For Questions That Don't Merit Their Own Thread

[Scarecrow71]

This may very well need to be in its own thread, and if that's the case I can certainly move it to one.  But I've been playing a lot of KSP (duh) and watching the new season of Another Life, and something I saw in one of the episodes got me to thinking about traveling vast distances quickly:  wormholes.  Wormholes are, in theory, created when space folds in on itself, creating entry and exit points at 2 spots that are far away from each other.  Kind of like rolling a piece of paper in on itself until the outer edges touch.  And this got me to thinking about how space bends in on itself, which leads to the following question:

Does gravity cause space to bend/fold in on itself....or is gravity the result of space folding in on itself?

[lrd.Helmet]

48 minutes ago, Scarecrow71 said:

This may very well need to be in its own thread, and if that's the case I can certainly move it to one.  But I've been playing a lot of KSP (duh) and watching the new season of Another Life, and something I saw in one of the episodes got me to thinking about traveling vast distances quickly:  wormholes.  Wormholes are, in theory, created when space folds in on itself, creating entry and exit points at 2 spots that are far away from each other.  Kind of like rolling a piece of paper in on itself until the outer edges touch.  And this got me to thinking about how space bends in on itself, which leads to the following question:

Does gravity cause space to bend/fold in on itself....or is gravity the result of space folding in on itself?

As far as I know, Gravity is the folding (bending) of spacetime. So; neither? 

Mass bends spacetime and that gives you gravity.

[JoeSchmuckatelli]

@K^2 has been successful in answering similar questions for me in the past.

One of the answers I've gleaned from what he and others have written is that Relativity and Newtonian physics are exceptionally good mathematical analogs to what is really going on.  Solid tools to help us understand the universe we live in.

Thus, in many ways matter acts as though it bends (warps) spacetime - but it may not literally curve space.  A distinction that is largely esoteric and moot for most of what people have used Einstein and Newton's work to understand and predict natural processes.

Edited November 11, 2021 by JoeSchmuckatelli

[JoeSchmuckatelli]

For fun, here's a little light reading on inflation 

https://www.cnn.com/2021/11/04/opinions/problem-with-the-big-bang-theory-lincoln/index.html

 

[ARS]

I've seen multiple times on works set in WW2, be it games or movies, that everytime there's a soldier carrying a flamethrower, if a bullet struck a backpack he carries, it usually blows up in a massive fireball, engulfing the user and anyone nearby. Is this true to real-life? Because AFAIK, military grade-flamethrowers does not use gas-based fuel, only liquid (because it sticks better to enemies), and liquid is not compressible, so if a bullet pierced the tank, the contents inside is not under pressure, so there's no sudden expansion that can cause the whole backpack to rupture. And if I remember, military flamethrowers are specifically designed to be slow-burning and hard to ignite to maximize the burn effect and only burns when you want it to burn

[mikegarrison]

16 minutes ago, ARS said:

I've seen multiple times on works set in WW2, be it games or movies, that everytime there's a soldier carrying a flamethrower, if a bullet struck a backpack he carries, it usually blows up in a massive fireball, engulfing the user and anyone nearby. Is this true to real-life? Because AFAIK, military grade-flamethrowers does not use gas-based fuel, only liquid (because it sticks better to enemies), and liquid is not compressible, so if a bullet pierced the tank, the contents inside is not under pressure, so there's no sudden expansion that can cause the whole backpack to rupture. And if I remember, military flamethrowers are specifically designed to be slow-burning and hard to ignite to maximize the burn effect and only burns when you want it to burn

Videogames and movies != reality.

However, flammable liquids are flammable.

Edited November 12, 2021 by mikegarrison

[JoeSchmuckatelli]

1 hour ago, ARS said:

only burns when you want it to burn

Until it doesn't.  There's a reason they went out of favor.

Flamethrowers have not been in the U.S. arsenal since 1978, when the Department of Defense unilaterally stopped using them ⁠— ⁠the last American infantry flamethrower was the Vietnam-era M9-7. They have been deemed of questionable effectiveness in modern combat. Despite some assertions, they are not generally banned, but as incendiary weapons they are subject to the usage prohibitions described under Protocol III of the Convention on Certain Conventional Weapons.

Wikipedia

[kerbiloid]

It's a petrol can with petrol.

The pressure is made either by compressed air in the other balloon, or by several powder charges,

So, it isn't more dangerous than a petrol can with attached high-pressure air balloon or a bandolier below.

Edited November 12, 2021 by kerbiloid

[DDE]

1 hour ago, JoeSchmuckatelli said:

Until it doesn't.  There's a reason they went out of favor.

Flamethrowers have not been in the U.S. arsenal since 1978, when the Department of Defense unilaterally stopped using them ⁠— ⁠the last American infantry flamethrower was the Vietnam-era M9-7. They have been deemed of questionable effectiveness in modern combat. Despite some assertions, they are not generally banned, but as incendiary weapons they are subject to the usage prohibitions described under Protocol III of the Convention on Certain Conventional Weapons.

Wikipedia

Well, to be precise they were replaced by various rocketborne incendiary projectors like the M202 FLASH, which have been eschewed in favor specialist projectiles for general-purpose launchers (e.g. SMAW), and the incendiary munitions have been supplanted by thermobarics, known in the US by the euphemism "novel explosives". Russian terminology is quiant but straightforward in still calling such launchers "flamethrowers".

@ARS

[K^2]

14 hours ago, Scarecrow71 said:

Does gravity cause space to bend/fold in on itself....or is gravity the result of space folding in on itself?

Gravity is a phenomenon caused by curvature.

If you stand inside a rotating room, it seems like there is a force pushing you out from a center. In fact, if you describe dynamics from perspective of coordinate system attached to the room, you actually have to add this term as an external force to make the math work. Likewise, if you are in an accelerating car, a similar kind of force pushes you into the car's seat. These are inertial forces, also known as fictitious forces. A bit of a terrible name, since they technically work the same way as absolutely any force, and therefore aren't any less real, but we're stuck with this historical terminology. Gravity is such an inertial force. The thing that makes gravity special is that no choice of coordinate system makes it go away. If you describe a rotating room from a non-rotating coordinate system, centrifugal force goes away. Instead, you have wall pressing into you to keep you going in circles. Likewise, a car's seat has to press into you to keep you accelerated along with the car.

You can't make gravity disappear from your equations of motion in the same way. You can choose a coordinate system where gravity isn't experienced locally - like if you are free-falling, you don't experience gravity. But the fact that ground is rushing to meet you at an accelerated rate suggests that gravity is still there, plotting your demise on the global scale. This is because of the space-time curvature. In flat-space time, which, technically, only exists in school physics problems, we can choose an inertial coordinate system that makes all fictitious forces go away - no gravity. In curved space-time, in general, that is not possible, and we call the force that accounts for this curvature "gravity".

The source of curvature is stress-energy tensor. That is a generalization of energy and momentum densities to arbitrary coordinate system. People often casually simplify it to just saying that energy causes gravity, but it's really energy, momentum, and how they flow that all together influence the curvature. In practice, though, unless you're dealing with something very exotic, like a neutron star or a black hole, or something very large, like clusters of galaxies, you really can reduce it to just how much energy there is.

And if you want to get super technical, it's not even that the stress-energy is directly causing the curvature, but rather that the stress-energy is the conserved current of the Poincare symmetry, which is the extrinsic symmetry of the Lagrangian, and is therefore the source of the gauge field, which happens to be related to the metric tensor, which determines the differential curvature. But that's, like, some years of lectures in a sentence, so I hope previous paragraphs provide you with some intuitive sense for gravity and curvature.

[kerbiloid]

Also we should not confuse the combat flamethrowers with the kitchengarden ones

These ones are gas-fueled, as they don't need the fuel get sticked to the target.

[JoeSchmuckatelli]

55 minutes ago, DDE said:

nown in the US by the euphemism "novel explosives

I used to laugh at the hype thermobarics caused among some folks.  "Russia's doomsday RPG round" - I mean, really?

Still, they're pretty cool - but not quite up to the hype.  (Of course, we Marines got to play with our own version (novel))

I might remind readers that we were using similar tech in Vietnam (Daisy Cutter).

...

I will say, however, that Russia did impress with some of the RPG rounds they sold to ISIL, Syria, et.al. 

Payback for Afghanistan in the 70s?

Edit: this last is said with a knowing grin - no need to get upset by 'political' talk; this is about tech, not politics.

[DDE]

5 minutes ago, JoeSchmuckatelli said:

I used to laugh at the hype thermobarics caused among some folks.  "Russia's doomsday RPG round" - I mean, really?

Still, they're pretty cool - but not quite up to the hype.

Oh, I fully agree. It's little more than a high-powered concussive round.

But, of course, VaCuUm BoMb!!1!

[kerbiloid]

44 minutes ago, JoeSchmuckatelli said:

I used to laugh at the hype thermobarics caused among some folks.  "Russia's doomsday RPG round" - I mean, really?

Still, they're pretty cool - but not quite up to the hype.  (Of course, we Marines got to play with our own version (novel))

+10!

When I was playing Microprose F-117, the "Fuel-Air" was effective only against "Fuel Depots" and "Trails"

***

Though it was discussed as an anti-sat weapon in space.

The charge releases the air-fuel cloud on approaching to the target and ignites it.

The charge misses, but the thin aluminium target gets crushed in the cloud by the overpressure.

[JoeSchmuckatelli]

22 minutes ago, kerbiloid said:

Trails

LOLz

22 minutes ago, kerbiloid said:

anti-sat weapon in space

Might work if you included an oxidizer  - but 'Fuel-Air' typically doesn't have anything but fuel in it.  (although the term is loose, most don't think of powdered metal as fuel)

 

 

* okay, we're on a rocket enthusiast forum... so hush. 

[magnemoe]

2 hours ago, JoeSchmuckatelli said:

Might work if you included an oxidizer  - but 'Fuel-Air' typically doesn't have anything but fuel in it.  (although the term is loose, most don't think of powdered metal as fuel)

* okay, we're on a rocket enthusiast forum... so hush. 

And that is why fuel air is so effective, you don't need to bring oxidizer. Its like comparing an jet and rocket engine. 
Add that in space an shock wave will die out very fast in vacuum, you are also usually moving very fast relatively to the target so the only reason to have an warhead is to generate an cloud of fragments to increase chance of hitting. 

[DDE]

26 minutes ago, magnemoe said:

you are also usually moving very fast relatively to the target

Can't entirely agree. You do if you're using direct ascent ASAT, you don't if you've arranged a close encounter from a similar orbit. A delay-fused shell that detonates inside the target might have some value... but there's no point in making it thermobaric.

[kerbiloid]

11 minutes ago, DDE said:

but there's no point in making it thermobaric.

The point was to avoid direct hit.

[SOXBLOX]

We've got a new pile of promotional material from Sukhoi for their Su-75 Checkmate fighter. It's a very pretty plane, IMO. Brings back the X-32 vibes with that intake...

It's called a light fighter, but it's the same size as the Su-57 Felon. Good inspirational material for fighters in KSP, especially with Runway Project here again.

This site did an analysis: https://www.thedrive.com/the-war-zone/43119/latest-images-of-russias-checkmate-fighter-shows-us-just-how-big-it-really-is

[kerbiloid]

And the front gear leg looks innovative.

Edited November 14, 2021 by kerbiloid

[Gargamel]

Ok....  working on a project for the home shop... and the thought was “I’ll need to make sure that’s level”.    When in actuality, I only need the table surface to be flat and co-planar with the saw it’s being built for.   
 

The term “level” is just a way to describe the perpendicularity of a plane to the force of gravity.     For most uses, the fact that dinner doesn’t slide off the table is good enough, for various values of good enough.      
 

But how precise can we be with the levelness of a plane?    “Down” is basically pointing to the barycenter of the Earth/Moon system, but that wobbles around slightly over time.  
 

Given a table top being made at the equator at sea level at a random location, to what degree of levelness can we achieve?   

[JoeSchmuckatelli]

8 minutes ago, Gargamel said:

Ok....  working on a project for the home shop... and the thought was “I’ll need to make sure that’s level”.    When in actuality, I only need the table surface to be flat and co-planar with the saw it’s being built for.   
 

The term “level” is just a way to describe the perpendicularity of a plane to the force of gravity.     For most uses, the fact that dinner doesn’t slide off the table is good enough, for various values of good enough.      
 

But how precise can we be with the levelness of a plane?    “Down” is basically pointing to the barycenter of the Earth/Moon system, but that wobbles around slightly over time.  
 

Given a table top being made at the equator at sea level at a random location, to what degree of levelness can we achieve?   

Interesting question.  Obviously, you get the co-planar issue of the saw for clean, square cuts... but hmmm.

From the old days, people used to use a plumb bob or plumb square to get a level... but there's probably an expected margin of error when using twine, but I'd guess that a long bob on a thin filament would be a good start (better than a bubble of air in oil).

Looking forward to more 'mathematical' answers!

[TheSaint]

5 minutes ago, Gargamel said:

Ok....  working on a project for the home shop... and the thought was “I’ll need to make sure that’s level”.    When in actuality, I only need the table surface to be flat and co-planar with the saw it’s being built for.   
 

The term “level” is just a way to describe the perpendicularity of a plane to the force of gravity.     For most uses, the fact that dinner doesn’t slide off the table is good enough, for various values of good enough.      
 

But how precise can we be with the levelness of a plane?    “Down” is basically pointing to the barycenter of the Earth/Moon system, but that wobbles around slightly over time.  
 

Given a table top being made at the equator at sea level at a random location, to what degree of levelness can we achieve?   

In practical terms, the most level you can make something is to float it on a pool of liquid metal, such as mercury. (In a perfect system water would perform the same function, but IRL it is far more susceptible to local vibration.) You obviously have to do everything you can to isolate the pool from disturbance: give it rubber isolation mounts, shield it from air currents, etc. Trying to think if there is anything else you could do to improve the accuracy.

[kerbiloid]

The table surface should be gimballed in two planes, with a heavy counterweight, automatically pointing at the barycenter.

Maybe even floating in a dense and viscous liquid for better accuracy.

(Glycerol?)

Like a good compass does.

[Gargamel]

1 hour ago, TheSaint said:

In practical terms, the most level you can make something is to float it on a pool of liquid metal, such as mercury. (In a perfect system water would perform the same function, but IRL it is far more susceptible to local vibration.) You obviously have to do everything you can to isolate the pool from disturbance: give it rubber isolation mounts, shield it from air currents, etc. Trying to think if there is anything else you could do to improve the accuracy.

Ohhh good point.   Floating plane, gimbals to account for any variance.   Nice.  
 

So let me rephrase to a hypothetical.    In degrees, how much would a fixed plane vary from truly level due to the wobble of the barycenter?