What’s the difference between a living collection of matter, such as a tortoise, and an inanimate lump of it, such as a rock? They are, after all, both just made up of non-living atoms. The truth is, we don’t really know yet. Life seems to just somehow emerge from non-living parts.
This is an enigma we’re tackling in the fifth episode of our Great Mysteries of Physics podcast – hosted by me, Miriam Frankel, science editor at The Conversation, and supported by FQxI, the Foundational Questions Institute.
The physics of the living world ultimately seems to contradict the second law of thermodynamics: that a closed system gets more disordered over time, increasing in what physicists call entropy. Living systems have low entropy. A messy lump of tissue in the womb, for example, can grow into a highly ordered state of a foot with five toes.
“We maintain this high sense of order for many, many decades,” explains Jim Al-Khalili, a broadcaster and distinguished professor of physics at the University of Surrey in the UK. “It’s only when we die that entropy and the second law of thermodynamics really kicks in.”
Quantum biology is one approach to understanding how living matter is different from inanimate matter. It is based on the strange world of quantum mechanics, which governs the microworld of particles and atoms. The idea is that living systems may use quantum mechanics to their advantage – promoting or halting quantum processes.
“Evolution has had long enough to fine-tune things or to stop quantum mechanics from doing something that life doesn’t want it to do,” explains Al-Khalili, who carries out research in the area. “It’s a newish area of science.”
One example, albeit still controversial, is photosynthesis, the process in which plants or bacteria absorb particles of sunlight, photons, and convert it to chemical energy. Some physicists believe a quantum property known as superposition – allowing a particle to be in many possible states, such as taking different paths, simultaneously – enables this process.
“A lump of energy [such as a photon] just randomly bouncing around should just be lost as waste heat,” explains Al-Khalili. “There’s a quantum mechanical explanation for how that photon follows multiple paths simultaneously.”
Al-Khalili and his colleagues are now using quantum biology to try to understand DNA mutations – a core part of life – and they’ve made some intriguing discoveries already. And while he isn’t convinced the approach will ever be able to explain consciousness, he argues we cannot rule it out.
Sara Walker, an astrobiologist and theoretical physicist working as a professor at Arizona State University in the US, favours another approach, however. She is trying to create a new physical theory of life based on information theory – which takes information to be real and physical.
Information seems to be crucial to life. Living organisms have an inbuilt set of instructions, DNA, which non-living things simply don’t have. Similarly, when living beings invent things, such as rockets, they rely on information, such as knowledge of the laws of physics, stored in their memory.
We can use the current laws of physics to predict how a planet evolves over time, for example whether and when nearby objects are likely to crash into it. But we can’t use the laws to explain how and when intelligent beings arise and decide to build rockets and satellites which they launch into orbit around the planet.
“I do think that there are laws of physics that are yet undiscovered that explain the phenomena of life, and I think those have to do with how information structures reality in some sense,” explains Walker.
Walker believes that living organisms are more complex and difficult to assemble from fundamental building blocks than inanimate, naturally produced objects, such as simple molecules. And when simple living beings exist, they seem to generate even more complexity – either by evolution or through construction.
So Walker believes life generates a sudden boost in complexity which may have a threshold that could be a fundamental feature in the physics of life. Another central part of her theory is time. “The deeper in time an object is, the more evolution is required to produce it.”
Walker has designed an experiment to look at how molecules are built up by joining smaller pieces together in various ways. She says the team hasn’t found any evidence that molecules with high complexity can be produced by non-living things. The ultimate goal is to pinpoint an origin of life in which a chemical system can generate its own complexity.
Not only could that help us understand how life arises from non-living building blocks, we could also use it to search for life on other worlds in the cosmos.