Number of stars in the visible universe: 30 billion trillion

Neil deGrasse Tyson being awesome.

When talking about the subject of life and civilizations on other planets in other star systems there's something called the Fermi paradox that basically says that "if there is so much life in the universe, why hasn't any of it contacted us?" -- that if there really is life in other star systems we should have some sign of them such as radio signals.

Well, the first thing to mention is that there is a huge difference between simply having life and having a civilization that can produce radio signals. On Earth it took a huge amount of time (a few billion years) to get everything "just right" to produce us, and until then there were periods of hundreds of millions of years of animals dominating the planet, then going extinct, then new types springing up, then disappearing, and finally we appeared, and the first commercial radio broadcasts in the US only began in the 1920s, so it really hasn't been all that long for us. If you were to spin a Wheel of Fortune-type wheel where each 100-year period since the beginning of the Earth had one cm of space (so that there was a chance of you being able to land your finger on it as you stopped it, the wheel itself would have a diameter of 143 kilometres. If you only went from when life began on Earth, it would have a diameter of 117 kilometres. The area since the dawn of history (let's say 10000 years or so) would be a metre long. Chances are, if you're taking a snapshot of Earth during its existence it would have life, but no civilization, and even if you landed on a point with civilization, chances are it wouldn't have anything close to a radio.

Plus!

There's also the fact that the universe is just big. A great site (which is actually the subject of the post) is this one here, atlasoftheuniverse.com. It's not flashy at all but there's a part of the site that gives you a great idea of just how big the universe is (start here and click once to zoom out each time), and once you start seeing that we are just a speck on the arm of a very normal galaxy and that there are actually clouds and superclouds of galaxies out there, it becomes very easy to see why it wouldn't be all that easy to contact us. Indeed, by the time you get out to a distance of a mere 250 light-years you've already covered one third of the stars we can see in the sky with the naked eye.

Like Neil deGrass Tyson, I find this idea exhilarating, not the least bit depressing.





Finally, there's a classic video here:



that also gives a good idea about the size of the universe as you take off from Chicago and move out into space. What's most interesting is the journey back, because every time you decrease the ratio by a factor of ten you've made it 90% of the way back from that scale (that is, if you're 10,000 km away then the first 9,000 km is 90% of the first distance, then the next 900 km is 90% of the remaining distance, the next 90 km is 90% of that remaining distance and so on), but it still takes a near eternity to get back.