When people try to picture black holes, they can’t help but imagine them as the hoovers of the Universe; pulling in and gobbling up everything and anything around them. But that couldn’t be further from the truth, because black holes don’t suck.

Think about the Solar System: 99.8 per cent of all the matter in the Solar System can be found in the very centre, in the Sun. It completely dominates the Solar System and is utterly massive in comparison to everything else. Even Jupiter, the ‘King of the Planets’,⁹⁰ is only 0.09 per cent of all of the mass in the Solar System. Earth is a piddly 0.0003 per cent of the share. Despite the Sun’s gravity dominating in this way, all the other inhabitants of the Solar System, from planets to asteroids and comets, happily orbit the Sun without ‘falling in’. As general relativity explains, the Sun curves space and the planets travel along that curved space. To get the Earth to move closer to the Sun, you would somehow need to take away some of the Earth’s energy, to disrupt the perfect gravitational balance it’s currently in.

The region around black holes is exactly the same. Sure, black holes are massive, but their dimensions are relatively tiny. Remember that if we collapsed the Sun down to a black hole the Schwarzschild radius would only be 2.9 km. Let’s imagine for a minute we could make that happen; at first we’d probably notice that someone had turned the lights out, but apart from that we wouldn’t notice a thing. Earth’s orbit wouldn’t change at all because the thing that it’s orbiting around hasn’t changed in mass, and the distance of Earth to the Sun hasn’t changed, so the pull of gravity would be exactly the same.

But anything too close to that roughly 6 kilometre-across Sun-turned-black hole probably wouldn’t be so lucky. The curvature of space near it would be dramatic, increasing the force exponentially. Anything further away, though, would just continue to orbit this theoretical black hole, forever tracing the same path through space in an endless loop. This is why when I say we are all orbiting a black hole at the centre of the Milky Way, there is no need to panic. Unless you spend your days terrified over the possibility that the Earth is going to fall inwards towards the Sun, then you can sleep soundly knowing that the Milky Way’s black hole is merely shepherding the Solar System around the Milky Way. The Solar System is not spiralling in to the centre. It’s on a very happy orbit; there’s no doomsday scenario at the end of time where we fall into a black hole.

In fact, it’s incredibly rare that anything makes it into black holes at all. It’s a wonder that some of them have managed to get so supermassive. Take the black hole at the centre of the Milky Way; at about 4 million times the mass of the Sun, it has an event horizon just seventeen times bigger than the Sun’s diameter. Just sit with that for a second; 4 million times the amount of matter found in the Sun, fitting well inside the orbit of Mercury. You’d think a behemoth like that wouldn’t struggle with accreting any matter that got too close, but that’s exactly what happened in early 2014.

Back in 2002, in the same year that the paper from Andrea Ghez’s group confirmed that the only thing the centre of the galaxy could be was a supermassive black hole, something a little weird-looking was spotted on images of the centre of the Milky Way. It turned out to be a gas cloud, and by 2012 people had worked out that it was most definitely on its way towards the danger zone around the Milky Way’s supermassive black hole. This was a once-in-a-lifetime chance for astronomers, because in the words of Douglas Adams: ‘Space is big. You just won’t believe how vastly, hugely, mind-bogglingly big it is.’⁹¹

Astronomers don’t really do experiments, as such. The entire Universe is our experiment and we observe it in different ways at different times and watch how it changes. It means that if you want to know how matter behaves when it gets too close to a black hole, you can’t just set that experiment up and make it happen. You have two options, either 1) simulate it happening on a computer and hope you didn’t miss any laws of physics or 2) wait around billions of years for it to occur. The fact that this gas cloud, dubbed ‘G2’, was going to come within spitting distance of the supermassive black hole at the centre of the Milky Way wasn’t just once-in-a-lifetime, it was a once-in-a-billion-years type of opportunity.

So, as the gas cloud was torn apart slowly over the next two years, the astronomy world held its breath, and by 2014 fireworks were expected! But instead, astronomers got more of a flop. It was Andrea Ghez’s group, using the Keck telescopes once again, that confirmed the G2 gas cloud was still intact. The gas cloud had looped around the centre of the galaxy relatively unscathed, despite passing as close as thirty-six light-hours from the black hole (about 2,375 times the size of the event horizon). Perhaps a star held it together against the pull of the black hole’s gravity? Who knows. But what this serves to demonstrate is that black holes aren’t just endless hoovers sucking material in. This gas cloud got as close as we’d ever seen something get before and it still didn’t ‘fall in’ to become part of the black hole. Sure, it got a bit beaten up – it looks less like a cloud and more like an aeroplane contrail now – but it lived to fight another day, or at least drift around space indefinitely.

I can’t help but anthropomorphise things in space when I think about events like this. I picture the G2 gas cloud streaking away from the black hole thinking phew! and warning every other gas cloud they happen upon not to go near the scary elephant graveyard⁹² at the centre of the galaxy. The story of G2 gets passed around and used as a cautionary tale for millennia by parents of little gas clouds: ‘Have a nice day, love; don’t stray too close to the black hole! You don’t want to end up like G2!’

And yet despite G2’s escape, some gas clouds still end up under a black hole’s control. We see this in the accretion disks around much more active supermassive black holes in the centres of other galaxies. Accretion disks are made of material that has made its way to the centre of a galaxy and not been as lucky as G2. Instead, it has been captured in orbit around the supermassive black hole. But as we just reasoned with planets around the Sun, material in orbit is not in danger of being ‘sucked in’ to the black hole. It will happily continue to orbit unless it loses energy in some way.

An accretion disk is an incredibly dense place. There’s a huge amount of gas moving at immense speeds. Collisions between particles, like atomic nuclei (having separated from their electrons and become a plasma because it’s so hot), are very common. These collisions are akin to those between balls in a game of pool. You give the white cue ball some energy by hitting it with the cue, and it then impacts with another ball, transferring that energy. Sometimes in those collisions the cue ball, if hit just right, will stop on impact, losing nearly all of its energy, and sometimes it will travel on with the other ball with a fraction of the energy it had before.

The same thing can happen to the particles in accretion disks; random collisions can transfer energy, imparting some particles with more energy so that they can move away from the black hole, and stealing energy away from others so that their orbit decreases. Enough of these random collisions can eventually strip a gas particle of enough energy so that it crosses the region around the black hole where you can have a stable orbit, and tumbles in beyond the event horizon to add to the mass of the black hole. Finally, one single particle has been accreted.

It can take over 500 million years for a supermassive black hole to accrete just half of all the matter in its accretion disk in this way as there’s a limit to how fast this process can occur around a black hole. Rather ironically, the limit is named after Arthur Eddington (who we met earlier), who doggedly argued against the existence of black holes for so long. To be fair, it’s a concept that doesn’t just apply to black holes, but to all things that glow, including all the stars in the Universe.

Eddington had always been focused on stars and their interiors. How were they powering themselves? How much power they did they produce? To answer these questions he started by focusing on how stars stopped themselves from collapsing. Like Kelvin, Eddington reasoned that for stars to be stable spheres that didn’t pulse in any way, the force of gravity crushing inwards must be balanced by the amount of energy released inside by whatever process powered stars. Since stars are hot, most astronomers assumed that it must be thermal energy alone pushing outwards, but Eddington added something extra: radiation pressure. Stars aren’t just hot, they also shine, giving out huge amounts of light which exert a pressure outwards, resisting the crush of gravity.

When light hits things it can transfer energy. In theory, if you could build a laser powerful enough, you could use it as a pool cue. While I fervently hope that laser pool becomes a sport at some point in the future, radiation pressure is actually used in many applications today, including to propel spacecraft with ‘solar sails’. The radiation pressure the solar sails receive as light from the Sun impacts them is akin to the pressure the sail on a boat feels from the wind. This isn’t science fiction; this was first demonstrated by JAXA (the Japanese space agency) on their IKAROS (Interplanetary Kite-craft Accelerated by Radiation Of the Sun) spacecraft in 2010. It deployed a 192 m² (just over 2,000 square feet) plastic membrane, pointed it at the Sun and managed to fly all the way to Venus.⁹³ It’s an exciting prospect, because there are no moving parts and no fuel that can run out, so craft powered this way could operate for much longer than we’re used to.

Radiation pressure is also something space agencies have to take into account when planning missions in the Solar System. Even with a more typically powered spacecraft, say for example on a trip to Mars, radiation pressure from the Sun’s light will push it off course, causing the craft to miss Mars by a few thousand kilometres. When spacecraft are launched and set on their merry way, they’re sent in slightly the wrong direction, knowing light from the Sun will bring them onto the right path.

So the forces from radiation pressure are definitely not something you can ignore. They’re enough to power spacecraft and, inside stars themselves during nuclear fusion, enough to resist the crush of gravity inwards. This perfect balance between gravity inwards and radiation pressure outwards in a star is known as the Eddington limit. It’s the maximum brightness a star can achieve. If exceeded, the force outwards will be larger than the force of gravity inwards and the star will start to shed some of its outer layers in a wind, or outflow. Because the only thing that radiation pressure in a star needs to resist is gravity, the Eddington luminosity is directly related to the mass of the star. The more massive the star, the brighter it can be.

Similarly, radiation pressure is also a big player in accretion disks around black holes. As the material falls into orbit around the black hole it is accelerated by gravity, heating it up, giving it a huge amount of energy so that it starts to radiate light. This light then exerts pressure outwards on other material trying to fall inwards onto the accretion disk. In a perfect scenario, there’d be a balance between the amount of matter falling onto the accretion disk and the radiation pressure outwards from the material already in the disk. In that case, the black hole would be growing by its maximum possible amount: its Eddington limit. If a glut of extra material falls onto the accretion disk, it will be blown away by radiation pressure, again in a wind or outflow. Black holes therefore have a natural control process to curb their gluttony when their eyes get bigger than their bellies: radiation pressure allows the accretion disk to let a burp rip every now and then.

Just like for stars, the Eddington limit for black holes is set by their mass. The bigger the black hole, the brighter the accretion disk can get, and the faster the black hole can grow (they have a higher ‘accretion rate’). A typical supermassive black hole of 700 million times the mass of the Sun would have an Eddington limit (or maximum brightness of its accretion disk) 26 trillion times brighter than the Sun.⁹⁴ If we assume about 10 per cent of the gravitational energy gained by the matter falling onto the accretion disk is radiated away, then (using E = mc²) we can work out that the maximum rate that a black hole 700 million times the mass of the Sun can grow by is three Suns’ worth of material every year.

But that’s just a maximum. Only approximately 10 per cent of galaxies have active supermassive black holes at their centre that are currently growing, i.e. they have an accretion disk. And the majority of those are accreting at less than 10 per cent of the maximum rate. Take our own supermassive black hole at the centre of the Milky Way; it is (thankfully) not that active right now. It is radiating at 10 million times less than its Eddington limit, only around a few hundred times brighter than the Sun, meaning it’s only growing by a ten billionth of the mass of the Sun every year. A meagre amount.

If there was enough gas funnelled to the centre of the Milky Way, towards the black hole, it could technically grow at a rate 10 million times more than that. But it doesn’t: because black holes aren’t endless hoovers. They don’t suck. There has to be some process that physically moves material towards the centre before it gets close enough to be caught up in the accretion disk and brought into orbit by the black hole’s gravity. If you think about it, black holes are less like hoovers and more like couch cushions: sat there in your lounge, unassuming, not sucking anything towards them. But if you happen to physically move something close to the edge of that couch cushion and it falls down the back, it’s lost down there for good.