For three days in April, 2005, I was a speaker and panelist at the NASA-sponsored “Physics for the 3rd Millennium II Conference” in Huntsville, Alabama, where twelve of us, including two Nobel Laureates, were invited to give 50-minute lectures about cutting-edge physics to an audience of NASA engineers, teachers, students, parents, and other interested attendees. In this column, I want to tell you about the work described in one of the talks, given by Dr. George Chapline of the Lawrence Livermore Laboratory.
The issue that Chapline addressed had previously been brought into sharp focus in a science fiction story, Poul Anderson’s widely reprinted “Kyrie”, in which an intelligent energy-being falls into a black hole while in telepathic contact with a nun in a nearby spaceship. Near the event horizon of the black hole where time stops due to gravitational time dilation, the being is trapped in agony, presumably for all eternity.
If a collapsing star has enough mass, the star cannot be stabilized by repulsion from the strong interaction, and so it collapses to a black hole. According to Einstein’s general relativity, as one approaches such a collapsed star, the increasing gravity field causes time to slow down until, on a surface called the “event horizon, ” time stops altogether and an infalling object “freezes” there. Curiously, the general relativity description of a fall through the event-horizon depends on the situation of the observer. As viewed by an outside observer, time effectively comes to a halt for the infalling object, so that it appears to freeze at the event horizon. However, from the viewpoint of the unfortunate observer who is falling through the event horizon and into the black hole, nothing unusual happens as the event horizon is crossed except that contact with the external universe is cut off.
Chapline argues that it is unreasonable that time comes to a halt in the external reference frame, while producing no observable consequences in the infalling observer’s frame. He suggests that quantum effects should become important near the event horizon.
This suggestion goes against the conventional wisdom. Since the 1950s, there has been a general agreement among the physicist practitioners of general relativity that quantum effects should become significant only at very small distance scales, and that quantum mechanics can be comfortably ignored elsewhere. Chapline raises objections to this rule. He argues that the infinite time dilation at the event horizon of a black hole creates a situation in which quantum effects are needed, even though it is a macroscopic system. Further, he points out that quantum mechanics (which in its current formulation is not compatible with general relativity) requires some universal time standard, so that clocks can be synchronized and cross-referenced everywhere in a system. The behavior of time near the event horizon prevents such synchronization. If general relativity tries to stop the clocks, Chapline argues that quantum mechanics will “fix” this problem by producing a phase transition in the infalling matter as the event horizon is approached.
To decide what should happen when quantum mechanics takes over, he considers an analogous situation, well described by quantum mechanics, which occurs in a vertical column of superfluid helium that has more pressure at the bottom than the top of the column due to the weight of the upper liquid. In such a situation, there is some particular vertical height at which the speed of sound in the liquid goes to zero. The sound waves encountering this zero-sound-velocity “barrier” should behave in a way that is analogous to the black hole situation in which the infalling particles attempt to cross the infinite time dilation barrier.
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