4 ms·

But there would be a spot where the pressure waves would cross?

thaumasiotes 5y ago
> But there would be a spot where the pressure waves would cross [simultaneously]? There could be, but it's not immediately obvious that there has to be. It does seem like certain points would need to be on 'cusps' of the wavefront, though, so something like getting the wave from the north and the northeast 'simultaneously'.
layer8 5y ago
Not necessarily. The set of points reached last by the waves could be multiple points, or a line, or multiple lines.
gnramires 5y ago
I think so. I think there is necessarily a circle "eversion" (turning it inside-out) with a crease. Whatever is the point of the crease would hear sound from all directions. Even with something like a figure 8 collapsing wavefront, eventually one or more creases should form.
- rocqua 5y ago
  If the crease is a line, would it not sound like comming from two opposite directions simultaneously?
Someone 5y ago
I think it’s highly unlikely that even half the pressure waves would simultaneously reach a single point on earth. The waves get reflected in all kinds of ways by the ground, mountain ranges, difference in air temperature, etc. If they did, however, I wonder how strong the force would be at its focus (if that’s the proper name) (If I did my math right, the antipodes of Tonga live in the south of Algeria, near the Niger/Mali/Algeria tripoint. I haven’t heard of weird phenomena happening there, but then, I don’t think that is the most populated and most twitter-addicted part of the world)
ajnin 5y ago
Sounds like the hairy ball theorem (https://en.m.wikipedia.org/wiki/Hairy_ball_theorem https://en.m.wikipedia.org/wiki/Hairy_ball_theorem)
- Tildey 5y ago
jcims 5y ago
There's an animation of the antipode from this eruption arriving in northern Africa and it's interesting how distorted the incoming wavefront is. https://twitter.com/weather_models/status/1483006436750028801 https://twitter.com/weather_models/status/148300643675002880...
cozzyd 5y ago
The earth is (almost) an oblate ellipsoid so most geodesics on earth aren't closed and even if they all meet (do they? I need to to think about this or play with GeographicLib) at one point they are of different lengths (imagine a point on the equator, the latitudinal path is quite different from the longitudinal path). And then there are different propagation velocities due to pressure, terrain, etc. So probably not.

> But there would be a spot where the pressure waves would cross [simultaneously]? There could be, but it's not immediately obvious that there has to be. It does seem like certain points would need to be on 'cusps' of the wavefront, though, so something like getting the wave from the north and the northeast 'simultaneously'.

layer8 5y ago

Not necessarily. The set of points reached last by the waves could be multiple points, or a line, or multiple lines.

gnramires 5y ago

I think so. I think there is necessarily a circle "eversion" (turning it inside-out) with a crease. Whatever is the point of the crease would hear sound from all directions. Even with something like a figure 8 collapsing wavefront, eventually one or more creases should form.

rocqua 5y ago
If the crease is a line, would it not sound like comming from two opposite directions simultaneously?

Someone 5y ago

I think it’s highly unlikely that even half the pressure waves would simultaneously reach a single point on earth. The waves get reflected in all kinds of ways by the ground, mountain ranges, difference in air temperature, etc. If they did, however, I wonder how strong the force would be at its focus (if that’s the proper name) (If I did my math right, the antipodes of Tonga live in the south of Algeria, near the Niger/Mali/Algeria tripoint. I haven’t heard of weird phenomena happening there, but then, I don’t think that is the most populated and most twitter-addicted part of the world)

ajnin 5y ago

Sounds like the hairy ball theorem (https://en.m.wikipedia.org/wiki/Hairy_ball_theorem https://en.m.wikipedia.org/wiki/Hairy_ball_theorem)

Tildey 5y ago

jcims 5y ago

There's an animation of the antipode from this eruption arriving in northern Africa and it's interesting how distorted the incoming wavefront is. https://twitter.com/weather_models/status/1483006436750028801 https://twitter.com/weather_models/status/148300643675002880...

cozzyd 5y ago

The earth is (almost) an oblate ellipsoid so most geodesics on earth aren't closed and even if they all meet (do they? I need to to think about this or play with GeographicLib) at one point they are of different lengths (imagine a point on the equator, the latitudinal path is quite different from the longitudinal path). And then there are different propagation velocities due to pressure, terrain, etc. So probably not.