Nobody doubts that many of tech’s unicorns are indeed real businesses and that some could be with us for decades. But because of low interest rates, tech companies are raising gobs of money from investors whose desperate need for returns has pushed them into riskier territory. Start-ups have begun attracting money from hedge and mutual funds that don’t usually invest in tech companies before they are public.

Valuations — and there is no real standard for determining how much a private company is worth — are inflating, leading some people to worry that investment decisions are being guided by something venture capitalists call FOMO — the fear of missing out.

In a recent analysis, Mr. Tunguz of Redpoint, who was in high school when the dot-com bubble burst, found that investors were paying twice as much for stakes in private technology companies as they were for those that were publicly traded.

He called it “a runaway train of late-stage fund-raising.” He also called it “a really weird time” and “a really hard environment to maintain financial discipline.”

The problem with the bubble question is nobody seems to agree on what exactly a bubble is. Robert Shiller, an economist whose work on stock prices earned him the 2013 Nobel Prize and who wrote the bubble book “Irrational Exuberance,” defined speculative bubbles as “a psychological epidemic” in which people put reason aside and instead buy into a story.

“It’s a complicated social phenomenon that gets people into trouble, just like smoking too much and drinking too much,” Mr. Shiller said.