Is the universe fine-tuned for life?

What exactly seems fine-tuned?

The Stanford Encyclopedia’s fine-tuning entry lists several cases that physicists have studied. Here are a few, in plain words.

• Gravity. If gravity were much weaker, galaxies, stars and planets would not form. If it were a little stronger, stars would be smaller and die sooner.
• The strong force. Stronger by more than about 50%, and almost all hydrogen would have burned up in the early universe. For making both carbon and oxygen in stars, the window is far narrower, as the 2000 study in Science found.
• The weak force. About ten times weaker, and long-lived stars like the Sun, which burn hydrogen, would not exist.
• The energy of empty space. Theory suggests it should be vastly larger than it is, by a factor somewhere between a 1 followed by 50 zeros and a 1 followed by 123 zeros, depending on assumptions. Yet only values a few powers of ten larger than the real one would still let galaxies form.

Some numbers in nature are also wildly different in size. The same entry notes that, between an electron and a proton, electromagnetism is stronger than gravity by roughly a 1 followed by 40 zeros. CERN puts it simply: a small fridge magnet out-pulls the gravity of the whole Earth. CERN’s page on extra dimensions opens with that very puzzle, why gravity is so much weaker than the other forces, and explores one possible answer.

Why is the number 137 famous?

The fine-structure constant, written with the Greek letter alpha, measures how strongly light and electric charges interact. It was introduced by Sommerfeld in 1916, according to NIST, the U.S. national measurement lab. It has no units. It is a pure number, very nearly 1/137, so its value is the same whether you use meters or miles.

That is why it fascinates physicists. Quanta Magazine reports that Richard Feynman called it “a magic number that comes to us with no understanding,” and that Paul Dirac called its origin “the most fundamental unsolved problem of physics.” Quanta adds that physicists have more or less given up trying to explain where the value comes from. Many now accept that the constants could be random, set by chance as the universe began. They still measure alpha as precisely as they can, because precise values test the Standard Model, the main theory of particles.

One detail surprises many readers. NIST explains that alpha grows at high energies, to about 1/128. So NIST calls the famous number 1/137 “not unique or especially fundamental.” It is the low-energy value.

Could the laws or the speed of light have been different?

Physicists have tested this idea seriously. A review by Jean-Philippe Uzan lists many checks: atomic clocks, light from distant quasars, the timing of pulsars, the afterglow of the Big Bang, and the making of the first elements.

In 2020, a team led by Michael Wilczynska studied light from a quasar so far away that the light left it when the universe was only 0.8 billion years old. They found no evidence that alpha has changed over time. When they combined these with older data, the data slightly preferred a model in which alpha varies across space. The team called this preference marginal, and it is not established. The Stanford Encyclopedia says ideas of constants that vary across space seem disfavored by the evidence.

What about the speed of light? In 1998, Andreas Albrecht and João Magueijo explored what would follow if light traveled faster in the early universe. They argued this could solve some cosmic puzzles, such as why space is so flat. Their paper is a theoretical proposal; it does not report any observation of faster light. M. J. Duff argues that asking whether the speed of light alone changed has no clear meaning, since its number depends on our choice of units. Changes in pure numbers such as alpha, he says, are a fair subject for testing. Not everyone agrees: Duff’s paper includes appendices answering criticisms from other physicists and remarks by Magueijo. Since 1983, the speed of light has been fixed by definition in our units, at 299,792,458 meters per second.

What are the main answers, and the strongest replies?

A designer

The Stanford Encyclopedia says many regard fine-tuning as the strongest form of the design argument that modern science offers. Defenders such as Swinburne and Collins argue that a life-friendly universe is more likely if God exists than if not. Leslie offers a famous story, as an analogy: a prisoner faces a firing squad, all the marksmen fire, and he is still alive. He wonders whether they meant to miss. Leslie and others argue it would be very implausible for him not to suspect it. (The analogy breaks in one place: we know firing squads have people who plan them, which is the very thing in question for the universe.)

Strongest reply: Elliott Sober and Jan Narveson argue we are in no position to predict what a bodiless cosmic designer would want or do. As Sober puts it, our judgments about design rest on what we know of human designers. Defenders such as Rota answer that the argument still has force, because a life-friendly universe without a designer would be so utterly unexpected. The debate continues.

Many universes

If there are countless universes with different constants, some will allow life, and we must live in one of those. Physicists such as Susskind, Greene and Tegmark regard this step to a multiverse as a rational response to fine-tuning. Our page Could there be other universes beyond ours? looks at it closely. Whether our own space goes on forever is a separate puzzle, covered in Is the universe infinite?

Strongest reply: Ian Hacking identified the “inverse gambler’s fallacy” in 1987. If you walk into a casino and see a lucky roll, you should not conclude that many rolls probably came before. Critics say inferring other universes from our lucky one makes the same mistake. Many philosophers reject that charge. McGrath, for example, argues that the casino story misleads, because we could not have found ourselves in a universe unfit for life. The Stanford Encyclopedia adds that concrete multiverse ideas are extremely difficult to test.

Luck and observer selection

Carter’s weak anthropic principle points out that we could only ever observe conditions that allow observers. The Stanford Encyclopedia adds a simple point, a view shared by writers such as Gould: any exact run in a long series of coin tosses is very unlikely, yet it needs no special explanation. Perhaps fine-tuning is like that.

Strongest reply: Leslie agrees that unlikely events do not always need explaining. But he argues that when good candidate explanations are on the table, we should not dismiss fine-tuning as how things happen to be.

We don’t know yet

Einstein hoped a final theory would fix every constant, leaving nothing to tune. Cosmic inflation, a brief burst of very fast expansion in the first instant, is the model case. If it is right, it explains why space is so flat, which older accounts treated as fine-tuned. Not everyone accepts its successes. See What is a theory of everything? for where that search stands.

Strongest reply: The Stanford Encyclopedia notes that string theory, a leading candidate, seems to allow an enormous number of possible sets of laws, not one. And unlike flatness, most other constants show no clear pattern that a theory might explain.

What do people often get wrong?

“Fine-tuned means someone tuned it.” In physics, the word only describes sensitivity. Whether anyone did the tuning is the open question.

“Every constant sits on a knife’s edge.” The ranges differ a lot. Fred Adams argues that life-friendly ranges are quite considerable, and that a universe could even have been more friendly to life than ours. Luke Barnes disagrees when many constants change at once.

“It is about humans.” The arguments concern life of any kind. The worry is that different numbers would give a universe with far less structure, such as no stars or no atoms beyond hydrogen.

Where can people of faith and skeptics agree?

• The physics is shared. The measured values and the sensitivity studies are the same for everyone. The disagreement is about what they mean.
• Each view has serious thinkers. The Stanford Encyclopedia lists careful defenders and critics of design, of the multiverse and of the luck answer.
• New physics can surprise both sides. Inflation may explain one case that once looked tuned, the flatness of space. The Stanford Encyclopedia doubts that similar accounts will come for most other constants.
• The wonder is real. Whatever explains it, a universe that makes carbon in stars is a remarkable thing to live in.

For the design argument alongside other arguments for God, see What are the main arguments for God’s existence?.

How can you check this yourself?

• Look up the number. NIST lists the official value of the inverse fine-structure constant, 137.035999177, on its public constants site.
• Read the abstract. The 2000 carbon study is free on arXiv (astro-ph/0007178). Note that it gives a window, not an exact point.
• Ask “compared with what?” Any claim of fine-tuning needs a range of possible values. Ask how that range was chosen.
• Separate the two questions. Whether the constants are sensitive is physics. What explains them is partly philosophy.

The answer stays the same after all this detail. Many physicists find that life depends on some constants in narrow ranges, and why nature has these values remains an open question.