What we’re pretty sure we know

Let’s start with some truths—things we are almost certain to be correct based on our current knowledge.

True: black holes exist

The idea of black holes has been around for hundreds of years, ever since scientists took the known laws of physics and determined what would happen at their most extreme. Although these laws of physics have held up against experimental testing, that’s still not a guarantee that these extreme scenarios really happen in nature.

So to prove black holes exist, we need to look at astronomical observations.

We cannot directly see black holes with any light we know how to detect. But we can see them indirectly, based on how they influence the matter around them. This includes observations of how objects move around black holes, or how light is distorted.

You find that there is something there that’s over four million times the mass of our sun but in a tiny area that produces absolutely no light. There’s nothing else we think that it could be other than a black hole.Amanda Bauer, via ABC News

Not satisfied with indirect evidence? Enter the gravitational wave discovery, first announced in February 2016 and with a second detection announced in June 2016. By directing two lasers beams down a long tunnel and looking for abnormalities in how the light travels, scientists detected a ripple in space itself. The ripple matched the predicted signal of two colliding black holes, giving us direct evidence for their existence.

From all that we’ve learnt, it’s either the case that black holes really do exist, or the laws of physics as we know them need to be tweaked. But for us to accept that latter option, those tweaked theories will need to be developed, tested, and proven—and still be able to explain simpler physics like why you don’t float away from Earth when you jump, or how we can use gravitational boosts of planets to get spacecraft to specific targets in the outer solar system.

True: you could use black holes to travel forward in time

Want to be a time traveller? Hang around an area with extreme gravity for a while. You can’t go back into the past, but if you got close enough to a black hole, you could travel to the future.

This isn’t a feature limited to black holes—it can happen wherever there’s enough gravity. It’s just that the effects only become noticeable if the gravity is really intense, so a supermassive black hole might be your prime ticket to the future … assuming you could get close enough. (That might be a challenge, seeing as how the closest supermassive black hole, Sagittarius A*, is about 26,000 light years away.)

Just don’t cross the event horizon GLOSSARY event horizonthe region around a black hole that can be thought of as an ultimate point of no return. Not even light can escape once it enters the event horizon. , or you’ll never be able to return and reap the rewards of your time travel!

Using the gravity near black holes to travel into the future isn’t the same idea as entering a black hole and using it as a wormhole. As we hope to make clear throughout this topic, passing the event horizon of a black hole would be a very bad idea.

True: black holes cause huge tidal forces

Another result of extreme gravity is extreme tidal forces. On Earth, the gravitational pull of the sun and moon creates the tides of our oceans. This is because the gravity of the sun and moon distort the ocean in different directions, depending on the time of day. If you were on a planet orbiting close to a supermassive or stellar black hole, the tidal forces could be extreme—possibly leading to scenarios like the breathtaking tide scene in the 2014 film Interstellar (though that particular sci-fi rendition still involves some questionable physics).

Black holes can even tear planets or stars apart!

But, like the possibility of forward time travel, huge tidal forces is not a phenomenon unique to black holes. Jupiter, for instance, is so massive that it causes its moons to squeeze and stretch, leading to a heated core and volcanic activity.

Busted myths

Just because black holes are bizarre doesn’t mean everything you hear about them is true. Here are some common misconceptions.

Myth: all black holes are black

We tend to imagine black holes as black spheres in the middle of space. But not all black holes fit this description, because light can still be emitted from the region outside the event horizon.

Some black holes power the brightest known objects in the universe, known as quasars GLOSSARY quasarsa massive celestial object that emits huge amounts of energy. From Earth, they appear as very bright stars. They are thought to have black holes at their centre. . As material gets close to the event horizon it starts to speed up—and heat up—shining brightly until it passes through the event horizon.

Another reason why black holes don’t necessarily appear to be black is due to the way they distort time. From the point of view of someone far away, a spaceship heading towards a black hole will never—literally never—appear to reach it. So if a bright object heads towards a black hole, its light will appear to float just outside the event horizon, getting redder and redder, and fainter and fainter, until it disappears. This is due to the phenomenon of red shifting GLOSSARY red shiftinglight waves emitted by an object in a region of strong gravity gets stretched. As an object’s light comes towards us from near a black hole, it will become red (which is more ‘stretched’ than other colours), and the object will eventually disappear from view. .

Black holes warp space-time so much that at the event horizon, time essentially stops. You’d see your clock running normally, and you’d just fall in—bloop, gone. But someone far away would see your clock ticking more slowly as you fell in … your fall would literally take forever.Phil Plait, via Crash Course Astronomy

Myth: black holes suck everything in

Perhaps the most prevalent myth about black holes is that they ‘suck’ matter towards them, like really powerful vacuum cleaners. Don’t worry! They’re not going to eventually consume everything in the universe, and you don’t need to be afraid of them … unless you plan on travelling VERY close.

Why? Well, even though black holes are extreme in many ways, they don’t have infinite mass—and it’s mass that determines the force of their gravity. Some black holes—known as stellar black holes GLOSSARY stellar black holesblack holes formed by the gravitational collapse of a massive star. —have about the amount of mass that very massive stars do. So, just as objects can orbit massive stars without falling in, the same is true of black holes. You could happily orbit a black hole forever.

Once you get close enough, the story’s different, and gravity will guide you in. But that’s the same as any massive object, like a planet or star.

Myth: all stars become black holes

Stellar black holes are formed when massive stars run out of fuel and collapse in on themselves in a cataclysmic implosion. But not all stars end their lives this way. If a star is average or small in size (like our sun), it will become a white dwarf GLOSSARY white dwarfa small, dense star that is typically the size of a planet. It forms when a low-mass star exhausts the fuel that drives its nuclear core. —a stable, low-energy star that doesn’t have enough mass to collapse into a black hole. If the star is much bigger, it will become either a neutron star GLOSSARY neutron starthe object left over after a supernova explosion. This is what happens at the end of a star’s life if it’s way more massive than our sun, but not quite massive enough to directly implode and become a black hole. or a black hole. A star needs to be around 20 times the size of our sun before it can become a black hole at the end of its life. You can compare these outcomes in this infographic by Khan Academy on the lifecycles of stars.

So our sun won’t eventually become a black hole, and neither will most stars in our universe.

Myth: event horizons cause spaghettification

Because gravity quickly gets more powerful the closer you get to a black hole, a person who (hypothetically) travels feet-first towards a black hole could have their feet pulled faster than their head, and their sides crushed inwards. As they continue to travel towards the black hole, the effect would be amplified until they are eventually ripped into a string of individual atoms in a process called spaghettification.

While researchers think that spaghettification is a real phenomenon, there tends to be a popular perception that it always happens at the event horizon of every black hole. But the point at which spaghettification happens varies, depending on the size of the black hole. For the biggest supermassive black holes, you could easily pass the event horizon without being spaghettified … and you might not even realise you’ve crossed the event horizon! In these cases, spaghettification wouldn’t happen until you’re already well beyond the event horizon.

Myth: the Large Hadron Collider will create black holes that destroy us all

There have been fears that the Large Hadron Collider (LHC)—the world’s largest particle accelerator GLOSSARY particle acceleratoran instrument that accelerates particles to high velocities, then collides them together with other particles. , credited with proving the existence of the Higgs boson—will be able to create tiny black holes that will destroy us all.

It’s true that a stellar or supermassive black hole would be bad news if created in the proximity of our home planet. But neither are a possibility. There are no stars massive enough in our local part of the galaxy to become black holes. Even if you could somehow harness all the mass of all the objects in our solar system, you’d still be far short of the amount needed to make one of these large black holes.

As for microscopic black holes, we’re not even sure they exist. And, if they do exist, while we don’t know how they’d work, we do know they’re not destructive.

The collisions that occur at the LHC are the sorts of collisions that happen in nature all the time. Cosmic rays regularly bombard Earth, slamming particles together with the same energies as those at the LHC. If those kinds of collisions—the sorts of collisions that have been happening for billions of years—create mini black holes, then they don’t appear to do any damage.

What we’re less sure of

As is always the case in science, there’s more to be discovered. Here are some of the questions that researchers are working on today.

Do tiny black holes exist?

While we do know that black holes come in different sizes (or, more accurately, different masses), we’re not entirely sure whether there’s some limit on how small they could get. Tiny black holes, also called micro black holes or mini black holes, are a predicted possibility.

Tiny black holes would be incredibly difficult to observe, as it’s thought they’d have little effect on the environment before almost instantly disappearing (due to evaporation by Hawking radiation GLOSSARY Hawking radiationa proposed process where particles are emitted by black holes over time. As black holes emit this radiation, they slowly lose mass. ). We may one day be able to create them in controlled environments like the LHC—but, as explained above, we needn’t be afraid of them.

How do supermassive black holes form?

Supermassive black holes have so much mass that they can’t have been formed by a typical dying star. One idea is that they form when galaxies (with black holes at their centre) collide, and then those galaxies collide with other galaxies. But it’s not clear that this can explain the size of today’s supermassive black holes based on the time they’ve had to form since the Big Bang.

Another idea is that the original ‘seeds’ for today’s supermassive black holes formed from gas clouds before the formation of stars and galaxies, or during the Big Bang itself.

Do all black holes evaporate?

It’s thought that, with enough time, black holes eventually will evaporate away by releasing particles through a process known as Hawking radiation. Tiny black holes are predicted to disappear almost instantaneously, and big ones are thought to take much longer (around 10¹⁰⁰ years for the biggest ones to evaporate—a gargantuan number compared to even the age of the universe). Because of this, it’s thought that our universe will eventually enter a ‘black hole era’—after stars stop forming and have mostly burned out, the universe will be dominated by these supermassive black holes that slowly, sloooowly evaporate.

Although Hawking radiation has yet to be experimentally verified, it is widely accepted among scientists today, as it fits in well with what we know about physics.

What happens inside a black hole?

If you were brave—or foolish—enough to head into a black hole, what would happen when you crossed the event horizon?

First, there’d be the physical effects on your body. Spaghettification is thought to eventually happen (sooner for stellar black holes than for supermassive black holes). Another idea is that, as travellers crossed the event horizon, they would be met with a super-heated wall of energy known as a firewall that would prevent them from travelling any further.

The idea of the firewall was introduced to try and solve a paradox that arises from our current understanding of black holes, known as the black hole information paradox GLOSSARY black hole information paradoxa puzzle relating to the physical information ‘lost’ inside a black hole. It’s thought that nothing is ‘lost’ in the universe—just converted into other forms. But it seems the information about objects that enter black holes appears to be lost forever. . But not all researchers are convinced by the idea of a firewall, and it’s unlikely we’ll ever know whether it’s really true … at least not before we reconcile some of the differences between general relativity and quantum mechanics.

Okay, so that’s some of the proposed ideas of what would happen to your body. What would happen to the passage of time? As explained above, we know that, compared to other people sitting far away, time will run at a different rate when you get closer to a black hole. Based on our current knowledge of physics, we think that things will get really weird at the event horizon … and that you might even see all of time pass you by.

Unless our understanding of general relativity gets a major revamp, we don’t think there’ll ever be a way to test what actually happens when you enter a black hole. So be cautious when anyone suggests they have a theory for what happens inside or at the event horizon. It probably won’t be a theory at all—theories need to make predictions that are testable and falsifiable.

What else don’t we know?

As is often the case in frontier science, we don’t even have all the questions, let alone the answers. Black holes certainly test the limits of our knowledge, and there’s plenty more for us to learn.