Saturday, November 12, 2016

Hydrogen or Antihydrogen?

Will alien civilizations in their asymptotic technology stage run on hydrogen or antihydrogen? The case for hydrogen is simple. It can be generated almost anywhere on their planet and transported, using some of the hydrogen itself for fuel, to any of the arcologies that aliens are expected to live in. It can be turned into electricity at the destination point. Even with our primitive knowledge of energy systems, it seems that making hydrogen can be done efficiently, meaning without much loss of energy. Splitting water into hydrogen and oxygen and only storing the hydrogen means that the atmosphere takes on the job of moving most of the weight needed for energy transformation. We assume their atmosphere has oxygen, as that is the product of photosynthesis, which in turn is the most efficient way for plants to grab energy.

At the destination end, hydrogen can efficiently be turned into electricity for those uses of electricity the alien civilization still has. We think of fuel cells, and hardly use any, but efficiency falls out at about 50%, without trying hard. Add in some centuries of R&D, and a much higher efficiency, or alternatively a more efficient replacement, should be on the table. With no pollution but only water as an output, it is a simple solution to keeping their atmosphere unaltered.

On Earth, distributing fuel was the common procedure prior to the invention of electricity distribution systems. For example, each home might get a coal delivery or a whale oil delivery or a kerosene delivery, and it would use them for heat or light. On alien planets with advanced technology, having hydrogen tanks delivered to wherever they were needed goes back to this method, and much fewer scarce resources are needed, e.g., copper. All kinds of engines can work on hydrogen, so whatever is convenient and efficient might be used for transport. Fuel cells are small in size, and could be used wherever needed; whatever the aliens invent to replace or improve them would be more so. Recall that technology is absolutely independent of who does the research. This means that all alien worlds would use the same energy system so if hydrogen works on one planet, all planets will figure it out and use the same thing. Even the details should work out fairly identically. For example, if an engine of type X is found to be the most efficient in power per weight on planet Y, planet Z will figure out exactly the same thing, and also use engines of type X.

If transportation proves to be only a small increment of cost for the hydrogen system, fusion plants or hydroelectric plants or solar farms or whatever they finally evolve into can be anywhere where transportation can travel. To be more explicit, if you need a power plant producing N terajoules equivalent of hydrogen for a city, if the plant is located near the city, and the transport from a distant location takes 5% of that amount, you need a power plant producing 1.05 N terajoules located at the more convenient site.

Some transport wouldn’t do well with hydrogen because of the low energy per weight ratio, considering tankage. The aliens would no doubt, if they had uses for it, like aircraft, come up with an efficient hydrogen to liquid hydrocarbon conversion system, using octane, dimethyl benzene, xylene or any of dozens of other options or even a combination, so they could easily fly aircraft or whatever they chose to use for long distance rapid transport. Depending on the conversion efficiency of the hydrogen to liquid hydrocarbon system, it might even be useful to have a secondary system for transporting octane around. Liquid hydrocarbons are very efficient in energy per weight, tankage included, and if produced in a process that withdrew carbon dioxide from the atmosphere for the conversion process, would be neutral in atmospheric contributions.

It may seem somewhat risky to assume we can figure out the energy system for an alien planet far away in the galaxy and far advanced over our technology, but we can understand that science for them is the same as science for us, and with an atmosphere arising from the evolution of energy-seeking cells, there are not many energy-efficient options that do not include a closed cycle. Perhaps they will find some way of simply packaging energy other than in chemical bonds in hydrocarbons.

One option is antihydrogen, the simplest neutral form of antimatter. We haven’t produced it and experimented with it so that we can't tell what the tankage requirements are. It may turn out that there is a high quantum barrier to annihilation, and so a simple hydrogen tank can be used for antihydrogen. All we know about is high energy reactions that are seen in accelerators. These may be incredibly misleading. If antimatter in a matter tank is stable, the energy to weight ratio goes up by many orders of magnitude. The waste products of annihilation are nothing. Annihilation produces pure energy and leave no residue behind.

There are a number of inventions the alien civilizations would have to come up to use antihydrogen. First, there would have to be a high efficiency way to make it. We have no clue at this point, but add a hundred years to our physics knowledge and it may be that there are simple ways. Second, there would also have to be a simply and compact way to cause it to undergo annihilation. If it is stable in tankage, this means it would be stable in contact with everything. Perhaps ionization would be enough to overcome the barriers. This would simplify energy production, but it would mean there would be risk from cosmic rays ionizing something in an antihydrogen tank, leading to an annihilation reaction. If this means a chain reaction might be initiated, we are talking about the equivalent of a nuclear bomb. On the other hand, if ionization does not work to produce annihilation, tanks are safer, but energy production is more difficult. At any rate, Earth physicists have a lot to work on for the next century or two.

If we ever build big telescopes, such as kilometer-sized dishes at a Lagrange point, and can see alien planets, we might be able to discern if they are using antimatter rockets in their solar system. This would be a clue that the antimatter production problem is solvable, but not that on-planet use was feasible. Given the distances, it will be quicker to figure it out ourselves rather than try to communicate over a few hundred light years and ask them about it.

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