- 4281 - BLACKHOLES - learn what we cannot see? Blackhole frustration comes from the fact that they seem to have two disparate natures: on the one hand, they are bizarrely simple creatures. But on the other, they completely defy our ability to explain them with our current understanding of physics.
-------------- 4281 - BLACKHOLES - learn what we cannot see?
- Blackholes are
stars that have gotten so large their gravity collapses them into a Blackhole
where not even light can escape. The
name is said to come from the Black Hole of Calcutta, an infamous prison that
you cannot escape from. It is a fitting name, for black holes are the ultimate
cosmological prison.
-
- Black holes are
defined by their “event horizon”, which is an imaginary line drawn through
space and time. There are no warning bells, no sign or signal or evidence of
its existence. But it’s there. And crossing it means simply, you can never turn around.
-
- If you compress
matter into a small enough volume you create an event horizon. You can enter
this region of space, but you cannot leave. You are simply cut off from the
wider universe, never to be seen or spoken to again.
-
- This impossibility
of return applies to all things in the universe, up to and including light
itself. And so even though these event horizons exist as mathematical
curiosities, they have a physical manifestation: they appear completely,
utterly, totally blank.
-
- Black holes exist
as their own separate domains in the universe, their internal affairs forever
hidden from view, the shades of the event horizon forever drawn against our
curious gaze. That is one of many reasons that black holes are perhaps the most
frustrating entities to inhabit the universe.
-
- Black holes were
discovered in the mathematics of Einstein’s general theory of relativity. Black holes are simple. We have our equations, as told to us by
Einstein and Schwarzschild and the rest, that instruct us as to the nature of
spacetime within and around them. Those equations tell us about event horizons.
They tell us what happens to hapless wanderers who stray too close. They tell
us about the gravity we experience near them.
-
- And to tell one
black hole apart from another, those equations have as free parameters only
three numbers. Three numbers! That’s all it takes to completely describe
everything you ever need to know about a black hole. Mass, electric charge, and
spin. That’s it. General relativity
will tell you the rest.
-
- This means that
black holes aren’t just hiders of light, they are hiders of knowledge. Of
information. They forget, or at least lock away forever, the secrets of what
forged them and what fell below the event horizons later. This is the called “no hair theorem”, a term
coined half as a joke (and reviled by Richard Feynman) that has nevertheless
stood the test of time.
-
- All those records
of what was and what could have been are wrapped in an event horizon, leaving
us on the outside with three simple and mute numbers to describe it.
-
- Scientists have
discovered gravitational waves stemming from black hole merger events that
suggest the resultant black hole settled into a stable, spherical shape. These
waves also reveal the combo black hole may be much larger than previously
thought.
-
- When initially
detected on May 21, 2019, the gravitational wave event known as “GW190521” was
believed to have come from a merger between two black holes, one with a mass
equivalent to just over 85 suns and the other with a mass equivalent to about
66 suns. Scientists believed the merger therefore created an approximately 142
solar mass daughter black hole.
-
- Yet, newly studied
spacetime vibrations from the merger-created black hole, rippling outward as
the void resolved into a proper spherical shape, seem to suggest it's more
massive than initially predicted. Rather than possess 142 solar masses,
calculations say it should have a mass equal to around 250 times that of the
sun.
-
- These results
could ultimately help scientists better test general relativity, Albert
Einstein's 1915 theory of gravity, which first introduced the concept of
gravitational waves and black holes.
-
- General relativity
predicts that objects with mass warp the very fabric of space and time, united
as a single, four-dimensional entity called "spacetime”", and that
"gravity" as we perceive it arises from that curvature itself.
-
- Just as a bowling
ball placed on a stretched rubber sheet causes a more extreme "dent"
than a tennis ball would, a black hole causes more curvature in spacetime than
a star does, and a star causes more curvature than a planet does. In fact, a
black hole, in general relativity, is a point of matter so dense it causes
curvature of spacetime so extreme that, at a boundary event horizon, not even
light is fast enough to escape the inward dent.
-
- This isn't the
only revolutionary prediction of general relativity, however. Einstein also
predicted that when objects accelerate, they should set the very fabric of
spacetime ringing with ripples called “gravitational waves”. And again, the
more massive the objects involved, the more extreme the phenomenon is.
-
- This means when
dense bodies like black holes spiral around one another, constantly
accelerating due to their circular motion, spacetime rings around them like a
struck bell, humming with gravitational waves.
-
- These ripples in
spacetime carry away angular momentum from the spiraling black holes, and that,
in turn, causes the black holes' mutual orbits to tighten, drawing them
together and increasing the frequency of the gravitational waves emitted.
Spiraling closer and closer, the black holes finally merge, creating a daughter
black hole and sending a high-frequency "chirp" of gravitational
waves echoing out through the universe.
-
- But there was one
thing Einstein got wrong about gravitational waves. The great physicist
believed that these ripples in spacetime would be so faint that they would
never be detected here on Earth after traveling across the universe for
millions, or even billions, of light years.
-
- Yet, in September,
2015, the twin detectors of the Laser Interferometer Gravitational-Wave
Observatory (LIGO) based in Washington and Louisiana showed Einstein was
incorrect. They detected GW150914, gravitational waves associated with merging
black holes located around 1.3 billion light-years away.
-
- The gravitational
wave signal was detected as a change in the length of one of LIGO's 2.5 miles
long laser arms, equivalent to a thousandth the width of a proton.
-
- Remarkably, since
then, LIGO and its fellow gravitational wave detectors, Virgo in Italy and
KAGRA in Japan, have detected many more such
events, reaching the point at which they are detecting one gravitational
wave event each week. Although, even among this cornucopia of gravitational
wave detections, GW190521 stands out.
-
- The merging
frequency of the two black holes behind the GW190521 signal, which are located
as far away as 8.8 billion light-years from Earth, was so low it was only
during the final two orbits of the black holes that the frequency became high
enough to reach the sensitivity limits of LIGO and Virgo.
-
- They found that
the instant the black holes collided, the resultant black hole was created with
a lopsided shape. Black holes are only stable when they have a spherical shape,
meaning that within milliseconds of the merger, the daughter black hole would
have to assume the shape of a sphere.
-
- Just as the shape
of a bell determines the frequency at which it rings, as this new black hole changed shape and
stabilized, the frequencies of the gravitational waves it rang out were
shifted. These so-called "ring down" gravitational waves contained
information about the mass of the daughter black hole and also the rate at
which it is spinning.
-
- The team found two
separate ring-down frequencies in the gravitational wave signal GW190521,
which, when considered together, give the created black hole a mass of 250
solar masses. That means it's considerably more massive than estimated by using
the spiraling gravitational waves. The detection of these ringdown
gravitational waves was shocking even to the team behind these findings.
-
- The James Webb
Space Telescope (JWST) has spotted the oldest black hole ever seen, an ancient
monster with the mass of 1.6 million suns lurking 13 billion years in the
universe's past. Those cameras enable
it to look back in time to our universe's beginnings, spotted the supermassive
black hole at the center of the infant galaxy “GN-z11” just 440 million years after the
universe began.
-
- And the space-time
rupture isn't alone, it's one of countless black holes that gorged themselves
to terrifying scales during the period about 100 million years after the Big
Bang when the young universe began glowing for a billion years.
-
- Closer to the
present-day, astronomers believe black holes are born from the collapse of
giant stars. But however they come to be, they grow by ceaselessly gorging on
gas, dust, stars and other black holes. As they feast, friction causes the
material spiraling into the black holes' maws to heat up, and they emit light
that can be detected by telescopes turning them into so-called active galactic
nuclei (AGN).
-
- The most extreme
AGN are quasars, supermassive black holes that are billions of times heavier
than the sun and shed their gaseous cocoons with light blasts trillions of
times more luminous than the brightest stars.
-
- Because light
travels at a fixed speed through the vacuum of space, the deeper that
scientists look into the universe, the more remote light they intercept and the
further back in time they see.
-
- To spot the black
hole the astronomers scanned the sky with two infrared cameras, the JWST's
Mid-Infrared Instrument (MIRI) and Near Infrared Camera, and used the cameras'
built-in spectrographs to break down the light into its component frequencies.
-
- By deconstructing
these faint glimmers from the universe's earliest years, they found an
unexpected spike among the frequencies contained within the light. This is a key sign that the hot material
around a black hole was beaming out faint traces of light across the universe.
-
- To prevent such a
cloud from cooling too quickly and collapsing into massive stars first, it must
also be beamed with ultraviolet light, likely from a nearby galaxy or black
hole. We need to learn much more about
blackholes that we can not see?
-
-
December 21, 2023 BLACKHOLES -
learn what we cannot see? 4281
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