Writing Relativity vs. Writing Quantum

Fall term classes ended yesterday, officially-- my last class was Friday-- so I'm shifting over to spend more time working on the sequel to How to Teach Physics to Your Dog, which involves talking to Emmy about relativity. Progress has been slower than last time, largely because the previous book was written while I was on sabbatical and before SteelyKid was born. But there's also a structural issue that's giving me some problems.

This is partly a matter of familiarity with the material-- I'm a low-energy physicist, so I've never needed to worry all that much about relativity. A bigger issue, though, is that I'm a low-energy experimental physicist, and that's better suited to talking about quantum than relativity.

The first book had a very natural structure to it, that fit nicely with the way I engage with science. Every chapter had a discussion of some odd quantum phenomenon, followed by a description of one of the many experiments demonstrating the relevant phenomenon in real systems. This was easy to set up, as there are a whole host of experiments demonstrating quantum effects-- interference of matter waves, quantization of light, quantum entanglement, etc. All of these have been done in the real world with familiar atoms and molecules.

Relativity, on the other hand, lacks this kind of convenient experimental support-- don't get me wrong, it's supported by just about as much experimental evidence as quantum mechanics is. But with a few exceptions, the experimental demonstrations of relativity are not as clean and clear as the experimental demonstrations of quantum phenomena, for a very simple reason: we have never accelerated anything heavier than a single atom to relativistic speeds.

This means that, while the experimental support is there, it's more subtle, and difficult to pull out in a way that is convincing to a non-physicist human, let alone a dog. There are clear and incontrovertible pictures of molecules behaving like waves, but there aren't any pictures of moving objects shrinking. We can infer that moving objects must shrink in the direction of motion from things like sea-level cosmic-ray muons, and the fact that time dilation has been directly observed, but that doesn't have the same impact as a figure showing that fullerene molecules sent through a diffraction grating produce an interference pattern.

It's not an unsurmountable objection, but it cuts a little against my natural inclination. I'm an empiricist by nature, and strongly prefer to deal in objectively measurable reality, a tendency that has only grown stronger after years of interacting with theorists on the Internet. Demonstrating that relativity is real relies heavily on thought experiments and appeals to mathematical elegance, and I'm much less comfortable with that sort of thing. It's also harder to plausibly get across to a dog.

I can do it, but it's a more difficult process for me. In the end, I'll be better for it-- I already see several ways to improve my discussion of relativity the next time I teach our modern physics class. But for the moment, it's making progress on the book slower than I'd like, leading to more procrastinatory blogging, among other things.

Which reminds me: back to work.

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Would Emmy know anything about GPS? That's a now everyday technology which requires relativistic adjustments (general as well as special) to work properly. The effects may be at the part per billion (or less) level, but they add up quickly, since the speed of light is roughly one foot per nanosecond and there are ~30 million seconds in a year.

The Olbers paradox would be another point you can bring up. Certainly Emmy would know that it gets dark at night, even when it's not cloudy, and you had a thread a while back discussing the point.

By Eric Lund (not verified) on 16 Nov 2010 #permalink

GPS is a good one, but it ends up involving both special and general relativity-- the GPS satellites are both high up and moving reasonably fast, so it's not that clean a demonstration. Similarly, there's the Hafele-Keating experiment putting clocks on jet planes, but again, you need both special and general relativity, and it's a little complicated to explain.

The recent optical clock experiments are really a life-saver in this regard-- the motional shift part of that paper is as clean and clear as you could possibly want.

... we have never accelerated anything heavier than a single atom to relativistic speeds.

[channeling Emmy:] Throw the ball harder!

By Pierce R. Butler (not verified) on 16 Nov 2010 #permalink

Funny, I'd think that QM would present many more challenges to the "objectively measurable reality" worldview, even there any piece of evidence is indirect and laden with interpretation, not to mention all that indeterminacy business. It might be one of those cases where the familiar seems obvious.

Why do you think that things "actually shrink"? Your phrasing is that of the Lorentz view that there is a specific preferred reference frame where things are their normal size and shape when they are at rest and that they get deformed when they move. That is not relativity, where all inertial coordinate systems are equivalent.

Maybe Emmy needs to ask "Do moving dog biscuits (or squirrels) really shrink?"

By the way, there is nothing subtle about the time it takes a particle to go around the CERN ring when you double or quadruple its energy.

By CCPhysicist (not verified) on 16 Nov 2010 #permalink

Why do you think that things "actually shrink"? Your phrasing is that of the Lorentz view that there is a specific preferred reference frame where things are their normal size and shape when they are at rest and that they get deformed when they move.

I would say (and in the current draft do say in response to a question from Emmy) that things "really shrink" in the sense that there is no measurement you can do of the size of a moving object that will give you a value other than the size in its rest frame divided by the Lorentz factor. If there is no way to measure something other than a "shrunken" value, then from the point of view of an observer watching that object go by, it really is smaller.

I say this to make a distinction between the situation that exists in relativity and a situation in which things "appear" to shrink due to some optical illusion sort of thing. While it's legitimate to phrase the relativistic situation as things "appearing" to shrink (as that's essentially how the contradiction between observers is resolved-- each observer thinks the other did the measurement wrong by making non-simultaneous measurements of the ends), I think that leaves a misleading impression that is worse than the misleading impression left by saying that moving objects really do shrink. If there's no legitimate measurement of length that you can do that will give you anything other than the Lorentz contracted value, then I think the distinction between "appears small" and "is small" is pretty well nonexistent.

It's funny how people in different branches of physics view the world differently. I spend a lot of time thinking about collider experiments, so relativity just seems like an obvious fact of daily life. Quantum effects usually seem more subtle.

I like your answer, but maybe you need to ground it in your experimentalist point of view: What we know is what we measure with our rulers and clocks. That is the epistemology of experiment in the language of relativity.

OUR rulers and clocks are always well behaved regardless of how fast we are moving, and relativity asserts that the laws of nature are always the same if we are careful to use our rulers and clocks. The object "really shrinks" in the strictly observational sense you describe rather than in the absolute sense assumed by Lorentz. The mistake, such as it was, of the Lorentz use of galilean relativity, was to assume that measurements made in one system will also apply in another system.

BTW, one place where you actually see that shrinkage is in the distortion of the electric field of a moving particle, which shows up in the force between two wires.

By CCPhysicist (not verified) on 16 Nov 2010 #permalink

Here are two examples of special relativity affecting ordinary household objects, and thus two possible candidates for examples in your book:

1.) For anyone who thinks that only high-energy accelerator jockeys routinely encounter relativistic effects, note that almost every ordinary citizen has purchased a home particle accelerator - The television picture tube. The electrons are moving fast enough that you have to use relativistic momentum when calculating how to deflect the beam to the right spot on the phosphor screen. Dogs sometimes watch TV, right?

2.) Gold - Why does gold have a golden color, and why does gold resist tarnishing so well? Turns out that both are a consequence of special relativity. There's an excellent short discussion here:


Okay, dogs aren't much interested in jewelry, but dogs sometimes _swallow_ jewelry, right?

By Emory Kimbrough (not verified) on 16 Nov 2010 #permalink

Maybe you need a faster dog or a slower speed of light? If Emmy were chasing as squirrel at 0.63c and the squirrel was running 0.68c (or 0.58c), you might have to serious teaching moment. Similarly, if the earth had much stronger gravity, Emmy might have interesting problems fetching the red ball as opposed to the blue ball. Then there's the twin puppy paradox and a journey to the Dog Star.

Regarding the "appears small" and "is small" distinction, I think it is really better to think of the space in which the object is embdedded being warped rather than the object itself. For example, "is small" might imply the presence of some elastic strain, whereas in fact there is none since the co-ordinates describing the extension (which is zero) change too. It is our notion of smallness and largeness that is changing rather than anything about the object itself.

Relativistic mixing of E and B fields, as highlighted by CCPhysicist, is a good example of a relativistic effect which leads to easily observable consequences such as a magnetic force between two current carrying wires.

Another topical example of a system where relativitic effects can be observed under lab conditions is in graphene, where electrons behave as massless Dirac fermions. No idea what relevance this would have to a dog, but might be interesting to consider.

As far as relativistic effects in everyday life, I understand that magnetism is actually a relativistic effect. This might be a good thing to bring up when explaining magnets to the dog.

Also, a clear explanation of how relativity + FTL travel leads to time travel would be good. Now *there's* something with direct relation to a dog's everyday experience! (kidding)

As a poet and English professor previously at a small liberal arts college like yours and now at a slightly larger private university, I very much appreciate how your research and teaching play into and off of each other--and that both intersect in your real (or relative?) life. It's pretty important that those who "strongly prefer to deal in objectively measurable reality" are able to talk with--even attempt to understand--those who are drawn to thought experiments and mathematical elegance. And vice versa.


Back in the sixties, some folks at MIT made a film about a time dilation demonstration they conducted using cosmic ray detectors. It looks like it was trying to do what you're talking about: