The most sensitive instrument ever made, the Laser Interferometer Gravitational Wave Observatory (Ligo) has bagged the physics Nobel for detecting an effect that Albert Einstein had predicted a century ago. Physics has been deeply lagged, with the laboratory often trailing decades behind the blackboard. But science fiction has sometimes been ahead of the curve, even while observing the rigour of science. One recalls an extraordinary short story of just 20 pages which worked with the same material as Ligo — Larry Niven’s Neutron Star, which appeared in the August 1966 issue of the American science fiction magazine If. It kicked off a run of Hugos scored by If, including an award for the magazine itself, besides Harlan Ellison’s classic of vengeful artificial intelligence, I Have No Mouth and I Must Scream.
Neutron Star is an unusual work concerning gravity, the fundamental force whose study has yielded a harvest of Nobels. Usually, science fiction speculates on the foundation of formal science. Niven, on the other hand, posed a question which, to the best of my knowledge, astrophysics was yet to ask. He answered it, too. The question was — assuming that a human was shielded from the gigantic electromagnetic and gravitational forces near a neutron star, could any other force prevent him or her from surviving the experience? Niven concluded that such a force exists, but it could be survived.
But first, this neutron star business. Big stars don’t burn down. They expand into supernovas, while their cores collapse under their own gravity. If they are extremely massive, gravitational collapse creates a black hole, whose gravity well is so intense that even light can’t escape it. If the star is a shade lighter, it could collapse into a neutron star, a very dense body the size of a small city, whose gravity mashes up the electrons and protons of matter into neutrons. Ligo detected gravity waves from two accelerating black holes in collision. In 1993, the physics Nobel went to Amherst researchers inferring gravitational waves from two interacting pulsars, which are neutron stars spinning several hundred times per second, and emitting electromagnetic “lighthouse beams”.
The first pulsar, and, therefore, the first neutron star, was observed in Cambridge in 1967, the year after Neutron Star appeared — it won a controversial Nobel, because the first observer, Jocelyn Bell Burnell, then a postgrad student, was excluded. It is amazing how accurate Niven’s observations were, considering that he was projecting from pure theory. For instance, he describes gravitational lensing — the gravity of a neutron star is strong enough to bend light, and stars passing behind it momentarily appear as haloes. And there is the odd reversal of the expected Doppler shift of light. Stars ahead of a speeding craft appear bluer and those behind it redder, as wavelengths are decreased or increased from the point of view of the observer. But Shaeffer sees the stars behind him blue-shifted. Light is falling into the star’s gravity well as rapidly as his ship and gaining energy, but since light can’t travel faster than light, the extra energy is expressed as a shorter wavelength.
Niven’s plot is simple, and it’s the science which makes the story. A brilliant, rich, pacifist and cowardly alien race — so cowardly that they even conceal the location of their home planet — known as the Puppeteers, enjoy a monopoly on an indestructible spaceship hull which is proof to matter and electromagnetic energy. But one hull returned from a research voyage to a neutron star with its crew dead. An unknown force had reached through the hull and smashed them to pulp. Their lucrative monopoly threatened, the Puppeteers blackmail Niven’s ever-reluctant hero Beowulf Shaeffer into discovering what it was, by repeating the dangerous journey.
He infers that it was the tides of gravity. Just as tides in the sea are caused by the differential pull of the moon on waters, depending on their distance, the pilot experiences a lethal gravitational differential across his body. “Giants’ hands gripped my arms and legs and head, gently but with great firmness, and tried to pull me in two.”
Shaeffer survived the experience by retreating to the centre of mass of the ship, where the differential was lowest. Niven was surprised when physicists tried to model the dynamics of his story, which is a tribute to its accuracy. The math was intense, and, sadly, it turned out that Shaeffer could not actually survive the exit from the gravity well.
I’ll leave you with a simpler, intuitive problem. When Shaeffer returned and told his employer about the tides of gravity, the Puppeteer did not immediately understand. Shaeffer caught on instantly, and blackmailed him for a million by threatening to disclose a crucial property of the alien’s home planet, a carefully kept secret. So the question is, if someone doesn’t instinctively understand what a tide is, what does it tell you about their planet?