Perception changes in physics

(My latest book God vs. Darwin: The War Between Evolution and Creationism in the Classroom has just been released and is now available through the usual outlets. You can order it from Amazon, Barnes and Noble, the publishers Rowman & Littlefield, and also through your local bookstores. For more on the book, see here. You can also listen to the podcast of the interview on WCPN 90.3 about the book.)

In an earlier post, I suggested (following Thomas Kuhn) that Gestalt-type switches can play an important role in the creation and adoption of new theories in science. Today I want to look at specific examples of such changes.

Take the case of a simple pendulum, made by hanging a small weight from a fixed point by a string and setting it in motion by pulling it back and releasing it. What had been observed from time immemorial is the weight swinging back and forth with decreasing amplitude before finally coming to rest at the lowest point in its trajectory. People used to interpret this motion as the pendulum weight, when released, ‘seeking’ (to use anthropomorphic language) to get to its final resting place at the lowest point in its trajectory, but initially overshooting the mark, trying again to get to the lowest point, overshooting again by a smaller amount, and so on, until it finally reaches its destination and stays there.

Viewed this way what the pendulum is ‘trying’ to do is to come to rest at the bottom but is prevented from doing so by overshooting it due to its motion. Hence the time taken from the instant of release to the final resting point would be the significant thing to measure to see if there are any patterns in this data. But we now know that the time taken to reach the lowest point in its trajectory is not a useful parameter and this is why this approach did not lead to any interesting results.

It took a Galileo to observe the same pendulum motion as everyone else but see it in a different way. He saw the fundamental aspect as an oscillation. In that view, what the pendulum is ‘trying’ to do is keep oscillating forever with the same amplitude but other factors prevent it from doing so, bringing it to rest. In this view, it makes sense to measure the period of oscillation (i.e. the time taken to go through one cycle) and this data does yield useful patterns, such as that the period is independent of the weight or the amplitude of motion (within certain limits), but does depend in a precise way on the length of the string.

The point is that how one views a phenomenon will determine what one chooses to measure. And what one measures determines what one will discover.

In the case of theories of motion in a straight line, the ancient Greeks saw the motion of bodies as headed towards something. In such a view, the key distance is the distance of the object from its final destination. It was only the reversal of worldview that saw the distance and elapsed time of the object from its starting point as the parameters worth measuring that yielded useful patterns of relationships that eventually culminated in Newton’s laws of motion.

Once someone had made this Gestalt-type switch and were able to articulate the new view to others, others quickly started seeing the same thing. What had been seen as a duck was now a rabbit, what was as two faces was now a vase. But not everyone will see the world in the new way. Those who are strongly wedded to the old way of looking at the world will resist making the switch. It may not be that they see the duck and are consciously rejecting it in favor of the rabbit. It may actually be that they do not even ‘see’ the duck. For them, the rabbit remains a rabbit and never becomes a duck.

In the actual case of the rabbit and the duck image, it has been my experience everyone sees both shapes within moments of it being pointed out to them, although there are small differences in the time taken for the realization to hit. But there are other examples of switches where people struggle for a long time. (These are taken from this site where you can see even more examples.)

A popular one that some have a hard time seeing is the one below. People initially tend to see either one image or the other but not both. Once they have locked onto one image, they find it hard to switch until they are told what to look for and specific features are pointed out.


The next one is even harder. It is not two images but requires one to see a single image instead of seemingly randomly scattered blobs. I initially could not see anything. Even after I was told what to look for, I still did not see it for some time until it suddenly ‘appeared’. Now that I have seen it, it seems obvious.


In both cases, most people do not see the picture on their own but need someone else to point out to them what they should be seeing before they suddenly see it for themselves. This was the particular genius of people like Copernicus, Galileo, Newton, Einstein, and (as I shall argue in the next post) Darwin. They looked at the same world that others did but saw it in a new way. And they were able to persuade others to see what they saw.

Next: Gestalt switches in evolution

POST SCRIPT: Buster Keaton film shorts

One of the funniest comics of the silent era was Buster Keaton. The Cleveland Cinematheque will show a series of his short films on Friday, November 13 at 7:30 pm. The films will be introduced by Robert Spadoni, professor of film studies at Case Western Reserve University. Accompanying the films will be live music, with pianist Shuai Bertalan-Wang playing the ragtime music of Scott Joplin.

For more details on location, admission prices, etc. see here. There is also a Facebook page about it.


  1. says

    For those who cannot make out the second image, it is that of a Dalmatian dog sniffing the ground near the base of a tree.

    Don’t be surprised if you still don’t see it for a while. If you need more guidance on how to see it, let me know and I’ll give more details of what to look for.

  2. says

    Hi Mano, I completely agree — there are so many concepts in math that only make sense when the appropriate analogy is used (negative numbers being debt, complex numbers being rotation).

    I think one of the difficulties of teaching and learning is understanding the mental model used to discover new facts, not just the results themselves.

  3. says

    Hi Mano, I must admit that the first time I saw the photo, I can’t see the old woman. The young lady image seems more dominant. Anyway, men often see the same subject in different light.

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