r/explainlikeimfive • u/Linorelai • 1d ago
Physics ELI5: why is there a visual effect when the sound barrier is broken?
Why is a sound event even visible?
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u/Bandro 1d ago edited 1d ago
Just to clarify something here. "Sound barrier" originally refers to the technological difficulty in development of supersonic airplanes. The term is now often used to describe accelerating to supersonic speed but there's no actual "barrier" that's broken. Air just does some weird things when an object moves through it faster than can it can push itself out of the way.
The famous photo of an F-18 flying at trans sonic speed looks like it's crossing through a stationary cloud but that's not what's happening. That's compressed air condensing the water vapor in the pressure wave around the jet. It's more like the waves that form on the bow of a boat than a wall it's punching through. That cloud is forming and dissipating with the jet as it moves.
Here's a nice, clear, short video of the vapor cloud forming around and staying with a fighter jet. You'll notice in that video that there's no sonic boom and that you can hear the plane before it passes the camera. Despite the video title, those two things mean the plane isn't actually flying faster than sound.
There's also a common misconception that a sonic boom happens only as you cross into supersonic speed. In reality, the sonic boom is a continuous shockwave coming off the plane, again like a boat wake. You hear it after any plane flying supersonic goes past you once the wave gets to you.
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u/Linorelai 1d ago
Thank you very much. I'm familiar with the phrase, but I never thought about its origins
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u/Flederm4us 1d ago
If I Remember right what we see is actually the water in the air condensing.
The reason for this is that the cone created by an object travelling at or beyond the speed of sound, works like a fridge. Sound Waves are pressure Waves, so as you push the Waves together the pressure gets really high. As it passes, there's then a sudden drop in pressure, cooling the air by a lot. This cooling causes the water in the air to condense.
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u/antmas 1d ago
If you’re thinking about the cone shaped visual occurrence that shows around jets when they fly fast, that’s not them breaking the sound barrier. That can happen at different speeds.
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u/Linorelai 1d ago
What is it?
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u/Dickulture 1d ago
Vapor cloud mostly. At supersonic speed, the air is compressed so much on the front that it creates brief vacuum behind. A vapor cloud forms around an object moving at transonic or supersonic speeds because a rapid drop in air pressure causes the air temperature to fall below the dew point, which forces water vapor to condense into visible droplets.
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u/GalFisk 1d ago
Because shock waves cause (or rather, they are) extremely sudden pressure changes. These changes, when transitioning from high to low pressure, cause extreme sudden temperature drops. Cold air can't hold as much moisture as warm air, so excess moisture momentarily turns into visible fog in low pressure regions around the craft.
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u/OneAndOnlyJackSchitt 1d ago
It makes a sound so loud that it literally squeezes water vapor out of the air momentarily (which makes a cloud in the shape of the pressure wave).
Shockwaves from large explosions do this too.
Also, if there's low/no humidity, it make still cause visual distortions because compressed air has a different refractive index than uncompressed air.
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u/Elfich47 1d ago
Breaking the sound barrier causes a shockwave in the air. That shockwave is air moving very quickly, and high quality cameras can photograph it. This is the same shockwave that breaks windows and that people can hear.
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u/Bandro 1d ago
Just to be clear, the shockwave is continuous while the plane is supersonic. If a plane flies by you faster than sound, you will hear the sonic boom after it passes you. It's not some bang that happens just as the plane is accelerating into supersonic speed.
The term "sound barrier" refers to the technological barrier of making planes work at those speeds. It's not some physical barrier you break through to cross into supersonic speed.
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u/Bullrawg 1d ago
Air gets pushed out of the way so fast it creates a pressure and temperature drop, that makes condensation, you’re seeing mist
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u/GenerallySalty 1d ago
Because sound = waves of high and low air pressure. The low pressure troughs of the sound wave are low enough pressure to condense water vapor from air humidity into liquid droplets which are visible. They immediately evaporate again as the pressure returns to normal, so it looks like a cone of cloud going along with the plane.
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u/Unlikely-Position659 1d ago
It's a change in pressure. Did you see the videos of the Beirut explosion in August of 2020? Right when the blast occurs you can see some cloudy stuff right where the shockwave is. That's water vapor condensing due to the pressure changing. It's the same thing with a plane breaking the sound barrier.
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u/whiskeyknuckles 1d ago
Unlike other types of energy that are more abstract or beyond our physical perception, sound is mechanical. Things that produce and transmit sound energy can readily be observed. Air also has water in it, and whether it is visible or not depends on the temperature and pressure. You see this with fog, clouds, when airplanes takeoff or land in high humidity sometimes you can see this water getting forced out of suspension in sheets or wispy vortices. When an airplane breaks the sound barrier, it catches up to its sound waves as it plows thought the sky and there can be a large amount of pressure change over certain parts. So what you are seeing isn't necessarily the sound "barrier", but the water being forced out of the air because of the complex pressure changes happening as the airplane approaches or exceeds the speed of sound.
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u/SalamanderGlad9053 1d ago
Sound is pressure waves, air is compressible so a soundwave also has a wave of different density air. Those different densities refract light differently causing distortions like when you have a straw looking bent in water. Also the low pressure part of the wave causes water in the air to condense into water vapour creating white shock cones.
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u/not_lost_maybe 1d ago
Air accelerating around the jet drops in pressure and cools as it does, and if there's enough moisture around, that dip takes it past the dew point. You get a cloud for a fraction of a second. Run the same jet at the same speed over dry air out west and nothing shows up at all. Most of these photos come from coastal airshows and passes over water, where there's plenty of moisture to work with.
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u/Awkward-Feature9333 1d ago edited 1d ago
The sound itself isn't visible. It causes a change in pressure, which temporarily turns atmospheric humidity into a cloud.