Welcome to The Nonlinear Library, where we use Text-to-Speech software to convert the best writing from the Rationalist and EA communities into audio. This is: re: Yudkowsky on biological materials, published by bhauth on December 11, 2023 on LessWrong.

I was asked to respond to this comment by Eliezer Yudkowsky. This post is partly redundant with my previous post.

Why is flesh weaker than diamond?

When trying to resolve disagreements, I find that precision is important. Tensile strength, compressive strength, and impact strength are different. Material microstructure matters. Poorly-sintered diamond crystals could crumble like sand, and a large diamond crystal has lower impact strength than some materials made of proteins.

Even when the load-bearing forces holding large molecular systems together are locally covalent bonds, as in lignin (what makes wood strong), if you've got larger molecules only held together by covalent bonds at interspersed points along their edges, that's like having 10cm-diameter steel beams held together by 1cm welds.

lignin (what makes wood strong)

That's an odd way of putting things. The mechanical strength of wood is generally considered to come from it acting a composite of cellulose fibers in a lignin matrix, though that's obviously a simplification.

If Yudkowsky meant "cellulose fibers" instead of "lignin", then yes, force transfers between cellulose fibers pass through non-covalent interactions, but because fibers have a large surface area relative to cross-section area, those non-covalent interactions collectively provide enough strength. The same is true with modern composites, such as carbon fibers in an epoxy matrix. Also, there generally are some covalent bonds between cellulose and lignin and hemicellulose.

Bone is stronger than wood; it runs on a relatively stronger structure of ionic bonds

Bone has lower tensile strength than many woods, but has higher compressive strength than wood. Also, they're both partly air or water. Per dry mass, I'd say their strengths are similar.

Saying bone is stronger than wood because "it runs on a relatively stronger structure of ionic bonds" indicates to me that Yudkowsky has some fundamental misunderstandings about material science. It's a non sequitur that I don't know how to engage with. (What determines the mechanical strength of bonds is the derivative of energy with length.)

But mainly, bone is so much weaker than diamond (on my understanding) because the carbon bonds in diamond have a regular crystal structure that locks the carbon atoms into relative angles, and in a solid diamond this crystal structure is tesselated globally.

This seems confused, conflating molecular strength and the strength of macroscopic materials. Yes, perfect diamond crystals have higher theoretical strength than perfect apatite crystals, but that's almost irrelevant. The theoretical ideal strength of most crystals is much greater than that of macroscopic materials. In practice, composites are used when high-strength materials are needed, with strong fibers embedded in a more-flexible matrix that distributes load between fibers.

But then, why don't diamond bones exist already? Not just for the added strength; why make the organism look for calcium and phosphorus instead of just carbon?

The search process of evolutionary biology is not the search of engineering; natural selection can only access designs via pathways of incremental mutations that are locally advantageous, not intelligently designed simultaneous changes that compensate for each other.

Growth or removal of diamond requires highly-reactive intermediates. Production of those intermediates requires extreme conditions which require macroscopic containment, so they cannot be produced by microscopic systems. Calcium phosphate, unlike diamond, can be made from ions that dissolve in water and can be transported by proteins. That is why bones are made with calcium phosphate instead of diamond. The implication that lack...