The limits of doing your science in metaphors

It’s vital not to confuse an illuminating explanation of science with the science itself.

Our astronomer writes:

Popular science is all about metaphors.  Scientists can use equations and complicated concepts talking among ourselves, but when explaining our work to the public we can’t assume that anyone has the necessary background.  (In a study I did a few years ago I found that even a little bit of mathematics helps a layman immensely.  I strongly encourage whatever you can do in that direction!)  We have to use metaphors–something “is like” something else; we tell a story that captures the important part of the equation.

It’s not easy to do well.  The story has to be correct in a scientific sense, but also easily grasped, and interesting to the audience.  It’s a bit like writing a formal sonnet that people will actually read.  (Our writer took up the challenge, and explained the difficulty of popular astronomy in a pair of Spencerian sonnets: The Skill)  And it’s never quite complete: the metaphor is going to be wrong somewhere.

For instance, there’s the balloon metaphor for the expansion of the universe.  Take a balloon; make a few marks on it to represent galaxies; slowly fill it with air.  Each of the “galaxies” gets farther away from the others, without any one of them being the center from which everything is flying.  And the total number of galaxies is finite, the universe is finite, without there being an edge.  (Understanding these “is like” features depends critically on your audience thinking only of the two-dimensional surface of the balloon and considering it a model of three-dimensional space.  They have to ignore the third dimension and the air going in, something I’m not sure always happens.)  In fact we don’t know that the universe is finite; from all we can see, it appears infinite; but it’s worthwhile understanding how it could be finite without having an edge.

The balloon is not like the universe in other ways.  It has a material surface that is expanding–each of the marks representing a galaxy is itself growing in size.  Space is not like that; galaxies themselves are not expanding.  And if we look at things we might express in numbers or equations, we find that the force resisting expansion–the tension in the rubber–gets bigger as the balloon gets bigger.  In the universe, the force resisting expansion is gravity, which gets weaker as things get farther away.  So the balloon metaphor is definitely not what you want as a base for the mathematics of your theory.

And all metaphors are wrong in some way.  A completely right metaphor would not be “is like,” but “is,” and that would have to include all the mathematical bits.  That takes us out of popular science.

What brought this subject up is an idea sent to me from a non-scientist, whose understanding of cosmology was necessarily built out of metaphors.  “What if the expansion of the universe is like a sponge absorbing water?”  That is (if I understand it correctly) something from outside the universe enters it, reacts with it, and the result is expansion.

Well, in any detail the metaphor doesn’t work (as the non-scientist freely admits).  As in the case of the balloon, you can’t use it as a guide to a mathematical theory.  But more generally, it goes in the wrong direction.  The mathematics comes first; the metaphors illuminate what the mathematics says.

[It’s not the case this time, but I have been irritated by paradoxers (a species I’ve mentioned before) and their ideas based on metaphors.  They send them to me, asking me to fill in the mathematical details, and graciously offer to share the resulting glory.  It’s like going up to a stuggling author and saying, “Hey!  You should write a bestseller!  Solve your money problems right away.  I’ll only take a fifty percent cut of the profits for giving you the idea.”]

But there is an excellent use for these ideas based on metaphorical understanding.  Take yours to a nice, approachable scientist and ask, “How is the expansion of the universe like/unlike a sponge absorbing water?”  It may take some time to answer, because the links between metaphor and equations are not simple.  But when you get the answer, you will have a much better understanding of the subject.