Page 8 - Science Focus (Issue 19)
P. 8

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

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