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Wowww! Remember how the Space Shuttle made two rapid sonic booms during entry? They always said it was one from the nose & one from the tail. You can see the two strong shockwaves ("booms") per aircraft in these photos, one from the nose & one from the tail, plus smaller shocks.
2/ These photos are using a technique that was only recently developed, Background-Oriented Schlieren (BOS) Photography. Traditional Schlieren photography is where you put an object into an apparatus so there is a spherical mirror behind it, and you use a special light source...
3/ It is complicated, but suffice it to say that this apparatus allows you to actually SEE the slight variations in air density around the object inside the apparatus, similar to seeing the air shimmering over a hot highway, or a mirage in a hot desert.
4/ The new BOS Schlieren dispenses with the apparatus so you can see the air density around objects out in the real world, like these airplanes. It is a surprisingly simple technique. All you need is an unchanging background behind the object that you photograph...
5/ You can use stars in the sky as your background, for example. Since we know where the stars are, any variation in air density will diffract the light slightly, making the stars look slightly out of place. We can calculate how much diffraction took place to move the stars.
6/ You can even use the ground under the airplane for this purpose. The ground is moving relative to the airplane but otherwise not changing. We can compare images of the ground in each picture, figure out how much it shifted per image, then subtract it out of all the images...
7/ You can see that there is no visible ground under the airplanes. It is all red. This is because it got subtracted out. But it didn't subtract out perfectly because there were variations in air density around the planes that diffracted the image of the ground slightly...
8/ The places where the ground didn't subtract out are where it is not red. The end result is that you can actually see variations in air density around the aircraft. These variations in air density are the shockwaves from the aircraft. So we can SEE the shockwaves! AMAZING!
9/ It is a surprisingly simple technique, but it requires excellent digital processing to make it possible. This is another example how the Digital Revolution is changing our lives, giving us new "senses" to see the world as we couldn't see it before.
10/10 The hardware to do this has been available for quite some time. What's new, making it possible, is the software (including the math) that teases the amazing information from photos. If you know kids who question the value of math, maybe this will help! :)
