Are protons unstable?
In case you're worried that the protons you're made of will disintegrate, transforming you into a puddle of elementary particles and free energy, don't sweat it. Various observations and experiments show that protons must be stable for at least a billion trillion trillion years. However, many physicists believe that if the three atomic forces are really just different manifestations of a single unified field, the alchemical, supermassive bosons described above will materialize out of quarks every now and then, causing quarks, and the protons they compose, to degenerate.
At first glance, you'd be forgiven for thinking these physicists had experienced some sort of mental decay on the grounds that tiny quarks are unlikely to give birth to behemoth bosons weighing more than 10,000,000,000,000,000 times themselves. But there's something called the Heisenberg uncertainty principle, which states that you can never know both the momentum and the position of a particle at the same time, and it indirectly allows for such an outrageous proposition. Therefore, it's possible for a massive boson to pop out of a quark making up a proton for a very short time and cause that proton to decay.
What is gravity?
Next there's the matter of gravity, the odd force out when it comes to small particles and the energy that holds them together. When Einstein improved on Newton's theory, he extended the concept of gravity by taking into account both extremely large gravitational fields and objects moving at velocities close to the speed of light. These extensions lead to the famous concepts of relativity and space-time. But Einstein's theories do not pay any attention to quantum mechanics, the realm of the extremely small, because gravitational forces are negligible at small scales, and discrete packets of gravity, unlike discrete packets of energy that hold atoms together, have never been experimentally observed.
Nonetheless, there are extreme conditions in nature in which gravity is compelled to get up close and personal with the small stuff. For example, near the heart of a black hole, where huge amounts of matter are squeezed into quantum spaces, gravitational forces become very powerful at tiny distances. The same must have been true in the dense primordial universe around the time of the Big Bang.
Physicist Stephen Hawking identified a specific problem about black holes that requires a bridging of quantum mechanics and gravity before we can have a unified theory of anything. According to Hawking, the assertion that nothing, even light, can escape from a black hole is not strictly true. Weak thermal energy does radiate from around black holes. Hawking theorized that this energy is born when particle-antiparticle pairs materialize from the vacuum in the vicinity of a black hole. Before the matter-antimatter particles can recombine and annihilate each other, one that may be slightly closer to the black hole will be sucked in, while the other that is slightly farther away escapes as heat. This release does not connect in any obvious way to the states of matter and energy that were earlier sucked into that black hole and therefore violates a law of quantum physics stipulating that all events must be traceable to previous events. New theories may be needed to explain this problem.
Are there additional dimensions?
Wondering about the real nature of gravity leads eventually to wondering whether there are more than the four dimensions we can easily observe. To get to that place, we might first wonder if nature is, in fact, schizophrenic: Should we accept that there are two kinds of forces that operate over two different scales—gravity for big scales like galaxies, the other three forces for the tiny world of atoms? Poppycock, say unified theory proponents—there must be a way to connect the three atomic-scale forces with gravity. Maybe, but it won't be easy. In the first place, gravity is odd. Einstein's general theory of relativity says gravity isn't so much a force as it is an inherent property of space and time. Accordingly, Earth orbits the sun not because it is attracted by gravity but because it has been caught in a big dimple in space-time caused by the sun and spins around inside this dimple like a fast-moving marble caught in a large bowl. Second, gravity, as far as we have been able to detect, is a continuous phenomenon, whereas all the other forces of nature come in discrete packets.
All this leads us to the string theorists and their explanation for gravity, which includes other dimensions. The original string-theory model of the universe combines gravity with the other three forces in a complex 11-dimensional world. In that world—our world—seven of the dimensions are wrapped up on themselves in unimaginably small regions that escape our notice. One way to get your mind around these extra dimensions is to visualize a single strand of a spiderweb. To the naked eye, the filament appears to be one dimensional, but at high magnification it resolves into an object with considerable width, breadth, and depth. String theorists argue that we can't see extra dimensions because we lack instruments powerful enough to resolve them.
We may never see these extra dimensions directly, but we may be able to detect evidence of their existence with the instruments of astronomers and particle physicists.
How did the universe begin?
If all four forces of nature are really a single force that takes on different complexions at temperatures below several million degrees, then the unimaginably hot and dense universe that existed at the Big Bang must have been a place where distinctions between gravity, strong force, particles, and antiparticles had no meaning. Einstein's theories of matter and space-time, which depend upon more familiar benchmarks, cannot explain what caused the hot primordial pinpoint of the universe to inflate into the universe we see today. We don't even know why the universe is full of matter. According to current physics ideas, energy in the early universe should have produced an equal mix of matter and antimatter, which would later annihilate each other. Some mysterious and very helpful mechanism tipped the scales in favor of matter, leaving enough to produce galaxies full of stars.
Fortunately, the primordial universe left behind a few clues. One is the cosmic microwave background radiation, the afterglow of the Big Bang. For several decades now, that weak radiation measured the same wherever astronomers looked at the edges of the universe. Astronomers believed such uniformity meant that the Big Bang commenced with an inflation of space-time that unfolded faster than the speed of light.
More recent careful observation, however, shows that the cosmic background radiation is not perfectly uniform. There are minuscule variations from one small patch of space to another that are randomly distributed. Could random quantum fluctuations in the density of the early universe have left this fingerprint? Very possibly, says Michael Turner, chairman of the astrophysics department at the University of Chicago and chairman of the committee that came up with these 11 questions. Turner and many other cosmologists now believe the lumps of the universe—vast stretches of void punctuated by galaxies and galactic clusters—are probably vastly magnified versions of quantum fluctuations of the original, subatomic-size universe.
And that is just the sort of marriage of the infinite and the infinitesimal that has particle physicists cozying up to astronomers these days, and why all 11 of these mysteries might soon be explained by one idea.
How Did We Get Here?
Astronomers cannot see all the way back in time to the origin of the universe, but by drawing on lots of clues and theory, they can imagine how everything began.
Their model starts with the entire universe as a very hot dot, much smaller than the diameter of an atom. The dot began to expand faster than the speed of light, an expansion called the Big Bang. Cosmologists are still arguing about the exact mechanism that may have set this event in motion. From there on out, however, they are in remarkable agreement about what happened. As the baby universe expanded, it cooled the various forms of matter and antimatter it contained, such as quarks and leptons, along with their antimatter twins, antiquarks and antileptons. These particles promptly smashed into and annihilated one another, leaving behind a small residue of matter and a lot of energy. The universe continued to cool down until the few quarks that survived could latch together into protons and neutrons, which in turn formed the nuclei of hydrogen, helium, deuterium, and lithium. For 300,000 years, this soup stayed too hot for electrons to bind to the nuclei and form complete atoms. But once temperatures dropped enough, the same hydrogen, helium, deuterium, and lithium atoms that are around today formed, ready to start a long journey into becoming dust, planets, stars, galaxies, and lawyers.
Gravity—the weakest of the forces but the only one that acts cumulatively across long distances—gradually took control, gathering gas and dust into massive globs that collapsed in on themselves until fusion reactions were ignited and the first stars were born. At much larger scales, gravity pulled together huge regions of denser-than-average gas. These evolved into clusters of galaxies, each one brimming with billions of stars.
Over the eons fusion reactions inside stars transformed hydrogen and helium into other atomic nuclei, including carbon, the basis for all life on Earth.
The most massive stars sometimes exploded in energetic supernovas that produced even heavier elements, up to and including iron. Where the heaviest elements, such as uranium and lead, came from still remains something of a mystery.
The Particle Data Group of Lawrence Berkeley National Laboratory has an excellent particle physics primer: particleadventure.org/particleadventure/index.html
Read the full National Research Council report on the 11 biggest questions: www.nationalacademies.org/bpa/reports/cpu/index.html