by Ben Miller
Right, so let’s find the gravity dial. It’s there on the far left, next to the dials for the other three fundamental forces: electromagnetism, the weak force, and the strong force.13 Straight away, we can see that something’s up. The dial for the strong force is on a linear scale, and set to 1, but the dials for the other three forces work in powers of ten. The electromagnetic force is set to –2, the weak force to –6, and gravity at a whopping –38.14 What happens if we reset with the green button, reduce gravity by a few clicks, then hit the red button to restart the universe?
Amazingly, we see something even more grand and life-friendly. We see the tiny flash of the Big Bang, followed by inflation, just as before. The first stars take a split second longer to form, but when they do they are truly gargantuan. Once again they go supernova, spreading the elements of the periodic table throughout their neighborhoods. Galaxies also form a little more slowly, grow larger, and the stars within them burn for longer. This is especially handy for stable hydrogen-burning stars like our Sun. With three clicks down on the dial, to –41, such stars last for much longer, giving evolution time to do its thing time and time again.
In other words, as far as life goes, lower gravity is good news. But what about an increase? Let’s go up four clicks, to –34, hit the red button and see what happens. Fascinatingly, as far as life is concerned, it’s a disaster. This time it takes much less matter to form stars, planets, galaxies and black holes. In comparison to the normal universe, galaxies are tiny and form much more quickly, with small stars so close-packed that they are constantly stealing one another’s planets. And the average lifetime of a star like the Sun decreases, lasting only hundredths of a second. Even if such a star manages to hold on to planets, there won’t be nearly enough time for life to emerge. Strong gravity means no life.
KNOTS IN COTTON
OK, so we’ve toyed with gravity. What happens if we tweak the lumpiness of the early universe? Let’s find out by locating another handy dial, Q, that controls the density variations in the fireball. Like the dials for the electromagnetic, weak, and gravitational forces, it’s on a logarithmic scale, set to –5.15 Let’s dial up the magnification on our model, light up the dark matter and hit the slo-mo button, so we can see exactly how this process works.
Firstly, if we really zoom in, we can see that the density variations start life as quantum fluctuations, which then get amplified as the universe is inflated. Over the next 0.4 of a second—corresponding to some 400 million years in the real universe—pockets of dark matter form, growing denser and denser, which hydrogen and helium then fall into. Eventually the ordinary matter becomes so dense that it collapses under its own gravity, lighting up as the universe’s very first generation of stars.
And some stars they are, too. Some are hundreds of times the mass of our own Sun, and when they explode as supernovae they shower their own particular corner of the universe with every chemical element imaginable. What’s left of their cores then collapses down to form a black hole, and continues to pull a swirl of matter and dark matter toward it. As the black hole starts to become supermassive, that swirl becomes a galaxy.16 And galaxies are the nursery beds for generations of stars and planets.
AVENUE Q
OK, so that’s the universe we’ve already won. That’s safe. Now let’s do what superbeings were put on this superplanet to do, and tweak the lumpiness of the primordial fireball to see how that affects the evolution of the universe. Let’s start by dialing Q down, to 1 part in 1,000,000, and see what effect it has.
The difference is striking. After we press the red button, we see the flash of the Big Bang, then inflation, then darkness. The fireball is too fine-grained for any part of it to collapse under gravity. The expansion wins out, and once again we end up with a thin gruel of hydrogen, helium, and dark matter particles. There are no stars, no planets, and no galaxies. Once again, the universe is a no-go zone for life.
What about increasing Q? Surely that will make glorious giant galaxies, like when we reduced gravity? Sadly not. If we dial Q up to –4, reset, and push start, before a second is up, huge regions of dark matter begin to clump together, precipitating enormous clumps of gas, more massive than entire galaxy clusters, that quickly collapse under gravity to form truly forbidding black holes. That’s not a reality any of us wants to live in.
THE STRONG FORCE
By now, I’m sure you’re getting the picture, but before we move on there’s one more tire I want to kick: the strong force. Without it, of course, there would be no atoms, because its job is to bind protons together to make atomic nuclei. You’ll remember that this happens in two places: firstly, in the Big Bang, where the nuclei of lighter elements like lithium and helium get made; and secondly, in big stars, which make all the rest.
That sounds pretty straightforward, and you might think that so long as the strong force is attractive, and can be felt when protons get within a short range of one another, all would be well. But there’s a complication. Protons also carry a positive charge, which tends to push them apart. At the same time that the strong force pulls together all the protons in a nucleus, the electromagnetic force is pushing them apart. In fact, one of the puzzles of the early twentieth century was how atomic nuclei managed to exist at all.
“Very well,” you might say, “let’s just make the strong force between two protons in a nucleus stronger than the electromagnetic force between them.” But that’s not how the problem is solved either. As it turns out, the strong force isn’t quite up to the job of holding two protons together in a nucleus; instead, the electromagnetic force wins and the two protons are forced apart. So how do atomic nuclei remain stable?
The solution, ingeniously, comes in the form of a neutral particle with roughly the same mass as a proton, which also feels the strong force, but doesn’t have any charge. No doubt you’ll remember that such particles are called neutrons. These gentle peacemakers are the diplomatic glue that holds atomic nuclei together. They also feel the strong force, but they don’t have any charge, so aren’t repelled by the positive charge on the proton. A deuterium nucleus, for example, which is made up of a proton and a neutron, is stable, as is a helium nucleus, which has two protons and two neutrons.17
FAITES VOS JEUX
Right. With all that in mind, let’s twiddle the dial for the strong force on our model universe and see what happens. First of all, let’s dial it up a bit. Surely, if we increase the attraction between protons and neutrons, making nuclei will get easier? And with more elements knocking around the periodic table, there will be even more chemistry and hence a greater abundance of life?
I’m afraid not. Let’s increase the strong force by a tenth, to 1.1 on the dial, and press the red button. Once again, we see the flash of the Big Bang, and half a second later the Dark Ages end, as the first giant stars burst into life and go supernova. Galaxies form just as before, but we notice something odd about the stars within them. They have exceedingly short lifetimes; in the compressed time of our model universe, they last maybe a millisecond or so. Once again, there’s not nearly long enough for evolution to produce life. And even worse, if we zoom in on the asteroids and planets, we see that none of them have any water. What’s going on?
It was all there in the Big Bang, but we missed it. Crucially, increasing the strong force has meant that all the protons fused into pairs, forming a new helium isotope with no neutrons, 2He. In other words, we have made a universe with no hydrogen. It’s the hydrogen burning phase in stars that takes time, so they burn much quicker. And without hydrogen to react with oxygen, there’s no water. Maybe that doesn’t rule out all life-forms, but it certainly rules out everything that has ever lived on Earth.
So what happens if we decrease the strong force to, say, 0.9 on the dial? Again, once we push the red button, we get a shock. We see the flash of the Big Bang and the short, furious burn of the first stars, but they fade without going supernova. Galaxies form as before, but again their stars burn only briefly before
fading into darkness. All stars have short lifetimes, big, medium, and small alike, and, even worse, without material from supernovae there are no planets. We have made another universe that is barren of life. What went wrong?
The answer, again, was in the intense heat and pressure of the primordial fireball. A decrease in the strength of the strong force has meant that protons no longer fused with neutrons to make the hydrogen isotope deuterium (2D), a vital stepping stone in the synthesis of 4He. Without deuterium to pave the way, we effectively turned off nuclear fusion like a tap. There are no other atoms except hydrogen, no chemistry, and therefore no life. Stars still light up, heated by gravitation, but without nuclear fusion reactions in their cores, they quickly lose energy as heat and cool into black holes.
Sobering, eh? We can’t tweak the strong force by more than a tenth in either direction without nixing the periodic table, chemistry, and life. Since the strong force competes with the electromagnetic force, that implies we can’t tweak the electromagnetic force by much either without having the same effect. Why? Reduce the electromagnetic force, and 2He will be stable, removing all hydrogen from the Big Bang. Increase it, and the Big Bang makes no deuterium, and hydrogen is all we get.
A PUT-UP JOB
And if none of that convinces you, try this extraordinary fact, first pointed out by Fred Hoyle: Every element heavier than helium should be rare in the universe. The reason is that there’s another roadblock early on in the fusion process that goes on inside stars. The easiest way to make larger and larger nuclei is to start with a helium nucleus, say, and keep adding a proton; however, there’s a problem. If we add a proton to a helium nucleus, we get lithium-5, which rapidly decays. If we squash two helium nuclei together, we get beryllium-8, which is also unstable. That means there’s no easy way to make boron-10, which has five protons, or carbon-12, which has six protons, or oxygen-16, which has eight. Yet carbon is the next most abundant element after hydrogen and helium, and oxygen is the next most common. In fact, apart from a scarcity in lithium, beryllium, and boron, elements all the way up to iron are commonplace. So what’s going on?
One possible route to making carbon is for three helium nuclei to fuse together, in what is called the “triple alpha” process. But there’s a problem. The first step is for two helium nuclei to come together to make beryllium-8, which is unstable. The chance of a third helium nucleus colliding with beryllium-8 in time to make carbon-12 is therefore vanishingly slim. Hoyle’s genius was to suggest that there might be a fortuitous resonance, in the form of an excited state of carbon, which exactly matched the energy of the beryllium-8 nucleus when capturing a helium nucleus. The experimentalists went to work, and, sure enough, exactly just such a state was discovered.
Without Hoyle’s resonance, next to no carbon would be made in stars, and—since to make an oxygen nucleus all you need is for a carbon nucleus to capture another helium nucleus—next to no oxygen. Give or take the odd rogue metal ion, all life on Earth is essentially made up of just a few elements: carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur. Can it be coincidence that these exact elements happen to be the most abundant in the universe? A change of a fraction of a percent in the strong force, and just a few percent in the charge on a proton, would cause the Hoyle resonance to disappear. And then where would we be? Literally nowhere.
AN ACCIDENT OF HISTORY
So how do we explain this fine-tuning? Is it written into the laws of physics? So far as we know, it’s not. The present laws of physics are a blank slate. They tell us the relationship between physical quantities, but not their absolute values. Newton’s Law of Gravitation, for example, tells us how the gravity of an object varies with distance. Double the distance, it says, and you get a quarter of the gravity. But how much gravity does a mass of one kilogram have at a distance of one meter? Newton can’t tell you. To work that out, someone somewhere has to do an experiment, make some measurements, figure out the strength of gravity per unit of mass and distance, and then plug it into Newton’s equation.
And it’s the same throughout physics. The Standard Model doesn’t predict the masses of the fundamental particles, for example, which is why the Higgs gave us the runaround. We only know their masses from experiment. With the partial information we had, we were able to put an upper limit on the mass of the Higgs, but that was about it. Likewise, General Relativity in cosmology doesn’t tell us the strength of gravity or the size of the cosmological constant. We have to build ourselves a telescope and have a look.
The truth is that over the past two centuries, experimental scientists have done an extraordinary job of keeping the theorists in business by making incredibly accurate measurements of all the constants that we find in nature. Don’t think for a second that any of the theorists like this situation. The grail they chase is a theory that will, of its own sweet accord, predict the universe that we find ourselves in. This elusive Theory Of Everything (TOE), as it is known, would make all experimentalists redundant overnight. Every detail of the universe—the mass of dark matter particles, the strength of gravity, the charge on an electron—would pop out like a rabbit from a hat. Instead of using quantum physics here and relativity there, everything would be a special case of the TOE. Cosmology and particle physics would be a job well done.
Big deal, you might say. We’re just not there yet. It’s not surprising our theories don’t describe the universe exactly as we find it; after all, we haven’t got the full picture. Our “laws” are really all special cases, like Kepler’s Laws of Planetary Motion turned out to be special cases of Newton’s Law of Gravitation, which are, in turn, a special case of General Relativity. Once we have a truly fundamental theory, we’ll have a truly detailed description. When we do, General Relativity will prove to be a special case of some as yet undiscovered Theory Of Everything, and we’ll be able to predict the rate of expansion of the universe before we measure it, not fudge it after.
But there’s an alternate argument. It says that the reason we can’t predict the fundamental constants is that they are a result of the universe’s history rather than a feature of any fundamental theory. The universe wound up the way it is by chance, not design. Why should we be surprised that gravity is weak, when if gravity were strong we wouldn’t be here? Maybe there are other universes out there where gravity is strong; they just don’t have people in them. The universe is fine-tuned for life because, if it weren’t, we wouldn’t be here.
THE MAN WHO BROKE THE BANK
The idea that the universe is fine-tuned for life is called the anthropic principle. The fundamental constants are what they are because otherwise we wouldn’t be here to ponder them. By sheer chance the strong force can overcome the electromagnetic force to create atoms, and gravity is weak, so those atoms form stars and planets. By remote accident, the atoms on those planets happen to have interesting chemistry, and the environment on at least one of those planets kick-started the very special set of chemical reactions we call Lady Gaga. We lucked out, simple as that.
Some would leave it there, content to believe that the universe is a lottery, and our numbers just happened to come up. Others—and I am one of them—would say that’s not a satisfying answer. To me, a freak fine-tuned universe begs further explanation. How many other tickets were there in the lottery? How many other universes failed to create life before ours succeeded?
The philosophers among you will no doubt protest that I am falling for one of the oldest logical faux pas in the book, a version of what is known as the Monte Carlo fallacy, after a famous Monaco gambling incident. Purportedly, on August 13, 1931, the roulette wheel at the Grand Casino in Monte Carlo landed on black twenty-six times in a row. Did anyone make any money? Yes, the casino did. Because after the wheel had landed black around fifteen times in a row, everyone started putting all their money on red. Time after time they piled high their chips, and time after time the wheel came up black.
So what was the flaw in the punters’ logic? Simply that the roulet
te wheel somehow “knew” how many times it had come up black, so was bound to come up red. Of course, the wheel knew no such thing, which is why everyone took a very deep bath. The lesson, of course, is that truly random processes know nothing of their own history.
Take lotteries, for example. The version in the UK asks you to choose six numbers between 1 and 49. After a short burst of excruciating live entertainment, the numbers are drawn live on television. The chances of matching all six numbers and winning the jackpot are 1 in 13,983,816.18 Are the odds better if you’ve played a great many times before and lost? Your cheating gambling heart might say “yes,” but your cool, probabilistic head definitely says “no.” Guinevere (one of the UK’s number-picking lottery machines) doesn’t know how many times you’ve played, or what your lucky numbers are. You could just as well win at your first attempt as at your nine hundred and ninety-ninth.19 Win the lottery one week, and you’ve got just as much—or as little—chance of winning it the next.
By analogy, then, am I committing the Monte Carlo fallacy when I say that a rare event like a fine-tuned universe implies lots of previous attempts which weren’t fine-tuned? Isn’t that just like saying, “Hank just won the lottery. He must have been playing for years.” At the risk of answering my own rhetorical question, yes, I think it is the same. Just because Hank’s numbers came up once doesn’t mean he’s played before and lost. But, then again, it doesn’t rule it out either. If we raid Hank’s house on a hunch, and find a cupboard full of scrunched-up lottery tickets, we wouldn’t be that surprised.