Gisela was contemplating the advantages of being crushed--almost certainly to death, albeit as slowly as possible--when the messenger appeared in her homescape. She noted its presence but instructed it to wait, a sleek golden courier with winged sandals stretching out a hand impatiently, frozen in mid-stride twenty delta away.
The scape was currently an expanse of yellow dunes beneath a pale blue sky, neither too stark nor too distracting. Gisela, reclining on the cool sand, was intent on a giant, scruffy triangle hovering at an incline over the dunes, each edge resembling a loose bundle of straw. The triangle was a collection of Feynman diagrams, showing just a few of the many ways a particle could move between three events in spacetime. A quantum particle could not be pinned down to any one path, but it could be treated as a sum of localized components, each following a different trajectory and taking part in a different set of interactions along the way.
In "empty" spacetime, interactions with virtual particles caused each component's phase to rotate constantly, like the hand of a clock. But the time measured by any kind of clock traveling between two events in flat spacetime was greatest when the route taken was a straight line--any detours caused time dilation, shortening the trip--and so a plot of phase shift versus detour size also reached its peak for a straight line. Since this peak was smooth and flat, a group of nearly straight paths clustered around it all had similar phase shifts, and these paths allowed many more components to arrive in phase with each other, reinforcing each other, than any equivalent group on the slopes. Three straight lines, glowing red through the center of each "bundle of straw", illustrated the result: the classical paths, the paths of highest probability, were straight lines.
In the presence of matter, all the same processes became slightly skewed. Gisela added a couple of nanograms of lead to the model--a few trillion atoms, their world lines running vertically through the center of the triangle, sprouting their own thicket of virtual particles. Atoms were neutral in charge and color, but their individual electrons and quarks still scattered virtual photons and gluons. Every kind of matter interfered with some part of the virtual swarm, and the initial disturbance spread out through spacetime by scattering virtual particles itself, rapidly obliterating any difference between the effect of a tonne of rock or a tonne of neutrinos, growing weaker with distance according to a roughly inverse square law. With the rain of virtual particles--and the phase shifts they created--varying from place to place, the paths of highest probability ceased obeying the geometry of flat spacetime. The luminous red triangle of most-probable trajectories was now visibly curved.
The key idea dated back to Sakharov: gravity was nothing but the residue of the imperfect cancellation of other forces; squeeze the quantum vacuum hard enough and Einstein's equations fell out. But since Einstein, every theory of gravity was also a theory of time. Relativity demanded that a free-falling particle's rotating phase agree with every other clock that traveled the same path, and once gravitational time dilation was linked to changes in virtual particle density, every measure of time--from the half-life of a radioisotope's decay (stimulated by vacuum fluctuations) to the vibrational modes of a sliver of quartz (ultimately due to the same phase effects as those giving rise to classical paths)--could be reinterpreted as a count of interactions with virtual particles.
It was this line of reasoning that had led Kumar--a century after Sakharov, building on work by Penrose, Smolin and Rovelli--to devise a model of spacetime as a quantum sum of every possible network of particle world lines, with classical "time" arising from the number of intersections along a given strand of the net. This model had been an unqualified success, surviving theoretical scrutiny and experimental tests for centuries. But it had never been validated at the smallest length scales, accessible only at absurdly high energies, and it made no attempt to explain the basic structure of the nets, or the rules that governed them. Gisela wanted to know where those details came from. She wanted to understand the universe at its deepest level, to touch the beauty and simplicity that lay beneath it all.
That was why she was taking the Planck Dive.
)
[Quantum physics and space-time relativity are explored at length in this densely packed hard science fiction story.] On a planet circling a black hole ninety-seven light years from earth, five physicists are preparing an experimental ship to descend into a nearby black hole. The crew for the suicide mission will be the physicists' clones, and although the originals don't expect the crew to survive or send back information about the structure of spacetime at the Planck scale, they are secretly hopeful their genetic counterparts will prove them wrong.
