Page 9 - Science Focus (Issue 19)

On April 10, 2019, a team of scientists made history light. Nothing escapes a black hole – not even light. the fastest-vanishing black holes vanish barely within the
by capturing the first ever image of a black hole. This And now a few myths: entire history of the universe, which is still a long time.
remarkable picture occupied headlines worldwide,
and rightfully so – we never thought we would ever be Myth: This picture was taken by one telescope. Myth: Black holes aren’t black
able to capture a black hole in an image that can be Busted. (since we can see them).
passed around the world. Busted.
The black hole in question is a supermassive one, This remarkable image was not taken by a single
2.4 billion times the mass of the Sun, at the center of telescope, but rather an amalgam of images taken Let’s make it clear here, a black hole itself is pitch
the M87 galaxy, about 53 million light years from Earth. across eight telescopes on Earth known as the Event black – it is defined such that once anything goes
Horizon Telescope (EHT) [3]. To capture a reasonably beyond the event horizon, it cannot escape. What
clear image of an object so far from Earth, the we can see in the image is the matter surrounding
telescope would require a very large aperture (the the black hole – a ring of matter and light that is
size of opening of the lens) to gather enough light about to disappear into the event horizon forever.
from it. Instead of building one large telescope, the The orange color you see in the photo, however,
team decided that they would align the different is artificially colored; it represents the intensity of
EHT telescopes at the black hole at the same time, radiation detected around the black hole, with the
creating a large, “virtual” telescope. The aperture of orange areas representing intense radiation and black
this telescope becomes the distance between the two representing areas with little to no radiation [4]. A
furthest telescopes – in this case nearly the diameter black hole also emits Hawking radiation, as mentioned
when the size of a star collapses to a small enough of the Earth! This vastly increases the resolution of the above, but this is so weak that we don’t expect to be
size – more specifically, smaller than the Schwarzschild telescope and allows us to take images of objects that able to observe them in the foreseeable future.
radius, . (Here M is the mass of the star, c is the are far away. The image you see is an amalgam of
The black hole at the center of the Messier 87 (M87) galaxy, in the data taken over eight sites in four days in April 2017 [4]; Myth: Once you fall into a black hole, you will be
constellation Virgo speed of light, and G is the gravitational constant.) it was finally put together using techniques of image torn apart.
Photo credit: Event Horizon Telescope Collaboration [1] restoration to reconstruct a singular image from the That is true.
To put things into perspective, if the Sun were a large body of data.
But first we have to answer the question – what black hole, it would have a radius of approximately This is due to a process known as spaghettification
exactly are black holes, and how are they formed? 2.95 km; in comparison, the current radius of the Sun is Myth: Black holes only get bigger. – caused by the gravitational gradient of a black
696,340 km. Actually, owing to the relatively tiny mass Busted! hole. Due to a marked difference in strength of the
of the Sun, it can never turn into a black hole – so we gravitational field, gravity is stronger at your head than
Black Holes: A Brief Introduction won’t have to worry about our Sun turning into a black One might think that black holes only swallow at your feet if you fall head-first into the black hole.
hole any time soon! matter – this was what scholars had believed for a This naturally causes you to be stretched vertically
Although the existence of black holes was long time…until one day Mr. Hawking came along. and compressed horizontally. These tidal forces are so
first speculated in the 18th century, they were first The black holes we can see in the sky are formed strong that they can rip a person apart long before
scientifically predicted by Einstein’s theory of general by a process known as gravitational collapse. In a One of Prof. Hawking’s greatest discoveries is you reach the center of a black hole; chances are you
relativity as a solution to the Einstein field equations. dying star, there is a reduction in internal pressure Hawking radiation, a kind of radiation emitted by will not survive the fall. (Needless to say, the editorial
Einstein’s theory of relativity can be elegantly summed due to the fusing of heavier elements. As the internal black holes. It is true that nothing escapes a black team does not recommend falling into a black hole to
up in twelve words from John Wheeler – “Space-time pressure continues to decrease, gravity causes the star hole, but what was not known at the time was that verify this myth.)
tells matter how to move; matter tells space-time how to further collapse onto itself. Eventually, the density of they also emit radiation themselves [5]. The precise
to curve [2].” the star becomes high enough that it creates a very mechanism of its emission is difficult to state in full here,
strong gravitational pull – and this is a black hole. You but the main idea is that due to quantum fluctuations, Side note: There is still so much to learn about
Mass bends the fabric of space-time itself; around may recall the concept of escape velocity from high black holes are bound to lose energy, for instance, in black holes, but hopefully this article has piqued
a black hole’s event horizon, the space-time is bent school physics; black holes are so small and massive the form of electromagnetic waves. If black holes emit your interest. If you want to know more, the
in a way that even light cannot escape. This occurs that the escape velocity is greater than the speed of more radiation than they absorb, then they will lose mass internet is filled with resources that will provide
and eventually evaporate, due to the famous equation, more information than this limited space. It’s a
E = mc , which tells us mass and energy are equivalent. good idea to learn something about Einstein’s
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MythBusters:Black Holes holes to evaporate and disappear: often we are holes requires a good grasp of first special, then
theory of relativity, since understanding black
That being said, it will take a long time for black
general relativity – good places to start include
PBS Space Time and MinutePhysics (see the QR
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of the universe (which is on the scale of 10 years). Even
流言終結者：黑洞篇 talking about 10 years, far, far older than the age codes on P.9).
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By Sonia Choy 蔡蒨珩

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