pour la page en français, clique ici
"if you wish to make an apple pie from scratch, you must first invent the universe" -- Carl Sagan
uh oh youve been bugged!
ok, you caught me, i put all my efforts into the french page on black holes and very little here. it's mainly because my real objective was to explain to my mom what i do. i'll eventually circle back to this page!
what's a black hole?
Black holes -- once a mathematical curiosity, a limiting case to Einstein's Field Equations with preposterous implications,
so preposterous even that the name that ended up describing it, "black hole", was somewhat of a playful expression of disbelief
-- are now ubiquitous in astrophysics.
No matter where you go, whether galactic or extra-galactic, early Universe or projected future, you'll likely find a black hole
somewhere along the way -- sometimes at the end of the road. They power some of the most energetic engines found in the Universe through
a process called accretion (more on this below), and, when a pair of them gets caught in each other's gravitational reach,
they enter into a dance of death, staring each other down as their forceful orbit precipitates them to a thunderous end. From two,
there is now only one. Their merger has sent out ripples in spacetime, which will now travel unimpeded until... until what?
Well, until they cross the path of a modest little stellar system made up of a middle-aged G-type main sequence star orbited by eight
planets -- four rocky, four gaseous -- and a couple asteroid belts. There, the gravitational ripples will encounter a strange device.
A few mirrors, a couple beam-splitters, two empty tubes inside which a laser beam is traveling uninterrupted. Nearly uninterrupted,
I should say. See, the strange device has the laser beam perform a round trip to meet back in the middle, where it is made to interfere
with itself. The gravitational wave isn't really bothered by the existence of this strange device. It has places to go, after all.
But it's now left a mark in its wake. The strange little device is rigged up to detect infinitesimally small fluctuations in the length
of its tubes. How small? Oh, about one part in $10^{21}$. That means that for every kilometer the laser travels, its path length might
shrink or grow by $\Delta L = L /10^{21} = $ 1 attometer, or a billionth of a billionth of a meter, or about the size of a quark.
Well worth a Nobel Prize or two,
in my humble opinion.
Black holes are notoriously elusive -- how do you find, in all the vastness of space, an object so dense and compact
that not even light can escape it?
You might not be able to look directly at its core--beyond its event horizon--but a black hole is a gluttonous and vociferous
little fellow that leaves characteristic hints in its wake that nothing else in this Universe can mimic*.
One such "hint" is the emission of gravitational wave, as I alluded to above. I have a soft spot for that one.
This is an exciting time to be an astrophysicist. There are entire sub-fields, thousands upon thousands of researchers,
dedicated to finding, cataloguing, measuring, evolving, monitoring, observing black holes, using a variety of complementary approaches.
(my method of choice is X-ray spectroscopy, but with a multimessenger emphasis. You can read more about it here)
*pure theorists are free to disagree -- coming up with plausible 'black hole mimickers' is an enticing conundrum, no doubt about it. I will be honest that I do find some
comfort in the principle of cosmic censorship, which states that the Universe has no choice but to hide singularities (and all their baggage) safely behind event horizons,
never to be discoverable by anyone outside it. These 'pockets' in the fabric of spacetime are the purest embodiment of "not my monkey, not my business**" there is.
Hint taken, I won't look. Nature cannot stop me from looking right outside it however, which feels like a loophole, but it's what pays my bills.
**nie mój cyrk, nie moje małpy -- "not my circus, not my monkeys" is the correct aphorism.
the basics of accretion disks
Accretion is the process through which gas caught in the orbit of a black hole releases
its angular momentum as heat and radiation as it slowly spirals inwards.
The gas settles into a disk over time, as the vertical components of its momentum are dissipated
through collisions.
Your basic recipe for an accretion disk would go something like this:
Gas has some initial angular momentum |$\vec{L}$|$=I \omega$, which is conserved.
$\cdot \quad$ $I=$ moment of inertia $\propto m \cdot r^2$ where $r=$ distance to the BH
$\cdot \quad$ $\omega=$ angular velocity
This has 2 main effects:
$\cdot \quad$ prevents instantaneous collapse into the BH (gas must dissipate its $\vec{L}$ through viscous stress)
$\quad \cdot \quad$ think of water forming a whirlpool as it falls down the drain
$\cdot \quad$ flattens out the accretion disk (centrifugal force resists collapse ⊥ rotation axis)
$\quad \cdot \quad$ think of pizza dough stretching as it's being spun
Results in:
$\cdot \quad$ gas settles on nearly circular orbits in the equatorial plane
$\cdot \quad$ small inward motion (angular momentum is slowly transferred outwards)
$\cdot \quad$ viscous stress (friction) heats up the gas, which starts radiating
The physical description of viscosity involves magnetic fields (magneto-rotational instability, MRI)
It's a mind-boggingly efficient process, where up to 40% (!!) of the gas' rest mass energy
($Mc^2$) is converted to luminosity.