PASADENA, Calif. — The future of computing may be in its past.

The silicon transistor, the tiny switch that is the building block of modern microelectronics, replaced the vacuum tube in many consumer products in the 1970s. Now as shrinking transistors to even more Lilliputian dimensions is becoming vastly more challenging, the vacuum tube may be on the verge of a comeback.

In a darkened laboratory here, two stories beneath the California Institute of Technology campus, two students stare through the walls of a thick plastic vacuum chamber at what they hope will be the next small thing — a computer chip made from circuits like vacuum tubes whose dimensions are each roughly one-thousandth the size of a red blood cell.

At stake is the future of what electronic engineers call scaling, the ability to continue to shrink the size of electronic circuits, which is becoming harder to do as they become as small as viruses.

It has been more than half a century since the physicist Richard Feynman predicted the rise of microelectronics, noting “there’s plenty of room at the bottom.” He used the phrase in 1959 when he speculated about engineering with individual atoms. Several years later, Gordon Moore, co-founder of Intel, wrote that the number of transistors that could be etched into silicon wafers would double at regular intervals for the foreseeable future.