*Brian Greene continues the WSF Live Forum all month long. Each day, he’ll answer one of your questions for this ongoing series that delves into the fundamental nature of space, time, and reality as we may or may not know it.*

**Why are so many physicists chasing string theory when, after 30+ years, there is still no means of testing it?**

*—C Gulow (via BoingBoing)*

The primary goal of string theory is to join the laws of quantum mechanics (the laws of the “small”) with the laws of general relativity (the laws of the “large”), into a single mathematically consistent framework. This is vital because in the standard formulation (without string theory, say), the union of quantum mechanics and general relativity leads to mathematical inconsistencies, sort of like what happens on a calculator if you divide 1 by 0. And since the universe is surely consistent, we need a mathematical description that’s consistent too. Without such a consistent formulation, for example, we don’t stand a chance of fully understanding the origin of the universe or what happens in the deep interior of black holes.

Melding quantum mechanics and general relativity is a tough nut to crack; to date, very few approaches claim success. String theory is one such proposal. But because the hostilities between general relativity and quantum mechanics only surface in extreme domains (as above, near the moment of the big bang or the singularity of a black hole, to give two examples), any theory—including string theory—that purports to meld them typically only shows its true colors in similarly extreme domains. That makes it hard to test the theory. As you point out, such is and has been the case with string theory for many years.

We continue to work on it for a number of reasons, here are three:

First, the problem of uniting gravity and quantum mechanics is one of the most important problems in fundamental theoretical physics. We can’t ignore this problem. And in the opinion of many practitioners (including me, but not all researchers by any means) string theory offers the most convincing and compelling of the proposed solutions. String theory naturally incorporates key breakthroughs from the past few decades of physics research, uniting them into a single mathematically persuasive theory that, at least on paper, combines general relativity and quantum mechanics. That’s impressive and to many is a sign that the theory is moving in the right direction.

Second, over the past few decades, string theorists have made remarkable strides in developing and understanding the theory. We’ve not hit brick walls that have thwarted progress. Although such progress has not, as yet, allowed us to make contract with experiments or observations, the rapid and impressive progress has kept the forward momentum of research going strong.

Third, over the past few decades as the theory has developed, it has made contact—sometimes quite profound contact—with other areas of physics. String theory inspired the whole research area of supersymmetry; string theory has made important contributions to our understanding of black hole entropy; string theory has offered impressive insights into aspects of the quark gluon plasma; string theory inspired new approaches to conventional calculations in quantum field theory; and we now realize that, more generally, there’s a profound connection between the very framework of quantum field theory (the standard approach for describing elementary particles) and string theory. In other words, string theory is not an isolated body of research taking place in some obscure corner of physics. Instead, string theory has tentacles that have reached far and wide into more conventional areas. This vibrant conversation among diverse areas of physics that string theory has allowed has also kept interest and enthusiasm strong.

To be sure, we won’t know if string theory is indeed the long sought unified theory that Einstein dreamed of finding, until we have explicit experimental or observational support. But progress has been sufficiently impressive, and the potential problems the theory can address sufficiently profound, that many have been and continue to be excited to vigorously press on with research. Thirty years might seem like a long time. But the depth of the problems being tackled is such that, in reality, thirty years is not that long at all.

*Brian Greene is co-founder of the World Science Festival and professor of physics and mathematics at Columbia University. His books include The Elegant Universe, The Fabric of the Cosmos, and The Hidden Reality. NOVA’s miniseries “The Fabric of the Cosmos” airs Wednesday nights on PBS.*

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