what is a ruby and why is it red?
why is the friction so low on a sled?
on a plot of P versus T,
sketch out the phases of helium three.
Janos loved simply stated questions - and took delight if he could make them rhyme. I had thirty minutes to answer. The first question was straightforward although there are answers at several levels of depth. The second turned out to be a trick question. The folklore answer is the pressure of a blade on the ice, lowers the melting point of the ice and gives a very low friction watery surface. I had skied, skated and fallen on the ice over the years, but I hadn't thought about what makes a surface slippery and hadn't questioned the explanation.
I worked out how much added pressure would lower the melting point and made a simple calculation - what if a skater had a few square inches of contact area and weighed two hundred pounds - say one hundred pounds per square inch at the blade. For this example the melting point drops less than a tenth of a degree F. If the ice is below that temperature in the first place, skating wouldn't work. This is something of a problem as I've skated, skied and fallen on much colder ice and snow.
At this point the problem becomes interesting. With the simple model broken one has to propose new models or think about ways to make measurements that would add insight. In an oral exam it would be foolish to propose a new model without offering a way to test it. I remember concentrating on possible measurements.
This turns out this is a difficult problem. The interface between two surfaces is largely hidden. Progress has been made in the decades since and there are two leading theories. Normal ice is a rigid hexagonal crystal structure of water molecules. At the surface this structure breaks down as there aren't any water molecules on the other side of the top molecules to hold them in place. The surface molecules form a semi-mobile liquid layer a few atoms thick. A very slippery layer. There are some problems with this as surfaces aren't very smooth at our scale A second theory holds that friction provides some local melting that goes down a few atoms creating a slippery liquid surface.1
Two theories and an incomplete understanding even though the underlying physics is well understood. There are a lot of interesting questions that haven't been worked out and this makes physics such an interesting sport. In the meantime enjoy Winter sport on your own or watching the experts. I'm a fan of Winter and its sport, but a warning for those that walk with me when ice is around. I'm not particularly observant of ice and am generally uncoordinated. This combination leads to falls. Since I've broken my left arm four times and my right arm zero, it probably makes sense to stay to my right.
Another interesting "obvious" question presented itself a few years before when I was an undergrad. The professor was famous for beautiful but deep simplicity. He liked oral exams and often started out with simple but impossible thought experiments to lead a student. It turned out this is how he played with ideas.
Imagine that fusion turned off in the Sun. Only that process shuts off - imagine that everything else stays intact. What would be observed from Earth?
Clearly a trick question. A similar question is sometimes posed in high school physics to get a student to consider the finite speed of light and the Sun's shape. In the simple case a black dot appears at the center of the solar disk and spreads grows to the edge. You can work out the rate if you like.
Far too easy and out of place - this was a quantum electrodynamics course. It was also about the time Ray Davis raised some interesting questions about how the Sun worked.
Sorting out how the Sun works has been one of the triumphs of science. During the industrial revolution the Sun was assumed to be a mass of something like a fossil fuel. Calculations indicated a very brief life - tens of thousands of years. This was in conflict with the geologic record. Darwin originally thought the Earth to be a few hundred million years old based on some contemporary geology, but didn't publish as the results from physics indicated a much smaller time. Physicists of the day didn't believe their calculations. They recognized there must be some unknown process at work, but at the time evidence was lacking. Like dark matter and dark energy now, it was a known unknown candidate.
Many threads came together. By the early 1800s physicists and chemists were using prisms to split light into a spectrum to identify molecules and atoms. Some curious souls tried this on telescopes as film became sensitive enough to create a record. Using this process helium was discovered - on the Sun before it was found on Earth. The industrial revolution had created a boom in physics to sort out thermodynamics and hopefully improve the efficiency of the machines of the day. The understanding of thermodynamics, in turn, helped create the intellectual foundations of quantum mechanics. Einstein showed the equivalence of mass and energy. Bits and pieces of understanding came together and the process of fusion - slapping a couple of atoms together to form a new heavier atom and liberating some energy - made itself apparent. The fundamental process of the Sun was beginning to emerge.
I often refer to fossil fuels as "nearly ideal." They are energy dense, easy to mine and process, inexpensive, easy to store and relatively safe. There are some unfortunate byproducts, but they drove the industrial revolution and continue to drive the economies of the planet today.1 Gasoline stores about one hundred times the energy per kilogram as the best rechargeable batteries. Coal stores about half as much as gasoline, but there are almost in the noise compared with nuclear processes. Specifically the energy from the mass difference when fusing hydrogen into helium has millions of times the energy density. It takes place naturally in stars, but sadly is very difficult to achieve on Earth.2
So back to the question. The process of solar fusion was almost completely worked out by the time I was a student. The mechanism is a bit complex, but basically four protons come together in a few steps to form a helium nucleus and release energy. Some of the excess energy is carried off by a neutrino and some off by very short wave light (gamma rays).3 The neutrinos don't interact strongly with matter and fly unimpeded through the Sun at nearly the speed of light. They fly out in almost unimaginable numbers At the Earth's distance there is a steady stream of about 70 billion per square centimeter per second. While most pass through the Earth as if nothing was there, a tiny fraction smash into things - things like solar neutrino detectors. If nuclear fusion were to suddenly stop we'd notice the solar neutrino flux would drop to zero. We'd see it about 500 seconds after fusion stopped - their time of flight to Earth.
Light is a bit trickier. Fusion only takes place in the inner core of the Sun. Gamma rays from the process bounce around as if they were in a huge spherical pinball machine. Along the way they downshift in energy so much of the energy is radiated as visible light by the time it makes it to the Sun's surface. My task was to estimate how long it takes for light energy to percolate out from the Sun's core to the surface where it could radiate to space. I came up with about 30,000 years. The best estimates are closer to 100,000 years, so this isn't bad for a quick back of the envelope.
Science often proceeds like this. One discovery building on another. It is often unclear exactly where the new ideas will come from and clever funding is something of a loaded dart game. Some questions respond more rapidly than others, but patience is part of the game.
Technology is somewhat like this. Exact paths are frequently unpredictable and practical uses can wait for decades while other supporting technologies become available.4 Technology is fueled by basic research. If you stop or slow basic research there still see may be progress for a few decades, but there is a point where progress fades. I fear the US and EU is currently in a partial retreat.
There is an additional complexity of mixing technology and sociology. In retrospect the fading of importance of the desktop web and the rise of the smartphone and social apps becomes clear if you look at basic sociology and anthropology and the notion of the strength of social links. The tech wasn't ready yet. If basic understanding isn't there the result is often a Cambrian-like explosion of new companies followed by a near extinction of most of them. Messy and unavoidable, but such is progress. To skate to where the puck will be and make it through this mess one must understand the technology path and sociology. Not an easy task.
1 I'm very interesting in getting away from their grip! Sadly they are so inexpensive, energy dense and easy to use and store that it is difficult finding substitutes.
2 People have been working on fusion reactors since the 50s. At first it was about 20 years off, but serious people tend to say it is at least 50 years off. My guess it will be 50 years off for quite some time. We can capture fusion energy from some future reactor or we can capture it from the Sun now.
Although the energy density from the mass difference in solar hydrogen fusion is tremendously high, the energy density in the core of the Sun isn't. It turns out the process is very slow at the temperature and pressure at the Sun's core - something like 275 watts per cubic meter. By comparison the metabolism of an average person is about 1450 watts per cubic meter - the Sun is closer to the metabolism of a lizard. The trick is a lot of hydrogen in the Sun, so it can burn for billions of years.
3 a very high level explanation here
4 It was very clear in the 90s that something like a smartphone would be revolutionary, but one had to wait on Moore's law, battery electrophysics and billions of dollars of infrastructure spending before practical devices emerged. I used my first touch screen in the late 80s and worked with a lab that focused on video gesture recognition at the same time.
For whatever reason I was making chocolate chip cookies. I give most away for obvious reasons, but good ones are worth the effort. Rather than give my favorite recipe(s) or get into the butter vs shortening wars, I'll offer a few tips that can improve any chocolate chip (or similar)
° Use good ingredients! Fresh high quality butter, sugar, flour, etc. I use pasteurized eggs so I can safely sample the batter. For vanilla use or make a good extract. For chocolate I use chop up a 60 to 70% bar. Usually Lindt. If I can get them I use chocolate discs from Valrhona .. they melt much more and help make a very chewy cookie.
° Chocolate's flavor comes out with salt. Fine grained sea salt - uniodized - is best. At these temperatures the iodized salt imparts an unnecessary flavor.
° Cream the butter and sugar until the mixture is very smooth. Over mixing isn't a problem. Be careful with the eggs. Don't over-mix when adding flour - over-mixing gives tough cookies.
° Chill the dough!! Wrap the dough in plastic warp and refrigerate for at least 24 hours. This allows the ingredients to combine and is a core secret used in most of the great cookies. Plus you have a supply of chocolate chip cookie dough in the fridge. Go ahead and have a bit if you've used pasteurized eggs:-)
° Use a heavy baking sheet or pan. The thin ones warp and lack the mass to average out some of the temperature differences in your oven. Stay away from dark metal. I like to line with a non-stick baking mat. Parchment paper is also good.
° Take the cookies out just before they are done enough for your tastes and let them cool on a rack to finish off the cooking.