Everyone has a favourite word. That combination of syllables, consonants and mouth shapes that somehow sparks joy. Tolkien might have had his precious ‘cellar door’ combination, but for me, my favourite word in the entirety of the English language is ‘spaghettification’. My mouth has to work overtime to even be able to say it, my fingers have to blaze across the keyboard to type it and my brain has to think really hard to remember how to spell it.¹⁰⁹ But I dare you to say it without breaking into a smile. You may even find yourself channelling Sean Connery as you proclaim ‘spaghettification!’

Now, as much as this sounds like a word I have made up to make myself happy, it is in fact a real astrophysical term; a phenomenon that black holes cause. All this information about black holes may have got you as excited as I am about them. You may even be wondering what it’d be like to visit a black hole, or perhaps get close enough to peer beyond the event horizon. Well, let me warn you now reader, that that is something you would never wish to do, for fear of being spaghettified.

The gravity around a black hole is so strong that if you fell towards it head first the gravity would be so much stronger at your head than at your feet that you would get stretched out like Elastigirl from The Incredibles. You would look more like spaghetti than a human; a long thin chain of atoms stretching all the way down to the centre of the black hole. We’ve seen this happening to gas clouds like G2 around the Milky Way’s central black hole, but also to stars, as they go from perfectly spherical to stretched thin.

This is all because of the gradient of the strength of the gravity around a black hole. Far enough away and it’s no different than the pull of a planet or a star, but get too close and the increase is exponential. It’s this gradient that causes the spaghettification; imagine being at the top of a very steep water slide, holding on where it’s flatter, but your feet are all the way at the bottom, lost over the edge. Strangely, with spaghettification it’s the less massive black holes you have to be more wary of, rather than the supermassive ones.

As a black hole gets more massive, its event horizon gets larger. The area of space that the black hole influences is much bigger, but the gravity gradient doesn’t get really steep until very near the black hole, sometimes well within the event horizon itself. But a less massive black hole has a smaller event horizon, and the gradient of gravity can get really steep outside it. Gravity is not stronger around the smaller black hole, but the strength changes more rapidly with every step closer you take. Think of it like mountains; the height of one mountain can be less than another, but the climb up can still be much steeper.

Or for the skiers out there, getting closer to a less massive black hole would be like cross-country skiing on the flat for a while before the slope all of a sudden becomes a super-steep black diamond run that could injure you. Thankfully, though, there’s a ski lift to take you away from danger (because in this analogy you haven’t crossed the event horizon yet). But getting closer to a supermassive black hole would be like being on a gentle beginner’s slope for a long time, before it gradually transitions to a steeper blue slope, then a steeper red slope, and then finally a super-steep black run that you could hurt yourself on but you realise too late there’s no ski lift to take you away and the only way is down. The Milky Way’s supermassive black hole is on the piddly side of supermassive, hence why the G2 gas cloud, when it looped around it back in 2014, got a bit spaghettified but otherwise escaped unscathed (it got the ski lift out of there, to labour that analogy).

So if you did desperately want to feel the effects of spaghettification you could in theory get closer to a less massive black hole and still escape, but your very shape would be irrevocably changed. This is what you would feel if you ‘fell into’ a black hole, but what would you actually see? Assuming you could somehow resist the stretching effects, perhaps in a spaghettification-proof spacecraft,¹¹⁰ what would you see out of the window? Well, thanks to general relativity, we have the equations to work out what would happen without any astronauts needing to make the ultimate sacrifice.

Let’s assume the black hole we’re falling into is not accreting material, so that we don’t blind/kill ourselves with high-energy radiation sat in the window seat of our spacecraft. From a great distance you wouldn’t see much, black holes are very dense after all, so size-wise they’re pretty small and you wouldn’t be able to spot them from far out. As you got closer, though, you would eventually be able to notice a small dark circle where there was no light whatsoever, marking the event horizon.

As you got ever closer to the black hole you’d start to think your mind was playing tricks on you. Black holes curve spacetime so much that they affect the path of light from behind and around them, messing with your sense of perspective. Approaching a typical object in space, like the Moon for example, on your journey from Earth, the object would get steadily bigger in your window directly proportional to how close you were. When you were halfway there, the Moon would look twice as big as it does on Earth. But with all the curving of light around them, black holes don’t behave the same as the Moon.

Black holes are like puffer fish; they make themselves look bigger than they truly are. The light from stars behind them gets bent to the side so that the area where no light is coming from looks bigger than it really is; an effect that is exacerbated as you get ever closer. So much so that if you were ten times the event horizon away from a black hole, the black hole would completely block your view looking out of your spacecraft window. Compare that to being ten times the size of the Moon away from the Moon, and the Moon would be about the size of your fist held in front of you at arm’s length.

Getting closer still, the black hole would continue to appear larger around you, with the darkness slowly engulfing your spacecraft from all angles as the black hole continues to bend light out and away from you. Looking backwards, you’d see not only the view back the way you came, but the view behind the black hole, bent into your eye line. A 360° view squished into an ever-shrinking circle, until at the event horizon it becomes a single dot of light: the light of the entire Universe bent into your eyes for one final glimpse, one look back over the shoulder, before you face the unknown.

I can’t tell you what happens next, as you cross the event horizon. Do you descend into darkness or into bright blinding light? Is there a star-like object there made of an exotic form of matter that we don’t know about that’s held up by another form of degeneracy pressure: the next stage in a star’s evolution from white dwarf to neutron star to something else? Has all the matter that has been trapped beyond the event horizon for billions of years turned to pure energy? Is there really a singularity? Only you would know, having crossed over, but you’d never be able to share what you found with the world.

Once beyond the event horizon, every direction would be ‘downhill’. Even if you turned around the way you came, every path would lead you to the centre. Perhaps you might panic and try to accelerate away from the centre to get back out, but that would just get you to the centre even faster. There’s no way out. Every single version of your future has you ending up at the very centre of the black hole. Space and time become one so that the future is a direction in space rather than in time. Your spacecraft wouldn’t be able to save you, just like it wouldn’t be able to stop tomorrow from coming.

That’s your perspective though: what you would see. But what if you had a friend who wanted to watch from a safe distance what happens when you fall in? Perhaps you might set up a system where you send a burst of light every minute, like a lighthouse, just to let them know you’re OK on the journey. To you in your spacecraft, you will send off those bursts every minute, on the minute. But that’s not what your friend will see. Because to you, getting ever closer to the black hole and its strong gravity, time would pass differently than to your friend at a safer distance. What feels like a minute to you could be an hour or more from their perspective.

This is something called time dilation; a concept that Einstein explained for moving objects in his theory of special relativity way back in 1905. This phenomenon had already been predicted for electrons orbiting atoms in 1897 by Northern Irish physicist Sir Joseph Larmor, but it was Einstein that linked this back to the very nature of time itself, as opposed to a property of electrons. Einstein derived the relation between the difference in time that passes and the difference in speed that two objects are moving at. The greater the difference in speed, the bigger the time difference, so much so that as you reach the speed of light time slows to a standstill.

The speeds that we can currently achieve for space travel don’t produce a time dilation that’s noticeable by current astronauts. For example, astronauts aboard the International Space Station, which orbits at an average altitude of 408 km at a speed of 27,500 km/h (17,000 mph) will experience around 0.01 seconds less time than those on Earth for every year they spend in space. After a year onboard, they touchdown back on Earth 0.01 seconds younger than they would be if they’d stayed at home.

This is called ‘kinetic time dilation’, an effect caused by increased speed. But there is a second type of time dilation: ‘gravitational time dilation’. Instead of a higher speed causing time dilation, it can also be caused by incredibly strong gravity; the stronger the gravity, the slower time passes for you relative to someone in lower gravity. This effect is not just noticeable around black holes; the gravity at the core of the Earth is stronger than at the crust, making the core ever so slightly younger than the crust. It also means that astronauts on board the International Space Station in a lower gravity than us on the ground experience time a little faster, actually cancelling out the effects of kinetic time dilation making them younger.

Time dilation has been tested and proved many times in many ways over the past century, but perhaps the most famous experiment was one designed by two Americans: physicist Joseph Hafele and astronomer Richard Keating. Hafele was an assistant professor in St. Louis in 1970 when he was preparing a lecture for students on relativity and time dilation. He ended up doing a quick calculation on the amount of time dilation a commercial airliner would experience with a typical airspeed of 300 m/s (670 mph) at a typical altitude of 10 km (33,000 ft). He realised the combination of time slowing down due to kinetic time dilation and time speeding up for the lower gravity would give an overall time difference of around 100 nanoseconds (0.0000001 seconds; remember human reaction time is 0.25 seconds, so this is a tiny fraction of a second).

To measure such a tiny difference, you need an incredibly precise clock; one that can measure to nanosecond precision. In 1955, the first such clock was built at the National Physical Laboratory in south-west London, using caesium atoms as the inbuilt time keeper. It’s not just the light from stars that can make electrons in atoms jump up orbits into excited states; we can use lasers to do this too. The electrons absorb a little bit of energy, jump up an energy level and then drop back down, emitting a very specific wavelength (or colour) of light. This is how we know what elements are present in nebula gas clouds that form stars; specific elements emit specific colours, like a fingerprint.

You can fine-tune this process even more; if the laser you use has the same wavelength of light as the wavelength given off by the electrons as they jump down, you hit a sweet spot and give the electrons just the right amount of energy to keep oscillating between their excited and normal states. We describe this as the atom and the laser being in resonance. If you can find that wavelength sweet spot with your laser then you know the exact frequency that the transition happens at, thanks to the wave speed equation that we all learn at school. For light, the speed of light is constant and so frequency and wavelength are intrinsically linked: speed of light = frequency × wavelength.

So for caesium atoms, we’ve found the laser wavelength sweet spot and know that the electrons jump up and down between the first two orbits when they’re in resonance 9,192,631,770 times a second. This is so precise that although the second used to be defined based on the Earth’s rotation as 1 ⁄ 86,400 of a single day, the second is now defined by a caesium atomic clock because it’s more precise (it’s also measurable anywhere in the Universe as well). Today’s caesium atomic clocks are so accurate that even in 100 million years, they won’t drop or gain a second (compare that to a typical mechanical wrist watch that drops around five seconds a day on average).

Back in 1970, atomic clocks weren’t as precise as they are today, but could still measure time to a few nanoseconds of precision. Hafele realised that two out of three things he needed to easily test the time dilation predictions of relativity were readily available to him: airplanes and atomic clocks. The third thing, which wasn’t readily available to him, was money. He spent another year as an academic beggar, asking many institutes for money to do the experiment, before meeting astronomer Richard Keating, who worked in the atomic clocks department at the US Naval Observatory. Atomic clocks were also used for nautical navigation at the time, as a much more useful replacement for the timing of Io’s eclipses. Keating helped Hafele obtain $8,000 of funding from the Office for Naval Research, $7,000 of which was spent hiring out commercial aircraft and crew. On each flight they had a seat for both Hafele and Keating and two seats for a passenger named ‘Mr Clock’.

They flew the atomic clock heading east around the world, and then two weeks later flew around the world heading west, comparing the time recorded on each clock to others that had been kept on the ground by the Naval Observatory. In this experiment, the airplanes are moving and the centre of Earth is the stationary reference point, since it doesn’t move as the Earth rotates. A plane flying east, in the same direction as the Earth rotates, has a higher relative velocity than a plane flying west in the opposite direction to the Earth’s rotation. So on the two flights, a different kinetic time dilation should occur (with the clock on the eastern flight losing time compared to the western flight). Combining this with the much stronger effect of gravitational time dilation (assuming the two planes fly at exactly the same constant altitude, which in reality won’t quite be the case) gives a total predicted time loss of 40 nanoseconds on the eastern flight and 275 nanoseconds gained on the western flight.

Hafele and Keating published their results in 1972, reporting a measurement of the time lost on the eastern flight of 59 nanoseconds (±10 nanoseconds due to measurement error, meaning the value could be anywhere between 49–69 nanoseconds) and time gained on the western flight of 273 nanoseconds (±7 nanoseconds). The agreement between predicted and measured values in this experiment is astonishing and is one that has been repeated many times since with the same results. It showcases just how accurate the predictions we can make with Einstein’s theories of special and general relativity really are. And a good job too, because GPS satellites in orbit around Earth suffer from this same kinetic and gravitational time dilation (the gravitational time dilation is what dominates); the clocks on board the satellites gain 38,640 nanoseconds per day compared to clocks on Earth. If we didn’t correct for this time gain, then GPS would be utterly useless in giving an accurate position within two minutes. These errors in positions would snowball by 10 km per day (or around 6 miles).

So even just above our heads, here on Earth relativity has a noticeable effect. Imagine, then, the effect of gravitational time dilation around a black hole a trillion times more massive than Earth. You on your spaghettification-proof spacecraft sending out a flash of light once every minute to your friend watching your journey towards the black hole, would not notice any difference in how time flowed. It would truly still feel like one minute to you and not like time had slowed down. But to your friend, those light flashes would take longer to arrive as your speed appeared to slow down as you got closer to the event horizon. A minute between flashes would turn to an hour, an hour to a day, a day to a year and a year to a century. In fact, someone watching you get ever closer would never actually see you cross the event horizon, appearing as if time had frozen for you, when in reality you crossed with no bother, feeling like only a few hours or days had passed since the start of your journey. The event of your light flash when you cross that point of no return would be for ever outside your friend’s possible powers of observation.

This freezing of time is an optical illusion created by the effects of gravitational time dilation, like the illusion of the black hole appearing much larger out of the window due to the curvature of space. Black holes in that sense really are the ultimate tricksters; we can’t rely on what we see. Instead, the equations of general relativity can open the door and reveal the truth, no matter how massive the black hole.