Hoping to Explain Everything
Over the past five years, some physicists have increasingly embraced “string theories” that appear to defy common sense: Among other seeming absurdities, these theories would expand the universe into 11 dimensions and perhaps even dispense with space and time.
What, one might ask, has led physicists to brazenly embrace such a bizarre scenario? Revolutions do not come lightly in physical law, and space and time will not go gently into the night.
These radical new views have gained ground because physicists have been repeatedly foiled in their attempts to find a consistent set of laws that rule the cosmos.
The puzzles they have encountered suggest that major revisions may be necessary in our present understanding of the physical world.
The mysteries include: The large-scale universe doesn’t mesh with the small; the vacuum is both empty and enormously heavy; most of the matter in the universe can’t be seen; matter shouldn’t exist.
Luckily, these cracks in the edifice of physics suggest places to look for answers.
Among the strongest hints that something important is missing in traditional theories is that large-scale events in the cosmos (ruled by gravity) and small-scale happenings (ruled by quantum mechanics) appear to operate under different--and mutually exclusive--sets of laws.
Indeed, physicists became attracted to the field of string theory because it alone resolves the glaring mismatch between gravity and the quantum world.
An equally serious (and related) problem is that empty space, to put it bluntly, OF SPACE, TIME AND STRINGS has a weight problem. Weighed by one set of scales, it is enormously heavy; measured by another, it weighs practically nothing at all.
“It’s an embarrassment to all theoretical physics, and it’s a particular embarrassment to string theorists because string theory is supposed to be a complete theory which describes everything,” said Harvard physicist Andrew Strominger.
Why does empty space weigh anything at all? The short answer is: because space never rests.
No particle or force field stands still in the inherently jittery subatomic world. That means that no quantity can be precisely anything--including precisely zero. So even empty space is constantly wiggling around like a fidgety child in school.
The “weight” of empty space is really a measure of its energy content--the combined fluctuations of every kind of particle and potential particle in the universe.
But there’s an even odder aspect to this energy of emptiness: Unlike normal energy and matter, which gravitate together, the energy of the vacuum is repulsive. In effect, it acts like anti-gravity.
Put these ideas together, and it means that the quantum mechanical “weight” of the vacuum is enormous--even though it behaves as if it has negative mass.
According to established theories, this repulsive energy should be pumping up the universe, propelling it onward and outward at a tremendous rate. In reality, however, its gravitational impact on the overall expansion of the universe appears to be zero, or very close to zero. In fact, the mathematical difference between the theoretical answer and the observed answer is a number so big that it has 120 zeros after it.
Such a colossal mismatch suggests that something major is awry with physicists’ understanding of the universe. As University of Texas physicist Steven Weinberg put it: “This must be the worst failure of an order of magnitude estimate in the history of science.”
The unreasonable heaviness of empty space has been puzzling physicists since the early part of this century. Indeed, Albert Einstein inserted a mathematical term for this strange repulsive energy into his original theory of gravity. He called it the “cosmological constant,” and later abandoned the idea as a mistake.
Mostly, physicists were able to sweep the problem under the rug.
Last year, however, observations of exploding stars on the far reaches of the cosmos indicated that some such repulsive force might actually be at work, suggesting that Einstein was mistaken about being mistaken. Once more, the energy of emptiness was propelled to the forefront of physicists’ minds. But whatever this energy is, it’s far more complex than the force Einstein had in mind.
For one thing, it’s probably not constant. Today the force--if it exists--is exceedingly weak. However, 15 billion years ago, physicists believe, the vacuum energy packed a much bigger punch than it does today. It was so strong, in fact, that it completely overwhelmed the attractive force of gravity, causing the universe to expand exponentially during the first instants of existence.
Solving the Universe’s Weight Problem
This chameleon-like constant raises a huge, unanswered question: Is it just a coincidence that the repulsive vacuum energy and the attractive gravitation energy seem to balance almost exactly during this particular era? Or is such a balance required for life to exist anywhere?
Whatever the answer, the weight problem of the universe suggests major missing pieces in the cosmic puzzle. At the very least, said UCLA physicist Roberto Peccei, “we’re not thinking about the vacuum in the right way.”
If the properties of empty space puzzle physicists, so do the properties of the matter that fills it. Among the questions desperately seeking answers:
Why does matter exist? Why are subatomic particles grouped into three distinct families? What is mass, and what produces it? What kind of matter is most of the universe made of?
Consider that physicists don’t know what 99% of the universe is made of. Most of the matter is presumed to be so-called “dark matter.” It’s seen only by its gravitational influence, and nothing else. All attempts to pin it down are so far inconclusive.
If current models can’t account for most of the matter or energy in the universe, said astronomer Margaret Geller of the Harvard-Smithsonian Center for Astrophysics, then they are obviously incomplete.
Whatever the dark matter is, physicists are fairly certain that most of it is “exotic”--that is, something entirely unlike the ordinary subatomic particles that make up people, planets and stars.
Yet even everyday matter defies easy explanations. The subatomic particles that make up chairs and stars alike are grouped into families with similar characteristics. Physicists suspect there must be a pattern behind the particle family tree, like the periodic table of elements, but no one has been able to find it.
Discovering such a table would help answer such questions as: Why are particles the way they are? Why do they have different masses and different electric charges? Where do they get their properties?
“To me, one of the burning questions is: What makes an electron an electron, and a top quark a top quark?” said physicist Chris Quigg of Fermi National Accelerator Laboratory outside Chicago. For now, no one knows how to answer that question.
Not only matter, but also the forces that pull and push matter around--like gravity and electricity--operate in unexplained ways. Electricity is trillions of trillions of times stronger than gravity. Physicists suspect that in the very early universe, all the forces were the same strength. How did they evolve into such very different entities?
Complicating matters further is the fact that the traditional distinction between force and matter has been blurred, and today they are both viewed as aspects of the same essential stuff.
For example, particles known as gluons--which provide the force that holds quarks together inside an atomic nucleus--can theoretically form into matter particles called glueballs.
“The distinction becomes a little fuzzed,” said Quigg. “I think this wall between force carriers and fundamental constituents is going to fall sometime soon.”
Figuring Out Why Matter Even Exists
The ultimate question about matter is: Why is there matter at all?
It is well known that matter and antimatter (which is opposite to matter in every respect) are always created in equal quantities. What’s more, they annihilate each other whenever they meet, and should cancel out each other exactly everywhere in the universe. Instead of something in the universe, there should logically be nothing.
And yet, physicists have calculated that by one-millionth of a second after the Big Bang, particles of matter outnumbered anti-particles by 1 part in a billion, an imbalance--though minuscule--big enough to create everything in the universe.
But why and how did the imbalance get there? The answer will require understanding how nothing managed to turn into something--a something that includes atoms and stars and us.
Or, as MIT physicist Alan Guth likes to put it, our universe appears to be “the ultimate free lunch. Needless to say, much of how this happened is still a puzzlement.”
Some clues to resolving these issues will come from the quiet scratchings of theorists teasing truth out of equations. Others will come from experiments being readied to map the cosmos on its grandest level and probe the atom at its smallest.
Whatever the physicists find, it’s all but certain that the trend away from viewing space, time, matter and forces as separate entities will continue. Instead, all will be seen as threads in a single tightly woven tapestry, whether it’s made of strings or something else.
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Coming in December: Experiments on the Universe
