Comment by gus_massa | Hacker News Reader

Comment by gus_massa | Hacker News Reader

At least this makes sense. Perhaps calling it a fractal is an unnecessary spin, but if it is linbaity enough to cover the spaceship theory I'm in.

More than a fractal, it is better to imagine it as a pile of recently fallen snow is a day with the correct climate (cold?), so the snow accumulates but it doesn't compact too much. So the density is much smaller than the density of ice that has been in a glaciar for years under a lot of pressure. Since the gravity is too low, it will no compact too much.

The main difference is that it's not a pile of real snowflakes, ut a pile of dust "snowflakes". Otherwise, the ice in the real snowflakes would have been partially evaporated by the sun and it will be classified as a comet.

Anyway, it would be interesting to read more technical details about the plausibility of this. Can the dust pile survive the micrometeorites? Does the color profile match a dust pile object. I really don't like the starship hypothesis paper, but at least it contains a good analysis of many of the (secondary) objections.

It makes more sense to call it a fractal rather than a pile. In the paper they reference, it is suggested that particles can form fractal like structures through a diffusion limited aggregation process[0]. One very important result in planetary science is the observation by the astronaut Donald Pettit of granular materials aggregating together in microgravity[1]* . In the experiment[2], it appears that some of the resulting structures resemble those formed in a diffusion limited aggregation process.

*It is also worth noting that this experiment was entirely ad-hoc and came about from Don basically just playing around with stuff in microgravity

[0]https://arxiv.org/pdf/1402.7132.pdf

[1]https://www.lpi.usra.edu/meetings/lpsc2004/pdf/1119.pdf

[2]https://www.youtube.com/watch?v=k0YuivnCXhM

For example, bismuth crystals grow in an interesting way and, like h20 are less dense as a solid than a liquid. At the utterly extreme temperature/pressure environments expereinced in the universe, it isn't hard to believe there are solids out there we can't begin to predict at present. Below are some 3D crystal growths at SEM and macro scales for earth examples

https://i.pinimg.com/originals/5d/bf/9c/5dbf9caa3ece888f2dc2...

https://www.doitpoms.ac.uk/miclib/micrograph_record.php?id=6...

"Perhaps calling it a fractal is an unnecessary spin,"

It is, or at least calling it self-similar is, as well as what the image implies: https://www.youtube.com/watch?v=gB9n2gHsHN4 That's a 3Blue1Brown video explaining why "fractal" and "self-similar" are not the same thing.

That video is about how "self-similar" is not sufficient for fractal. (The rest of the requirement is some notion of fine detail at every scale, which the video focuses on.)

It is widely agreed that self-similarity is required for fractal, although often interpreted as quasi/statistical/multiple similarity

https://en.wikipedia.org/wiki/Fractal

Put another way, some things are self-similar but lack detail (for example, a straight line), and some things have detail but not similar detail at all scales (for example, a meterstick with ticks at every millimeter) and some things have infinite detail but no similarity (for example, purely entropic noise that cannot be compressed or expressed as a structure)

The core idea of a fractal is something having fractional dimension. It just happens that the ones we know of are self-similar. If we had a formula that generated different shapes ad infinitum, I would also call that a fractal.

It may be "self-similar" in a particular mathematical way, but fractal does not imply "self-similar" in the way people expect.

I think you meant "not necessary".

And can that loose pile of dust survive even a slight rotation. If it is long and thin, the dust near the tips should separate.

There are mechanisms by which tiny particles may bond over time without becoming more dense. The only prerequisite is that individual atoms or molecules can move on the surface of the particles. This is true for a wide range of conditions, but usually is extremely slow.

The article claims that if it has the snowflake properties it isn’t long and thin and still produces what we saw.

> More than a fractal, it is better to imagine it as a pile of recently fallen snow

But snow flakes are fractals.

But a pile of snow is not.

And snow ‘falls’ in gravity. This aggregated somehow else.maybe by tenuous electrostatic attraction (some ice particles get positively ionized when a photo kicks out an electron, some get negatively ionized when an electron attaches to them, & cetera and eventually they clump together to make a roughly electrical neutral object. The. Hydrogen bonds kick in once it’s electrically neutral (assuming it’s made of water or other polar molecules).

Why does it have to be something else? Couldn't the particles have precipitated gravitationally inside a smallish dust cloud?

It's Wired. They have to use "fractal" a few times an issue or their articles of incorporation call for a new board.

It’s Phil Plait on the Syfy blog Bad Astronomer, not Wired. He does occasionally get a bit click-baity in his headlines but the content is pretty solid, particularly on the subject of omuamua

Oh wow, you're right. I saw the green-underlined all-caps text and assumed Wired. That and the word fractal.

Funny you mention that. I seem to remember them classifying it as a “meme that died from overexposure” in their regular feature sometime back in 1997 or thereabouts.

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