Where does the government get money from to keep paying treasury bond interest?

This seems like a Ponzi scheme. They buy money from the public with bonds, then when the first investors want their money back, they just borrow it from more investors.

I don't see how they can keep doing this (faster than inflation or the rate of economic growth) without either raising taxes, causing more inflation, taking on more debt, the government becoming a greater and greater percentage of the GDP, or reducing government services.

1) It's not the Ponzi scheme considering that the government does receive an income.

2) Just like in Finance, we consider the debt relative to your income. In order to establish this, we look at the National Debt relative to the GDP(adjusted for Purchasing Power Parity).

Here is a ranking list of the National debt in relation to GDP, from the CIA website: https://www.cia.gov/library/publications/the-world-factbook/rankorder/2186rank.html

We're not that far from other countries.

Simple answer: From the government's income... our taxes.

They borrow it from China. China holds the mortgage on the U.S. Almost completely. "Happy Days Are Here Again!" (That's the Rebublican theme song. It comes from the Great Depression.)

The last couple years have seen big surpluses for the United States, so they're not actually borrowing much money from the public, and in theory are paying some of it back.

A large part of government revenues go towards paying the interest on the national debt. In theory, in years when there is a strong economy, the government also pays back the national debt, while in years when there is a weak economy, the government borrows to increase spending. This sometimes doesn't happen because of politics - it's easier to justify more money for hospitals, schools or tanks instead of the debt, and hard to justify higher taxes.

One reason governments sometimes fail to pay down their debt is because they believe that by spending the money elsewhere, they can cause the GDP to grow. As the GDP grows, the ratio of debt-to-income will shrink, and the debt becomes less of an issue. ...Again, in theory. ;-)