“The beginnings of a speculative bubble may be forming but it is impossible to tell when it will end,” Siegel wrote in his weekly WisdomTree commentary Monday.

Siegel, who retired from Wharton last week after a 45-year career as a finance professor, pointed to the meteoric rise in semiconductor stocks—which are seen as the vendors of the “pickaxes and shovels” of the AI revolution—as one example of bubble-like market behavior. To his point, the iShares Semiconductor ETF (SOXX), which tracks U.S.-listed chip stocks, has soared over 57% in the past 12 months alone.

“It is impossible to tell,” Siegel wrote, “whether semiconductors’ action today is akin to 1997-98 and the internet—or more like 1995, like tech bulls think. Or 2000, like bears think.” He compared the current rise of chip stocks to the internet darlings during the dot-com era.

The former professor did admit that tech stocks are “expensive” in general, but he also noted there’s a reason for that—earnings growth. As of last week, after 80% of S&P 500 constituents reported fourth quarter earnings, the tech sector saw earnings growth of more than 20% year-over-year, according to Fisher Investments.  Wall Street expects that trend to continue as well, with analysts forecasting 17% year-over-year EPS growth and 9.1% revenue growth for the sector in 2024, according to Charles Schwab. 

Nvidia, the special one

Despite strong tech earnings, some analysts have argued that the meteoric rise of Nvidia, the semiconductor and graphics processing unit (GPU) giant, is evidence that a tech bubble is already here. But Siegel pushed back on that claim on Monday. 

“I do not think NVIDIA is dramatically over- or under-valued,” he wrote. “This has been a special company, and this is a special time when the demand for semiconductors is seemingly unlimited as the strong investment cycle is supporting chips and training new artificial intelligence (AI) models.”

Siegel argued that not only can Nvidia “rise further,” but “much of the tech momentum may continue for some time.”

His comments echo those of billionaire entrepreneur Mark Cuban, who told Fortune that “we are not in a tech bubble” just last week. Cuban, who made a big chunk of his $7.2 billion fortune selling Broadcast.com before the dot-com bubble burst, said that there really aren’t many similarities between today’s stock market and that of the late ‘90s.  

Big tech valuations, while elevated, seem to back up Cuban’s argument. In March 2000, right before the dot-com bubble burst, the top 7 stocks in the tech-heavy Nasdaq Composite traded at nearly 90 times their forward earnings, on average. Today, that number is below 35. 

Still, at the end of the day, Siegel warned that “tech stocks momentum will run dry” eventually. “In a run such as this, the saying is ‘Stairs up, elevator down.’ And that elevator ride can be quite swift!” he warned.

For investors, Siegel argued that in spite of the threat of a bursting tech bubble, it’s important to focus on the long haul. After all, it’s “very difficult to time exits,” he noted, and no one wants to miss the big returns that come when a bubble is inflating.