For Questions That Don't Merit Their Own Thread

[darthgently]

17 hours ago, kerbiloid said:

If the entropy is always growing, how can the Universe expansion be determinate?

Dark enthalpy is probably involved

[kerbiloid]

8 hours ago, darthgently said:

Dark enthalpy is probably involved

But the entropy is a measure of chaos.

How can the Universe keep expanding determinately, when the growing entropy should make its further evolution chaotic.

[farmerben]

Is an ocean planet really colonizable?  How?

[darthgently]

1 hour ago, farmerben said:

Is an ocean planet really colonizable?  How?

Seasteads?  Google "seastead"

I'd think the main problem with ocean worlds would be monster waves

[AckSed]

24 minutes ago, darthgently said:

I'd think the main problem with ocean worlds would be monster waves.

Maybe so, but the frequency - in the hertz sense - of the waves would be much lower. If we assume that the seafloor is smooth, then I imagine the water currents would wrap around like the magnetic field on the sun - different material, same fluid dynamics.

If the frequency dips low enough it could be akin to a tide.

Edited September 6 by AckSed

[darthgently]

2 hours ago, AckSed said:

Maybe so, but the frequency - in the hertz sense - of the waves would be much lower. If we assume that the seafloor is smooth, then I imagine the water currents would wrap around like the magnetic field on the sun - different material, same fluid dynamics.

If the frequency dips low enough it could be akin to a tide.

Not the same dynamics.  What you are calling "like tides" is referred to as swell.  Big swell can turn into huge waves if the seafloor rises in the direction of travel.  If there are varying winds from coriolis, and there would be, especially with no land masses to tame them, then there will be wind generated waves from varying directions and distances.  The waves from different places would add and subtract as waves do at an given location.  On a water world the peaks and troughs could be tremendous.  The deeper the oceans, the more tame, but any rise in the sea floor would make waves pile up and that is a lot of energy.  There would be monster waves at times with no continents to diffuse them.   The difference between day and night insolation would be the main driver, as it is on Earth.   Our moon does cause tides, but the waves are nearly completely from solar heating via the wind blowing the water surface for weeks on end.

Consider how hurricanes form and how land diffuses them also.  They form over warm water and spin from coriolis.   Maybe some form of motion-dampening kelp-like sea life and no shallower water anywhere could tone things down but there would almost certainly be monster soliton waves still just as there are on Earth.   Also, the lower the frequency of a water wave, the faster it travels.   

I remember reading something about soliton waves on a water world theoretically being capable of travelling at supersonic speeds (airspeed, not speed of sound in water) repeatedly around the planet.  Perhaps as permanent dynamic features if the feedback systems work it that way.

Unrelated, but interesting, I recall that the rock that wiped out the dinosaurs is calculated to have created tsunamis that were supersonic also.

Anyway, I would never underestimate the weather or the ocean on any world until it was very well understood

[kerbiloid]

The ocean planet would turn into either into Supervenus, or into Underuranus.

[Nuke]

11 hours ago, darthgently said:

Seasteads?  Google "seastead"

I'd think the main problem with ocean worlds would be monster waves

and hurricanes, with no land to break them up.

[magnemoe]

5 hours ago, farmerben said:

Is an ocean planet really colonizable?  How?

You come in an starship, Its probably easier to work in orbit using asteroids and water is so common in the outer solar system. 
Its like mars, in the short time, an science base makes sense. 

[Jacke]

On 9/5/2024 at 4:39 AM, kerbiloid said:

If the entropy is always growing, how can the Universe expansion be determinate?

These are complex concepts.  It's best to start with the background and definitions, where Wikipedia is good.

Chaos is usually used to refer to systems who's development is strongly affected by even small changes in the initial state.
https://en.wikipedia.org/wiki/Chaos

Entropy is a thermodynamic property that is difficult to understand and isn't directly observable, but usually only measurable in its changes.  It measures disorder of a system and limits how much energy can be put to useful work.  It's also a classical concept and like temperature, it isn't quite there when at the level of subatomic particles who's behaviour is bound by Quantum Theory (which isn't even in a complete version, only incorporates Special Relativity and also Background Dependent).  Entropy is very much a product of observing classical systems at scales about the same size as ourselves.
https://en.wikipedia.org/wiki/Entropy

The expansion of the Universe is directly observed, it has happened and is happening.
https://en.wikipedia.org/wiki/Expansion_of_the_universe

What happens in detail to the Universe in the past and up to now can be observed to a degree.  How to understand it is rather more complex.  Whether it is determinate--I assume you mean predictable--is something that can only be observed over a lot of time, a lot longer than any of our lifetimes.

[kerbiloid]

1 hour ago, Jacke said:

Entropy is a thermodynamic property that is difficult to understand and isn't directly observable

Mathematical definition:

S_m = k_m ln W, wkere W is number of possible states of the system

Physical definition:

1/T = k_ph dS_ph/dE, wkere E is total energy of the system

As the Universe is a closed and adiabatic system, and it is expanding (i.e. a physical process, which inevitably means growth of entropy), then it's evolving to the state of maximum entropy, i.e. heat death ^(tm), where its further expansion is impossible, or its evolution is unpredictable, i.e. chaotic.

1 hour ago, Jacke said:

The expansion of the Universe is directly observed, it has happened and is happening.

It is presumed, based on interpretation of the observed star position and light wavelength in the visible part of physically observable Universe.

 

And thus, how can be we sure that the Universe is in its predictable part of its evolution, instead of chaotically shivering.

Edited September 7 by kerbiloid

[farmerben]

What is the weight to energy ratio of hydrogen fuel cells?  I googled it but can't find simple off the shelf hydrogen fuel cells with their mass and energy output posted like the ones in KSP.

 

 

[RKunze]

On 9/7/2024 at 4:41 PM, kerbiloid said:

And thus, how can be we sure that the Universe is in its predictable part of its evolution, instead of chaotically shivering.

Because it has been "shivering" in the same direction (outwards) for as long as we can observe - which means for the last dozen billion years or so (thanks to the finite speed of light, we actually can look back in time). And there is absolutely no indication that this trend will stop (in fact, it seems to be speeding up).

[kerbiloid]

1 hour ago, RKunze said:

in fact, it seems to be speeding up

Somebody is inputting energy?

[AckSed]

3 hours ago, farmerben said:

What is the weight to energy ratio of hydrogen fuel cells?  I googled it but can't find simple off the shelf hydrogen fuel cells with their mass and energy output posted like the ones in KSP.

The Toyota Mirai is a hydrogen fuel-cell car. A 2020 report cites a 2018 report on a 2017 model:

Quote

The  Toyota Mirai fuel cell stack is reported to have a gross power of 114 kW, with a volume of 37 L  and mass of 56 kg, corresponding to a (gross) power density of 3,100 W/L and specific power of  2,000 W/kg.

Back when the nickel-hydrogen batteries on the ISS were being replaced in 2017, someone proposed fuel cells. Their presentation said: "75kW~90kW [energy usage of ISS] of fuel cell weights around 60.7kg-73.2kg based on commercial PEMFC."

Sadly, I can't find anything more current beyond a 2019 ESA study on regenerative fuel cells, and you have to ask nicely for the actual report: https://connectivity.esa.int/projects/fuel-cells-extra-large-telecommunication-satellite

[farmerben]

Methane fuel cells are available as well.  They are crazy expensive though.  

I'm actually designing a catamaran to take the Northwest Passage.  I'm looking at the best way to power it and to heat the whole boat.  Perhaps fuel cells could be built into the oven.  You can cook on the heat generated by fuel cells.  Open the oven door and blow a fan across it to heat the whole boat.  Of course there are numerous approaches you could take for heat, the priorities are lightweight and fewest number of potential failure points.  

[Jacke]

This is all much more complex than what I think you realise.  Not easy to connect the assumption behind each of those formulae.  As well, each symbol needs definition.  And even to speak of the number of possible states of the entire Universe is lacking something, especially as....

On 9/7/2024 at 2:41 PM, kerbiloid said:

As the Universe is a closed and adiabatic system, and it is expanding (i.e. a physical process, which inevitably means growth of entropy)....

Well, the Universe is now on a large scale flat.  Which leads to complexities like given a particular spot, there are places that will never be observable because Expansion means they're beyond the Light Speed Horizon, AKA the Cosmological Horizon.  They are parts of the same Universe but no information can ever be received from or sent to them.

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

This is a System that even Classically is well outside of anything considered when Thermodynamics was formulated in the 19th Century.  I have no idea how to adjust Thermodynamics to the current Structure of the Universe.  Or even if that gives anything useful.

 

Edited September 14 by Jacke

[ARS]

Is there a specific reason why Kirov-class have rear-mounted turrets? And why the rest of her sisters only has 1 turret instead of 2?

[DDE]

3 hours ago, ARS said:

Is there a specific reason why Kirov-class have rear-mounted turrets? And why the rest of her sisters only has 1 turret instead of 2?

The latter is easier to explain: the 130 mm wasn't ready in time, so the two 100 mm are an ersatz. Plus, they seem to have wanted to place the Granit battery closer to the CoM of the ship, whereas the guns are really an afterthought.

[ARS]

5 hours ago, DDE said:

The latter is easier to explain: the 130 mm wasn't ready in time, so the two 100 mm are an ersatz. Plus, they seem to have wanted to place the Granit battery closer to the CoM of the ship, whereas the guns are really an afterthought.

Isn't Kirov the first of her class? And all of them are armed with 130mm? I thought they put it like that because their doctrine is fire all missiles at US carrier strike group, turn back, and use the extra firepower of their guns to cover their back?

Edited September 17 by ARS

[kerbiloid]

Traditions, traditions.

Spoiler

[DDE]

16 hours ago, ARS said:

Isn't Kirov the first of her class? And all of them are armed with 130mm? I thought they put it like that because their doctrine is fire all missiles at US carrier strike group, turn back, and use the extra firepower of their guns to cover their back?

The build schedule got mixed up AFAIK.

Anyway, their doctrine is to hunt down Polaris-carrying submarines arouns the inshore launch positions with their ASROCs. Everything else is defensive armament to facilitate that mission.

There weren't any Polaris carriers anymore by the time the Kirovs were actually ready, though.

[magnemoe]

3 hours ago, DDE said:

The build schedule got mixed up AFAIK.

Anyway, their doctrine is to hunt down Polaris-carrying submarines arouns the inshore launch positions with their ASROCs. Everything else is defensive armament to facilitate that mission.

There weren't any Polaris carriers anymore by the time the Kirovs were actually ready, though.

The US made an nuclear artillery shell for the the 16" guns of the Iowa battleships. Before program was finished all the Iowas was mothballed. The 16" nuclear shell was retired before the Iowas was reactivated for the Vietnam war. 
 

[farmerben]

How smart is it to stack hydrofoils vertically?

My idea is for a proa sailing vessel in which the smaller windward hull foils.  But instead of one foil it could be a multitiered foil, like a biplane wing.  You would be able to pilot it such that at ideal power only the bottom foil is exposed.  But the upper foils allow it to lift even at less than full power.  In choppy seas the upper foil will slam in and out of the water but it might be OK.  

[K^2]

On 9/6/2024 at 12:37 AM, kerbiloid said:

How can the Universe keep expanding determinately, when the growing entropy should make its further evolution chaotic.

Fine grain entropy is conserved. Coarse entropy is growing. In context of a system governed by quantum mechanics, it allows for chaotic observables while being entirely on rails. The easiest (though, not the only) way to interpret this is through the lens of MWI, where it's pretty easy to show that the fine grain entropy of the universe is simply fixed at zero (there is exactly one state), while any particular timeline becomes more chaotic over the course of evolution. The increase in available state-space as the universe inflates leads to significantly more entropy/chaos in the "future" direction, setting up the arrow of time. Consequently, there is no practical difference if our universe is expanding from a point due to a Big Bang, or if it's collapsing in on itself. The future universe is in the future because it's bigger. And all of the chaos is a consequence of observation, while all underlying physics is fully deterministic, because the universe as a whole doesn't have such silly notions as order or chaos or time or entropy. It's just a multi-dimensional manifold with some symmetries, and we're just too small to perceive it that way.

Physics is the most depressing of sciences.