On a sofa in the corner of the room, a cat is purring. It seems obvious that the cat is an example of life, whereas the sofa itself is not. But should we trust our intuition? Consider this: Isaac Newton assumed a universal time flowing without external influence, and relative time measured by clocks – just as our perception tells us. Two centuries later, Albert Einstein dropped the concept of universal time, and instead introduced a concept of time measured only locally by clocks. Who before Einstein would have thought that time on the Sun, the Moon, and even on each of our watches runs at slightly different rates – that time is not a universal absolute? And yet today our cellphones must take this into account for a GPS to function.

Science has made amazing strides, uncovering a deep and often counterintuitive understanding of physical reality. We understand a lot about the atoms in the human body and the stars in the night sky: much more than we do about the individual human as an example of life. In fact, life scientists continue to debate the exact definition of life. It was Aristotle who first said that life is something that grows and reproduces. He was fascinated by the mule, a cross between a horse and donkey that is always sterile. But just because the mule was sterile, you couldn’t call it dead. The debate is endless: some say that life must metabolise, that is, take in compounds, turn them to energy, and release some waste. But do jet engines qualify? In short, there is no theory and therefore no measuring apparatus that can confirm or refute our assumption that the cat is alive and the sofa is not, nor even that you are alive as you read this.

This is not for lack of trying. An important step to understand the fundamental principles that could explain life was put forward by Erwin Schrödinger, one of the fathers of quantum physics. Schrödinger is perhaps best-known for his thought experiment of a cat that is both alive and dead, thus existing in two states at once (called a superposition in physics). But he is also highly regarded for a series of lectures he delivered in 1943 at the Dublin Institute for Advanced Studies under the thought-provoking title ‘What Is Life?’ The lectures were published as a book in the subsequent year, and gained fame for inspiring generations of scientists to understand life at a deeper level. Most widely noted is his influence on the molecular biologists James Watson and Francis Crick to search for the structure of DNA, which Schrödinger predicted as an ‘aperiodic crystal forming the hereditary substance’.

In What Is Life?, Schrödinger explained an apparent incompatibility between life and the second law of thermodynamics, which holds that the entropy, or disorder, of a physical system always increases. How can life increase order when the Universe must always decrease its order, as mandated by the second law? Schrödinger’s answer was that life has to feed on ‘negative entropy’ or free energy – what we more typically call food and sunlight. The second law mandates that the amount of order in sunlight that is destroyed when it heats up our planet must be greater than the amount of order generated by a growing plant.

There is, however, a difference between showing that life is compatible with the laws of physics and making the stronger claim that life is explained by them. Schrödinger in particular was intrigued by how life can act in such an ordered manner despite constant exposure to the noise inherent in molecular dynamics – something that remains an unanswered question to this day. Even harder is explaining how life emerges from nonlife.

Schrödinger himself was left puzzled by such questions, commenting towards the end of What Is Life? that perhaps a ‘new type of physical law’ might be necessary to explain life. In the more than 70 years since Schrödinger’s book was published, there have been many attempts to define life or at least pin down its key properties. But, so far, an explanatory theory remains elusive.

It might be that our intuition of life is wrong, based on flawed perceptions because we cannot see directly the relationship between the cat and the sofa. Driven by the idea that universal rules or laws are true everywhere, not just for cats or sofas or atoms or galaxies, physics has a long history of discovering the hidden structure in our world. Take, as an example, the law of conservation of energy: that no process will create or destroy energy, whether a galaxy or a cat. An important step in recognising this law was realising that kinetic energy and heat can be unified. In trying to determine a physics of life, we must consider all examples of life to fundamentally be part of the same universal phenomenon. Otherwise, ‘life’ is not an objective property, it is a collection of special cases. If a ‘new type of physical law’ is indeed necessary to explain life, as we think, and as Schrödinger hinted, then we must radically rethink not only our approach to life, but also to physics. The ‘laws of life’ should be universal in the same sense that the laws of physics are. We should expect them to apply not only to life on Earth, but also on other worlds, and not just in biology, but also in physics in general.

Debate surrounding whether life is governed by its own special laws, or whether the same laws apply within and without life, is not new. Vitalist philosophers argued that there is something special about living organisms – a vital force – that separates the living from the nonliving, and in chemistry separates the organic from the nonorganic. The vital force was supposed to explain why bacteria shouldn’t spontaneously appear in dirt. In 1859, Louis Pasteur, after whom the process of pasteurisation is named, showed that bacteria cannot spontaneously generate. His conclusion was that all life must arise from other life.

But today we know that it is not lack of a vital force that prevents spontaneous generation of a bacterium. Instead, something else is missing: information. By this we mean the structure of the bacterial cell, and the contents of its genome. This information has been acquired over billions of years of evolution and can’t be regenerated instantaneously from systems with no information, no matter how much dirt you have. In a way, this information is the real vital force: it is what separates living from nonliving. However, rather than being some mystical force possessed only by special bits of matter, it emerges over time and space via the process of selection. To understand the origin of life, we must learn what bits of matter can acquire such information, and how they do so.

To answer this, we must go beyond the ideas of Claude Shannon, who spearheaded the theory of information in 1948 in part by ignoring meaning. For example, in studying radio signals or other communications, Shannon was not concerned with the content of the message transmitted. Instead, he was interested in whether the bits transmitted reduced uncertainty in a receiver about the state of the sender. To accomplish this, he counted only possibilities. If tomorrow might see rain or shine, both equally likely, and you see a 100 per cent accurate weather report saying it will rain, the number of possibilities is reduced two-fold. You gain one bit of information. The same holds true if you flip a coin and learn which side faces up, or if you observe a black hole to see whether it rotates to the left or to the right. In all these examples, you gain precisely one bit of information by your observation. Information is simply the reduction in the number of expected possibilities, in essence reducing uncertainty. This is true no matter what the message says – the meaning of a signal is irrelevant to Shannon’s theory.

If we found dead Martian cats, we will have found life on Mars

To develop a theory of life, we need to put the meaning that Shannon removed back into information, and, by extension, also into physics. When billiard balls hit each other, they become correlated – you can predict something about one ball from looking at the other. This is information in the Shannon sense: it is just about reducing the number of possible states that the first ball could be in once you know something about the second. On the other hand, when life gains information about its environment, this information isn’t about predicting the likelihoods of arbitrary states. It is what we call ‘functional information’ – information about how to survive better in the environment, and how to reproduce that information in the next generation.

Another polymath of the 20th century looked at reproduction and information transfer from a mathematical point of view. John von Neumann was a pioneer of many areas including the early development of computers, game theory and economics. He recognised that to get organisms, or any reproducing machine – which he called universal constructors – to build themselves, they need a program specifying how to do it, and a means to copy this plan and transfer the copy to the new entity. Constructors are more sophisticated than things such as sofas. Sofas can be generated only by information, but constructors can also generate new things by processing information (this includes mutant variants of themselves, which allows evolution to work in the first place) – that is, they can use information. A cat is a programmable constructor in this sense and so are you, but the sofa is not. In all these cases however (sofas and cats etc), a process of evolution is necessary to generate them.

A theory of information that could explain living systems will thus have to account for two aspects of information – how the information is acquired, and how it is used. This information was acquired over evolutionary time, for example through selection, via survival of the fittest. The use of the information is, as Schrödinger pointed out, accomplished by siphoning off negative entropy to pay for the increase in organisation that organisms need to survive, which they can do because they are constructors with information about how to produce themselves.

At this point, it is instructive to distinguish two concepts pertaining to acquisition and use of information: we’ll refer to them as life and alive. Alive refers to the continual use of negative entropy. It is the opposite of dead. While alive, a cat can use the negative entropy it acquired in the form of food to generate order in its cells, to construct itself. Things that are alive are constructors. A dead cat is not able to do that anymore. Being alive requires a process of maintaining homeostasis, that is, overcoming perturbations and maintaining the balance of the organism, overall.

Life, on the other hand, refers to the process that generates the required information. It also generates things that are alive and the information required to produce them. Also included in life are cases where information is acquired to generate things that aren’t alive, such as sofas. This is the process that generates information that is copied over time and across different physical things: from parent to offspring or among individuals. Life generates long threads of information through time – what evolutionary biologists call lineages: lineages of cats, leading from the ancestors of all cats to the cat sitting on the sofa; lineages of information leading from the first seats to the sofa; and lineages leading from the origin of life to the bacteria in the cat’s gut. These lineages would be the ones that scientists call ‘open-ended’, that is, they are candidates for continual evolution and expansion of novelty. Known life is our biosphere, including all lineages that have existed on Earth and the future lineages that will evolve from them. Alive things are the generating systems for expanding this process in space and time by constructing new possibilities. Thus, the cat and sofa are both life, but only the cat is alive.

To see why the distinction between life and alive is useful, we can consider a common example used in debates about definitions of life: viruses. There is a disagreement in biology about whether viruses are alive. They don’t have their own homeostatic mechanism, and piggyback on that of host cells to borrow the machinery needed to generate more viruses. By common criteria, therefore, viruses are not alive because they are not constructors able to reproduce themselves. But they can still be life. Viruses, such as cells, cats or humans, emerge through a long process that gathered functional information. If we want to explain the origin of life, we have to be able to explain how this process emerged. A dead cat is not alive, but it is life – it also emerged through a long process of acquiring functional information. If we can explain the origin of dead cats, we will have explained the origin of life, and if we found dead Martian cats, we will have found life on Mars. It is thus important to distinguish the organism, the thing that is alive, from the process – life – that generated the information that it uses.

It is possible that every time life arises in the Universe, things will appear that we would call alive, or that share some of the features of what we think of as being alive. They will probably use information and negative entropy to maintain themselves. But it is not at all clear that all life will lead to things that are alive everywhere that life exists, nor is it clear what features of being alive are necessary for open-ended evolution and expansion of a biosphere.

Imagine you have built a sophisticated 3D printer called Alice, the first to be able to print itself. As with von Neumann’s constructor, you supply it with information specifying its own plan, and a mechanism for copying that information: Alice is now a complete von Neumann constructor. Have you created new life on Earth?

Biological lineages, such as our ~4 billion-year-old one, might be some of the oldest physical structures on earth because they keep regenerating themselves

Suppose you then rig up Alice so it acquires (through your design) more information: it can use rocks and the minerals derived from them as raw materials to make new 3D printers. Are Alice and her offspring (Bob, Charley, Daisy and Eve) now life? Getting annoyed with continually having to find raw materials for all the little 3D printers running around, you decide to equip one offspring, Eve, with even more acquired information: solar panels for energy that enable Eve to go out by itself and use that acquired information to hunt for minerals. Is Eve now life?

For this latest design you gain acclaim for engineering the first ever self-reproducing 3D printer that is completely autonomous and doesn’t require supervision. You put a lot of information and programming into making your constructor. Although you care deeply for your creation, your insurance company worries about the unfavourable losses if Eve ever takes over the Earth. So, you figure out a way to get your little autonomous 3D printer sent on the next mission to Mars as a stowaway. Imagine Eve has a happy existence in a hidden valley on Mars, and goes on to produce many copies of itself. Humanity discovers the valley a few million years later to find the process of evolution on 3D printers generated a wide variety of them that are quite different from your original design – small ones, big ones, blue ones, red ones, ones that hunt other 3D printers for resources, and so on.

The value of this thought experiment is that it allows us to play with the notions of life and alive. Intuitively, we feel that life emerged, or maybe that the new robots are even alive. Did the 3D printers become life? If so, at what point did they become life? Are they alive?

The 3D printers form a lineage. On Mars, novel functions were discovered via evolution, branching off new lineages, each with independently derived functions developed through selection. But back on Earth, the origin of this lineage was your engineering. And you yourself are part of a nearly 4 billion-year-old lineage going all the way back to the origin of life on Earth. Notice that the 3D printer lineage and your lineage aren’t really separate. The 3D printers would not have appeared without a long history of accumulation of functional information on Earth, just as you never would have appeared without that long history. It is for this reason that, to truly study independent examples of life, we must discover alien life and not just make it in the lab.

It is impossible to remove our own agency in attempts to produce artificial life: anything we create is part of our own lineage, and therefore represents the same life. Biological lineages, such as our ~4 billion-year-old one, might be some of the oldest physical structures on Earth because they keep regenerating themselves. Not only that, but they are also powered by information to continually expand and evolve.

But let us look at the functional information that makes us. Some was acquired around 4 billion years ago when life got started, but most was acquired over evolutionary time. We learned how to build our bodies around 540 million years ago, and to digest cooked food maybe in the past million years. We are a bundle of information, acquired at different stages, some acquired by the bacteria in our gut, and some acquired through culture. From the lineage viewpoint, the 3D printers were always life, there was no nonlife/life transition in our thought experiment. But something did change over the course of their evolution – they acquired functional information over millions of years on Mars, and were able to use this information to do novel things – that is, they came closer to what we would call alive. It is in this sense that we might consider the 3D printers to represent new examples of alive things, each generating new possibilities for life.

Our aim is to understand life on Earth and on other worlds, and to understand its origin. But in order to achieve this we might have to fundamentally change the way we think about life. A cat is an instance of an organism that is alive. But it is also a 4 billion-year-old lineage of independently derived functional information. The question of whether Schrödinger’s cat is alive or dead might not be the right one – in either case, it is life.

In the corner of the room a 4 billion-year-old lineage is sitting, currently in the shape of a cat. That is life.

Published in association with the Santa Fe Institute, an Aeon Strategic Partner.