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.
Starting with a detailed analysis of the Michelson-Morley experiment, perhaps the most famous failed experiment in history, is a good start. I take my students through the 19th-century idea of a hypothetical “ether” which was thought to permeate all of space and thus provide an intangible medium through which light waves could propagate, and how Michelson and Morley set up an experiment in the 1880s to detect this ether by measuring very slight shifts in the speed of light relative to the motion of the Earth. Of course, as any student of physics knows, this experiment is so famous precisely because it failed so utterly – the Michelson-Morley experiment detected absolutely no shift in the speed of light, no matter what the conditions!
What this means is that, no matter how fast the relative motion of the observer is to the platform launching the light, they will always measure the same value as an observer on the platform. The platform could be moving at 99% the speed of light, and both observers will still measure the same value for light speed.
Once this fact (and it is an experimental fact, verified repeatedly over the last 120 years) starts to sink in with my students, they can start to learn how it leads to all manner of strangeness. Some of these effects include time dilation, where the rate at which time is observed to pass is seen as slower when the clock is moving relative to the observer. This is another bold claim on my part, but it follows logically from an analysis of the constancy of light speed as outlined by the Michelson-Morley experiment. I not only argue this from the standpoint of logic, but I actually have my students perform an experiment whereby they confirm the reality of time dilation for themselves, thus cementing the notion in their minds.
And last, but not least, as the lessons go on I show my students how other consequences of Einstein’s theory of relativity have led to the development of some important forms of modern technology: nuclear power plants operate on the principle of mass-energy equivalence (E = mc^2), and the GPS system wouldn’t function were the effects of time dilation through general relativity not taken into account. So these seemingly weird and outlandish ideas, which my students so rightly question with skepticism in the beginning, turn out to not only be true but lead to devices (think GPS receivers) they can hold in the palm of their hands.
Wow, that’s powerful stuff.
And therein lies an inherent danger: the desire to cling on to some kind of “far out” or oddball idea because of the “coolness” factor. This is where so many physics cranks, charlatans, and New Age gurus of various stripes take advantage of the credulous – they make the extraordinary claim but then play off the gullible who don’t demand the extraordinary evidence to back it up.
Relativity theory is cool not just because it proposes outlandish ideas… it is cool because it is real. It really does describe the way in which the universe around us functions, and that reality is borne out by decades of hard work, experimental verification, and – yes – skeptical thinking about the claims.
As my students have learned by the time they leave my class at year’s end, being skeptical isn’t about simply pooh-poohing and dismissing claims, it is about applying a sharpened and well thought-out reasoning process to distinguishing good claims from bad claims. And without this key aspect of skepticism, reality would be very much lost to us.
Matt Lowry is a high school & college physics professor with a strong interest in promoting science education, skepticism and critical thinking among his students and the population in general. Towards these ends, he works with the JREF on their educational advisory board, and he also works with a number of grassroots skeptical, pro-science groups. In what little spare time he has, he blogs on these and related subjects at The Skeptical Teacher.