> 1a. A quick look at that last glacial maximum shows time frames on 10’s of thousands of years. So the ice would be accumulating for around ~10-15 thousand years then declining to where we see it today possibly.

A fair bit too quick for the ice accumulation phases. 10,000-15,000 years sounds more like the length of an interglacial, ie. what we’re in right now. It has always taken several times the length of an interglacial to transition to the next glacial maximum (though transitions back into interglacial are typically more abrupt). Maybe you are reading the graphs the wrong way around? 15,000 years is about the length of the phases in the last million years when going from glacial to interglacial.

Even before the Mid-Pleistocene Transition — before the glacial-interglacial cycle started turning from a 40,000 year cycle into the current 100,000 year cycle — it was taking at least 20,000 years to get from minimum to maximum ice volume.

For the last million years or so we are within the territory of the 100,000 year cycle, with much more drawn out changes (at least going from interglacials to glacials). For instance the previous interglacial ended about 115,000 years ago and the last glacial maximum began about 30,000 years ago.

The core is thought to be solid or immensely viscous. If anything settles it would be on the outer layer of the inner core. Gravity is about 4.3m/s^(2) here and the radius is about 20% of the whole earth. From seismic wave analysis the density of the inner core is 3% less than iron. This means most likely that it’s mostly iron and lighter elements. Gold isn’t lighter than iron but I guess there could be very small traces.

The terrestrial version are called Tetrapods (literally "four feet"), which originated about 350 million years ago, with modern descendants being amphibians, reptiles, birds, and mammals (though some have subsequently lost tails like us, or feet like snakes).

However, these limbs are just modifications of pectoral and pelvic fins of our fishy ancestors. The true origin of this body plan is 435 million years ago, with gnathostomes, the jawed vertebrates. This is the origin of the two pairs of fins, which eventually became limbs.

For vertebrae it started with fish. Four fins, head and tail.

A good book about the line of fish to vertebrate is Your Inner Fish by Shubin

For insects it started earlier.

It probably originated simply from practical reasons... You want your head at the front to sense where you're going, and you want waste behind you so you don't run into it/eat it again. The limbs on the sides therefore is the best spots (of course there are exceptions to this body plan... Sponges, jellyfish, etc)

Velocity is relative. That includes a velocity of zero. There's no such thing as being "at rest" in an absolute sense. Right now you consider yourself at rest, but to the billions of neutrinos passing through your body every second you're travelling at nearly the speed of light! So the idea that things at rest stay at rest is the same idea that things moving with a constant velocity maintain their velocity. If they don't feel like the same idea, then you haven't yet come to a full appreciation of the principle of (Galilean) relativity.

The reason that it's hard at first to wrap your mind around the principle of relativity is that the forces due to gravity and friction dominate our lives. In our everyday experience, there's very much a difference between being at rest and moving with respect to the air, nothing we push keeps moving in a straight line forever at a constant speed, and everything that goes up comes down. Coming to terms with Newton's 1st Law requires understanding that we're surrounded by complex "special cases" that hide the underlying simplicity of inertia. It's not intuitive, but you can build up an intuition here.

On force...

While force is related to phenomena you're familiar with (e.g., pushing and pulling), ultimately it's an abstract quantity, and it's simply a fact that (net) force is directly proportional to acceleration, not velocity. If it helps, you can regard Newton's 2nd law as a definition: the net force acting on a body is defined as the body's acceleration scaled by its mass.

Also, don't confuse the net force acting on an object with any particular force acting on it. When you push the box and it moves at a constant velocity, it has no net force, but you are still applying a force to it. Think of it like this: the table is trying to bring the box to rest, and you're applying the force that's preventing that from happening. As soon as you stop pushing, the only remaining force acting on the box is due to friction with the table, and since the net force is now not zero, the box's velocity changes as it slows to a stop.

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Source for the 250k?

The answer is time dilation, essentially. Even though our spacetime is globally curved, it's more intuitive to think about a flat spacetime, since that removes ambiguity about the definitions of distances and relative velocities. In this scenario, the most distant objects are receding at velocities arbitrarily close to the speed of light. That means that even though they might have traveled a great distance from their origin point, arbitrarily little time has passed for them, due to time dilation. So we can receive light from these objects that tells us the state of the universe at arbitrarily early times, even though they could be quite distant from the origin point for the universe.

(I'll also note that a common answer to this question is that the universe didn't begin at a point. While that would also resolve the problem, it's not something we can say for certain.)

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Yes, galvanic corrosion can occur when aluminum bronze is in contact with steel because these two metals have different electrode potentials, which means they will react when they come into contact with each other in the presence of an electrolyte (such as saltwater). This reaction can cause the aluminum in the bronze to corrode, which can lead to structural damage if left unchecked. To prevent galvanic corrosion in these situations, it is important to use appropriate insulation or coatings to separate the two metals.

> Space is a real thing that can expand. If you’ve heard phrases like “the fabric of spacetime” or “the spacetime continuum”, these are actually real, not just some sci-fi mumbo jumbo. You can imagine a big rubber sheet, on which all the planets and stars and everything are sitting. If you label this sheet with a grid and stretch it out, you’ll see that stuff gets further apart, but it doesn’t change position on the grid. That’s how space expands: it doesn’t move things, it just makes the distance between them bigger. (Note: don’t take this analogy too far: unlike rubber, space can stretch infinitely, and it doesn’t “snap back” into place).

This is kind of a problematic way of thinking, because there isn't any objective sense in which space or spacetime can move or stretch. Those kinds of effects only ever represent subjective choices, often made to simplify a mathematical problem. They are coordinate choices, specifically. The only objective property of a point in spacetime is its (tensor) curvature.

For example, the idea of space expanding is a coordinate choice. It's equally valid to just say that objects are moving apart.

(How, then, can things recede "faster than light"? Just as it's not possible to uniquely define the angle between arrows drawn at different places on a curved sheet, relative velocities of distant objects in curved spacetimes are not meaningful.)

Wow you must tectonophysics! Great clarifications! I’m glad to hear the updates on my now ancient knowledge of the D”.

The sand keeps the clay from shrinking as it dries because the sand particles act as a filler, providing spaces for moisture to escape and preventing the clay particles from packing too closely together as they dry.

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Instantaneous communication leads to quite a few problems of the "grandfather paradox" type. For example, the relativity of simultaneity means that according to some observers, the response to a query will be given before the query itself.

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But if the grids don't get bigger they are the same distance, always. Otherwise the two objects are, in fact, moving. If there is no center of the universe then where'd the big bang happen?

Btw I'm really not trying to sound like I'm arguing. I'm actually enjoying this conversation most people I talk to just go "oh, huh, yea that's crazy....so did you want to order something?"

Yes exactly, our Milky Way blocks a lot of the view. Plus most of them are irregular and small (compared to our Milky Way at least) and then it's easy for things to "get lost"

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The Local Group is what's known as a galaxy group, with a few dozen galaxies, in contrast to larger galaxy clusters with hundreds or thousands of galaxies. So yes, it's sparse compared to the giant clusters, but not exceptionally sparse. The more massive clusters are much rarer than the small groups.

https://www.mpe.mpg.de/2040034/clusters_and_groups_of_galaxies

What seem particularly rare are isolated "void galaxies", but there are a few known. https://esahubble.org/images/potw1545a/

That's an awesome model. Thanks for sharing!

> There were periods of time when Indonesia was accessible by foot

This rough map of now-submerged river systems is what really made it "real" for me.

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