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.Its powerful magnetic field, amplified during the collapse, traps charged particles rather as the much tinier magnetic field of Jupiter does.Electrons in the rotating magnetic field emit beamed radiation not only at radio frequencies but in visible light as well.If the Earth happens to lie in the beam of this cosmic lighthouse, we see it flash once each rotation.This is the reason it is called a pulsar.Blinking and ticking like a cosmic metronome, pulsars keep far better time than the most accurate ordinary clock.Long-term timing of the radio pulse rate of some pulsars, for instance, one called PSR 0329+54, suggests that these objects may have one or more small planetary companions.It is perhaps conceivable that a planet could survive the evolution of a star into a pulsar; or a planet could be captured at a later time.I wonder how the sky would look from the surface of such a planet.Neutron star matter weighs -about the same as an ordinary mountain per teaspoonful - so much that if you had a piece of it and let it go (you could hardly do otherwise), it might pass effortlessly through the Earth like a falling stone through air, carving a hole for itself completely through our planet and emerging out the other side - perhaps in China.People there might be out for a stroll, minding their own business, when a tiny lump of neutron star plummets out of the ground, hovers for a moment, and then returns beneath the Earth, providing at least a diversion from the routine of the day.If a piece of neutron star matter were dropped from nearby space, with the Earth rotating beneath it as it fell, it would plunge repeatedly through the rotating Earth, punching hundreds of thousands of holes before friction with the interior of our planet stopped the motion.Before it comes to rest at the center of the Earth, the inside of our planet might look briefly like a Swiss cheese until the subterranean flow of rock and metal healed the wounds.It is just as well that large lumps of neutron star matter are unknown on Earth.But small lumps are everywhere.The awesome power of the neutron star is lurking in the nucleus of every atom, hidden in every teacup and dormouse, every breath of air, every apple pie.The neutron star teaches us respect for the commonplace.A star like the Sun will end its days, as we have seen, as a red giant and then a white dwarf.A collapsing star twice as massive as the Sun will become a supernova and then a neutron star.But a more massive star, left, after its supernova phase, with, say, five times the Sun’s mass, has an even more remarkable fate reserved for it - its gravity will turn it into a black hole.Suppose we had a magic gravity machine - a device with which we could control the Earth’s gravity, perhaps by turning a dial.Initially the dial is set at 1 g* and everything behaves as we have grown up to expect.The animals and plants on Earth and the structures of our buildings are all evolved or designed for 1 g.If the gravity were much less, there might be tall, spindly shapes that would not be tumbled or crushed by their own weight.If the gravity were much more, plants and animals and architecture would have to be short and squat and sturdy in order not to collapse.But even in a fairly strong gravity field, light would travel in a straight line, as it does, of course, in everyday life.* 1 g is the acceleration experienced by falling objects on the Earth, almost 10 meters per second every second.A falling rock will reach a speed of 10 meters per second after one second of fall, 20 meters per second after two seconds, and so on until it strikes the ground or is slowed by friction with the air.On a world where the gravitational acceleration was much greater, falling bodies would increase their speed by correspondingly greater amounts.On a world with 10 g acceleration, a rock would travel 10 x 10 m/sec or almost 100 m/sec after the first second, 200 m/sec after the next second, and so on.A slight stumble could be fatal.The acceleration due to gravity should always be written with a lowercase g, to distinguish it from the Newtonian gravitational constant, G, which is a measure of the strength of gravity everywhere in the universe, not merely on whatever world or sun we are discussing.(The Newtonian relationship of the two quantities is F = mg = GMm/r2; g = GM/r2, where F is the gravitational force, M is the mass of the planet or star, m is the mass of the falling object, and r is the distance from the falling object to the center of the planet or star.)Consider a possibly typical group of Earth beings, Alice and her friends from Alice in Wonderland at the Mad Hatter’s tea party.As we lower the gravity, things weigh less.Near 0 g the slightest motion sends our friends floating and tumbling up in the air.Spilled tea - or any other liquid - forms throbbing spherical globs in the air: the surface tension of the liquid overwhelms gravity.Balls of tea are everywhere.If now we dial 1 g again, we make a rain of tea.When we increase the gravity a little - from 1 g to, say, 3 or 4 g’s - everyone becomes immobilized: even moving a paw requires enormous effort.As a kindness we remove our friends from the domain of the gravity machine before we dial higher gravities still.The beam from a lantern travels in a perfectly straight line (as nearly as we can see) at a few g’s, as it does at 0 g.At 1000 g’s, the beam is still straight, but trees have become squashed and flattened; at 100,000 g’s, rocks are crushed by their own weight.Eventually, nothing at all survives except, through a special dispensation, the Cheshire cat.When the gravity approaches a billion g’s, something still more strange happens
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