Nobel Physics 1901-1910 Trivia Quiz

Answer: An X-ray is light that is more energetic than visible light.

Basically, light can be understood as being made up of particles - photons - that are nothing more than bundles of energy. Each photon carries a distinct energy, which defines its wavelength and hence its color; the full range of possible photon energies defines the electromagnetic spectrum, which runs from radio waves at the low-energy end, to the visible range that our eyes can see, all the way up to ultraviolet, X-rays, and gamma rays.

Visible light has a relatively low energy, and it doesn't take a very dense material to block it; at higher energies, X-rays pass right through skin, muscle and fat, and are stopped only by higher-density objects like bones and metal. An X-ray image is all about the contrast between high-density and low-density areas, which is why images of organs can be taken if they're first injected with a relatively high-density substance like iodine.

Röntgen, a German physicist, discovered the X-ray in 1895 while performing experiments with vacuum tubes. He was one of many working in the field; it was good luck (and good experimental instincts) that got him there first.

Answer: Two or more states share the same energy, despite being physically different.

In the late nineteenth century, scientists measuring atomic emission spectra -- the range of light emitted from a certain type of atom -- realized that, instead of looking like rainbows, the spectra consisted of narrow lines at particular wavelengths that were the same for each type of atom. The locations, widths and separations of these lines were carefully catalogued, but their origins were unknown. We now understand that the spectral lines represent the energy difference between different electron states (since a photon is emitted when an electron drops in energy), but it was a grand mystery at the time: electrons had barely been discovered, and atomic nuclei were totally unknown!

Zeeman observed a curious effect: a single spectral line would split into two or more closely spaced lines in the presence of a magnetic field. He went to his colleague Lorentz, a theorist who was also instrumental in developing special relativity -- and who had proposed these experiments himself. Lorentz had been working on incorporating electrons into classical electrodynamics, and he came up with a clever explanation for the phenomenon: electrons were oscillating back and forth inside the atom, giving off light in the process, and a magnetic field disturbed their paths and changed the frequency of the light. The field split the degeneracy, inducing a slight energy change in the different oscillations that made them just distinguishable from each other. The theory was beautiful, although we now know that it is incorrect. Nevertheless, it was very fruitful as the basis for the investigations that would eventually lead to quantum mechanics and a true understanding of emission spectra.

This was the first Nobel Physics Prize to be split between two people, and the committee consequently took pains to prove that their decision was "not only justified, but just," observing that this project represented a perfect marriage between theory and experiment.

Answer: Its nucleus is unstable, and tends to decay to a lighter nucleus by releasing radiation.

An element is defined by the number of protons in its nucleus; different isotopes mean different numbers of neutrons in the nucleus. Some of these are more stable than others. In the same way that a pencil balanced on its tip will inevitably and spontaneously fall over, an unstable nucleus will eventually spontaneously decay to a lighter nucleus (transition to a heavier nucleus would require an input of energy). To do this, the unstable nucleus must jettison some of the excess material and energy; this constitutes radiation. Depending on the particular isotope, several types of radiation could be emitted: alpha radiation (a helium nucleus); beta radiation (an electron); gamma radiation (a photon); a neutron; or two (relatively) heavy daughter nuclei, if the decay is via spontaneous nuclear fission.

Though the Nobel committee didn't know it at the time, spontaneous radioactivity was the first observed signature of the weak nuclear force, one of the four fundamental forces of nature. You can tell how mysterious the whole thing was by the classification scheme of the different types of radiation: alpha, beta, and gamma are simply the first three letters of the Greek alphabet. They didn't give helium nuclei, electrons and photons fancy names to make it difficult for introductory physics students; they did it because they had no idea what they were looking at!

By the way, the tale of Becquerel's initial discovery is a classic. He was studying uranium salts as part of an investigation into fluorescence. He wanted to conduct an experiment involving bright sunlight, and he prepared by wrapping the salts in black paper and mounting them on a photographic plate. When he went to remove the samples from his desk drawer, he discovered that the plate had already been fully exposed - and he immediately set about exploring this new phenomenon. As with the discovery of penicillin, so many scientific breakthroughs have happened solely because of a happy accident - but it takes an observant scientist to recognize the accident for what it really is!

Answer: Since argon is a noble gas, argon atoms rarely undergo chemical reactions.

One of the great problems of the eighteenth and nineteenth centuries was the seemingly basic question: What is air? By the time of Rayleigh's experiments, scientists thought they had the question answered: air was made of nitrogen (78%), oxygen (21%) and water vapor (about 1%). Rayleigh shook this understanding when he pursued a strange error during a measurement of the mass of nitrogen. He noticed that nitrogen isolated from air was heavier than nitrogen isolated from ammonia gas, and pursued the error until he realized that the "nitrogen gas" from air must contain a second element that accounted for the extra mass. After painstakingly isolating the argon (its name derives from the Greek for "lazy," since it doesn't interact easily with other elements), Rayleigh realized that it accounted for about 1% of air. (The percentages don't add up because they're rounded, by the way.)

This may sound more like chemistry than physics to the modern observer, but as the first noble gas to be identified on Earth, argon played a crucial role in developing an understanding of the atom. Noble gases tend not to form chemical bonds because the atom's electrons completely fill its orbitals (available electron quantum states); no spot is left empty. Other elements form chemical bonds by sharing electrons to fill up orbital vacancies. One of the most important successes of quantum mechanics was in explaining the behavior already familiar to chemists and students of the periodic table.

Answer: The rays are emitted by the cathode.

A cathode ray tube works by applying a high voltage between electrodes (an anode and a cathode) at either end. The anode has positive charge, and the cathode has negative charge - thus it serves as the electron source. The high voltage strips the electrons from the cathode, sending them on their way to the anode; a phosphorous material was often placed in the tube at the beginning, so that experimenters could see the path of the rays by the light it emitted. Von Lénárd's technique for moving the cathode rays out of their tubes thus allowed much more precise study, and paved the way for Thomson's identification of the rays as electrons (Question 6). Incidentally, cathode ray tubes eventually evolved into modern CRTs, the basis of computer monitors and television screens for decades.

Von Lénárd's other great contribution to physics was his study of the photoelectric effect. Experimenters had already noticed that, when illuminated with ultraviolet light, certain metals emitted a sort of ray; von Lénárd showed that these rays had many of the same characteristics as cathode rays, and that their energy depended on the wavelength of the light that had been used. His observations provided the basis of the work for which Albert Einstein would win the 1921 Nobel Prize - but von Lénárd, a nationalist and a jealous man, saw this not as an advancement of science but as an insult to German physics. He spent the years leading up to World War II as an ardent Nazi (despite his own Jewish ancestry) and promoter of "Deutsche physik," rejecting relativity and driving his Jewish colleagues from German universities. It was a cruel and sad last chapter to the story of this great physicist; if only good researchers were always good human beings, as well.

Answer: Measure how the moving particle curves in a magnetic field perpendicular to its motion.

Thomson's experiment -- still a standard way of measuring the mass-to-charge ratio m/q -- relies on the way that charged particles interact with electric and magnetic fields. A voltage V (like the one in a cathode ray tube) accelerates the particle in a line, according to qV=(mv^2)/2 where v is the velocity of the particle as it leaves the electric field. When the particle enters a magnetic field B that's perpendicular to its velocity, it travels in a circle of radius R: qBv=(mv^2)/R. If you (like Thomson) know V and B and can measure R, then you can combine those two equations to get (m/q)=(B^2 R^2)/(2V). Voila! You've measured the mass-to-charge ratio! If you're looking at electrons, you'll find -- just as Thomson did -- that this ratio is enormous: the particle is very, very light. In fact, it's almost two thousand times lighter than a proton, which has the same charge!

Thomson went on to perform one experimental coup after another, discovering the existence of isotopes (atoms of the same element with different numbers of neutrons) in 1913 and helping to invent the mass spectrometer a few years later. His "plum pudding" model of the atom -- a continuous ball of positive charge with negative electrons studded in it, like plums in a pudding -- was discredited by his student Ernest Rutherford in 1909, but it had helped to kick off a very fruitful era of atomic physics exploration.

There is an interesting footnote to Thomson's Nobel-winning discovery of the particle properties of an electron: his son, George Paget Thomson, shared the 1937 Nobel Prize for discovering that an electron also has the properties of a wave. It's not often that a single family has contributed so much to both sides of a paradox!

Answer: Special Theory of Relativity (Einstein)

The 1887 experiment conducted by Albert Michelson and Edward Morley in Cleveland is renowned as the most important failed experiment of all time. Until that time, scientists had assumed that light waves must have a medium to carry them, just like sound waves and water waves. This "luminiferous ether" must fill the vacuum of space. Since the Earth's orbit took it through constant changes of direction at a speed of 30 kilometers per second, it was thought that this would produce an "ether wind," which could be measured by its effects on the speed of light.

Michelson's and Morley's experimental skill allowed them to build an instrument (an interferometer) so precise that they should have been able to measure a difference in the speed of light and plot the Earth's motion through the ether - but they found nothing, measuring a speed consistent with zero. (This means that the result was not exactly zero, but the errors were large enough that it could have been.) Physicists toyed with the idea that the Earth's motion might be dragging the ether with it, but as other experiments accumulated more evidence the beautiful idea of the ether had to be abandoned. Einstein's Special Theory of Relativity (which was published in 1905, although he may not have known of this result) is now considered the solution to this problem. In his theory, light travels through empty space - no material or medium required! - and its speed is the same from any perspective. No part of the universe is absolutely at rest; they can all be considered to be moving. If Michelson and Morley had achieved a different result, relativity would have been shown to be wrong.

Answer: The addition (or superposition) of two waves, yielding a composite wave pattern

A simple wave is composed of peaks and troughs; the distance from peak to peak (the same distance as from trough to trough) is a wavelength. Light of different colors has different wavelengths; for example, red light has a longer wavelength than blue light does. Now suppose you have two waves of red light (same wavelength). If you start both waves at the same moment -- one peak occurring at the same time as the other -- you get constructive interference: the peaks and troughs add to each other, and you get a wave with the same wavelength but twice the height. If you let one wave lag behind the other, however, the two waves will cancel each other out when they add, and you get a straight line -- no light at all.

In Lippmann's clever method (devised in 1891), a silver emulsion is sandwiched between a glass plate and a reflecting surface. Light hits the glass plate travels through the emulsion, and reflects back -- causing interference with itself. The resulting wave patterns react with the emulsion (which is sensitive to light), and when the plate is processed this results in a change in the way that the emulsion reflects and refracts white light. Shine a diffuse light source (for example, indirect sunlight) on the plate, and voila! A color photograph!

This method was the first to produce stable colors on a photographic plate, although it never saw widespread use.

Answer: Damped oscillation

Oscillation is a familiar concept; it's the motion of a wave (or a pendulum, or a spring), moving back and forth, always returning to its original position. But in the real world, the oscillator doesn't return to its exact original position: oscillations are damped by friction or air resistance, and the size (or amplitude) of the oscillation is a little less each time. Damping can be very useful -- shock absorbers in a car are designed to damp the shaking as quickly as possible -- but it has to be controlled when it's causing severe energy loss.

Braun tracked the source of the wireless telegraph damping to loss in the electrical circuit that connected the oscillator to the antenna; by changing the coupling, he was able to drastically reduce the energy loss in both the transmitter and the receiver, making radio signals a truly long-distance form of communication for the first time.

Together with the efforts of other scientists, Braun and Marconi's innovations made radio possible -- but this period of widespread scientific cooperation quickly degenerated into lawsuits and acrimony. Marconi's success was a clear example of standing on the shoulders of giants; as the Nobel presentation speech put it, he had "the honour of the first trials," and he deserved credit for "his ability to shape the whole thing into a practical, usable system." But Marconi wanted credit for more, and he pursued and received patents for ideas and inventions that had been developed by other people -- including Braun and Nikola Tesla. The lawsuits that came out of this mess were not resolved until 1943 -- six years after Marconi's death.

Answer: Forces between neighboring molecules

Neither gravitational effects nor Heisenberg's uncertainty principle has a noticeable effect on the equation of state (although if van der Waals had accounted for the latter, it would have been worth a Nobel Prize in clairvoyance: Heisenberg didn't formulate his principle until 1926), and competent experimenters would have accounted for any outside heating effects on their own. Van der Waals's contribution was to realize that molecules can attract each other - which leads to lessened pressure on the outer shell of the gas, as surface molecules are attracted inward. He also recognized that, since each molecule occupies a finite amount of space, there is a limit to how far the gas can be compressed: there has to be room for the molecules!

The van der Waals equation of state, which he proposed in 1873, represented the first major improvement over the ideal gas law; it even predicts the formation of a liquid phase at a certain combination of temperature and pressure! It also marked a new and very fruitful way of thinking about molecules, which led to further improvements down the line. It's no longer used in current research, but is still studied by every student of statistical mechanics and thermodynamics.