Filed by KOSU News in Science.
June 4, 2011

The U.S. government is $14.3 trillion in debt. When we first neared the trillion-dollar mark in 1981, President Ronald Reagan said that the height of our debt amounted to a stack of $1,000 bills about 67 miles high. That’s somewhere in the thermosphere.

Today, that pile of $1,000 bills would be floating in space, more than 900 miles above the Earth. There aren’t any $1,000 bills in circulation anymore, so here’s an astronomical analogy about today’s debt: If you stack up 14.3 trillion dollar bills, the pile would stretch to the moon and back twice.

Unless you’re Buzz Aldrin, that’s hard for most of us to visualize. So just how big a number can our brains process?

For help with this gargantuan problem, we turned to our Math Guy, Keith Devlin of Stanford University. He tells NPR’s Scott Simon the biggest number human beings can really comprehend is much, much smaller. Seven, in fact.

“If you show someone, an adult person, up to seven objects, they will instantly be able to recognize how many there are,” Devlin says. “If the collection’s bigger than seven, we have to visualize them in groups, or we have to count them.”

That goes for even the smartest of us. “That’s just a basic feature of the human brain,” Devlin says. “In fact, we were only able to develop all of our science, technology, architecture and so forth because we invented numbers around 10,000 years ago.”

“It really does come down to having numbers,” Devlin says. “Without numbers, we would be essentially innumerate — in all senses of the word.”

Even so, 14.3 trillion is a pretty big number to comprehend. “It’s essentially meaningless,” Devlin says. “People often say, ‘This is an astronomical number’ — well, it’s actually way beyond astronomical; the best estimates the astronomers have is that there are about 200 billion stars in the Milky Way, and about 150 billion galaxies.”

One analogy that might help visualize those trillions starts, again, with the dollar bill. “A dollar bill is slightly more than 16 square inches, and I like to think of it as carpeting an area,” Devlin offers.

So for example, one million dollar bills would cover roughly two football fields. Take a billion dollar bills — about four square miles — and you could cover a small town, perhaps something the size of the Stanford campus.

“But if you go all the way to the national debt at $14.3 trillion, appropriately enough, I think, that would exactly cover President Obama’s home state of Illinois,” Devlin says. [Copyright 2011 National Public Radio]