Richard J. Field
Chemistry
University of Montana
"Scotty, warp nine please!"
"Aye, aye, sir! Warp nine it is."
The concepts of time-warps and black holes are thoroughly imbedded in our culture, but few claim to understand them or their source in Einstein' s theory of general relativity.
Kip Thorne makes these concepts accessible and traces their nearly century-long development, especially with regard to the collapse of stars and the formation of black holes, and describes the history as well as the physical and intellectual bases of relativistic physics. He hopes in this labor of love to convince the reader that black holes indeed do exist, even though none has yet been directly observed.
While the style of the book is often that of a precise physicist, it reads easily and transmits Thorne's delight in having participated for 30 years in an intense personal and intellectual adventure. Thorne also relates much personal history of the people involved in this fascinating, collaborative endeavor and explains how theoretical and experimental physics research was done. The book is intended for a well-educated general audience, and at times requires careful study. But, it is not necessary to understand the detailed physics to understand the concepts and history.
Albert Einstein, still the best known physicist in the world, made seminal contributions in most areas of theoretical physics. His extraordinary strength was a matchless intuition into how the physical world must be constructed. This gift also proved to be tragic, as other people built on his work and made predictions he regarded as outrageous. Black holes are an abstract prediction that Einstein simply walked away from, as he also did from quantum mechanics.
Why, then, are black holes and time-warps referred to as "Einstein's outrageous legacy?" Einstein, a classical, nineteenth-century physicist, believed in an objective reality understandable in terms of our ordinary experience and anything not thus understandable he considered outrageous. Although Einstein developed his theory of relativity to explain particular experiments and used physical reasoning to do so, he stated his results in a way that mathematicians and theoretical physicists were able to generalize and to use to make abstract predictions. Einstein did not participate in or believe much of this work, but it is Einstein's legacy because it was done within his profound relativistic framework.
In 1905, Einstein recognized that there is no such thing as absolute time or absolute space. Space and time must be measures relative to something, e.g., the position of the Earth relative to the Sun, or time relative to an observable event. Only the speed of light is absolute; it must be independent of the motion of the person who measures it. Finally, whatever be their nature, the laws of physics must treat all states of motion equally; no reference point is better than another. These facts led Einstein to his special theory of relativity, which applies to objects moving in a straight line and at a constant velocity.
Relativity requires space and time to be linked to the relative motion of observer and observed. Thorne uses the analogy of a car with a line of firecrackers on its-roof traveling down a straight highway at a constant speed near that of light. A stationzuy sheriff observes it. If the firecrackers are set off simultaneously in the car's time as it passes the sheriff, the light flashes will not seem simultaneous to the sheriff because some must travel further than others to reach him/her.
Special relativity does not lead to anything outrageous. However, Einstein extended relativity to objects not traveling at a constant velocity or following a curved trajectory, i.e., masses subject to acceleration. When objects change their speed they feel a force such as that felt when driving a curvy road or in an elevator that is speeding up or slowing down. He recognized that such forces are closely related to gravity and mass, which thus get tangled up with space and time. The description of this' entanglement is Einstein's general theory of relativity. Building on Minkowski, Einstein realized that this absolute, four-dimensional spacetime is curved, just as is the three-dimensional spacetime of the surface of a growing sphere. In 1915, Einstein formulated the Einstein field equations (EFE), which describe how mass curves spacetime, thus completing the Laws of general relativity. Hilbert immediately cast Einstein's physically derived equations into a completely general but very abstract mathematical form. And then the outrageous fun began! Theme gives a readable and joyful account of both the physics and the fun.
Whenever careful analysis of a physical phenomenon leads to a general mathematical result, it is investigated for new physical insights. Theme shows this to be a tricky business for two reasons that make physics research more of an art than people realize. The complete equations are usually very complex and their rigorous analysis most always impossible. The EFE are particularly difficult. Judgments must be made of how much physics to include, leaving out less important details. Even after this, mathematical approximations still must be made to solve the equations. Often the equations are investigated at extreme conditions, e.g., very high pressure, temperature or density. It is never known how far an incomplete equation can be extrapolated before its approximations fail. Controversies develop as different approximations are made and mistakes occur.
The collapse of stars to black holes was one of the frrst predictions made by mathematical analysis of the EFE. But the motion of a black hole goes back to late eighteen century physics, which theorized that if light can be treated as a particle, as Newton did, and is thus subject to gravity, then objects in principle can become so massive that light is unable to escape their gravity; they are black. The question became in the 1920s whether such an object could result from collapse of an aging star. Relativity theory suggested yes.
Schwarzschild found a solution to the EFE. If a star of a particular mass gets smaller than a critical circumference, then its light will be red-shifted out of existence by the spacetime curvature of the star's own mass. The term "black hole" was coined for such a beast much later by J. A. Wheeler because in spacetime a very massive object sits in a deep depression much like a heavy rock on a thin rubber sheet. Anything with mass falls into the hole if it gets too close. Since this motion isolated Einstein's observation of the physical universe, he considered it outrageous.
Although there is observational evidence that stars collapse to very dense objects, they are thought to collapse to a black hole only if they have sufficient mass. Lighter stars collapse to white dwarfs or neutron stars. In 1939, Oppenheimer and Snyder calculated that stars had to be more massive than six suns to collapse to black holes rather than to white dwarfs or to neutron stars.
Work on black holes slowed in 1942 when physicists were called from basic research to do war-related research. This work did have an impact on black-hole research. In the late 1940s, war-related technologies constructed radio and X-ray telescopes which discovered intense stellar emitters of radio waves and X-rays. A potential source of such radiation is electrons spiraling into a black hole! While absolute, "smoking-gun," observational evidence of black holes did not appear, much circumstantial evidence accumulated.
The time warp at the surface of a collapsing black hole near a critical circumference was calculated to be extreme, even outrageous; an observer on its surface sees continued collapse, but a distant observer sees no further movement. It was deduced that the exact theoretical description of black holes requires unification of relativity with quantum mechanics, an effort that is still incomplete. It was predicted that spinning black holes produce gravity waves. It was realized that a quantum-mechanical mechanism may exist for the evaporation of black holes. Finally, it was deduced that the warpage of spacetime in the vicinity of a large black hole may be large enough to penetrate to another part of curved spacetime, thus leading to a worm hole through which an object can disappear in one part of the universe and appear in another almost instantly, even though the direct route through spacetime may be very long. This is roughly equivalent to an overweight Montanan falling through the Earth to China. Now that's outrageous !
While Thorne makes a strong argument that black holes exist, there is not yet direct verification; many physicists are not at all as convinced as Thorne is that black holes exist. But so far no observation has contradicted even the most abstract predictions of the general theory of relativity.
Another outrageous legacy of Einstein' s is that he has a history of being correct in such matters!