Astrobiologists Rule Out Silicon Life Over Water Decay and Sand Waste

What If Alien Life Were Silicon-Based? — PBS Space Time

PBS Space Time

Silicon cannot displace carbon as the backbone of extraterrestrial biology because its molecules break down in water and oxidize into unmanageable solid sand rather than exhaled gas.

AI-assisted summary of What If Alien Life Were Silicon-Based? — original source. Check the original for full context.

Source published:

Key ideas

  • Despite silicon making up over 28% of Earth's crust compared to carbon's 0.02%, terrestrial biology almost entirely ignores it due to the extreme instability of complex silicon molecules in water. (6:41)
  • Silicon reacts with an overwhelming thermodynamic affinity for oxygen, trapping vast quantities in solid silicate rock and creating a severe metabolic challenge where respiration would generate solid silica sand instead of easily exhaled gas. (9:30)
  • While carbon-in-water remains the most cosmologically viable biochemical basis, extreme low-temperature liquid hydrocarbons or sulfuric acid clouds could theoretically support alternative silicon-based life. (6:41)
  • Earth organisms like diatoms demonstrate biological manipulation of silicon by building outer protective cell walls from hydrated silica, though their internal metabolic machinery remains entirely carbon-based. (12:11)

Detailed notes

## Background Published by PBS Space Time on YouTube on January 26, 2023, and hosted by Matt O'Dowd, this episode investigates whether extraterrestrial biology could be founded on silicon rather than carbon. According to the video description, although all known life is carbon-based, silicon shares key chemical properties and is far more abundant across terrestrial planets, prompting the question of whether silicon-based alien life could realistically exist.

## TL;DR Host Matt O'Dowd examines the physical chemistry behind hypothetical silicon-based biology, narrowing down the periodic table to just seven potential elements capable of building complex covalent molecular backbones. While silicon mirrors carbon's valence structure and is thousands of times more abundant in Earth's crust, it faces severe biochemical hurdles: it disintegrates in water, bonds irreversibly with oxygen into immobile silicates, and leaves solid silica (sand) rather than gaseous carbon dioxide as a respiratory byproduct. Although speculative environments like cryogenic hydrocarbon lakes or sulfuric acid clouds could theoretically host silicon polymers, carbon in liquid water remains overwhelmingly superior, making Carl Sagan's concept of 'carbon chauvinism' chemically sound. The episode concludes with a viewer mailbag exploring Tegmark's Mathematical Universe, Roger Penrose's Three Worlds ontology, Theia remnants within Earth's mantle, and anatomical corrections.

## Atmosphere & style The presentation blends rigorous, college-level physical chemistry with lively, self-aware science-fiction humor. Matt O'Dowd speaks with energetic intellectual clarity, illustrating dense thermodynamic and molecular bonding principles through evocative diagrams before pivoting smoothly to deadpan jokes about alien space slugs and coughing up sand. The viewer mailbag and correction segments foster an engaging, nerdy camaraderie, gracefully handling pedantic audience critiques with self-deprecating wit.

## Bulletin - [06:41](https://www.youtube.com/watch?v=469chceiiUQ&t=401s) Despite silicon making up over 28% of Earth's crust compared to carbon's 0.02%, terrestrial biology almost entirely ignores it due to the extreme instability of complex silicon molecules in water. - [09:30](https://www.youtube.com/watch?v=469chceiiUQ&t=570s) Silicon reacts with an overwhelming thermodynamic affinity for oxygen, trapping vast quantities in solid silicate rock and creating a severe metabolic challenge where respiration would generate solid silica sand instead of easily exhaled gas. - [06:41](https://www.youtube.com/watch?v=469chceiiUQ&t=401s) While carbon-in-water remains the most cosmologically viable biochemical basis, extreme low-temperature liquid hydrocarbons or sulfuric acid clouds could theoretically support alternative silicon-based life. - [12:11](https://www.youtube.com/watch?v=469chceiiUQ&t=731s) Earth organisms like diatoms demonstrate biological manipulation of silicon by building outer protective cell walls from hydrated silica, though their internal metabolic machinery remains entirely carbon-based.

## Key takeaways - [00:00](https://www.youtube.com/watch?v=469chceiiUQ&t=0s) Science fiction frequently proposes silicon-based life due to silicon's identical valence configuration to carbon and its high planetary abundance. - [01:59](https://www.youtube.com/watch?v=469chceiiUQ&t=119s) Biochemistry requires a dynamic thermodynamic middle ground balancing molecular stability with chemical reactivity, avoiding irreversible thermodynamic sinks. - [03:34](https://www.youtube.com/watch?v=469chceiiUQ&t=214s) Only covalent bonds provide the structural modularity needed for life, as ionic bonds dissociate too easily and metallic bonds form rigid crystalline lattices. - [03:34](https://www.youtube.com/watch?v=469chceiiUQ&t=214s) Elements in row 4 and below on the periodic table possess electron clouds too distant from their nuclei, making their covalent bonds too fragile for stable biochemical structures. - [03:34](https://www.youtube.com/watch?v=469chceiiUQ&t=214s) Eliminating unsuitable elemental groups leaves only seven viable candidates for life's scaffolding: hydrogen, carbon, nitrogen, oxygen, silicon, phosphorus, and sulfur. - [06:41](https://www.youtube.com/watch?v=469chceiiUQ&t=401s) Silicon is the only element capable of mimicking carbon's ability to assemble extended polymers, rings, and complex molecular backbones. - [06:41](https://www.youtube.com/watch?v=469chceiiUQ&t=401s) Water, the universe's most abundant and effective solvent, rapidly destabilizes and destroys complex silicon-based molecules. - [06:41](https://www.youtube.com/watch?v=469chceiiUQ&t=401s) Alternative solvents such as liquid hydrocarbons on Titan or sulfuric acid on Venus present trade-offs, including severely restricted solubility at cryogenic temperatures or extreme chemical corrosiveness. - [09:30](https://www.youtube.com/watch?v=469chceiiUQ&t=570s) Silicon's exceptionally strong thermodynamic affinity for oxygen traps the element permanently in unyielding silicate minerals, out of reach for biological synthesis. - [11:16](https://www.youtube.com/watch?v=469chceiiUQ&t=676s) Aerobic respiration using silicon would yield solid silicon dioxide (silica or sand), posing a catastrophic excretion problem equivalent to exhaling a kilogram of mineral solids daily. - [12:11](https://www.youtube.com/watch?v=469chceiiUQ&t=731s) Diatoms provide an empirical proof of concept on Earth that biology can synthesize and manipulate crystalline silica structures to protect carbon-based cellular machinery. - [13:49](https://www.youtube.com/watch?v=469chceiiUQ&t=829s) The overwhelming biochemical efficiency of carbon in water leads to scientific 'carbon chauvinism', suggesting first contact with extraterrestrial biology will almost certainly involve carbon. - [15:44](https://www.youtube.com/watch?v=469chceiiUQ&t=944s) Sir Roger Penrose's Three Worlds ontology presents physical reality, mathematical forms, and conscious thought in a mutually emergent, circular relationship rather than a hierarchy rooted strictly in mathematics. - [18:21](https://www.youtube.com/watch?v=469chceiiUQ&t=1101s) Large Low-Shear-Velocity Provinces at Earth's core-mantle boundary may represent unmixed, dense mantle remnants of the impactor protoplanet Theia. - [18:21](https://www.youtube.com/watch?v=469chceiiUQ&t=1101s) The Fermi Paradox is likely resolved not by a single insurmountable barrier, but by a chain of cumulative, moderately difficult evolutionary and planetary filters.

## Detailed walkthrough ### [00:00](https://www.youtube.com/watch?v=469chceiiUQ&t=0s) The Sci-Fi Dream of Silicon-Based Life

Every mechanism of living biology encountered so far is anchored entirely around carbon. Because astrobiology seeks to estimate the prevalence and diversity of life across the cosmos, science must confront a fundamental question: is carbon really the only viable foundation, or merely an Earthly accident? In popular science fiction, the leading candidate for alternative biology has long been silicon, appearing in everything from the lethal acid-blooded xenomorphs of the *Alien* franchise to the colossal asteroid-dwelling exogorth (space slug) in *Star Wars*.

Writers naturally gravitate toward silicon because it sits directly beneath carbon on the periodic table, possessing nearly identical valence configurations and valence chemistry. Furthermore, it is vastly more abundant on terrestrial planets than carbon. When astrobiologists refer to life as "carbon-based," they do not simply mean organisms consume or incorporate the element; rather, carbon forms the structural scaffolding for every functional macromolecule, including DNA, RNA, amino acids, proteins, and the complex cellular architecture assembled from them. The bond is so absolute that organic chemists routinely omit the letter "C" entirely from diagrams, marking molecular carbon backbones with simple zigzagging lines.

Matt O'Dowd highlights the total identity between the element and biology: “Biochemistry is by definition the chemistry of Carbon.” Whether life on other worlds could ever swap that foundational atom for silicon depends on the physical chemistry of molecular stability and energetic balance.

[01:59](https://www.youtube.com/watch?v=469chceiiUQ&t=119s) The Goldilocks Zone of Chemical Energy: Reactivity vs. Stability

Physical systems inevitably tumble down potential energy gradients toward the lowest achievable energy state. A stick of dynamite illustrates this thermodynamic push: nitroglycerin holds considerable chemical potential energy within delicate bonds. Because it takes minimal activation energy to rupture those tenuous connections, the molecule rapidly dissociates into smaller, low-energy products, releasing heat and gas in a runaway chain reaction. That reaction is strictly one-way because reassembling those fragmented molecules back into nitroglycerin requires far more energy than the ambient environment can supply.

Living systems cannot operate on unidirectional chemical reactions. If the molecular components of a cell descend into an unreactive thermodynamic sink, metabolic chemistry halts permanently, which is equivalent to death. Conversely, organisms cannot rely on molecules so fragile that they detonate or unravel under normal ambient conditions; molecular machines must survive long enough to replicate, store genetic code, and catalyze metabolic pathways.

Matt O'Dowd summarizes this essential metabolic prerequisite: “In other words, you want a balance of reactivity and stability.” Achieving that dynamic middle ground requires specific types of chemical bonds and elemental architectures.

[03:34](https://www.youtube.com/watch?v=469chceiiUQ&t=214s) Whittling Down the Periodic Table to Seven Elements

To find which chemical elements could hypothetically support an open-ended biochemical network, one must look at atomic bonding behaviors. Among the three primary bond types—ionic, metallic, and covalent—only covalent bonds supply the requisite structural nuance. Ionic bonds dissolve or dissociate far too easily, while metallic bonding yields rigid crystalline lattices that lack molecular variety. Covalent bonds, by contrast, permit neighboring atoms to share electrons to complete their outer valence shells, opening up limitless combinatorial arrangements. Carbon possesses four valence electrons and requires four more to achieve an octet; it can satisfy this by binding four individual hydrogen atoms to form methane, or by linking with other carbon and oxygen atoms to assemble ethane, ethanol, and infinitely branching chains.

Evaluating the periodic table reveals that most elements are incapable of this chemistry. Ionic and metallic formers are immediately eliminated, noble gases are ruled out due to inertness, and halogens fail because they rarely bind to multiple atoms simultaneously and tend to react with violent volatility. Heavy elements sitting in row 4 and below must also be discarded: their sprawling electron clouds are held so loosely by their distant nuclei that the resulting covalent bonds are catastrophically fragile. This atomic scaling explains why arsenic is lethal to terrestrial biology; cells mistake arsenic for phosphorus due to their identical valence count, but incorporating the heavier, row-4 atom creates brittle molecules that snap and fatally disrupt cellular machinery.

```mermaid flowchart TD accTitle: Periodic Table Elimination for Life Scaffolding accDescr: Elimination process discarding unviable elemental groups to leave only seven candidate building blocks. subgraph Discarded["Discarded Elements"] A["Metals & Ionic Formers<br/>(Rigid lattices / weak stability)"] B["Noble Gases & Halogens<br/>(Inert or explosive / mono-bonding)"] C["Row 4+ Elements<br/>(Large nuclei yield brittle bonds)"] end subgraph Viable["Seven Life Candidates"] D["Row 1 & 2 Elements:<br/>H, C, N, O"] E["Row 3 Elements:<br/>Si, P, S"] end Discarded -.->|"Screening Criteria"| Viable ```

After eliminating unsuitable rows and groups, only seven candidate elements remain: hydrogen, carbon, nitrogen, oxygen, silicon, phosphorus, and sulfur. Six of these make up the bedrock of Earth life, with carbon reigning supreme because it uniquely forms both 1D chains and 2D rings with itself. As Matt O'Dowd notes when evaluating chemical options: “Covalent bonds are really the only option.”

- **Valence mechanics**: Carbon shares 4 electrons to complete its valence shell of 8, forming the baseline methane molecule ($CH_4$). - **Combinatorial modularity**: Substituting hydrogens and joining carbon backbones produces ethane ($C_2H_6$), ethanol ($C_2H_5OH$), and extensive polymers. - **Arsenic toxicity mechanism**: Arsenic mimics phosphorus in cellular uptake, but its larger atomic radius causes structural collapse in synthesized biomolecules. - **Core chemical survivors**: Hydrogen, Carbon, Nitrogen, Oxygen, Silicon, Phosphorus, Sulfur.

[06:41](https://www.youtube.com/watch?v=469chceiiUQ&t=401s) The Silicon Paradox and the Solvent Dilemma

Silicon is the only element on the periodic table that matches carbon’s ability to create extended covalent polymers, rings, and complex molecular scaffolds. Chemists can synthesize synthetic silicon oils that replicate the texture and behavior of organic petroleum-based oils. Yet despite this biochemical mimicry, terrestrial biology almost completely ignores silicon, using it far less than trace metals like iron. This non-use is an extraordinary evolutionary paradox given the geological abundance: silicon makes up over 28% of the Earth's crust by mass within silicate minerals, whereas carbon makes up a mere 0.02%.

The primary barrier preventing silicon from becoming the foundation of Earth life is its behavior in water. Complex silicon molecules are highly unstable in aqueous environments, breaking down rapidly when exposed to $H_2O$. Life demands a fluid solvent to allow enzymes and substrates to circulate and interact. While water is chemically abundant, thermally stable, and broadly considered the ultimate solvent across the universe, its destructive interaction with silicon chemistry presents a devastating hurdle.

Matt O'Dowd underlines this critical incompatibility: “The instability of most silicon molecules in water is actually a huge blow to the viability of silicon-based life.”

Alternative liquid solvents could hypothetically host silicon organisms, though each introduces severe physical drawbacks: - **Liquid hydrocarbons**: Cryogenic lakes of methane and ethane (such as those found on Saturn's moon Titan) stabilize complex silicon polymers. However, extreme cold severely restricts solvent capacity, making it difficult to suspend the large, sophisticated macromolecules required for life. - **Sulfuric acid**: Common in the upper cloud decks of Venus, concentrated sulfuric acid supports diverse silicon interactions. Yet its extreme chemical corrosiveness may tear nascent proto-biological precursors apart before reproduction can establish itself. - **Atmospheric sci-fi parallels**: The acidic blood of the cinematic xenomorph mirrors a theoretical silicon-in-sulfuric-acid system, prompting O'Dowd to joke that humans should rethink Venusian floating cloud colonies before unleashing space horrors.

[09:30](https://www.youtube.com/watch?v=469chceiiUQ&t=570s) The Oxygen Trap and Energy Generation

A speculative workaround for aqueous silicon biology involves alternating silicon with carbon or oxygen atoms into hybrid scaffolding backbones. However, these hybrid polymers offer zero evolutionary or structural advantage over straightforward hydrocarbon chains. Furthermore, silicon suffers from an overwhelming chemical bias: it has an extreme thermodynamic affinity for oxygen.

Silicon binds far more readily and tightly to oxygen than it does to other silicon atoms or any other candidate element in the biological toolkit. When oxygen is present, silicon-based molecules fracture, reacting irreversibly to form remarkably stubborn silicon-oxygen bonds. Matt O'Dowd observes: “Silicon forms stronger bonds with oxygen than with any of the other ‘building blocks of life’, and certainly more so than with other silicon atoms.”

Because silicon reacts with oxygen along a steep, one-way path, the vast majority of Earth's crustal silicon is trapped in unyielding silicate rock, permanently out of reach of biological synthesis. In contrast, carbon forms atmospheric carbon dioxide ($CO_2$), a gas that easily dissolves in liquid and remains bio-available for primary producers to capture via photosynthesis.

The sole advantage of this intense bond strength lies in metabolic energetics. Terrestrial animals power their cells by converting high-energy $O_2$ bonds into lower-energy $CO_2$ waste via aerobic respiration, capturing the released energy through mitochondrial phosphorylation to produce ATP. Because the silicon-oxygen bond is even more stable, a hypothetical alien metabolizing silicon with oxygen could extract substantial energy from the reaction—provided it could overcome the profound physical challenge of managing the reaction's solid, rock-like waste products.

[11:16](https://www.youtube.com/watch?v=469chceiiUQ&t=676s) The Waste Product Dilemma: Exhaling Sand vs. CO2

```mermaid flowchart TD accTitle: Metabolic Byproduct Comparison: Carbon vs Silicon accDescr: Flowchart illustrating the cellular waste products of carbon-based versus silicon-based respiration.

subgraph CarbonRespiration["Carbon Respiration"] C1["Carbon + O2"] --> C2["CO2 (Gas)"] C2 --> C3["Exhaled effortlessly (~1 kg/day)"] end

subgraph SiliconRespiration["Silicon Respiration"] S1["Silicon + O2"] --> S2["SiO2 (Silica / Sand)"] S2 --> S3["Dense solid byproduct"] S3 --> S4["Requires coughing sand or excreting glass beads"] end ```

When considering alien respiration based on silicon, converting oxygen ($\text{O}_2$) into silicon dioxide ($\text{SiO}_2$) seems thermodynamically favorable at first glance. Silicon dioxide is even more stable than carbon dioxide ($\text{CO}_2$), which means the reaction can liberate a greater amount of energy. However, this creates a catastrophic biological waste problem. Carbon-based organisms produce $\text{CO}_2$, a gas that can be effortlessly expelled through respiration. By contrast, silicon dioxide is silica—solid sand. As Matt O'Dowd quips, invoking a familiar pop-culture sentiment: “It’s coarse and rough and gets everywhere, I hate it.” Solid silica accumulating inside cells or respiratory organs would be fatal.

The sheer volume of solid waste presents an enormous biomechanical hurdle. An average human exhales roughly one kilogram of carbon dioxide every day. A silicon-based counterpart would therefore need to physically eliminate a comparable mass of solid mineral waste:

> “A regular person exhales about a kilogram of Carbon Dioxide throughout the day, meaning Silicon aliens would need to cough out a good amount of sand. Or they poop glass beads or something - probably there’s a way around the issue.”

While hypothetical organisms might evolve mechanisms to excrete glass beads or expunge gritty minerals, doing so demands significant physical and metabolic overhead. In biological evolution, any minor thermodynamic or energetic disadvantage becomes decisive over evolutionary timescales.

[12:11](https://www.youtube.com/watch?v=469chceiiUQ&t=731s) The Chemistry of Silicon Life and the Reality of Living Crystals

Synthesizing the biochemistry: under very specific, highly specialized conditions, silicon possesses enough chemical diversity to sustain complex molecular structures. However, these environments are far less chemically versatile and accommodating than the mild conditions that nurture carbon-based life. If carbon is available on a world, prebiotic chemistry will almost inevitably favor it.

Science fiction writers have long portrayed silicon-based life as living rocks or animated mineral crystals simply because that is how silicon commonly manifests on Earth. Yet carbon-based animals do not resemble pencil graphite or walking diamonds. Nonetheless, Earth does host organisms that use silicon as a primary structural element: diatoms.

```mermaid flowchart LR accTitle: Diatom Structural Architecture accDescr: Diagram showing the hybrid structure of diatoms using silicon cell walls around carbon-based cellular machinery.

subgraph DiatomArchitecture["Diatom Architecture"] direction LR SW["Rigid Silica Wall (Hydrated SiO2 Crystal)"] CM["Internal Cellular Machinery (Carbon-based organelles)"] SW -->|Encapsulates| CM end

CM -->|"Photosynthesis"| O2["Produces ~50% of Earth's O2"] ```

Diatoms are single-celled photosynthetic organisms that construct rigid, crystalline outer cell walls out of hydrated silica. These intricate, glass-like shells protect the organic machinery inside. Floating in oceanic colonies, diatoms are critical to Earth's biosphere, generating roughly half of the oxygen we breathe. While their internal biochemistry remains strictly carbon-based, diatoms serve as an empirical proof of concept that biology can successfully manipulate and integrate solid silicon into functional structures.

[13:49](https://www.youtube.com/watch?v=469chceiiUQ&t=829s) Carbon Chauvinism and Exotic Habitats

Ultimately, silicon-based life appears to be overrepresented in science fiction relative to its real-world cosmological probability. The chemistry of carbon dissolved in liquid water is so overwhelmingly efficient, robust, and versatile that alternative schemes may struggle to compete. Matt O'Dowd points out:

> “In fact, carbon-in-water is such an astoundingly good option for biochemistry that there may be no other game in town. Even Carl Sagan called himself a carbon-chauvinist for this reason.”

Because of this chemical supremacy, humanity’s first contact with extraterrestrial biology will likely be with organisms that share our fundamental carbon backbone. Nevertheless, extreme planetary environments should not be ruled out. In cryogenic liquid hydrocarbon seas, such as those on Saturn's moon Titan, or amidst the concentrated sulfuric acid clouds of Venus, alternative solvent chemistries might permit exotic silicon-based biochemistry to arise.

[14:33](https://www.youtube.com/watch?v=469chceiiUQ&t=873s) Brilliant.org: Quantum Objects

Matt introduces the episode sponsor, Brilliant.org, a STEM learning platform featuring interactive, problem-solving lessons designed for learners at all experience levels. He highlights their course on **Quantum Objects**, which investigates microscopic phenomena where subatomic particles violate classical intuition and probability. The course guides students through building new equations of motion and algebraic measurement frameworks, exploring how quantum mechanics underpins modern technologies like lasers, transistors, and quantum computers.

[15:44](https://www.youtube.com/watch?v=469chceiiUQ&t=944s) Mailbag: Tegmark's Platonic Universe vs. Penrose's Three Worlds

O'Dowd transitions to viewer comments, first addressing the episode on Max Tegmark’s Mathematical Universe Hypothesis (MUH), which posits that fundamental physical reality is identical to an abstract mathematical structure.

- **Foundational Symmetries vs. Equations:** Viewer *onebronx* argued that the root of our universe is unlikely to be the complicated Standard Model Lagrangian or general relativity field equations, but rather simpler underlying principles such as the principle of stationary action and core symmetry groups. O'Dowd agrees, noting that if the MUH holds, our universe likely stems from very simple core rules from which complex mathematical relationships and physical laws emerge hierarchically. - **Sir Roger Penrose’s Three Worlds Ontology:** Viewer *S. Lee Odegard* asked why Sir Roger Penrose’s three-realm model of reality was excluded from the episode. O'Dowd explains that the concept was cut strictly due to time constraints, but outlines Penrose's triadic, mutually emergent ontology: 1. **The Physical Realm** emerges from the **Platonic Realm** (physical regularities depend on mathematical forms). 2. **The Mental Realm** emerges from the **Physical Realm** (physical brains generate conscious thought). 3. **The Platonic Realm** emerges from the **Mental Realm** (conscious minds conceive mathematical concepts extending far beyond immediate physical reality).

Unlike Tegmark, who elevates mathematics to the single ultimate foundation of reality, Penrose views these three domains in a circular, co-dependent relationship without granting ultimate primacy to mathematics. Viewers interested in the mathematical depth of this framework are directed to Penrose’s comprehensive book, *The Road to Reality*, which O'Dowd jokingly warns is as demanding as an intensive, full-semester university physics course.

[18:21](https://www.youtube.com/watch?v=469chceiiUQ&t=1101s) Mailbag: Theia Mantle Blobs and the Fermi Paradox

The discussion turns to the prior episode on high-resolution simulations of the Moon-forming collision between Earth and the protoplanet Theia.

```mermaid flowchart TD accTitle: Theia Remnants and Mantle Plumes accDescr: Diagram illustrating the proposed origin of LLSVPs from Theia impactor material.

T["Theia Impactor"] -->|"Giant Impact"| E["Early Earth"] E -->|"Dense, unmixed remnants sink"| LLSVP["LLSVPs (Core-Mantle Boundary)"] LLSVP -->|"Deep Mantle Plumes"| Basalt["Ocean Island Basalts"] Basalt -->|"Isotopic Analysis"| Prim["Primitive Isotope Signatures"] ```

- **Relics of Theia inside Earth:** Viewer *markcentral* asked whether remnants of Theia could still exist intact inside the Earth. O'Dowd confirms this is an active hypothesis: geophysicists have identified two massive, dense regions at the core-mantle boundary known as Large Low-Shear-Velocity Provinces (LLSVPs). Some researchers propose these structures are unmixed, dense chunks of Theia's mantle that sank during the collision. Basaltic lavas originating from plumes above these regions exhibit anomalous, "primitive" isotopic ratios, hinting at reservoirs unmixed by the giant impact. - **The Giant Moon as a Great Filter:** Viewer *Alice* inquired whether the requirement for a massive moon—which stabilizes planetary axial tilt and helps maintain a robust dynamo—could act as a hard filter resolving the Fermi Paradox. O'Dowd suggests the resolution is rarely a single bottleneck:

> “My personal intuition is that the extremely hard filter needed to explain the Fermi paradox is actually a chain of somewhat hard filters, and the large moon may be one of these.”

Additional filters likely include planetary architecture (protective gas giants in outer orbits paired with inner rocky planets) and the specific radiation profile and lifespan of the host star.

[20:07](https://www.youtube.com/watch?v=469chceiiUQ&t=1207s) Corrections: Animal Hands, Bat Anatomy, and Moons

O'Dowd concludes with lighthearted corrections from previous episodes:

- **Avian Hands vs. Bat Wings:** In a prior segment on supercritical fluids, O'Dowd joked that birds are animals with hands large enough to swim through gas. Viewer *Xovvo* delivered a detailed anatomical correction, explaining that bats have elongated, membranous hands supported by digits II through V, whereas bird wing surfaces consist of feathers mounted on a tiny, fused skeletal hand called a carpometacarpus with reduced digits. Taking the pedantic critique in stride, O'Dowd quipped that he learned “even casual jokes should be subject to rigorous research and fact-checking.” - **Titan vs. Triton:** In a verbal slip, O'Dowd had previously said Titan (Saturn’s moon) when referring to Triton (Neptune’s moon). He playfully deflects the blunder onto his Australian accent, claiming that Aussies simply omit 'r's after 't's—and concludes with the deadpan claim that Australian birds, unlike all others, actually do have giant hands.

## Notable quotes > [06:41](https://www.youtube.com/watch?v=469chceiiUQ&t=401s) The instability of most silicon molecules in water is actually a huge blow to the viability of silicon-based life. — Matt O'Dowd

> [13:49](https://www.youtube.com/watch?v=469chceiiUQ&t=829s) In fact, carbon-in-water is such an astoundingly good option for biochemistry that there may be no other game in town. Even Carl Sagan called himself a carbon-chauvinist for this reason. — Matt O'Dowd

> [09:30](https://www.youtube.com/watch?v=469chceiiUQ&t=570s) Silicon forms stronger bonds with oxygen than with any of the other ‘building blocks of life’, and certainly more so than with other silicon atoms. — Matt O'Dowd

> [18:21](https://www.youtube.com/watch?v=469chceiiUQ&t=1101s) My personal intuition is that the extremely hard filter needed to explain the Fermi paradox is actually a chain of somewhat hard filters, and the large moon may be one of these. — Matt O'Dowd

> [00:00](https://www.youtube.com/watch?v=469chceiiUQ&t=0s) Biochemistry is by definition the chemistry of Carbon. — Matt O'Dowd

## Mentioned PBS Space Time, Matt O'Dowd, Carl Sagan, Max Tegmark, Sir Roger Penrose, Alien, Star Wars, Brilliant.org, The Road to Reality

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Life as we know it is carbon-based, but does it have to be this way? There’s another element on the periodic table that shares some of the key properties of carbon but is far more abundant on most planets. I’m talking about silicon. So is there silicon-based life out there?

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What If Alien Life Were Silicon-Based?

[Source](https://www.youtube.com/watch?v=469chceiiUQ)

中文简报

天体生物学家因水解与沙质废弃物排斥硅基生命可能 硅无法取代碳成为外星生物学的骨架，原因在于其分子会在水中分解，且氧化后会变成难以处理的固体沙子而非呼出的气体。 主持人马特·奥多德探讨了假想硅基生命背后的物理化学机制，将元素周期表缩小到仅有七种可能构建复杂共价分子骨架的元素。尽管硅具有与碳相似的价电子结构，在地壳中的丰度更是后者的数千倍，但它面临着严重的生物化学障碍：它在水中会发生解体，与氧不可逆地结合形成无法移动的硅酸盐，并且代谢排出的呼吸副产物是固体二氧化硅（沙子）而非气体二氧化碳。尽管低温碳氢化合物湖泊或硫酸云等假想环境理论上可以容纳硅聚合物，但碳与液态水的组合仍具有绝对优势，从而使卡尔·萨根所说的“碳沙文主义”在化学上站得住脚。本集最后以观众来信问答环节结束，探讨了泰格马克的数学宇宙、罗杰·彭罗斯的三世界本体论、地幔内部的忒伊亚残余物以及解剖学修正。 尽管硅占地壳成分的28%以上，而碳仅占0.02%，但由于复杂的硅分子在水中极不稳定，地球生物几乎完全弃用了硅。 (6:41) 硅与氧具有极强的热力学亲和力，从而将大量氧锁在固体硅酸盐岩石中；同时这也构成了严重的代谢挑战，即呼吸作用会产生固体二氧化硅（沙子），而不是易于呼出的气体。 (9:30) 虽然水环境中的碳仍是宇宙中最具可行性的生物化学基础，但极低温液态碳氢化合物或硫酸云在理论上可以支持替代性的硅基生命。 (6:41) 地球生物（如硅藻）通过利用水合二氧化硅构建外层保护性细胞壁，展示了对硅的生物学操控能力，但其内部代谢机制仍完全基于碳。 (12:11)

Reading summary

While science fiction frequently looks to silicon as an alternative chemical foundation for life, Matt O'Dowd of PBS Space Time explains that the strict thermodynamic and solvent requirements of biochemistry make carbon vastly superior across the universe. Screening the Periodic Table for Biochemistry (3:34) Biochemical systems require an open-ended combinatorial toolkit balanced between stability and reactivity. Molecules cannot descend into permanent thermodynamic sinks or be so fragile that they dissociate under ambient conditions. Covalent bonds are the only viable mechanism to build complex biological scaffolding, as ionic bonds dissociate too easily and metallic bonds form rigid, uniform lattices. Noble gases and halogens fail due to inertness or volatile mono-bonding, while elements in row 4 and below have large atomic radii that generate catastrophically brittle covalent bonds, which explains why arsenic lethally disrupts cellular machinery by substituting for phosphorus. The Abundance Paradox and Solvent Incompatibility (6:41) Silicon shares carbon's valence structure and is far more abundant on terrestrial planets, composing over 28% of Earth's crust by mass compared to carbon's 0.02%. It is also the only other element capable of assembling extended chains, rings, and complex polymers. However, terrestrial biology almost completely ignores it because complex silicon molecules are fundamentally unstable in liquid water, rapidly breaking down upon contact. Alternative Solvents and Their Severe Trade-offs (6:41) To bypass the destructive effects of water, theoretical silicon life would require non-aqueous solvents, but each candidate introduces physical compromises. Cryogenic liquid hydrocarbons like the methane and ethane lakes on Saturn's moon Titan can stabilize silicon chains, but extreme cold significantly restricts solvent capacity, making it difficult to suspend large macromolecules. Concentrated sulfuric acid, such as that found in Venusian clouds, supports silicon chemistry but possesses intense corrosiveness that could tear nascent prebiotic precursors apart before replication can begin. The Oxygen Trap and Solid Waste Crisis (9:30) Silicon exhibits an extreme thermodynamic affinity for oxygen, binding to it far more readily than to itself or other elements. This one-way chemical path locks crustal silicon into unyielding silicate rock, unlike carbon, which forms atmospheric carbon dioxide that easily dissolves in liquid and remains bioavailable. While oxidizing silicon yields substantial metabolic energy, its primary byproduct is silicon dioxide (silica), a dense solid. Biological Management of Mineral Waste (11:16) Because humans exhale roughly one kilogram of gaseous carbon dioxide each day, an equivalent silicon-based organism would face the fatal biomechanical challenge of expelling a comparable mass of solid sand or glass beads from its respiratory structures. While Earth organisms like diatoms demonstrate that biology can incorporate hydrated silica into rigid protective cell walls, their internal machinery remains strictly carbon-based. O'Dowd concludes that carbon-in-water chemistry is so versatile and efficient that prebiotic chemistry will almost inevitably favor it whenever carbon is present. While exotic silicon biologies might emerge in extreme niches like hydrocarbon seas or acid clouds, humanity's first discovery of alien life will most likely share our fundamental carbon architecture.

精炼阅读摘要

天体物理学家兼主持人 Matt O'Dowd 指出，尽管硅在元素周期表中具备与碳相似的价键结构且在类地行星表面丰度极高，但严苛的物理化学限制决定了碳水化学在宇宙中占据近乎绝对的生命主导地位，使得纯硅基生命在现实演化中极其罕见。 元素筛选与共价键必要性 (3:34) 生命系统需要在反应活性与分子稳定性之间维持微妙平衡。生物代谢若陷入不可逆的热力学低能态便会导致化学反应停滞，而过于脆弱的分子结构又无法稳定维持遗传编码与酶催化。通过对周期表键合模式的系统筛选，离子键因在液体中极易解离被排除，金属键形成的晶体晶格缺乏多样性，只有共价键能够通过共享外层电子构建灵活多元的分子骨架。第四周期及以下的重元素（例如砷）因原子核对电子云束缚弱、形成的共价键极易碎裂而无法构成骨架，最终仅有氢、碳、氮、氧、硅、磷、硫七种元素具备构建生命网络的潜力。 水溶剂矛盾与替代介质局限 (6:41) 地质数据显示，硅在地球地壳质量中占比超过 28%，而碳仅占 0.02%，但地球生物圈却几乎完全绕过了硅。这一现象的核心阻碍在于硅化合物与水的化学不相容性：复杂的硅骨架分子在水性环境中极易水解碎裂，而液态水恰恰是宇宙中最丰富且热力学稳定的通用溶剂。若将目光转向非水溶剂环境，土卫六（Titan）上的低温液态甲烷和乙烷湖泊虽能维持硅聚合物的稳定，但极低温度严重削弱了溶解大分子的溶剂能力；金星高空浓硫酸云层虽然允许丰富的硅化学反应，但其强烈的强腐蚀性可能在原初生物演化出自我复制机制前将其解体。 氧气陷阱与固态代谢废弃物 (9:30) 硅的另一大致命缺陷是其对氧具有极高的热力学亲和力。硅与氧结合的倾向远远超过与其他硅原子或生命元素的结合，导致含硅分子极易发生不可逆反应，将硅元素永久固锁在无法被生物合成利用的硅酸盐岩石中；相比之下，碳形成的二氧化碳气体易溶于水且能参与光合作用循环。在代谢层面，尽管硅与氧反应释放的能量高于碳的有氧呼吸，但其生成的副产物是固态二氧化硅（沙子或石英）。以普通人类每日呼出约一公斤二氧化碳为参照，同等规模的硅基异星生物必须应对排出大量固体砂石或玻璃珠的机械负担，这种代谢损耗在长期自然演化竞争中极为不利。 碳沙文主义与硅的结构性应用 (12:11) 科幻创作常将硅基生物刻画为能够活动的岩石或晶体，但正如碳基生命并不表现为石墨或金刚石一样，这种具象化并不准确。地球上确实存在大规模利用硅元素的生物，例如单细胞光合生物硅藻（diatoms）。硅藻利用水合二氧化硅合成立体的保护性外壳，同时依靠内部纯碳基机制进行代谢，贡献了地球上近一半的氧气供应，证实了生物系统具备调动固态硅构建结构的能力。然而，正如 Carl Sagan 所承认的“碳沙文主义”，碳在水溶剂中的高效率与适应性确立了其宇宙级的垄断地位，人类未来寻找外星生命大概率仍将以碳基为主。 宇宙本体论与地质演化探讨 (15:44) 在观众互动环节中，节目进一步探讨了基础物理模型与行星演化假说。针对 Max Tegmark 的“数学宇宙假说”，O'Dowd 倾向于宇宙深层源于简单的基本对称性或最小作用量原理，而非繁复的高阶场方程；同时对比了 Roger Penrose 的“三个世界”本体论，即物理、意识与柏拉图数学世界之间的相互循环涌现，而非单向主导。在地质演化方面，针对早期原行星 Theia 与地球碰撞的模拟，地幔底部发现的大型低剪切波速区（LLSVPs）可能正是 Theia 沉入地核边界的致密地幔残片；而稳定行星自转轴与驱动发电机效应的巨大天然卫星，则可能构成大过滤器（Great Filter）假说中的关键环节之一。 总体而言，尽管硅基生命在严寒碳氢化合物湖泊或强酸环境中存在理论上的微小窗口，但由于水解脆弱性、对氧不可逆的锁定以及排出固态排泄物的生物力学劣势，碳水化合物体系仍是宇宙间最无可匹敌的生命化学基石。

Detailed notes — zh

## Background 该视频由科普频道 PBS Space Time 于 2023 年 1 月 26 日发布在 YouTube 平台上。节目由物理学家 Matt O'Dowd 主持，旨在从化学键合与天体生物学的角度，探讨外星生命是否可能打破地球的碳基范式、转而以丰度更高的硅元素为基础构建。 ## TL;DR 节目系统对比了碳与硅的化学特性，解释了生命为何需要兼具稳定性与反应性的骨架分子。尽管硅在元素周期表中与碳同族且地壳储量更丰富，但纯硅骨架在水中极易水解，且极易陷入与氧形成极强共价键的“氧陷阱”。此外，硅基呼吸代谢若生成固态二氧化硅（沙子），将带来难以克服的排泄与能量损耗难题。主持人总结指出，虽然硅基生命在某些非水极端环境（如液态烃或强酸云层）中理论上存在极小可能，但碳基化学在宇宙中仍具有压倒性的进化优势。 ## Atmosphere & style 全片洋溢着严谨、生动且充满极客幽默的科学探索氛围。Matt O'Dowd 运用通俗的比喻和精准的化学图解，将枯燥的周期表筛选与键能原理讲得引人入胜；穿插其中的《异形》与《星球大战》科幻梗，以及片尾坦率接受观众纠错的自嘲互动，让整期硬核科普在保持高学术密度的同时，流淌着轻松诙谐的对话感。 ## Bulletin - [06:41](https://www.youtube.com/watch?v=469chceiiUQ&t=401s) 硅在地壳中的质量丰度超过28%，是碳（约0.02%）的上千倍，但地球生命由于化学特性的巨大差异完全选择了碳基路线。 - [09:30](https://www.youtube.com/watch?v=469chceiiUQ&t=570s) 纯硅分子在液态水中极易发生水解断裂，且硅极度亲氧并会自发锁死在二氧化硅（SiO2）的深层能量井中，无法维持活细胞所需的动态代谢平衡。 - [11:16](https://www.youtube.com/watch?v=469chceiiUQ&t=676s) 硅基生命若以氧气进行呼吸代谢，副产物将是固态二氧化硅（沙子），使生物面临每日需咳出或排泄大量固态晶体的严峻物理排泄障碍。 - [13:49](https://www.youtube.com/watch?v=469chceiiUQ&t=829s) 尽管卡尔·萨根等学者倾向于全宇宙普遍由碳主导的“碳沙文主义”，但在土卫六液态烃湖泊或金星浓硫酸云层等非水极端环境中，硅基生化仍存理论可能。 ## Key takeaways - [00:00](https://www.youtube.com/watch?v=469chceiiUQ&t=0s) 硅由于在元素周期表中位于碳的正下方且具备相似的4价成键能力，成为科幻与天体生物学中最主流的非碳生命候选元素。 - [01:10](https://www.youtube.com/watch?v=469chceiiUQ&t=70s) 生命本质上是一场受控的持续化学反应，分子骨架必须兼具足够的物理结构稳定性与易于打散重组的动态代谢反应性。 - [03:29](https://www.youtube.com/watch?v=469chceiiUQ&t=209s) 经由排除金属键、离子键、惰性气体、剧烈卤素以及第四周期以上高半径元素后，周期表中仅剩氢、碳、氮、氧、硅、磷、硫7种可行共价骨架元素。 - [06:41](https://www.youtube.com/watch?v=469chceiiUQ&t=401s) 液态水是全宇宙最理想的生命溶剂，但纯硅骨架分子在水中极易水解碎裂，成为硅基生命在常态宜居行星上立足的致命短板。 - [08:00](https://www.youtube.com/watch?v=469chceiiUQ&t=480s) 非水溶剂如液态烃（如土卫六）中分子溶解度极低且代谢极慢，而浓硫酸（如金星）腐蚀性极强，对构建复杂硅基生命系统带来全新挑战。 - [09:30](https://www.youtube.com/watch?v=469chceiiUQ&t=570s) 硅原子对氧原子具有极其强烈的化学亲和力，一旦结合形成坚固的硅-氧键（Si-O）便陷入单向反应死锁，难以如碳循环般灵活重构。 - [11:16](https://www.youtube.com/watch?v=469chceiiUQ&t=676s) 碳代谢副产物为极易呼出的气态二氧化碳，而硅氧化副产物为固态二氧化硅晶体，会导致生物面临排出大量沙子或玻璃珠的巨大能耗困境。 - [12:36](https://www.youtube.com/watch?v=469chceiiUQ&t=756s) 硅基外星生命在理论上不一定表现为矿石或晶体形态；地球上的微观硅藻便成功利用二氧化硅构建了坚固的细胞壁结构。 - [13:49](https://www.youtube.com/watch?v=469chceiiUQ&t=829s) 卡尔·萨根戏称自己为“碳沙文主义者”，因为碳在液态水环境中的全方位生化优势，使我们在宇宙中更有可能遭遇碳基外星生命。 - [15:44](https://www.youtube.com/watch?v=469chceiiUQ&t=944s) 罗杰·彭罗斯爵士的“三界”本体论指出柏拉图数学世界、物质物理世界与主观心智世界三者构成相互涌现的闭环，而非泰格马克那般赋予数学绝对首要性。 - [18:21](https://www.youtube.com/watch?v=469chceiiUQ&t=1101s) 地球地幔与地核交界处的“大型低剪切波速区”（LLSVPs）具有原始同位素特征，被部分地质学家推测为原始撞击行星忒伊亚的残留碎块。 - [19:05](https://www.youtube.com/watch?v=469chceiiUQ&t=1145s) 解释费米悖论的“大过滤器”未必是单一的硬性事件，更可能由大月球的诞生、外层气态巨行星护盾及母恒星稳定性等一连串严苛条件串联而成。 ## Detailed walkthrough ### [00:00](https://www.youtube.com/watch?v=469chceiiUQ&t=0s) 碳基之外的猜想与硅基生命的科幻魅力 宇宙中已知的所有生命形式，其本质底色都是碳。不论是微观层面的生物分子结构，还是复杂有机体的运作，生命系统无一例外地围绕着碳原子构建。然而，如果我们希望探寻宇宙中生命的真正多样性与潜在丰度，就必须提出一个根本性的问题：“这是唯一的选择吗？”在科幻作品中，最受欢迎的非碳替代方案正是硅基生命——例如《异形》（Alien）系列中令人胆寒的异形生物，抑或是《星球大战》（Star Wars）中生活在小行星深处的巨型太空蠕虫（Exogorth）。 科幻作家对硅情有独钟并非凭空捏造。正如主持人 Matt O'Dowd 所说：“它确实是最有希望的非碳候选者，原因就在于两者的化学相似性。”（“Because it really is the most promising non-carbon prospect due to its chemical similarity.”）在元素周期表中，硅恰好位于碳的正下方；在丰度上，硅在地球地壳中的含量甚至远远超过碳。既然如此，生命能否真正基于硅建立？如果能，为什么地球上的我们没有演化成硅基生物？ - **地外生命多样性假设**：探索生命形式是否必然局限于碳，是天体生物学研究的核心前提。 - **流行文化表征**：《异形》系列中带有酸性体液的硅质外骨骼生物、《星球大战》中的巨型太空蠕虫等。 - **核心悖论**：硅在自然界不仅具备与碳相近的化合价和成键方式，在大部分行星表层的分布也更为广泛，但地球生物圈却几乎完全由碳主导。 --- ### [01:10](https://www.youtube.com/watch?v=469chceiiUQ&t=70s) 生命为何由碳构成：反应性与稳定性的精妙平衡 当我们说地球生命是“碳基”时，并不只是指生命频繁使用碳，而是指构成生命的全部分子机器底层完全依赖碳化学。碳链与碳环构成了巨大分子多样性的骨架，从 DNA、RNA、氨基酸、蛋白质，到由它们堆叠组装出的复杂组织。生物化学在本质上就是碳化学——生化学家在绘制分子式时甚至省略碳原子，仅用折线转角代表一个碳原子的存在。这种对碳的极端偏爱，究竟是生命诞生之初的随机偶然，还是自然规律下的绝对必然？ 答案在于碳成键时所展现的“金发姑娘”特征：兼具恰到好处的反应活性与结构稳定性。自然界的所有系统都在自发寻找能量最低状态。以炸药（如硝化甘油）为例，它处于高能不稳定状态，只要微小的活化能打破脆弱化学键，就会释放出巨大的能量并生成小分子产物；这一过程是不可逆的单向反应，因为小分子不可能自发重组成硝化甘油。生命本质上是一场持续进行的受控化学反应。如果反应是不可逆的单向进行，所有原料最终都会跌落进过度稳定的死寂分子中，化学反应终结便意味着死亡。生命既需要参与工作的分子具备足够的寿命和稳定性，又需要其具有打散重组的灵活性。 ```mermaid flowchart TD accTitle: 生命对化学骨架的稳定性与反应性要求 accDescr: 展示为什么生命需要处于过分稳定和过分活泼之间的中间平衡态。 subgraph Extremes ["极端的化学状态"] A["过高反应性 / 不稳定（如硝化甘油）<br/>微小扰动即剧烈释放能量，单向不可逆"] B["过高稳定性 / 惰性分子<br/>化学反应陷于低谷，无法发生代谢转化（死亡）"] end subgraph Balance ["生命的平衡状态"] C["碳骨架的理想平衡<br/>高稳定性确保分子长久存在<br/>适度反应性维持动态代谢网络"] end A -.->|"无法维持精细结构"| C B -.->|"无法驱动化学循环"| C ``` - **生物化学的符号惯例**：生化学家绘制分子结构时通常隐去“C”，直接用折线顶点指代碳原子。 - **热力学驱动**：自然自发倾向于最低能量态；过于脆弱的化学键会导致链式失控瓦解，过于稳固的化学键会导致分子被彻底锁死。 - **生命的本质**：“生命基本上就是一种正在持续进行的化学反应。”（“Life is basically an ongoing chemical reaction.”） --- ### [03:29](https://www.youtube.com/watch?v=469chceiiUQ&t=209s) 周期表大筛选：为什么只有共价键与前三周期元素入选？ 从整个元素周期表出发，可以通过排除法筛选能够承载生命的骨架元素。化学键主要分为三大类：离子键、金属键和共价键。离子键结合力过于松散易变；金属键倾向于形成规则排列的金属晶体，缺乏构成生命所需的化学多样性。只有共价键是唯一可行的路径。在共价键中，原子通过与邻近原子共享电子来填满最外层的价电子层。例如碳有 4 个价电子，倾向于达到 8 电子的稳定结构；它与 4 个各自贡献 1 个电子的氢原子结合，形成了甲烷（$CH_4$）。通过移除或替换氢原子，分子可以轻松组装出乙烷、乙醇等形态万千的化合物。 基于此，生命骨架的候选池被大幅压缩： 1. **排除离子与金属倾向元素**：剔除元素周期表中绝大多数倾向于形成离子键或金属键的金属与碱金属。 2. **排除稀有气体与卤素**：稀有气体化学极度惰性，几乎不反应；卤素反应极其剧烈且难以同时与多个原子建立稳固的共价连接。 3. **排除第四周期及以下元素**：原子半径过大导致原子核对外层价电子束缚较弱，形成的共价键机械强度极低，化合物容易自发断裂。例如砷（As）与磷（P）同族同价，细胞若误将砷纳入分子中，脆弱的化学键会导致重要细胞器破碎或代谢中断，这也正是砷毒性的生化根源。 经过层层筛选，全周期表仅剩下 7 种核心候选元素：氢（H）、碳（C）、氮（N）、氧（O）、硅（Si）、磷（P）和硫（S）。除硅以外的 6 种构成了地球生命的六大基本元素（CHNOPS）。而在这些元素中，碳是绝对无可替代的基石，因为只有碳能够以多种几何形式自身相连，形成稳定的一维长链和二维网络。 | 排除类别 | 代表元素 | 排除原因 | |:--- |:--- |:--- | | **金属与碱金属** | 钠、镁、铁等 | 主导键型为离子键或金属键；金属键产物结构单调（晶体），离子键水中易离解。 | | **稀有气体** | 氦、氖、氩等 | 价电子层完全饱和，极度惰性，几乎不参与化学成键。 | | **卤素** | 氟、氯、溴等 | 单官能团倾向，难以同时连接多个骨架原子，反应过于剧烈甚至具爆炸性。 | | **第四周期及以下** | 砷、硒等重元素 | 原子半径过大，原子核对电子控制力弱，形成的共价键过于脆弱，易导致结构解体。 | | **最终候选元素** | H, C, N, O, Si, P, S | 能够形成多样且稳固的共价分子；除 Si 外均为地球生命的六大核心元素。 | --- ### [06:41](https://www.youtube.com/watch?v=469chceiiUQ&t=401s) 硅的潜质与致命短板：水环境中的脆弱性 既然筛选出的 7 种元素中包含硅，且硅具有与碳相同的 4 个价电子，能够模仿碳合成链状、环状化合物（例如物理性状与碳氢油脂极为相似的硅油），为什么生物进化完全抛弃了硅？地球地壳中硅的质量占比超过 28%（广泛存在于硅酸盐岩石中），而碳仅占约 0.02%，硅的储量是碳的上千倍。生命弃用硅而选择极其稀缺的碳，证明硅作为生命骨架存在难以克服的物理与化学缺陷。 首要缺陷是硅基化合物在水环境中的极度不稳定性。生命系统必须依赖溶剂——一种能够让各种分子机器自由移动、碰撞和反应的液体介质。水凭借其高比热容的热稳定性、卓越的溶解能力以及在全宇宙的极高丰度，成为了生命的理想溶剂。然而，绝大多数复杂的纯硅分子在水中极易发生水解反应而破裂。Matt O'Dowd 强调：“水分子对于大多数硅分子的不稳定性，实际上对硅基生命的可行性造成了致命一击。”（“The instability of most silicon molecules in water is actually a huge blow to the viability of silicon-based life.”） 如果脱离水环境，硅基分子在液态烃（如气态巨行星冰冷卫星上的甲烷湖）或浓硫酸（如金星大气层）中表现出更强的稳定性： - **液态烃环境**：环境温度极低，导致大型复杂的生化分子溶解度极差，分子运动和代谢节奏极度缓慢。 - **浓硫酸环境**：化学腐蚀性极其强烈，对原始生命的孕育和有机演化构成严峻挑战。针对这一点，Matt O'Dowd 调侃道：“从另一面来看，异形就是硅基生物而且流着强酸血液——它们基于‘浓硫酸环境中的硅’完全讲得通。如果金星生命真是长成那个样子，我们还是赶在太空恐怖片上演之前，把‘浮空云城’建设计划取消算了。” --- ### [09:30](https://www.youtube.com/watch?v=469chceiiUQ&t=570s) 无法挣脱的“氧陷阱”：硅的强键与死锁 在水中维持硅骨架的唯一替代方案是将硅与碳或氧交替结合（形成硅氧链或硅碳链），但这类结构相比纯碳骨架没有任何明显的生化优势。更严重的问题在于：硅原子对氧原子有着近乎极端的化学亲和力。硅与氧结合所释放的键能，远超过硅与自身成键或与其他“生命元素”成键的能量。 这导致了不对称的“单向陷阱”：含有硅的复杂分子在遇到氧气时会自发断裂，转而生成极其坚固且难以拆解的硅-氧键（Si-O）。对于生命运作而言，这种过度的稳定性打破了前面提到的反应平衡，把物质锁死在反应路径的终点。这也解释了地球上绝大部分硅都以坚硬岩石（二氧化硅及硅酸盐）的形式封存，无法被早期生命摄取；反观碳元素，其与氧形成的二氧化碳（$CO_2$）在常温常压下是气态，并且能够轻松通过光合作用等生化路径被捕获和解离。 尽管极高强度的硅-氧键对构建柔性骨架是灾难性的，但这种释放能量的剧烈反应在理论上可能为外星代谢提供能量。地球真核生物的线粒体通过氧化代谢，将高能的氧分子和碳水化合物转化为低能的 $CO_2$ 分子，并在此过程中把能量转移储存在 ATP 中。理论上，硅基外星生命也能通过硅与氧的结合来获取能量，但这同时也带来了全新的生物化学排泄与产物处理难题。 - **硅-氧键能过高**：反应不可逆，一旦形成二氧化硅即陷入深层能量井，无法像有机分子那样灵活循环。 - **地质固化 vs. 气态循环**：地球上的硅深埋在岩石矿物中难以动用，而碳以气态 $CO_2$ 的形式弥散于大气与水体，极易参与碳循环。 - **代谢产物隐患**：碳代谢生成的 $CO_2$ 可直接通过呼吸排出，而硅氧化生成的产物（如固态 $SiO_2$）则给生物代谢带来了巨大的物理排泄障碍。 ### [11:16](https://www.youtube.com/watch?v=469chceiiUQ&t=676s) 代谢排泄的物理困境：二氧化硅与沙子 如果硅基生命利用氧气进行代谢，将氧气转化为二氧化二氧化硅（$\text{SiO}_2$），在表面上看似乎非常理想，因为二氧化硅比二氧化碳（$\text{CO}_2$）更为稳定，反应能够释放出更多的能量。然而，核心症结在于物理状态：二氧化碳是一种气体，能够轻易通过呼吸排出体外；而二氧化硅则是二氧化硅晶体——也就是沙子。正如 Matt O'Dowd 借用《星球大战》的梗调侃道：“它又粗又硬，而且到处都是，我讨厌它。”这种物质绝非任何生物希望在细胞或肺部逐渐堆积的东西。 这种排泄负担在规模上极为惊人。一个普通人类每天通过呼吸呼出大约一千克的二氧化碳，这意味着等效的硅基外星人每天必须咳出相当数量的沙子，或者像排泄玻璃珠一样把它们拉出来。尽管生命可能进化出某种变通方案，但这绝对不如呼出气体那样便利和低能耗。对于生命而言，微小的能量节省都关乎生死存亡。在特定条件下，硅元素虽然理论上具备支撑外星生物化学所需的多样性，但这些环境极其严苛特殊，远不如维持碳基生命的环境理想。Matt O'Dowd 总结指出：“看来如果生命有使用碳的选择，它就应该这么做。” ```mermaid flowchart TD accTitle: 碳基与硅基代谢排泄对比 accDescr: 对比碳基生物呼出气态二氧化碳与硅基生物排泄固态二氧化硅的代谢差异 subgraph Carbon["碳基呼吸代谢"] C1["有机碳化合物 + O2"] --> C2["释放能量"] C2 --> C3["副产物：气态 CO2"] C3 --> C4["低能耗呼出体外（人类约 1 kg/天）"] end subgraph Silicon["硅基呼吸代谢"] S1["硅氢/有机硅化合物 + O2"] --> S2["释放更高能量（更稳定）"] S2 --> S3["副产物：固态 SiO2（二氧化硅/沙子）"] S3 --> S4["高能耗排出：咳出沙子或排泄玻璃珠"] end ``` --- ### [12:36](https://www.youtube.com/watch?v=469chceiiUQ&t=756s) 硅基生命的外形假想与地球上的“硅藻” 数十年来，科幻作家常常将硅基生命描绘为活生生的水晶或行走的岩石，这主要是因为硅在地球上最常见的形式就是岩石和矿物。但正如碳基生物不会长成行走的钻石或铅笔芯一样，没有任何生化法则规定硅基外星人必须呈现晶体形态。然而，大自然在地球上确实演化出了某种“活体硅晶体”——硅藻（Diatoms）。 硅藻是单细胞藻类，它们将硅元素融入自己的细胞壁中，构建出坚硬的二氧化硅晶体外壳，将细胞内的所有结构包裹其中。这些美丽的微观晶体在海洋中漂浮并连成长链，默默进行光合作用，制造了地球上多达一半的氧气。需要明确的是，硅藻并不是真正的硅基生命，它们的内部生化机制完全是碳基的，仅仅是细胞壁采用了硅。但它们无可辩驳地证明了一个概念：生命的某个关键构件（如细胞壁）完全可以用硅来打造。 - 硅藻是微观单细胞生物，其二氧化硅构成的硬质外壳展现了硅在生物结构中的可行性。 - 地球大气中约 50% 的氧气由这些微小的含硅生物光合作用产生。 - 科幻作品倾向于把硅基生命固化为矿石或晶体形态，但生物的化学本质不决定其宏观外观必须是单一矿物。 --- ### [13:49](https://www.youtube.com/watch?v=469chceiiUQ&t=829s) “碳沙文主义”与极端环境下的生命可能 纵观宇宙化学，硅基外星人在科幻作品中的出场频率，恐怕远高于它们在现实宇宙中的实际概率。碳在液态水环境中的化学优势实在太具压倒性，以至于生物化学可能几乎没有其他可行替代方案。就连著名天文学家卡尔·萨根（Carl Sagan）也曾为此戏称自己是个“碳沙文主义者”。当我们最终与外星智慧生命进行第一次接触时，对方在生化层面上很可能与我们大同小异。 不过，科学家们并未彻底排除硅基生命在某些非水环境中的存在可能。例如土卫六（泰坦，Titan）表面零下上百度的液态烃湖泊，或是金星高层大气中浓硫酸构建的酸性云层。在这些水无法稳定存在或极其恶劣的环境中，独特的硅基生物或许能够打破常规，极大地拓宽我们对宇宙时空中生命多样性边界的认知。正如 Matt O'Dowd 所说：“甚至卡尔·萨根也因为这个原因称自己为碳沙文主义者。” --- ### [14:33](https://www.youtube.com/watch?v=469chceiiUQ&t=873s) 赞助商信息：Brilliant.org 与量子物理课程 Matt O'Dowd 感谢了平台赞助商 Brilliant.org 对 PBS 节目的长期支持。Brilliant 是一个专注于 STEM 领域的在线互动学习平台，专为全年龄段、无论专业背景或经验水平的求知者设计，其核心是通过实践性、互动性的课程与解题练习培养用户的思维方式。 他在本期推荐了该平台上的《量子对象》（Quantum Objects）课程。该课程深入探讨了微观世界的物理规律，带领学员探究原子与基本粒子那些看似违背经典力学与直觉概率的奇异行为。学员可以通过实验模拟构建新的运动方程、物理定律，以及基于代数而非纯数字的新型测量体系，不仅能深刻理解激光、晶体管等现代技术的底层物理，还能为进一步探索正在发生的量子信息与量子计算革命奠定基础。 --- ### [15:44](https://www.youtube.com/watch?v=469chceiiUQ&t=944s) 观众答疑：数学宇宙假说与罗杰·彭罗斯的“三界”本体论 在上一期关于马克斯·泰格马克（Max Tegmark）“数学宇宙假说”的讨论中，网友 onebronx 提出：宇宙的基本数学可能并非标准模型拉格朗日量或广义相对论方程，而是平稳作用量原理与一组自洽的对称性。Matt O'Dowd 对此表示赞同，他指出如果数学宇宙假说成立，宇宙更可能是由极简规则而非一组复杂的方程式所定义；物理世界从数学世界中涌现，并不妨碍复杂的数学结构本身是从更简单、更基础的数学结构中涌现出来的。 随后，观众 S. Lee Odegard 指出本期节目应提及罗杰·彭罗斯爵士（Sir Roger Penrose）的“三界”本体论（Three Worlds Ontology）。Matt O'Dowd 承认这一概念原计划纳入节目，后因篇幅受限而删减。在彭罗斯的模型中，现实由三个相互循环涌现的领域构成： 1. **物理世界（Material World）**涌现自**柏拉图数学世界（Platonic World）**——若无数学规律性，物理世界便无法稳定存在； 2. **心智世界（Mental World）**涌现自**物理世界**——物质构成的大脑产生了主观意识与思维； 3. **柏拉图数学世界**又反过来涌现自**心智世界**——心智能够构想出超越物理现实的完整数学语言体系。 彭罗斯并没有将绝对的本体论优先权赋予数学世界，这与泰格马克的观点大相径庭。Matt O'Dowd 评价道：“彭罗斯并没有赋予柏拉图世界首要地位——这也是我最终决定略过它的原因，也许会留给以后的节目。”他推荐观众阅读彭罗斯的著作《通往现实之路》（*The Road to Reality*），但也打趣地提醒道，读完这本书的强度堪比修完一整学期的高强度物理专业课。 ```mermaid flowchart TD accTitle: 罗杰·彭罗斯的三界本体论循环 accDescr: 展示柏拉图数学世界、物质物理世界与主观心智世界之间的相互涌现循环关系 Platonic["柏拉图数学世界<br/>（数学规律与抽象概念）"] Material["物质物理世界<br/>（基本粒子、能量与大脑）"] Mental["主观心智世界<br/>（意识、思维与感知）"] Platonic -->|"规律赋予物理现实存在性"| Material Material -->|"物质大脑产生意识活动"| Mental Mental -->|"意识构想并拓展超越现实的数学"| Platonic ``` --- ### [18:21](https://www.youtube.com/watch?v=469chceiiUQ&t=1101s) 观众答疑：地幔中的忒伊亚遗迹与大过滤器假说 针对月球大碰撞起源的超高清模拟视频，网友 markcentral 提出了一个引人入胜的假说：撞击地球的远古原行星“忒伊亚”（Theia）的部分完整残骸，至今是否仍埋藏于地球地幔深处？Matt O'Dowd 解释说，在地球地幔与地核交界处，确实存在着两团被称为“大型低剪切波速区”（LLSVPs）的高密度异常构造。部分地质学家推测，这些区域正是大碰撞中未完全熔合的忒伊亚残余物质。从这些区域上方地幔热柱玄武岩中检测到的同位素比例显示出“原始”特征，暗示它们并未在撞击中与地球物质彻底混匀。 另一位观众 Alice 提问：如果形成大月球属于极小概率事件，且一个异常强韧的铁核对保护生命演化至关重要，这是否构成了足以解释费米悖论的“硬过滤器”（Hard Filter）？Matt O'Dowd 回答说，目前学界虽有类似猜测，但尚无定论。他表达了自己的直觉判断：“我个人的直觉是，解释费米悖论所需的极其严苛的过滤器，实际上是一连串具有一定难度的过滤链条，而巨大的月球可能只是其中之一。”其他潜在的环节还包括太阳系的特殊行星架构（外围有巨大气态巨行星提供引力护盾、内圈仅有小型岩质行星）以及母恒星的极其特殊的稳定性等。 --- ### [20:07](https://www.youtube.com/watch?v=469chceiiUQ&t=1207s) 幽默勘误：鸟类与蝙蝠的解剖学差异，以及澳洲口音 在节目的最后，Matt O'Dowd 幽默地回应了此前评论区指出的两处疏漏。在关于超临界流体的剧集中，他曾开玩笑说“有动物长着足够大的手可以在气体中游泳，它们被称为鸟类”。多位观众（特别是网友 Xovvo）指出，蝙蝠才是手部飞行的更好例证。鸟类的手骨实际上极度退化并愈合成“腕掌骨”（carpometacarpus），仅保留萎缩的第二和第三指（第一指形成小翼羽），其升力主要依靠漫长羽毛构建；相比之下，蝙蝠的翼膜则是从手掌延伸到第二至第五指的指尖，是真正意义上的“手”。 对于这番硬核解剖学科普，Matt O'Dowd 自嘲道：“我从错误中学到了很多——不仅关乎飞行类动物手部的大小，也让我明白即使是随口说的段子，也应该经过严格的研究与事实核查。”随后，他还承认自己之前把海王星的卫星海卫一（Triton）口误说成了土星的卫星土卫六（Titan）。面对那些调侃这是否是“澳洲口音吞掉了 T 后面的 R”的评论，他顺水推舟地打趣说，以后凡是口误都可以算作地道的澳洲发音，并且坚称“澳大利亚的鸟类全都不一样，它们的手都硕大无比”。 ## Notable quotes > [11:16](https://www.youtube.com/watch?v=469chceiiUQ&t=676s) 如果生命有使用碳的选择，它就应该这么做。 — Matt O'Dowd > [18:21](https://www.youtube.com/watch?v=469chceiiUQ&t=1101s) 我个人的直觉是，解释费米悖论所需的极其严苛的过滤器，实际上是一连串具有一定难度的过滤链条，而巨大的月球可能只是其中之一。 — Matt O'Dowd ## Mentioned PBS Space Time, Matt O'Dowd, Brilliant.org, 异形 (Alien), 星球大战 (Star Wars), 巨型太空蠕虫 (Exogorth), 硅藻 (Diatoms), 卡尔·萨根 (Carl Sagan), 马克斯·泰格马克 (Max Tegmark), 罗杰·彭罗斯 (Sir Roger Penrose), 通往现实之路 (The Road to Reality), 忒伊亚 (Theia), 大型低剪切波速区 (LLSVPs) ## Video description Thank you to Brilliant for Supporting PBS. To learn more go to https://brilliant.org/SpaceTime/ PBS Member Stations rely on viewers like you. To support your local station, go to:http://to.pbs.org/DonateSPACE Sign Up on Patreon to get access to the Space Time Discord! https://www.patreon.com/pbsspacetime Life as we know it is carbon-based, but does it have to be this way? There’s another element on the periodic table that shares some of the key properties of carbon but is far more abundant on most planets. I’m talking about silicon. So is there silicon-based life out there? Check out the Space Time Merch Store https://www.pbsspacetime.com/shop Sign up for the mailing list to get episode notifications and hear special announcements! https://mailchi.mp/1a6eb8f2717d/spacetime Search the Entire Space Time Library Here: https://search.pbsspacetime.com/ Hosted by Matt O'Dowd Written by Fernando Franco Félix & Matt O'Dowd Post Production by Leonardo Scholzer, Yago Ballarini & Stephanie Faria Directed by Andrew Kornhaber Associate Producer: Bahar Gholipour Executive Producers: Eric Brown & Andrew Kornhaber Executive in Charge for PBS: Maribel Lopez Director of Programming for PBS: Gabrielle Ewing Assistant Director of Programming for PBS: John Campbell Spacetime is produced by Kornhaber Brown for PBS Digital Studios. This program is produced by Kornhaber Brown, which is solely responsible for its content. © 2023 PBS. All rights reserved. End Credits Music by J.R.S. Schattenberg: https://www.youtube.com/user/MultiDroideka Space Time was made possible in part by Our Patreon Supporters Big Bang Supporters Tim O’Rourke Steffen Bendel Bryce Fort Peter Barrett David Neumann Leo Koguan Sean Maddox Alexander Tamas Morgan Hough Juan Benet Vinnie Falco Fabrice Eap Mark Rosenthal Quasar Supporters Glenn Sugden Alex Kern Ethan Cohen Stephen Wilcox Christina Oegren Mark Heising Hank S Hypernova Supporters Ryan Moser David Giltinan Ivari Tölp Vyce Ailour Ken S Gregory Forfa Kirk Honour drollere Joe Moreira Bradley Voorhees Marc Armstrong Scott Gorlick Paul Stehr-Green Ben Delo Scott Gray Антон Кочков John R. Slavik Mathew David S. Cerutti Donal Botkin John Pollock Edmund Fokschaner chuck zegar Jordan Young John Hofmann Daniel Muzquiz Gamma Ray Burst Supporters Joe Pavlovic Jswcp Ryan McGaughy Mark West Hypothasies Justin Lloyd Chuck Lukaszewski Edward Hodapp Kevin Tillery Cole Marc Forand D. Delair Walter Montalvo Andrea Galvagni Jerry Thomas Nikhil Sharma Alexander Gruber Jonathan Cordovano John Anderson Scott Hannum Paul Widden Bradley Ulis Craig Falls Kane Holbrook Ross Story teng guo Mason Dillon Harsh Khandhadia Thomas Tarler bsgbryan Susan Albee Frank Walker Matt Q MHL SHS Terje Vold James Trimmier Anatoliy Nagornyy comboy Andre Stechert Paul Wood Kent Durham jim bartosh Nubble Ramon Nogueira The Mad Mechanic Ellis Hall John H. Austin, Jr. Diana S Faraz Khan Almog Cohen Alex Edwards Ádám Kettinger MD3 Endre Pech Daniel Jennings Cameron Sampson Geoffrey Clarion Russ Creech Jeremy Reed David Johnston Web Browser Michael Barton Mr T Andrew Mann Isaac Suttell Oliver Flanagan Bleys Goodson Robert Walter Mirik Gogri Mark Delagasse Mark Daniel Cohen Nickolas Andrew Freeman Shane Calimlim Tybie Fitzhugh Robert Ilardi Eric Kiebler Craig Stonaha Graydon Goss Frederic Simon Tonyface John Robinson Jim Hudson A G David Barnholdt David Neal justahat John Funai Tristan Bradley Jenkins Daniel Stříbrný Luaan Cody Brumfield Thomas Dougherty King Zeckendorff Dan Warren Patrick Sutton John Griffith DFaulk Sam Long What If Alien Life Were Silicon-Based? [Source](https://www.youtube.com/watch?v=469chceiiUQ)

Source quotes

[11:16] Matt O'Dowd: 如果生命有使用碳的选择，它就应该这么做。 [18:21] Matt O'Dowd: 我个人的直觉是，解释费米悖论所需的极其严苛的过滤器，实际上是一连串具有一定难度的过滤链条，而巨大的月球可能只是其中之一。

Source quotes

[6:41] Matt O'Dowd: The instability of most silicon molecules in water is actually a huge blow to the viability of silicon-based life. [13:49] Matt O'Dowd: In fact, carbon-in-water is such an astoundingly good option for biochemistry that there may be no other game in town. Even Carl Sagan called himself a carbon-chauvinist for this reason. [9:30] Matt O'Dowd: Silicon forms stronger bonds with oxygen than with any of the other ‘building blocks of life’, and certainly more so than with other silicon atoms. [18:21] Matt O'Dowd: My personal intuition is that the extremely hard filter needed to explain the Fermi paradox is actually a chain of somewhat hard filters, and the large moon may be one of these. [0:00] Matt O'Dowd: Biochemistry is by definition the chemistry of Carbon.

Original transcript

[0:00] Thank you to brilliant.org for supporting PBS. [0:02] Life as we know it is carbon-based,  but does it have to be that way? [0:07] There is another element on the  periodic table that shares some [0:10] of the key properties of carbon but  is far more abundant on most planets. [0:14] I’m talking about silicon. So could  there silicon-based life out there? [0:24] All of the chemical machinery of life -  all biochemistry - is based around the [0:29] carboan atom. At least as far as we know. If  we want to understand the possible diversity [0:34] and abundance of life in the universe, we  need to ask whether this is the only option [0:39] Perhaps the most popular alternative - at least in  science fiction - is silicon-based life. We have [0:45] the xenomorphs of the Alien series to the giant  space worm in Star Wars and many, many others. [0:52] So why are scifi writers so into silicon-based  life? Because it really is the most promising [0:58] non-carbon prospect due to its chemical  similarity. It’s also more abundant on Earth [1:04] than carbon. So, can life be based on silicon?  And if so, why aren’t we silicon-based? [1:10] When we say life is carbon-based, we don’t  just mean that life uses carbon a lot - we [1:15] mean that literally every aspect of the  molecular machinery that makes up life is [1:20] based around the chemistry of carbon. Carbon  chains and rings form the scaffolding for [1:26] an enormous variety of molecules which  enable the raw machinery of life - from [1:30] DNA & RNA to amino acids & proteins  to all the structures built by these. [1:34] Biochemistry is by definition  the chemistry of Carbon. In fact, [1:38] biochemists don't even write Carbon in their  formulas, they just draw bent lines and they [1:43] know that's supposed to be a Carbon atom. So why  is carbon so favored by life? Is it just that the [1:50] first life form happened to be carbon based,  so everything else got stuck with that atom? [1:54] Or is carbon-based chemistry really the  only way life can happen anywhere, ever? [1:59] Ultimately, carbon’s incredible utility comes  down to the way it bonds with other elements, [2:05] which enable just the right  balance of reactivity and [2:08] stability. To understand this,  we need to think about energy. [2:12] Nature will always try to get to the lowest energy  state that it possibly can. A stick of dynamite [2:18] for example, is in a high energy state due to  all the energy stored in the chemical bonds of [2:24] its nitroglycerine. If the atoms find a way  to a lower energy state they’ll go for it, [2:30] explosively releasing that energy in the  process of converting the nitro to smaller [2:35] molecules. Nitroglycerine is unstable  because it only takes a little energy [2:40] to break its tenuous bonds, which in turn  releases a lot more energy than went in, [2:45] initiating a chain reaction. That reaction  only goes in one direction, because it’s [2:50] not energetically favourable to reassemble  all the smaller molecules back into nitro. [2:56] Life is basically an ongoing chemical  reaction. You don’t want the chemistry [3:02] of life to be unidirectional. You don’t  want all the raw ingredients to end up in [3:08] the same overly-stable molecules, with no  way back. That means the reaction is over, [3:12] which for life means …well, death. At the  same time, you want the molecules doing [3:19] the work of life to be stable enough - they  need to last long enough to do their jobs. [3:24] In other words, you want a balance  of reactivity and stability. [3:29] Let’s figure out what elements might  be good for reaching this balance. [3:34] And to start we’re going to open up to the entire  periodic table and see what we can rule out. [3:39] The key is in what types of chemical bonds a given  element can make. There are three main bond types: [3:45] ionic, metallic, and covalent.  Ionic bonds are way too unstable, [3:50] while metallic bonds result in repeating  structures - crystals - which don’t have [3:54] the chemical variety needed for life.  Covalent bonds are really the only option. [4:00] In these, atoms share electrons with  one or more neighbors in order to fill [4:05] up their outer or valence shells. For example,  carbon has 4 valence electrons. It wants 8 for [4:12] maximum stability. It achieves that stability  by connecting to 4 hydrogen atoms and sharing [4:18] their single electron. This also gives each  of those hydrogen atoms one more electron [4:23] to fill up its own valence shells. The  result of this arrangement is methane. [4:29] The cool thing about covalent bonds is that there  are countless ways to do this electron sharing, [4:33] enabling enormous diversity  and modularity. For example, [4:37] if we remove a hydrogen from two methane  molecules and stick them together you have [4:43] ethane. Now replace one of the hydrogens  with an oxygen and we have ethanol. [4:49] Covalent bonds are the key to the complex and  resilient molecular structures needed for life. [4:55] With this in mind we can look at the Periodic  Table and rule out all the elements that form [4:59] mostly ionic or metallic bonds, which is actually  most elements. We’re keeping things that are good [5:05] at forming covalent bonds, even if they  also sometimes bond in those other ways. [5:10] We can also rule out Noble Gasses since they are  barely reactive at all, and the Halogens, because [5:15] they’re just too much trouble. It's hard to get  them to form covalent bonds with more than one [5:20] element at the same time, and when they react with  something it's usually in an explosive manner. [5:25] Finally, we’re going to scrap elements in rows 4  and below. Although these may form covalent bonds, [5:31] these atoms are so large that the  nucleus doesn't hold the electrons [5:35] with enough strength, leading  to dangerously weak molecules [5:39] By the way, this is the reason Arsenic is toxic.  Cells can accidentally build molecules using [5:45] Arsenic in place of Phosphorus due to them having  the same number of valence electrons. When that [5:50] happens the resulting molecules will break,  destroying important structures in the cell, [5:54] or interrupting important processes, and  if this happens too much the cell will die. [5:59] In the end we are left with only seven elements:  Hydrogen, Carbon, Nitrogen, Oxygen, Silicon, [6:05] Phosphorus, and Sulfur. Of these, all but silicon  are considered fundamental building blocks of life [6:11] on Earth. Other elements are used by life for  very specific functions, but hydrogen, carbon, [6:17] nitrogen, oxygen, phosphorus, and sulfur are  the most important. And of these, carbon is [6:22] by far the more critical because it provides the  scaffolding of all biochemistry. The reason it’s [6:28] good for this is that it forms strong bonds with  itself, and does so in a variety of ways. In fact, [6:35] of the 6 building blocks of life, only carbon  can form both 1-D and 2-D structures with itself. [6:41] In our process of identifying elements that  may be useful for life we got to the 6 main [6:46] building blocks plus silicon. But why  did silicon get dropped by evolution? [6:51] This seems weird because silicon  is chemically similar to carbon, [6:56] also having 4 valence electrons. Silicon  can imitate Carbon and form long chains, [7:01] and rings, and the resulting molecules often have  similar properties to molecules made with Carbon. [7:07] To give you just one example we can make  Silicon oils, and they feel and look just [7:13] like other kinds of oil. It’s the only  other element capable of forming complex [7:18] covalent scaffolds and supporting a large  diversity of molecules. Despite all of this, [7:24] Earth life makes less use of silicon than it  does of many other trace elements like iron. [7:29] This seems extra strange when you remember  that silicon accounts for more than 28% of [7:34] the mass of Earth’s crust, in all them silicate  rocks. On the other hand, Carbon makes up only [7:38] around 0.02% of the crust. This tells us that  carbon must have substantial advantages over [7:45] silicon. So let’s talk about some of the  issues with silicon as a basis for life. [7:51] One huge one is that silicon-based molecules  tend to be far more reactive to water than [7:56] carbon-based. In fact, although  silicon chemistry is extremely [8:00] diverse, most complex silicon  molecules are unstable in water. [8:05] You can see how this would be a problem for  silicon-based life on our very watery Earth. [8:10] In general, life needs a solvent - a  liquid medium that allows molecular [8:15] machinery to move around and interact. Water  is incredible for this. Its thermal stability, [8:21] power as a solvent, and abundance  through the universe make it the best, [8:25] and perhaps even the only decent choice of the  base solvent for life. The instability of most [8:32] silicon molecules in water is actually a huge  blow to the viability of silicon-based life. [8:37] However water is not the only option. A much  greater variety of silicon-based molecules [8:44] are stable in liquid hydrocarbons - which  are present on the moons of the gas giants, [8:49] and even in sulphuric acid, which  is abundant in Venus’s atmosphere. [8:54] Both cases have their issues - liquid hydrocarbons  are cold, which among other things means they have [9:00] trouble holding in solution the large, complex  molecules that known life relies on. Sulphuric [9:07] acid is chemically extremely aggressive,  perhaps too much so for life to get started. [9:13] On the other hand, xenomorphs are silicon-based  and they do have acidic blood - makes sense that [9:19] they’re silicon -in-sulphuric-acid-based.  If that’s what Venusian life is like, [9:24] let’s cancel the cloud city construction  program before the space horror begins. [9:30] There is one way to potentially get silicon-based  life in water - and that’s by coupling silicon [9:35] with either carbon or oxygen for our scaffolding  molecule. Chains of these pairs of atoms can [9:40] produce some chemical diversity. However neither  case seems to provide significant advantage over [9:46] just using carbon chains. Another reason that  silicon seems implausible as a scaffolding [9:52] element for life is that the stuff really,  really loves forming bonds with oxygen. [9:57] Silicon forms stronger bonds with oxygen than  with any of the other “building blocks of life”, [10:02] and certainly more so than  with other silicon atoms. [10:05] Silicon-based molecules are  easily broken by oxygen, [10:09] and silicon-oxygen bonds are hard to  break in turn. That means reactions [10:14] tend to go in one direction when there’s  oxygen available. And the strength of the [10:19] silicon-oxygen bond is perhaps the main reason  that silicon is rarely used by life on Earth. [10:24] Although there’s an awful lot of  it, Earth’s silicon is locked up in [10:28] rocks and is inaccessible to life. On the  other hand, most of Earth’s carbon is in [10:34] CO2 in the atmosphere, which is relatively  easily accessible through photosynthesis. [10:39] There is one sense in which the strength of the  silicon-oxygen bond could be useful for life, [10:44] and that’s in energy generation. Let’s  think about where our energy comes from. [10:50] It’s from the conversion of oxygen molecules  - which contain a ton of energy in their [10:54] bonds - into CO2 molecules - which contain very  little. The simplest way to get that energy is [11:00] to set something on fire, which works great  for steam engines but not so great for cells. [11:05] The mitochondria in our eukariot cells  liberate that energy with a few more steps, [11:11] finally storing it in the ATP molecule. So  could silicon-based aliens get energy by [11:16] converting O2 into silicon-dioxide?  On the surface this looks good, [11:21] because SIO2 is even more stable than  CO2, so there’s more energy to liberate. [11:28] The problem is that while CO2 is  a gas that can be expelled easily, [11:33] silicon-dioxide is silica … it’s sand. It’s  coarse and rough and gets everywhere, I hate it. [11:41] It’s not something you want building up in your  cells or your lungs. And this wouldn't be a small [11:48] amount of sand. A regular person exhales about  a kilogram of Carbon Dioxide throughout the day, [11:53] meaning Silicon aliens would need to cough out  a good amount of sand. Or they poop glass beads [12:00] or something - probably there’s a way around the  issue. But it’s certainly not as convenient and [12:04] energy-cheap as exhaling CO2. And with life,  any little saving in energy is a big deal. [12:11] Let’s summarize. In certain conditions silicon  potentially has the chemical diversity for an [12:17] alien biochemistry, but those conditions are  pretty specialized, and are far less ideal [12:22] that the ones that support carbon-based life. It  seems that if life has the option of using carbon [12:29] then it should do so. But it’s not impossible that  in highly unusual environments silicon-based life [12:36] But what would that life even look like? [12:40] For decades science fiction authors have  imagined silicon based life as variations [12:44] of living crystals or rocks, that's because that’s  the common form of silicon on Earth is just that. [12:51] But we carbon-based creatures don’t look  like walking diamonds or pencil leads, [12:55] so there’s no reason to think of silicon-based  aliens would look particularly crystally. [13:01] Well, actually. There is a reason. And that’s  because living silicon crystals already exist [13:07] on Earth. Let me present you to Diatoms. These  are single celled organism that have incorporated [13:14] Silicon into their cell walls, making them  rigid, and basically just Silica crystals [13:19] that encapsulate all the parts of the cell inside  them. These beautiful living crystals live simple [13:25] lives. They just float around in the ocean  in long chains photosynthesizing, producing [13:31] half of the oxygen that we breath. Now to be  clear, Diatoms are not Silicon based life, [13:37] it's just their cell walls. Everything inside  them is based on Carbon. But they are at least [13:43] a proof of concept that one important component of  life - the cell wall - can be built from silicon. [13:49] OK, so what have we learned? Well, silicon-based  aliens are probably slightly over-represented in [13:56] fiction compared to their likely frequency  out there in the universe. In fact, [14:01] carbon-in-water is such an astoundingly good  option for biochemistry that there may be no [14:07] other game in town. Even Carl Sagan called  himself a carbon-chauvinist for this reason. [14:13] We’ll probably be more similar than different to  the alien species of our first-contact. But let’s [14:19] not ignore the hydrocarbon lakes of Titan or the  acid cloud-scapes of Venus, where silicon-based [14:26] critters may expand our notion of what is  possible in the variety of life across space time. [14:33] Thank you to Brilliant.org for supporting PBS. [14:37] Brilliant is an online learning platform for STEM  with hands-on, interactive lessons. Brilliant is [14:42] for curious learners, both young and old,  professional and inexperienced. Brilliant [14:46] courses teach you how to think via interactive  lessons and problem-solving activities/exercises. [14:53] For example, Brilliant.org offers  a course called Quantum Objects. [14:57] The principles of quantum mechanics came  from investigating microscopic phenomena, [15:02] the bizarre behavior of quantum objects like atoms  and elementary particles that often appear to [15:07] contradict classical mechanics and probability.  In this course, you'll explore experiments of [15:13] quantum objects and use them to construct new  equations of motion, new laws of physics, and [15:19] a new system of measurement based not on numbers  but on algebras. By the end, you’ll gain a new [15:26] appreciation for how the physics of the small  enables lasers, transistors, and other modern [15:33] technologies that define our world. Then you'll  be ready to dive into the ongoing revolution of [15:38] quantum information and computing. To learn more  about Brilliant, go to brilliant.org/spacetime [15:44] Today we’re doing comment responses for the  last two episodes - the one about the new [15:49] simulations that show how our moon formed from  a giant impact with higher resolution than ever [15:55] before. And the one on Max Tegmark’s mathematical  universe hypothesis which proposes that the most [16:02] fundamental reality is a platonic realm of  mathematics. We’ll start with the latter. [16:07] onebronx opines that the basic math of our  Universe is likely not the SM lagrangian [16:15] and the GR equation, but the principle of  a stationary action and a specific set of [16:23] mutually-consistent symmetries. I absolutely  agree on the core point - which is that IF [16:31] the mathematical universe hypothesis is true,  a given universe probably shouldn’t defined [16:36] by the platonic existence of a complicated  set of equations, but rather a very simple [16:41] set of rules. In the mathematical universe  hypothesis, the physical world is emergent [16:46] from a more fundamental mathematical world.  But that doesn’t mean that complex mathematical [16:51] structures can’t also be emergent from simpler,  more fundamental mathematical structures. [16:57] S. Lee Odegard mentions that we should have  included Sir Rodger Penrose's three worlds [17:03] ontology, in which reality is described as three  realms of existence, each merging from the other. [17:09] I agree, and this was totally going to be in the  episode but we ran out of space for it. To give an [17:15] overview - Penrose proposes this way of thinking  about the circular emergence of reality. We have [17:22] the material world as emerging from a platonic  mathematical world - in the sense that without [17:27] the possibility of mathematical regularities,  the material world couldn’t exist; and then [17:34] the mental world emerges from the material - in  the sense that material brains generate mind; [17:41] and finally the platonic world emerges from  the mental - in the sense that minds are able [17:47] to conceive of the full language of mathematics  even beyond its manifestation in the physical. [17:53] This is pretty different from Tegmark’s idea  that the platonic math world is the ontological [17:59] precursor to everything else. Penrose doesn’t  grant primacy to the platonic - which is why [18:05] I ultimately decided to leave that  out - perhaps for a future episode. [18:09] If anyone wants to read about this, get Penrose’s  book Road to Reality - but fair warning, [18:15] reading it is a full semester intensive physics  course. Which is, by the way, a recommendation. [18:21] OK, and now on to the far less abstract  idea that the moon formed when a planet [18:26] that we name Theia smashed  into the newly formed Earth [18:29] markcentral mentioned an idea I’d never  heard before. That lumps of relatively [18:35] intact Theia material may still  be present in the earth’s mantle. [18:40] There are these high density blobs of material  that sit at the boundary between Earth’s mantle [18:46] and core called the Large low-shear-velocity  provinces. Some scientists have suggested that [18:53] this is material from the planet that collided  with Earth in the Moon’s formation. There is some [18:58] loose evidence that isotopic ratios in basalt  thought to be from lava plumes rising from the [19:06] regions above the LLSVPs have “primitive” isotopic  ratios consistent with material that wasn’t mixed [19:13] in the giant impact. Very cool possibility,  though obviously a lot more work is needed. [19:18] Alice asks: If the moon is a very unlikely event,  and an unusually strong iron core is necessary [19:26] to protect the development of life, does this  provide a sufficiently hard filter to resolve [19:31] the Fermi paradox? The answer is: we don’t know,  but people have speculated that this might be the [19:37] case. My personal intuition is that the extremely  hard filter needed to explain the Fermi paradox [19:43] is actually a chain of somewhat hard filters,  and the large moon may be one of these. Others [19:51] include having a planetary composition like ours  - large protective gas giants in the outer solar [19:57] system but only small terrestrial planets in  close, a star of a very particular quality, [20:03] and so on. Check our video on the rare  earth hypothesis for more on this. [20:07] I’d like to address a comment about my  comment response from the supercritical [20:13] fluids episode where I joked that there are  animals with hands large enough to swim in [20:18] gas - they’re called birds. Several people noted  that bats would be a better example here because, [20:26] as Xovvo notes, and I quote “Bat wings are formed  from membranous extensions from palm to fingertip [20:34] between digits two through five. Bird hands  are actually very small and have completely [20:40] fused together into a structure called the  carpometaparpus, tipped by greatly reduced [20:46] digits II and III (digit I bearing the alula,  anterior to the rest of the carpometacarpus). [20:53] Birds just have very long feathers to form  their flight surface. So … I’ve learned a [21:01] lot from my mistake - both regarding the size  of flighted animal hands, and that even casual [21:06] jokes should be subject to rigorous research and  fact-checking. My sincere apologies to everyone [21:11] Several people noticed that I flubbed when  naming Neptune’s moon Triton. I misspoke [21:17] and said Titan, which is of course Saturns moon. [21:20] Thanks to those in the comments who wondered if  it might just be my Australian accent - like, [21:25] we don’t pronounce r’s after t’s or something.  Yeah, that’s it - just remember any time you hear [21:31] me misspeak it’s actually just how Aussies  pronounce the actual correct word. Also, [21:35] all Australian birds have gigantic hands,  unlike everywhere else in the world.