# Tag Info

Accepted

### What is the largest fraction of Earth's sky that an astronomical object could take up (visually) before ripping Earth or itself apart?

The calculation turns out to be pretty straightforward and relies on two equations. Say the two objects have masses $M_p$ and $M_s$, with radii $R_p$ and $R_s$, with the $p$ denoting the more massive ...
• 101k
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### If money were no object, is there an element or alloy that would make for better train tracks than common steel?

10% Gold 90% Platinum I know, it surprised me too. Its the most wear resistant metal alloy known. To quote tires made from it would only lose a single layer of atoms after a mile of constant skidding ...
• 81.2k

### If money were no object, is there an element or alloy that would make for better train tracks than common steel?

So, we can look at the real world to see what goes into a tracks longevity. As expected, there is no easy answer. According to this your standard gravel bed might last 15 years, despite the tracks in ...
• 2,456

### How does one accurately deduce scale from any given fantasy map (to the best one can)?

Travel time. Mostly maps for the books are made because position and size of various regions matters to the story. Let's say a hero travels by foot from town A to town B following specific path on the ...
• 2,523
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### How do human girls who are sterilised at age 9 develop as they mature?

They Would Look Unusual The best bet for an approximate equivalence here would be Estrogen Insensitivity Syndrome. The default body plan is female, and initial development (including uterine ...
• 32k

### If money were no object, is there an element or alloy that would make for better train tracks than common steel?

Water. But, to be fair, you would have to replace the trains, too. If money is no impediment, an artificially built, fast-running water channel can be used to transport massive amounts of load with ...
• 9,084
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### Physics feasibility of a light-gas driven rifle cartridge? Diagram example included

No, this would not work. The disc between the propellant and light gas can't be driven faster than a bullet would by the same propellant, and it can't in turn compress the gas any faster than it is ...
• 38.2k
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### Amount of antimatter needed to propel large spacecraft?

I don't think you can mark a answer to be correct if it uses classical mechanics, but whatever. $\frac{1}{2}mv^2$ definitely does not work at these speeds anymore, plus there is a factor missing. ...
• 2,456

### If money were no object, is there an element or alloy that would make for better train tracks than common steel?

Aluminum bronze. Basically nearly corrosion proof mild steel. Can't be hardened but it is a bearing surface so just use more of it to support the load if cost is no object. I've heard it is difficult ...
• 11.1k

### How small can a nuclear weapon get?

One gigaton is $4\times10^{18}$ joules. Deuterium fusion releases about $10^{12}$ joules/g, so you're looking at $4\times10^6$ grams (about 9000 lbs or 4.5 tons) of deuterium, minimum. In practice ...
• 1,451
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### Can two species from the same genus but with a different sex determination system interbreed and have fertile (or at least sub-fertile) offspring?

It's possible, but it would result in more than 2 possible sexes. In 2015, the clawed frog Xenopus tropicalis was discovered to have 3 sets of sex chromosomes: Y, W, and Z. This results in 3 different ...

### Did I just discover that the ether that my etheric keels interact with is four-dimensional?

When doing dimensional analysis, the usual way is to work with dimension symbols time (T), length (L), mass (M), electric current (I), absolute temperature (Θ), and optionally amount of substance (N) ...
• 89.7k

### How much better would metallic hydrogen be than conventional explosives?

I don't believe that metallic hydrogen would be a good 'explosive' to use against armoured targets despite its likely effectiveness against unarmoured targets. The trouble with metallic hydrogen is ...
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### Do the mountains formed by a divergent boundary form on either coast of the resulting channel, or on the part that has not yet separated?

For the land portion of the rift there would be a rift valley with volcanic ridges on either side. For the portions in the water there will be an underwater ridge. If enough magma rises under the ...
• 946

### What kind of scanner can detect a hidden chip in someone's body?

The hard-science tag is kinda overkill for this one. Please read it's wiki. X-Rays have been successfully used to scan a single atom. There is no chip that can be manufactured today that can't be ...
• 123k

### How many different mines and extraction industries would a colony need in order to be able to produce equipment similar to what we have now?

For high tech stuff you need a lot of mines. A car needs 76 elements to work well. If you want a peak performing car, you need a fairly wide array of elements. A lot of them are gonna be fairly hard ...
• 39.1k

### Zero energy transfer between Earth and Moon with space elevators

You are asking a pretty involved question that would need simulations to be properly answered. Just conceptually, we know that if a Space Elevator tether extends beyond Geostationary orbit, the tip of ...
• 2,456
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### Ammonia transporting blood

Water. Ammonia and water are fully miscible. They dissolve into each other easily in all proportions, so you don't really need any special carrier molecules. Higher temperatures will drive ammonia out ...
• 42.3k

### Do the mountains formed by a divergent boundary form on either coast of the resulting channel, or on the part that has not yet separated?

This depends on what you mean by mountain. Rift valleys form a smattering of small isolated volcanoes. They occur everywhere in the area and are mostly a side effect of the upwelling mantle magma that ...
• 81.2k
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### How close can two planets be within the habitable zone?

NOTE: I'm not particularly happy with this answer. Notably because I've not used the equations myself and don't know if I'm representing them fairly. If another answer can improve on this, I'd be ...
• 123k

### How does one accurately deduce scale from any given fantasy map (to the best one can)?

Look for the market towns in populated agricultural areas. Since everyone needs to be within a day’s travel of market, those towns should be around 15 to 20 miles apart, as they are in Europe. This ...
• 20.3k
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### How could a K star develop an [O VII] Forbidden Line at 22.1 Å

The O VII complex you see in x-ray spectra around 22 Å arises from hot gas, like you said. The emissivity of the line peaks at around $2\times10^6$ K (see the AtomDB database, Foster et al. 2012), ...
• 101k

### If money were no object, is there an element or alloy that would make for better train tracks than common steel?

If money is no object / the distribution of elements is different (making it more common, perhaps?) tungsten could do the trick. It only begins to oxidize at extreme heats, and is already used in the ...

### Did I just discover that the ether that my etheric keels interact with is four-dimensional?

You started with a formula for lift in terms of surface area, but then you plugged in a volume instead of an area into that equation and noticed that the units weren't consistent. This is to be ...
• 5,602
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### Global warming in an apocalyptic future?

Contrary to most answers, I will refer to the authoritative source on the matter. Which is the IPCC, you can find all their reports here : https://www.ipcc.ch. if you are more broadly interested on ...
• 494

### How could a remote controlled bacterium work?

This is completely impractical. If you've been inspired by the conspiracy theories about vaccines and 5G, please note that those theories are obvious nonsense to anyone with a little knowledge of both ...
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### Calculating the total thermal energy contained within an object

The theorem of equipartition of energy states that molecules in thermal equilibrium have the same average energy. Using the theorem, you can derive a formula for the average kinetic energy of ...
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### Would lenses work in a universe with four spatial dimensions?

You are running into pretty big conceptual issues here. Especially for a Hard Sci-Fi question, there simply is no correct answer. General Relativity for instance, which is what all of modern physics ...
• 2,456