Life Beyond the Earth
Life Beyond the Earth ASTR 3420
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Date Created: 09/21/15
by Lawrence M Krauss and Glenn D Starkman Eternal Life Eternal life is a core belief of many of the worlds religions Usually it is extolled as a spiritual Valhalla an existence without pain death worry or evil a world removed from our physical reality But there is another sort of eternal life that we hope for one in the temporal realm In the conclusion to Origin of Species Charles Darwin wrote quotAs all the living forms of life are lineal descendants of those which lived before the Cambrian epoch we may feel certain that the ordinary succession by generation has never once been broken Hence we may look with some confidence to a secure future of great lengthquot The sun will eventually exhaust its hydrogen fuel and life as we know it on our home planet will eventually end but the human race is resilient Our progeny will seek new homes spreading into every corner of the universe just as organisms have colonized every possible niche of the earth Death and evil will take their toll pain and worry may never go away but somewhere we expect that some of our children will carry on Or maybe not Remarkably even though scientists fully understand neither the physical basis of life nor the unfolding of the universe they can make educated guesses about the destiny of living things Cosmological observations now suggest the universe will continue to expand foreveri rather than as scientists once thought expand to a maximum size and then shrink Therefore we are not doomed to perish in a fiery quotbig crunchquot in which any vestige of our current or future civilization would be erased At first glance eternal expansion is cause for optimism What could stop a sufficiently intelligent civilization from exploiting the endless resources to survive indefinitely Yet life thrives on energy and information and very general scientific arguments hint that only a finite amount of energy and a finite amount of information can be amassed in even an infinite period For life to persist it would have to make do with dwindling resources and limited knowledge We have concluded that no meaningful form of consciousness could exist forever under these conditions The Deserts of Vast Eternity Over the past century scientific eschatology has swung between optimism and pessimism Not long after Darwin39s confident prediction Victorianeera scientists began to fret about the quotheat deathquot in which the whole cosmos would come to a common temperature and thereafter be incapable of change The discovery of the expansion of the universe in the 1920s allayed this concern because expansion prevents the universe from reaching such an equilibrium But few cosmologists thought through the other implications for life in an ever expanding universe until a classic paper in 1979 by physicist Freeman Dyson of the Institute for Advanced Study in Princeton N itself motivated by earlier work by Jamal lslam now at the University of Chittagong in Bangladesh Since Dyson39s paper physicists and astronomers have periodically reexamined the topic see quotThe Future of the Universequot by Duane A Dicus John R Letaw Doris C Teplitz and Vigdor L Teplitz Scienti c American March 1983 A year ago spurred on by new observations that suggest a drastically different long term future for the universe than that previously envisaged we decided to take another look Over the past 12 billion years or so the universe has passed through many stages At the earliest times for which scientists now have empirical information it was incredibly hot and dense Gradually it expanded and cooled For hundreds of thousands of years radiation ruled the famous cosmic microwave background radiation is thought to be a vestige of this era Then matter started to dominate and progressively larger astronomical structures condensed out Now if recent cosmological observations are correct the expansion of the universe is beginning to acceleratei a sign that a strange new type of energy perhaps springing from space itself may be taking over Life as we know it depends on stars But stars inevitably die and their birth rate has declined dramatically since an initial burst about 10 billion years ago About 100 trillion years from now the last conventionally formed star will wink out and a new era will commence Processes currently too slow to be noticed will become important the dispersal of planetary systems by stellar close encounters the possible decay of ordinary and exotic matter the slow evaporation of black holes Assuming that intelligent life can adapt to the changing circumstances what fundamental limits does it face In an eternal universe potentially of in nite volume one might hope that a sufficiently advanced civilization could collect an infinite amount of matter energy and information Surprisingly this is not true Even after an eternity of hard and welleplanned labor living beings could accumulate only a finite number of particles a finite quantity of energy and a finite number of bits of information What makes this failure all the more frustrating is that the number of available particles ergs and bits may grow without bound The problem is not necessarily the lack of resources but rather the difficulty in collecting them The culprit is the very thing that allows us to contemplate an eternal tenure the expansion of the universe As the cosmos grows in size the average density of ordinary sources of energy declines Doubling the radius of the universe decreases the density of atoms eightfold For light waves the decline is even more precipitous Their energy density drops by a factor of 16 because the expansion stretches them and thereby saps their energy see illustration below Figure l Dilution of the cosmos by the expansion of space affects different forms of energy in different ways Ordinary matter orange thins out in direct proportion to volume whereas the cosmic background radiation purple weakens even faster as it is stretched from light into microwaves and beyond The energy density represented by a cosmological constant blue does not change at least according to present theories As a result of this dilution resources become ever more timeeconsuming to collect lntelligent beings have two distinct strategies let the material come to them or try to chase it down For the former the best approach in the long run is to let gravity do the work Of all the forces of nature only gravity and electromagnetism can draw things in from arbitrarily far away But the latter gets screened out oppositely charged particles balance one another so that the typical object is neutral and hence immune to longerange electrical and magnetic forces Gravity on the other hand cannot be screened out because particles of matter and radiation only attract gravitationally they do not repel Surrender to the Void Even gravity however must contend with the expansion of the universe which pulls objects apart and thereby weakens their mutual attraction In all but one scenario gravity eventually becomes unable to pull together larger quantities of material Indeed our universe may have already reached this point clusters of galaxies may be the largest bodies that gravity will ever be able to bind together see quotThe Evolution of Galaxy Clustersquot by J Patrick Henry Ulrich G Briel and Hans Bohringer Scienti c American December 1998 The lone exception occurs if the universe is poised between expansion and contraction in which case gravity continues indefinitely to assemble ever greater amounts of matter But that scenario is now thought to contradict observations and in any event it poses its own difficulty after 1033 years or so the accessible matter will become so concentrated that most of it will collapse into black holes sweeping up any lifeeforms Being inside a black hole is not a happy condition On the earth all roads may lead to Rome but inside a black hole all roads lead in a finite amount of time to the center of the hole where death and dismemberment are certain Sadly the strategy of actively seeking resources fares no better than the passive approach does The expansion of the universe drains away kinetic energy so prospectors would have to squander their booty to maintain their speed Even in the most optimistic scenario 7 in which the energy is traveling toward the scavenger at the speed of light and is collected without loss 7 a civilization could garner limitless energy only in or near a black hole The latter possibility was explored by Steven Frautschi of the California Institute of Technology in 1982 He concluded that the energy available from the holes would dwindle more quickly than the costs of scavenging see iIIustI ation beIow We recently reexamined this possibility and found that the predicament is even worse than Frautschi thought The size of a black hole required to sweep up energy forever exceeds the extent of the visible universe Figure 2 Energy collection strategy devised by physicist Steven Frautschi illustrates how difficult it will be to survive in the far future 10 or so years from now In many cosmological scenarios resources multiply as the universe 7 and any arbitrary reference sphere within it blue sphere eXpands and an increasing fraction of it becomes observable red sphere A civilization could use a black hole to convert matteri plundered from its empire green spherei into energy But as the empire grows the cost of capturing new territory increases the conquest can barely keep pace with the dilution of matter In fact matter will become so diluted that the civilization will not be able to safely build a black hole large enough to collect it The cosmic dilution of energy is truly dire if the universe is expanding at an accelerating rate All distant objects that are currently in view will eventually move away from us faster than the speed of light and in doing so disappear from view The total resources at our disposal are therefore limited by what we can see today at most see beIow The Worst of All Possible Universes Among all the scenarios for an eternally expanding universe the one dominated by the soicalled cosmological constant is the bleakest Not only is it unambiguous that life cannot survive eternally in such a universe but the quality of life will quickly deteriorate as well So if recent observations that the expansion is accelerating see quotSurveying SpaceiTime with Supernovaequot by Craig Hogan Robert P Kirshner and Nicholas B Suntzeff Scientific American January 1999 are borne out we could face a grim future Cosmic expansion carries objects away from one another unless they are bound together by gravity or another force In our case the Milky Way is part of a larger cluster of galaxies About 10 million lightiyears across this cluster remains a cohesive whole whereas galaxies beyond it are whisked away as intergalactic space expands The relative velocity of these distant galaxies is proportional to their distance Beyond a certain distance called the horizon the velocity exceeds the speed of light which is allowed in the general theory of relativity because the velocity is imparted by the expansion of space itself We can see no farther If the universe has a cosmological constant with a positive value as the observations suggest the expansion is accelerating galaxies are beginning to move apart ever more rapidly Their velocity is still proportional to their distance but the constant of proportionality remains constant rather than decreasing with time as it does if the universe decelerates Consequently galaxies that are now beyond our horizon will forever remain out of sight Even the galaxies we can currently see 7 except for those in the local cluster 7 will eventually attain the speed of light and vanish from View The acceleration which resembles in ation in the very early universe began when the cosmos was about half its present age The disappearance of distant galaxies will be gradual Their light will stretch out until it becomes undetectable Over time the amount of matter we can see will decrease and the number of worlds our starships can reach will diminish Within two trillion years well before the last stars in the universe die all objects outside our own cluster of galaxies will no longer be observable or accessible There will be no new worlds to conquer literally We will truly be alone in the umverse Expanding universe looks dramatically different depending on whether the growth is decelerating upper sequenceor accelerating lower sequence In both cases the universe is in nite but any patch of spacei demarcated by a reference sphere that represents the distance to particular galaxies 7 enlarges que sphere We can see only a limited volume which grows steadily as light signals have time to propagate red sphere lf expansion is decelerating we can see an increasing fraction of the cosmos More and more galaxies ll the sky But if expansion is accelerating we can see a decreasing fraction of the cosmos Space seems to empty out Not all forms of energy are equally subject to the dilution The universe might for example be lled with a network of cosmic stringsi in nitely long thin concentrations of energy that could have developed as the early universe cooled unevenly The energy per unit length of a cosmic string remains unchanged despite cosmic expansion see quotCosmic Stringsquot by Alexander Vilenkin Scienti c American December 1987 Intelligent beings might try to cut one congregate around the loose ends and begin consuming it If the string network is infinite they might hope to satisfy their appetite forever The problem with this strategy is that whatever lifeforms can do natural processes can also do If a civilization can figure out a way to cut cosmic strings then the string network will fall apart of its own accord For example black holes may spontaneously appear on the strings and devour them Therefore the beings could swallow only a nite amount of string before running into another loose end The entire string network would eventually disappear leaving the civilization destitute What about mining the quantum vacuum After all the cosmic acceleration may be driven by the soecalled cosmological constant a form of energy that does not dilute as the universe expands see quotCosmological Antigravityquot by Lawrence M Krauss Scienti c American January If so empty space is filled with a bizarre type of radiation called GibbonseHawking or de Sitter radiation Alas it is impossible to extract energy from this radiation for useful work If the vacuum yielded up energy it would drop into a lower energy state yet the vacuum is already the lowest energy state there is No matter how clever we try to be and how cooperative the universe is we will someday have to confront the finiteness of the resources at our disposal Even so are there ways to cope forever The obvious strategy is to learn to make do with less a scheme first discussed quantitatively by Dyson In order to reduce energy consumption and keep it low despite exertion we would eventually have to reduce our body temperature One might speculate about genetically engineered humans who function at somewhat lower temperatures than 310 kelvins 986 degrees Fahrenheit Yet the human body temperature cannot be reduced arbitrarily the freezing point of blood is a firm lower limit Ultimately we will need to abandon our bodies entirely While futuristic the idea of shedding our bodies presents no fundamental difficulties lt presumes only that consciousness is not tied to a particular set of organic molecules but rather can be embodied in a multitude of different forms from cyborgs to sentient interstellar clouds see quotWill Robots Inherit the Earthquot by Marvin Minsky Scientific American October 1994 Most modern philosophers and cognitive scientists regard conscious thought as a process that a computer could perform The details need not concern us here which is convenient as we are not competent to discuss them We still have many billions of years to design new physical incarnations to which we will someday transfer our conscious selves These new quotbodiesquot will need to operate at cooler temperatures and at lower metabolic rates 7 that is lower rates of energy consumption Dyson showed that if organisms could slow their metabolism as the universe cooled they could arrange to consume a finite total amount of energy over all