The search for life beyond Earth is hampered by a fundamental conundrum: there is no agreed definition of what life actually is.
Living organisms are so extraordinary, so unlike other complex systems we know, that they seem to be made of some sort of magic matter. And for centuries it was widely supposed that organisms were indeed infused with a unique essence or life force to animate them.
A hundred years ago, however, these mystical musings gave way to a mechanical view of life, according to which organisms are just immensely complex machines obeying the same basic laws of physics as non-living systems.
Yet doubt has persisted. Seventy-five years ago, the theoretical physicist Erwin Schrödinger, one of the founders of quantum mechanics, delivered a series of lectures in Dublin entitled What is Life? He openly questioned whether life could in fact be explained by known physics, urging his colleagues to keep an open mind about the possibility of fundamentally new laws.
Schrödinger pointed to life’s ability to buck the trend of the second law of thermodynamics, the universal tendency for physical systems to become progressively more disordered over time. Life, by contrast, creates order from order. So maybe something was missing from the mechanistic model. But what?
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