Read Online Beyond the God Particle by Leon M. Lederman, Christopher T. Hill - Free Book Online
Authors: Leon M. Lederman, Christopher T. Hill
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half a mile, a feat that impressed the Cambridge dons, including J. J. Thomson, the Cavendish director at the time. Later Thomson would declare, “I have never had a student with more enthusiasm or ability for original research than Mr. Rutherford.” 8
By 1909, Rutherford and his students were shooting tiny subatomicprobes, called “alpha particles,” at a piece of thin gold foil. They were carefully measuring the way in which the particles were slightly deflected as they scattered off of the heavy gold atoms in the target foil. The alpha particles were produced from the radioactive decay of the element radium, which naturally accelerated them to high energies—there were no man-made particle accelerators in those days. Alpha particles were now known, thanks to Rutherford, to be very heavy compared to an electron, and with a little energy they have a very tiny quantum wavelength and are therefore capable of probing deep inside the atom.
One day something utterly unexpected happened. The alpha particles were usually deflected only slightly by their passage through the gold foil, and the scientists were measuring in detail this “forward scattering.” They decided, simply as a sanity check, to see if there was any “backward scattering.” To their astonishment they found that one in 8,000 alpha particles bounced back toward the source. As Rutherford remembered it, “It was as if you fired a fifteen inch artillery shell at a piece of tissue paper and the shell came back and hit you.” 9 What was happening here? What kind of thing inside the atom was repelling the positively charged and massive alpha particle?
No one before Rutherford had devised any way of mapping the inner shape of the atom. According to the “raisin bread” model of the atom of J. J. Thomson, the alpha particles should always have bullied their way straight through the atom—always! The atom was like a big glob of shaving cream, and the alpha particles were rifle bullets. Rifle bullets would tear straight through globs of shaving cream. Imagine seeing a rifle bullet occasionally deflected and ricocheting backward upon colliding with a blob of shaving cream. Such was the observation of Rutherford and his students.
Rutherford devoted his full energies to understanding this remarkable discovery. According to his detailed calculations there was only one way that any alpha particles could ever be deflected backward. That could only happen if the entire mass, and positive charge, of the atom was concentrated into a tiny volume in the center of the atom—the “atomic nucleus” was discovered! The nucleus's hefty mass and large positive charge could repel the positively charged alpha particles that came within its range and deflect them through a large angle, even kick them backward. It was as if within the glob of shaving cream there were dense, hard ball bearings thatcould cause bullets to collide and deflect. The electrons were orbiting this dense central charge of the atom. The raisin bread theory of the atom of J. J. Thomson was tossed in the trash-bin. An atom now resembled a tiny solar system, with miniature “planets” (electrons) orbiting a dense “dark star” at the center (nucleus), and it was all held together by electromagnetism.
Further experiments indicated that the nucleus was indeed tiny—one-trillionth of the volume of the atom—even though most of the mass of the atom, more than 99.98 percent of it, resided in the nucleus. At the time of this discovery, within this tiny solar system model of an atom, all the classical laws of physics were still thought to be rock solid, just as in the macroscopic solar system with the sun and its planets. The same laws of classical physics were believed to work in the atom just as they did everywhere else—until Niels Bohr showed up.
THINKING THROUGH THE ATOM
Niels Bohr 10 was a young theoretical physicist from Denmark who was studying at the Cavendish Lab, and he happened to attend a
By 1909, Rutherford and his students were shooting tiny subatomicprobes, called “alpha particles,” at a piece of thin gold foil. They were carefully measuring the way in which the particles were slightly deflected as they scattered off of the heavy gold atoms in the target foil. The alpha particles were produced from the radioactive decay of the element radium, which naturally accelerated them to high energies—there were no man-made particle accelerators in those days. Alpha particles were now known, thanks to Rutherford, to be very heavy compared to an electron, and with a little energy they have a very tiny quantum wavelength and are therefore capable of probing deep inside the atom.
One day something utterly unexpected happened. The alpha particles were usually deflected only slightly by their passage through the gold foil, and the scientists were measuring in detail this “forward scattering.” They decided, simply as a sanity check, to see if there was any “backward scattering.” To their astonishment they found that one in 8,000 alpha particles bounced back toward the source. As Rutherford remembered it, “It was as if you fired a fifteen inch artillery shell at a piece of tissue paper and the shell came back and hit you.” 9 What was happening here? What kind of thing inside the atom was repelling the positively charged and massive alpha particle?
No one before Rutherford had devised any way of mapping the inner shape of the atom. According to the “raisin bread” model of the atom of J. J. Thomson, the alpha particles should always have bullied their way straight through the atom—always! The atom was like a big glob of shaving cream, and the alpha particles were rifle bullets. Rifle bullets would tear straight through globs of shaving cream. Imagine seeing a rifle bullet occasionally deflected and ricocheting backward upon colliding with a blob of shaving cream. Such was the observation of Rutherford and his students.
Rutherford devoted his full energies to understanding this remarkable discovery. According to his detailed calculations there was only one way that any alpha particles could ever be deflected backward. That could only happen if the entire mass, and positive charge, of the atom was concentrated into a tiny volume in the center of the atom—the “atomic nucleus” was discovered! The nucleus's hefty mass and large positive charge could repel the positively charged alpha particles that came within its range and deflect them through a large angle, even kick them backward. It was as if within the glob of shaving cream there were dense, hard ball bearings thatcould cause bullets to collide and deflect. The electrons were orbiting this dense central charge of the atom. The raisin bread theory of the atom of J. J. Thomson was tossed in the trash-bin. An atom now resembled a tiny solar system, with miniature “planets” (electrons) orbiting a dense “dark star” at the center (nucleus), and it was all held together by electromagnetism.
Further experiments indicated that the nucleus was indeed tiny—one-trillionth of the volume of the atom—even though most of the mass of the atom, more than 99.98 percent of it, resided in the nucleus. At the time of this discovery, within this tiny solar system model of an atom, all the classical laws of physics were still thought to be rock solid, just as in the macroscopic solar system with the sun and its planets. The same laws of classical physics were believed to work in the atom just as they did everywhere else—until Niels Bohr showed up.
THINKING THROUGH THE ATOM
Niels Bohr 10 was a young theoretical physicist from Denmark who was studying at the Cavendish Lab, and he happened to attend a
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