TOI correspondent from Washington: The United States’ national debt crossed $40 trillion on Tuesday, a number so large that even breaking it up with commas can scarcely make it comprehensible. As in a 4 followed by 13 zeroes: $40,000,000,000,000!Number-crunching economists reached for various metrics to convey the scale of the crisis. The US economy produces roughly $33 trillion worth of goods and services a year. So the debt is now bigger than the entire annual economic output (GDP) of the country. Per American, that works out to roughly $118,000; for a family of four, about $472,000. Divvying it another way, the US is adding debt at a rate of roughly $90,000 every second.To put that in perspective for readers in India, the $40 trillion debt America has racked up is roughly 10 times the size of India’s entire annual economy. In other words — although debt versus GDP is not an apples-to-apples comparison — the US owes roughly ten times the value of everything India produces in a year. America’s debt mountain is therefore not merely bigger than India’s economy; it is about ten Indian economies stacked on top of one another.Economists also put the best possible spin by pointing out that the raw number itself is less important than debt relative to the size of the economy and the government’s ability to service it. Just the US debt held by its public is roughly equal to annual GDP, and the Congressional Budget Office says that, under current law, the ratio could rise to 120% of GDP by 2036 and 175% by 2056.So how did the world’s richest country get here? In the simplest terms, Washington has been spending more than it collects in taxes — year after year, administration after administration. Wars, tax cuts, the 2008 financial crisis, Covid stimulus, an ageing population and rising Social Security and Medicare costs have all added to the bill. The pandemic was particularly spectacular: debt exploded as the government tried to prevent an economic catastrophe.But the extraordinary thing is that America has continued borrowing heavily even after the emergencies have passed. Federal budget deficits are still running at levels normally associated with wars or recessions. The awkward question for Washington now is whether the borrowing eventually stops gently — or with a crash.So who is lending America $40 trillion? Surprisingly, America is not mainly borrowing from China. The biggest lenders are actually Americans and American institutions – pension funds, mutual funds, banks, insurance companies, households and investment funds. Foreign governments and investors also buy what is euphemistically called “US Treasury securities”: Japan held about $1.12 trillion in June, Britain about $940 billion, China about $633 billion and India roughly $200 billion.But why lend to a country this deeply in debt? Because US “Treasury securities” remain the world’s favourite financial equivalent of a steel safe with a very large lock. They are highly liquid, easily traded and backed by the world’s largest economy — as well as by Washington’s long-established record of paying its debts. In other words, America has been able to borrow on an extraordinary scale because the world keeps saying: Yes, Uncle Sam, we’ll take your IOU. For now.President Trump has repeatedly promised to reduce the national debt, including by using tariff revenue. But the arithmetic has not exactly been taking orders from the White House. Amid the irresistible American urge to splurge, Trump is proposing a $1.5 trillion military budget, roughly a 50% increase from recent levels and an unprecedented level of Pentagon spending, arguing that a stronger military is essential to national security.So America is essentially trying to lose weight while ordering a second dessert; spending like a drunken sailor on shore leave — and then promising the bartender that tariffs will somehow pay the tab.And debt is not merely about the amount borrowed. It is also about the interest bill. Interest payments are now approaching $1 trillion a year. That means an increasing share of the federal budget is going simply toward servicing yesterday’s borrowing rather than paying for today’s schools, roads, defence, healthcare or anything else Washington wants to spend money on.As the debt grows, investors could eventually demand higher interest rates to keep buying Treasury securities. That would feed through into mortgage rates, business borrowing costs and the government’s own financing costs, potentially slowing investment and economic growth. The US would then face the unpleasant menu economists have been warning about for years: higher taxes, slower spending, cuts to benefits, faster economic growth, inflation — or some combination of all of them.Washington would have to bring spending and revenues into better balance: restrain the growth of entitlement programmes, control discretionary spending, reconsider tax cuts, raise more revenue and — hardest of all — persuade voters that there is no painless option. There is no magic button marked “Pay Off $40 Trillion.”The CBO’s is essentially warning that the longer Washington waits, the more painful the eventual adjustment becomes. That is America’s peculiar fiscal paradox: the world’s richest country can afford an enormous debt — until one day the debt becomes so enormous that it starts deciding what the world’s richest country can afford. Or as one economist might put it, there is no such thing as a free lunch — just a lunch somebody has put on the national credit card.