Nobel Physics 1911-1920 Trivia Quiz

Answer: Its temperature

The heat radiation of objects is more famously known as blackbody radiation; in nature, a perfect black body is one that absorbs all the light that strikes it, so that its thermal radiation makes up all the light observed to come from it. Perfect black bodies don't exist in nature, but there are objects (such as stars) that come close. Before Wien's 1893 paper, physicists had managed to determine a theoretical model for blackbody radiation (still considered a triumph of thermodynamics) and relate the total radiation to the body's temperature -- but an understanding of the spectrum (the distribution of energy emitted at each wavelength of light) remained elusive.

Enter Wilhelm Wien, who (as Max von Laue would later breathlessly write) "led us to the very gates of quantum physics." In 1893, after an incredible amount of effort, Wien published a strikingly simple formula. The wavelength (in meters) of the peak of a blackbody spectrum was equal to a constant (about 0.003 meter-Kelvin) divided by the black body's temperature (in Kelvin). Suddenly, spectral analysis grew up. Simply by looking at the light from a star, scientists could determine its temperature, which led to an understanding of the stages of a star's life. By measuring the human body temperature, scientists could determine that living people emit light in the far infrared, information which is now the basis of night vision goggles. In 1894 he developed a law for short-wavelength blackbody radiation -- and the challenge of extending it to longer wavelengths inspired Max Planck to begin the first investigations of quantum mechanics.

Wien, a skilled experimentalist as well as a theoretician, also helped to identify the proton and to develop the principles of mass spectroscopy. He received the Nobel Prize in the prime of his scientific life.

Answer: It expands as it heats.

As an object heats up, it absorbs energy -- in this case, electromagnetic energy (which is the nature of light itself). This tends to speed up its constituent particles (the energy has to go somewhere!), which in turn means that, on average, those particles are further apart: thermal expansion in action. (A few materials, such as water, actually _contract_ when heated over small temperature ranges, but positive thermal expansion is far more common.) The sun valve works by combining four metal rods in a glass tube. Three of the rods are polished, so that they don't absorb much sunlight; the fourth is blackened, so that it absorbs nearly all the sunlight that hits it. As it heats, it expands, closing the valve and shutting off the supply of fuel; when it cools, it contracts, opening the valve and allowing the fuel to flow. This innovation allowed thousands of lighthouses around the world to use acetylene and other fuels in a cost-effective way, eliminating the need for year-round, on-station lighthouse keepers in many situations. The sensitivity of the valves could even be tuned so that the valve would open not only at sunset, but also when fog or clouds made the sun too dim.

Dalén spent most of his professional life working on applications for acetylene. Other scientists had had the idea of dissolving acetylene in acetone fluid, which made it inert -- but there was always the chance that, as the volume of acetone diminished (through temperature or pressure changes), explosive pockets of acetylene might form between the top of the liquid and the top of the container. Dalén's invention of Agamassan (nicknamed Aga) solved the problem: Aga is a porous, elastic material that fills up the remaining space in the container and prevents the acetylene from forming dangerous concentrations.

Sadly, despite his monumental contributions to safety, Dalén was gravely injured in an acetylene explosion in 1912, while testing safety devices on transport containers; his long convalescence prevented him from accepting his prize in person. Though he recovered well, he never regained his sight -- which did not stop him from continuing to invent applications for acetylene. Cutting-edge science is all too often dangerous work.

Answer: Putting it through cycles of expansion and compression.

In general, a gas cools as its pressure is reduced, reflecting the fact that the molecules in a lower-pressure gas collide with each other less frequently. The liquefaction of a gas like helium is a more complicated problem, however: the phase transition between a gas and a liquid typically requires both a decrease in temperature and an increase in pressure. It's a tricky business to attain ever-lower temperatures by liquefying ever-more stubborn gases.

In the 19th century, physicists developed several cyclical methods of approaching the problem. Both the Siemens cycle (1857) and the Hampson-Linde cycle (1895) begin with a compression stage, which heats the gas in lowering its volume. The next step is to cool this heated gas, first in a lower-temperature bath, then in a heat exchanged. In the Siemens cycle, the gas is further cooled in an expansion machine; in the Hampson-Linde cycle, the last cooling stage involves passing through a porous plug, which has no moving parts but is less efficient for hotter gases. After this last stage, the chilled gas passes through the heat exchanger the other way (cooling the compressed gas), and then enters a compressor to repeat the cycle.

Kamerlingh Onnes was not the first to liquefy a gas at low temperatures. In fact, he was able to use liquid air, liquid oxygen and liquid hydrogen as low-temperature baths when he cooled his helium, which has a boiling point of 4.4 K at one atmosphere of pressure. When he used the Hampson-Linde cycle to bring his liquid helium down to 0.9 K -- less than one degree centigrade above absolute zero -- it was said, somewhat facetiously, that his laboratory was the coldest place on Earth.

Answer: Its electrical resistance drops abruptly to zero below a critical temperature.

In a normal conductor -- like the copper wires that carry electricity in homes and businesses -- some electrical power is always lost because the current does not flow freely: it encounters some resistance in the conductor and power dissipates as heat. Although this resistance was found to be lower in colder wires than in warmer ones, no one conceived of a perfect conductor until 1911, when Kamerlingh Onnes saw the resistance of mercury drop to zero when the sample reached a temperature of just 4.2 degrees Celsius above absolute zero. This was the first superconductor! Over the next few years, scientists worked to identify other superconductors (like lead and niobium nitride) as well as other characteristics of the strange materials. For example, in a phenomenon known as the Meissner effect, superconductors act to prevent the presence of an internal magnetic field; if a magnet is brought close by, it induces currents on the superconductor's surface that effectively cancel the magnetic field inside.

Modern materials physicists are engaged in an intensive search for "high-temperature superconductors" -- materials that are in a superconducting state without the aid of super-cold cryogens. Some success has been found with various families of ceramics that have a critical temperature of about 150 degrees Celsius above absolute zero -- and if we can ever develop superconductors that operate at room temperature (about 300 degrees above absolute zero), humanity's power problems will be dramatically reduced. Imagine transmitting electricity over long-distance power lines with absolutely no loss! The full promise of Kamerlingh Onnes's discovery has yet to be realized.

Answer: The internal structure of a crystal is around the same size as an X-ray wavelength.

This experiment actually settled two outstanding questions: what are X-rays, and what makes a crystal crystalline? The whole thing hinges on diffraction, which is a phenomenon that occurs when a wave, like light, strikes an obstacle. When the wavelength - the distance from one peak of the wave to the next - is around the same size as the slit or gap it's traveling through, you get an interference pattern as the part of the wave coming from one side of the slit cancels out (or adds to) the part of the wave coming from the other side. If you're shining light through the gap and you measure what's arriving on the other end, you see a characteristic interference pattern, with dark patches where the waves cancel and light patches where the waves add to each other.

In 1912, when von Laue did his experiments, physicists were very puzzled by the identity of X-rays. How did they have such high energy? If they were a type of light, why couldn't you reflect them or refract them? If they were particles, why weren't they bent in a strong magnetic field? Independently, they were also puzzled by the question of crystal structure; many scientists subscribed to a theory holding that crystals were arrangements of atoms in regular, geometric patterns with space in between, but there seemed no way to prove it. Von Laue realized that there was a way to attack both questions. If X-rays were waves of light with very short wavelengths (as one would expect from their very high energies), and if crystals were really made up of atoms arranged in lattices, then the gaps in the crystal structure should be about the same size as the wavelength of an X-ray - and X-rays should diffract as they pass through crystals. He and his assistants, Walter Friedrich and Paul Knipping, performed this double test by shining X-rays through a copper sulphate crystal onto a photographic plate, answering both questions definitively. This result not only answered two fundamental questions, but also opened up a whole new field of investigation: the use of X-ray diffraction to study crystal structures in depth.

(So, why couldn't scientists reflect or refract X-rays? The trouble is their very short wavelengths: most materials have an index of refraction very close to 1 in the X-ray range, which means that there is no appreciable refraction. Also, materials tend to absorb X-rays, which makes reflection harder, too.)

Von Laue was a German physicist and did this work in Munich. He is remembered not only as an excellent scientist but also as a gentleman: when he shared his Nobel prize money with his assistants, no one was surprised. Famously, he resisted the Nazi insistence on a "pure" German physics - without such "degenerate" notions as general relativity - and was the only member of the Berlin Academy of Sciences to protest when the Academy's vice president claimed that Einstein's resignation was "no loss."

Answer: Consider the structure plane by plane, reducing a three-dimensional problem to two.

The two Braggs were father and son, and the younger Bragg - who went by Lawrence - had trouble his whole career with people who thought that the pair's prize-winning work was primarily his father's. It was Lawrence, however, who had the key insight while he was studying for his doctoral degree. Von Laue had proved that crystals had a regular, lattice structure, so if you could take very thin slices of crystal you should get one sheet after another. Lawrence realized that you could treat the sheets independently from each other: when you shine X-rays through a crystal, some of them interact with the first sheet, some with the second sheet, some with the third, and on and on. The result is the Bragg equation, a deceptively simple relationship between the incidence angle of the X-rays on each sheet of atoms; the distance between two adjacent sheets; and the wavelength of the X-rays. If you know the wavelength of the light, you can thus rotate the crystal to map out the lattice spacing in every direction.

William, fired by his son's calculations, constructed a device that accurately delivered X-rays to crystals and measured the resulting diffraction patterns. They studied one structure after another with such precision that, as the Chairman of the Nobel Committee for Physics observed in his presentation speech, the biggest source of error was the physical constants -- the numbers like h and c that you can nowadays look up in textbooks!

Lawrence, who at 25 became by far the youngest person to win the Nobel Prize, went on to extend his X-ray diffraction techniques to the study of molecular biology. Four of his coworkers at the Cavendish Lab went on to win Nobel Prizes for discoveries made by X-ray crystallography, coincidentally all in 1962: Francis Crick and James Watson (in Medicine, for working out the structure of DNA), and Max Perutz and John Kendrew (in Chemistry, for working out the structure of globular proteins).

Answer: The number of protons in its nucleus

The periodic table of the elements was assembled based on the way that atoms of different elements behave chemically, and allowed the key insight that an element further down in the periodic table had a higher atomic weight. We now know that the atomic weight is proportional to the number of protons and neutrons in the nucleus, but it's the number of protons that determines an element's chemistry. A neutral atom has the same number of electrons as protons, and undergoes chemical interactions based on how those electrons are distributed in the atom.

We know that now - but in 1917, this was all very mysterious. After all, the very knowledge that there WAS a nucleus - a small, dense concentration of mass and positive charge in the middle of an atom - was only six years old! Neutrons wouldn't be discovered for another fifteen years. Barkla's studies of secondary radiation were an earthquake in the field. Others had noticed that materials tended to give off radiation when X-rays passed through them, but Barkla realized that there were two types of secondary radiation: reflected X-rays from the original transmission, and "characteristic" radiation whose wavelength depended on the material. We now know that this characteristic radiation occurs when an X-ray ionizes an atom, knocking off an electron. When this electron leaves a gap in an inner (lower-energy) orbital, an electron from an outer orbital drops down to fill the gap, and the extra energy is emitted in the form of a photon. The wavelength of the photon depends on the energy gap between the two orbitals involved. Nowadays, this X-ray fluorescence is a common tool for identifying which elements are present in a sample.

Barkla soon realized that his X-rays were scattering from electrons, and that the frequency with which they scattered could tell him how many electrons there were in an atom to start with. And since his atoms were neutral, that number was the same as the number of protons - and it quickly became clear that it was the number of protons that mattered when it came to X-ray fluorescence and other characteristics of the elements. Suddenly the periodic table revealed a beautiful underlying structure. The elements didn't just march up in irregular intervals of atomic weight; instead, each one had an atomic number exactly one higher than its predecessor. This realization had a thrilling sidebar: at last, scientists could predict exactly how many holes remained in the periodic table, marking elements that had yet to be discovered.

Answer: At any given frequency, the absorber can only emit energy in multiples of a certain minimum energy.

In physics, a perfect absorber - something that absorbs all the light that strikes it, so that none is reflected, and so that all the light it gives off is from thermal emissions - is known as a black body. Stars are usually modeled as black bodies

Before Planck solved the problem in 1900-1901, there had been a number of attempts to develop a mathematical model for the shape of a blackbody spectrum. Planck began by laboriously working out the form of the equation from the available experimental data, and then devoted himself to deriving the equation theoretically. He did it by considering a box - a cavity - with walls that reflected all the light on the inside, except for the light that was able to escape through a pinhole. The cavity contained several oscillators, each with its own characteristic frequency. (The frequency of a wave is the number of complete cycles per second; higher frequency means more energy carried by the wave.) This simple system produces the right equation - but only if the energy of oscillator was quantized. Say that an oscillator had a frequency f. Its energy could be 0, or hf, or 2hf, or 3hf, where h is what would become known as Planck's constant - but it couldn't be anywhere in between (like half an hf or hf times pi). When a physical quantity has to have a discrete value that is an integer multiple of some minimum quantity, instead of being allowed to vary continuously, it's said that the quantity is quantized.

Initially, Planck regarded this as nothing more than a mathematical trick, and he saw it as something that applied only to his theoretical oscillators inside the blackbody cavity - not to light as it traveled through space. Others, however, took the idea and ran with it. Soon the idea of light as being made up of photons - discrete bundles of electromagnetic energy equal to h times their frequency - took hold (largely thanks to Albert Einstein). Planck's quantization of energy became the founding principle of quantum mechanics, which was named in its honor.

Planck was slow to accept the theory he had founded, but is remembered with great respect and affection for his rigor and his moral grounding; the 80-odd Max Planck Institutes, which conduct basic research in Germany, are a testament to his reputation. In the 1930s, he resisted Nazi efforts to stamp out so-called "Jewish physics" (namely relativity and quantum mechanics). He outlived his first four children, including a son who was executed in 1945 for his role in a plot to assassinate Hitler.

Answer: Electron energy levels in an atom

You can see the whole visible spectrum via the refraction of white light in a prism, but that's continuous - a "spectral line" is something different. Imagine that you're looking at that rainbow spectrum after it's passed through, say, hydrogen gas. You'll see that the spectrum is interrupted by dark stripes: light at those particular frequencies has been absorbed by the hydrogen, so it isn't available for you to admire afterward. Why does hydrogen preferentially absorb only light at certain frequencies? Well, if an atom is going to absorb light, it's going to gain energy (that is, enter an excited state), and it needs a place to put that energy. This means that the atom can only absorb light that's exactly the right energy to kick an electron into an available higher-energy state. That makes a dark absorption line at that place in the spectrum. (Excited atoms tend to decay to lower-energy states, releasing light with the same energy as the gap between the levels; that makes bright emission lines in an otherwise dark spectrum.) Each element has its own distinctive spectral lines. This is how Stark earned the first half of his Prize, by the way: the "canal rays" were really fast-moving, excited ions (positively charged atoms), and Stark observed a distinctive frequency shift in their emission lines depending on whether the ions were moving toward his apparatus or away from it.

Often, there are several states available to an electron that all have the same energy. For example, the electron might have spin up or spin down, or it might have a different angular momentum but the same approximate distance from the nucleus. These states are degenerate - they're different, but they're the same energy - until something happens to break the symmetry and separate the energies. Stark used a very large electric field to break a degeneracy between states with different angular momentum. With no field on a hydrogen atom, four available states all had the same energy, showing a single spectral line; with the field applied, they split into three different lines. (The degeneracy was only partially broken.) One of quantum mechanics' early successes was its explanation of the Stark effect.

It's ironic, then, that Stark turned so violently against quantum mechanics (and against relativity). Despite his skill as an experimentalist, he was more attached to his own conception of the world - and his own racist views - than he was to scientific evidence. Along with 1905 Nobel Physics laureate Philipp von Lenard, he sought to forge an "Aryan physics" free of the "Jewish" influences of Albert Einstein and others. In fact, one of the postwar denazification courts sentenced him to four years in prison as a "major offender." (On appeal, another court downgraded that to "lesser.") It was a shameful legacy for a man who had earlier achieved such great things.

Answer: Invar

At first blush, this may not sound very much like physics, but these "anomalies" are in fact tremendously useful. One of the foundations of modern science is the concept of reproducibility: you should be able to repeat someone's measurement and get a consistent result. To do that, though, you need to have agreement on what the numbers mean. Common units, like the metric system, make it possible to interpret and repeat measurements from other groups.

How do you define a meter, though? Nowadays, meters are defined based on the speed of light, but in Guillaume's time, accurately knowing the length of a meter required a precise replica of the prototype meter, a platinum-and-iridium bar of the right length. But metals tend to expand and contract when their temperature changes. In a high-precision experiment, fractions of millimeters matter, and you don't want a situation where you get different results in summer and in winter. The prototype meter was fairly temperature-stable, but platinum is very expensive. Guillaume was sure he could do better. And, in 1896, after long, thorough, and careful experimentation with nickel steel alloys, he did do better: Invar (named for "invariable"), which is 36% nickel and 64% iron. And, not only is it cheaper than platinum-iridium, but it's also more stable! (At least up to around 225 degrees Celsius. It's still not clear exactly how Invar's temperature stability works, but it seems to be partly due to its ferromagnetism - and that temperature is where that internal magnetism begins to break down.)

Invar is still the primary component of surveying equipment and other high-precision instruments. Guillaume lived to see it widely adopted, along with a related alloy called elinvar. At the time he received his prize, he was the Director of the International Bureau of Weights and Measures, in Sèvres, France.