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Physicist Brian Greene on Making Sense of String Theory (Transcript)

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Brian Greene

In this TED Talk (2008) Physicist Brian Greene explains how our understanding of the universe has evolved from Einstein’s notions of gravity and space-time to superstring theory, where minuscule strands of energy vibrating in 11 dimensions create every particle and force in the universe.

Listen to the MP3 Audio here: Brian Greene Making sense of string theory


In the year 1919, a virtually unknown German mathematician, named Theodor Kaluza suggested a very bold and, in some ways, a very bizarre idea. He proposed that our universe might actually have more than the three dimensions that we are all aware of. That is in addition to left, right, back, forth and up, down, Kaluza proposed that there might be additional dimensions of space that for some reason we don’t yet see.

Now, when someone makes a bold and bizarre idea, sometimes that’s all it is — bold and bizarre, but it has nothing to do with the world around us. This particular idea, however — although we don’t yet know whether it’s right or wrong, and at the end I’ll discuss experiments which, in the next few years, may tell us whether it’s right or wrong — this idea has had a major impact on physics in the last century and continues to inform a lot of cutting-edge research.

So, I’d like to tell you something about the story of these extra dimensions.

So where do we go? To begin we need a little bit of back story. Go to 1907. This is a year when Einstein is basking in the glow of having discovered the special theory of relativity and decides to take on a new project, to try to understand fully the grand, pervasive force of gravity. And in that moment, there are many people around who thought that that project had already been resolved. Newton had given the world a theory of gravity in the late 1600s that works well, describes the motion of planets, the motion of the moon and so forth, the motion of apocryphal apples falling from trees, hitting people on the head. All of that could be described using Newton’s work.

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But Einstein realized that Newton had left something out of the story, because even Newton had written that although he understood how to calculate the effect of gravity, he’d been unable to figure out how it really works. How is it that the Sun, 93 million miles away, that somehow it affects the motion of the Earth? How does the Sun reach out across empty inert space and exert influence? And that is a task to which Einstein set himself — to figure out how gravity works.

And let me show you what it is that he found. So Einstein found that the medium that transmits gravity is space itself. The idea goes like this: imagine space is a substrate of all there is.

Einstein said space is nice and flat, if there’s no matter present. But if there is matter in the environment, such as the Sun, it causes the fabric of space to warp, to curve. And that communicates the force of gravity. Even the Earth warps space around it.

Now look at the Moon. The Moon is kept in orbit, according to these ideas, because it rolls along a valley in the curved environment that the Sun and the Moon and the Earth can all create by virtue of their presence. We go to a full-frame view of this. The Earth itself is kept in orbit because it rolls along a valley in the environment that’s curved because of the Sun’s presence. That is this new idea about how gravity actually works.

Now, this idea was tested in 1919 through astronomical observations. It really works. It describes the data. And this gained Einstein prominence around the world. And that is what got Kaluza thinking. He, like Einstein, was in search of what we call a unified theory. That’s one theory that might be able to describe all of nature’s forces from one set of ideas, one set of principles, one master equation, if you will.

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So Kaluza said to himself, Einstein has been able to describe gravity in terms of warps and curves in space — in fact, space and time, to be more precise. Maybe I can play the same game with the other known force, which was, at that time, known as the electromagnetic force — we know of others today, but at that time that was the only other one people were thinking about. You know, the force responsible for electricity and magnetic attraction and so forth.

So Kaluza says, maybe I can play the same game and describe electromagnetic force in terms of warps and curves.

That raised a question: warps and curves in what? Einstein had already used up space and time, warps and curves, to describe gravity. There didn’t seem to be anything else to warp or curve. So Kaluza said, well, maybe there are more dimensions of space. He said, if I want to describe one more force, maybe I need one more dimension. So he imagined that the world had four dimensions of space, not three, and imagined that electromagnetism was warps and curves in that fourth dimension.

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