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Other problems stand in the way of large-scale conversion to solar energy. Engineers have yet to figure out effective ways to store heat from the sun for more than three days or to tap solar energy for power production without filling huge tracts of land with reflectors or photovoltaic cells. Even legal technicalities must be resolved before use of solar energy can become practical. A study by Arthur D. Little suggests that the courts might be required to decide whether everyone has an equal right to sunlight, a question that will certainly arise the first time someone tries to put up a building that casts a shadow on a neighbor's solar collector.
With all of these obstacles yet to be overcome, ERDA does not expect solar power to provide more than 1% of U.S. energy needs during the next quarter-century, or even as much as 25% by the year 2020. But solar radiation may yet become a major means of meeting the needs of the earth for energy. Regardless of how great they may be, the earth's supplies of coal, oil and natural gas are finite. Long after these resources have been exhausted, the sun's golden apples will still be ripe for harvesting.