A summer school student writes about a phenomenon which some speculate to have connections with quantum mechanics.
BY DIKSHA BHATT & DEBDUTTA PAUL
ಈ ಲೇಖನವನ್ನು ಕನ್ನಡದಲ್ಲಿ ಓದಲು ೨ನೇ ಪುಟಕ್ಕೆ ಹೋಗಿ.
We have all seen oil droplets float on water, but have you noticed oil droplets floating on oil? Or even better, oil droplets walking on the oil surface?
During the Summer School for Women in Physics held at ICTS between the 29th of May and the 9th of June, 2023, we performed an experiment that helped us do just that — we made oil droplets float on the surface of the same oil. We could also make the droplets walk on the surface using an oscillator.
An oscillator is an electrical device that can vibrate the surface above it at a frequency that the users can choose. Typically, it vibrates fast — about ten to a hundred times a second.
We placed a black dish on the oscillator and filled it with the transparent liquid: silicone oil. When we turned the oscillator on, we saw the liquid on the dish vibrate. When we slowly increased the frequency to about 37 hertz causing 37 vibrations of the surface each second, the liquid formed a perfect checkerboard-like structure.
The limit beyond which the liquid starts forming such patterns is called the Faraday instability. The wave patterns are called Faraday waves.
After observing the Faraday waves with silicone oil, we took a skewer and flicked the silicone oil’s surface slightly to make tiny droplets. The liquid’s particles form bonds and lead to the formation of droplets. These bonds help them remain intact without merging with the rest of the liquid.
The silicone oil droplets looked like small diamonds floating on the surface. But what would happen if we turned on the oscillator?
This time around, we made observations directly and by capturing slow-motion videos. As we turned the oscillator on, we saw the droplets vibrate with the liquid.
The slow-motion videos had a surprise waiting for us.
By zooming in, we observed that the droplets created small waves as they struck the surface. When we increased the oscillation frequency to about 37 hertz, the droplets started walking on the surface! While some slowly moved over the surface, a few came together.
We observed the walking droplet phenomenon only at a sufficiently large oscillation frequency. Each droplet impact generates a wave that vibrates out of sync with the droplet’s vertical motion on the surface. As the drop hits the surface, its wave collides with it at an angle, pushing it forward.
It was a fun experiment. But it can also bridge the macroscopic physics of fluid mechanics and the microscopic physics of quantum mechanics.
Quantum mechanics was developed in the twentieth century to solve numerous unanswered questions, like the stability of atoms. Fundamental particles like electrons, physicists noticed, behave very differently from objects around us. While Young’s double-slit experiment established light as a wave, the photoelectric effect showed that it consists of individual particles, now called photons.
Modern quantum mechanics solved the riddle by placing importance on the act of observation. It says that a particle behaves differently when the observer makes a measurement of it compared to when the observer leaves it alone. So, it says there is no way to know the position and velocity of a particle until an observer measures them.
Although consistent with experimental observations, some physicists have traditionally considered this interpretation inadequate. They have proposed alternate theories to explain experiments involving particles.
One such theory, the pilot-wave theory, states that a particle is always guided by its wave function, but its position and velocity exist even when the observer is not measuring them.
Our silicone oil droplets guided by waves appear visually similar to a pilot wave guiding the particles and the particles sustaining the pilot wave.
Although physicists have studied this possible correspondence with advanced experiments, they still debate it. While some physicists claim that visual resemblance can lead to a deeper understanding of quantum mechanics, others argue that the correspondence is only a coincidence. Moreover, the theory of pilot waves has yet to be widely accepted as an alternative to modern quantum mechanics.
Nevertheless, our experiment fascinated us, advanced our fluid and quantum mechanics knowledge, and exposed us to scientific debates. Who knew that a few drops of oil could do that?
Diksha Bhatt and Sharmada Iyer attended the Summer School for Women in Physics held at ICTS between the 29th of May and the 9th of June, 2023.
Debdutta Paul thanks Divya Jaganathan for inputs.