The ties that bind the universe, and their origin in the superforce
It’s a question straight out of Hollywood. Take two imposing but very different beasts, and set them against one another. We have had Alien vs. Predator and King Kong vs. Godzilla; how about Gravity vs. the Strong Force? Or the Weak Nuclear Force vs. Electromagnetism? You won’t be surprised to hear that the answers to such questions are unattainable. The reason for that might come as a surprise, however.
If physicists’ suspicions are proved right, we are not dealing with four forces, but one. Just as a skilled puppeteer can control more than one marionette, there seems to be one superforce behind what we see as the different forces of nature. It could be that gravity, electromagnetism and the strong and weak nuclear forces (see table: How the Superforce Split) were once united.
In the preface to his great work Philosophiae Naturalis Principia Mathematica, Newton wrote that he harbored a deep suspicion that all the phenomena of nature “depend upon certain forces by which the particles of bodies, by some causes hitherto unknown, are either mutually impelled toward each other, and cohere in regular figures, or are repelled and recede from each other.” The forces of nature, in other words, are at the core of physics.
This idea was in marked contrast to what had gone before. The Greek mode of scientific investigation was to assume and respect the role of a “prime mover,” an ultimate cause that also governed notions of justice and morality. Seeking purely physical mechanisms for natural events, without searching for the ethical and moral dimensions they related to, was just not done. But we now know that the forces of physics hold true for everyone but mean nothing in moral terms. Gravity, to paraphrase the Gospel of Matthew, “sendeth rain on the just and on the unjust.”
Not all the forces are so inclusive. The electromagnetic force, for example, only acts between particles containing electrical charge. The strong force only acts over a short range, and between the particles in the nucleus. This raises a question. If they are all so different, why do we believe they are all of the same origin? To answer that, we look first at our ideas of gravity—and where they fall short.
Gravity, to us the weakest of the forces, was the first to be tamed. Newton made the initial move in his universal law of gravitation, offering a formula that described how any bodies with mass would interact. In Newton’s scheme, the pull of gravity accounted for the motions of the planets with an astonishing degree of accuracy. Newton’s gravitational ideas fell short in two ways, however. One was that they offered a description but no explanation of gravity. The other was that they did not describe every facet of how gravity works in the universe: some phenomena defied explanation.
The precession of the perihelion of Mercury is perhaps the most famous example. The perihelion is the point of closest approach in an elliptical orbit. Mercury’s trip round the Sun has just such a point, which moves, or precesses, with successive orbits. The precession is a result of the gravitational pull of the other planets in the solar system, and, in 1845, the French astronomer Urbain Joseph Le Verrier used Newton’s law to work out what it should be. There seemed to be an error. Le Verrier’s calculation missed the observed precession by 43 seconds of an arc per century. Every hundred years, the calculations were out by just one hundredth of a degree, but they were wrong nonetheless.
Fortunately, Einstein’s general theory of relativity provided the required correction. Relativity describes the gravitational fields as arising from the influence of mass and energy on the fabric of the universe: gravity comes from a warping of space–time. It is an astonishingly successful theory, and has never failed an experimental test. Nonetheless, for all relativity’s grand successes in describing what we see in the universe, a proper explanation for the why and how of gravity remains elusive. And until we have one, we cannot be sure that gravity really is so weak—especially when we examine the next force to succumb to science.
Electromagnetism is a much stronger force than gravity. Take two electrons: the electromagnetic repulsion between them is 1043 times larger than their mutual gravitational attraction. But this relative strength may be an illusion. The clue lies in the fact that electromagnetism is a unification of two theories: electricity and magnetism.
In the 1840s, the English physicist Michael Faraday had come up with the concept of a field to explain why iron filings formed lines when scattered around a magnet. To Faraday, these “lines of force” were associated with some physical properties of the space around the magnet. The link to electricity came easily: Faraday also discovered that a changing magnetic field creates an electric field.
But there was a complication. When Faraday’s friend James Clerk Maxwell tried to pull Faraday’s discoveries, and the equations that described them, together, he could only make sense of the result if he added another factor into the mix. It is not enough that changing magnetic fields create electric fields. The converse must also be true: changing electric fields, Maxwell said, must create magnetic fields.
Maxwell’s new equations glowed with a beautiful consistency: electricity and magnetism were two sides of the same coin. This unification led to another beautiful result. When Maxwell looked at the consequence of a changing magnetic field growing an electric field, which grew a magnetic field in turn, and so on for infinity, he realized he had discovered the root of electromagnetic radiation. What’s more, the speed of propagation of this disturbance was the speed of light. Light, it became immediately clear, is an electromagnetic wave.
The significance of this discovery is hard to overestimate. It led to the discovery of the electromagnetic spectrum, to radio waves and gamma rays and everything in-between. It showed how energy could be transferred from point to point through space, doing away with the idea of some ghostly interactions that had no physical source. Perhaps most importantly, it paved the way for an instant revolution in physics. Maxwell’s equations didn’t work when the source of radiation was moving relative to an observer, an observation that prompted Einstein to resolve the anomaly with special relativity (see What is Time?) in 1905. What’s more, the unification of electricity and magnetism was only the start. We now know that another of nature’s forces is delivered from the same hand.
Einstein was highly motivated by the idea of unification. After the success of relativity, he spent his life trying to construct a “unified field theory” that pulled electromagnetism out of the geometry of space–time, just as he had done with gravity. As a result, he and his few followers ignored the development of quantum theory. Einstein had never liked it, and hoped it would go away.
But it didn’t, and explorations of the new theory, along with the fast post-war development of particle physics, pointed to the existence of two new forces: the strong and weak nuclear forces. Einstein never addressed these, but carried on playing exclusively with electromagnetism and gravity. By the time he died in 1955, physics had moved on without him.
It is a particular shame because the weak nuclear force, which acts between the particles of the nucleus—the neutron and the proton—and has an extremely short range of 10–17 meters, is now known to be closely related to the electromagnetic force. We know this because the weak force is responsible for “beta” radiation, where an atom emits an electron or its positively charged counterpart, a positron. The beta-emission of an electron involves a neutron turning into a proton, which can only happen if a “W boson,” the source of the weak force, is emitted first: it is this particle that then decays to produce the electron.
The link was made stronger when we realized that the weak force and the electromagnetic force result from the same process, known to physicists as “spontaneous symmetry-breaking.” This is rather like what happens when you assemble a crowd of strangers in a room. As they get talking, some will find common points of interest in one area, some in another, and, given enough time, they will form into distinct groups that end up talking about different things. Initially, there was “symmetry”: there was nothing to distinguish the strangers from one another, no way to group them. But, as they talked, that symmetry was spontaneously broken, and groups formed.
In the 1960s, Steven Weinberg, Sheldon Glashow and Abdus Salaam showed that the same process of spontaneous symmetry breaking created the electromagnetic and weak forces from another force. They named it the “electroweak force,” and suggested that it had only existed in its unbroken form in the high-energy conditions at the beginning of the universe. The work was a masterstroke, and won the trio the 1979 Nobel Prize in physics. The theory made specific theoretical predictions: the existence of the W and Z bosons, for example, which were found, complete with all the assumed characteristics, in 1983.
Perhaps most important of all, though, this breakthrough suggested that the seemingly different forces might not be all that different at heart, even though the weak force acts over the shortest ranges, and on uncharged neutrons, whereas the electromagnetic force acts over enormous distances, and on charged particles. In fact, not only can we not say which is the stronger force, we suddenly find ourselves facing a shocking question.
If the electromagnetic and the weak forces were once the same force, who is to say that spontaneous symmetry breaking didn’t give rise to all of the forces of nature? Perhaps we can’t say one force is the strongest, simply because they are all manifestations of one ancient superforce. In order to explore that possibility, we have to consider the remaining element: the strong nuclear force.
Just as the weak force had to exist in order to explain beta-decay, the mutual repulsion between protons in a nucleus made the strong force a necessity, otherwise the nucleus could not hold together. Strong is an appropriate name for the force: typically, it appears to have a hundred times the strength of the electromagnetic force that would tear the nucleus apart. Measuring its strength was the easy part of taming the strong force, however; explaining its existence was much more difficult. It’s not enough to know that such a gargantuan force is the only way that a nucleus can hold together. What creates it?
The ideas behind this strong force were developed in the early 1970s. It was known that quarks make up the protons and neutrons in the nucleus. Each quark has a characteristic that physicists call its color. For this reason, the theory tying the strong force to the quarks is called “quantum chromodynamics,” or QCD. According to QCD, the strong force binds quarks together using an interaction that, unlike the electromagnetic and gravitational forces, does not diminish with distance. The force grows stronger as quarks move further apart just as if they were connected by a spring.
This peculiar property, which emerges from the equations of QCD, gives the strong force the power to bind quarks together wherever they might be found. Its nature is borne out by the fact that, despite many searches, we have never found a free-roaming lone quark. QCD says that the strong force is created by a boson known as a gluon. Gluons were seen in experiments for the first time in 1979. The theory was already on a firm footing by that time, however: when quarks, complete with their predicted characteristics, began to be spotted in particle accelerators in the late 1960s and early 1970s, QCD was considered a proven theory.
But what really excited physicists was the fact that QCD is built on the same symmetry-breaking idea as the electroweak force. It seemed entirely plausible that they could be closely related—and pulled together into one description of the behavior of matter: the “grand unified theory,” or GUT. And here is where the quest hits the skids. After three decades of searching, we are still not sure if the strong force really is from the same stable as the electroweak force.
The problem is that unification is far from straightforward. What is required is another symmetry, like the strangers gathered in a room scenario—but this time there are even more of them. Somehow this bunch of indistinguishable strangers has to spontaneously break up in a way that describes five different types of particles—the three differently colored quarks and the electron and its associated neutrino—and three forces.
The unification is almost impossible to recreate on Earth: reaching the energy for this symmetry breaking requires particle accelerators 100 billion times more powerful than the Large Hadron Collider (LHC), our most powerful atom-smasher. However, there are other ways to test the idea. According to any grand unified theory, quarks must be able to change into electrons and neutrinos, and physicists’ best-looking candidate for this grand theory (known as SU(5) because of the five particles that arise from it) has just such a process up its sleeve. It involves the proton in a kind of radioactive decay, and makes a prediction about how often that will happen.
It’s just a shame it gets it so very wrong. The theory says a proton will last approximately 1033 years before decay. About a quarter of a century ago, physicists built huge tanks of highly purified water surrounded by detectors that would register such an event occurring. From the theory, and the number of protons in their tanks, they expected a few decays per year. So far, though, they have seen nothing. We still have another shot, though—and this one might be vindicated in the LHC. It is called “supersymmetry.”
Supersymmetry arises from the fact that physicists split particles into two camps: the fermions, such as the electron and the quarks, which make up matter; and the bosons, such as the photon and the gluon, which create the forces. These two different kinds of particles follow two different sets of rules. And supersymmetry, or SUSY, says each one has a “superpartner” from the other camp that will behave the same in any experiment.
That is possible because the essential difference between fermions and bosons comes from the quantum property known as spin. Bosons have integer spin—1, 2, 3 and so on—while fermions have spins that are half-integers: 1/2, 3/2 etc. SUSY involves applying a kind of perspective change, something akin to looking at a clock from the front or the back. This change of view alters quantum spin (just as the sense of rotation of the clock’s hands is different when viewed from the back), but not other qualities, such as electric charge or quark color.
It might sound like a convenient fiction, but it is a highly respected mode of thinking that stands amongst the best ideas in physics. The burning question, of course, is whether it is true. Besides spin, one other property of the superpartner particles is changed: their mass. They are much, much heavier than the set of particles that we are familiar with. That means that, thanks to E = mc2, they will only exist at high energies. Thankfully, however, the LHC’s collision energy of 14 TeV should be high enough to see the lightest of them, which are thought to come into play at around 1 TeV.
Though it sounds promising, these particles are still difficult to detect. They hardly interact with normal matter, and will fly out of the machine almost without trace. That means the only hint of supersymmetry might be some energy missing from the LHC’s detectors. Since other theories suggest that some normal particles might be disappearing into other, “hidden” dimensions of reality, it’s a recipe for false positives and missed sightings.
If we do see unquestionable sightings of supersymmetric particles, though, we can feel sure that the grand unified theory of the forces of nature is on solid ground. It will be entirely reasonable to assume that the strong, weak, and electromagnetic forces arise from a common source, a “prime mover,” as the Greeks would say. There is just one fly in the ointment, though. What about gravity? Is that also part of the unification, or is it a separate entity? If we can’t say there is a strongest force, can we at least say that gravity is the weakest?
Gravity certainly is weak. When we draw the likely unification diagram for the forces, showing the energy where they (might) unite, it’s hard to put gravity on there, unless your graph is bigger than the known universe. While the other forces converge from a factor of 100 or so apart, gravity is simply off the scale. But there is a get-out. It is enormously technical, but, boiling it down, it says that the gravitational interaction depends on mass, which is proportional to the energy involved. In the SUSY picture, when considering the high-energy conditions of unification, gravity drops into the picture at an alluring scale—almost, but not quite, where the other forces unite.
It’s not a fully convincing answer, but it does suggest that gravity and all the other forces of nature might spring from one ultimate force. This superforce only existed in the first moments after the universe was formed. In that situation, asking which of the forces is stronger is like asking which of the particles is more particle-like. Though different, they are all aspects of one characteristic. Gravity vs. electromagnetism just won’t work; they appear to be fighting from the same corner.