By Michio Kaku

Time Magazine chose him as Man of the Century. Albert Einstein had three great theories. His first theory of Special Relativity (1905) gave us E = mc², which led to the atomic bomb and unlocked the secret of the stars. His second great theory was General Relativity (1915), which gave us space warps, the Big Bang, and black holes. But many don't realize that his greatest theory was never finished: "a theory of everything". Einstein's crowning achievement was to havebeen the unified field theory, an attempt to "read the mind of God".

But on the third try, Einstein failed. He spent the last 30 years of his life chasing after an equation, perhaps no more than one inch long, that would explain all physical phenomena. Everything from Creation, to supernovas, to atoms and molecules, perhaps even DNA, people, and love was to be explained by this equation. If discovered, it was to have been the ultimate achievement of 2,000 years of investigation into the nature of space and matter, ever since the Greeks asked what was the smallest particle and the smallest unit of space. Although there are many unresolved questions, today the leading and, in fact, only candidate for the Theory of Everything is ‘superstring’ theory, defined in 10 dimensional hyperspace. Superstring theory, in turn may one day answer some of the deepest questions of the universe, such as:

• What happened before the big bang?
• Is it possible to build a time machine?
• Can we punch a hole in space?

CalTech

Albert Einstein never managed to "read the mind of God"

Not only has the power of this theory startled the world of mathematics and shaken the world of physics, it is also the craziest theory ever proposed.

Today, we realize that the entire universe is governed by four fundamental forces:

• The gravitational force, which keeps us from flying into outer space, and prevents our sun (a gigantic hydrogen bomb) from exploding outward.
• The electromagnetic force, which light up our cities and energizes our lasers and our computers.
• The strong and weak nuclear forces, which lights up the stars and galaxies.

Gravity can be described by Einstein's general relativity theory. Matter warps the space surround it, thereby creating the "force" of gravity.

Imagine an ant walking on a crumpled sheet of paper. The ant would say that there was a mysterious "force" which pulled it left and right. But we know that there is no "force" pulling the ant; there is only the crumpled sheet of paper pushing the ant left and right. Gravity does not pull: empty space pushes.

SOHO

The colossal power of a coronal mass ejection from our sun

The other three forces can be described by the quantum theory. The quantum theory has a tortured history. Back in the 1950s, when scores of strange new "fundamental" particles were flooding out of our atom smashers, J. Robert Oppenheimer (father of the atomic bomb) was so frustrated that he declared, "the Nobel Prize in physics shall go to the physicist who does NOT discover a new particle that year." There were so many particles, each given strange Greek names, that Enrico Fermi said, "if I had known there would be so many particles, I would have become a botanist rather than a physicist."

But after decades of wandering in the wilderness (and spending billions of tax payers dollars) physicists have unified these three quantum forces into what is called the Standard Model, based on a zoo of bizarre particles, called quarks, leptons, Higgs bosons, Yang-Mills particles, gluons, W-bosons and more. Remarkably, all known physical phenomenon can, in principle, be described by these two great theories, relativity and the quantum theory.

CERN

A segment of the largest tunnel in the world: the LEP particle accelerator

Although these two great theories represent the two pillars upon which ALL physical knowledge is based, the fundamental mystery is why these two theories are so different in almost every way. The first theory is based on the curvature of smooth surfaces, which describes the world of the very large.

The second theory is based on tiny discrete packets of energy (called "quanta") and explains the world of the very small, such as atoms and nuclei.

But why should nature, at the most fundamental level, create two totally dissimilar theories? Sadly, every attempt to merge these two theories has failed. Some of the greatest minds of the century have tackled this problem, only to be unsuccessful.

As physicist Freeman Dyson has pointed out, the road to the unified field theory is "littered with the corpses". Neils Bohr once attended a meeting when Nobel Laureate Wolfgang Pauli was presenting his version of the unified field theory. Bohr stood up and said, "Mr. Pauli, we in the back are all convinced that your theory is crazy. But what divides us is whether your theory is crazy enough!"

This is perhaps the greatest challenge of all time, to unite all four fundamental forces into a consistent, coherent picture. At present, the sole candidate for the theory of everything is superstring theory.

CERN

Even particles leave tracks in the BEBC (Big European Bubble Chamber)

Superstrings and the 10th Dimension

Superstring theory combines relativity and quantum in an elegant, intuitive way. First, it describes the myriad of quantum particles of nature because each particle represents a "note" on a vibrating string. Think of a violin string. No one says that A or B is more fundamental than C. What is fundamental is the string itself.

Superstring theory says that, if we had a supermicroscope and could peer at an electron, we would see a string vibrating in a certain mode. The string is extremely small (10 to the minus 33 centimeters!) so that the electron looks like a point particle to us. If we shake the string, so it vibrates in a different mode, then the electron can turn into something else, such as a quark, the fundamental constitute of protons and neutrons. Shake it again, and the string could vibrate in the mode which describes photons (the quanta of light). Shake it again and it turns into a graviton (the quanta of gravity).

In fact, the collective set of vibrations corresponds to the entire spectrum of known particles. Instead of postulating millions of different particles, one only has to postulate a single object, the superstring. The sub-atomic particles are notes on the superstring. Our bodies are symphonies of strings, and the laws of physics are the laws of harmony of the superstring.

The superstring theory can also explain gravity. When the superstring moves in space and time, splitting and rejoining into other strings, it forces the space-time surrounding it to curl up, just as Einstein's equations predict. In other words, even if Einstein never dreamed up general relativity, we might have discovered it through superstring theory.

CERN

Dimensions in the quantum world: the decay of a kaon leaves tracks behind it

Hyperspace

Superstring theory, of course, has its detractors. Many point out it predicts the universe is defined in 10 dimensional hyperspace, which sounds more like science fiction than real physics. It's indisputable that the universe exists in four dimensions (3 spatial dimensions and one time dimension). Every object in the universe, from the tip of your nose to the farthest star, can be located by giving just 3 co-ordinates (length, width, and height). If we also give the time, then we can describe every event in the universe with just four numbers. For example in New York, we might say to a friend, "meet me at 42nd street and 5th avenue, on the 25th floor, at 12:00." Thus, four numbers (42,5,25,12) completely specifies this event in space-time.

Superstring theory, however, predicts the universe should exist in 10 dimensions, not four. To explain where the other six dimensions went, physicists believe that the universe originally existed in 10 dimensions. However, at the instant of the Big Bang, for reasons we don't understand, six of the 10 dimensions "curled up" and collapsed, while the other four dimensions expanded rapidly. In some sense, our universe expanded at the expense of a twin universe which collapsed down to microscopic size.

Other critics of superstring theory point out that an atom smasher powerful enough to test the superstring theory would have to be the size of the galaxy. The theory is untestable. I think this criticism is a bit silly. Most science is done indirectly, not directly. No one has ever been to the sun or seen a black hole, yet we know what the sun is made of and we have found 20 galactic black holes in space. Similarly, we might be able to detect echos of the 10th dimension from the Large Hadron Collider (LHC), now being build outside Geneva, Switzerland. There is a small hope that we will be able to find "sparticles," (or superparticles) which would represent higher vibrations of the superstrings.

Personally, I think that the problem will be solved by pure mathematics. Once the theory is solved completely, it should yield not just the origin of the universe, but it should also perfectly match the masses of the quarks, leptons, Higgs particles and others.

NASA

Gateway to the future? A black hole.

Time Travel?

Although a quantum theory of gravity has immediate practical application there is one budding area of physics devoted to a novel application of quantum physics: time travel. Oddly enough, Einstein's equations admit the possibility of time travel. But it may take the full power of the unified field theory to calculate whether it's really possible or not. Back in 1949, Einstein's next door neighbor at the Institute for Advance Study, the great mathematician Kurt Goedel, demonstrated Einstein's own equations allowed for time travel. If the universe rotated, and you went around the universe, you could arrive back before you left!

In his memoirs, Einstein pointed out that Goedel's solution could be dismissed on "physical grounds." Our universe expands, it doesn't rotate. But this leaves open the possibility that if the universe rotated, then time travel would be common place!

Since then, literally hundreds of solutions of Einstein's equations have been found which yield time travel solutions. They include:

• An infinite, spinning cylinder. This allows for time travel if one travels around the cylinder.
• Cosmic strings. They allow for time travel if the cosmic strings collide.
• A spinning black hole. This collapses into a spinning ring (not a point), so anyone falling through the ring might actually fall through a wormhole (the Einstein-Rosen Bridge) which, like Alice's Looking Glass, connects two different regions of space and time.
• Negative matter. If enough negative matter were to be found,then it might open up a wormhole large enough so that a trip through time wouldn't be any more jarring than a ride on an airplane.
• Negative energy. Similarly, an intense concentration of negative energy can also open up a wormhole. A crude version of "warp drive" can be obtained if one stretches the space in front of you and compress the space behind you via negative energy.

A Theory of Everything may also help explain the sticky paradoxes found in time travel stories,such as the grandfather paradox (what happens if you kill your ancestors before you are born). Because the entire universe must be quantized, it’s possible the universe splits in half when you alter the past. The "river of time" forks into two different rivers.

If you go back in time to save President Kennedy from being assassinated, you will only save someone else's President Kennedy. Your own past cannot be changed.

NASA

A black hole can be seen by its deflection of starlight

But don't expect any amateur inventor to announce the invention of a time machine anytime soon. Negative matter has never been seen (it falls up, not down) and you need a fantastic amount of both negative and positive energy, called the Planck energy (which is a quadrillion times larger than the energy of the LHC). When Michael J. Fox jumped into his plutonium-fired De Lorean car in ‘Back to the Future’, we can calculate that his plutonium power source does not have enough energy to open a hole in space-time. Even if we could buld one the stability of these time machines is in question. We don't know if they will be stable enough to transport us safely back in time.

Outlook

At present, superstring theory has emerged from being a fringe theory of physics to becoming one of the dominant areas of research, generating tens of thousands of papers. The pace of research is feverish. Edward Witten of the Institute of Advanced Study, one of the principle researchers in string theory, recently made another discovery, that there might even be a hidden eleventh dimension. But the truth is that no one is smart enough to completely solve the theory and settle intriguing theoretical questions about what happened before the Big Bang and if time travel is possible.

Perhaps a young person reading this article will become inspired to solve the greatest problem of fundamental physics!

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