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Week 0: Introduction What we're trying to achieve in this course. Imagine that it is ten years after you graduate. You may be an engineer, a scientist, or in some quite different job. But let's say you have a problem to solve. This course aims to equip you with some very powerful mental tools to help you solve that problem. Some of these tools are specific bits of physics knowledge. But mostly, it's a different way of looking at the world. This approach goes by many names - "Going to back to first principles", "Problem solving strategies", "Modelling", and even "Thinking like a physicist". Why does this matter? Because it will affect the whole future of the human race. Will the future look like this: Or like this? Ninja Physics/Paul Francis 2 Will science and technology continue to advance, leading us into an unimaginable interstellar future, or will progress slow down and stop, leaving an overpopulated and polluted earth to fight over finite resources? Is scientific progress slowing down? Many very respectable researchers believe that scientific progress is slowing down. Others disagree. Why might you think scientific progress is slowing down? We think that science is advancing rapidly now - with such wonders as the internet, smartphones and DNA sequencing. But compare them to the discoveries of 100-200 years ago, like radio, electricity, cars, jet engines, artificial fertiliser and antibiotics. It's hard to argue that the recent changes have had anything like such a big effect on people’s lives. Why might things be slowing down? Possibly because science is becoming harder. Galileo could revolutionise science by rolling balls across a table - nowadays it takes the multi- billion-dollar large hadron collider, run by teams of thousands of researchers. You could argue that the "low hanging fruit" has been picked. There were a certain number of scientific discoveries which could rapidly be turned into very useful inventions - things like electricity (light bulbs, telegraph, TV) and thermodynamics (steam engines, cars, jet engines). But they were all discovered a long time ago - more recent discoveries like the standard model of particle physics, or General Relativity, have had little or no effect on people's day-to-day lives. The Ninja Physics/Paul Francis 3 number of scientists in the world keeps growing rapidly, yet the growth in economic productivity due to science seems to be constantly dropping. The cumulative volume of human knowledge doubles every ten years, yet humans are not getting ten times smarter or able to learn stuff ten times faster every ten years - can our brains really cope? On the other hand, there are people who believe that scientific progress is accelerating. They point out that exponential growth (like we are seeing in computer speeds) has as increasing effect with time. Doubling something that is already very large has a much bigger effect than doubling something small. And even though the easier scientific problems have been solved, we have much more powerful tools to help us solve future problems. An artificial-intelligence- supported cyborg using quantum metrology can solve problems that would have baffled Einstein. And we know that there is a lot more to learn because nature can produce marvels (like a mosquito) that our technologies cannot even come close to matching. So which is it going to be - science grinding to a halt, or accelerating into the future? That's up to you. If you don't do it, as the scientists and engineers at one of the world's greatest universities, who will? References: https://en.wikipedia.org/wiki/The_Great_Stagnation - a book arguing the progress is slowing down. https://en.wikipedia.org/wiki/Race_Against_the_Machine - a book arguing the opposite. Ninja Physics In this course, we will be focusing on what I like to call "Ninja Physics", but is also called "Back-of-the-envelope" or "Fermi problems". Ninja Physics/Paul Francis 4 The basic idea is to very quickly come up with an approximate answer to a problem, using lots of approximations and simplifications to get to the essential physics quickly. I call it Ninja physics because it is very fast and very powerful, but often doesn't stand up well in the light of day. It's an alternative to standard "Samurai" physics, which is also very powerful, and is all about playing by the laws, but is much slower and more cumbersome. Why do Ninja Physics? It seems rather strange to deliberately make approximations. Why don't we just do things properly and get the "right answer"? Let's go back and think why we might need to do physics in the first place. We are probably trying to solve a problem, or come up with some new technology to improve life. So let's imagine that ten years from now, you have been given a problem to solve. It will probably be a very difficult problem (top students like you won't be given the easy problems). You will probably have no idea where to start. That's where Ninja physics comes in. It allows you to play with ideas quickly, and get your mind around what are the key factors. You can try out ideas and quickly see if they are at all sensible. For even the most brilliant researchers, only 1 in 10 of their ideas turns out to work (if all your ideas work, you are clearly having unimaginative, unambitious ideas). Ninja physics can rapidly eliminate most of the ideas that won't work, allowing you to focus on the few that will. Once you've identified the idea or ideas that have a real chance of working, Ninja physics helps you work out what factors are crucial to its success, and which are just minor details. You can then focus your efforts on better understanding the most important factors. Ninja Physics/Paul Francis 5 And only then, once you've found an idea that will work, and you've identified what are the most important factors, is it time to put in the details and try to get a very accurate "right answer". Basic Principles There are three basic principles of Ninja Physics: ● Go to first principles ● Build a simplified model ● Focus on what matters most What are "First Principles"? This means trying to understand the deep fundamental reasons behind something. Often it means trying to understand a situation in terms of the most fundamental laws of physics. It can mean asking "why" repeatedly, as you try to understand a situation in terms of very basic principles. What is a "simplified model"? Scientists and engineers can never calculate the full complexity of any real world situation. We always have to simplify it somehow to make it tractable. The trick is to choose the best simplifications - simplifications that make a problem solvable but don't mean that your model is no longer like reality. How do you focus on what matters most? To begin with, only worry about things that would change your answer by more than an order of magnitude (a factor of roughly 10 or more). At this stage, don't bother yourself about relatively unimportant things like factors of two, pi etc. Once you are sure you have the big stuff nailed down, then you can start worrying about factors of three etc. I'll show you lots of examples of how to do all this as the course goes on! Ninja Physics/Paul Francis 6 Week 1: The Physics of Normal Cars Flying Cars Venture capitalist Peter Thiel famously said "We wanted flying cars, instead we got 140 characters". He was bemoaning what he sees as a slow-down in technological progress. While there has been rapid progress in computers and smartphones, many other technologies like transport and energy are advancing very slowly. Ninja Physics/Paul Francis 7 (graphs from http://foundersfund.com/the-future/ ) Note the immense advance in computer chip power, and the complete lack of recent major advances in either productivity or travel speed across the North Atlantic (though this immense advance in computing power was only achieved by a vast increase in research resources - http://marginalrevolution.com/marginalrevolution/2016/12/depressing-paper-great- stagnation.html, and many think that this rapid advance in computing power is coming to an end - see - https://www.theguardian.com/technology/2017/jan/26/vanishing-point-rise-invisible- computer) Flying cars would be amazing. They would solve congestion problems in our cities (though they wouldn't solve parking problems unless they could fly themselves home after dropping us off), and transform the way we live and the shapes of our cities. Countless science- fiction shows have featured flying cars, ranging from Star Wars to Back to the Future. And yet we don't have them. Every year or two someone claims to be on the brink of launching a commercial flying car, yet it never quite seems to "take off". Why? Is it just that researchers are being slack and unambitious? Or is there some fundamental physical reason why flying cars are hard? And if so, what is this fundamental reason and can we get around it some-how? Ninja Physics/Paul Francis 8 This is a good job for Ninja physics - trying to understand what the fundamental physics of a flying car might be, and see what, if anything, is limiting it. This is what "Going back to first principles" is all about. How can we go about analysing something like the feasibility of flying cars? One approach is to try and figure out particular possible designs, and how feasible they are.

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