Geraint F. Lewis’ day job involves creating synthetic universes on supercomputers. They can be overwhelmingly bizarre, unstable places. The question that compels him is: how did our universe come to be so perfectly tuned for stability and life?
For more than 400 years, physicists treated the universe like a machine, taking it apart to see how it ticks. The surprise is it turns out to have remarkably few parts: just leptons and quarks and four fundamental forces to glue them together.
But those few parts are exquisitely machined. If we tinker with their settings, even slightly, the universe as we know it would cease to exist. Science now faces the question of why the universe appears to have been “fine-tuned” to allow the appearance of complex life, a question that has some potentially uncomfortable answers.
The deeper we look at the universe, the simpler it appears to be. We know that everyday matter is built from about 100 different atoms. They, in turn, are composed of a dense nucleus of close-packed protons and neutrons, surrounded by a buzzing cloud of electrons.
Peering deeper, we find that protons and neutrons are themselves made of quarks – of which there are six distinct types. But two dominate the universe: the up-quark and the down-quark. There are also six leptons of which the electron is the most famous.
The four fundamental forces glue matter together. Two of them, the strong and the weak force, only inhabit the sub-atomic world. Everyday life is dominated by the electro-magnetic force and gravity.
These building blocks of the universe come with tight specifications and they never vary. Wherever you are in the universe, the mass of the electron, the speed of light (light is an electromagnetic wave), and the strength of the gravitational force is the same. In physics, we encounter these so-called fundamental constants so often, we barely give them a second thought. We just plug them into our equations and calculate the properties of matter and energy to our heart’s content.
As a cosmologist, I can use these immutable laws of physics to evolve synthetic universes on supercomputers, watching matter flow in the clutches of gravity, pooling into galaxies, and forming stars. Simulations such as these allow me to test ideas about the universe – particularly to try to understand the mystery of dark energy (more on this later).
This plug-and-play approach to science has also given us a masterful ability to operate in our real universe. We blasted the Rosetta spacecraft 510 million kilometres into the solar system with such pinpoint precision it could land its probe on a three-kilometre-wide speeding asteroid. We’ve designed an instrument so sensitive it could detect the gravitational waves reverberating from two black holes that collided 1.3 billion years ago. Every aspect of our modern technological world is underpinned by plug-and-play science.
While our ability to make use of the fundamental constants is impressive, they also pose a mystery. Why do they have the values they do?
So now, I invite you to join me in imagining a universe, a universe slightly different to our own. Let’s just play with one number and see what happens: the mass of the down-quark. Currently, it is set to be slightly heavier than the up-quark.
A proton is made of two light-ish up-quarks plus one of the heavy-ish down quarks. A neutron is made of two heavy-ish down-quarks plus one light-ish up-quark. Hence a neutron is a little heavier than a proton.
That heaviness has consequences. The extra mass corresponds to extra energy, making the neutron unstable. Around 15 minutes after being created, usually in a nuclear reactor, neutrons break down. They decay into a proton and spit out an electron and a neutrino. Protons, on the other hand, appear to have an infinite lifespan.
This explains why the early universe was rich in protons. A single proton plus an electron is what we know as hydrogen, the simplest atom. It dominated the early cosmos and even today, hydrogen represents 90% of all the atoms in the universe. The smaller number of surviving neutrons combined with protons, losing their energy to become stable chemical elements.
Now let’s start to play. If we start to ratchet up the mass of the down-quark, eventually something drastic takes place. Instead of the proton being the lightest member of the family, a particle made of three up-quarks usurps its position. It’s known as the Δ++. It has only been seen in the rubble of particle colliders and exists only fleetingly before decaying. But in a heavy down-quark universe, it is Δ++ that is stable while the proton decays! In this alternative cosmos, the Big Bang generates a sea of Δ++ particles rather than a sea of protons. This might not seem like too much of an issue, except that this usurper carries an electric charge twice that of the proton since each up-quark carries a positive charge of two-thirds.
As a result, the Δ++ holds on to two electrons and so the simplest element behaves not like reactive hydrogen, but inert helium.
This situation is devastating for the possibility of complex life, as in a heavy down-quark universe, the simplest atoms will not join and form molecules. Such a universe is destined to be inert and sterile over its entire history. And how much would we need to increase the down-quark mass to realise such a catastrophe? More than 70 times heavier and there would be no life. While this may not seem too finely tuned, physics suggests that the down-quark could have been many trillions of times heavier. So we are actually left with the question: why does the down-quark appear so light?
Things get worse when we fiddle with forces. Make the strength of gravity stronger or weaker by a factor of 100 or so, and you get universes where stars refuse to shine, or they burn so fast they exhaust their nuclear fuel in a moment. Messing with the strong or weak forces delivers elements that fall apart in the blink of an eye, or are too robust to transmute through radioactive decay into other elements,
Examining the huge number of potential universes, each with their own unique laws of physics, leads to a startling conclusion: most of the universes that result from fiddling with the fundamental constants would lack physical properties needed to support complex life.
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