The Physics of Relativity and a Lesson in Skepticism
Posted by mattusmaximus on August 5, 2011
I recently had a guest post appear over at the JREF Swift blog, and I wanted to share it with you all here. Enjoy!
I often spend at least a few days or, if I’m lucky, a few weeks addressing the topic of modern physics (that is, post 19th-century physics) in my high school classes towards the end of the year. And the topic I spend the most time on is Einstein’s theory of relativity, something which never fails in gaining the interest of my students, despite the fact that summer vacation is just around the corner. It’s one thing to talk about Newton’s laws, force diagrams, and vectors, but once you get to that “good stuff” like light speed, time travel, and whatnot the students perk right up. That’s precisely why I teach the topic at the end of the year when it is most difficult to keep classes on track.
Whenever I introduce this topic I start off with a very basic review of the physics of relative motion – many students roll their eyes at this introduction as “too simple” because it is a rehash of simple vector addition. For example, if you are traveling down a road in a bus that is moving at 50 mph and you throw a ball in front of you at a speed of 20 mph (from your viewpoint), an observer on the side of the road will see the ball moving at 50 mph + 20 mph = 70 mph, assuming there is no acceleration involved. But here’s the rub, and quite an extraordinary claim on my part: that idea is wrong!
Now that usually gets my students’ attention. How can this simple rule of velocity addition be wrong?! Don’t we use these rules all the time in the world around us to do everything from plan out plane routes to driving down the freeway? When I drop the “this rule of velocity addition is wrong” bomb on my classes, it is wonderful to see the immediate skepticism on display in both the students’ questions and mannerisms. Some of them even look at me as if I’ve lost my mind.
And this is a good thing, folks. By the end of the school year, I want my students to feel free to openly express their skepticism as an exercise in critical thinking. They should question me about a claim so bold as “the velocity addition we’ve used all year is wrong”, and they should demand a really good argument as to why my claim is accurate. And I should have to work hard to justify the claim, and I do. …