Space and Time

[1] http://arxiv.org/abs/1312.0613

* do actually I believe there was early-epoch life sophisticated enough to think up physical theories? No, but what does that matter?

Wow. Non-dark matter is ~5%, so non-xray-hot-halo matter is 0.5% of the matter universe? That's all the rocks, all the stars, all the exotic objects we can count. Half a percent?

However, something always bothers me about the Fermi paradox. I've never seen the most obvious resolution mentioned (at least to me). I think that we are just not (yet) able to detect any civilization given the maximum technological state a civilization may reach. Many people assume that because technological progress has been exponential last century for humans, that this can be projected indefinitely into the future. Similarly like some thought about indefinite economic growth. But this is nonsense, it is much more likely we will plateau at some point. The Seti-project listens for radio signals with star strength, assuming that a proper developing civilization is able to manipulate stars as radio beacons. I find this highly unlikely that any civilization will ever reach such technology or that it is even physically possible. I think it isn't.

The only thing one can say is that apparently in this galaxy, no civilization seems far enough 'progressed' yet to have colonized the entire galaxy, as I also think that crossing galaxy boundary will be a no go for any civilization. But will humans ever undertake such a quest? It will mean building generational ships, enough DNA diversity to make a new civilization feasible (minimum 5000 people?), finding a good habitable planet from here or be doomed as traveling will take thousands of years and will be tremendously costly in terms of effort, energy, materials and personal costs. How likely is that we will ever do that for every planet in the galaxy? Technology to turn one selves inward (e.g. VR) seems a much more likely turning point at which outward exploration might not seem that interesting anymore.

The logic of why things happen (why elliptical galaxies or small galaxies are not good for life) is only easy to explain if you master the subjects very well.

Some say that the Sun is young and so if there's other intelligent life, some would naturally have a 7 or 8 gyr lead on us. But the contra is that the young sun evolved just when the metallicity of the gas was high enough to actually form rocky planets and the solar system.

The article notes that the universal star formation rate has been decreasing for a while, since about redshift 2. So everything that's going to get a start has already gotten it. The vast majority of possible planet host environments are hostile because of SNe, AGN feedback, stellar winds etc. etc. etc. in the article.

Short of the long, sure we could say that we are approximately at the time when one would expect to see the emergence of advanced life give or take a couple billion years.

HOWEVER, this article neglects the fact that there are still billions of stars similar in aspect to the sun, that have cosmically long lifetimes, high metallicities and relatively quiescent environments.

From Kepler we know that planet formation is not the exception, it is the norm; at this juncture, it is impossible for us to say if rocky planet formation is an exception or a norm. You have to understand that the Kepler mission was not designed to figure that out, it was designed just to figure out if planets were common. The next major ESA planet mission, PLATO (and I think NASA may have a similar project in the works), will answer that question, and until that time, it's entirely plausible that the galaxy is filled with goldilocks planets.

(For reference, the Kepler spacecraft's sensitivity to earthlike planets in the habitable zone is minimal-- it's a function of size of the planet vs distance from the star, every time the planet goes in front of the star (function of distance) a small signal (function of size) is added to a time series that can be coadded over and over to remove noise (so maybe it takes 7 or 8 orbits to verify a planet). This is why Kepler mostly finds molten rock planets and gas giants. The distribution of planets we know about today _is a function of the instrument, not the actual planet distribution_.

The more interesting projects are the ones attempting to take planetary spectra. A spectra of a planet's atmosphere will tell you the chemical components of that atmosphere and is performed by taking a spectra of the star and then subtracting the spectra of the planet in front of the star (the difference between the two being the planet. There are certain chemicals in atmospheres, called biomarkers, that are indicative of life (ozone, for example, only exists in meaningful quantities because of photosynthetic life). This is an incredibly difficult and ambitious project: you need high res spectra at large signal to noise to even see these biomarkers which means probably multiple nights of 8-10 hour observations on modern 8m class telescopes, in the 30m telescope era, maybe it's more plausible; you need the right type of rocky planet, of which we know few; you need that planet to be in the exact right spot in its orbit in front of the star; and of course god needs to make it not rain after your 20 hour flight to Hawaii.

For all we know, not only are planets the norm, not only are rocky planets the norm, but life is also the norm.

Thus, not seeing advanced life is a function of _our instruments_ not indicative of the fact that advanced life is somehow new, or rare. Even with all the notes in the article, there could be advanced life with several billion years head start on us.

A thought I haven't devoted much time to is that perhaps interstellar travel is just downright impossible on a meaningful scale at a reasonable speed.

... Unless of course life arose from a panspermia situation, and we are the actual results of the most sophisticated form of interstellar travel in the universe. Perhaps we are a colony of some monolithic species, packing their genetic blocks into chunks of ice and tossing them willy-nilly about the galaxy...