Photographing black holes
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| Image of Black Hole and its shadow - M87 Galaxy |
So, here it
is the landmark achievement of the decade and one of the most significant moments in the history of humankind!
The first-ever photograph of a black hole.
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| A zoomed image of the black hole at the centre of M87 Galaxy |
All the theories
built on mathematical models make sense now. Here, it is. Its proof. Its
photograph, for seeing is believing!
As we all know,
at 06:30 pm (IST), April 10, 2019; the first-ever picture of a black hole was released
by the National Science Foundation.
Targeted galaxies for the operation were Sagittarius A*, the black hole at the centre of our galaxy (closer in cosmic
terms, 26000 light-years) and Messier87 the largest galaxy that we know of (54 million light-years) away.
And there it
was the image of the black hole and its shadow. Both appearing approximately
equal in size. Well, M87 is quite far. It's not like we can blame anyone. XD
The event horizon of Sagittarius A* is 20-million-kilometre-wide with a mass of about 4.31± 0.38 million solar
masses (1 solar mass = 2*1030 Kg). The event horizon of Messier87 has an angular diameter of 42 micro-arc seconds, implying that it would take 23 quadrillion black holes of equivalent size to fill the entire sky. It has a mass 6.5 billion times as great as our sun.
So, after
all the celebrations, we cosmo nerds would be very much interested in knowing
how we obtained these images.
Obtaining the image was no easy task. It required a telescope that was as large as our planet
itself, which we all know is well not possible for now. So, what did we do?
It was made
possible by VLBI (very-long-baseline interferometry) - a technique used in astronomy
to obtain high-resolution images of the sky using a network of telescopes across
the planet. It can, with the aid of high-tech computing come close to mimicking
the sharpness of a hypothetical telescope nearly as large as the planet! It is
commonly used to image distant cosmic radio sources, such as quasars, although it is also sometimes
used to study stars.
VLBI did a great job, indeed. But do you know an interesting fact about it? It works on a
two-decades-old principle. The principle of Young’s double-slit experiment! The
light source is replaced by a distant source of radio waves like a black hole
and the slits are replaced by radio antennas on the telescope.
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| Young's Double Slit Experiment |
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| Very-long-baseline interferometry (VLBI) |
Since Earth is rotating, the antennae are in motion relative
to the black hole and receive the radio waves at different times. When these
signals are allowed to interfere with each other, they produce an interference
pattern that is processed at a central location to recreate the state of the black hole, whether visually or any other way.
For this purpose, radio telescopes were used.
Radio waves have a greater wavelength than visible light. So,
radio telescopes have an inherently poorer angular resolution than optical
telescopes. For example, a 50-meter-wide telescope will have an angular
resolution of ~41.2 arc second. An optical telescope
of the same size will have an angular resolution of 0.004 arc-second (An arc second = 0.000277778 degrees).
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| Angular diameter |
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| Arc seconds, arc minutes and degrees |
Why aren't we using visible light then? Well, we do have good enough reasons.
Astronomical
objects that do emit radio waves encode certain information in them which the
visible radiation does not carry. The
radio waves of wavelength 1.3 mm – that the EHT (Event Horizon Telescope) tracks are not absorbed or
scattered by the dust in the Milky Way, or in Earth’s atmosphere, allowing antennas
on the surface to capture them. But this, in turn, does require a telescope
dish antenna to be wider than Earth. How do we solve that? Here, to our aid comes into
picture gravitational lensing. Gravitational lensing can magnify the whole view by five times. This reduces the size of the telescope required to a size of a few thousand kilometres. Trust me,
this was much more manageable.
So, the problem got resolved to some
extent. Now what?
Here, enters VLBI aka very-long-baseline
interferometry. In VLBI, because multiple telescopes are receiving the
radio signals, the angular resolution of a so-called interferometric telescope
is defined differently. It is the ratio between the telescopes in the array,
called the baseline. If, say, the baseline is 1000 kilometres, the angular resolution
of an array of telescopes already becomes 20,000 times better.
With the advent of atomic clocks,
telescopes could be placed on different continents because the clock was kept
in sync using international protocols.
To sum it up, a telescope receives a
radio signal, a computer sticks a timestamp on it and sends it to the receiver.
The receiver collates such data from different telescopes and creates the characteristic
interference pattern. Using this pattern, a processor recreates the source of
all the radio waves at different locations, together with the time at which
each signal was received.
There are also many systems in
between to stabilize and improve the quality of the signal to coordinate
observations between the telescopes, etc. But the basic principle is the same
as in Young’s Double-slit experiment two centuries ago.
The EHT has
over eight ground-based participating radio telescopes spread over North and South America, Europe,
The Pacific Ocean and Antarctica.
The EHT can study the Sagittarius A* - the site only when there are clear skies over all these telescopes at the same
time. This is about one week per year – which makes each observation very
precious.
After all this hard work, we obtained a
pixelated image of the black hole. And here it is…
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| The black hole at the centre of galaxy Messier87 |
The credit's due where the credit's due. It would not have been possible
without the stars and the other bodies that died being torn apart by the black
hole. This is the accretion disk and a blurred event horizon that we are
seeing in this picture.
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| Her smile says it all |
I hope I could help you in
understanding this discovery better.
A homage to Thomas Young, Albert Einstein, Karl Schwarzchild, and Stephen Hawking.
With this article, our series of articles on the black holes come to an end.
It was a delight to write. I hope you understood it better too.
"If I have seen further than others, it is by standing upon
the shoulder of giants.”
