Wealth really is a relative concept. Some treat millionaire status as passé (after all, there are about three million millionaires in the United States alone), and these days it takes billionaire status to get much attention. On the other hand, there are over two billion people getting by on less than $1,000 per year.

John D. Rockefeller is held to be the world's first official billionaire, achieving that status in 1916 largely through his ownership of Standard Oil. From that point nearly a century ago, wealth has multiplied to the point where the richest men in the world top out at around $50 billion (by comparison, Rockefeller's wealth in modern inflation-adjusted terms would amount to about $400 billion). The question then, is how long it will be before the world sees its first trillionaire. (More than 70 years after his death, this man remains one of the great figures of Wall Street. See J.D. Rockefeller: From Oil Baron To Billionaire.)

One trillion dollars is a phenomenal sum of money. In present day terms, $1 trillion is roughly the nominal GDP of Mexico or South Korea. One trillion dollars is also enough money to buy Exxon Mobil (NYSE:XOM) and Apple (Nasdaq:AAPL), with enough left over to buy Google (Nasdaq:GOOG), and Nike (NYSE:NKE) and Agilent (NYSE:A) - and still have enough left over to be a billionaire with the money in the bank.

Who Won't Get There
The first trillionaire is not going to come from the current ranks of the world's richest people. Carlos Slim and Warren Buffett both have sizable and very healthy business interests, but they are both in their 70s. Even if Slim could achieve the exceptional rate of return of 25%, after tax, every year indefinitely, it would take almost 14 years to leverage his $54 billion fortune into $1 trillion, and that's assuming he put it all on the line.

While Bill Gates arguably still has youth on his side at age 55, his interests have changed. Even if Gates were more interested in amassing more wealth than giving it away through his Gates Foundation, Gates would have to find a new "new thing" to invest in, as Microsoft (Nasdaq:MSFT) shows few signs of producing the sort of growth it would take to lift Gates' stake to $1 trillion.

The Scale of the Problem
In many respects, wealth begets wealth. Rich people have attractive investment options that just simply are not available to "regular people." That said, there gets to be a point where the level of one's wealth becomes an impediment to the rate of return. To double a $100 billion stake is tantamount to finding another Vietnam, and those opportunities are not commonplace. When investors hear Warren Buffett speak of the difficulty he has in finding suitable opportunities for his cash, consider that he has much less than $50 billion or $100 billion to actually put to work.

On top of those issues are obstacles related to government policy. The sort of monopolistic and robber-baron activities that created the world's first self-made multi-millionaires in the 1800s are now largely illegal across almost the entire world. Moreover, taxes in general are higher now and governments offer fewer loopholes and shelters than in the past. This is not to say that a creative and motivated entrepreneur will not find ways around these obstacles, but it seems fair to say that the business of getting hyper-rich has gotten harder with time. (If you think economists are only concerned with numbers, check out some who are more like economic philosophers. Learn more in The Austrian School Of Economics.)

The Will and Appetite for Risk
Less quantitative, but certainly important, is the role of psychology. Simply put, it seems that most people find it is hard to stay as hungry and aggressive when they have ample wealth as when they were poor and had little to lose or fall back on if things failed. Consider the cases of Paulson, Soros or Simons. All of these men are quite wealthy and certainly have shown a large degree of comfort with the use of leverage in their investing. The problem is, though, is that it is hard to imagine that any of these men see the need to take on that kind of risk.

Could Paulson et al apply 10-to-1 (or greater) leverage and make a play for $1 trillion? Perhaps. But these men already have built what is quite likely to be multi-generational wealth; why throw that away on a risky and improbable gamble? What could someone buy with $1 trillion that is not available at $1 billion and is worth risking it all?

A Handful of Candidates
So is there anyone alive today who could approach trillionaire status? For purposes of this column, government leaders and dictators are excluded; it is not unthinkable that an individual or family could rule a petrostate and be "worth" $1 trillion, or more if the value of those resources in the ground is included in the calculation, but that really is not in the spirit of this piece.

Facebook's Mark Zuckerberg is only 26 and reportedly worth around $7 billion. Clearly, that is a fine start. If Zuckerberg can find a way to grow his wealth 10% a year, every year (excluding taxes), he would be a trillionaire before his 60th birthday. But consider how impossibly large Facebook would have to become to fuel that sort of wealth. With his ownership stake, Facebook would have to grow to become ten times the current size of Exxon Mobil to make him a trillionaire.

One off-the-board candidate to consider would be Craig Venter. Famous as the founder of Celera Genomics, and for overseeing research that has led to what arguably amounts to the first example of synthetic life, Venter is both brilliant and keenly ambitious. Although he does not appear to be currently targeting cancer as a subject of his research (focusing instead on synthetic biology that could be applied to clean fuels), imagine what a cure for cancer could be worth. The U.S. currently spends $90 billion a year on broadly-defined "cancer care" and a true cure would seem to be a multi-trillion dollar opportunity. Then again, clean biofuel is nothing to sneeze at either; it likely will not make Venter a trillionaire, but the idea cannot be wholly dismissed. (Find out how he went from selling soft drinks to buying up companies and making billions of dollars. To learn more, check out Warren Buffett: The Road To Riches.)

The Bottom Line
Beyond these two men, it is anybody's guess who will achieve the lofty perch of $1 trillion. Inflation will make the job a little easier (a million dollars isn't worth what it used to be), but it is still an incredible and mind-boggling goal. Considering that nobody would have predicted that Gates would become a billionaire from computer software (who even thought of "personal" computers when he began?) or that Al Mann would become a billionaire from insulin pumps, it is likely that the world's first true trillion-dollar idea will come from somebody's imagination and would seem ridiculous today.