“I checked it very thoroughly,” said the computer, “and that quite definitely is the answer. I think the problem, to be quite honest with you, is that you've never actually known what the question is.”
“But it was the Great Question! The Ultimate Question of Life, the Universe and Everything,” howled Loonquawl.
“Yes,” said Deep Thought with the air of one who suffers fools gladly, “but what actually is it?”
So wrote Douglas Adams back in 1979, and the galaxy has been pondering it ever since. We know the Answer is 42, but what we really need is an answer to the question of what the Question was to get that Answer.
There are so many things that 42 could be the answer to. In math alone, it’s the number of partitions of 10 – that is, the number of ways you can write 10 as a sum of positive integers; it’s the first number equal to the sum of its own nonprime proper divisors; the number of triangulations of a heptagon; and it was the last natural number below 100 to reveal a representation as a sum of three cubes – that last one only occurred in 2019.
It turns up in the FIFA World Cup: it’s the number of all possible outcomes of each group stage, counting up all wins, losses, and draws. It’s the precise offset in degrees from a light source that produces a rainbow, which is all sorts of delightful.
Unfortunately, when it comes to human lives, 42 has a much less fun claim to fame: it’s pretty much the worst age to be.
At least, that’s the result of a study that followed tens of thousands of people across three countries back in 2015. Happiness, it turns out, tracks with age in a U-shape: it’s highest at the beginning and the end, and we’re all doomed by statistics to become crotchety old bastards in the middle for a bit.
“The change-in-life-satisfaction function crosses the zero x-axis at ages 42.3 in the [British Household Panel Survey], 40.1 in the [Household Income and Labour Dynamics in Australia study], 41.4 in the [German Socio-Economic Panel] and 46.9 in the [Medicine in Australia Balancing Employment and Life study],” noted the authors. “By implication, these are the ages at which well-being reaches a minimum.”
And yes, if you were wondering, the average of those figures… is 42.
It makes sense, actually, because 42 is also the average age of a midlife crisis. That’s according to data from Spotify, at least: it’s the “specific point when middle-aged listeners drop their sophisticated singer-songwriters, their ‘best of the 80s, 90s and today,’ and spontaneously start listening to teeny-bopper pop again,” noted Caitlin Dewey for The Washington Post back in 2015.
The change in music taste at that age can’t be attributed to parenthood, and it only lasts for a couple years or so – but it’s definitely there, and, perhaps unexpectedly, it’s more pronounced in women. Don’t worry, though: by about 45, you’ll be back listening to painfully outdated golden oldies once again.
After all, apparently once you hit 42 years old, things can only get better.
We live on a rotating rock named Earth, orbiting a sun named, um, The Sun, which, in turn, orbits the center of the Milky Way, which itself orbits a supermassive black hole named Sagittarius A*.
It won’t always be this way. One day (whatever that may mean in this situation), our Sun will run out of hydrogen and stop being able to create helium via nuclear fusion, as it currently does. Without that outward pressure, it will fall prey to gravity, which will make the core extremely hot and dense, igniting the outside shell and forming a red giant big enough to gobble up its nearest planets.
That won’t be the end of the Sun: when the temperature in the core gets high enough, nuclear fusion will start again – but this time, it will be turning helium into carbon, and fast. Eventually, it will blast out a planetary nebula and collapse into a white dwarf roughly the size of the Earth, and from there, a black dwarf – a type of star so old that none are currently known. But for the Earth, there’s a reasonable chance that our time in the universe will end in the jaws of a gigantic red star that we once relied on to provide us with life.
And the really sad part? Technically, we won’t even have reached middle age.
“Throughout all of these changes, the Sun and our Solar System will continue to orbit around the Milky Way’s center, completing a full orbit every ~250 million years or so,” wrote astrophysicist and author Ethan Siegel in October 2023. “The time to return to our starting point is known as a galactic year, and has about a ~10% uncertainty on how long it actually takes. Meanwhile, in term of stellar evolution, we are quite confident that the Sun will last roughly 10–12 billion years from the moment nuclear fusion first ignites in its core until the red giant phase begins, a track that we’re just a hair over 4.5 billion years into, at present.”
With this information, we can figure out – more or less – how many galactic years the Earth-Sun system gets to enjoy before one cosmic dance partner finally destroys the other. And the result? You guessed it.
“42 is an answer that’s extremely consistent with the best data we have,” Siegel wrote. “It may yet turn out to be the exact answer to this question, although superior data will be required to know for certain.”
When it comes to “ultimate questions”, you could do worse than one concerning the fundamental nature of existence itself. And while we’ll leave the existentialism to the philosophers – this isn’t IFLP after all – there’s one pretty important place that 42 crops up in cosmology: the Hubble Constant.
“It's a measure of how fast the universe is expanding at the current time,” Wendy Freedman, an astrophysicist at the University of Chicago who has spent her career measuring the constant, told BBC Future back in 2021.
And it really does fulfill the remit of “everything”, too: “The Hubble Constant sets the scale of the Universe, both its size and its age,” she explained.
How’s that, you ask? Well, let’s start by thinking about how big the universe is. We know the observable universe is about 93 billion light-years in diameter, with us slap bang in the middle of it – but what about the unobservable universe?
“We can only make inferences based on the laws of physics as we know them, and the things we can measure within our observable Universe,” Siegel wrote in 2018. But the best data we have suggests that the entire universe – the bits we can’t see as well as the bits we can – must be at least 250 times bigger than the observable part.
“This means the unobservable Universe, assuming there's no topological weirdness, must be at least 23 trillion light-years in diameter, and contain a volume of space that's over 15 million times as large as the volume we can observe,” Siegel explained. “If we're willing to speculate, however, we can argue quite compellingly that the unobservable Universe should be significantly even bigger than that.”
And it’s for this nigh-impossible question that the Hubble Constant is key.
See, the universe may be unfathomably huge, but it’s still expanding – and if we want to know how big it is, we need to know precisely how fast that’s happening. There are two main ways to measure this speed: we can either look at the nearest galaxies to our own and figure out how quickly they’re moving away from us, which is known as “late universe” measurement; or, we can extrapolate it from lumps and bumps in the cosmic background radiation, or “early universe” measurement.
While estimates of the Hubble Constant go back all the way to Georges Lemaître in 1927 – two years before Edwin Hubble got in on the game, in fact – modern estimates have the advantage of a vast wealth of empirical data from spacecraft such as the Planck mission. Now, there’s a bit of a cosmic paradox here, because you’ll find significantly different results for the Hubble Constant depending on how you measure it, and nobody really knows why, but since the Planck mission measured cosmic background radiation, that’s what we’ll concentrate on here.
And according to the most current of those estimates, the Hubble Constant is… 42.
“We have taken two measurements for the constant,” Richard Saunders, a senior lecturer at Cambridge University’s Cavendish Astrophysics Laboratory, told The Independent in 1996 “and the average of them is, well, it's 42.”
Saunders had just headed up a project to measure the Hubble Constant using the Ryle radio telescope – an array of what look like huge satellite dishes, but are in fact specialized antennae and radio receivers that scan the sky to pick up cosmic background radiation. He had measured the Hubble Constant in km/s/Mpc, or kilometers per second per Megaparsec – interestingly, this means that the Hubble Constant is technically a frequency, rather than a speed or acceleration – and here’s the really weird thing: he was wrong… but the answer is still 42.
The most up-to-date estimates of the Hubble Constant these days, which use data from the Planck mission, put the value a little higher. “There are multiple possible cosmologies that can reproduce the patterns we see [in the cosmic background radiation],” explained Siegel in a 2021 column on the constant. “But […] the best-fit value comes in at 67-68 km/s/Mpc for the expansion rate […] There’s very little actual wiggle-room.”
Let’s take the middle of that interval, then, for balance – call it 67.5 km/s/Mpc. Well, what is that in imperial? That is, in miles/s/Mpc?
Oh. It just so happens to be 42.