Читать книгу Astrobiology - Charles S. Cockell - Страница 63

The neutron star dwelling cheela raise an important point about whether life can be made of different states of matter, or even exclusively made from one state of matter. We are made up of solids and liquids, and we exchange gases with the environment. Consider a liquid-only life form. How would information be encoded to allow such a life form to reproduce or repair itself? What about a gas cloud intelligence, such as that found in astronomer Fred Hoyle's science fiction novel The Black Cloud? Would gas molecules be too disordered to allow for information storage, movement, and processing, and how would such an entity evolve in the first place? Can clouds of gas ever reproduce and make similar copies of themselves while adapting to their environment, or are gas molecules generally either too energetic or too dispersed to make systems of reproducing, evolving matter? One could continue with a list of such ideas, and you might want to list every conceivable different type of matter life form you can think of and list what the limitations of such life forms might be. An alternative, and fun, exercise is to consider our own liquid–solid make-up from the point of view of an alien made of a different state of matter and think about some of the things that might make our physics seem unlikely to them. Maybe the unpredictable interactions at solid–fluid interfaces make our own construction unlikely? These sorts of deliberations bring us back to the basic question: Is life on Earth a reflection of universal principles governing the evolution of living things or are we simply narrow-minded in our outlook?

Neutron degenerate matter is associated with huge gravitational forces. When a neutron star has a close companion, it pulls material to it. This material flies down to the surface of the star and collides with the surface, releasing energy. This energy is emitted mostly as X-rays and is modulated with the neutron star spin.

The typical characteristics of a neutron star are that they have a mass greater than 1.4 solar masses, a radius of 10–80 km, and a density of ∼10¹¹ kg cm⁻³.

There is an upper limit to the mass of a neutron degenerate object, the Tolman–Oppenheimer–Volkoff limit, which is analogous to the Chandrasekhar limit for white dwarfs. The precise limit is unknown. Above this limit, a neutron star may collapse into a black hole (Figure 3.23).

Figure 3.23 A photograph of a black hole. The supermassive black hole is at the center of the galaxy M87. The orange halo is superheated material being swept up into black hole.

Source: Reproduced with permission of Event Horizon Telescope collaboration.

The composition of the interior of a black hole is unknown. Does the interior of black holes contain quark degenerate matter, where the neutrons have themselves disintegrated into their constituent subatomic quarks? Quark degenerate matter has also been suggested to occur in hypothetical “quark stars.”

At the center of a black hole is thought to be a singularity or singularity ring where density is infinite. This can be regarded as some of the most extreme matter in the Universe.

These extreme types of matter are not very relevant for life, although as we have seen, that has not stopped speculation about possible life forms inhabiting objects made of these types of matter. These ideas aside, one reason for investigating these unusual types of matter is that they remind us that the ordinary matter from which known life is made is a tiny subset of the matter in the known Universe. If ordinary matter constitutes say 5% of the matter in the Universe and 99% of that is plasma, then the gases, liquids, and solids that make up biological systems are made from about 0.05% of the types of matter in the Universe.