A Trumpet, Fire, and Physics

In the case of a recent viral video of a trumpeter playing into a tube that’s on fire, making the flames dance higher and lower in time to the music, we asked experts at UConn to explain just how the device – called a Rubens’ tube – worked

In the case of a recent viral video of a trumpeter playing into a tube that’s on fire, making the flames dance higher and lower in time to the music, we asked experts at UConn to explain just how the device – called a Rubens’ tube – worked ()

In the midst of an infinite scroll – you know, the kind where you’re lying on the couch after a long day just thumbing through your social media feed, watching whatever random video clip pops up next – was this little recording of a trumpeter.

He’s wearing a red shirt and sitting at a long metal tube, poised to play, with the bell of his brass trumpet snugged right up to the end of the tube.

The tube is on fire.

As the trumpeter – who I later learned is the musician Moises Alves – begins to play, the flames start to dance. Higher. Lower. Faster. Slower. Parallel columns of flickering fire seemingly floating atop the tube.

The video abruptly ends with a chaotic panning shot to the floor, like someone forgot to stop their phone while still recording, and the whole thing is over in 30 seconds. I watched it again and again and again.

It was mesmerizing.

I showed my spouse. I showed coworkers in University Communications at UConn. Have you seen this viral fire trumpet video? It’s so amazing to watch, but how does this even work? Sound waves or something, right?

Could someone actually explain the science behind what’s making the fire dance atop that tube?

As it turns out, someone at UConn certainly can – and was eager to do so.

“With a Rubens’ tube, when you play certain frequencies, you set up what’s called a standing wave,” says George Gibson, a professor and head of the Department of Physics in UConn’s College of Liberal Arts and Sciences.

“A standing wave is a pattern of nodes, where nothing’s happening, and antinodes, where there’s a lot of action,” he says, “and so, the flames respond to whether it’s at a node or an antinode. That’s why you develop a stable pattern of the flames either being high or low or high or low.”

All waves – including sound waves, Gibson explains – are a disturbance in a medium.

“The medium has to be able to return to equilibrium,” he says, “and there’s some force which does that. For sound waves, the restoring force is pressure.”

The cavity inside the Rubens’ tube isn’t empty. It’s piped full of gas – in this case, flammable propane gas, which has a different density than the air we breathe. As sound waves are projected into the tube by a speaker, or from the bell of a trumpet, they bounce off the opposite end of the tube and interfere with each other, creating a standing wave with regions of high and low pressure.

Those regions of pressure will cause the fire burning off the propane from holes drilled along the top of the tube to move based on where the nodes and antinodes are positioned at a given time within the tube.

The height and intensity of the flame depends on factors like the volume of the sound and the amount of gas pressure within the tube. The length of the tube itself will determine what frequency is resonant inside of it to form the standing wave.

Gibson designed and teaches a course at UConn on the physics of music. In fact, he was teaching it this summer as an online class when we reached out to him about an idea to recreate this viral video with a UConn expert explaining the science behind it.

Gibson isn’t just an expert on the physics of music, he’s also a researcher, a student of physics history, and a proponent of using music as a way to learn about physics.

“With classical mechanics, which deals with objects and projectiles, unfortunately a lot of people’s common sense isn’t correct,” he says. “If you toss a baseball up, at the top, when it turns around and starts coming down, if you ask what’s the acceleration at the top, most people will say zero, because it’s not moving. But it’s not zero. It’s still the same acceleration. So, you have to break down people’s intuition and common sense, which can be difficult.”

Music is different, though – most people haven’t really thought much about the waves that carry the sounds we hear or the music we enjoy.“They don’t have any preconceived notions, and their intuition is usually correct,” Gibson says. “And so, it actually makes it very easy to make a connection and to teach physics, because modern physics is basically wave theory.”

On a deeper level, it is not surprising to Gibson that almost all of the founders of quantum mechanics had connections with music – they played music themselves, or studied consonance and dissonance. The Rubens’ tube itself is named for a German physicist – Heinrich Rubens – who Gibson says doesn’t get enough credit for his contributions to the origins of quantum mechanics.

“If you have an electric stove, and you look at the coils, as you turn it on and it starts heating up, it’s first a dull red, then a brighter red, then orange, then yellow, then even white as you get hotter and hotter,” says Gibson.

But in the 1890s, nobody had any clue as to how you got from the temperature to the color.

“It was so fundamental,” Gibson says. “Heinrich Rubens was very interested in that. He studied the infrared tail of the spectrum very accurately. He was invited to the house of a fellow German physicist, Max Planck, for a dinner party, and he showed him his data. Max Planck got so excited about it, he stayed up all night and actually came up with the first equation that described what is called the ‘blackbody spectrum’.”

Planck, Gibson shares, is said to have woken his wife in the middle of the night playing Beethoven’s “Ode to Joy” excitedly on the piano as he formulated the Nobel-prize winning equation that contained the first clues as to the strange nature of quantum mechanics.

But if Rubens was so busy helping to build the foundations of modern physics, why did he design this dancing fire tube?

Because he was also an educator, Gibson explains, and the tube offered a visual means of demonstrating otherwise invisible sound waves.

Rubens used it to teach.

And as it turns out, a student group on campus, the UConn Engineering Ambassadors, has a Rubens’ tube that they use for that same original educational purpose – their Rubens’ tube is part of a series of STEM demonstrations they offer to K-through-12 schools around Connecticut.

When we assembled their Rubens’ tube – first for a demonstration at UConn’s Fire Station and later on the darkened stage at the UConn School of Music’s von der Mehden Recital Hall – those in attendance got to view this wonder of sound and pressure and waves close-up. And to learn about the science behind the fire that danced to the trumpet’s melody.

And even then, knowing how it worked exactly, and knowing what was happening inside of that tube, it was mesmerizing.

Very special thanks to Professor George Gibson, the UConn Engineering Ambassadors, UConn School of Fine Arts Graduate Fellow Eric Rizzo, UConn Music Professor Louis Hanzlik, the UConn Fire Department, and the von der Mehden Recital Hall for their invaluable contributions to this project.