Life presents a curious contrast: a cat embodies life, whereas a sofa does not, yet both are part of the broader context we call life. This paradox is illuminated by information theory.

Consider the scene: a purring cat rests on a sofa. Our instinct tells us the cat is alive, while the sofa is not. But should we rely solely on this intuition? Historical perspectives offer insight. Isaac Newton proposed a universal, unchanging time, while two centuries later, Albert Einstein challenged this notion, demonstrating that time is not absolute but varies based on local measurements. Few would have anticipated that time flows differently on the Sun, the Moon, and even on our wristwatches. Yet, this understanding is essential for technologies like GPS.

Science has made tremendous progress, revealing a complex and often counterintuitive picture of physical reality. While we have extensive knowledge about atoms and celestial bodies, our comprehension of life itself remains limited. The definition of life is still debated among scientists. Aristotle suggested that life is characterized by growth and reproduction, yet the mule—a sterile hybrid of a horse and donkey—defies this definition. Despite its sterility, the mule is not considered dead. The discourse continues: some argue that life must involve metabolism—intake, energy conversion, and waste expulsion. But does a jet engine fit this criterion? Ultimately, no existing framework can definitively validate the assumption that the cat is alive and the sofa is not, nor can it confirm your own existence as you read this.

Efforts to grasp the fundamental principles of life have been ongoing. Erwin Schrödinger, a prominent figure in quantum physics, contributed significantly to this dialogue. His thought experiment featuring a cat that is simultaneously alive and dead—existing in a state of superposition—challenges our understanding. In 1943, he delivered influential lectures titled "What Is Life?" at the Dublin Institute for Advanced Studies, which were later published as a book. This work inspired generations of scientists, including James Watson and Francis Crick, who sought the structure of DNA, which Schrödinger anticipated as an "aperiodic crystal forming the hereditary substance."

In "What Is Life?", Schrödinger addressed the apparent conflict between life and the second law of thermodynamics, which states that entropy, or disorder, in a system tends to increase over time. How can life create order if the universe inherently trends toward disorder? Schrödinger proposed that life consumes "negative entropy" or free energy—what we typically define as food and sunlight. The second law necessitates that the energy lost in sunlight must exceed the order generated by a growing organism.

However, demonstrating that life aligns with physical laws is different from asserting that it can be fully explained by them. Schrödinger was particularly intrigued by how life maintains order amidst the chaotic dynamics of molecular interactions—a mystery that remains unsolved. Moreover, the emergence of life from non-life poses an even greater challenge.

Schrödinger himself expressed uncertainty about such matters, suggesting the potential need for "new types of physical laws" to explain life. Since the publication of his book over 70 years ago, numerous attempts have been made to define life and identify its core characteristics, yet a satisfactory explanatory theory has yet to materialize.

Our intuitive grasp of life may be flawed due to our inability to directly observe the intricate relationship between living beings and inanimate objects. Physics has a rich history of uncovering hidden structures within the universe, often revealing universal rules that apply across various contexts—be it galaxies or cats. For example, the law of conservation of energy states that energy cannot be created or destroyed. Recognizing this principle involved understanding the unification of kinetic energy and heat. In seeking a physics of life, we must consider all manifestations of life as interconnected aspects of a universal phenomenon. If a "new type of physical law" is indeed required to elucidate life, as Schrödinger suggested, we must fundamentally reassess our views on both life and physics. The principles governing life should be as universal as those of physics, applicable not only to life on Earth but potentially to other worlds, transcending biological boundaries.

Debates regarding whether life is governed by distinct laws or if the same laws apply universally are not new. Vitalist philosophers posited that living organisms possess a unique "vital force" that differentiates them from non-living entities. This force was thought to explain why bacteria do not spontaneously arise from dirt. However, in 1859, Louis Pasteur demonstrated that bacteria do not spontaneously generate; rather, life emerges from other life.

Today, we understand that the absence of a vital force does not preclude the spontaneous generation of bacteria. Instead, what is lacking is information—specifically, the structural and genomic information contained within a bacterial cell. This information has evolved over billions of years and cannot be instantaneously replicated from systems devoid of it, no matter the abundance of raw materials. In this sense, information serves as the true vital force, distinguishing living from non-living entities. However, rather than being an esoteric force inherent in specific materials, information emerges through the processes of selection over time and space. To comprehend the origin of life, we must investigate which forms of matter can acquire such information and the mechanisms through which they do so.

To advance this understanding, we must expand beyond Claude Shannon's foundational work in information theory, which focused on the quantification of information while disregarding its meaning. For instance, in his studies on communication and signals, Shannon was unconcerned with the content of messages. Instead, he measured the reduction of uncertainty for a receiver based on the transmitted bits. If tomorrow's weather prediction equally suggests rain or shine, and a reliable report indicates rain, the possibilities are halved, yielding one bit of information. This principle applies to various contexts, whether flipping a coin or observing celestial phenomena. Information, in essence, is the reduction of expected possibilities—its meaning remains irrelevant to Shannon's framework.

If we were to discover deceased Martian cats, it would imply the existence of life on Mars.

To formulate a comprehensive theory of life, we must reintegrate meaning into information, and by extension, into the realm of physics. When billiard balls collide, they establish a correlation—knowledge of one ball's state informs predictions about another. This represents information in Shannon's terms, focusing on possible states. However, when living organisms acquire information about their surroundings, it transcends mere predictions; we refer to this as "functional information," which pertains to survival and reproduction.

John von Neumann, a polymath of the 20th century, examined reproduction and information transfer from a mathematical perspective. He identified that for any self-replicating entity—termed universal constructors—to autonomously reproduce, they require a program that dictates their construction and a means to transmit this blueprint to new entities. Unlike sofas, which can be generated solely by information, constructors, like cats and humans, can innovate and produce novel variations through the processing of information. Evolution relies on this capacity for variation.

An effective theory of information that elucidates living systems must account for two facets: how information is acquired and how it is utilized. This information is acquired over the course of evolution through mechanisms like natural selection. The utilization of this information, as Schrödinger noted, involves harnessing negative entropy to facilitate the organization necessary for survival, enabled by constructors that possess knowledge of their own replication.

It is beneficial to delineate two concepts related to the acquisition and utilization of information: we will refer to them as "life" and "alive." "Alive" signifies the ongoing use of negative entropy, contrasting with the state of being dead. While a cat is alive, it utilizes the negative entropy sourced from food to cultivate order within its cells. Living entities act as constructors. In contrast, a deceased cat lacks this capability. The state of being alive necessitates maintaining homeostasis, effectively managing disturbances while preserving the organism's equilibrium.

Conversely, "life" embodies the process that generates essential information, facilitating the emergence of living entities and the knowledge required for their existence. This process also encompasses instances where information is harnessed to create non-living objects, such as sofas. Life engenders continuous streams of information over time—what evolutionary biologists term lineages: lineages of cats stretching from their common ancestors to the feline on the sofa, lineages of information evolving from the earliest seats to modern sofas, and lineages from the origin of life to the bacteria residing in a cat's digestive system. These lineages are classified as "open-ended," indicating their potential for continuous evolution and the emergence of novelty. Our biosphere embodies known life, inclusive of all lineages throughout Earth's history and the future lineages yet to evolve. While both the cat and sofa can be considered "life," only the cat is "alive."

The distinction between "life" and "alive" proves useful when examining contentious examples in biological debates, such as viruses. There is ongoing disagreement regarding whether viruses should be classified as living entities. Lacking independent homeostatic mechanisms, viruses rely on host cells to replicate. By conventional criteria, they are deemed non-living as they cannot self-replicate. Yet, they still represent "life" as they arise from extensive processes that accumulate functional information. If we aim to elucidate the origin of life, we must also explain the emergence of non-living entities, such as a deceased cat. Should we succeed in explaining the origin of dead cats, we may unravel the mysteries of life's origins. Consequently, if we were to encounter deceased Martian cats, we would have found evidence of life on Mars. It is crucial to differentiate between the organism—what is alive—and the overarching process—life—that generates the information it utilizes.

Every occurrence of life in the universe may yield entities we classify as "alive" or exhibiting characteristics associated with life. These entities will likely rely on information and negative entropy for self-maintenance. However, it remains uncertain whether all forms of life will result in living beings across various environments, nor do we fully grasp which attributes of being alive are essential for the ongoing evolution and expansion of biospheres.

Consider the hypothetical scenario of designing a sophisticated 3D printer named Alice, capable of self-replication. Much like von Neumann's constructors, Alice is provided with a blueprint and a mechanism for duplicating that information, rendering her a fully autonomous constructor. Have we birthed a new form of life on Earth?

Now, suppose you enhance Alice's capabilities, enabling her to source raw materials like rocks for constructing additional 3D printers. At this juncture, are Alice and her offspring—Bob, Charley, Daisy, and Eve—classified as "life"? Frustrated by the constant need to procure materials for all the miniature 3D printers, you decide to endow Eve with solar panels, granting her the autonomy to seek minerals. At this point, is Eve now "life"?

Your innovative design garners acclaim for creating the first self-replicating 3D printer that operates independently without oversight. While you cherish your invention, your insurance company fears potential losses if Eve were to dominate Earth. Thus, you devise a plan to send Eve as a hidden passenger on a mission to Mars. Envision Eve flourishing in a secluded Martian valley, replicating herself over millennia. Humanity, millions of years later, discovers this valley and finds a diverse array of 3D printers, each differing from the original design—some small, others large, some colored, and some predatory toward other printers.

This thought experiment allows us to explore the concepts of "life" and "alive." Intuitively, we might conclude that life has emerged, or that these new machines are "alive." If they are considered "life," when did this classification occur? Are they indeed alive?

The lineage of 3D printers evolves in Mars's environment, leading to novel adaptations and branching into new lineages through evolutionary processes. Conversely, the origin of this lineage on Earth was your engineering. Importantly, the 3D printer lineage and your lineage are interconnected; the 3D printers could not have emerged without the extensive history of functional information accumulated on Earth, just as you could not exist without that same legacy. This underscores the necessity of discovering extraterrestrial life to study independent instances of life, rather than merely creating it in controlled environments.

Our own agency inevitably influences our endeavors to generate artificial life—anything we construct is inherently part of our lineage, thereby representing the same life. Biological lineages, such as our own, which spans approximately 4 billion years, may be among Earth's oldest physical entities due to their capacity for self-renewal. Furthermore, they draw upon information to continuously evolve and expand.

Let us examine the functional information that constitutes our being. Some of this information was acquired around 4 billion years ago when life first emerged, while much was accumulated through the course of evolution. Our ability to build complex bodies developed around 540 million years ago, and our adaptation to digest cooked food is a recent advancement, occurring over the past million years. We are a composite of information, acquired at various stages, some from the bacteria inhabiting our intestines and some through cultural evolution. Viewed from a lineage perspective, the 3D printers were always "life," and no clear transition from non-life to life occurred in our scenario. However, their evolution over time on Mars allowed them to accumulate functional information, enabling them to perform new tasks—bringing them closer to what we consider "alive." Thus, the 3D printers may represent novel examples of living entities, each contributing new possibilities for life.

Our overarching goal is to comprehend life on Earth and other celestial bodies while investigating its origins. To achieve this, we may need to fundamentally shift our conceptualization of life. A cat exemplifies an organism that is alive, yet it also embodies a lineage of functional information that has evolved over 4 billion years. The question of whether Schrödinger's cat is alive or dead might not be the most relevant inquiry—in both scenarios, it represents "life."

In the corner of the room, a lineage that has persisted for 4 billion years resides, currently taking the form of a cat. That is life.

Sara Walker is an astrobiologist and theoretical physicist at Arizona State University, where she serves as deputy director of the Beyond Center for Fundamental Concepts in Science, associate director of the ASU-Santa Fe Institute Center for Biosocial Complex Systems, and assistant professor in the School of Earth and Space Exploration.

Michael Lachmann is a professor at the Santa Fe Institute in New Mexico, focusing on the intersection of evolution and information, particularly regarding the origins of life.

Edited by Pam Weintraub

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