Why don't shockwaves expand rearwards when they expand out?

Why don't shockwaves expand rearward when they expand out? In this for some reason really big picture, you can see the shocks expanding out past the actual plane. Why don't they also expand rearward? And for that matter, forward?

If it did do this, I'm guessing it'd make something that looked like a triangle.

• You're going to have to clarify what you mean by 'expand rearward' and 'expand out'. Your question doesn't make any sense to me. Note that this image is a 2D perspective on a 3D phenomena. The actual main shock at the nose of the aircraft is really a cone. The imaging technique used to generate this picture mostly captures a slice -- but with some averaging of the 3d behavior into that slice. Mar 13 at 20:54
• oh okay, will do. Basically, because shockwaves are high pressure, I assumed that it'd want to expand, like any high pressure gas does. Mar 13 at 21:34
• Also, expanding out meaning going further from the plane, like seen in the photo. Expanding rearward meaning the shock expanding into the red triangle area. Mar 13 at 21:51
• Shocks are discontinuities. Mar 14 at 1:56
• A shock isn't a region of high pressure, it's a discontinuity in pressure (see my answer). It's a contour line on a map, not a collection of molecules whose volume could increase. Mar 14 at 16:27

A shock doesn't expand (spread out), simply by definition. The shock is where pressure and density change discontinuously. If that gets smeared to cover the red triangle superimposed on the photo, then it becomes continuous, so it's no longer a shock.

A related kind of spreading is a Prandtl–Meyer expansion fan, but that may be fancier than what this question is asking.

A shock is an abrupt (thin) change in properties.

As you get far from a body, the need for the shock may dissipate and the shock may cease to exist.

However, when there is a shock, it is thin -- by definition.

• It might help to understand that a sound wave (better example: light wave) is an energy form that does not simply dissipate quickly by equalizing pressure). It takes much longer, unfortunately resulting in sonic booms. Mar 14 at 16:45

The short answer is because the thing that is making the shock wave is moving faster than the shock wave is able to expand.

That's why a shock wave forms, rather than sound rippling out in all directions - It's caused by all the ripples piling up.

• Ah okay, that makes sense. So basically the air can't expand backward because it's moving forward so quickly? Mar 13 at 23:05
• Well, the sound is expanding backwards relative to the plane, so you will hear it after it passes, but there is no rearward expanding shockwave. Mar 13 at 23:38
• Oh okay. So last question because I don’t want to keep asking questions, but if sound can expand backwards, why can’t the shockwaves? Mar 13 at 23:46
• Because it's being pushed forward continuously? Mar 14 at 2:33
• "Expand" is being used in different ways here. Are there better verbs we could use? Mar 14 at 15:08