麻豆传媒 tackles eight of the deepest challenges faced by science 鈥� from reality and consciousness, to free will and death, in The Big Questions special features.
It鈥檚 all-out war. The hostilities have begun. With guns blazing, daily salvos are being fired by both sides. Welcome to the conflict raging within the rarefied world of theoretical physics, where a civil war has erupted over string theory and a theory of everything.
The stakes are high. A genuine unified field theory that can unite all the physical laws of the universe into a single theory would be the crowning achievement of 2000 years of investigation into the nature of the matter. This is the holy grail of physics, and would be a landmark in human intellectual thought. It would, in the words of Einstein, allow us to 鈥渞ead the mind of God鈥�.
Advertisement
The focus of the ongoing civil war is string theory, which claims to be able to unify all the physical forces into a single coherent picture. It says that all the blizzard of particles we see in nature 鈥� the quarks, electrons, neutrinos and the rest 鈥� are nothing but vibrations of tiny pieces of superstring, like musical notes on a violin string.
On one side, leading the charge, are the cynics. They claim that no matter how many glossy, slick TV documentaries or breathless articles appear on string theory, the fact is that a direct test is impossible, period. Definitive proof would require an atom smasher the size of the Milky Way. Furthermore, they claim that the brightest minds of the entire planet have been suckered into working on a theory that may turn out to be a wild goose chase, a theory of nothing. They are like children following the Pied Piper.
The defenders of the faith fire back. They cite a series of experiments that may in the coming years probe the periphery of string theory, through cosmic rays and the Large Hadron Collider (LHC) near Geneva, Switzerland. The true believers hope to find evidence of new particles and mini black holes that may vindicate parts of the theory.
The defenders also note that string theory has no rival. They claim that, for all the bellyaching, the cynics have yet to propose a single serious alternative of their own. They are suffering from sour grapes. String theorists say to the critics it is time to put up or shut up.
So who鈥檚 right? Actually, both have a legitimate point of view. But far from signalling a collapse in physics, this debate is actually rather healthy. It鈥檚 a sign of the vitality of theoretical physics that people are so passionate about the outcome. Science flourishes with controversy.
It鈥檚 easy to see why emotions are so feverishly high. The search for a theory of everything has fired up the imagination of mystics and scientists since the dawn of recorded history. Socrates himself once said: 鈥淚t seemed to me a superlative thing 鈥� to know the explanation of everything, why it comes to be, why it perishes, why it is.鈥�
Around 500 BC, the Pythagoreans analysed the vibrations of a lyre string and unravelled some of the mathematics behind music and harmony. They then made the bold conjecture that the universe itself could be explained purely in terms of the mathematics of musical harmony. And nearly all the giants of 20th-century physics have proposed their version of a unified field theory. As Freeman Dyson said, 鈥淭he ground of physics is littered with the corpses of unified theories.鈥�
In hindsight, it is easy to see why all these attempts at unifying all the forces have failed. Remarkably, general relativity and the quantum theory together represent the sum of all physical knowledge at the fundamental level. One huge piece of the jigsaw puzzle, the nuclear force, was only understood in the last few decades.
Any unified theory must reconcile gravity with quantum theory, whose most advanced version is the standard model of particle physics, with its bizarre collection of quarks, leptons, gluons and bosons. But that鈥檚 not all. Any new theory must yield finite answers; its mathematics must not produce any troubling infinities.
Satisfying these two deceptively simple criteria has been devilishly difficult. They quickly rule out all previous attempts at unified field theories. So far, the only theory that fulfils them is string theory. The standard model of particles simply emerges as the lowest vibration of the superstrings. And as the strings move, they force space-time to curl up, precisely as Einstein鈥檚 general relativity predicts. Hence, both theories are neatly included in string theory. And unlike all other attempts at a unified field theory, it can remove all the infinities.
Curiously, string theory does much more. Much, much more. It has trillions upon trillions of solutions that don鈥檛 look anything like our universe. There is a multiverse of solutions. Many of these parallel universes seem uncannily like our own, but a vast number differ sharply from ours, with a different list of subatomic particles.
So string theory is temporarily stuck, awaiting the next breakthrough. At present, no one knows how to find the solution that corresponds to our specific universe among the many parallel universes string theory contains. If this is not possible, then the theory has no predictive power.
This recent lull has opened the door to the cynics. The latest volley was launched by Peter Woit of Columbia University in New York. He alleges that the finest young minds in physics are blindly following this string fad, and he fears it could derail an entire generation of physicists.
I smile when I hear this criticism. You see, there is truth in what he says, but it is nothing new; physics has always been a victim of fads, fashions and bandwagons. We physicists don鈥檛 like to admit it, but in the cutting edge of physics, as in all human endeavours, it鈥檚 always hard to get a job if you are not working on the latest 鈥渇ashion鈥�.
Ironically, the most pathetic victims trampled by this herd mentality have been string theorists themselves. I started working on string theory in 1969, a year after it was born. By 1974 it had been thoroughly crushed by quantum chromodynamics, the theory of quarks. I remember the crash quite well. Droves of physicists left string theory, or were thrown out of work. We joked that the place to find string theorists was on the unemployment line. Sadly, many of my friends did not survive the debacle: they either switched fields or were thrown out of physics entirely. For 10 dark years, string theorists wandered in the wilderness. Only the true believers, those willing to suffer severe deprivation and humiliation, kept the home fires burning.
My personal point of view is that we live in a spoilt society, always demanding immediate results. We pop pills, push buttons, flip channels and demand instant gratification. The media whips this up, lavishing praise when you are on the rise, and dumping on you when you are down.
Fortunately, science is not determined by popularity contests, media hype or blogs. Science is not a beauty contest. We forget that real advances in physics may take years or even centuries to gestate. For example, the theory of black holes was introduced in 1783 by John Michell in an article in Philosophical Transactions of the Royal Society. He calculated that there were 鈥渄ark stars鈥� so massive that 鈥渁ll light emitted from such a body would be made to return to it by its own proper gravity鈥�.
Michell鈥檚 dark star was ridiculed for centuries and Arthur Eddington even said that there should 鈥渂e a law of nature to prevent a star from behaving in this absurd way鈥�. Since a dark star is invisible, by definition, it was impossible to ever verify Michell鈥檚 theory. This was the first theory in science that appeared, by its very nature, hopelessly untestable. It would take 200 years for Michell鈥檚 鈥渦ntestable鈥� theory to be vindicated by space telescopes, and then only indirectly.
We forget that most discoveries are done indirectly, not directly, so we don鈥檛 need atom smashers the size of the Milky Way galaxy to prove string theory. In 1825, the philosopher Auguste Comte challenged the world of science, stating that there was something that would forever be beyond our grasp: what the stars are made of. Just a few years later, Joseph von Fraunhofer and others used atomic spectra to determine that the sun was made of hydrogen.
鈥淢ost discoveries are made indirectly. we don鈥檛 need an atom smasher the size of the milky way to prove string theory鈥�
Although no one has ever visited the sun, we now know what the sun is made of by analysing these indirect 鈥渆choes鈥� of sunlight. Similarly, no one has ever visited a black hole, but we can indirectly analyse the material falling in.
Furthermore, many other 鈥渦ntestable鈥� theories ultimately become testable. The physicist Wolfgang Pauli introduced the concept of the neutrino in 1930, a particle so elusive it could pass through a block of solid lead the size of an entire star system and not be absorbed. Pauli said, 鈥淚 have committed the ultimate sin; I have introduced a particle that can never be observed.鈥� It was 鈥渋mpossible鈥� to detect the neutrino, so it was considered little more than science fiction for several decades. Yet today we can produce beams of neutrinos.
But today, when we look at the standard model, even the critics are impressed by one tantalising, intriguing fact: while all the forces of nature have different strengths at the energies we encounter every day, the standard model predicts that they have the same strength at one very much higher energy. This suggests that a single 鈥渟uperforce鈥� existed at the beginning of time. String theory gives us the most compelling theory of this superforce, which reduces to vibrations of superstrings.
Nobel laureate Steven Weinberg likens this to the existence of the North Pole. For centuries, early explorers had maps of the Earth with a huge hole at the top. Compass needles all converged to a mythical point called the North Pole, but no one had ever seen it. It took centuries to finally prove its existence. So instead of waiting for galactic-sized atom smashers to test string theory, we should examine experiments in which indirect tests of string theory will soon be conducted.
Among them is the LHC outside Geneva, which will be turned on next year. It is perhaps capable of producing 鈥渟particles鈥�, or super particles, which represent the higher vibrations predicted by superstring theory as well as other supersymmetric theories.
Then in 2015, the LISA gravitational wave observatory will be launched into space, where it will be capable of detecting space-time shock waves released at the instant of the big bang. There is hope that LISA will be sensitive enough to test several 鈥減re-big bang鈥� theories, including versions of string theory.
Meanwhile a number of labs are investigating the presence of higher dimensions by looking at deviations from Newton鈥檚 famed inverse-square law at the millimetre scale. If there is a fourth spatial dimension at these scales, then gravity should fall by the inverse cube, not inverse square. The latest version of string theory predicts up to 11 dimensions.
Ongoing experiments at many labs may eventually detect and study dark matter in the laboratory, since the Earth is moving in a cosmic wind of the stuff. String theory makes specific, testable predictions about the physical properties of dark matter because it is probably a higher string vibration. And various other experiments hope to detect the presence of mini black holes and other strange objects by analysing anomalies in cosmic rays, whose energies can easily exceed those attainable in the LHC. Between them, cosmic ray experiments and the LHC will open a new, exciting frontier beyond the standard model.
But what happens if the next generation of experiments is still not sensitive enough to indirectly prove the theory? All is still not lost.
One alternative is to assume that any theory of everything will have to be supplied with one more bit of information: the initial conditions. To understand this idea, take Maxwell鈥檚 theory of electromagnetic waves, which has an infinite number of solutions. In order to describe a laser or the radiation inside a microwave oven, you have to specify the 鈥渂oundary conditions鈥� 鈥� the system鈥檚 initial state 鈥� by hand. Similarly, since each solution of the string equations is an entire universe, one might have to specify the initial state of the universe at the instant of the big bang. Then string theory will describe how this universe will evolve into our present-day universe.
This does not leave everyone happy. As the mathematician Alan Turing once said, 鈥淪cience is a differential equation. Religion is a boundary condition.鈥�
I have my own point of view. To me, the fundamental problem is that string theory is not yet in its final form. It was discovered quite by accident in 1968, and it has been evolving backwards ever since. 鈥淪tring theory is 21st-century physics that fell accidentally into the 20th century,鈥� quips Edward Witten of the Institute for Advanced Study in Princeton.
Unfortunately, the necessary 21st-century mathematics has not yet been discovered. The theory is smarter than we are. String theory has evolved backwards. It was accidentally discovered as a full-blown quantum theory, and only later did physicists reveal the classical theory behind the model, which was a vibrating string. Now we are trying to find the fundamental principle behind string theory.
I like to compare it to wandering in the desert, and stumbling over a tiny pebble. When we push away the sand, we find that this 鈥減ebble鈥� is actually the tip of a gargantuan pyramid. After years of excavation, we find wondrous hieroglyphics, strange tunnels and secret passageways. Every time we think we are at the bottom stage, we find a stage below it. Today, finally, we think we are at the very bottom, and can see the doorway.
One day, some bright, enterprising physicist, perhaps inspired by this article, will complete the theory, open the doorway, and use the power of pure thought to determine if string theory is a theory of everything, anything, or nothing.
Read more: The biggest questions ever asked
