Physicist Sabine Hossenfelder presents an innovative scientific method for detecting signs of life beyond Earth. The video discusses current research approaches in the field.
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Physicist Sabine Hossenfelder examines a proposal to use assembly theory to search for life on exoplanets. Proposed by a group of biologists in 2023, the theory attempts to quantify complexity by counting the steps needed to assemble an object. Hossenfelder illustrates the idea with the complicated process of setting up a smart IKEA lamp, then describes how the researchers want to apply it to combinations of molecules observed in planetary atmospheres.
Many searches for extraterrestrial life look for molecules associated with life on Earth. The difficulty is that those molecules may also arise from geological or chemical processes. Dimethyl sulfide, for example, is produced by life on Earth but can also be generated geologically. The paper’s authors argue that combinations of molecules requiring more than about 15 assembly steps are unlikely to arise without biological selection. They propose searching for such combinations in particular ratios. Hossenfelder notes that the approach does not assume life elsewhere must resemble life on Earth.
She gives the proposal a 3 out of 10 on her “bullshit meter”: she likes the application, but doubts that the assembly index is a good general measure of complexity. She argues that a better starting point is to examine the equations governing a system and how they can be simplified by identifying assembled things.
0:00Every scientific discipline has an elephant in the room that no one’s talking about. In physics that elephant is complexity. We all know what we mean by it: plants, people, IKEA manuals. But just what is complexity? I think that answering this question is a precondition for understanding life and consciousness. A few years ago we saw the first serious attempt at quantifying complexity, called “assembly theory” and today I have an update. The same group argued, quite convincingly I think, that their method offers an unbiased way to look for life on other planets that sidesteps most of the problems that astrophysicists currently have. Let’s have a look. My joke about IKEA manuals didn’t come from nowhere, I recently tried
0:45to, ugh, add an IKEA lamp to an IKEA shelf. You’d think that shouldn’t be so difficult, but it turned out that it wasn’t just a lamp but a “smart lamp”. The lamp was so smart, it didn’t come with a power cable. So I had to order that. Then it turned out it doesn’t have a switch, it needs a remote control. Which needs a battery. So I had to order that. Then the remote needs to be paired to the lamp which wasn’t in the manual, so I had to watch several YouTube videos just to learn that the US-version works differently. It took me a full hour to simply get the lamp to turn on and off. I’m not telling you this to complain about IKEA. Or maybe not just. But as an example for how the complexity of life is increasing. And how we naturally measure this complexity by how
1:31many steps the assembly takes. And this, in a nutshell, is the idea behind “assembly theory”. The idea was proposed by a group of biologists in 2023. Basically, they take all the possible ways you can assemble an object and then ask what’s the shortest way to do it. They also take into account that to assemble big things you need to make the smaller things first, so that gives you a history-dependence that induces a selection. As a physicist I’d say, well, I’d have done it differently. Because complex objects like humans assemble less complex objects like lamps. I’ll tell you how I’d go about this in a moment, but first let me tell you the interesting development, which is that the same group that came up with
2:19assembly theory has said, look, we can use this to quantify how likely an exoplanet is to have life. You see, the current searches for alien life have a big problem. They usually look at the atmosphere of the planet and search for molecules that are associated with life on earth. But there is always the question of whether the same molecules could have come about instead by geological or chemical processes that do not require any complex intermediate steps. This has been the major reason why astrophysicists have not yet agreed that we have found life on other planets. Dimethyl sulphide is a good example. Yes, on earth it’s a by-effect of life. But it can also be produced geologically. As I said in an earlier video, I think if you look at the data, they have pretty much
3:06found life on other planets. They just can’t agree on how to quantify their discoveries. The authors of the new paper now say look, this is the perfect application for our theory of complexity. They calculate the probability of how difficult certain observed combinations of molecules are to assemble and conclude that molecule combinations requiring more than about 15 assembly steps are unlikely to arise without biological selection. So, they say, this is what we should look for. The combined appearance in certain ratios. The big benefit of this method is that it does not require assumptions that life on other planets must be in any way similar to life on earth. By the way, you can find a written version of our science news on Substack at sciencewtg.substack.com.
3:55I give this a 3 out of 10 on the bullshit meter. I like it because it shows just what understanding complexity would be good for. It’s a 3 because while it might work for molecules, I don’t think that in general this assembly index is a good way to quantify complexity. The basic issue with this measure of complexity is that it starts with things assembling other things without ever defining what a thing or an assembly is. This is why personally I think one needs to start by looking at the equations that govern a system and see how much they can be simplified by identifying “assembled things”. Basically I think that complexity in the end is just effective field theory applied. As a corollary, biology will eventually be reduced to physics.
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