Celebrity Scientist Brings Time Travel, Cosmic Theory Down to Earth

Stefan Lovgren
for National Geographic News

March 26, 2004

If anyone is to realize Albert Einstein's dream of one day producing one unified theory to explain the nature of the universe, Brian Greene seems like a pretty strong candidate. And if he succeeds, there is even a chance we may actually understand what he's talking about.

Greene, who teaches physics and mathematics at Columbia University in New York City, is popularizing cutting edge physics. His 1999 bestseller The Elegant Universe, took lay readers on a lucid stroll through the theoretical physics brain-twister known as string theory. In his new book, The Fabric of the Cosmos: Space, Time, and the Texture of Reality, Greene dives into even deeper questions of space, time, and where the universe is going.

In a recent telephone interview, Greene spoke with National Geographic News about everything from time travel to the possibility that our world is merely a hologram.

Your previous book described string theory. What does that entail?

At its core [string theory] is a new proposal for the fundamental constituents of matter and energy … a little vibrating filament. It's hard to say what it's made of, but the word "energy" kind of captures it. It's a vibrating filament of energy that lies within every particle.

An electron is literally a vibrating loop of string. Just as the strings in a violin can vibrate in different patterns, producing different musical notes, these strings can also vibrate different ways. If you take all the elementary constituents of the universe and magnify them sufficiently, the only difference between them would be how the string is vibrating.

We think [string theory] realizes the dream that Einstein articulated, the dream of finding an essential idea that might be able to describe everything in the physical universe. He searched in vain for this unified theory and never found it, and we think that this approach, this string theory, may well be what he was looking for.

What is your new book about?

The book traces our ever changing conception of reality over the last 300 years. It shows there have been a number of major upheavals in our thinking, which are closely intertwined with upheavals in our understanding of what space is and what time is.

Our senses evolved in order for us to survive in the wilderness. But there's no reason to expect that senses that evolved for that purpose would be at all good at revealing the true nature of the universe, the true underlying reality. In fact, experience and our senses provide a very poor guide to the true nature of the world.

Science has revealed a strange layer of reality just beneath the surface of the everyday world. That's what the book is about—this layer of reality beneath everything.

Give us an example of how our experience may be a poor guide for understanding what's going on.

Take time, for example. If we are to trust our everyday familiar experiences, we would all think—as [Isaac] Newton did—that there's a universal notion of time.

That the universe has sort of a built-in clock?

Right. But we learned from Einstein's insights, and through experiments that prove it, that time is not like that. Each of us carries our own individual clock. As we move relative to each other, our clocks tick off time at different rates.

If you and I were to synchronize our watches and were to move around relative to each other and then come back, our watches would differ. That is completely unfamiliar based on everyday experience.

The effects that I'm describing are small at the velocities that we can achieve in daily life. That's why we don't notice it, because our senses are poor. But if we could move at the speed of light, then [these] effects would become as plain as day. We would recognize that we have been fooled for the past few thousand years simply because our senses were only being presented with slow motion relative to light speed.

Many people are blown away by the idea of time travel. Is that a possibility?

As technology gets better and better, one can imagine that we will gain the ability to go faster and faster. Will we ever achieve near-light-speed motion? I don't know. But imagine that we do.

You might travel out to space for six months, explore the universe, turn around and come back another six months later. In the Newtonian way of thinking, in the everyday way of thinking, you'd say a year has elapsed.

So if you left in 2004, it will be 2005 on Earth. But if you're going near the speed of light, when you come back one year will have elapsed on your clock, but on Earth it may be that ten years have elapsed, or a hundred, or ten thousand, or even a million years, depending upon how near to light speed you got.

The bottom line is that time travel is allowed by the laws of physics. The laws of the universe allow you to see what Earth will be like a million years from now. To my mind, that's stunning. And when most people realize the implications of these ideas, they're completely bowled over and no longer have an issue as to why they should think about it.

So time depends on how fast we move.

It also depends on the strength of the gravitational force that we experience. If you were to go to the edge of a black hole [where there's a lot of gravity], time for you would again slow down relative to someone who's far away from the black hole.

The bottom-line summary is that the passage of time depends very much on circumstance and environment, whereas everyday experience suggests that time doesn't care about circumstance and environment. It just ticks away relentlessly, uniformly. That notion of time is false.

In your book you also propose that there are as many as 11 dimensions. How do you come to that conclusion?

There's literally an equation in string theory, which tells us how many dimensions the universe should have. In other theories, you simply put in the number of dimensions. [String] theory doesn't require that you put it in. It gives the answer to you.

Basically, the equation tells you that there must be a certain number of vibrational patterns at the string of a given energy. The more dimensions there are, the more vibrations the string can execute. If the world truly had three spatial dimensions, the string would not execute enough vibrational patterns to solve the equation.

In four dimensions it gets closer, but it's still short. In five, it's closer still but still short. You must go all the way up to ten dimensions before you get the right number. That's why we come to this conclusion that string theory demands that our world have more than left-right, back-forth, and up-down.

In describing the universe, you argue that 5 percent of it can be characterized as familiar matter, 25 percent is dark matter, and, controversially, 70 percent may consist of dark energy. Explain that, please.

The familiar matter is basically protons and neutrons, the basic ingredients in atoms, which are in anything that we encounter in daily life. We have found through observations of the cosmos that if you were to weigh the universe, only 5 percent of that weight would be composed of protons and neutrons.

If you ask what that other 95 percent is, I can't really tell you. [Probably] 25 percent is some kind of matter, but almost certainly not protons and neutrons.

The remaining 70 percent is not made up of material, but is made of some kind of haze-like gas that we give the name dark energy because it doesn't give off light. We're struggling to figure out what 95 percent of the universe actually is.

You're basing your entire work on string theory, but the physical evidence doesn't exist to corroborate what you're suggesting. What if you're wrong?

If we're wrong, we'll gladly put the theory aside and head back to the blackboard. I feel very wedded to the notion of trying to find the deep truth about the laws of the universe, but I'm not wedded to any particular theory. If it's wrong I would like to know tomorrow, so I can stop working on it.

I think it's very unlikely that string theory will be wrong, but I think it's quite possible it might be just a step for some other theory.

What kind of technological advances do we need to test these theories?

It would be asking too much to travel at the speed of light. There's a new machine, an atom accelerator that's being built in Geneva, which may be able to test certain features of string theory in the next decade.

I'm personally hoping that astronomical observations might be the key to confirming some of these ideas. You can imagine that when the universe was really tiny, in the very beginning, strings were small, but they'd be roughly the size of the universe. They could be the imprint on the universe.

As the universe expands, those imprints can be stretched out. They may leave a subtle imprint on the sky, for instance.

Others are hoping to test these theories by looking down. I'm going in the other direction by looking up.

In the final chapter of your book, you suggest that the world may be a hologram. That sounds very Matrix-like.

It's a very speculative idea that seems to, strangely enough, naturally emerge from string theory. Basically, the fundamental laws of the universe don't really operate in the environment around us. They may operate on sort of a distant bounding surface and give rise to the familiar world that we experience in much the same way that a thin piece of plastic, when illuminated correctly—if it's a hologram—can yield a three-dimensional image.

It might be that the deep laws are more like the thin piece of plastic existing on a thin bounding surface. Everything we know might be akin to a holographic projection of those distant laws.

Do you believe there is going to be time when we truly understand the nature of the universe—a final theory that we can all agree on?

I do think so. Every step of the way, when we've gone deeper, our theories have actually gotten simpler. They may sound more complicated, but when you truly study them, there really is a drive toward simplicity.

Its quite possible there's a limit to how simple things can become. And that may be when we've reached the final formulation of the deep laws.

You're a celebrity now. I bet your class at Columbia is full.

Yeah, a good number of students show up.

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