strict requirement that any electromagnetic signal (or, more
generally, any physical interaction) has to satisfy in order for it to be observed by
us today. Because of the short crossing-time of our galaxy (∼ 10
5 years), the time
interval we can directly observe is very small, compared to the age of the universe,