Thomas Young performed an incredible experiment in 1801. He shone sunlight though two holes and found that the beams of light coming out the other side were interacting with each other. This shouldn’t happen because light is made of lots of little particles called photons (it’s these particles that hit the back of our eyes and enable us to see). We know that these particles don’t interact with each other, so how can two beams made up of these particles interact?
To figure this out you need to look at the pattern of interaction, or as it’s properly called, interference. And when you do that, you discover that these two beams of light are acting like waves; they are interfering like waves would interfere. Young’s Double Slit experiment showed that particles like photons travel like waves. In his experiment, these particles were travelling though BOTH slits. Something only a wave can do. And that’s the fundamental weirdness of quantum mechanics, that the building blocks of our universe are particles and waves.
As a science presenter I’m always on the lookout for cool ways to demonstrate these weird ideas. The classic way is to use a wave tank – a shallow tank of water with waves travelling across the surface. When the wave front meets a barrier with two holes in it, you see the same pattern of interference that Young saw when he did his experiment with light.
That’s because the pattern of interference is a phenomenon of waves in general. That got me thinking, is there some other kind of wave I could use to show this kind of interference? Then it hit me, I could use sound! So instead of two holes or two slits, I’d use two speakers. And instead of a tank of water I’d use an audience of people!
If I could get everything just right I should be able to produce a sound that only some people in the audience can hear. Those are the people for whom the waves from the two speakers interfere constructively. That just means that when the two waves reach their ears, the peaks from one should be arriving at the same time as the peaks from the other, and the troughs from one should be arriving at the same time as the troughs from the other.
But there should also be people in the audience for whom the waves interfere destructively, where the peak of one wave arrives at the same time as the trough of another, cancelling each other out.
All I needed was an audience and an outdoor space (sound reflected off walls would ruin the experiment (I think!)). So when the Cheltenham Science Festival asked if I had any experiments I’d like to do outside I jumped at the chance, with the caveat that this wasn’t the type of experiment you see in a science show where you know it’s going to work. This is more like an experiment you might see in a lab where you don’t know what the outcome will be!
The short story is, it worked. Sort of. Well there were places in the audience where the sound was much quieter that it would have been were it not for the interference.
BUT, I wanted to go one better than Young. I wanted to try something you can’t do with light. I wanted to change the wavelength of sound coming from one of the speakers. When you do this, it causes the pattern to sweep around the audience. So my plan was to get them to act like volume meters, putting their hands in the air when the sound they could hear was loud and put their hands down when the sound was quiet. What I hoped to create was a sort of automated Mexican wave. Did it work? Judge for yourself…
Take a look at some more weird and wonderful videos on Steve’s Nerdy Blog