Expansion, Einstein, Black Holes, and More
The idea that we live in an expanding universe probably raises lots of questions for you, such as “What is space expanding into?” or “Where is the center of the expansion?” Scientists can answer such questions; for example, modern science tells us that the answer to the first question is that space is not expanding into any pre-existing space (it is space itself that is growing with time), and the answer to the second question is that the expansion has no center (and no edges). However, these answers probably won’t make much sense until you first learn what Einstein’s theory of relativity tells us about the nature of space, time, and gravity.
We won’t cover relativity in this course, but it’s worth noting that although a complete understanding of it requires advanced mathematics, it is possible for middle school students to understand the basic ideas. Moreover, these basic ideas lead to many fascinating ideas that you may have heard about; for example, relativity explains both the famous equation E=mc2 and the exotic objects known as black holes. There are many excellent books and videos that can introduce you to the ideas of relativity, including a book written by the author of this textbook (called What is Relativity?).