What are the physics of life? That is more than just a philosophical question – it has practical implications for our search for life elsewhere in the galaxy. We know what Earth life looks like, on a number of levels, but finding it on another planet could require us to redefine what we even mean by life itself. A new paper from Stuart Bartlett of Cal Tech and his co-authors provides a new framework for how life could be defined that could reach beyond just what we understand from our one Pale Blue Dot.<\/p>\n
According to their framework, life isn\u2019t just chemical or thermodynamic – it\u2019s informational. In particular, it involves processing what they call Semantic Information (SI). By their definition, SI is information that helps and organism survive – such as \u201cdon\u2019t eat that red berry\u201d. Syntactic Information, on the other hand, is just raw data and can be dismissed as \u201cnoise\u201d by life without any consequence to its viability.<\/p>\n
Viability is really one of the keys to understanding the framework. The authors define life as having an intrinsic, relatively simple goal – don\u2019t die. On the other hand, something that has complex chemical and thermodynamic systems, such as a rock or a hurricane, has no such qualms about its demise. Life has to constantly monitoring it surroundings, in terms of its available food, the exterior temperature, and whether there are any toxins around. Those pieces of information are all SI – meaning they have a material impact on the viability of the life form. On the other hand, a rock doesn\u2019t really care what toxins or food are round, nor what the temperature is.<\/p>\n
Fraser discusses why space exploration is so important to our evolution as a species.<\/p>\n
This understanding represents a deep dive into one pillar of the Lyfe Framework, originally put forward by Dr. Bartlett as a way of understanding \u201cuniversal life\u201d. It consists of four pillars – Dissipation (energy use), Autocatalysis (growth), Homeostasis (stability) and Learning (information processing). The definition of SI described in the paper nicely fleshes out more details about that fourth pillar.<\/p>\n
One of the most interesting parts of this framework of thinking is implications for Origins of Life research. Typically, scientists have thought of the origin of life not as when the first specific molecule like DNA was produced, but for the Information Transition, when molecules started processing the information around them to enhance their own viability. Luckily, unlike so many abiogenesis theories, this one comes with a test to be able to falsify it.<\/p>\n
A Chemical Garden is a structure that self-assembles by mixing metal salts into a solution. The experiment also requires a tool called an Epsilon Machine, which is a type of algorithm used, in this context, to act as a pattern generator but to hide the patterns behind layers of complexity. If you hook this algorithm up to a function generator to generate electrical signals, it could effectively mask a complex pattern into those signals. Hook the output of the function generator up to the Chemical Garden, and you could potentially see what appear on the surface to be \u201cdumb\u201d minerals reacting in a way that reflects the hidden pattern developed by the algorithm. Measuring the growth of the garden with the internal state of the Epsilon machine provides the falsifiable evidence needed to prove whether or not a Chemical Garden could start processing information – in particular by matching its internal structure to that of the pattern provided by the Epsilon Machine.<\/p>\n
Fraser talks about how we can search for aliens using traditional methods.<\/p>\n
Admittedly, this is a space and astronomy blog, and despite the fact that Dr. Bartlett also works for the SETI Institute and there are a number of astronomers as co-authors on the paper, what does all of this have to do with space? There are two immediate consequences of the framework – one having to deal with the minimum size of life, while the other has to do with how we could potentially find it on alien planets.<\/p>\n
In order to detect its surroundings to improve its viability, cell structure must have sensors that can actually monitor their environment. Physics imposes a limit on the minimum size of a cell that is capable of housing this sensory equipment at .4um. Any smaller than that and the Brownian motion would throw off the cell\u2019s ability to accurately detect its surrounding environment. It would do this in two ways. First it could literally spin the cell around so quickly that it loses its orientation, not able to act on whatever information it processes. Second, it would cause molecules to randomly be picked up with the cell in fits and starts that are representative of its actual environment, essentially causing an inaccurate reading that could trick the cell into doing something that hurts its viability rather than helps it.<\/p>\n
Ultimately, what this means is that the minimum size of a cell should be the same everywhere – whether that\u2019s on Earth or on Titan (which the paper uses as an example). It should also hold for life on other planets, though how we would detect a group of cells that are .4um in diameter from light years away is a mystery at this point.<\/p>\n
Fraser discusses the impliactions of what happens if we finally find extraterrestrial life.<\/p>\n
However, we could find life on other planets by \u201cpoking\u201d it. In the context of alien life, if we send a signal to another planet, and it responds with a \u201csmart\u201d response, that\u2019s a pretty clear-cut case for evidence of life. We just have to broaden our definition of what a \u201csmart\u201d response is. Most likely it would be an increase in the information that comes from that particular planet, though searching out that information itself is a difficult process. We could also use Assembly Theory to measure how complex the molecule on a different planet is. In some cases, there could be molecules that are too complex to be formed by random chance and must have been driven by a process that uses information, such as evolution.<\/p>\n
Frameworks are generally helpful in providing ways to think about the world, and this one is no different. It also comes with some falsifiable tests that interest scientists can run relatively inexpensively. There\u2019s still a debate about whether we should be sending signals to other worlds, rather than just receiving them, but the fact that we could potentially find life by the way it reacts to those signals is something else to consider as part of that argument. For now, this framework is a useful tool in an astronomers toolkit – but, if it passes some falsifiability tests, it might someday help reframe our conversation about how life in the universe got started.<\/p>\n
Learn More:<\/p>\n
S. Bartlett et al. – Physics of Life: Exploring Information as a Distinctive Feature of Living Systems<\/a><\/p>\n UT – Failing to Find Life Tells Us a Lot About Life in the Universe<\/a><\/p>\n UT – Finding a Better Way to Distinguish Life from Non-Life<\/a><\/p>\n