Why Does the Universe Seem Fine-Tuned for Life?

Inside the Sun, hydrogen fuses into helium, and a small part of the mass turns into energy. That part is 0.7 percent. It looks like a number you would forget at once. But the astronomer Martin Rees argues that this number matters a great deal. If it were 0.6 or 0.8 percent, the universe would have very different elements and probably no life like ours. Why should our existence depend on such a narrow range?

What does it mean that the universe is fine-tuned?

Fine-tuning is the observation that several basic numbers in physics seem to lie in narrow ranges that allow life. If these numbers were a little different, there would be no long-lived stars, no carbon chemistry and nobody to measure anything. Such numbers are called constants of nature. Our best theories use them, but they do not explain their values. Physicists measure them.

Fine-tuning is first a claim about physics. It says that, as far as we know, the constants could have had other values, and that most other values would rule out life. The claim says nothing yet about why the values are what they are. That second question is where scientists and philosophers disagree. The claim is also not about humans in particular. It is about any kind of life that needs complex chemistry.

What would change if one number were slightly different?

Let us take the 0.7 percent from the opening and follow it step by step.

1. What the number measures. Four hydrogen nuclei can fuse into one helium nucleus. The helium nucleus weighs 0.7 percent less than the four hydrogen nuclei together. The missing mass becomes energy, and this energy makes the Sun shine.
2. Where the number comes from. The 0.7 percent depends on the strong nuclear force, the force that holds the parts of an atomic nucleus together. A stronger force gives a larger number, and a weaker force gives a smaller one.
3. Turn the number down to 0.6 percent. In this case, Rees argues, a proton could no longer stick to a neutron. But that pair is the first step on the way from hydrogen to helium. So the universe would contain almost only hydrogen. With only one element, there would be no chemistry worth the name.
4. Turn the number up to 0.8 percent. Now the force is a little stronger. Two protons could stick together directly. According to Rees, hydrogen would then have fused into heavier elements soon after the Big Bang. Little hydrogen would be left to fuel long-lived stars like the Sun, or to form water.
5. Look at the window. On this account, life needs the number to sit somewhere between about 0.6 and 0.8 percent. Ours sits at 0.7.

There is a more extreme case. Empty space itself seems to carry energy, which speeds up the expansion of the universe. Physicists describe it with the cosmological constant. Simple estimates from theory give a value larger than the measured one by up to about 120 orders of magnitude. That is a factor of 1 followed by 120 zeros. Yet if the real value were only a few orders of magnitude larger, space would have expanded too fast for galaxies to form.

How do scientists and philosophers explain it?

There are four main answers. They are not all exclusive, and none has won.

The first answer is the anthropic principle. It says that we can only observe a universe that allows observers. In a universe without stars, nobody would ask the question. This is true, but critics point out that it does not explain everything.

The philosopher John Leslie used a comparison. Imagine you face fifty trained marksmen, and all of them miss. You could not wonder about it if they had hit you. But your survival still needs an explanation, for example that they missed on purpose.

The second answer is the multiverse. Some theories in cosmology suggest that our universe may be one of very many, each with different constants. If there are enough of them, a few will allow life, and we must live in one of those. Together with the anthropic principle, this would explain the fine-tuning. Critics raise two problems. Other universes cannot be observed, so the idea is hard to test. And the philosophers Ian Hacking and Roger White argued that other universes do not make this universe any more likely to allow life.

The third answer is design. A creator chose the values so that life would be possible. Defenders argue that a life-friendly universe is much more likely if a designer exists than if the values were chosen by chance. Critics answer that we cannot know what a designer would want, so we cannot say what the design idea predicts.

The fourth answer is that a deeper law of physics will one day fix the values. Then the constants could not have been different. But this answer only moves the question. We would then ask why the deeper law allows life.

What fine-tuning teaches you

The most useful lesson is about selection effects. What you can observe is often filtered by the conditions that make the observation possible. So when a pattern looks surprising, ask: “Could I have seen anything else?”

This question helps in many everyday situations.

• Success stories. You read interviews with founders of successful companies, and many of them took big risks. But founders who took the same risks and failed are not interviewed. The filter decides what you hear.
• Money. Lists of investment funds often show only the funds that still exist. Funds that did badly were closed and disappeared. So the funds you see look better than funds did on average.
• Health claims. “My grandfather smoked all his life and lived to 95.” You hear this story because he lived. The smokers who died early do not tell it.

Physicists use the same reasoning in their work. In 1987, Steven Weinberg asked how large the cosmological constant could be if galaxies, and therefore observers, were to exist. He argued that it should be small but probably not zero. In 1998, two teams of astronomers found that the expansion of the universe is speeding up, which points to a small positive value. They won the Nobel Prize in Physics in 2011. Physicists still debate how much this success shows.

How strong is the fine-tuning argument?

A common mistake is to treat fine-tuning as a finished proof, either of a designer or of a multiverse. It is neither. It is a puzzle with several serious objections.

The first objection is that we only know life as we know it. Perhaps other kinds of life could exist under other constants. This objection is weaker than it sounds, because many changes would remove stable atoms or stars altogether. But it shows how little we know about what life needs.

The second objection concerns changing one number at a time. Rees’s example changes the strength of the nuclear force and keeps everything else fixed. Later calculations found that plenty of hydrogen survives even if the force is up to about 50 percent stronger. So some windows are wider than popular books suggest.

The third objection is about probability. Claims such as “a chance of 1 in 10 to the power of 120” assume that we know how likely each value of a constant is. We do not. If any number is possible and every number is equally likely, every limited range has a probability of zero. Then the numbers lose their meaning.

The fourth objection concerns a famous story. Fred Hoyle is often said to have predicted a special energy state of carbon in 1953 because life needs carbon. The historian Helge Kragh showed that Hoyle and his colleagues did not link the prediction to life at the time. That link was added in the 1980s.

My view is this. Some cases, especially the cosmological constant, are real puzzles that physicists themselves take seriously. But physics alone cannot decide between luck, a multiverse and a designer. Fine-tuning is a good reason to look for deeper theories. It is not a proof of any worldview.

Learn more

No single person discovered fine-tuning. The astronomer Brandon Carter introduced the term “anthropic principle” in 1973 in Kraków, at a conference marking the 500th birthday of Copernicus. Earlier, in 1953, Fred Hoyle’s work on carbon had shown how sensitive the making of elements in stars is. John Barrow and Frank Tipler collected many examples in their book The Anthropic Cosmological Principle (1986). John Leslie developed the philosophical arguments in Universes (1989). Martin Rees later explained the six most important numbers for a general audience.

Rees’s short book Just Six Numbers (Affiliate Link) is the best next step if you want each constant explained without formulas. For a related question, read Why Is the Night Sky Dark? Olbers’ Paradox, Explained. It shows how a plain observation, the dark night, tells us that the universe has a limited age.