We need to cast off our biological blinkers in the search for life beyond Earth and turn to physics, says cosmologist Charley Lineweaver What’s wrong with the definition of “life”?

Charley: Almost everything. For a start, we don’t really have one. When you ask a biologist, “Are viruses alive?”, half of them will say yes, and half will say no. We don’t want a terrestrial-life-only definition. We want one that could apply elsewhere in the universe. This is important because NASA is trying to build an instrument to detect life elsewhere.

So we can’t just head off and look for the molecules of life as we know it?

Charley: Imagine if some historian wanted to know about the origin of France. Most French people now have passports. So imagine the historian went out on an archaeological dig to look for French passports. That would be ridiculous. France as a nation has evolved over time, and only in the modern version do you get passports. Same for life. You could say all life today has DNA, but in the past, who knows?

Many biologists define life as anything that undergoes Darwinian evolution.

Charley: We pretend that makes sense, but if you look it makes no sense at all. What is the unit of Darwinian evolution? Is it the gene? Is it the cell? Is it a multicellular organism? Is a city evolving? How about Gaia? Is that a life form?

So how would you define life?

Charley: To the extent that the question makes sense, as a “far-from-equilibrium dissipative system”.

What do you mean by that?

Charley: A system that feeds on the free energy associated with the gradients in the environment. For example, a hurricane is a dissipative structure because it feeds on the free energy of air pressure, humidity and thermal gradients. Hurricanes dissipate the free energy – they undo the gradients and bring the environment closer to equilibrium. That’s what life is. We take chemical energy and undo it – move it closer to equilibrium.

Why would you want to draw the line there?

Charley: If you’re going to draw a line anywhere between life and non-life, it makes sense to draw it on physical criteria. If you’re going to be black and white, let’s use the physicists’ black and white. But you really should give up the black and white and go to a spectrum.

By your definition, a hurricane and a star are both alive. Are you really saying that?

Charley: If you want to chain me to a black and white view, then yes, I am. The distinction between far-from-equilibrium dissipative systems and near-equilibrium features is clearer than the traditional boundaries that people put on life and non-life. It makes more sense to me to increase the size of what we call “life” than to continually search for a meaningful boundary where there is none.

Isn’t it a bit disappointing to claim we have found extraterrestrial life, then point to a star?

Charley: I’m moving the bar in what I consider to be a reasonable way. People should be disappointed, not at my moving the bar, but in the unrealistic expectation that there should be a bar where we have traditionally placed it.

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