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Comment by amelius | Hacker News Reader

No collisions?

IIRC two galaxies can pass through each other with no collisions, because the space between the stars is so large compared to the size of the stars. Quite how well that applies to Spongebob is an open question.

Space between atoms is also very large compared to the size of atoms.

Yet, making two everyday objects pass through each other is not that easy ...

To the size of their core, sure. But a collision happens way before the atom cores are close to eachother, as you already mentioned. Therefore it does not compare as an analogue.

Additionally, the densities of the systems are also very different (not in absolute terms of course)

Electromagnetism is some 40 orders of magnitude stronger than gravity. Also gravity only attracts, it doesn't repel like 2 atoms approaching each other. So it is really quite different.

Are we assuming the objects retain form? If I smash two iron cubes together and apply enough gravity, eventually I'll get fusion of the atom cores. As the highly-compressed charged particles start to collide more-and-more, some will repel, some will fuse.

As long as the 2nd law is in play, I could see the two objects as being passed through each other. One side you have particlesA[cub1_particles, new_fusion_particles] and on the other side you have particlesB[cube2_particles, new_fusion_particles]. Both of these would sum to the same thermodynamic energy of [cube1, cube2].

Right?

You don't quite RC, because of the interstellar medium (and sometimes the circumgalactic medium). In a galaxy-galaxy collision, individual stars are almost collisionless, depending on the stellar density of the galaxies. However, if there's even a small fraction of the normal amount of gas in both galaxies, the interaction will be very X-Ray bright from gas friction. The behaviour of gas tends to dominate many if not most galaxy-galaxy collisions.

Good opportunity to plug https://savechandra.org/why/

NGC 2207 https://www.chandra.harvard.edu/photo/2014/ngc2207/ ("Colliding galaxies like this pair are well known to contain intense star formation. Shock waves — like the sonic booms from supersonic aircraft — form during the collision, leading to the collapse of clouds of gas and the formation of star clusters")

NGC 1232 https://chandra.harvard.edu/photo/2013/ngc1232/ (the text below the image is excellent).

Stephan's Quintet https://chandra.harvard.edu/photo/2009/stephq/ (be sure to use the "View Wavelengths" buttons right below the image, and mouse over the image for annotations)

There are lots of other examples. While brief <https://en.wikipedia.org/wiki/Interacting_galaxy#Galaxy_coll...> is decent, and can take you to the longer https://en.wikipedia.org/wiki/Galaxy_merger, which is pretty encyclopedic on that branch of galaxy-interaction outcomes.

Spongebob pretty clearly doesn't account for gas. The initial velocities are also not really comparable to a galaxy-galaxy interaction. And of course it's in two spatial dimensions, so fewer ways that any pair of particles might avoid each other than in our three.

And to answer the grandparent comment, the gravitational interaction of particle-particle close calls in Spongebob are suppressed, so no collisions. Gravitation is the only force modelled, so no clumping. I arrived at that from scattered Q&A comments in the twitter thread, but also it's pretty clear from eyeballing.

I was referring to star to star collisions, which I understand is incredibly rare (but not impossible).

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