|Speaker:||John Marburger, Science Advisor to the President and Director of the Office of Science and Technology Policy in the White House|
|Topic:||“The Origin of Size”|
President Collins called the 2149th meeting of the Philosophical Society of Washington to order at 8:20 p.m. on September 13, 2002. The Recording Secretary read the minutes of the 2148th meeting and they were approved.
The speaker for the 2149th meeting was John Marburger, Science Advisor to the President and Director of the Office of Science and Technology Policy in the White House. The title of his presentation was, “The Origin of Size.”
Quantum concepts are more than a century old, but they remain fundamental to our understanding of matter. Simple properties like the size of objects have surprising quantum origins. This lecture reminds us that ordinary features of the universe are manifestations of an extraordinary world beneath our perceptions.
The speaker described a recent article in the New Your Times on “Science's Ten Most Beautiful Experiments.” He noted that No. 5 on the “most beautiful” list was Thomas Young's double-slit light interference experiment done 200 years ago to show the wave characteristic of light. The speaker than described how in 1897, J. J. Thompson exposed electrodes in an evacuated tube to a strong electric field that pulled pieces off one electrode and slammed them into the side of the container where they caused a glow, showing clearly the electrons acted like little particles of electricity. However, over the next few years, using thought experiments and the developing rules of quantum theory, physicists also showed the electron's wave properties, strikingly demonstrated in a 1961 experiment ranked No. 1 on the Top Ten list.
Quantum theory began shortly after 1900 with Max Planck's hypothesis that it takes more energy to excite high frequency ultraviolet light than low frequency infrared light and the needed energy is proportional to the frequency of the light. Reversing the logic and using electrons, de Broglie argued that since electrons have mass, and mass is equivalent to energy according to Einstein, electrons must be associated with something that vibrates. Further, the vibration something should trace out a wave, and sure enough, measurements confirmed de Broglie's idea.
The speaker then introduced the subject of size by pointing out that nature comes in two varieties, the ponderous and the delicate, where ponderous things such as planets and baseballs are undisturbed by acts of observation and follow Newton's laws. Delicate things such as electrons and photons are different, for which we need quantum mechanics.
The speaker then described the impact of delicate things on ponderous things, such as a ponderous mousetrap. Consider for a moment the mouse, a truly delicate creature that can register its presence through a whisker's touch upon the trap whereupon we would hear a sharp click. We would then see the trap bar snapped shut—but no mouse. The evidence for the mouse is manifest, but the mouse itself remains elusive. Quantum mechanics is about clicks in detectors and relates the clicks to a wave that describes its energy and motion according to the quantum facts of life.
With an array of such mousetrap detectors and a source of electrons, we can make a bar chart which could have a single hump, or two or three irregular humps—looking something like a wave. This is the basic wave-like phenomenon of quantum theory and matches Schrödinger's wave as a probability for detector clicks. The speaker showed how Heisenberg's uncertainty relation emerges from these ideas.
The speaker then described an approach to sizing things. If you imagine smaller components of anything that has a definite size, the fundamental components of such things can have no other size than zero. But if they have no size, then the things they form must consist of point-like, zero sized components with intervening spaces. If this sized matter is stable, then any effort to increase or decrease the spacing must meet with resistance. Thus, there must be both attractive and repulsive forces that balance to hold the point components in place. Our best theory to date is called the Standard Model, which consists of 12 particles, their associated antiparticles and four forces—gravity, electromagnetic, the strong force, and the weak force. However, none of these four forces acts to prevent an atom's electrons from spiraling into the nuclei. In fact, there is a fifth force in nature, quantum pressure.
Knowing electrons are confined to the vicinity of nuclei, we can infer the Schrödinger wave for electrons is a standing wave on the order of the atomic size. The uncertainty relation tells us there is a corresponding spread in momenta. However, the associated motion would cause the electrons to spread beyond the size of the atom. There needs to be a confining force which is the electric attraction between the electrons and the nucleus. However, the original momentum spread associated with confinement gives rise to a “quantum pressure” that causes the Schrödinger waves to expand. This is the origin of the “fifth force” that stabilizes all matter.
The size of atoms can now be obtained in a straightforward fashion by balancing the quantum pressure from the uncertainty relation against the electrostatic force between the nucleus and the electrons. The result agrees closely with known atomic sizes.
The speaker then pointed out that things made of atoms cannot be arbitrarily large. For example, in star-sized objects, gravity forces confine the atomic electrons to such small volumes that the “uncertainty energy” is big enough to make it energetically favorable for the electrons to combine with the protons in the nuclei and convert them to neutrons. The size of the resulting neutron star may then be determined by balancing the quantum pressure of the nuclei against gravity. There does not appear to be a limit to this process, but normally its final stages occur within the concealing boundary of a black hole, where the force of gravity is so strong that even light rays cannot escape confinement.
Mr. Marburger then closed his presentation and kindly answered questions from the floor. President Collins thereupon thanked Mr. Marburger for the Society, welcomed him to membership, announced the next meeting and made the usual parking announcement. He then adjourned the 2149th meeting to the Social Hour at 9:42 p.m.
New York Times: “Science's 10 most beautiful experiments”|
Office of Science & technology Policy (Includes speaker's files of his talks including this talk and his talk on Michael Frayn's play “Copenhagen”)
J. Marburger talk on “Copenhagen”
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