What is the Anthropic Principle?

In his 1988 book ‘A Brief History of Time’ the British theoretical physicist Stephen Hawkins (1942-2018) stated:

‘The laws of science, as we know them at present, contain many fundamental numbers, like the size of the electric charge of the electron and the ratio of the masses of the proton and the electron. … The remarkable fact is that the values of these numbers seem to have been very finely adjusted to make possible the development of life.’

In this post we will talk about the view that the laws of physics, and the properties of the Universe as a whole, are somehow finely tuned to allow our existence. The term ‘anthropic principle’ was coined in 1973 by the Australian physicist Brandon Carter (1942-) to describe this viewpoint. However, these ideas had been circulating for decades beforehand.



Brandon Carter – image from Wikimedia Commons

Since 1973 the ideas behind the anthropic principle have been reviewed in many books, both popular science and those aimed at the more specialised reader. They were developed in detail in a book written in 1986 by the theoretical physicists John Barrow and Frank Tipler called ‘The Anthropic Cosmological Principle’.

Background: atoms, the atomic nucleus and the four fundamental forces

Before we go into detail about the anthropic principle it is worth discussing, at a high level, the structure of matter and the fundamental forces which drive the way that everything in the Universe behaves.

All ordinary matter in objects like ourselves, planets and stars is made up of atoms. An atom consists of a central nucleus, which has a positive electric charge, surrounded by a cloud of negatively charged electrons. Atoms are very small, typically around 0.0001 microns in diameter (a micron is a millionth of a metre). However the nucleus, which contains nearly all the mass of the atom, is much, much smaller, typically around 0.000 000 001 microns in diameter.

The nucleus consists of a number of protons, which have a positive electric charge and neutrons which have no electric charge. Because the electrons have a negative charge, and the number of protons and electrons in an atom is always the same, atoms have a net charge of zero.

• The number of protons in the nucleus is called the atomic number and determines the atom’s chemical properties. You may remember from high school chemistry that the atomic number gives the position in the periodic table.
• The number of neutrons in the nucleus does not affect the chemical properties of the atoms. In fact, all elements have different versions of themselves called isotopes, which have a different numbers of neutrons but the same number of protons.

The simplest possible atomic nucleus is that of hydrogen, which consists of a single proton. Ordinary hydrogen atoms have no neutrons, but a small fraction of naturally occurring hydrogen atoms are deuterium or heavy hydrogen which have one proton and one neutron. Atoms which have 2 protons (regardless of the number of neutrons) are helium atoms, those which have 3 protons are lithium atoms and so on. The element with the highest atomic number which naturally occurs on Earth is uranium, which has 92 protons.







An atom of the most common isotope of carbon has 6 protons and 6 neutrons in the nucleus surrounded by 6 electrons. Other isotopes of carbon are found on Earth which have 7 and 8 neutrons in the nucleus.

The Universe is governed by four fundamental forces. All other interactions, such as the combination of two hydrogen atoms and one oxygen atom to form a water molecule, are due to these fundamental forces.
Gravity – an attractive force which acts on all particles having mass.
Electromagnetic force – This only acts on electrically charged particles and can be an attractive or repulsive force. If two particles have the same charge, such as two protons, the force is repulsive. If two particles have different charges, such as a proton and an electron, the force is attractive.


Strong force – this force acts on fundamental particles called quarks. There are six types of quark, which have the rather odd names of up, down, charm, strange, top, and bottom.
a proton consists of two up quarks and a down quark
a neutron consists of one up and two down quarks



Internal structure of a proton – image from Wikimedia Commons

Because protons and neutrons are made up of quarks they too are acted on by the strong force. The force between protons and neutrons which is sometimes called the ‘residual strong force’ or ‘nuclear force’ only works over extremely short ranges, of less than 2.5 femtometres, where 1 femtometre (fm) is one thousand trillionth of a metre. It is this nuclear force which binds protons and neutrons together into atomic nuclei. For more details on the strong force see the notes at the bottom of this post.
Weak force – this acts over a short distance, 0.01 fm to 0.1 fm. It is this force which is responsible for a particular type of radioactive decay called beta decay. Without the weak force there would be insufficient oxygen produced in stars to support life (Clavelli 2008).

In addition to these four forces there are other fundamental parameters such as the mass of the proton and electron and the average densities of ordinary matter, dark matter and dark energy in the Universe. 

Fine tuning

One thing that is apparent is that the relative strength of the four forces (and other fundamental parameters such the density of matter in the Universe) appear to be finely tuned to enable the production of stars, planets and the eventual evolution of intelligent life. If they were only slightly different, the Universe would be a very different place and life would never emerge. To illustrate this I’ll give some examples below.
Hydrogen is the most common element in the Universe. In the early Universe 10 to 1000 seconds after the Big Bang, when it was at a temperature of billions of degrees, about 25% of the primordial hydrogen was converted to helium by nuclear fusion. If the strong interaction had been only 2% stronger than its current value then all its hydrogen would have been converted to helium in the first few minutes of the Universe’s existence. There would be no hydrogen compounds in the Universe, such as water which is, as far as we know, essential for life.
If the strong interaction were only a few percent weaker, then deuterium (heavy hydrogen) would not be stable. This would mean that certain elements essential for life such as nitrogen and phosphorous, which are made in the centre of stars by nuclear reactions which fuse deuterium with other nuclei, would not be formed to any appreciable degree.
If the electromagnetic interaction were three times stronger, and all other forces the same strength, then any element heavier than carbon (atomic number = 6) could not form. For these elements, the repulsive electromagnetic force between the protons in the nucleus would be stronger than the attractive nuclear force holding the nucleus together. Therefore elements such as nitrogen and oxygen on which life is based would not exist.


If gravity were a thousand times stronger, then stars would be much smaller and burn their nuclear fuel more quickly. Instead of living for ten billion years, a typical star would live for about 10 million years. As readers of this blog will recall, it took hundreds of millions of years from when the Earth was formed until the emergence of the first single-celled lifeforms. So, in a Universe with stronger gravity, these mini-suns and would have stopped shining before even the first steps in evolution had started.


If the strength of all the forces were the same, but there were much less matter in the Universe, then the way the Universe evolved would have been very different. In our Universe, initial unevenness in the distribution of matter in its early stages eventually became the structures such as stars and galaxies which we see today. If the density of ordinary matter were 10% of its current value, then structures such as stars and galaxies would not have formed.

The strong and weak anthropic principles

In his 1973 work Carter distinguished between the strong anthropic principle (SAP) and weak anthropic principle (WAP). Since then there have been many slightly different definitions of the WAP. The one below is from Barrow and Tipler (1986):

‘the observed values of all physical and cosmological constants are not equally probable but they take on values restricted by the requirement that carbon-based life can evolve..’

The WAP is generally accepted by most astronomers. In fact it has been criticised as a tautology – a statement which must be true. For if conditions were very different, so that life couldn’t evolve, humanity wouldn’t be around to observe them. Even so, it is remarkable how finely tuned the Universe is.

Many physicists believe in the multiverse – a collection of possibly an infinite number of other universes.



In other universes different relative strengths of the fundamental forces might apply, so they would look very different from our Universe In some of them there might be more or fewer than four fundamental forces, perhaps even more than three dimensions of space. It seems likely that in the vast majority of these other universes conditions are such that life can never evolve.

While the WAP is generally accepted, the SAP is more contentious. The definition given by Barrow and Tipler is as follows:

‘The Universe must have those properties which allow life to develop within it at some stage in its history.’

A variation to this definition was proposed by the physicist John Archibald Wheeler (1911-2008) in 1977, which he called the participatory anthropic principle.

‘Observers are necessary to bring the Universe into being’

What the SAP is saying is that we cannot have a universe which doesn’t have, or have the potential to have, any observers. In some way the purpose of the universe it to give rise to intelligent observers. The SAP is not generally accepted by most astronomers. One particular criticism of it is that we cannot falsify it by observing a ‘dead universe’ in which observers cannot exist. This is because other universes are, by definition, unobservable.

Final Anthropic Principle

Even more controversial is the Final Anthropic Principle (FAP). This idea was developed by Barrow and Tipler. They could see little point in having a Universe which has as its purpose giving rise to intelligent observers and these intelligent observers then become extinct. The FAP is defined as follows:

Intelligent information-processing must come into existence in the universe, and, once it comes into existence, it will never die out.

In the final chapter of The Anthropic Cosmological Principle they outline a future of the Universe in which the FAP is true. In this universe billions of years after the Big Bang intelligent carbon-based life (i.e ourselves ) eventually emerges. Over a long period of time as the civilisation develops it evolves into different forms of life, which are more robust and better able to survive the harsh conditions of interstellar travel and the long timescales needed. These new lifeforms will not be carbon based but could be for example intelligent self-replicating robots. The civilisation then spreads to neighbouring stars and eventually spreads throughout the galaxy.



Eventually it spreads to neighbouring galaxies and through the entire Universe. If in the far future, the Universe’s expansion slows down and stops and it starts contracting, it may end in what is termed a singularity of infinite density where the Universe will come to an end and space and time will cease to exist. Barrow and Tipler called this point in time the Omega Point.



At the Omega Point life will have gained complete control of the Universe and will be able to process an infinite amount of information. The final two sentences of The Anthropic Cosmological Principle state:

‘At the instant the Omega Point is reached, life will have gained control of all matter and forces not only in a single universe, but in all universes whose existence is logically possible; life will have spread into all spatial regions in all universes which could logically exist, and will have stored an infinite amount of information, including all bits of knowledge which it is logically possible to know. And this is the end’

In general the scientific community reacted with scepticism to the FAP. To many it smacks of pseudo-science. In a review of The Anthropic Cosmological Principle the American science writer Martin Gardner (1914-2010) called it a ‘Completely Ridiculous Anthropic Principle’ which he suggested should be abbreviated to CRAP. In addition, although in the 1980s many astronomers believed in a Universe which wold would eventually collapse into singularity this is not supported by current observations. The general consensus is that we live in a Universe which will expand forever and so there will be no Omega Point.

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