A 200 Year Old Lesson: Scientific Predictions Are Worthless Unless Tested
“So the next time you run across what appears to be a theoretical absurdity, either because you believe such a thing must be so or cannot be so, don’t forget the vital importance of putting it to the experimental test! It’s the only Universe we have, and no matter how solid the footing of our theoretical predictions, they must always be subject to the scrutiny of unrelenting and continuous tests. After all, you never know what secrets the Universe will reveal about itself until you look!”
For centuries, Newton’s theoretical predictions were as unassailable as physics got. His ideas about mechanics, gravitation and optics passed test after test after test. Yet around the dawn of the 19th century, one class of observations appeared to run counter to his assertions: light appeared to exhibit a wave-like nature. The phenomena of diffraction and interference could not be well-explained by a corpuscular theory of light. Towering scientific figures such as Fresnel, Fraunhofer and Poisson calculated what they expected from a wave-like theory under various conditions, with Poisson getting the most absurd result. In theory, light that was shined around a spherical obstacle should produce a shadow… with a brilliant bright spot at the center. This was ruled a victory by Newton for all, proving the wave nature of light’s absurdity.
We all know that a shadow is a dark area or shape produced by something coming between rays of light and the surface which it was directed towards. But does a shadow have parts that are brighter or darker?… How would that even work?
In what was supposed to be a disproof of a theory submitted by
Augustin-Jean Fresnel that light propagates in waves, french mathematician and physicist Siméon Poisson theoretically predicted that if it were the case instead of the theory that light propagated in corpuscles of light (which he favored), the consequence would be that there would exist an on-axis bright spot in the shadow of a circular obstacle, where there should be complete darkness. This was interpreted as absurd and enough to disprove the theory, until François Arago did the experiment and was able to observe the predicted spot.
In the gifs above you see a demonstration of the experiment in action. Where it is shown that a bright spot appears at the center of a circular object’s shadow due to lights wave properties of diffraction. Shown is the laser shadow of a 2€ coin, at an effective distance of 62 meters. And the bright spot at the center of the coin is clearly visible.
This experiment played an important part of the wave theory of light and is a common way of showing that light has wave properties, as long as it meets conditions of the Fressnel diffraction and the light being a diverging laser beam or a “point source”. So we don’t see this phenomenon happening in our every day life. But we do see an amazing phenomenon that puts our intuition about lights and shadows in perspective.