Even with a smartphone and Google at your fingertips, some things are just hard to wrap your brain around. Take, for example, the seemingly-improbable idea that energy does not flow continuously, but is released in discrete packets called quanta. Or the mind-numbing notion that the entirety of the cosmos, spanning some 93 billion light-years across, may be just one in a multitude of parallel universes.
That's where Michio Kaku comes in. The theoretical physicist has built a robust secondary career as a mass-market science popularizer, untangling some of physics' knottiest and most far-flung concepts — like quantum theory or the multiverse — and streamlining them for the public. His latest best-selling book, The God Equation, chronicles the long quest to create a "theory of everything," which would combine Einstein's model of general relativity with quantum theory, and potentially unlock new understandings of space and time. Kaku also co-founded string field theory, which he believes is the strongest candidate for such an equation.
Kaku caught up with Discover to chat about what inspired him to embark on this quest as a young child, why subatomic particles are like notes on a vibrating string, and what we can learn from science fiction.
Q: In your book, you describe an all-encompassing “theory of everything,” as the holy grail of physics. Why is a theory like that so important?
A: When Newton worked out the laws of gravity and mechanics, that set into motion what eventually became the Industrial Revolution, which lifted humanity out of agrarian misery and poverty. When Maxwell and Faraday worked out the laws of electricity and magnetism, that set into motion the Electric Revolution, which gave us electricity, radio, TV, dynamos, and generators. When Heisenberg and Schrödinger worked out the laws of the quantum and the atom, that gave us lasers, transistors, computers and the internet. So, every time physicists explain a force of nature, it alters the destiny of the human race and the world economy.
And now, we are on the verge of a theory of everything, which can unite all the forces of the universe via an equation perhaps no more than an inch long. Eventually, this may once again alter the destiny of humanity. It may also answer the deepest questions about the universe, such as: Can we break the light barrier and reach the stars? What happened before the Big Bang? Are there other universes and dimensions? What lies on the other side of a black hole? Is time travel possible? Are wormholes possible?
Q: The quest for this theory captivated some of history’s most famous scientists; I’m thinking of Albert Einstein and Stephen Hawking. What was it about a theory of everything that first ignited your imagination?
A: When I was 8 years old, something happened which changed my life. All the newspapers said that a great scientist had just died. But they printed a picture of his desk, with an open book. The caption said that the greatest scientist of our time could not finish this book. I was fascinated. What could be so complicated that a great scientist could not finish it? What could be so important? Over the years, I discovered that this man’s name was Albert Einstein, and that this book was the unfinished unified field theory, which could unify all the laws of nature into a single equation. I was hooked. I had to know what was in that book, and why he could not finish it. This became the focus of my life.
Today, the leading (and only) candidate for this theory of everything is called string theory. I have had the privilege of working on this theory since 1968. My contribution was — along with professor Keiji Kikkawa — to create string field theory, which can summarize string theory in an equation about 1-inch long. However, it is not the final theory, since now we know that membranes can also exist along with string.
Q: When I was much younger, I found the abstraction and mathematics involved in hard sciences like physics intimidating. How would you describe string theory to a high-schooler? Is the theory’s elegance and beauty lost when you explain it like that?
A: To paraphrase Einstein, he once said that if a theory cannot be explained to a child, then the theory is probably worthless. By this, he meant that all great theories are based on a simple, elegant physical picture, a single principle, a paradigm, that reveals the secrets of a theory. The rest is tedious math.
To understand string theory, imagine a rubber band, which represents a tiny, tiny electron. If you stretch the rubber band, it vibrates at a precise frequency. If you twang the band, it vibrates at a different frequency — call it a neutrino. If you twang it again, it becomes a different frequency; call it a quark. In fact, there are an unlimited number of frequencies that the band can vibrate, corresponding to an infinite number of possible sub-atomic particles.
So all the subatomic particles of nature are like musical notes on a tiny vibrating string. So what is physics? Physics is the harmonies you can create on a vibrating string. What is chemistry? Chemistry is the melodies you can create on colliding strings. What is the universe? The universe is a symphony of vibrating strings. And what is the mind of God, that Einstein wrote about for the last 30 years of his life? The mind of God would correspond to cosmic music resonating through the universe. That’s why I titled my latest book The God Equation: The Quest for a Theory of Everything.
The kernel of this idea was proposed over 2,000 years ago by the great mathematical Pythagorus. He realized that a lyre string can vibrate with an infinite number of musical notes, each one corresponding to a resonance or frequency. He then proposed that the vast diversity of matter that we see around us is nothing but the harmonies found on strings. Only music, he thought, was rich enough to make sense of the vast complexity of the universe. This picture gives us an elegant, simple way to explain why the universe is so diverse.
Q: You note that the biggest problem with string theory is that we don’t yet have any testable, concrete evidence for it. How close are we to finally being able to prove it?
A: Already, the Japanese, Chinese and Europeans are debating whether to build a machine even bigger than the Large Hadron Collider [the world's largest and most powerful particle accelerator]. One of its goals might be to verify the existence of a new symmetry, called supersymmetry, which is the symmetry of the string. If we can find these super particles, called “sparticles,” it might prove the correctness of this path.
Currently, the “theory of almost everything” is called the Standard Model, which describes the low energy behavior of sub-atomic particles. The problem, however, is that it is one of most awkward, clumsy theories ever proposed. It has 36 quarks and anti-quarks, about 20 free parameters that are inserted by hand, three identical and redundant families of particles, and never mentions gravity. No one, not even the creators of the Standard Model, believes it to be the final theory. It is a theory only a mother could love.
But using pure math (and not gigantic atom smashers), one might prove the correctness of the theory. If the theory can calculate the precise mass and properties of familiar particles like the electron, proton and neutron using pure math, from first principles, it would prove the theory to be correct.
So perhaps there is a bright, industrious student reading this article, who might be able to derive the properties of the familiar particles using pure math, then my advice would be: Tell me first. Then we can publish together and split the Nobel Price money between us.
Q: You also mention another criticism of string theory — that it predicts a multitude of universes, and therefore, an infinite number of potential solutions. How would scientists ever be able to say that they’d landed on the right one?
A: All great theories have an infinite number of solutions. For example, Newton’s laws can predict the motion of rocks, cannon balls, rocket ships, meteors. You have to specify, from the outside, that Newton’s laws will be used to explain the motion of a rock, not a rocket. These are called initial conditions. Once you specify what the initial conditions are, you can predict its motion.
Likewise, like all great theories, you have to tell string theory what it is describing, like an electron or a quark. This information is input from the outside. But string theory is different from all other theories, because its initial conditions are the Big Bang itself. Hence, you have to specify the conditions of the Big Bang at the instant it was created, which are unknown.
There are at least two ways out of this problem. One possibility is to experimentally find the initial conditions at the instant of the Big Bang, and then let string theory describe how the universe evolves from there. Already, if you make some reasonable estimates of the nature of the Big Bang, there are solutions of the string theory which describe the evolution of our universe since then. String theory can already do this. This is already a great achievement.
But a more ambitious path is to demand that string theory select out its own initial conditions. At present, no one on the Earth is smart enough to do this — prove that our universe is the only one selected out by string theory. (Perhaps all the other universes are unstable). At present, some of the greatest minds on the planet are painfully cataloging the vast number of solutions of string theory, hoping that a way might emerge which selects out the correct theory out of this jungle.
Q: The idea of the “multiverse,” or that there’s a multitude of parallel universes, seems to be everywhere in pop culture. I’m thinking of everything from shows like Stranger Things to some of the upcoming Marvel movies — it's even in the title of Dr. Strange in the Multiverse of Madness. How does the science behind the theory actually work?
A: Ordinary quantum mechanics gives rise to the multiverse. When I teach quantum mechanics to graduate students, I explain that, in some sense, electrons can be two places at the same time. This, in turn, makes possible atoms, lasers, electronics, the atomic bomb, stars, etc. When grad students ask me, “How is that possible?” the answer is usually, “Get used to it. That’s just the way the universe is.”
But one way to explain how an electron can be in two places at the same time is to assume that the universe splits in half. So our time line, which usually runs in a straight line, has a fork in the road and splits. Like the branches of a tree, one time line can give rise to a multiverse of other time lines.
String theory is a quantum theory. So, the situation gets worse. Now, we have multiple universes, not just multiple time lines. Einstein gives us a picture of the universe as a bubble which is expanding. String theory replaces the single bubble/universe of Einstein with a bubble bath of universes, with bubbles merging or splitting in half. In fact, the Big Bang is just the collision of two baby universes, or the splitting of a universe into two universes.
Then the next question is, “Is Elvis Presley still alive in a parallel universe?” If the multiverse picture is correct, then the answer is yes. But travel between parallel universes, for example, is far beyond our technology.
Q: The first book of yours I read, Physics of the Impossible, explores the plausibility of different sci-fi conceits, like force fields, hyperspace and time travel. Now you’re teaching a course about the physics of science fiction at the City College of New York. Why are you drawn to these concepts, and what can we learn from them?
A: When I was 8 years old, on Saturday morning, I used to watch the old Flash Gordon series. Ray guns. Cities in the sky. Invisibility rays. Cities under the oceans. I was hooked. Years later, I realized that my two loves, Einstein’s unified field theory, and science fiction, were related. In order to understand whether science fiction is possible, plausible, or impossible, you need physics.
I used to go to sci-fi movies and count the number of laws of physics that are violated. I don’t do that anymore, because I realize that if we can one day use the quantum theory to master nanotechnology, then many of the magic tricks found in Harry Potter movies are actually physically possible (but practically difficult). For example, Mother Nature is a master nanotechnologist. She can take a bunch of french fries and hamburgers and convert it into a baby in nine months. Atom for atom, Mother Nature (through ribosomes) can reform organic matter into a human being. So if one day we can master nanotechnology, we will have the power of a magician.
Q: Do you have a personal favorite sci-fi technology or concept?
A: My favorite science fiction novels are the Foundation series by Asimov, because they force you to imagine a galactic civilization 50,000 years into the future, when new laws of physics open up. Things that we consider impossible (like breaking the light barrier) might become possible.
For example, all the current laws of physics actually break down at the Planck Energy, a quadrillion times more powerful than the Large Hadron Collider. The stability of space-time begins to break down at the Planck Energy. Empty space becomes a space-time foam (in the words of Stephen Hawking) with baby universes darting in and out of the vacuum. With enough energy concentrated in one place, we might be able to “boil space.” (At the Planck Energy, empty space begins to look like boiling water, with each bubble representing a wormhole to another universe.) So we have to reanalyze things which we once thought were impossible, such as breaking the light barrier, or time traveling, or traveling through higher dimensions. All bets are off.
This interview has been edited and condensed for clarity.