Why Humans May Never Leave the Solar System, with Charles Liu

A StarTalk Special Edition in the Cosmic Queries format: Neil deGrasse Tyson, Chuck Nice and Gary O’Reilly put seven listener questions to the astrophysicist Charles Liu. The title covers only the first. Asked whether unlimited energy (Iron Man’s arc reactor) would open up the stars, Liu says no: relativity caps any ship at the speed of light however much energy it carries, so crossing the galaxy still takes tens of thousands of years. That leaves generation ships or robots. He turns it into the Fermi question. A species travelling near light speed could spread through the Milky Way in a few hundred thousand years, a sliver of the galaxy’s age, so where is everyone? Either they are somewhere we have not looked, or they are hiding. Nice adds that probes would come first, as ours have. Liu does not say we should stay home: “that doesn’t mean we shouldn’t go.”

The second question is the one in the trailer. Are the cosmic web and the brain’s neural network related? Liu’s answer is that both are fractal but measurably different fractals, so they share a mathematics and nothing physical. Tyson sharpens it: counting eight planets and eight children does not make the children planets. A listener then asks whether the multiverse is apophenia, a pattern read into the equations. Both say no. Liu’s reason is that the equations are open-ended and permit many solutions; Tyson’s is that maths, unlike a gambler on a streak, has no bias toward patterns.

The rest moves quickly. The Planck length is a “horizon” beyond which the theories stop working, not a place where time runs backwards. An early civilisation seeding life across the galaxy, which Liu answers through a Star Trek episode, is unnecessary given how fast evolution diversifies. We know space itself expands because galaxies in a cluster move among themselves while the cluster recedes as a whole. Last comes the question of whether a civilisation born late enough would see no other galaxies and no trace of the Big Bang. Both answer yes at once. Tyson says he loses sleep wondering which chapter we have already lost; Liu says he is fine with it, since there would still be a galaxy to study.

Some of the facts need adjusting. The best-known comparison of the brain and the cosmic web (Vazza and Feletti, 2020) did not compare fractals: it compared network statistics, found similar complexity, and noted that the patterns were not simple fractal ones. Tyson’s essay “The Perimeter of Ignorance” is from 2005, not 2009. The Milky Way’s oldest disc is about 13 billion years old, not 10 billion. Monkeys and the human line split 25 to 30 million years ago, not 10 to 20. The Star Trek episode (The Next Generation, season 6, episode 20), the five probes leaving the solar system and the Planck length are as stated.