Chapter 19
In This Chapter
Revisiting Einstein’s view of the universe
Realizing that expansion is accelerating
Resurrecting Einstein’s cosmological constant
Trying to unify physics
Pursuing Einstein’s dream
A s I explain in Chapter 18, Einstein developed a theory of the universe using the field equation of his general theory of relativity. His initial calculations indicated that the universe is either expanding or collapsing, which Einstein considered a mistake. His model was of a static, finite universe — after all, no one had observed the universe expanding.
To make his calculations fit the observations of the time, Einstein introduced a correction factor called the cosmological constant. Unfortunately for Einstein, in 1929, astronomer Edwin Hubble showed that the universe is expanding. Einstein should’ve trusted his field equation.
How is it possible that the greatest mind of the 20th century could make what he later called his “greatest blunder”? In this chapter, I show you why Einstein’s blunder turned out to be not such a blunder after all. I also explain how Einstein’s work led physics and astronomy throughout the 20th century and is still guiding these fields today.
Reevaluating Einstein’s Universe
In recent years, Einstein’s cosmological constant has been resurrected by scientists to help them understand new, puzzling findings about the behavior of the universe. It turns out that Einstein may have been right all along. To see why, in this section I take a closer look at how Einstein introduced his cosmological constant into his equations for the universe.
Energy creates gravity
In the universe according to Isaac Newton, planets, stars, galaxies, apples, and baseballs feel a force, gravity, whose strength depends on their masses and their separation from each other (see Chapter 4). In Einstein’s universe, the strength of the gravitational field isn’t just due to mass and distance, but also to energy and pressure.
Einstein realized that energy contributes to gravity. Why? Because E = mc2 says so (see Chapter 11). A bird in flight has slightly more mass than when it is perched on a tree limb. The tiny amount of mass increase comes from the energy of the bird’s motion. Any form of energy contributes to the total mass. If the bird basks in the sun, its mass will also increase. The Earth and the warm, flying bird will attract each other with a slightly larger force than when the bird is on a shaded branch. (Not that you could measure the additional force — the increase would be about a one-hundredth of a trillionth of a pound!)
Negative pressure creates antigravity
According to the theory of general relativity, pressure also creates gravity, although the contribution of pressure is usually very small. How small? Consider this: The mass of the air in a room — and its contribution to gravity — is comparatively very small. And the pressure of the air contributes less than a hundred billionth of the small amount that the mass contributes.
Normally, pressure pushes outward, like the air pressure in your car’s tires. What would happen if pressure were negative? In general relativity, negative pressure creates repulsive gravity — antigravity!
In Einstein’s universe, planets, stars, and galaxies attract each other with attractive gravity. Like in Newton’s universe, this attractive gravity comes from the masses of the planets, stars, and galaxies. But in Einstein’s universe, the planets, stars, and galaxies attract each other with additional gravity from all the forms of energy that they have and from the positive pressures that they exert.
What about the negative gravity? In 1917, when Einstein was developing his model, no one knew about negative pressure. His equations told him that negative pressure was a possibility. Without negative pressure, which produces repulsive gravity, Einstein’s universe — like Newton’s — could collapse.
Einstein developed his model of the universe 12 years before Hubble’s discovery of the expansion of the universe (see Chapter 18). The observed universe wasn’t expanding and certainly wasn’t collapsing. It was static. Einstein saw that his equations allowed for repulsive gravity. If the universe had the right amount of this antigravity, it would counterbalance the pull of gravity, preventing the universe’s collapse and maintaining its perfect, static equilibrium.
That’s what the cosmological constant accomplished. By adding this term, Einstein’s universe became filled with this exotic form of energy that created antigravity and maintained the universe in balance. For Einstein,
The cosmological constant exerts a repulsive gravitational force; it creates antigravity.
If Einstein hadn’t included the term in his equations, he would’ve realized that to prevent the gravitational collapse, the stars in the universe had to be moving away from each other. He would’ve predicted the expansion of the universe 12 years before Hubble discovered it.
As I explain in Chapter 18, Hubble showed that it isn’t the stars that are moving away from each other, but the galaxies — the island universes that he had discovered. The stars themselves stay more or less at the same distances from each other inside their galaxies.
Exploring the Runaway Universe
With the discovery of the expansion of the universe in 1929, there was no need for the cosmological constant, and Einstein finally gave it up. As Einstein wrote on a postcard that he sent to the German mathematician Hermann Weyl, “If there is no quasi-static world, then away with the cosmological term.”
Attention shifted to a new question: Is the universe going to expand forever, or is it going to slow down, stop, reverse course, and collapse? According to the theory of general relativity, the answer depends on the density of matter and energy in the universe — on how much matter is out there.
Discovering dark matter
Just two years after Hubble’s discovery of the expansion of the universe, astronomer Fritz Zwicky of Caltech was observing a cluster of thousands of galaxies about 370 million light-years away. He discovered that the outer galaxies in the cluster were moving too quickly for his taste. His calculations showed him that at the speeds he was measuring, these outer galaxies should be thrown away from the cluster; yet, they remained together. To keep them together required gravitational attraction from about 100 times more galaxies than he was seeing. And these extra galaxies weren’t there. Zwicky concluded that additional mass may exist that doesn’t give off light.
In the 1970s, Vera Rubin and Kent Ford at the Carnegie Institution in Washington, D.C., observed the motions of the stars in many galaxies. They concluded that these motions could be explained only if there was a huge halo of invisible dark matter surrounding these galaxies.
Now, more than three decades later, astronomers think that the universe is filled with dark matter, an invisible form of matter that can be “seen” only through its gravitational pull on the visible matter that makes up the stars and galaxies.
What is dark matter? No one knows. What scientists do know, however, is that it isn’t your ordinary, run-of-the-mill type of matter. Dark matter isn’t made of protons, neutrons, electrons, or any of the known building blocks of ordinary matter.
Although they don’t know what dark matter is, astronomers have been able to figure out with a great deal of precision that there is as much as five times more dark matter than ordinary matter in the universe. What we see with our eyes and telescopes isn’t everything that’s out there.
Speeding away: Accelerating expansion
For astronomers and physicists, 1998 was a great year. Two independent groups of researchers, the Supernova Cosmology Project and the High-z Supernova Search Team, came out with the same result: The expansion of the universe is speeding up.
Saul Perlmutter of Lawrence Berkeley National Laboratory and Brian Schmidt of the Australian National University headed the two research teams. They and their researchers were actually trying to measure the deceleration of the universe, not its acceleration. They were doing so using what are known as supernovas, tremendous explosions of stars that are usually much more massive than our sun (see Chapter 13).
The supernova standard candle
Supernovas are classified into types according to their spectral features. One particular type of supernova, called Type Ia supernova, comes out of the explosion of a star the size of the sun that’s part of a binary system: a situation where two nearby stars orbit around each other. When this star uses up its nuclear fuel, it begins to pull matter from its companion, growing in size until it reaches about 11/2 solar masses. (In other words, it has one and a half times the mass of our sun.) At this point, called the Chandrasekhar limit, the pressure and temperature inside the star are so large that they trigger a nuclear explosion.
Because these explosions always occur exactly when the star reaches the Chandrasekhar limit, these supernovas shine with the same brightness. And they are bright. From Earth, you can see them halfway across the visible universe. The Hubble Space Telescope can see them when they are much farther away.
Why is this so important? Because it provides astronomers with what they call a standard candle, a way to measure the distances to galaxies. A standard candle is a star that always burns with the same brightness. It’s like having known light sources spread throughout the universe. If you know, for example, that all cars have 100-watt headlight bulbs, you can figure out how far the cars are from you by measuring how dim their headlights look from your location.
A long acceleration
Using this wonderful yardstick, Perlmutter and Schmidt’s teams discovered that, for the past 5 billion years, the expansion of the universe has been speeding up.
In 2002, NASA outfitted its Hubble Space Telescope with a new instrument, the Advanced Camera for Surveys. This camera turned the Hubble into a supernova-hunting machine. With this new capability, Adam Riess of NASA’s Space Telescope Science Institute confirmed that, early in the history of the universe, its expansion was slowing down. But about 5 billion years ago, the slowdown stopped for some time, after which the universe’s expansion started its current acceleration.
How can this happen? The accepted model of the universe, which was based on general relativity — essentially Alexander Friedmann’s model (which I describe in Chapter 18) — says that the matter, energy, and pressure in the universe generate attractive gravity that pulls in and slows down the expansion. How can the expansion speed up? What’s worse, why did it change from a well-behaved expansion that slowed down to a runaway expansion?
Reviving the Cosmological Constant: Einstein Was Right After All
Something is pushing the galaxies apart now. Some source of energy is fueling the expansion of the universe. And it permeates the universe. What can it be?
It turns out that the best way to explain the runaway expansion of the universe is with Einstein’s cosmological constant. The cosmological constant fills the universe with repulsive gravity — just what’s needed to speed up the expansion.
The key to how the cosmological constant works is the contribution to gravity from negative pressure. The negative pressure, which creates repulsive gravity, is constant throughout the universe; it doesn’t decrease with distance (like the part of gravity that comes from ordinary matter does). And it doesn’t need the presence of matter or energy to operate. The cosmological constant operates in empty space; it’s a property of empty space.
Tracking changes in gravity
The rate of expansion of the universe depends on a battle between two giants: the attractive and the repulsive parts of gravity (see Figure 19-1). As the universe expands and the galaxies move farther apart, the attractive part of gravity decreases. However, the repulsive part of gravity is the same throughout the universe and stays fairly constant even with the expansion. Which part wins out depends on how close the galaxies are to each other.
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Figure 19-1: The attractive part of gravity decreases with expansion, but repulsive gravity stays fairly constant. |
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When our universe was starting out, its galaxies were closer together than they are now. As a result, regular attractive gravity was much stronger than repulsive gravity and held the galaxies more tightly together, slowing the universe’s expansion (see the left side of Figure 19-1).
But after some time — about 5 billion years — the galaxies were far enough apart for the attractive part of gravity to decrease and match the strength of the repulsive gravity. At this point, the two canceled each other out; galaxies no longer felt gravitational force of any kind from other galaxies (see the middle diagram in Figure 19-1). But, even with no gravitational force, the galaxies kept moving away from each other, according to Newton’s first law (see Chapter 4).
Gravity didn’t disappear within galaxies, only between them. Attractive gravity from the mass and energy of objects depends on distance. And stars within a galaxy are close enough to each other that attractive gravity is stronger than repulsive gravity. The galaxies stay together with their stars bound to each other by gravity, as always.
As the galaxies continue to move apart, the attractive part of gravity decreases even more, and the constant repulsive gravity wins out, speeding the expansion of the universe (see the right part of Figure 19-1). This is the epoch that we are living in now.
In 1998, after their astonishing observations, Perlmutter and Schmidt suggested that Einstein may have been right: The universe does have a cosmological constant. It’s what drives the current acceleration and what caused the earlier slowdown. Michael Turner, of the University of Chicago, proposed a new name for the cosmological constant: dark energy.
Taking a baby picture of the universe: Space is flat!
In 2001, another remarkable result came in from a NASA satellite, the Wilkinson Microwave Anisotropy Probe or WMAP. This NASA mission is taking a baby picture of the universe, measuring the leftover heat from the big bang, the origin of the universe. The WMAP satellite discovered that the universe is flat.
How is it possible to take a baby picture of the universe? The key is to realize how the universe began. A hundred thousand years after the universe began its expansion, protons and electrons came together for the first time to form atoms. Because atoms are neutral, photons (the bundles of energy I discuss in Chapter 16), which interact only with charged matter, started their independent existence, separate from matter. Two hundred thousand years later, the universe became transparent to light.
The lifetime of a photon is infinite. The photons from those early days of the universe have traveled through the universe undisturbed, giving us the baby picture that the WMAP is trying to snap. There are about 400 million of these original photons for every cubic meter of space today. (Here’s a fun fact for your next party: If you tune a television set between channels, you see static; about 1 percent of that static is caused by the original photons.)
Today, 14 billion years later, these photons form the Cosmic Microwave Background, or CMB as scientists call it. When the photons first started their journeys, the temperature of the universe was about 3,000 kelvins. (As I explain in Chapter 5, a kelvin is the unit of the absolute scale of temperature.) This temperature was extremely uniform throughout the universe. Since then, the photons have been cooled as the universe expands. Today, the temperature of the CMB is only 2.7 kelvins, and it’s still extremely uniform.
The results from WMAP show the small temperature changes of the CMB. The light captured in the baby picture image is from a region that’s about 13 billion light-years from Earth. The image features patterns that represent tiny temperature differences, changing only by millionths of a degree. By measuring the distance between two regions and knowing the distance that the photons have traveled since their birth, scientists have been able to draw a big triangle in the sky with sides joining those two regions and the Earth. The angles of this triangle add up to 180 degrees, with a high precision. And that means that space is flat, not curved like everyone had been assuming for almost a century.
This finding is amazing. And there’s more. If space is flat, the density of the universe is equal to the critical density (the middle value between an open and a closed universe, as I explain in Chapter 18). This density is a very small quantity. Write 28 zeros after the decimal point, and place a 1 in the 29th position. That’s the fraction of a gram of matter (and energy) in every cubic meter of space.
From the discovery that space is flat, scientists have been able to use WMAP data to figure out the breakdown of matter in the universe. Their calculations show that ordinary matter — the atoms that make up the sun, the Earth, living things, stars, galaxies . . . everything we see in the universe — adds up to only 5 percent of the critical mass. (This result agrees very well with other calculations based on the nuclear processes that took place in the early universe.)
The data from WMAP also shows that dark matter accounts for 25 percent of the mass of the universe. That leaves 70 percent of the mass of the universe unaccounted for. The only candidate for the missing 70 percent today is dark energy, Einstein’s cosmological constant. Figure 19-2 illustrates the breakdown of ordinary matter, dark matter, and dark energy.
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Figure 19-2: Ordinary matter accounts for only 5 percent of the total mass of the universe. The rest is invisible dark matter and dark energy. |
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Think back to the Type Ia supernova studies I discuss in section “Speeding away: Accelerating expansion.” These studies indicated that the universe’s accelerated expansion could be explained by a push from repulsive gravity. More specifically, they indicated that dark energy (the cosmological constant) needed to contribute about 70 percent of the universe’s critical density. That’s a remarkable result. A completely independent experiment is telling us that dark energy accounts for the exact amount that’s not accounted for already by dark and ordinary matter!
In science, nothing is ever proven. All scientists ever hope to see is that their predictions agree with observations. Right now, everything seems to be fitting together. The dark matter studies, the supernova studies of the accelerated expansion, and the WMAP data all point to the same conclusion: The universe has a flat geometry and is made up mostly of stuff we can’t see.
What Is the Cosmological Constant?
What is dark energy, anyway? What is this exotic energy form that Einstein introduced in 1917?
Remarkably, the answer doesn’t come from relativity but from the other branch of physics that Einstein originated, quantum physics. And the answer that quantum physics gives us is stranger than anything else I’ve discussed so far in this book. Dark energy, the cosmological constant, is the energy of the vacuum, the energy of nothing.
Creating particles out of the blue
This strange idea of the energy of the vacuum is at the very heart of today’s physics. But today’s physics has its roots in Einstein and the theories that were developed with his ideas, like quantum physics. When quantum physics was developed in the 1920s, physicists had a powerful tool that could answer questions like
What is a force?
How does the force between two electrons work?
How does the electron know that there is another electron nearby so that it can repel it?
With general relativity, Einstein provided the answer to a question similar to this last one. The Earth knows that the sun is nearby because the sun curves the space around it. The Earth is moving in this curved space as best it can. It tries to move along a straight line, and the straight line in this curved space is the almost circular orbit around the sun. The force becomes geometry.
What about electrons? How do they know of each other’s presence? In 1927, the English physicist Paul Dirac combined special relativity, James Clerk Maxwell’s electromagnetism, and quantum physics, and he came up with quantum field theory. Two years later, U.S. physicists Richard Feynman and Julian Schwinger and, independently, Japanese physicist Sin-Itiro Tomonaga developed Dirac’s theory into a beautiful new quantum theory of the electron that’s called quantum electrodymamics or QED. This theory tells us what a force is and how an electron knows that there is another electron nearby.
According to Maxwell, when an electron accelerates, it radiates an electromagnetic wave (see Chapter 6). That’s how your cordless phone at home works: The base unit accelerates electrons back and forth along the antenna, and the accelerated electrons generate the electromagnetic wave that travels to your receiver at the speed of light. Quantum physics says that this electromagnetic wave consists of photons. Accelerating electrons produce photons.
That’s how electrons know of each other’s presence: They send photons back and forth. If two traveling electrons meet each other, they exchange photons. The next questions you may ask are
Where do these photons come from?
Do the electrons carry photons around in case they meet other electrons?
How do they know how many electrons they are going to encounter?
The push of the vacuum: The Casimir effect
In 1948, the Dutch physicist Hendrik Casimir came up with a clever idea to demonstrate that space is filled with virtual particles. He proposed placing two uncharged metal plates in an empty chamber. When the plates are moved very close to each other but not touching, the spacing between the plates excludes virtual particles larger than a particular wavelength. Because particles with longer wavelengths can still appear outside the plates, there are more particles outside than inside. The imbalance should push the plates together.
When Casimir first proposed it, his idea was just an idea — a thought experiment — because the technology of the 1940s and early 1950s wasn’t up to the task. But in a few years, several scientists attempted the experiment, getting encouraging results. In 1977, an experiment done at the University of Washington confirmed the Casimir effect, as it’s now called.
The answers to these questions may seem even more strange. Electrons don’t carry photons with them. According to QED, the photons are created out of nothing. Their existences are allowed by Heisenberg’s uncertainty principle (see Chapter 16). The uncertainty principle says that you can’t determine with complete accuracy the position of an electron and at the same time measure where and how fast it’s going. It also says that you can’t measure simultaneously and with complete accuracy the energy of a particle and for how long the particle has that energy.
And this uncertainty is the clue to everything. It turns out that Heisenberg’s uncertainty principle allows things to happen under the radar. You can borrow as much energy as you need and, as long as you pay it back within the short period of time allowed, you can use it for anything. If an unscrupulous kid “borrows” $100 from his parents, quickly invests the money in fast-rising stock, sells the stock at $110, and returns the $100 before his parents notice, he makes $10 from nothing, from money he didn’t have. And his parents are none the wiser.
This illegal scheme is allowed in the quantum world. You “borrow” energy to create a particle, for example. The energy to create the particle comes out of nowhere, out of the blue. (Actually, it comes out of the black, the vacuum.) You borrow it, use it, and create your particle. The particle does its thing, lives out its life, and dies. You recoup your energy and give it back to the vacuum. If all of that happens within the time allotted, everything is fine.
Particles that are created out of the vacuum are called virtual particles:
A virtual particle is one that’s created out of nothing and that exists only for the duration allowed by Heisenberg’s uncertainty principle.
Virtual particles exist for some fleeting moment and disappear. Real particles, on the other hand, exist for a long time or forever. Real photons are eternal.
Filling up the vacuum
As you can see, the conclusions that QED arrived at are very strange. Yet, the theory works — better than any other theory that humans have ever created. QED is the most accurate theory ever. Its predictions have been found valid to one part in a billion!
QED tells us that the vacuum isn’t really empty. The vacuum is filled with these virtual particles that suddenly appear, only to disappear almost immediately. The vacuum, teeming with these virtual particles, has energy.
In 1967, the Russian physicist Yakov Zel’dovich showed that the energy of the vacuum is actually the energy that Einstein needed to create antigravity. The vacuum energy is the same as the cosmological constant, the dark energy that permeates the universe and is now driving its accelerating expansion.
Looking to Unify All of Physics
Einstein (and everybody else) thought that he’d made a big mistake by introducing his cosmological constant, and he reluctantly retracted it when he saw the evidence of the expansion of the universe. But his great blunder wasn’t really a blunder. We now know that his cosmological constant, the dark energy (as scientists call it today), is the repulsive gravity that’s causing the universe to accelerate its expansion. Dark energy is also the main component of the universe.
Even though he missed the opportunity to predict Hubble’s discovery of the expansion of the universe, Einstein ended up being right, after all. But he wasn’t alive to have the last laugh.
Einstein was at the University of Berlin when he published his cosmological constant paper. He’d moved there in 1914 from the Polytechnic in Zurich, lured by an offer of a professorship without teaching obligations. He also served as the director of the new Kaiser Wilhelm Institute for Physics. He remained in Berlin until 1933, when the danger of the Nazi regime forced him to leave Europe for the United States.
After the extraordinary success of the general theory of relativity and the confirmation of the bending of light by Arthur Eddington’s eclipse expedition in 1919 (see Chapter 12), Einstein began to think about the next step. The general theory of relativity was the first extension. With it, he’d been able to develop a model of the universe. It was time to go beyond. Now he wanted to extend general relativity and merge it with the rest of physics into one, single unified field theory.
Recasting relativity into a five-dimensional spacetime
In 1919, there were only two fields in physics: the electromagnetic field and the gravitational field. Electromagnetism was itself the result of the unification of the electric field and the magnetic field. (As I explain in Chapter 6, that was Maxwell’s great achievement and what, in part, got Einstein excited about physics when he was in college.)
Einstein wasn’t the only scientist thinking about unifying electromagnetism and gravity. For example, Hermann Weyl, at the University of Zurich, developed equations attempting to unify general relativity and electromagnetism. And the German mathematician Theodor Kaluza rewrote Einstein’s gravitational field equations into a spacetime of five dimensions (instead of four, as Einstein had done).
Kaluza’s universe had four space dimensions, plus one of time. In space, you can go up and down (that’s one dimension), left and right (two), and front and back (three). What could the fourth dimension be? Kaluza said that the additional dimension was the electromagnetic field.
Einstein liked Kaluza’s approach but quickly saw its shortcomings. The main one was that no one had ever seen a fourth space dimension. There was no evidence of another direction in space.
Leaving his work unfinished
Even with its shortcomings, Einstein was encouraged by Kaluza’s theory and soon began to work along similar lines. In 1922 and 1923, he published five papers with different variations of Kaluza’s and Weyl’s ideas.
In 1925, Einstein published a paper with his first complete version of a unified field theory, only to reject it quickly after publication. He’d seen that it wouldn’t work. In a letter to a colleague, he said, “I have once again a theory of gravitation-electricity; very beautiful but dubious.”
The following year, the Swedish physicist Oscar Klein extended and cleaned up Kaluza’s theory and came up with a clever idea, one that scientists have revived recently. He said that the reason we don’t see Kaluza’s fourth space dimension is because it is curved, rolled up in a tiny circle that you can’t see. It’s a hidden dimension.
Einstein continued working on different attempts at a unified field theory, publishing his results and rejecting them later. Some attempts were extensions of the Kaluza–Klein theory, and others were different approaches.
Einstein died at 1:15 a.m. on Monday, April 18, 1955. He was 76. The day before, he had called his secretary on the phone, asking for a notebook in which he’d been working on his latest version of the unified field theory. He was too ill to make any progress on it. He died knowing that his work was unfinished.
Reviving Einstein’s Dream
Einstein’s lifelong dream of a unified field theory was unfinished but not forgotten. Today, similar ideas are once again being pursued by many physicists in the attempt to develop one single theory that can encompass all of physics.
In hindsight, we know that Einstein’s approach couldn’t work. When he started, there were only two known fields, electromagnetism and gravity. Since then, scientists have discovered two other fields:
The strong nuclear field that generates the force that keeps nuclear particles together
The weak nuclear field that regulates beta decay
Today, there are four fields in nature: the electromagnetic field, the gravitational field, the strong field, and the weak field. In their search for unification, physicists are looking for the single field that gave birth to these four fields.
Unifying the first two fields
The year before Einstein died, two physicists at Brookhaven National Laboratory, C.N. Yang and Robert Mills, started working on an idea that Heisenberg had come up with. Heisenberg realized that if you replaced every proton in the universe with a neutron, the nuclear field would remain unchanged. (Physicists call this type of process a symmetric operation.) From this apparently simple idea, Yang and Mills came up with a set of new fields. (Their symmetry operation was the motion of a particle in a circle in a fifth dimension.) After some analysis, they identified one field as the photon. QED theory had shown that the photon was the carrier of the electric and magnetic fields.
The other two fields described charged photons, and no one had seen such a beast. The Yang–Mills theory sat for decades until the 1960s, when three physicists, working independently, succeeded where Einstein had failed. Well, sort of.
Steven Weinberg and Sheldon Glashow of Harvard University and Abdus Salam of Imperial College London contributed key pieces to the first unification of fields achieved after Maxwell’s unification of the electric and magnetic fields. Weinberg, Glashow, and Salam unified the electromagnetic field with the weak field, the field that regulates beta decay. One of the steps that made this unification possible was the identification of Yang and Mills’s charged photons as the carriers of the weak force.
The three physicists called their new field the electroweak field.
The electroweak field unifies the electromagnetic field and the weak field, which is responsible for beta decay.
The charged photons, the carriers of the electroweak force, were identified as the W and Z particles. The W and Z particles were discovered in a set of extremely delicate, ingenious, and complicated experiments in 1983 by a large team of physicists at CERN, the European laboratory for nuclear physics, led by the Italian physicist Carlo Rubbia. The experiments had to recreate conditions that existed during the early universe, when these fields existed. They don’t exist today in nature. (The electroweak unification resulted in Nobel Prizes in physics in 1979 for Weinberg, Salam, and Glashow and in 1984 for Rubbia and Simon van der Meer, the engineer who designed the experiment at CERN.)
Attempting the next step
Encouraged by the success of the electroweak theory, several scientists started working on the next unification. The ones who have gotten the furthest are Sheldon Glashow and Howard Georgi of Harvard University. Their theory, which was refined later by Helen Quinn and by Weinberg, also uses symmetry arguments to introduce a set of new fields that have been identified with the W and Z particles of the electroweak field and with the carriers of the strong nuclear force (called gluons). Their Grand Unified Theory, as they call it, attempts to unify the electroweak force with the strong nuclear force.
Unlike the electroweak unification, Glashow and Georgi’s grand unification hasn’t been confirmed experimentally. And there are some problems with the theory. The theory originally predicted that protons should decay into other particles. After the prediction was made, several laboratories around the world started delicate experiments to look for the proton decay. None of the experiments found any evidence of that. Newer refinements to the theory don’t include the requirement that protons must decay, but these versions of the theory haven’t been confirmed yet either.
Today, most physicists think that the electroweak unification is correct and that the ideas behind the grand unification are probably correct. They believe that in time, these ideas will be refined into a successful theory. However, in recent years, another very different unification approach came up that’s keeping many physicists busy.
Tying it all with strings
Gravity was Einstein’s starting point in his attempts at a unified field theory. You may have noticed that none of the modern approaches at unification have started with gravity. In fact, none include gravity.
That’s because scientists realized that starting with gravity wasn’t going to work. The main reason is that general relativity, the theory of gravity, is a field theory but not a quantum field theory, like the electroweak theory. Before gravity can be integrated with the other forces, it must be quantized. Because the other forces operate in the realm of quantum physics, gravity must be modified so that it also operates in this realm. And so far, no one has been able to do that.
But there is another approach to unification: superstring theory. This theory proposes to marry general relativity with quantum gravity. With this theory, gravity doesn’t have to be separately quantized and then unified. Rather, gravity becomes quantized automatically.
The theory started in 1968 with a young postdoctoral fellow at CERN trying to untangle the complicated mess of the many particles that were being generated in the particle accelerators around the world. Gabriele Veneziano discovered that a mathematical expression that the Swiss mathematician Leonhard Euler had discovered two centuries before would match very precisely the data on the strong nuclear force. But he didn’t know why it worked.
Two years later, several other scientists showed that Veneziano’s formula actually described the quantized motion of subatomic strings connecting particles that felt the nuclear force. At this point, however, almost no one was interested in this obscure approach. Physicists were busy working on the grand unification.
But a couple of physicists were interested. John Schwartz of Caltech and Michael Green of Queen Mary College in London kept up the work on these strange strings, cleaning the infant theory of certain mathematical inconsistencies. By 1984, they succeeded. The new version of the theory, now called superstring theory, proposes that electrons and all other elementary particles are tiny vibrating strings of energy. These strings are truly one-dimensional entities, with length but no thickness.
These strings vibrate like guitar strings, and each particle vibrates with a characteristic vibration. From E = mc2, we know that the energy of the vibrating string gives the mass of the particle. Other properties of the particle, such as the electric charge, are also encoded in the strings. In fact, in superstring theory, all the properties of the particles that scientists observe in the laboratory can be calculated directly from the theory.
What makes superstring theory so appealing is that it automatically incorporates gravity with the three other forces of nature. Although it is currently a work very much in progress, it promises to one day be the true unified field theory. If it proves to be the right approach in the years to come, it would fulfill Einstein’s lifelong dream, opening for us a window into the deepest mysteries of the universe.
