The theoretical possibility of the existence of black holes has existedfor nearly a century. However, scientists still do not completely comprehendthem. This is despite the 30 years of convincing observational evidence thathas been collected over time. Black holes are immensely dense entities. For example, a black holewith 4 or 5 times the mass of the sun would occupy a spherical volume a fewkilometres across.

They curve the fabric of space and therefore change thestrength of gravity, increasing the gravitational attraction to them. 1If a person were to approach a black hole, their feet would accelerate fasterthan their head, in a process called ‘spaghettification’, which would pull youapart. 2 The first hint at the possibility of black holes arose in the 18thcentury, when geologist John Michell theorised that a star 500 times largerthan the sun would create such a quantity of gravity that even light could notescape.

Michell used 3rd Astronomer Royal James Bradley’s figure forthe speed of light, and Newtonian gravity, to estimate the size of a bodyneeded to have an escape velocity equal to the speed of light. He came to anestimate of 500 times the mass of the sun. This concept started a debate aboutthe possibility of dark stars. Eventually, natural philosophers came to theconclusion that Newton’s Laws meant that light isn’t affected by gravity, andhence light always leaves a celestial body, regardless of the mass. 3This resolution was accepted for a few centuries until Einstein published hisGeneral Theory of Relativity, which proved that gravity does in fact affectlight. Following Einstein’s publication, German mathematician KarlSchwarzschild found that celestial objects can become so dense that they formgravitational traps. The size of the trap, also known as the Schwarzschildradius, depends on the mass inside. Once an object (or light), passes throughthis radius (or event horizon), it never escapes.

4 After this, astronomers had to accept that black holes could exist.However, the problem was that they couldn’t observe them, as no light isemitted (or any other radiation types). The solution arose in the early 1970s. Thefirst x-ray telescopes were put into space and revealed an odd bright x-raysource in the Cygnus constellation, which is 8000 light years away. It wasdecided that this source was a superheated cloud of gas spiralling into a blackhole (Cygnus X-1). As the gas was accelerated into the strong gravitationalfield outside the event horizon, it was heated to millions of degrees andemitted x-rays. This is known as the black hole ‘feeding’. 5  It used to be thought that seeing a non-feeding black hole wasimpossible, but advances in radio telescopes have changed this.

In the futureit may be possible to see black hole silhouettes against a background of brightstars but this is not easy. The silhouettes are extremely small. However bycombining many observations from radio telescopes across the world, it maybecome possible to view these silhouettes, and could be plausible to see gasclouds falling into these black holes. By observing the gas clouds, we can seehow fast a black hole is spinning.

If the black hole is spinning, generalrelativity tells us that it will form a vortex in the fabric of space. Thistwisted region is called the ‘ergosphere’. Gas cloud observations reveal a newway to investigate black holes, and the validity of general relativity inextreme environments. 6 There are many different types of black hole. The smallest are stellarblack holes, which are several times the mass of the sun.

They’re formed whenmassive stars explode as supernovae. They orbit the centre of their galaxy andare the most prevalent kind of black holes. 7 Another type isintermediate mass black holes. These are larger than stellar black holes. Theyalso orbit the centre of their galaxy, and contain a few hundred or thousandsolar masses.

Astronomers are unsure of how they form, however there are threemain theories. The first is that they are formed when stellar black holes mergeby means of accretion. The second is when massive stars collide in densestellar clusters; the black holes form when the collision collapses. The thirdtheory is that they are primordial black holes created in the Big Bang, whenthe space-time continuum was crushed so much that tiny regions would sealthemselves off from the rest of the cosmos.

8 The final type ofblack hole is supermassive black holes. These are millions or even billions oftimes larger than the sun. They sit at the centre of every galaxy but only takeup the volume of an average solar system. In 90% of galaxies they are inactive.However, in the other 10%, these black holes are constantly feeding fromsurrounding celestial bodies. 9 There are a few theories as to howthese black holes were formed. One is that they are created out of the collapseof huge gas clouds during the early stages of galaxy formation. Another is thatthey’re the result of stellar black holes consuming a lot of material overmillions of years, growing to supermassive black hole sizes.

A final idea issupermassive black holes are generated from a cluster of stellar black holesforming and then merging. 10 In the 1970s, Stephen Hawking used quantum theory to suggest a way tofind primordial black holes and also to prove that black holes aren’t completelyblack. Quantum theory describes the universe on its smallest scales. A key ideaof the theory is that energy comes in discrete packets.

Quantum theory alsodescribes how particles behave. For example, particles can be difficult tolocate. Physicists can calculate where they expect a particle to be but itcould be elsewhere in a small region around the estimated position. This means thatparticles moving close to a boundary can appear to jump the boundary – a processknown as tunnelling.11 Hawking claimed that a particle couldtunnel from the inside of a rotating black hole and escape, lowering the blackhole’s mass. This process continues and accelerates, with the black hole losingmore mass until it disappears in a sudden release of gamma rays. This processmost likely applies to primordial black holes as they’ll evaporate faster thanthey can consume, however no evidence has been seen as of yet. 12 The existence of black holes is now accepted but there’s still anunease surrounding them.

Mathematically, they’re very similar to the Big Bang.They share an identical feature – the singularity (point of infinite densityand zero volume). Inside the black hole, the singularity is assumed to be thelast resting place of matter because gravity crushes matter into increasinglysmaller volumes.

The problem is, as the volume approaches zero, there’s notheory that can be used to study the resulting singularity. Another issue is the black hole information loss problem. To theoutside universe, only the mass, electric charge, and angular momentum of ablack hole are visible. Everything else (e.g.

what has fallen into the blackhole) is lost. This goes against reversibility which is the ability for aprocess to proceed equally well either in the forward or reverse temporal directions.However, once something crosses the event horizon, we can’t ever find out whatit was or recover it. Everything taken in by a black hole has effectively been erasedfrom the universe. Even tunnelling particles can’t give us this information. Despitethis, unifying gravity with other forces of nature using string theory couldallow investigation of this problem. String theory suggests that black holes don’thave singularities – their volume is a compressed ball of subatomic strings.These are the building blocks of nature that give us particles of matter.

Thestrings would store the fundamental information on the things that have enteredthe black hole. In this case, matter wouldn’t pass through the event horizon tothe singularity – it would compress itself onto the surface of the strings andmerge. The black hole’s Schwarzschild radius would increase in size to makeroom for the new material.

The curvature of space would get slightly steeperbecause the black hole has taken in more mass. This theory states that theblack hole expands to make more room for the new information.

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