Event Horizon Telescope Captures First Black Hole Image in the link below
By Luis
Mendoza
September 13, 2024
https://greekreporter.com/2024/09/13/black-hole-image-sagittarius-a-milky-way/
The Event Horizon Telescope just managed to capture the
first images of the black hole at the centre of the Milky Way, Sagittarius A.
The
Event Horizon Telescope, a collaboration of radio telescopes throughout the
world that work in unison to capture images of supermassive black holes, just
achieved its best picture yet. This new development could set the path for
images of the ring of light around a black hole that are a staggering 50
percent sharper.
This
could allow us to see previously unseen details of black holes and could
potentially even produce footage of the massive phenomenon.
So how
does the Event Horizon Telescope manage to capture these images? In principle,
the telescope works with a very long baseline interferometry. This process
involves diving into a network of telescopes worldwide
that all work together to observe the same object in space. The network of
telescopes then combines data and continues to operate under two principles.
The
first one is that the wider the distance between the two farthest telescopes in
the network, the better resolution the images have. This has been implemented
to a great extent by the EHT, given that its southern telescope is at the south
pole, whereas its northernmost is in the Greenland Telescope.
The
Event Horizon Telescope managed to capture the black hole in the center of our
Milky Way Galaxy
The
black hole, known as Sagittarius A, in the center of our galaxy was pictured by
the Event Horizon Telescope for the first time ever. However, that is not the
only black hole that the EHT pictured.
The
telescope also managed to pick up images from the black hole at the center of
the elliptical galaxy M87,
making these the first two black holes pictured by humanity.
But how
exactly did the Event Horizon Telescope manage this? As we previously
mentioned, the two telescopes are located at the South Pole and Greenland meaning
the network covers the planet from top to bottom.
This
was not the only factor that contributed to the imaging of the two black holes.
The two images were captured at a radio wavelength of 1.3 mm. At this
wavelength, the so-called “photon ring” of the black hole appears blurred as in
the case of Sagittarius A, where the photon ring was pictured around the black
hole’s circumference as a dark shadow.
Sagittarius A pictured by the EHT. Credit: EHT Collaboration
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Having read the claim (in red) above written by
Luis Mendoza, let me now write a bit more about Black Holes, and a
question I asked after that below:
A black hole is the remnant of a very
massive star that collapses below a certain radius such that light can no
longer escape, and the object is no longer visible. It is a characteristic
radius associated with every quantity of mass.
They are formed when massive stars
collapse at the end of their life cycle. They have masses ranging from about 5
to several tens of solar masses. But there are also much more massive ones
we shall mention a little later.
Black holes are considered a region
of spacetime where gravity is so strong that nothing, not
even light and other electromagnetic waves, is capable of
possessing enough energy to escape it. Einstein's theory
of general relativity predicts that a sufficiently
compact mass can deform spacetime to form a black hole.
The boundary of no escape is called the event horizon. A black
hole has a great effect on the fate and circumstances of an object crossing it,
but it has no locally detectable features according to general
relativity. In many ways, a black hole acts like an ideal black body,
as it reflects no light. Quantum field theory in curved spacetime predicts
that event horizons emit Hawking radiation, with the same
spectrum as a black body of a temperature inversely proportional
to its mass. This temperature is of the order of billionths of
a kelvin for stellar black holes, making it essentially
impossible to observe directly.
After a black hole has formed, it can
grow by absorbing mass from its surroundings. Black holes may also be form from
the remnants of supernova explosions, which may be observed as a type
of gamma ray burst. These black holes are also referred to
as collapsars.
The presence of a black hole can be
inferred through its interaction with other matter and with
electromagnetic radiation such as visible light. Any matter that falls toward a
black hole can form an external accretion disk heated by friction,
forming quasars, some of the brightest objects in the universe. Stars
passing too close to a supermassive black hole can be shredded into streamers
that shine very brightly before being "swallowed. If other stars are
orbiting a black hole, their orbits can be used to determine the black hole's
mass and location. Such observations can be used to exclude possible
alternatives such as neutron stars. In this way, astronomers have identified
numerous stellar black hole candidates in binary systems and
established that the radio source known as Sagittarius A at the core of
the Milky Way galaxy, contains a supermassive black hole of about
4.3 million solar masses.
Supermassive black holes of
millions of solar masses (M☉) that may form by absorbing other
stars and merging with other black holes, or via direct
collapse of gas clouds. Supermassive black holes may exist in the
centres of most galaxies.
Until now, all the black holes in our
galaxy identified by astronomers were between five and fifteen solar masses.
Neutron stars, by contrast, are generally no larger than about 2.1 solar
masses, since anything larger than 2.5 solar masses would collapse to form a
black hole. When LIGO and Virgo jointly detected gravitational waves caused by
a black hole merger, they were 31 and 25 solar masses, respectively.
Surrounding a black hole is the event
horizon as the black hole’s surface. Inside this boundary, the velocity needed
to escape the black hole exceeds the speed of light, which is as fast as
anything can go. So whatever passes into the event horizon is doomed to stay
inside it – even light. Because light can’t escape, black holes themselves
neither emit or reflect it, and nothing about what happens within them can
reach an outside observer. But astronomers can observe black holes thanks to
light emitted by surrounding matter that hasn’t yet dipped into the event
horizon.
Event Horizon Shadow:
An event horizon is the boundary
marking the limits of a black hole. The event horizon captures any light
passing through it, and the distorted space-time around it causes light to be
redirected through gravitational lensing. These two effects produce a dark zone
that astronomers refer to as the event horizon shadow, which is roughly twice
as big as the black hole’s actual surface.
At the event horizon,
the escape velocity is equal to the speed of light.
Since general relativity states that nothing can travel faster than
the speed of light, nothing inside the event horizon can ever cross the
boundary and escape beyond it, including light. Thus, nothing that enters
a black hole can get out or can be observed from outside the event horizon.
Likewise, any radiation generated inside the horizon can never escape
beyond it. For a nonrotating black hole, the Schwarzschild
radius delimits a spherical event horizon. Rotating black holes have
distorted, non spherical event horizons. Since the event horizon is not a
material surface but rather merely a mathematically defined demarcation
boundary, nothing prevents matter or radiation from entering a black
hole, only from existing one.
Photon Sphere:
From every viewing angle, thin rings of
light appear at the edge of the black hole shadow. These rings are really
multiple, highly distorted images of the accretion disk. Here, light from the
disk actually orbits the black hole multiple times before escaping to us. Rings
closer to the black hole become thinner and fainter.
Accretion Disk:
The main light source from a black hole
is a structure called an accretion disk. Black holes grow by consuming matter,
a process scientists call accretion, and by merging with other black holes. A
stellar-mass black hole paired with a star may pull gas from it, and a
supermassive black hole does the same from stars that stray too close. The gas
settles into a hot, bright, rapidly spinning disk. Matter gradually works its
way from the outer part of the disk to its inner edge, where it falls into the
event horizon. Isolated black holes that have consumed the matter surrounding
them do not possess an accretion disk and can be very difficult to find and
study.
Accretion disk has a funny shape when
viewed from most angles. This is because the black hole’s gravitational field
warps space-time, the fabric of the universe, and light must follow this
distorted path. Astronomers call this process gravitational lensing. Light
coming to us from the top of the disk behind the black hole appears to form
into a hump above it. Light from beneath the far side of the disk takes a
different path, creating another hump below. The humps’ sizes and shapes change
as we view them from different angles, and we see no humps at all when seeing
the disk exactly face on.
Doppler Beaming:
Viewed from most angles, one side of
the accretion disk appears brighter than the other. Near the black hole, the
disk spins so fast that an effect of Einstein’s theory of relativity becomes
apparent. Light streaming from the part of the disk spinning toward us becomes
brighter and bluer, while light from the side of the disk rotating away from us
becomes dimmer and redder. This is the optical equivalent of an everyday
acoustic phenomenon, where the pitch and volume of a sound – such as a siren –
rise and fall as the source approaches and passes by. The black hole’s particle
jets show off this effect even more dramatically.
Schwarzschild Radius:
The Schwarzschild radius is
the radius of a sphere surrounding a black hole where
nothing can escape, not even light. The Schwarzschild radius was
named after the German astronomer Karl Schwarzschild who
calculated this exact solution for the theory of general
relativity in 1916.
Distortion of Light:
We expect light surrounding outside the
Event Horizon as shown in the photo in the link above to be evenly distributed as a uniformly
shaped ring rather than with irregular projection.
This is because we expect a black
hole that has collapsed and crushed into a sphere will have the greatest
volume with the smallest surface area due to its immense gravity pulling in
everything inside, including light outside the event horizon, and hence the
ring of light ought to be perfectly round with no projections all round its
surface. But why was the light shown in the photo not perfectly ringed?
Could its irregular shape of the
light shown in the photo be its accretion disk rather than the light from the
event horizon as claimed by “Event Horizon Telescope Captures First Black Hole
Image” written above by Luis Mendoza
It raises important questions about the nature of the image and what it reveals.
However, the irregular shape of the
light in the image might also be from the accretion disk rather than the event
horizon. The distorted shape could also be the result from gravitational
lensing, where the extreme gravitational field of the black hole bends light
from the surrounding matter, such as the accretion disk. The light ring’s
irregularity, rather than being perfectly spherical, reflects the complexities
of the black hole’s environment, including relativistic effects like Doppler
beaming, where different parts of the disk appear brighter or dimmer depending
on their motion relative to the observer.
Unfortunately, the photo captured by
the Event Horizon Telescope (EHT) also did not give us two very important data,
namely, the distance between the black hole and the EHT camera, and the angle
of the field of view. If it did, we can easily calculate out the mass of the of
the black hole in Sagittarius A
Here is how we can calculate it:
The Schwarzschild radius (R) is given
as 2GM / c2
where G is
the gravitational constant = (6.693(34) ×10−11 m3⋅kg−1⋅s−2), M is
mass of the black hole, and c is the speed of light
(299,792,458 metres per second).
Let the angle of the field of view captured by EHT camera be given as θ
Let tan θ = diameter of black hole /
distance of EHT from black hole. R is half its diameter.
Rearranging the equation, the mass of
the black hole would be:
Rc2 = 2GM
M = Rc2 / 2GM
The black hole’s mass using
Schwarzschild radius using key data like the distance between the black hole
and the telescope, along with the field of view angle, would allow for such
calculations.
Had the image of the black hole given
us just these two simple measurements, we could easily tell if the image was an
ordinary black hole, or a super massive one claimed to be in Sagittarius from
the centre of our Milky Way Galaxy.
However, the mass of Sagittarius
A has already been estimated at around 4.3 million solar masses, largely
through the study of the orbits of nearby stars. The EHT image, while
groundbreaking, is more about visualizing the black hole’s “shadow” and the
photon ring, rather than offering direct measurements of mass.
I believe some data may not be readily
available or shared with the public, but astrophysicists involved with the EHT
project have access to vast amounts of observational data, which they analyse
using sophisticated models to derive information like mass, spin, and other
properties.
My approach to questioning the image
and considering alternative explanations is exactly how we make scientific
progress by constantly probing and testing assumptions, not just in astronomy
or astrophysics, but also in all fields of science, including in medical
research where I spent part of my life
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