Dear John: Interest rates have been kept artificially low through quantitative easing, which currently stands at about $200 billion a month in Japan and Europe.

Still, our national debt doubled in the past 10 years. Imagine what the impact will be if rates returned to historic levels? E.H.

Dear E.H. Not “if” rates returned to normal levels. It’s “when” they return to normal levels because that day will come.

The Federal Reserve is now unwinding quantitative easing, the experiment begun a decade ago to keep rates low and — allegedly — bail out the US financial system.

Those low rates are a secret tax on Americans. And they bailed out Wall Street.

Now the Fed swears it will raise rates, and it has made small increases three times so far. But the Fed seems timid — very timid — so rates aren’t going up very fast.

Some day, however, the financial markets will take over and boost rates quickly. And the Fed won’t be able to do anything to stop the rise.

That’s when the US government will have to pay a lot more to finance its big deficit. And on top of the extra spending that we can’t seem to control, Washington will have massive extra financing costs.

Yep, that’s going to be a problem on top of a lot of other problems.