laws. For example, when Albert Einstein thought hard about the most basic principles of kinematics, not just in the everyday world but at speeds near that of light, he was led to the special theory of relativity, with its bizarre melding of space and time. Later, once particle accelerators were built, it became possible to give particles extremely high energies and thus to show that supposedly fundamental particles like the proton actually had substructure—quarks and gluons. Accelerators gave birth to the whole new field of particle physics, whose laws are so different from those of classical physics. It will be quite remarkable if more “new” physics is not uncovered by probing matter under more extreme conditions.

Fortunately, the approaches to satisfying these twin reasons are identical. The problem must be attacked from both ends on the one hand by using the universe as a giant cosmic laboratory and watching it perform experiments and on the other by carrying out controlled experiments on Earth that are tailored to simulate, as closely as possible, astrophysical conditions. Neither approach by itself is complete. The cosmic laboratory includes the astrophysicist only as a silent witness, a decoder of distant events from fragmentary clues, rather like a historian or an archaeologist. The experimental physicist has more immediate control but cannot recreate the extraordinary range of conditions that occur in the universe. The two approaches are therefore complementary and should be pursued in parallel.

Particle accelerators exist on Earth that can raise protons to energies 1,000 times greater than their energies at rest. Many trillions of particles can be accelerated, from which a few very rare and valuable events are culled. However, for the foreseeable future, building a terrestrial accelerator with sufficient energy to explore directly the unification of all the forces is inconceivable. By contrast, cosmic ray protons are created in distant, astronomical sources with energies some 300 million times greater than those produced by the largest particle accelerators on Earth (see Box 6.1). These collide with atoms in the upper atmosphere, and the products of these collisions are observed on the ground as sprays of particles called air showers. Thus, cosmic ray protons can be used to explore physics at much higher energies, but only with rather primitive diagnostics. Both accelerators and cosmic ray experiments are needed to obtain a complete picture.


Many cosmic environments for testing physical laws are associated with stars or their remnants. The interiors of stars have temperatures of several millions of degrees, hot enough to drive the nuclear reactions that

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