Elon Musk, a serial entrepreneur who was a co-founder of PayPal and the electric car company Tesla Motors, sent people in California into a tizzy on Monday when he released a white paper outlining a hypothetical high-speed transportation system called the Hyperloop.

There were a number of curious questions about the Hyperloop, which Mr. Musk’s white paper claims will be able to travel at up to 800 miles an hour and transport people from San Francisco to Los Angeles in 30 minutes. While physicists agree that technically, on paper, this is possible, economists seem to agree that technically, on paper, the price tag of $6 billion is impossible.

Richard White, a professor of American history at Stanford and author of “Railroaded: The Transcontinentals and the Making of Modern America,” said Mr. Musk’s project was “dramatically overblown and the price is drastically underestimated.” He said in an interview that Mr. Musk’s document outlining the price of the project was “just pie in the sky.”...