Parallel Worlds (43 page)

Another goal of
the LHC is to create conditions not seen since the big bang itself. In
particular, physicists believe that the big bang originally consisted of a
loose collection of extremely hot quarks and gluons, called a quark-gluon
plasma. The LHC will be able to produce this kind of quark-gluon plasma, which
dominated the universe in the first 10 microseconds of its existence. In the
LHC, one can collide nuclei of lead with an energy of 1.1 trillion electron
volts. With such a colossal collision, the four hundred protons and neutrons
can "melt" and free the quarks into this hot plasma. In this way, cosmology
may gradually become less an observational science and more an experimental
science, with precise experiments on quark- gluon plasmas done right in the
laboratory.

There is also
the hope that the LHC might find mini-black holes among the debris created by
smashing protons together at fantastic energy, as mentioned in chapter 7.
Normally the creation of quantum black holes should take place at the Planck
energy, which is a quadrillion times beyond the energy of the LHC. But if a
parallel universe exists within a millimeter of our universe, this reduces the
energy at which quantum gravitational effects become measurable, putting
mini-black holes within reach of the LHC.

And last, there
is still the hope that the LHC might be able to find evidence of supersymmetry,
which would be a historic breakthrough in particle physics. These particles are
believed to be partners of the ordinary particles we see in nature. Although
string theory and su- persymmetry predict that each subatomic particle has a
"twin" with differing spin, supersymmetry has never been observed in
nature, probably because our machines are not powerful enough to detect it.

The existence of
superparticles would help to answer two nagging questions. First, is string
theory correct? Although it is exceedingly difficult to detect strings
directly, it may be possible to detect the lower octaves or resonances of
string theory. If particles are discovered, it would go a long way toward
giving string theory experimental justification (although this still would not
be direct proof of its correctness).

Second, it would
give perhaps the most plausible candidate for dark matter. If dark matter
consists of subatomic particles, they must be stable and neutral in charge
(otherwise they would be visible), and they must interact gravitationally. All
three properties can be found among the particles predicted by string theory.

The LHC, which
will be the most powerful particle accelerator when it is finally turned on, is
actually a second choice for most physicists. Back in the 1980s, President
Ronald Reagan approved the Superconducting Supercollider (SSC), a monstrous
machine 50 miles in circumference which was to have been built outside Dallas,
Texas; it would have dwarfed the LHC. While the LHC is capable of producing
particle collisions with 14 trillion electron volts of energy, the SSC was
designed to produce collisions with 40 trillion electron volts. The project was
initially approved but, in the final days of hearings, the U.S. Congress
abruptly canceled the project. It was a tremendous blow to high-energy physics
and set the field back for an entire generation.

Primarily, the
debate was about the $11 billion cost of the machine and greater scientific
priorities. The scientific community itself was badly split on the SSC, with
some physicists claiming that the SSC might drain funds from their own
research. The controversy grew so heated that even the
New York Times
wrote a critical editorial about the dangers that "big
science" would smother "small science." (These arguments were
misleading, since the SSC budget came out of a different source than the budget
for small science. The real competitor for funds was the Space Station, which
many scientists feel is a true waste of money.)

But in
retrospect, the controversy was also about learning to speak to the public in
language they can understand. In some sense, the physics world was used to
having its monster atom smashers approved by Congress because the Russians
were building them as well. The Russians, in fact, were building their UNK
accelerator to compete against the SSC. National prestige and honor were at
stake. But the Soviet Union broke apart, their machine was canceled, and the
wind gradually went out of the sails of the SSC program.

TABLETOP ACCELERATORS

With the LHC,
physicists are gradually approaching the upper limit of energy attainable with
the present generation of accelerators, which now dwarf many modern cities and
cost tens of billions of dollars. They are so huge that only large consortiums
of nations can afford them. New ideas and principles are necessary if we are
to push the barriers facing conventional accelerators. The holy grail for particle
physicists is to create a "tabletop" accelerator that can create
beams with billions of electron volts of energy at a fraction of the size and
cost of conventional accelerators.

To understand
the problem, imagine a relay race, where the runners are distributed around a
very large circular race track. The runners exchange a baton as they race
around the track. Now imagine that every time the baton is passed from one
runner to another, the runners get an extra burst of energy, so they run
successively faster along the track.

This is similar
to a particle accelerator, where the baton consists of a beam of subatomic
particles moving around the circular track. Every time the beam passes from one
runner to another, the beam receives an injection of radio frequency (RF)
energy, accelerating it to faster and faster velocities. This is how particle
accelerators have been built for the past half century. The problem with
conventional particle accelerators is that we are hitting the limit of RF
energy that can be used to drive the accelerator.

To solve this
vexing problem, scientists are experimenting with radically different ways of
pumping energy into the beam, such as with powerful laser beams, which are
growing exponentially in power. One advantage of laser light is that it is
"coherent"—that is, all the waves of light are vibrating in precise
unison, making it possible to create enormously powerful beams. Today, laser
beams can generate bursts of energy carrying trillions of watts (terrawatts) of
power for a brief period of time. (By contrast, a nuclear power plant can
generate only a paltry billion watts of power, but at a steady rate.) Lasers
that generate up to a thousand trillion watts (a quadrillion watts, or a
petawatt) are now becoming available.

Laser
accelerators work by the following principle. Laser light is hot enough to
create a gas of plasma (a collection of ionized atoms), which then moves in
wavelike oscillations at high velocities, like a tidal wave. Then a beam of
subatomic particles "surfs" in the wake created by this wave of
plasma. By injecting more laser energy, the plasma wave travels at faster
velocity, boosting the energy of the particle beam surfing on it. Recently, by
blasting a 50- terrawatt laser at a solid target, the scientists at the
Rutherford Appleton Laboratory in England produced a beam of protons emerging
from the target carrying up to 400 million electron volts (MeV) of energy in a
collimated beam. At Ecole Polytechnique in Paris, physicists have accelerated
electrons to 200 MeV over a distance of a millimeter.

The laser
accelerators created so far have been tiny and not very powerful. But assume
for a moment that this accelerator could be scaled up so that it operates not
just over a millimeter but over a full meter. Then it would be able to
accelerate electrons to 200 giga electron volts over a distance of a meter,
fulfilling the goal of a tabletop accelerator. Another milestone was reached in
2001, when the physicists at SLAC (Stanford Linear Accelerator Center) were
able to accelerate electrons over a distance of 1.4 meters. Instead of using a
laser beam, they created a plasma wave by injecting a beam of charged
particles. Although the energy they attained was low, it demonstrated that
plasma waves can accelerate particles over distances of a meter.

Progress in this
promising area of research is extremely rapid: the energy attained by these
accelerators is growing by a factor of 10 every five years. At this rate, a
prototype tabletop accelerator may be within reach. If successful, it may make
the LHC look like the last of the dinosaurs. Although promising, there are, of
course, still many hurdles facing such a tabletop accelerator. Like a surfer
who "wipes out" riding a treacherous ocean wave, maintaining the beam
so that it properly rides the plasma wave is difficult (problems include focusing
the beam and maintaining its stability and intensity). But none of these
problems seems insurmountable.

THE FUTURE

There are some
long shots in proving string theory. Edward Witten holds out the hope that, at
the instant of the big bang, the universe expanded so rapidly that maybe a
string was expanded along with it, leaving a huge string of astronomical
proportions drifting in space. He muses, "Although somewhat fanciful, this
is my favorite scenario for confirming string theory, as nothing would settle
the issue quite as dramatically as seeing a string in a telescope."

Brian Greene
lists five possible examples of experimental data that could confirm string
theory or at least give it credibility:

1.
    
The tiny mass of
the elusive, ghostlike neutrino could be experimentally determined, and string
theory might explain it.

2.
   
Small violations
of the Standard Model could be found that violate point-particle physics, such
as the decays of certain subatomic particles.

3.
   
New long-range
forces (other than gravity and electromagnet- ism) could be found
experimentally that would signal a certain choice of a Calabi-Yau manifold.

4.
  
Dark matter particles could be found in the laboratory and
compared to predictions of string theory.

5.
   
String theory
might be able to calculate the amount of dark energy in the universe.

My own view is
that verification of string theory might come entirely from pure mathematics,
rather than from experiment. Since string theory is supposed to be a theory of
everything, it should be a theory of everyday energies as well as cosmic ones.
Thus, if we can finally solve the theory completely, we should be able to
calculate the properties of ordinary objects, not just exotic ones found in
outer space. For example, if string theory can calculate the masses of the
proton, neutron, and electron from first principles, this would be an
accomplishment of first magnitude. In all models of physics (except string
theory), the masses of these familiar particles are put in by hand. We do not
need an LHC, in some sense, to verify the theory, since we already know the
masses of scores of subatomic particles, all of which should be determined by
string theory with no adjustable parameters.

As Einstein
said, "I am convinced that we can discover by means of purely mathematical
construction the concepts and the laws . . . which furnish the key to the
understanding of natural phenomena. Experience may suggest the appropriate
mathematical concepts, but they most certainly cannot be deduced from it . . .
In a certain sense, therefore, I hold it true that pure thought can grasp
reality, as the ancients dreamed."

If true, then
perhaps M-theory (or whatever theory finally leads us to a quantum theory of
gravity) will make possible the final journey for all intelligent life in the
universe, the escape from our dying universe trillions upon trillions of years
from now to a new home.

[Consider] the
view now held by most physicists, namely that the sun with all the planets will
in time grow too cold for life, unless indeed some great body dashes into the
sun and thus gives it fresh life—believing as I do that man in the distant
future will be a far more perfect creature than he now is, it is an intolerable
thought that he and all other sentient beings are doomed to complete
annihilation after such long-continued slow progress.

—Charles Darwin

According to Norse legend,
the final day
of reckoning, or Ragnarok, the Twilight of the Gods, will be accompanied by
cataclysmic upheavals. Midgard (Middle Earth) as well as the heavens will be
caught in the viselike grip of a bone-chilling frost. Piercing winds, blinding
blizzards, ruinous earthquakes, and famine will stalk the land, as men and women
perish helplessly in great numbers. Three such winters will paralyze the
earth, without any relief, while the ravenous wolves eat up the sun and the
moon, plunging the world into total darkness. The stars in the heaven will
fall, the earth will tremble, and the mountains will disintegrate. Monsters
will break free, as the god of chaos, Loki, escapes, spreading war, confusion,
and discord across the bleak land.