2. ENERGY AND POWER
The Sun radiates enormous power, mostly in the optical and infrared bands of the electromagnetic spectrum. To get an idea of this power, consider that every square centimeter of the Sun's surface puts out the power of a 6500 Watt lamp. That means that the entire Sun puts out 4 x 1026 Watts. The Earth captures less than 10-9 (one billionth) of this power; the rest escapes to cosmic space (except for the tiny fractions captured by the other planets). The Sun's radiation is by far the main source of energy on Earth and controls the Earth's ecology. Without sunlight there would be no rain, no water, no plants, and no life (except, possibly, archaea, or cyanobacteria, a form of life found in hydrothermal vents, or sulpherous hot springs at the bottom of the sea.)
Nowadays, human beings have tapped another source of energy on Earth: fossil fuels (petroleum, natural gas, coal). (In fact, the energy stored in fossil fuel also was put there by sunlight, hundreds of millions of years ago.) But the power we take from fossil fuel is tiny compared to sunlight. For example, the entire US consumes about 100 "quads" of fossil fuel per year -- equivalent to about 800 billion gallons of gasoline per year. But this power consumption is no greater than the sunlight that falls on a patch of Arizona desert 100 km x 100 km. If we knew a way to build (for a reasonable price) a solar power plant that would capture such sunlight and turn it into electricity, we would save a lot of gasoline!
We are burning fossil fuel fast, and the petroleum may run out in a few decades (I heard however about different estimates when this is going to happen, everything from 20 to 100 years). As you will read below, the Sun's energy comes from
nuclear fusion, by which four atoms of hydrogen are converted into one atom of helium. Mankind has found a way to produce energy from fusion -- the hydrogen bomb! (Actually, the reactions in the H-bomb do not convert ordinary hydrogen into helium; they are fusion reactions between an element called lithium and rare isotopes of hydrogen called deuterium and tritium.) H-bombs produce a lot of energy, but nobody knows how to use this energy except to do damage. A great dream of scientists is to develop a technology for controlled thermonuclear fusion -- some kind of reactor that could produce electrical energy from fusion reactions that take place in a non-explosive way. The most promising fuel for such reactions is deuterium - a rare isotope of hydrogen. In ordinary water, only one hydrogen atom in 6000 is deuterium. But even so, the energy that is released by the fusion of deuterium atoms into helium is so great that the water on Earth would provide an inexhaustible source of energy -- if we could figure out how to make such a reactor. For example, I estimate that fusion of the deuterium in one gallon of ordinary water would produce about as much energy as burning 1000 gallons of gasoline! But, despite substantial investment in controlled thermonuclear fusion research (by the US and several other nations) for more than 30 years, scientists have not found a way to build a reactor that will produce useful energy from fusion at a reasonable price.