Einstein circa 1905
There are a lot of people who, to this day, deny the truth of Einstein’s special relativity (SR). I’m not even referring to the OPERA superluminal neutrino debacle—an anomaly that was eventually found to be caused by a misconnected fiber optic cable. No, I’m referring to laymen who deny SR because it goes against common sense.
A Google search will find such people readily. Most of the time, their arguments aren’t even worth refutation, since it’s obvious in most cases that they haven’t mastered even the simplest algebra, much less sophomore-level physics. (I am planning to use this gem in my Modern Physics class in the spring as a homework problem: for 20 pts., find the elementary flaw in this person’s logic.) However, as a working physicist, I sometimes find myself dismissing such people too readily: it is easy, and self-gratifying, to call such people cranks.
A person who doubts SR is not necessarily a crank. After all, relativity is very counter-intuitive, and our brains have been exquisitely fine-tuned by natural selection to perceive the world as inherently classical. In fact I will go so far as to say that if you accept SR whole-cloth, without any mathematical or scientific background, then you’re basically showing a blind faith in science in the same way that Iotians have a blind faith in “The Book”. I would rather beginning physics students showed some skepticism; it makes their final “conversion” that much more intellectually pleasing.
I think the main problem with perceptions of SR is the way it is normally presented. My thesis is this: most physicists are teaching it wrong. And as a consequence, many people who have studied SR come away with a misguided notion of what SR is all about.
The old way to teach SR begins with Einstein’s two postulates. The first is that the laws of physics should be the same, in any inertial reference frame. The second is that the speed of light is the same for all inertial observers. There is then an obligatory picture of a train and lightning bolts, and talk about how simultaneity isn’t preserved in SR. This leads (usually after a lengthy derivation) to time dilation and length contraction. And then, out of the blue, there might be talk of the twin paradox and the obligatory pole vaulter in the barn.
Shudder. Such a pedagogically confusing approach! No wonder very few first-time SR students “get it” at all.
The original train picture from Einstein’s 1916 book
This approach has a long history. In Relativity: The Special and General Theory (1916), by Einstein himself (!) the discussion begins with the two postulates, and there is then a diagram of a train and a discussion of simultaneity (see above). Seriously? I’m not blaming Einstein, mind you; I’m blaming the textbook authors today who can’t let go of that stupid train. It’s been almost 96 years. Get over it. Hop off that train, please. There are more intuitive approaches that are easier for the layman to grasp.
Here’s the approach I use in my classes. This is not the only approach, of course, nor do I claim it is the best approach. However in my experience (admittedly, just one data point) this approach is a better way to get students to gradually accept SR. The trick is to present information one plausible chunk at a time, and then only gradually derive all the weird stuff. Thus, without realizing it, the students have been convinced of the truth of SR despite themselves. If you start with simultaneity and time dilation and length contraction then half of the students will get turned off immediately (because their common-sense alarms will be blaring full-force).
- Talk about classical (Galilean) relativity. That is, discuss how the laws of physics should be the same in any (inertial) reference frame you choose.
- Talk about coordinate transforms: how you can take the spatial coordinates of an object (x,y,z) and find what the coordinates (x’,y’,z’) would be in a different coordinate system.
- Talk about how some coordinate transforms are “good” and some are “bad”. For example, a translation in space such as x’ = x – L preserves distance, but a rescaling transform like x’ = ax does not.
- Mention how the good ol’ Pythagorean theorem s2 = x2 + y2 + z2 gives you an invariant quantity (s2) that is preserved under “good” transforms.
- Mention that the (experimental) behavior of light throws a monkey wrench into this analysis. For whatever reason, all observers measure the same speed c for light, and this actually makes things a little harder. (Don’t do any math at this point!)
- Here you should start talking about time as being a 4th dimension. The earlier you introduce the idea of an event P as a point P=(x,y,z,ct) in space-time, the better.
- State Einstein’s postulate about the speed of light.
- Show that the light postulate implies that s2 = x2 + y2 + z2 is no longer an invariant quantity, when talking about transforms as applied to space-time.
- If s2 = x2 + y2 + z2 is no longer invariant, can we modify the formula in any way so as to make s2 invariant, while still preserving the light postulate? The answer is yes; and so you should derive the 4D version of the distance formula, s2 = x2 + y2 + z2– c2t2.
To me, this is the core idea of SR. Everything else follows from the invariant interval s2. One should no longer think of our existence as being 3D; time represents another “direction”. And it turns out that the time you perceive depends upon your vantage point (time is “relative”), just like position.
For example, suppose you are looking at a row of trees. From one location, the trees are lined up in front of you (they all share the same x-coordinate). From another vantage point, they are separated by 1 m each (x=0, 1m, 2m, 3m, etc.) No one, not even Galileo, would find this controversial.
But now imagine that you think of time as just another “direction”. Why is it so hard to believe that your time coordinate could have one value in one reference frame, and another value in a different frame? Why is it so hard to believe that events that are simultaneous in one frame are not simultaneous in another?
Time dilation and length contraction follow from this in a straightforward way. And they are much easier to visualize if you buy into the paradigm (I’ll say it again) that time is another “direction”, and therefore relative just like position.
[Caveat: I do know that time is special, in the sense that there’s a minus sign in the s2 = x2 + y2 + z2– c2t2 formula. That minus sign is crucial. But discussing its importance should be deferred to a later (pun-intended) time.]
If you’re interested, here’s the rest of my SR program:
10. Discuss space-like, time-like, and light-like intervals, and the ideas of proper length and proper time.
11. Show how the Galilean boost (which is a “good” transform in classical relativity) must be modified into the Lorentz boost in order to preserve s2.
12. Show how the Lorentz boost implies length contraction and time dilation.
13. Discuss relative velocity in SR.
14. Discuss so-called paradoxes like the twin paradox and the pole vaulter paradox.
The discussion can then go into advanced topics: momentum, energy, E=mc2, forces, etc. However, with the foundation I have described, I believe these topics are much easier to present.
I’m sure there are some great professors out there who have had great success with the “traditional” program of SR instruction. I’m sure Feynman could teach circles around me, even with the train and lightning bolts. But I prefer this different approach, as I have presented it, and I hope others will realize that there’s more than one way to explain special relativity.
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