*Matt Strassler* # Progressive Summary Galileo established the Principle of Relativity, which states that we can't tell if we are in motion when we are in a bubble separated from the rest of the world. Ie the experience of motion is always relative to something else. This also means that there is no one correct experience of motion. Depending on what's around us, we can either be stationary, or moving in any direction. Newton based his 3 laws of motion on this fundamental principle. The 1st law, which is about inertia, or the fact that objects will continue coasting in the absence of friction, is a direct result of this principle. The Principle of Relativity holds that in the absence of friction, coasting doesn't take any more energy than staying still. This is what Newton's law of inertia says. By contrast, Aristotle held that an object will eventually come to rest if an external force isn't there to push it along. It says there is a fundamental difference between stationary and moving objects. The Principle of Relativity helped to dethrone humanity from its privileged perch at the center of the universe. We are on a rotating earth, orbiting a sun, which is itself on the periphery of the Milky Way. We hear scientists say that the universe is 14.8 billion years old, but in actual fact, what we can say is that the observable universe is 14.8 billion years old. # Definitions radioactivity - the process by which an atom is transformed from one type to another principle of relativity - it is impossible to detect if an object is in motion or at rest if it is in a bubble separated from everything else # Chapter Notes ## 4 - Armor Against the Universe > The language spoken among particle physicists is similar to English in many ways, but it is a dialect of its own, blending ordinary language, scientific jargon, and something more insidious: false friends. A false friend is a word in another language or dialect that sounds just like one you know, making it seem as though it needs no special definition. > Would you believe that Earth’s air is made of metals? For many physicists, a metal is a solid crystalline material that conducts electricity, just as in English, but astronomers use that word for any atom whose nucleus was forged inside a star. By that definition, which includes every element except hydrogen and helium, not only Earth’s atmosphere but also its living beings are all predominantly made of metals. > Few words are worse, in this regard, than matter. It has at least two contradictory definitions used by particle physicists and at least two others used by astronomers. All the definitions agree that atoms (and thus all ordinary objects) are examples of matter, while light is not. But they disagree as to whether more exotic objects, such as Higgs bosons, neutrinos, and antiprotons, are matter. It’s not even clear that “dark matter,” a term used widely by astronomers and particle physicists alike, is actually matter. > For the opening sections of this book, the most important thing to remember is the following: matter is a substance out of which objects can be made, whereas mass and energy are properties of objects, not substances. More generally, it will be crucial in this book to distinguish substances from properties. > Regarding mass and matter, there’s another issue raised by what many of us were taught as schoolchildren. In my chemistry class, I learned that the mass of an object is the “quantity of matter” inside it. It’s a definition that was introduced by Newton himself. He argued that the more matter an object is made from, the more difficult it is to change its motion. We will soon see why this viewpoint works well in chemistry class and in daily life. But in subatomic physics, serious flaws appear. We’ll soon encounter objects that have mass but are not (by most definitions) made from matter. Other objects that are clearly matter (by all definitions) may in the past have had no mass whatsoever. These are not the only problems. The definition of mass used in modern physics does not refer to matter at all. # Quotes # References