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add fermi paradox example

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NunoSempere 2024-01-20 14:02:59 +01:00
parent b99a9cb3f5
commit bb91d78859
2 changed files with 131 additions and 8 deletions
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@ -5,21 +5,144 @@
#include <stdio.h>
#include <stdlib.h>
double sample_loguniform(double a, double b, uint64_t* seed){
return exp(sample_uniform(log(a), log(b), seed));
}
int main()
{
// set randomness seed
uint64_t* seed = malloc(sizeof(uint64_t));
*seed = 1000; // xorshift can't start with a seed of 0
*seed = UINT64_MAX/64; // xorshift can't start with a seed of 0
int n = 1000000;
double* xs = malloc(sizeof(double) * n);
for (int i = 0; i < n; i++) {
xs[i] = sample_to(10, 100, seed);
double fermi_naive(uint64_t* seed){
double rate_of_star_formation = sample_loguniform(1,100, seed);
double fraction_of_stars_with_planets = sample_loguniform(0.1, 1, seed);
double number_of_habitable_planets_per_star_system = sample_loguniform(0.1, 1, seed);
double rate_of_life_formation_in_habitable_planets = sample_lognormal(1, 50, seed);
double fraction_of_habitable_planets_in_which_any_life_appears = -expm1(-rate_of_life_formation_in_habitable_planets);
// double fraction_of_habitable_planets_in_which_any_life_appears = 1-exp(-rate_of_life_formation_in_habitable_planets);
// but with more precision
double fraction_of_planets_with_life_in_which_intelligent_life_appears = sample_loguniform(0.001, 1, seed);
double fraction_of_intelligent_planets_which_are_detectable_as_such = sample_loguniform(0.01, 1, seed);
double longevity_of_detectable_civilizations = sample_loguniform(100, 10000000000, seed);
// printf(" rate_of_star_formation = %lf\n", rate_of_star_formation);
// printf(" fraction_of_stars_with_planets = %lf\n", fraction_of_stars_with_planets);
// printf(" number_of_habitable_planets_per_star_system = %lf\n", number_of_habitable_planets_per_star_system);
// printf(" rate_of_life_formation_in_habitable_planets = %.16lf\n", rate_of_life_formation_in_habitable_planets);
// printf(" fraction_of_habitable_planets_in_which_any_life_appears = %lf\n", fraction_of_habitable_planets_in_which_any_life_appears);
// printf(" fraction_of_planets_with_life_in_which_intelligent_life_appears = %lf\n", fraction_of_planets_with_life_in_which_intelligent_life_appears);
// printf(" fraction_of_intelligent_planets_which_are_detectable_as_such = %lf\n", fraction_of_intelligent_planets_which_are_detectable_as_such);
// printf(" longevity_of_detectable_civilizations = %lf\n", longevity_of_detectable_civilizations);
// Expected number of civilizations in the Milky way;
// see footnote 3 (p. 5)
double n = rate_of_star_formation *
fraction_of_stars_with_planets *
number_of_habitable_planets_per_star_system *
fraction_of_habitable_planets_in_which_any_life_appears *
fraction_of_planets_with_life_in_which_intelligent_life_appears *
fraction_of_intelligent_planets_which_are_detectable_as_such *
longevity_of_detectable_civilizations;
return n;
}
ci ci_90 = array_get_90_ci(xs, n);
printf("Recovering confidence interval of sample_to(10, 100):\n low: %f, high: %f\n", ci_90.low, ci_90.high);
double fermi_paradox_naive(uint64_t* seed){
double n = fermi_naive(seed);
return (n > 1 ? 1 : 0);
}
double result;
for(int i=0; i<1000; i++){
result = fermi_naive(seed);
printf("result from fermi_naive: %lf\n", result);
printf("\n\n");
}
printf("result from naïve implementation: %lf\n", result);
// Thinking in log space
double fermi_logspace(uint64_t* seed){
double log_rate_of_star_formation = sample_uniform(log(1), log(100), seed);
double log_fraction_of_stars_with_planets = sample_uniform(log(0.1), log(1), seed);
double log_number_of_habitable_planets_per_star_system = sample_uniform(log(0.1), log(1), seed);
double log_fraction_of_planets_with_life_in_which_intelligent_life_appears = sample_uniform(log(0.001), log(1), seed);
double log_fraction_of_intelligent_planets_which_are_detectable_as_such = sample_uniform(log(0.01), log(1), seed);
double log_longevity_of_detectable_civilizations = sample_uniform(log(100), log(10000000000), seed);
// printf(" log_rate_of_star_formation = %lf\n", log_rate_of_star_formation);
// printf(" log_fraction_of_stars_with_planets = %lf\n", log_fraction_of_stars_with_planets);
// printf(" log_number_of_habitable_planets_per_star_system = %lf\n", log_number_of_habitable_planets_per_star_system);
// printf(" log_fraction_of_planets_with_life_in_which_intelligent_life_appears = %lf\n", log_fraction_of_planets_with_life_in_which_intelligent_life_appears);
// printf(" log_fraction_of_intelligent_planets_which_are_detectable_as_such = %lf\n", log_fraction_of_intelligent_planets_which_are_detectable_as_such);
// printf(" log_longevity_of_detectable_civilizations = %lf\n", log_longevity_of_detectable_civilizations);
double log_n1 =
log_rate_of_star_formation +
log_fraction_of_stars_with_planets +
log_number_of_habitable_planets_per_star_system +
log_fraction_of_planets_with_life_in_which_intelligent_life_appears +
log_fraction_of_intelligent_planets_which_are_detectable_as_such +
log_longevity_of_detectable_civilizations;
printf("first part of calculation: %lf\n", log_n1);
/* Consider fraction_of_habitable_planets_in_which_any_life_appears separately.
Imprecisely, we could do:
double rate_of_life_formation_in_habitable_planets = sample_lognormal(1, 50, seed);
double fraction_of_habitable_planets_in_which_any_life_appears = 1- exp(-rate_of_life_formation_in_habitable_planets);
double log_fraction_of_habitable_planets_in_which_any_life_appears = log(1-fraction_of_habitable_planets_in_which_any_life_appears);
double n = exp(log_n1) * fraction_of_habitable_planets_in_which_any_life_appears;
// or:
double n2 = exp(log_n1 + log(fraction_of_habitable_planets_in_which_any_life_appears))
However, we lose all precision here.
Now, say
a = underlying normal
b = rate_of_life_formation_in_habitable_planets = exp(underlying normal)
c = 1 - exp(-b) = fraction_of_habitable_planets_in_which_any_life_appears
d = log(c)
Now, is there some way we can d more efficiently/precisely?
Turns out there is!
Looking at the Taylor expansion for c = 1 - exp(-b), it's b - b^2/2 + b^3/6 - x^b/24, etc.
When b ~ 0 (as it is), this is close to b.
But now, if b ~ 0
c ~ b
and d = log(c) ~ log(b) = log(exp(a)) = a
*/
double log_rate_of_life_formation_in_habitable_planets = sample_normal(1, 50, seed);
printf("log_rate_of_life_formation_in_habitable_planets: %lf\n", log_rate_of_life_formation_in_habitable_planets);
double log_fraction_of_habitable_planets_in_which_any_life_appears;
if(log_rate_of_life_formation_in_habitable_planets < -32){
log_fraction_of_habitable_planets_in_which_any_life_appears = log_rate_of_life_formation_in_habitable_planets;
} else{
double rate_of_life_formation_in_habitable_planets = exp(log_rate_of_life_formation_in_habitable_planets);
double fraction_of_habitable_planets_in_which_any_life_appears = -expm1(-rate_of_life_formation_in_habitable_planets);
log_fraction_of_habitable_planets_in_which_any_life_appears = log(fraction_of_habitable_planets_in_which_any_life_appears);
}
printf(" log_fraction_of_habitable_planets_in_which_any_life_appears: %lf\n", log_fraction_of_habitable_planets_in_which_any_life_appears);
double log_n = log_n1 + log_fraction_of_habitable_planets_in_which_any_life_appears;
return log_n;
}
double result2;
/*
for(int i=0; i<1000; i++){
result2 = fermi_logspace(seed);
printf("result from logspace implementation: %lf.2\n", result2);
printf("\n\n");
}
*/
printf("Size of uint64_t: %ld", sizeof(uint64_t*));
free(seed);
}