I've been a bit slack on keeping up with Arxiv for a while, so let's see what's hiding in the in-box.
2022-01-04
Hyper-Fast Positive Energy Warp Drives
Now
that sounds like a challenge to anyone not wanting to "out-geek their inner Trekkie", or however they describe it.
What's it all about? It's on the "gr-qc" section of Arxiv, which is "general relativity & quantum cosmology", so from the off, I'm not expecting a claim of a working model, ready to go out and speak to Vulcans.
Abstract
Solitons in space–time capable of transporting time-like observers at superluminal speeds have long been tied to violations of the weak, strong, and dominant energy conditions of general relativity. This trend was recently broken by a new approach that identified soliton solutions capable of superluminal travel while being sourced by purely positive energy densities.
Yes, previous suggestions of how to travel faster than light have been "hampered" by needing some way of generating "negative energy density" volumes of space, which nobody has any idea of what it means, or how to make one. So staying on the positive side of that question is probably a good idea - at least if you're wanting to talk about things that might actually be realisable.
[continued] This is the first example of hyper-fast solitons satisfying the weak energy condition, reopening the discussion of superluminal mechanisms rooted in conventional physics. This article summarizes the recent finding and its context in the literature. Remaining challenges to autonomous superluminal travel, such as the dominant energy condition, horizons, and the identification of a creation mechanism are also discussed.
OK, it's more of a review article than a research report, but that's OK.
The first 18 references are about the negative energy density thing. It still doesn't mean much to me, but at least I got that bit right. A soliton is a shaped block of space time that in some way differes from the surrounding smooth space time by means of it's rapidly varying curvature. I think. What does Wiki say? a self-reinforcing wave packet that maintains its shape while it propagates at a constant velocity - so it's not just a space-time thing, but a general wave phenomenon. And then ... the paper goes down a rabbithole of maths, way over my head.
There are still numerous challenges between the current state of physical warp drive research and a functioning prototype.
Ohhh, that sounds fun ...
The most glaring challenge is the astronomical energy cost of even a modest warp drive, currently measured in solar masses where kilograms is closer to the threshold of human technology.
Spoilsports!
the next hurdle to approach is modeling the full life cycle of a physical warp drive (creation, acceleration, inertial motion, deceleration, and diffusion).
Similarly more spoilsportery. Such tedious attention to uninteresting mere engineering. How are we meant to vibrate the dilithium crystals if someone insists that we make the damend things first?
The last hurdle I will mention is the full characterization of the sourcing fields, whether it be a plasma or other state of matter and energy. [...] the specification of the drive geometry only is an incomplete description of the full solution. Stress-energy sources must be specified to close the system.
More mere engineering. Is this paper meant ot be an inspiration to dreamers, or some sordid little chain which we have to slip, to touch the rest of the universe.
It sounds fascinating, but it's not a huge rallying cry to the Trekkies of the world. Sad. but they're used to it, I'm sure.
On thermodynamics of compact objects
ArXiv (where I recently discovered that the "X" isn't an "X" but a "chi", so the pronunciation is "archive"). Sounds dull, but since we can't yet find a way around those pesky laws of thermodynamics, I suppose we'd better pay attention to them. My first question is, are they talking about "compact bodies" which aren't gases, or bodies that are compact enough to have gone out the other side of atoms to being piles of fundamental particles. with no free internal space? Oh, it's black holes, so the inner workings of the bodies are thoroughly hidden from us. How convenient! No details to worry about.
Oh, no, they're not hiding the details : "focusing on self-gravitating compact systems without event horizons" means they don't have any convenient "Veil of Cosmic Censorship" (a.k.a "event horizon") to hde the details from the rest of the universe. How do they manage that? "The key step is the appropriate identification of thermodynamic volume [...] which is in general different from the geometric volume." Ah, that makes a degree of sense to me - if the spacetime were flat, a metre here is the same size as a metre over there, and a right angle to this straight line here defines a plane which is parallel to a right angle from the same straight line over there ; but explicitly they're not looking at a flat space time but a curved one, so you can't rely on either of those identities of translation. And 75 equations later, we get to a summary. Which is expressed i nterms of the Equation of State. of the material. (They're looking at photon gases, or fundamental particles, not atoms, so we shouldn't need to worry about "chemistry".)
Sidebar - Equation of State
I remember seeing these in Mike "Plutokiller" Brown's planetary science course, but I need to refresh mt memory. They provide a link between the pressure on a material and it's density. Wiki puts it slightly differently : "an equation of state is a thermodynamic equation relating state variables, which describe the state of matter under a given set of physical conditions, such as pressure, volume, temperature, or internal energy." That included the pressure-density relation (density being related to volume for a particular bit of matter), plus others.
The classical ideal gas law EoS is
p * V = n * R * T (Eq. 1 ; pressure, volume, number of moles in system, Rydberg constant and temperature, respectively)
or
p = R * T * (n/V) (Eq 2)
where n/V is clearly an expression of density.
If you're holding volume constant (so, doing no work, because the pressure vector doesn't move (if you think in terms of a piston model) you get a temperature- pressure relation. If you hold the temperature constant, you get a pressure volume (or pressure-density) relationship.
When you get into QM systems, you have to worry about Fermi or Bose-Einstein statistics. which is a more complicated study.
"For relativistic gas in particular, i.e. with EoS ρ = 3p" How did they get from Eq 2 (or one of the more complex QM expressions) to implying that R * T = 1 / 3 ?
So ... I don't understand what this paper is trying to say. It reminds me that I need to go back to Plutokiller's class notes - and it was a good class! - because I remember thinking I understood the EoS stuff then, but I don't seem to any more.
Rumour has it that Plutokiller is revising and updating that class. No mention on Coursera's website, but I'll keep ear open.
The influence of a fluid core and a solid inner core on the Cassini sate of Mercury
There's a sudden flurry of papers about "Cassini states", also for the Moon. It's something to do with the rotational interactions between outer and inner components. I guess there was a conference recently.
Mercury behaves like a rigid body (in terms of how it's spin axis and it's axis of rotation about the Sun relate - the so-called Cassini state) BUT we know it has a substantial liquid core (umm, how? I'll havve to check --- magnetic field?) so what is going on. The authors infer that there may be a large interior solid inner core. Earth has one, but at rather different T & P conditions. So ... probably the chemical composition of Mercury's core is different ot Earth's. Since there are still arguments going at the ~10% level about the composition of the Earth's core, that's not wildly constraining.
There's an update there for Mercury's properties. Stick that into the big astronomy database file.
Otherwise, not a lot of news.
Isostatic Modelling, Vertical Motion Rate Variation and Potential Detection of Past-Landslide in the Volcanic Island of Tahiti
What's this doing on Arxiv - it should be on Earth Arxiv. Anyway, In Tahiti, a coastline uplift of 80-110 m occurred 872 kyr ago after a giant landslide - that sounds like a bad hair day. Tahiti is considered a "stable" island and a tie point for reconstructing global eustatic sealevel variations, so finding a point deformation of the surface there and modelling the post-landslide deformation of the mantle underpinning of the island affects all the rest of the world's sealevel curves. Not by a lot, but definitely by a bit.
Is Tahiti really 6000m above sea level? That sounds incredible. I don't trust GeoMapApp's data sources. Wikipedia says "Highest elevation ... 2,241 m (7352 ft)" Ah, maybe they've got a ft elevation model, and attached it to a metres bathymetry model. That's a bit un-funny.
Update 2022-01-16 - Volcanic eruption, high explosivity in neighbouring Tonga yesterday. These "high islands" of the Pacific are active volcanoes.
What sort of size of volcano are we talking about? (Link to seabed image https://app.box.com/s/4omk3pi5roy1lui05sz8mw3rsowmpbtv ) It's about a 10km wide structure rising 750-800m above the seabed, 50-odd km behind the Tonga Trench. There's a line of volcanoes (and "high islands") close to the trench, and this is a step further back, sourced from deeper off the descending slab. (link to cross section (You can also see the trench-edge primary melt volcanoes and the more distal more andesitic line of volcanoes including the erupting one. "Andesitic volcano" and "good neighbour" don't generally appear in the same sentence.) Volcanism is reported as andesitic, which is appropriate to the reported explosivity of the eruption. The surface islands enclosed two sides (about 1/5) of the perimeter of the summit caldera. SI Global Volcanism database entry
2022-01-08
Well, it sounds serious. But ... if you're wanting to move the Earth outwards by (so much), you need to transfer a comparable amount of material inwards from wherever (the asteroid belt, in this discussion). But the asteroid belt weighs, about 1/2000 of the Earth. If you move that amount (several million asteroids) all the way to the surface of the Sun, you'd expect to move the Earth out by a corresponding amount - fractions of a percent.
What the energy and pollution costs of that would be, to not even address the actual outstanding problem of global warming, let alone the next generation's contribution ... Mr Sohrab Rahvar doesn't seem to have considered that. He calculates an expression for the change of power of light delivered to the Earth proportional to the asteroid mass and a factor for the change of angle - as my gut feeling at the start told me. And the temperature change would be the 4th root of that.
There's a minor point buried away in the details - the impact factor (how close the asteroid close-passer gets to Earth is set to 6400 km - a comfortable miss by 30km. For certain values of "comfortable" which many people wouldn't find very comfortable. That''l be fun.
Someone is trying to steal Avi Loeb's thunder!
This may be a suitable one for Slashdot's peanut gallery.
I did this one as a comment, attached to "Giant lasers simulate exoplanet cores prove they're more likely to have life, which was a fairly overblown piece about high pressure EoS for iron and it's magnetic effects. Great, you can generate a magnetic field at bigger planet sizes than Earth, but so what? The Earth gets most of it's radiation protection from it's atmosphere, not it's magnetic field. (More of an issue for Mars-size bodies, but so? The overblowing is about super-Earths and sub-Neptunes, not Mars-a-likes.)
A recent paper on ArXiv addresses the question of how you for an inner solid iron core in the molten iron core of a planet - which is believed to be necessary to produce the turbulent flow necessary to produce a self-stimulating dynamo.
The problem is that the "iron catastrophe" involved in separating the iron of a protoplanet from the rock it is mixed with, and it then settling to the middle of the planet, releases quite a lot of energy. (Depending on the composition, possibly enough to melt essentially all the protoplanet.) That leaves the initial core hot and molten, and you then need to nucleate iron crystals to form the solid core. Which would normally need a significant degree of undercooling (cooling the mixture below it's nominal melting point). Which is hard to achieve in the middle of a planet, possibly under thousands of km of magma ocean.
So, a new paper models a different way of forming the necessary crystals. Rather than going from the melt directly to a hexagonal close-packing (hcp) crystal (which is the energetically most favourable end product), they propose a two stage process of first forming a body-centred-cubic (bcc) close packing crystallite, which then rearranges to a hcp as it's growing. They propose that the energy barriers to that two step process would be lower, so the process rates would be higher.
Which is an interesting wrinkle on the details of core formation, but probably a bit less than practically useful, since people are still arguing at the several-percent level on the composition-temperature-pressure phase diagram for core formation. I haven't followed the details for years, but the last time I looked people were still arguing over whether the Earth's core contained several % (atom) of oxygen, sulphur, potassium, or all three (in addition to bulk iron and nickel) to get the right combination of viscosity, radiogenic heating, resistivity (conductivity) and magnetic permittivity, at appropriate temperatures and pressures.
Of course, the crystals formed would probably be quite pure iron, because you're essentially running a planet-scale zone refining process. So your melt composition is going to be constantly changing. Regardless of any continuing additions to/ losses to the overlying mantle bottom layer. Don'tcha just love reality against theory?
And I'm just about caught up with IArxiv.
