Monsters of the Cosmos: Black holes

by Chris Phoenix Clarke

“Now I am become Death, the destroyer of worlds” – Bhagavad Gita, Verse 32, Chapter 11

BLACK HOLES are the most terrifying, yet least understood features of the cosmos.  It is thought that a supermassive black hole — that is, one millions of times the mass of our sun — resides at the centre of every adult galaxy (including our own), and that quasars — the brightest and most distant (and, indeed, oldest) objects known in the Universe — might be the source of their turbulent creation.

I hope to take you on a journey into the heart of these mythical structures:  from violent births in the hearts of collapsing stars, to primordial beasts as old as the Universe itself; from white holes and wormholes, to escape velocities and event horizons.  Perhaps what follows might even make you question your very understanding of the fabric of reality, and indeed the Universe, itself.

So, first things first; how do they form?

To put it bluntly our Sun is a bit nondescript.  It’s a bit average.  For all its toiling fury and life-enabling radiation, the Sun is the cosmic equivalent of a vodka orange – nothing too exciting, but it’s hardly a  boring glass of soda water either.  When it dies it will quite uneventfully shed its outer layers and compress into a white dwarf star, seeing out the remainder of its days glowing gradually fainter as the aeons pass by.  Much like an ageing Guinness drinker propping up the local pub, our Sun is certainly no vomiting teenage binge drinker like its more massive counterparts.  In stark contrast, these stars getting on 15 times the mass of the Sun are simply too large to become white dwarfs and end their days in a quite spectacular fashion:  their respective cores undergoing cataclysmic implosions of truly astronomical proportions; the collapse causing the violent expulsion of each of the star’s outer layers into space, leaving behind dense and strange objects so mind-boggling it almost defies belief.

A 'feeding' black hole devouring a nearby star.
Artist’s depiction of a ‘feeding’ black hole devouring a nearby star.  Image credit: NASA

These objects are known as stellar black holes — something that challenges our most fundamental of intuitions on the most grandest of scales.

Smaller stars undergo nuclear fusion in their cores at a much slower rate than giant stars, ensuring that they burn less brightly and live a good while longer.  The comparison of giant stars being the ‘rock gods’ of the Universe is usually a good way of looking at it.   Rather than living a life of excess and partying and possibly dying from an overdose, our Sun is the much more sedate and conservative type, blending in with the crowd and living well into the age where beige clothing is appealing.  But leaving the alcohol-related anthropomorphic analogies aside for a moment, like everything else in the Universe, stars have finite lifetimes

When stars between about 0.5 and 1.4 solar masses pop their celestial clogs they implode, leaving behind the aforementioned white dwarf star; the fatal core collapse having been halted by something known as electron degeneracy pressure*.  This is the outward pressure caused by the electrons in the core obeying a quantum mechanical rule applying to all fermions (a family of particle of which an electron is a part of) called Pauli’s exclusion principle.  It states that no two fermions with the same energy level can occupy the same space, meaning that the core can only reach a certain density before the electrons simply cannot get any closer together, for fear of treading on each other’s sub-atomic toes, so to speak.

*as a point of interest, it is worth noting that the recent supernova in galaxy M82 is thought to have been a white dwarf star that gained mass from a binary companion (another star in mutual orbit around the other).  The result was a core-collapse supernova that is currently visible in the night sky at the time of writing [25th January 2014].

Then again, when stars with cores above this critical mass (otherwise known as the Chandrasekhar limit) die, their cores initiate a collapse of such energy that each electron decides it’s ‘had quite enough of this s**t’ and attaches itself to the nearest available proton — much like a toddler clinging to their mother’s leg — to form a neutron, enabling the collapse to continue to even more compact volumes and densities.  Then, just like the electrons before, the neutrons themselves create their own outward pressure for similar reasons and the collapse is halted.  The resulting object is known as a neutron star.  So dense is this stellar remnant (the star having been well over 2 million km in diameter before,  is now just 15 km wide) that one teaspoon-full would weigh as much as mount Everest!   (see previous blog ‘Pulsars, magnetars and neutron stars‘).

Artist's impression of a supernova explosion.
Artist’s impression of a supernova explosion.  Image credit: NASA

But it’s really the heavy-set kebab munchers with cores in excess of 15 solar masses that, via their own immense gravity, crush down so violently that even the neutron pressure cannot tolerate the force, and the result of this super-sized implosion is the object known as the stellar black hole**.

**another way for a stellar black hole to form is for an existing neutron star to gain mass; to accrete matter from somewhere else.  This can be achieved in a binary star system containing a red giant star and a neutron star, for example, with the latter gravitationally ripping off streams of gas and plasma from the former –- thus increasing its mass.  If the critical mass is breached then core-collapse will start and an implosion into a black hole is inevitable.

That’s a bit racist!  Why are they called black holes?

Considering the exotic nature of these awesome monsters, they are actually quite aptly named.  Far removed from the dubiously-coined terms for the sub-atomic quark particles (such as ‘up’, ‘down’, ‘charmed’ and ‘strange’ flavours), or the way biologists seem to over complicate the naming of even the most simple of life’s processes, black holes are essentially black and do represent a cosmic hole of sorts.

According to Einstein’s seminal paper on general relativity (see previous blog: ‘Once upon a spacetime‘), space is inextricably linked with time and in the presence of gravity space is curved.  In short, this means that space and time are no longer two separate entities, but instead exist as one conjoined entity known as spacetime, and that wherever mass is found, spacetime bends in towards it.  The larger the mass, the more the curvature of the spacetime around it.  The familiar analogy of a bowling ball being placed upon a taught sheet of rubber is one way to visualise it, with the rubber bending in as the weight of the bowling ball warps the sheet.  Analogous to a marble being rolled towards the bowling ball would be the planets in orbit around the Sun.

The curvature of spacetime around a mass is the definition of gravity itself; it’s the act of ‘falling’ into this curved space that gives the illusion of a force pulling something in towards it.

Such is the curvature of spacetime, or gravity, generated by a black hole that the situation arises in which nothing can escape its attraction once ‘fallen in’ (this, the event horizon, will be explained more in a second).  Light, in all its unparalleled performance, is nigh-on fast enough to free itself, meaning that, to quote The Eagles – Hotel California: ‘you can check out any time you like, but you can never leave’, rendering the black hole forever invisible to our curious eyes.  It is, to describe it as Terry Pratchett might, so dark that it is devoid of colour; it is the blackest of all really black things***

***sometimes when a black hole is ‘feeding’, the matter being sucked in forms a super-heated accretion disc around the perimeter of the event horizon.  The process can also produce jets of radiation expelled at the poles.  This enables the black hole to have its effects on in-falling matter directly observed, but still not those within the actual black hole itself.

Okay, but what actually is a black hole?

The key ingredient to black holes is their titanic gravity .  When a supergiant star dies and undergoes core-collapse as part of a supernova explosion, what’s left is an object so compact that it’s been hitherto impossible for human minds to contemplate.  Indeed, the predictions of Einstein’s general relativity suggest that these remnants are actually infinitely dense but occupy a space of zero volume — that is, they are infinitely small.  This ‘singularity’ as it is known, is clearly counter-intuitive; how can something be infinitely dense and yet take up no space whatsoever?  Moreover, the physics break down as soon as these infinite numbers start being introduced — usually meaning some part of the theory is wrong or incomplete — and that, in fact, there is probably no singularity at all — just something we are yet to understand.

So in answer to the question ‘what is a black hole?’, and as far as is currently understood, black holes are regions of space containing an object that is infinitely dense, but occupies no volume.  It is infinitely small — a singularity.  It sounds almost retarded, but we can actually calculate the mass of a black hole by its effect on nearby gas and stars — thus defining a particular value for its mass even though the object itself has zero volume and is infinitely small!  You might well be thinking that something is amiss here — that scientists need to go back and carefully review their life choices, and you wouldn’t be the first, or indeed last, to share this opinion!

The truth is no one knows; it is simply conjecture (more on the ‘funkier’ theories later on).  One of the aims of physics in the last 100 years has been to unite our two best theories of the Universe — general relativity and quantum mechanics — to give one overriding explanation of how everything works.  We’ll then see what the mathematics has to say about black holes and their dubious singularities…

‘Event horizon’ is a 1997 film starring Laurence Fishburne, what does it have to do with black holes?

The event horizon of a black hole.
The event horizon of a black hole.  Image credit: Answers magazine

What we can establish by using Einstein’s theory of general relativity is how black holes warp, or bend, the surrounding spacetime to such an extent that nothing can escape the pull upon breaching a certain critical distance from the centre.  This radius is known as the event horizon, or Schwarzschild radius, and it is the distance at which the escape velocity of the black hole equals the speed of light.  This ‘point of no return’ at which a black hole’s escape velocity is equal to the speed of light (300,000 km per second) is easily understood by picturing a fast-flowing river that becomes ever-more difficult to paddle your kayak against, until the current becomes so great that your heroic paddling counts for naught as the kayak tumbles over the waterfall.  Analogous to the waterfall is the black hole’s event horizon.  Simply put, it is the maximum distance from the black hole’s centre inside of which an outside observer cannot see into; events inside this zone are forever unknowable****

****Stephen Hawking recently argued that the event horizon is not actually a well-defined boundary as Einstein’s general relativity would have us believe but, instead, more of a fuzzy, fluctuating area of spacetime caused by quantum effects (Nature, 2014).

Just as a rocket being launched from Earth needs to hit a certain speed to be able to counter the effects of gravity and travel into space, so too does anything encountering a black hole (if it intends to leave).  This speed is known as the escape velocity.  The gravity generated by the warping of space around the mega-dense black hole is so huge that the required escape speed is in excess of the speed of light.  This means that something would need to travel faster than the speed of light to leave the confines of a black hole, and as light by its very nature cannot travel faster than itself, this poses a problem.  What it actually does is render the inside of the black hole forever black (as light is trapped by the gravity) so no information can ever leave to hit our eyes in order to see it.

If we can’t see them, how do we know they exist?

Due to their inherent blackness it is only possible to infer a black hole’s existence indirectly, that is, the effect they have on surrounding matter, and from doing so it is also possible to determine the mass of the black hole.  The two main indications of a black hole’s existence are the speed of orbiting stars and gas around a central region of space (observed as being empty), accretion disks of orbiting material, and energetic outbursts known as radiation jets.

The G2 gas cloud will encounter our very own supermassive black hole in 2014.
The G2 gas cloud will encounter our very own supermassive black hole in 2014.  Image credit: Sunday Times

A perfect example, potentially at least, is the impending feast of Sagittarius A* — the supermassive black hole at the centre of the Milky Way.  In the coming months leading up to summer 2014, the black hole–some four million times the mass of the Sun–will play host to an incoming gas cloud three times the mass of the Earth  (BBC 2014), tugging at it gravitationally until it rips apart and becomes ‘spaghettified’ as the black hole slowly feeds and devours — dragging the head of the gas cloud inwards faster than the tail, thus causing the metaphor of elongation commonly found in Italian cuisine.  This cosmic banquet has the prospective chance to brighten Sagittarius A* by a factor of 10,000; an event unlikely to occur at such close proximities again for several hundred years, and will allow astronomers and astrophysicists to finally have some light shed on an otherwise very shadowy supermassive spectre at the heart of our very own spiral galaxy.

This event is not too dissimilar to a deep-sea oil rig setting off thousands of flare guns one-by-one in the dead of night — illuminating the  sky for miles around as the location is quite clearly  revealed.  Of course, there’s always the chance things don’t go to plan, such as the gas cloud’s trajectory being slightly off or a small wayward star masquerading behind the veils of the cloud; both possibilities potentially causing the entire mass to career past the black hole rather than being sucked in like vermicelli.  Only time will tell.

Wormholes, white holes and parallel universes

Black holes are bread and butter for science fiction advocates and inventive imaginations.  No more exotic a phenomenon could have been dreamt of, let alone one that is posited to actually be real and considered science fact.  Many wondrously wacky ideas have been exclaimed regarding the ‘insides’ of black holes, and if we’ve learnt anything in science since the days of Copernicus and Newton it’s to never discount any theory as crazy by its merits alone.  It is often said that sufficiently advanced technology is indistinguishable from magic, and the same applies for new discoveries.  No one truly knows what goes on inside a black hole, but I’ve picked perhaps the most hopeful  and altogether enticing of the current bunch of theories, namely: wormholes and white holes.

White holes are the hypothesised opposites of black holes.  They are impossible to enter and can quite happily emit light and matter into Space.  Claimants of this belief hold that at the singularity of a black hole is a white hole doing its reverse thing out into another universe.  If you imagine a sand timer — the opening funnel at the top collects all the sand and directs it down to the centre, at which point it falls out of the opening funnel at the bottom.  Analogous to this is the black hole collecting all light and matter and funnelling it into the singularity, which in actual fact is a white hole spewing it back out into another universe.  Some even go as far to say that white holes are actually the causes of ‘big bangs’, and that our universe is actually a kind of waste product from another universe’s black hole – our big bang having been caused by the associated white hole.

In the immortal words of Wayne’s World — Wayne: “No way!”, Garth: “Way!”

But 1990’s-movie shock aside, it’s no less valid an idea than current standard cosmological models stating that everything in the Universe was created at a single point.

Wormholes, or Einstein-Rosen bridges to go by their proper name, are hypothesised tunnels connecting two regions of spacetime and are an active, ongoing area of theoretical physics research.  There is much debate about what kind of wormhole could exist, but a certain type, at least, is predicted from the equations of Einstein’s general relativity (a theory that has withstood all scrutinies thrown at it, in what is now its 99th year).

Traversing the wormhole's 'throat' can be quicker than light travelling outside the wormhole.
Traversing the wormhole’s ‘throat’ can be quicker than light travelling outside the wormhole.  Image credit: Edobric

Some proponents suggest intra-universe wormholes to be a possibility — that is, tunnels connecting 2 different regions of spacetime in the same universe.  If the geometry of space is curved like many believe, it would therefore be allowable to travel faster than the speed of light, although not in the traditional sense.  It would simply be faster to traverse the wormhole from one location to the other than it would be for light to ‘go the long way around’ by travelling in the spacetime outside the wormhole.  If you imagine sprinting at maximum speed around the side of a mountain to get to the other side, someone walking through a tunnel bored through its centre could conceivably get to the other side before you did.  Of course, light travelling in the wormhole with you would always reach the other location before you did, in the same way as a sprinter running through the tunnel will always beat the person walking.

There’s all sorts of complications with these wormholes, however.  To be traversable they seem to require ‘exotic’ matter in the form of some sort of negative-energy mass in order to stabilise them, and some scientists claim that wormholes can only exist in one direction in the singularities of black holes.  The latter would only bring you out into another region of spacetime inside a different black hole, meaning that you could never escape out past the event horizon in this new region.

The quantum-mechanical
The quantum-mechanical “Schrödinger’s cat” paradox according to the many-worlds interpretation. In this interpretation, every event is a branch point; the cat is both alive and dead, even before the box is opened, but the “alive” and “dead” cats are in different branches of the universe, both of which are equally real, but which do not interact with each other.  Image credit: Christian Schirm

Perhaps a more compelling argument is that of inter-universe wormholes.  These wormholes actually bring the traveller out into another region of spacetime in a parallel universe, resolving certain paradoxes which may arise in intra-universe wormholes, such as time travel and causality violations (the traveller would be brought out into a separate universe that doesn’t interact with the first, so they would never run into said paradoxes).  This theory is an interpretation of quantum mechanics, whereby reality is more like a tree of many branches, where every possible outcome does happen and there is an infinite number of parallel universes.  Standard quantum mechanics (I say standard; there is absolutely nothing orthodox about quantum theory!) uses something called a wavefunction collapse which asserts that before something is observed every outcome is possible, until collapsing into just one reality the moment it is observed.  Schrödinger’s cat being the usual analogy to use at this point, but I’ll refrain from delving any deeper into quantum mechanics here (see picture above).

Quasars and supermassive black holes

Finally we get to the real monsters of the cosmos: supermassive black holes.  It is hard to fathom just how powerful, how huge and how destructive these leviathans of space really are.  It is consensus amongst physicists that one lies at the centre of every adult galaxy and that the formation of these giants is down to the brightest phenomena in the Universe: quasars.

Supermassive black holes range in the order of  a hundred-thousand to a a few-billion times the mass of the Sun, whereas stellar black holes are classified as forming from stars up to about 30 times the mass of the Sun.  Intermediate black holes, as they are known, are strangely lacking from our observations of the cosmos which suggests that the formation of supermassive black holes is fundamentally different to that of stellar black holes.

It is worth mentioning at this point that evidence of supermassive black holes has been observed in the very early universe, implying that their creation is something that occurred during the first generation of galaxies.  Some theories assert that newly-forming galaxies are turbulent and chaotic places and that existing stellar black holes are constantly fed and grow to larger, more intermediate sizes.  They then encounter other black holes doing the same and a kind of cannibalistic black-hole chain-reaction occurs, eventually going on to form one colossal supermassive black hole at the centre of the galaxy.  This cannibal-esque feeding frenzy forms what is known as a quasar — an active galactic nucleus that takes the form of enormous jets of energy being emitted from the edges of the supermassive hole’s event horizon as an orbiting accretion disc of stars, gas and other black holes is fed into the monster’s mouth.

Quasars seem to have been a feature of almost all newly-forming galaxies during the first generation in the early universe.  They are therefore some of the oldest detectable sources of light we have observed — which also make them the most distant.  Such is the energy output of a quasar that it is possible to observe one 28 billion light years away!

Another theory puts forward the existence of primordial supermassive black holes — those that formed as a direct consequence of the extreme pressures of the Big Bang, and would explain the origins of supermassive black holes at the centres of the first galaxies.

So one might say, if some of the previously-stated theories are combined, that a black hole in another universe created a white hole in our Universe (the Big Bang) which, in turn, created the primordial supermassive black holes of the first galaxies.  These eerily-enormous galactic engines allowed for the continued evolution of stars which, themselves, lived out their lives to eventually collapse into stellar black holes.  And so the grand black-hole recycling system goes on and on…  One might even say that black holes are the fundamental mechanism by which our Universe (and, potentially, an infinite amount of other universes) is built upon, and without which, there would be no Universe to reside inside so we could sit here and ponder their importance in the first place.

So it seems that stellar black holes are as ubiquitous in the Universe as they are unusual and, although related in a sense to supermassive black holes, form very differently to their monstrous big brothers.  These celestial older siblings are likely found at the centre of every known galaxy and are perhaps key to the Universe’s evolution and continued survival.  Whatever really lurks in these regions of dark space, whether complicit with current theory or not, will doubtless be, at the very least, just as exotic as the limitations of our intelligence have allowed us to imagine.

Written by Chris Phoenix Clarke, January 2014.

References:

Clarke C.W., 2014. Space City. Available at https://www.facebook.com/Spacecityofficial/posts/581437771950444 [accessed 29th January 2014]

Nature, 2014. Stephen Hawking ‘There are no black holes’. Available at http://www.nature.com/news/stephen-hawking-there-are-no-black-holes-1.14583 [Accessed 29th January 2014]

BBC, 2014. Black hole’s ‘big meal’ could spark fireworks. Available at http://www.bbc.co.uk/news/world-middle-east-25678737 [accessed 29th January 2014]

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