It's a hot summer day in July, and as you look for shade, or duck into an air conditioned building, It's hard to believe the source of all that heat is 93 million miles away. What would it be like to approach the sun's surface, a blazing 10 thousand degrees, or swim in its center, at 15 million degrees? Try to wrap your mind around that number for a moment, 15 million degrees. Protons and electrons fly about in a frenetic subatomic dance that is the hallmark of heat. Yet they soon crash into one another and bounce off in different directions, as the immense weight of the sun packs them into a fluid 150 times as dense as water. Within this high energy soup, four protons will occasionally come together at high speed and form a helium nucleus. (This occurs in steps;two protons come together, then a third, then a fourth.) So hydrogen fuses into helium, and energy is released. Heat accumulates in the center, travels to the surface, radiates into space, and gives life to you and me.
How efficient is this process? How much energy is released? In other words, what is the power output per unit volume? If you have a glass full of hydrogen plasma in your living room, at high temperature and pressure, with a very tight lid keeping it all inside, could you use it to power a city? A building? Your house? The answer is none of the above. Energy production per cubic centimeter is very low, about the same as that of a resting salamander. A warm blooded animal such as yourself liberates much more energy than the sun. And yet the sun produces, and releases, huge amounts of heat. The secret is its size. Each cubic meter only generates 276 watts, but there are a lot of cubic meters in there! Heat builds up inside until the center is 15 million degrees, while the surface radiates heat out into space.
If it were not so, if the sun produced energy rapidly, like a thermonuclear bomb, it would never last for 5 billion years, and life would not evolve on earth. Once again the physical constants of the universe are fine-tuned to support life, by the hand of God or the anthropic principle, as you prefer. Either way, this amazing fact, the sun's rather paltry energy production, is not so amazing after all. It has to be so, or we would not be here to talk about it.
Although the laws of physics make life possible, they conspire against us as we try to harness the power of the sun here on earth. In the 70's I read optimistic articles in Scientific American about controlled fusion, and the dream of low cost power with virtually no environmental impact, especially when compared to fossil fuels and nuclear reactors. That was before global warming; the need for fusion power is even greater today! So how about it - compress some hydrogen under 250 billion atmospheres to 150 times the density of water, and heat it to 15 million degrees, and you're on your way. Unfortunately, recreating these conditions in the lab, even for a millisecond, is an enormous technological challenge, and after all that it's still not enough. Nobody can power a city with a fuel that releases energy like a salamander sitting on a rock, looking in your direction and blinking once or twice. Yes, we have a few tricks up our sleeve, like using other nuclei that fuse at lower temperatures and pressures, but the task remains daunting at best, far more difficult than sending humans to Mars. Still, if commercial fusion power could be achieved, the economic, environmental, and social benefits would be enormous. I believe this is an investment worthy of our research dollars, though it is difficult to perform a cost benefit analysis when the odds are long and the payoff is large - somewhat like a lotto ticket. Call me an optimist if you will - I do buy a lotto ticket now and then, and I believe fusion power is just a couple centuries away.