The Great Filter

The Fer­mi Para­dox cre­ates an inter­est­ing dilem­ma from sci­ence.  What is the fil­ter that seems to be pre­vent­ing con­clu­sive evi­dence of alien civ­i­liza­tions? In oth­er words, what is the ‘Great Fil­ter’? We have sev­er­al options:

1. Plan­ets capa­ble of sup­port­ing life:  That might be a decep­tive descrip­tion.  What is a plan­et capa­ble of sup­port­ing life. The first indi­ca­tion is that it must have liq­uid water present.  This is cer­tain­ly nec­es­sary but it isn’t suf­fi­cient. A bet­ter cri­te­ria would be the world hav­ing suf­fi­cient­ly long time to sup­port life for life to evolve far enough to have intel­li­gent life. Such exam­ples include evo­lu­tion of mag­net­ic field, and evo­lu­tion of atmos­pher­ic chemistry.
2. Prob­a­bil­i­ty of life becom­ing intel­li­gent life: There are some poten­tial hur­dles for this that might make this num­ber extreme­ly slow. First off, 99.9% of all species have gone extinct, giv­ing us a prob­a­bil­i­ty of a par­tic­u­lar species sur­viv­ing at 0.001.  But there may be oth­er bio­log­i­cal hur­dles to intel­li­gence that are not account­ed for in the mere sur­vival of a par­tic­u­lar species. An evo­lu­tion­ary line would need to be formed that make it past these bio­log­i­cal hur­dles, dri­ving the num­ber even low­er. Some of these hur­dles con­sist of devel­op­ment of Eucary­ot­ic cells, assem­bly of mul­ti-cel­lu­lar organ­isms, adap­ta­tion to plan­e­tary changes, and evo­lu­tion of cog­ni­tive mechanisms.
3. Prob­a­bil­i­ty of intel­li­gent life devel­op­ing nec­es­sary tech­nolo­gies for inter­stel­lar com­mu­ni­ca­tions: This is tech­no­log­i­cal evo­lu­tion from the first tools to the under­stand­ing of radio and elec­tron­ics.  On Earth, human­i­ty has gone over these tech­no­log­i­cal hur­dles in var­i­ous places at var­i­ous times.  The devel­op­ment of tech­nolo­gies is tied to many fac­tors includ­ing: avail­abil­i­ty of mate­ri­als, cul­tur­al norms, eco­nom­ic fac­tors,  per­ceived need, and envi­ron­men­tal limitations.
4. Life­time of a civ­i­liza­tion: There might be poi­son pill tech­nolo­gies that end a civ­i­liza­tion. In the past, we’ve seen civ­i­liza­tions come and go. His­to­ri­ans have pos­tu­lat­ed mul­ti­ple caus­es for the col­laps­es. In fact, some pos­tu­late that cer­tain col­laps­es set back human­i­ty’s tech­no­log­i­cal progress by a cou­ple hun­dred years or more. Human­i­ty hap­pened to devel­op the hydro­gen bomb about the same  time as they devel­oped the means of inter­stel­lar com­mu­ni­ca­tions. The dri­ving ques­tion has been: will we sur­vive long enough to make con­tact? In the video below John Michael Godi­er dis­cuss­es the Vul­ner­a­ble World Hypothesis.