Calls for wealth taxes have been gathering steam over the last few years. The recent AI boom, which has minted many more billionaires and centibillionaires, has intensified the clamor for governments to do something about wealth inequality. Those calls reached a crescendo in the days after Elon Musk became the world’s first trillionaire.

California has taken the lead. Its Billionaire Tax Act is a proposed ballot initiative that will be put before the state’s voters this November. It aims to apply a one-time 5 percent tax on billionaire wealth, spread over five years. If the measure passes, the revenues raised will be used to fund healthcare and enhance other social welfare services.

The wealth taxes that are being discussed around the world today would apply to both financial and non-financial assets such as real estate, private jets, yachts, and expensive artwork. In America, much of the focus has lately been on the massive stock holdings of technopreneurs. The rapid rise of the stock prices of AI-related companies has increased some fortunes to eye-popping levels, making them juicy targets for politicians.

A one-time wealth tax, such as the one being proposed in California, is unlikely to ruin any billionaires. Rather, it could generate much-needed revenues to help people who find themselves in dire straits. It would also be a humane thing to do. But Google co-founder Sergey Brin reportedly spent tens of millions of dollars trying to defeat the ballot initiative.

The only thing I can think of is that Mr. Brin doesn’t trust the politicians when they say it will be one and done. He and his fellow billionaires perhaps suspect that after the first haul, the politicians might keep coming back to squeeze more money out of them to fund ever-growing social programs.

My biggest concern in this debate, at least in America, is the nature of the wealth that governments are trying to tax. Many economists and commentators have repeatedly warned that all the hype about AI has made stock markets quite frothy. Listening to those voices, I get the sense that some of what we are considering as wealth today could very well be smoke. No one really knows how much of anything is real stuff that we can confidently take to the bank.

Musk became a trillionaire because he owns the vast majority of the shares in his company SpaceX, which went public recently. The company’s current stock valuation is based on future cash flows that may or may not materialize. One skeptical market veteran has referred to those cash projections as “hopes and dreams.”

We should be a bit careful about designing social programs on the basis of revenues we expect to receive from taxing such paper wealth. As politicians everywhere know quite well, once voters get a taste of certain government benefits, it is impossible to withdraw them even if they become unaffordable in future. The last thing we want to do as a society is to start something that we cannot sustain.

I share the view that AI has great promise. But I have a sneaky feeling that to some extent, we may be trying to tax ghosts.