Rampant enthusiasm is buoying tech shares to levels that defy gravity. Invest with caution, our columnist says.

© Janet Mac

John D. Rockefeller, the Gilded Age oil baron, became America’s first billionaire in 1916. Like Elon Musk’s ascension, the milestone made headlines.

© Associated Press

Strong investor interest in SpaceX could be good news for the A.I. rivals, which have signaled that they, too, intend to go public this year.

Elon Musk said he had initially given SpaceX less than a 10 percent chance of succeeding. His rocket company has come a long way.

© Andres Kudacki for The New York Times