Commercial flight

"Thank God, men cannot as yet fly, and lay waste the sky as well as the earth!" ― Henry David Thoreau

Since the 1960s, air travel has surpassed other forms of ridership in intercontinental travel.  In the United States,  1.73 million people travel on domestic flights in the United States on any given day.  However, it is doubtful that flight will maintain this kind of dominance into the century, if commercial flights will remain viable at all.

In some ways, the writing is already on the wall.  The number of air carriers is down 13% from its peak in 2001 as airlines downsized in response to rising oil prices in the past  few years.  Most jets use kerosene as fuel—like other hydrocarbons, its high energy density makes it ideal for this application.

Can other power sources step in?  While there have been a number of achievements in solar powered flight, it is provide passenger air travel on a large scale.  The amount of power required to travel through the air can be easily calculated.$$ \text {Power} = \frac{\text {Weight} \times \text{Speed}}{\text{Glide Ratio}}$$For a Boeing 747, the requirement is at least 3 MW, which would require at least 2,000 square meters of solar panels—an area twice as large as the wings of the 747.

Even if a substitute power source is found, high speed rail is often capable of reaching speeds comparable to jet liners, and could well provide for long distance domestic travel in the future.  Furthermore, if demand for intercontinental flights falls too much, the maintenance of the global infrastructure for managing flights may become untenable.  Flight will likely remain possible for governments, armed forces, and the very wealth; however, my expectation is that cheap commercial flights will become unavailable to most people within the following decades.