What if Planet 9 is a Primordial Black Hole? (2019)
arxiv.org
arxiv.org
I'm curious what math led to that funny exponent in eq.(5): ρ(r) ~ r^{-9/4}.
You don't just have to get the probe there. (And which probe? You have to design and build it first.) You have to get it there, have it get data, and have the data come back. Otherwise you've just thrown a rock, which, yeah, if we spent a fortune we could certainly throw a rock out vast distances very quickly.
So, you either:
Do a flyby, which requires a slower speed. New Horizon's speed was good for gathering data from Pluto and sending it all back. The hypothesized PBH is small enough to fit in your duffle and probably much darker.
Enter orbit. Easy enough with a 5-10M object. But the orbit has to be a useful orbit. So either you burn a tremendous amount of delta-v to reduce your tremendous speed so you get a compact-enough orbit; which requires even more energy to get out there, or you craft the orbit such that the object itself helps slow you: but that takes time and more speed means more time.
Studying it might allow us to finally unify GR and QM.
1. Humans aren't going to survive in space for 10 years. It's questionable that they'd even survive a trip to Mars without getting riddled with cancer from the cosmic radiation. Sure, if you built a big enough ship to provide some really effective shielding, it's technically possible, but that ship would be enormous and far beyond our current capabilities. I don't think it's feasible at all to launch such a ship from Earth; it would need to be assembled in space.
2. Project Orion is just an idea on paper; it's not within current technology, because no one ever built it. We don't "have the technology" at all. We don't even have the technology to land humans on the Moon. We did decades ago, but we no longer do: all the people who knew how to do that are retired or dead, so we'd have to start over. Of course, we can build powerful rocket motors easier now since we do so regularly now, so building equivalent Moon-landing capability is no longer as difficult as in the 60s, but a lot of things would have to be partially re-invented (e.g., the lander itself, the rover, etc).
3. Does your time estimate include the time needed to decelerate, so the ship doesn't just zip by the black hole with barely any time to collect data? (And if there's people on this ship, they might want to return to Earth...)
Can anyone imagine something concrete?
Even the Soviets didn't say they were dumb and wasteful.
(I tend to be a bit milder, and just say that the moon landings should be thought of as spending on entertainment, not on science. Manned space flight in general is very cost-ineffective, if you care about scientific bang for your buck.)
I don’t think it’s hypocritical to say that the moon landing was worth it but gearing up to go to a distant black hole is not*
* Is also shiny and has movies
* It's cheaper by a good number of orders of magnitude
* It has massive commercial value (bringing home piles of gold. Well, platinum metals)
* It has actual military implications, which is the main motivator for any government progrem
* It gets to call the military research "planetary defense", and there are more cool movies about that.
No government is getting its act together on that. A distant black hole is right out.
But yes, agreed, it's way too expensive. And so we're not even going to make that happen. That was the point - it's more attractive than neighboring black holes along several different axes, and it's still not worth it.
Refine / cut up your asteroid on the lunar surface, then you need a relatively small amount of dV to put your payloads into a decaying orbit around earth (~1/4 the dV to get into low earth orbit from the surface). Enough heat shielding, and you'll be able to crash your payload into the ocean somewhere for recovery.
Frankly, the biggest issue is that you'd end up flooding the market.
Second - humans need to locate this deposit. Remotely. In the asteroid field. We are still finding out deposits in the habitable Earth regions, because that is a hard task, talking about locating deposits in the Belt is an impossible task, and will remain so for a long time.
Third - we need to get there. Excluding flyby's, the best humanity managed is to deliver two 1 ton vehicles to the Mars. And there we had a luxury of aerobraking to save a lot of fuel. Belt is way farther, there is no aerobrake possibility and we will have to deal with other asteroids along the way. We don't have such tech.
Fourth - we will need to strap engines to the asteroid, remotely, with an hour signal lag. Engines which don't exist in any form today. And we need to get fuel to them somehow. Impossible task.
Fifth, we need a Moon colony with robots or humans.
Sixth - high energy tech on the Moon.
Seven - launch facility on the Moon.
And I've probably missed a few other impossible hurdles, writing this. Then IF all of that would exist, you would need it to be at least as cheap as Earth based mining, which is a pipe dream. Any space tech is more expensive by design, sometimes orders of magnitude more expensive. AND finally there need to be an infinitely elastic market on Earth for this new source of materials. Imagine you will sell a billion tons of platinum tomorrow, the price would crash and never ever recover. You see this yourself in your last sentence.
Not if you launch first, and present after ;)
I can think of PLENTY of billionaires to send :)
I assume you object to billionaires because they use a disproportionate share of the world's resources? Spending even more resources on them doesn't seem like it would make that better?
But small black holes with the same mass as the Earth have the same sucking power as the Earth since it's the mass that does the sucking.
We'd all freeze to death in a few days, but the earth would go right on orbiting as if nothing happened.
If our moon turned into a black hole of its same mass tomorrow there would be even less of an effect. We'd notice we could no longer see the moon, but we'd still have tides just like before.
I'm guessing this refers to https://en.wikipedia.org/wiki/Cherenkov_Telescope_Array ?
The actual exponent and its workings-out almost certainly comes from Gott 1975, which I do not remember ever having read <https://adsabs.harvard.edu/full/1975ApJ...201..296G>, so I cannot do it any sort of justice at this time, but see my comment on Gunn 1977 below.
Theres an early history of pre-NFW power-law dark matter density profiles swept up in ref [34] of the Planet 9 preprint, which is cited just before the equation that piqued your curiosity.
[34] is <https://ui.adsabs.harvard.edu/abs/1985ApJS...58...39B/abstra...> which turns out to be part of a 1984 Princeton doctoral thesis, and I am unfamiliar with it although I recognize its author Edmund Bertschinger as well-known from his later 1980s-2000s work in cosmological perturbation theory and cosmological simulations.
[34] also lists Gunn & Gott 1972, Gunn 1977 and Gott 1975 in the references section (bottom of 1st to top of 2nd column). Surprisingly the Planet 9 preprint lists none of these papers.
However, in a way that could be serendipitous: you might enjoy the "funny exponent"s in many the equations of [34], since some involve powers of -8/9, 8/3, -5/2, among others.
I wrote but abandoned an attempt to tease out the detail using the following two seminal papers.
Gott & Gunn 1972, "On the Infall of Matter into Clusters of Galaxies and Some Effects on their Evolution" <https://ui.adsabs.harvard.edu/abs/1972ApJ...176....1G/abstra...> (PDF via top right box), contemplating a Friedmann universe with a spherical homogeneous overdensity that collapses into a galaxy cluster.
Gunn 1977 <https://ui.adsabs.harvard.edu/abs/1977ApJ...218..592G/abstra...> contemplates a spherical but inhomogeneous perturbation, and arrives at a power law of density \propto r^{-9/4}. It relies on Gott 1975 for that, however. This paper is the basis for a "universality" result which indicates that the shape of an overdensity's boundary is essentially preserved over time, even as the matter within the overdensity collapses into various structures. That universality was convenient at the time to avoid having to treat spirals specially, and is in effect claimed as applicable to a PBH equipped with a DM microhalo in the preprint linked at the top.
The "funny exponent" appears in the text just after eqns (8)-(9b) in Gunn 1977.
All of the NASA/ADS abstracts linked in my comment have PDFs available in the top right box on the page.
From the abandoned first attempt (which grew in length and messiness) I'll save two things: the "curious" equation's r_eq and \rho_eq are fixed at the transition from radiation-domination to matter-domination at z ~ 3200 (47 thousand years after the big bang), after which radiation pressure is insufficient to keep perturbations from growing. The transition is also when scale factor a \propto t^{2/3}, and energy density \rho \propto a^{-3(1+w)} where w is close to 0; self-gravitation in the overdensity causes its \rho to drop more slowly, and furthermore draws in matter from the surrounding ~average density shell.
Renormalization.
Huh? Explain. How does that fit?
It's just a power-law distribution for the density of a halo that forms early around the "Planet 9" primordial black hole.
The density ~ r^{{3/2}^2} is a result from 1970s studies of structure formation shortly after Vera Rubin's work indicated that dark matter halos surrounded spiral galaxies. Rubin & Ford 1970 <https://ui.adsabs.harvard.edu/abs/1970ApJ...159..379R/abstra...> emissions regions of M31, begat Gott & Gunn 1972 <https://ui.adsabs.harvard.edu/abs/1972ApJ...176....1G/abstra...> infall of matter onto galaxy clusters begat Gott 1975 <https://adsabs.harvard.edu/full/1975ApJ...201..296G> structure formation and elliptical galaxies begat Gunn 1977 <https://ui.adsabs.harvard.edu/abs/1977ApJ...218..592G/abstra...> formation of massive galactic halos.
These are foundational to the paper at the top's ref [35], which is quoted just before the "funny exponent".
Roughly, let's start with an expanding Friedmann universe, where we treat galaxy clusters as motes of dust, and smear that out into one or more fluids representing nonrelativistic matter and radiation and other relativistic matter, with an identical energy-density at every point in space in an equatorial slicing, with each slice succeeding a spatially-smaller slice and preceeding a spatially-larger slice. At small scales, radiation pressure stabilizes any overdensities or underdensities in the nonrelativistic and relativistic fluids. Eventually expansion causes a transition from radiation-dominance to matter-dominance, allowing overdensities and underdensities to grow.
The papers by Gott and Gunn above consider the evolution of a spherical perturbation, an overdensity, starting at that transition, studying whether shortly after the transition from radiation-dominance, halos can support the evolution of generic galaxies and galaxy clusters. They can; and the "Planet 9" paper at the very top (albeit by way of its ref [34]) takes that further and applies this halo logic to primordial black holes based on that "universality" result (in Gunn 1977, penultimate paragraph).
Essentially what happens is that the nonrelativistic matter in an overdensity self-gravitates and so sticks around as an overdensity from one slice to the next bigger slice. Moreover, neither cold dark matter nor a primordial black hole radiates that early in the universe, so the overdensity can't revert to average through dissipation. Furthermore, the overdensity's gravitation draws in matter from the "shell" outside it (i.e., the rest of the universe); that's the "infall". If the infall is dissipationless, it sticks around in shells well outside the centre of the overdense perturbation.
All of this combines so that the density of an overdense perturbation drops much more slowly than the density in the rest of the universe. The latter drops like cosmic_time^{3/2} while the perturbation's density drops like cosmic_time^{9/4} (in these 1970s papers; in the age of fast computers one would use a profile like NFW <https://en.wikipedia.org/wiki/Navarro%E2%80%93Frenk%E2%80%93...>, and in this "Planet 9 is a PBH" context one might want to think about "bottom-up" hierarchical mergers of microhalos at all length scales: in that case, is "Planet 9" more likely to be a microhalo without a small black hole?).
In the case of the "Planet 9" PBH the (very small) black hole and its (relatively dense) halo of dark matter are essentially noninteracting except for gravitation, because the horizon is so small and cold dark matter basically feels nothing but gravitation. Gravitation (by way of the equivalence principle) can impart some fairly gentle accelerations that can alter the halo, as discussed in the paper linked at the very top.
Practically all of the results above were found using a linear approximation to General Relativity, checked by N-body numerical simulations (they got up to N=32k or so by the time non-power-law halo distributions were found to fit observations better in the 1990s). For the "Planet 9" PBH it is easy to imagine nonlinearities from interactions with early supernovae, however, if the "capture mechanism" is in-place-formation or an early entrainment to the local standard of rest. Even thinking about using a full nonlinear solution, I don't understand how renormalization might be used and or how it would generate the "funny exponent".
https://news.ycombinator.com/item?id=23993716 - 3 years ago, 119 comments
https://news.ycombinator.com/item?id=28167058 - 2 years ago, 153 comments
Einstein was able to predict how light was bent around the Sun and Eddington confirmed it right away, it was like Babe Ruth pointing to the stands and hitting a home run.
The lag between a phenomenon being predicted or model sped by theoreticians and actually observed is getting longer and longer in fundamental physics, I mean neutrino oscillations were hypothesized in 1957. The fact that you can’t get a Nobel prize posthumously means a theoretician might never get a Nobel in fundamental physics ever again. So they’ve got to do something speculative like this to have a possibility of a legacy unless you are Ed Witten and can convince people you are a genius without any appeal to experiment whatsoever.
It does point to a programme of observations to try to catch P9 in a gravitational snare and look really hard in that area with all kinds of telescopes and has the double prize of possibly finding non gravitational evidence for DM.
Personally I think interstellar travelers would use FFPs as a resource but the question of how a civilization that lives on an FFP (imagine something like Pluto cut up into small (5000km) ringworlds) finds the next one seems pretty tough to me.
The state of fundamental physics. There's still plenty of new and exciting stuff at all the scales from molecules to continents.
For example, there's still lots of exciting work happening in trying to predict the angle of repose that a heap of sand forms.
You can think of him maybe like the Velvet Underground of physicists. They never achieved much popularity themselves, but virtually every rock band of the past 40 years that has gotten popular cites them as an influence, and many artists would rather have that as a legacy than popularity. Similarity, I think a lot of physicists would rather be well known and influential to other physicists rather than becoming the next Michio Kaku or someone else who shows up on television a lot.
As it so happens, he's also the guy behind this paper: "Searching for a Black Hole in the Outer Solar System", https://arxiv.org/abs/2004.14192
:-)
A lot of other questions might not really be "real" in various senses like: it's interesting to speculate that the interior of a quantum black hole is entirely unlike a classical black hole but you're not going to have anyone take a look and come back and tell us and we can just speculate if something kills you at the apparent horizon or not (so many bad ideas including the idea there is an "information paradox" come out believing the classical picture of the black hole interior which is probably just wrong), the "hierarchy problem" and various allergies to fine tuning are really human preferences or things like
https://en.wikipedia.org/wiki/Anomalous_magnetic_dipole_mome...
where between the experimental errors and the possibility that theorists aren't quite doing the math right and that the answers to 1-4 might account for any difference (I wouldn't be surprised it is if 1-4 have the same answer)
The experiments for (1) and (2) are devilishly hard, there are accelerator observations of CP violations that are a line on (3), but the cosmic scale of (3) and (4) imposes its own difficulties.
Really there are a lot of grad students chasing a moderate number of postdocs who hope to get one of very few permanent positions and out of it all there is a tiny amount of glory to be had.
Condensed matter physics lacks the cosmic difficulties but it isn't dramatically better. How superconductivity works in cuprates
https://en.wikipedia.org/wiki/High-temperature_superconducti...
is still quite mysterious after 35 years. I would name check Mark Newman as a standout in the "complex systems" area but the real accomplishment he made in my mind wasn't finding an explanation for "universal" power laws in complex systems but instead proving we didn't know what we were doing when we plotted our statistics on log-log paper and drew a line... And he published about that in a statistics journal not a physics journal but it's OK because the paper is in arXiv anyway.
The hierarchy problem is a real problem. The bare Higgs mass, for instance, might be a brute fact (but then again, it might not be), but the effective Higgs mass is some (likely very complicated) consequence of more fundamental physics at energy scales beyond the standard model. We just don't know what it is. But even if those more fundamental constants aren't explicable, they're definitely an explanation.
The vast majority of what theoretical physicists do is "just math" except that unlike math, it's aimed at a problem posed by nature rather than a problem imagined up on the basis of what seems most interesting.
Not really. The distinction, to the extent that there even is one, is mostly a matter of motivation and methodology. Broadly generalizing: mathematical physicists (i.e. mathematicians) are interested in physical theories that seem like they might need interesting new mathematical tools to understand, while theoretical physicists are interested in theories that seem like they might have something to tell us about the true structure of whatever the object of interest is. There are other differences downstream of this, of course:
- mathematical physicists are rigorous while theoretical physicists will let it lapse if that gets them physically correct answers
- physicists care less about inconsistencies in theories they know are wrong anyway
- mathematicians stay long after all the physical content has been mined out (a significant fraction of just-plain-mathematics is the end result of this process)
and so on. But you can find people anywhere between these poles.
Witten can (and sometimes does) produce world-class mathematics when he wants to, but most of his work is on the physics side of the spectrum.
Such as Hawkings, Dawkins, Tyson, Cox.. but probably little is known about their actual work. (Exception being Hawkings in that list although he did pass away some years ago so would not count)
Things in virtual freefall that can flex (and everything can) do so in response to forces (e.g. thrusting, but also heat stresses, say), and will continue to do so if they start unless you take care to damp them and dump them into heat. There's nowhere for the vibration to "go" unless you design one in. Sometimes the structure of the craft itself has enough damping for practical purposes, especially when you take care to isolate large vibration sources (the ISS has a Sorbothane damper for the treadmill, for example), but when your big floppy (i.e. light) mirror surface has to stay put on a nanometre scale, it's not so simple.
It's a bit like the difference between a tuning fork glued down flat to a table and one hanging from a string.
In the sense of building things that last in space.
Fundamentally they're all somewhat similar in that there's a flexible and/or sloshing thing that doesn't have a huge mass that it's rigidly connected to. Spacecraft deployed in space usually have smaller forces on them (no air or water and the hard acceleration is done) but are also much flimsier due to being ultra-light. Telescopes are even worse as even a tiny vibration can ruin the usefulness of the optical paths.
Vibration of telescopes is an issue on Earth as well, e.g.: https://opg.optica.org/ao/abstract.cfm?uri=ao-53-21-4651
In particular, consider how you would damp an undesired movement by a satellite. A naive approach would be to apply thrust in the opposite direction. However, the control can't be exact, leading to thrust -> thrust <- over and over, eventually to the measurement limit of the thruster's control.
With a large mass, it's replaced with a spring, and converted to heat.
Thanks!
In space, there is nothing to damp the oscillations. They will just continue without active features of the craft to damp them. If they continue unabated a section may reach a resonant frequency, which can quickly cause failure. Even if it doesn't, those vibrations can cause cyclic loading failures, or just affect the stationkeeping of the craft or it's usefulness in gathering scientific data.
To make a spacecraft resistant to oscillations requires devices like gyroscopes or friction dampers, or long weighted booms which decrease the magnitude of oscillations. Making the craft rigid helps, but the larger it is, the less rigid it will be. And to make it more rigid, or to include more anti-oscillation devices, means more weight. That's an important limiting factor when you need to get the object up into orbit.
One misunderstanding some people have is to think that there are no external forces on free-floating structures. This is untrue. Most importantly, they are all affected by the solar wind, which is a generally constant pressure pushing the object away from the sun. Of course they will also be affected by gravity, and if close enough to the earth they will interact with the atmosphere. (There's not really a clean cutoff to where our atmosphere ends and space begins.) As a result, spacecraft have to perform some amount of stationkeeping maneuvers, which involves applying a force on one section of the craft. That itself will cause further oscillations, because the force can never be transferred perfectly to the entire body. (Imagine pushing a piece of paper in the air with a single finger. Yes, you can get the paper to move in a direction, but you cannot get all segments of the paper to move in exactly the same manner when exerting force at only one point.)
So forces on spacecraft are inherently unavoidable, and oscillations happen any time a force is applied. Oscillations are challenging to control in a free-floating vacuum environment, and become more problematic the larger a craft is. This results in fundamental issues with operating very large spacecraft. That's not to say it is impossible. But in space it's not a simple solution to say "just build it bigger."
So many things to learn about... thanks!
Also, your description of using the ground to dampen oscillations has very similar implications to electricity and ground... not a coincidence?
It's also the reason why any Moon base would be buried under layers of regolith to protect it. Glass domes on the moon are sci-fi fantasy.
A 2mm hole in a Soyuz docked to the station was fixed with a bit of Kapton tape and some epoxy, and was detected by a very small pressure drop in the crew spaces.
Larger rocks are more of a problem, but quite rare. Your dome is much more likely to get smashed by someone mixing up the pedals in a rover.
https://www.livescience.com/how-many-moon-meteorites
> "if you pick a square kilometer patch of ground, it will be hit by one of those pingpong-sized meteoroids once every thousand years or so"
We've landed in visibly quite smooth areas; for example: https://www.flickr.com/photos/nasa2explore/48299974871
Hence my question.
1. Space is big.
2. Space is REALLY big!
3. This thing would be tiny.
4. This thing would be invisible.
Combine these four together and you'll quickly realize just how difficult even scoping out the probe's requirements and such would be. Would be cool though.
"Searching for a Black Hole in the Outer Solar System", https://arxiv.org/abs/2004.14192
"And one would like to launch hundreds of spacecraft (at least) in different directions so that some would come within dozens of AU of Planet 9, rather than hundreds of AU."
Their example of a neat-but-good-luck project could actually achieve the goal is [1]. The spec requires ground lasers that's powered by a 1GW nuclear power plant for propulsion. And, "According to The Economist, at least a dozen off-the-shelf technologies will need to improve by orders of magnitude." [2]
[1] https://breakthroughinitiatives.org/initiative/3 [2] https://en.wikipedia.org/wiki/Breakthrough_Starshot#Technica...
If you can get a spacecraft somewhere in the vicinity of a black hole of decent mass (like a 10km asteroid equivalent) we could probably detect changes in trajectory of the spacecraft from the gravitational attraction.
Or, here's an idea, blow out a tonne of radar chaff in the vicinity of the black hole and watch for how it disperses via radar.
We could also try detecting Hawking radiation that a BH should generate, though that might be pretty faint (I have not done the math).
You'd have to look for a very precise spike in the EM spectrum at the electron-position annihilation energy equivalent.
Despite being raised on space opera and hard-sf and having a science degree, I am pretty bearish on most space exploration. Having said that, a nearby black hole wouldn't merely be nifty. For reasons we do not understand, the event horizon stops honoring conservation of baryon number (and lepton number, and other numbers). I consider this important because replication of that would allow for direct conversion of uncharged mass to energy, just as any neutral particle crossing the event horizon has its energy (eventually) returned to us. If Hawking radiation is true (we have yet to experimentally verify it), then we know that mechanism exists. Can we do it without a black hole?
I agree tho, #LaunchTheProbe #NASA
[1] - https://en.wikipedia.org/wiki/Lunar_Crater_Radio_Telescope
But still, every hypothesis for explaining dark matter is close to dead right now. Theoreticians reopening closed cases is a good thing.
As an aside, it takes the first visitors about 30 years to reach it, and they've got better tech than what's available to us today. It's pretty far out.
How close could a reinforced probe orbit? Would they just drop test masses in and observe what happens? Maybe drag a kuiper belt object in to make a little accretion disk?
A supernova-born black hole ( https://en.wikipedia.org/wiki/Stellar_mass_black_hole ) would be far more massive than the sun - so the entire solar system would obviously revolve around it, not the sun.
Also, the birth of such a black hole is a supernova explosion - which would not leave any solar-system-forming remains kicking around nearby.
https://en.wikipedia.org/wiki/Stellar_parallax_method#Histor...
You “just” need a way to get the matter there and a way to capture that energy.
this seems to be a paper describing something that could be true, probably isn't, and doesn't advance science as a whole at all.
https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.12...
It advances scientific knowledge by providing a testable prediction. The prediction can be used to collect data and determine whether there indeed is a black hole orbiting the Sun beyond Neptune.
do the thing, get the results, and if they're significant, then author a paper.
I think there is a lot of resume oriented science, these days, because publishing lots of papers looks good on your resume. i mean, we can't let actual science get in the way of our science careers, right? we gotta publish papers telling people what we're thinking about, and how to do those things.
Many theories were proposed before they were confirmed or refuted in data. Einstein's theory of general relativity is a famous example which was proposed before it could be confirmed by Eddington during a solar eclipse. Black holes themselves were also predicted by Penrose and Hawking before their existence was confirmed.
The Higgs Boson is a great example of this, theorized 40 years before it could finally be discovered at the LHC, a machine which wouldn't have been possible at all with the technology of the time when the Higgs boson was proposed as the computing capability to digest the Petabytes of data generated simply did not exist.
Plus, with these things there's a bit of a chicken and egg issue where part of the impetus for building these expensive machines is to test theories which have been gaining support, so if those theories weren't being shared, there's no way to really say if the machines to test them could even exist.
Neutrinos, time crystals and gravitational waves are other notable examples of things where it would not be practical to expect theoreticians to wait the decade(s) for technology to catch up before presenting their theories.
https://www.youtube.com/watch?v=aY985qzn7oI
His skype rant to SETI leadership about how nobody is taking the idea of contact or evidence of alien civilizations seriously, is really sad.
It is the gravitational drag of a moving solar system moving around in a spiraling milky way galaxy around a theoretical black hole in the center.
Glad to know there is a similar theory being proposed.
Checking to see if outer planetary motion can be explained in terms of PBH and working out the math as part of that is perfectly normal astrophysics, and good science. Read the paper, it's perfectly fine =)