Page 9 - Science Focus (Issue 19)
P. 9

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
 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
        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).

 By Sonia Choy 蔡蒨珩

   4   5   6   7   8   9   10   11   12   13   14