### Brian Greene on space and time

Tonight at the Spark speaker series, Brian Greene walked us through some exciting developments in the scientific understanding of our universe.

Space and time are the most compelling of scientific ideas because "we experience them, constantly." Yet scientists still struggle with what space and time are, and what if anything they mean to the universe's existence. To help us comprehend space and time, Dr. Greene provided an overview of some pivotal moments in scientific history.

**Newton's laws of motion**

First, we examined Isaac Newton's three laws of motion, developed in the 17th century. Newton viewed space and time as absolutes. In fact, his laws didn't mean anything absent a definition of space and a definition of time. A contemporary, Gottfried Liebniz, disputed Newton's certainty about the absoluteness of space and time. And yet Newton's laws remained generally accepted for at least the next two centuries.

**Einstein's theory of relativity**

Then in the early 20th century, Albert Einstein "came to very different conclusions." Einstein devoted 10 years to understanding how gravity works, and his theory of relativity postulated that space and time are not absolute. Instead, Einstein argued, gravity affects space and time. In terms of cosmology, "an object like the sun, merely by its presence in the fabric of space, deforms that space." Yet space and time are interconnected.

**Quantum mechanics and Heisenberg's uncertainty principle**

But then quantum mechanics began to pose problems. Werner Heisenberg's uncertainty principle states that the smaller you go, the bigger the uncertainty. What appears placid on the surface may instead be rent by "quantum jitters." The result is that if space and time are real, then they are subject to quantum uncertainty.

Einstein's theory of relativity does a great job of describing space and time on a large scale. Heisenberg's uncertainty principle does a great job of describing space and time on a microscopic scale. Yet the two laws seem unable to coexist peacefully.

**String theory: a unifying theory?**

String theory may address the gap between Einstein and quantum mechanics. Dr. Greene warned that string theory is "an unproven theory — I need to stress that."

Strings may be the fundamental building block of all matter. These strings vibrate in patterns, and different vibrations give rise to different particles. If true, then string theory reduces the jitteriness of space postulated by Heisenberg's uncertainty principle. It "allows quantum mechanics and general relativity to come together."

Yet string theory requires three fundamental changes in our understanding of space and time. First, there are more dimensions than previously believed — specifically, 10 instead of the 3 we currently are able to see. Second, space can rip. And third, our universe is created by one big slice of space that collides with another every trillion years or so.

**Proving string theory**

How can string theory be proven? One possibility is the Large Hadron Collider currently under construction in Geneva, Switzerland. While not as powerful as the Superconducting Super Collider once planned for Texas, the Swiss facility may be able to demonstrate the existence of gravitons, or the energy shed when two particles collide.

Scientists strive for the truth, no matter how difficult to obtain. String theory may or may not be correct, but it is at the forefront of today's scientific explorations of how space and time function in the universe.

**Join the discussion**

We would like to thank Dr. Greene and the audience for a fascinating exploration of space, time, and the universe.

What thoughts would you like to share? Do you find string theory a palatable explanation for how space and time operate in our universe?