If you want to put the book or screen ten times further away, the letters need to be ten times the size to have the same angular size. That applies whether or not you put flat mirrors you put in the light path.

Not directly. But an initially polished gold object will dull as it wears; as its surface is roughened by contact with other objects. A harder gold alloy will wear more slowly than a softer one.

Pure gold is rather soft so jewellery (and circulating coinage historically) is typically made of an alloy with the gold content described in either karats (24 is nearly pure) or millesimal fineness (1000 is pure). For example 22 karat / 916 fine and 18 karat / 750 are commonly used, with 18 karat being harder than 22 which is harder than 24. The alloying elements are usually silver and copper but other elements may be used, typically to change the colour of the alloy but the hardness will also be affected.

A fission bomb must start with a subcritical assembly and detonates by making it become supercritical. If you want a large amount of fission fuel, making a shape that's not already supercritical is a significant design constraint.

Economics is also a factor. A pure fission or boosted fission weapon gets most of its yield from the fission of either highly-enriched uranium or weapons-grade plutonium. Those are expensive.

(Boosted fission = you put a little bit of fusion fuel in the middle. It doesn't make much yield directly but it makes lots of neutrons to cause more fission in the plutonium. Without boosting only a small percentage of the fuel actually fissions.)

A thermonuclear weapon, AKA hydrogen bomb, uses a (nowadays always boosted) fission primary to cause a larger fusion reaction. Typically, but not always, the fusion then creates additional fission in a natural or depleted uranium tamper, much cheaper per kg than highly-enriched uranium. The yield is predominantly from the fusion and the tamper fission, and a relatively small amount of expensive material is needed for the primary.

Almost all current nuclear weapons are thermonuclear. It's considered to be the best design even for relatively low-yields of a few tens of kilotons. I think North Korea and possibly Pakistan are the only states using pure or boosted fission weapons.

> Wouldn't a 50 meter change compound over 120million years?

The effect on the size and shape of Earth's orbit would not "compound", it would still be small. The effect on Earth's position around its orbit does add up though.

Even without large impacts the solar system is chaotic, in the mathematical sense. We can make good predictions for the next few million years. But we cannot say, for example, what season it will be in the northern hemisphere exactly 100 million current-day years from now.

I would be very surprised if any creature resists chlorine trifluoride. That stuff will set materials such as concrete, sand, and asbestos on fire upon contact and reacts explosively with water. Teflon and some metals resist attack by ClF3, the metals by the formation of a surface metal fluoride layer, and neither are found in known lifeforms to my knowledge.

Dioxygen difluoride is also up there. It's nicknamed "FOOF" for a reason, blowing up on contact with solid ethanol, liquid methane, and water ice to name a few.

There are acidophile and alkaliphile organisms, but I suspect superacids would destroy all known life too.

https://www.science.org/content/blog-post/sand-won-t-save-you-time

https://www.science.org/content/blog-post/things-i-won-t-work-dioxygen-difluoride

Outside the realm of chemistry a high enough temperature, intense enough ionising radiation, or extremely strong magnetic fields will destroy all known molecules whether living or not. (In the magnetic field case, this is way beyond anything we can produce on Earth, but neutron stars will do it.)

Question. Do the (valence) quarks in a proton or neutron in a nucleus stay part of that one proton/neutron? Or is there exchange of quarks between protons and neutrons? (Or is it unknown? Or is the question meaningless?)

The darkest places on Earth, on a moonless night, are pretty close to what it would be like in instellar space. Although even then there are light sources not present in interstellar space such as the planets, zodiacal light, and airglow.

https://skyandtelescope.org/astronomy-resources/light-pollution-and-astronomy-the-bortle-dark-sky-scale/

Bortle 1 locations on land are rare indeed.

Near a star, on the other hand, the starlit side of a spaceship will be highly visible, while the unlit side will be virtually black. But all spacecraft and decent-sized celestial bodies glow in the infrared.

Yes. Trivial demonstration: Disintegrate the molecule into its component atoms by heating it up to several thousand Kelvin, then repeat the same steps that made it in the first place.

This might not be the most practical process, with steps like "let a tree grow" and "bury the dead tree for 100 million years".

Birds are members of a group, saurischia, best known for having "reptile-like" hips. Even though not all members of the group have that characteristic, that doesn't affect the validity of the group. In cladistics, which is by the far the dominant approach to taxonomy nowadays, taxons should be monophyletic groups or "clades" comprising a common ancestor and all its descendants.

There is current debate and research as to whether the traditional saurischia is a monophyletic group or not, and some classifications put ornithiscians as closely related to theropods with saurischia either redefined without theropods included or not used at all. But bird anatomy does not by itself invalidate the traditional view.

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

Because the hips of ornithiscians are similar to the hips of birds. Current theory is it's a case of convergent evolution with the "bird hipped" layout having evolved three or four times in the dinosaurs.

Archeopteryx was known when the saurischian-ornithisician division was proposed by Harry Seeley in 1888, but I don't know if archeopteryx's relation to other dinosaurs was really known back then. Even if it was, Seeley might not have cared that a bird was in saurischia.

More like a disintegration field. You know how atoms are round? Not in a magnetic field of 10^5 Tesla or more they aren't. The magnetic field is strong enough to distort the electron orbitals into narrow rods and ordinary molecules just fall apart. https://www.osti.gov/etdeweb/biblio/6961623 To borrow a phrase from Randall Munroe, "you would stop being biology and start being physics."

Oh, and the vacuum becomes birefringent - the speed of light depends on its polarisation.

The NuSTAR telescope has mirrors that reflect photons up to 79 keV, although only at glancing incidence (photon travelling close to parallel to the mirror surface). Astronomers typically regard energies below 100 keV as X-rays, but physicists regard photons emitted by atomic nuclei as gamma rays regardless of energy and some are in the range observed by NuSTAR, for example gamma rays emitted by decay of Titanium-44 in supernova remnants.

I don't know what the record is for photon energy reflected.

Edit: By contrast, the higher energy instrument on the Fermi gamma ray space telescope observes gamma rays from 20 MeV to 300 GeV, so at the high end that's over 3 million times as energetic as what NuSTAR observes. It does not use mirrors or lenses. Instead incoming gamma rays create electron-positron pairs and the telescope has a stack of detector layers that track their direction allowing the gamma ray direction to be determined fairly precisely, described in extreme detail by https://arxiv.org/abs/0902.1089 (scroll to end for the pictures)

INTEGRAL, working at lower energies from around 15 keV to 10 MeV, uses another approach, coded aperture masks. This is essentially the same idea as a pinhole camera, but with multiple pinholes and using computer process to unscramble the overlapping images.

On the contrary, in a star undergoing fusion the conditions may well cause nuclear reactions such as induced fission, making the effective half-life of unstable nuclides shorter than their half-lives when isolated. In the same way that uranium and plutonium in a nuclear reactor are fissioned much faster than their natural half-lives of millions or billions of years. I predict that would mask the effect of gravitational time dilation which has been calculated to be tiny by other answers.

While it's not a heavy nuclide, Lithium-7 is destroyed in fusing stars, and its presence or absence helps distinguish red dwarfs from brown dwarfs.

To add to the other answer, terminal velocity itself depends on altitude and projectile orientation. As altitude increases, air density decreases and terminal velocity increases, towards an extremely high value in outer space much faster than any natural object. Therefore any object that has come from outer space must "enter Earth's atmosphere", something that is itself not sharply defined, faster than its terminal velocity.

Most objects hitting planetary atmospheres are not travelling straight down either. The Earth Impact Effects Program states the most common angle for meteor impacts as 45 degrees. Orbital spacecraft making a re-entry, either controlled or uncontrolled, typically enter at much shallower angles. On the other hand sounding rockets follow a trajectory that's appoximately straight up and down.

The orbital mechanics works. As mentioned, a 1:1 orbital resonance. https://en.wikipedia.org/wiki/3753_Cruithne is a good example. If you look at the charts, it makes a close approach to Earth once a year, but how close these approaches are varies over a few centuries.

The question is whether such a body can preserve enough cometary activity to look like a comet to the naked eye. Any object in a 1:1 orbital resonance can never get more than 2AU from the Sun so it doesn't get any real break from the solar heating that drives cometary activity. Active asteroids, also known as main belt comets, are known, but most orbit more like 3AU out.