An attempt to understand the Nobel Prize winning Science of 2019
Every year, I would read that Nobel prizes have been awarded to certain distinguished individuals at some of the top research institutes. On further reading, I would realize that their research is almost completely incomprehensible to all but a few people across the world. This year, I have tried to read and blog about their research, if only to convey in layman’s terms what these individuals have achieved.
The Nobel Prize for Chemistry this year was awarded to John Goodenough, M Stanley Whittingham, and Akira Yoshina for the development of a safe and efficient Lithium-Ion based battery. I shall be following this article for the exposition.
Batteries have a simple enough principle- one element (element A) gives away electrons, and another (element B) collects electrons. This depositing and collecting should happen naturally (without any external input). Then electrons travel from element A to element B, forming an electric current in the process. The giving away of electrons happens at the the negative end of the battery or the anode, and the collection of electrons happens at the the positive end of the battery or the cathode.
One potential problem to avoid is the following: say that we need to light a bulb that lies on the path between the anode and the cathode. Then we need to ensure that the electrons only pass through the bulb. If the anode and cathode come in direct physical contact, then we will find a short circuit. This short circuiting is in fact a major problem in the manufacturing of batteries, and Akira Yoshino solved this problem in lithium batteries, amongst others, in his Nobel prize winning research.
The Voltaic cell, or the first battery ever produced, was made up of alternating layers of tin/zinc and copper plates. These plates were exposed to air.
But wait. All of these are metals: and we know that metals have a propensity to lose electrons. What will make one of them gain electrons? As one might know from a previous Chemistry class, it depends on the relative reduction (or oxidation) potentials of the elements. As zinc/tin have a greater propensity to lose electrons as compared to copper, they will do so. Copper, on exposure to air, forms CuO. In this state, copper is in a state . On receiving excess electrons at the cathode, the copper gains those electrons to again form Cu. This completes the circuit, and we have a current. In fact, the French monarchy was so impressed by the very first demonstration of the Voltaic cell that they made Volta a count! This battery created a voltage of 1.1 V.
We now come to the ubiquitous lead acid battery- both electrodes of which are lead, and the electrolyte contains sulfuric acid (). Clearly, as both electrolytes have lead, we don’t have a clear idea of which side should see the loss of electron and which side the gain! Turns out that one side has oxidized lead- (so lead is in state ). The non oxidized state sees Pb lose two electrons to form (and then ), whilst the other side sees gain two electrons to form . This battery creates a voltage of 2 V.
In the quest of making batteries that are lighter and produce higher voltages (the Voltaic battery was huge), scientists inevitably stumbled upon Lithium. It has a density of , which makes it ideal for batteries for watches, phones, etc. However, it is extremely reactive with water and air (as opposed to the Voltaic cell, which worked in fact only when exposed to air). This turns out to be a major problem that would take decades to effectively solve.
Two important developments happened in the 50s and 60s- one was that propylene carbonate was discovered to be an effective solvent for alkali metals (like lithium). Also, Kummer started studying ion transfer in solids. Note that atoms in solids are relatively rigidly fixed, and not free to move around very much. However, he noticed that sodium ions could move as easily within solids as within salt melts (salt melts would have a less rigid structure as compared to solids, and hence facilitate an easier transport of ions). The phenomenon of ion transfer would become important in the development of lithium batteries.
Now here’s the important difference for lithium batteries- we don’t want lithium to lose electrons at the negative electrode only for lithium ions to form compounds with the electrolyte. Lithium, being extremely reactive, would surely cause such reactions to be difficult to control. We want these lithium ions to float on to the other (positive) side, and just settle in between the atoms of that electrode. This process is called Intercalation Hence, we want a cathode which allows lithium ions to settle in, move about easily (easy ion transfer), gain back electrons if so desired, etc. Metal chalcogenides () were considered to be amongst such options.
One of the first such metal chalcogenides () to be considered was Titanium Sulfide- . A voltage of 2.5 V could be recorded in such lithium batteries, and Exxon started manufacturing these batteries. However, remember the problem of ensuring that the cathode and anode are not in physical contact? Dendrites of lithium started forming at the negative side (quite obvious, as lithium ions would travel from the negative to the positive side), which would eventually touch the positive side, short circuiting the battery. This was a huge setback for the lithium battery development.
Scientists eventually hit upon this idea- what if the positive lithium ions didn’t have to travel all the way from the negative electrode to the positive electrode? What if they could settle inside the negative electrode itself? They then started searching for materials that would make this possible. Akira Yoshino solved this problem- by considering heat-treated petroleum coke. This could form the anode, and allow lithium ions to settle in through intercalation at the anode (negative end) itself.
John Goodenough, on the other hand, found a material for the cathode that would increase the voltage of the cell from 2.5 V to 4-5 V. Instead of , he considered another metal chalcogenide- . Oxygen atoms are smaller than sulfur atoms (as in ), and would allow lithium ions to move about more easily. This would allow an easier gain of electrons by these ions, and hence a higher voltage. Moreover, lithium ions are especially mobile in close-packed arrays- and had exactly that structure. If the cathode and anode are both only meant to “house” the lithium ions, where would the lithium ions come from (if not from the anode plate)? The electrolyte- containing in propylene carbonate, along with lithium metal.
Hence, Yoshino and Goodenough, together, produced a much more powerful and stable lithium battery, and their research truly changed the world. The computer or mobile phone you might be reading this on is evidence enough.
Punchline: Peebles is the man behind the mathematical foundations of dark matter and dark energy!
We shall now begin with some background.
Cosmic Microwave Background (CMB) radiation- At the moment of the Big Bang (approximately 13.8 billion years ago), the universe was insanely hot, as one might expect. Electrons and nuclei were too excited (literally!) to combine to form elements. Charged particles would interact with photons (light), and hence light would not be able to travel long distances without being interfered with. 400,000 years of this madness, and then things cooled down (to around 3000 K). Charged particles no longer interacted with photos, allowing light to travel intergalactic distances, and telling us earthlings of galaxies far away in space (and possibly time). Electrons and nuclei could combine to form elements. Note that the energy of light travelling across an expanding universe would decrease because of redshift (this has caused the temperature of this radiation to drop from 3000 K to 2.7 K). The word “redshift” refers to a shift of frequency/energy from a higher (violet) region to a lower (red) region. In this case, the frequency has shifted to even lower than red- the microwave region. This, friends, is called the cosmic microwave background radiation- the radiation that has been travelling since 400,000 years after the big bang. And it is not the same in all directions!!
Let me try and explain the last line of the previous paragraph. Suppose you had an instrument using which you could measure the intensity/frequency, etc of the cosmic microwave background radiation ( CMB radiation is easily detectable on Earth, and its first detection also won the Nobel prize). Then if you turn in the instrument around in all directions, you will find a slight change in intensity, frequency, etc. This property, of not being the same in all directions, is called anisotropy.
We shall now derive some basic equations that are relevant to an expanding universe. We know that the universe is expanding in every direction. But what is the mechanism of this expansion? Expanding relative to what? These are some common questions that often trip the budding scientist. Let us, for purposes of illustration, imagine that the whole universe is an expanding balloon of radius – not just the rubber boundary, but the air inside too. Consider a mass $m$ on the boundary of the balloon. Then the energy of this mass is . Clearly, the first term is the kinetic energy and the second term the potential energy. Here .
A little rearrangement of this gives . Here , and may be interpreted as curvature. A fundamental question in cosmology has been- does the universe have positive curvature (is shaped like a ball) or negative curvature (is shaped like a horse’s saddle at each point)? Or is it flat (zero curvature)? Turns out that it is very nearly flat. However, arriving upon this answer was not easy, and took decades of cutting edge scientific work. Peebles was instrumental in arriving upon this answer, which lay upon understanding that the universe is at critical energy density (and not more or less, which would be characterized by positive and negative curvature respectively).
One of the fundamental properties of the universe that is studied in cosmology is energy density- how much energy does the universe pack in a given volume (given ball), and how does this density change when that volume volume itself expands (as the universe is expanding)? The equation tells us that matter can be converted into energy, or is just another form of energy. The energy density contained in matter changes by a factor of when the given volume expands from a ball of radius to a ball of radius . The energy density contained in radiation (say in light) changes even more, because an expanding universe creates redshift (loss of energy) as explained above. When a ball expands from radius to , the energy density in radiation changes by a factor of . Let us now try and bring these facts together.
Baryons (traditional matter that humans can perceive) form around 5% of the mass/energy of the universe. If baryons were the only matter in the universe, then our theories of gravity would predict a vastly different universe than what we can see. Galaxies would not form, and we would all be floating subatomic particles in space. To come up with a concept of matter that makes gravitational clumping of planets and galaxies etc possible, scientists came up with dark matter. However, this dark matter behaves like ordinary matter under expansion of the universe, in that its matter/energy density decreases (by a factor of ) on expansion. Hence, the energy density from matter and dark matter would become more and more sparse with time. This does not explain the energy density of the observable universe, as can be measured by cosmologists. Scientists then came up with the concept of, wait for it, dark energy. The energy density of dark energy doesn’t decrease with the expansion of the universe. Almost sounds like a cop out! But the presence of both dark matter and dark energy have been confirmed by multiple scientific experiments since the time of their conception. Dark energy should form 69% of the total energy in the universe.
Where does dark energy show up mathematically? Let us think back to the equation . Let us now add a constant to the right, to get . Note that . Hence, the equation looks like . As the size of the universe (or ) grows, the term comes to dominate, regardless of how small the value of is (it is scientifically predicted to be quite small, actually). Hence, looks more and more like a quadratic equation, which means that looks like a line . A velocity that grows linearly with suggests a non-zero acceleration. Hence, the larger the side of the universe, the faster the galaxies recede from each other! As is supposed to denote dark energy, it is this dark energy that causes galaxies to accelerate away from each other.
Now what was Peebles’ contribution to all of this? Turns out that when Penzias and Wilson observed CMB radiation in 1964, the theoretical basis for such a radiation (at 10 K) had already been laid in Peebles, and in fact Penzias and Wilson could understand the import of their discovery only after talking to Peebles.
Another contribution of his was the following: scientists used to think that both light elements (like hydrogen and helium) and heavier metals (like iron) were produced right during the big bang. However, Peebles clarified that only light metals could have been produced during the earlier stages of the universe, and that too when the temperatures had dropped enough to convert deuterium (a hydrogen isotope) to helium. If the matter density at this moment was high, then large amounts of helium would have been produced, otherwise lower.
Anisotropies in the CMB- Energy/temperature of CMB radiation is affected by two factors: (1) if the radiation is climbing out of a deep potential well (say getting away from an object with high gravitational attraction), then the radiation loses lots of energy in the process of climbing out, hence causing a lowering of temperature. (2) During decoupling (separation) of the radiation from charged matter (400,000 years after the big bang), the potential energy between the charged particles and photons is converted to energy of the photons, raising their temperature.
Remember that CMB radiation tells us about the state of the early universe. In the early universe, fluctuations in density would cause acoustic waves to travel in the hot plasma (acoustic waves are waves with frequencies in the acoustic region. They can vary across other parameters though). These acoustic waves would inevitably leave an imprint on CMB radiation (although they themselves would not be CMB radiation). These waves can be of different frequencies (all within the acoustic region however), and there can be a different power associated with each frequency. The power spectra of these acoustic waves tell us a lot about the early universe, and also help us detect dark energy!
The first peak is formed when baryonic (normal) matter and dark matter fall towards the centre of mass (perhaps) under the influence of gravity. Note that even such a collapse can produce acoustic waves, much like a building collapsing can send outwardly radiating cracks through the structure. Now after this collapse, radiation, with its energy increased because of this collapse, forces matter out again. This produces the second peak. However, the radiation cannot force dark matter to come out, as dark matter does not interact with radiation (that is the reason why we cannot see it or perceive it in other ways). This dark matter exerts a gravitational force on the baryonic matter, and causes the latter to collapse again, causing the third acoustic peak. Because it is only the same baryonic matter that comes out and then collapses again, the height of the third peak is exactly the same as the height of the second peak. The relative heights of the peaks tell us that baryonic matter is only 5% of known matter, and that dark matter is 26% of known matter. The rest of the 69% is dark energy.
What was Peebles’ contribution to all of this? He insisted on including the cosmological constant , which brought dark energy to the fore, and helped explain the heights of the acoustic peaks. He also accurately calculate the anisotropy of CMB radiation as , which was experimentally confirmed. He also predicted that anisotropies are visible in CMB radiation only at large scales, and that at small scales these anisotropies are mitigated due to diffusion. This was also experimentally confirmed.
Peebles is perhaps the rock around which our understanding of the composition of the universe revolves. A laureate amongst laureates.
Michel Mayor and Didier Queloz
Planets revolve around stars right? Almost. In any “solar system” (system of a star and its planets), both the star and the planets revolve around a common centre of mass. Analyzing this stellar motion, however small, is the most promising way of detecting whether that star has accompanying planets, because the gravitational pull of the planets would perturb the motion of the star in observable ways. The planets themselves would not be observable directly because of their extremely small size and distance. This is the kind of analysis that Mayor and Queloz did to detect earth-like planets around distant stars, kickstarting this whole field.
But how would one observe the motion of stars? Would we see them moving across the sky, and then make deductions? No. If one were an observer in the plane of the rotation of the star about the common centre of mass, sometimes the star would be coming towards us, and at other times it would be going away. Hence, Doppler effect would help us study the motion of the star.
The way that Doppler spectroscopy would work before was that scientists would compare the spectra of stars with the spectra of gases like hydrogen fluoride (HF), and then make deductions about the motions of stars. This was a fairly restrictive technique as only bright stars could be analyzed this way. Michel Mayor instead used the new fiber cable-linked echelle spectrograph called the ELODIE spectrograph, using which all kinds of stars, of low and high brightness, could be analyzed. Clearly, this opened up a lot more stars with potential planets to scientists.
Soon, using this spectrograph, Mayor and Queloz observed the star 51 Pegasi to have a revolution period of just 4 days, which helped them study many periods of this star, and hence its motion in great detail. Soon they deduced that it had a Jupiter-like planet 51 Pegasi b at an astonishing distance of 0.05 AU. Earlier, scientists had thought that a Jupiter-like planet would have to be at a large distance from its star. However, this discovery turned that prediction on its head. It was later hypothesized that such planets were probably formed at a large distance, but migrated closer to such stars due to gravitational attraction and other effects.
Mayor and Queloz started this revolution with a slightly improved spectrograph and suspended beliefs (about how far a Jupiter-like planet should be from its star), and now that revolution has yielded 4,000 exoplanets and 3,000 planetary systems. Moreover, the method of detecting planets has moved from studying Doppler effects to studying the reduction in brightness of stars when planets pass in front of them.
Hopefully, we shall soon discover life in an exoplanet, and end our isolation in the universe.
Thanks for reading!