As discussed in my previous two posts our Universe appears to be very finely tuned and is governed by four fundamental interactions. A relatively small increase or decrease in the strength of one of these interactions, such as making the force of gravity weaker or stronger, would make the emergence of life impossible.
The four fundamental interactions (also known as the four forces). Although, sometimes considered to be fundamental, the nuclear force which binds together protons and neutrons into atomic nuclei, isn’t. It is caused by the strong interaction.
So, this fine-tuning raises the natural question.
Why do these forces have their particular relative strengths?
For example: why is the electrostatic attraction between a proton and an electron in a hydrogen atom 39 orders of magnitude stronger than the gravitational attraction? Why is the attractive nuclear force, between two protons 10-15 metres apart, 100 times stronger than the electrostatic repulsion between them?
From my point of view there can be three possible explanations. In this post I’ll go through each one but it’s up to you to decide which you prefer!
Possibility 1 There is some deep underlying theory which explains our Universe
This hypothetical theory is often called the theory of everything . If such a theory exists it would be an all-encompassing, theoretical framework of physics that fully explains all physical aspects of the Universe. The relative strengths of the four forces and perhaps even the density of matter, dark matter and dark energy (which are needed to explain the way our Universe evolves) would follow from this theory. A theory of everything would have to include quantum theory (which Einstein’s theory of general relativity doesn’t) and would explain for example what happens at the very centre of a black hole. Does a singularity of zero volume, and thus infinite density, really occur?
For decades physicists have been looking for such a theory, and despite many advances in theoretical physics, none has ever been found.
Finding a theory of everything would follow on from the work performed over the last 200 years to discover the underlying physical laws of our Universe. In the late 19th century James Clark Maxwell, one of the greatest ever scientists, unified electricity and magnetism into electromagnetism. He was able to show that light was a wave of oscillating electric and magnetic fields. In the nineteenth and early twentieth centuries, physicists determined that many forces, which had been thought of as separate phenomena e.g. the friction which occurs when an object moves against a rough surface, air resistance and air pressure were caused by forces between molecules. The root cause of these intermolecular forces is the electromagnetic force.
As discussed in a previous post the chemical bonds binding atoms together into molecules, on which the whole of the science of chemistry is based, are the result of a combination of the electromagnetic force and quantum theory – which explains how the world operates at very small scales
A theory of everything might not explain our Universe
However, it is possible that even if a theory of everything exists (which of course it may not!) it does not explain why our universe is finely tuned. It could leave the relative strength of the four forces as free parameters meaning that they could have any values. We just happen to live in a universe with the particular values necessary to support life. It is also possible that even if the relative strengths of the four forces were explained by the theory, the values for the density of matter, dark matter and dark energy in our Universe which are crucial for the emergence of galaxies, stars and eventually intelligent life were not.
Possibility 2 Our Universe has been set up to allow for the emergence of life
Another possibility is that the fundamental parameters have been set in some way (perhaps at the instant of the big bang when our Universe came into existence?) In other words, the relative strength of the four forces and the amount of matter, dark matter and dark energy have been in some way selected so that, over billions of years galaxies, stars and planets will form, and intelligent life will eventually emerge on at least one planet. If this is the case, then a whole lot of questions naturally arise.
- How are the values of these fundamental parameter set to give rise to a universe on which life can emerge?
- Is there some form of ‘creating entity’ setting these parameters to the correct values?
- Is this creating entity part of our Universe? If so, this causes a problem because such a creating entity would have not have existed before the big bang in which our Universe came into existence.
If we then assume the ‘creating entity’ is not part of our Universe, then we can ask.
- Which universe does the ‘creating entity’ belong to?
- How was this universe created?
Closely related to this is the simulation hypothesis . Put simply this means that the universe we live in isn’t real. Instead everything we experience is a simulation created by a more advanced intelligence. The laws of physics which govern the Universe are just rules which have build into the simulation. The vast majority of cosmologists reject the simulation hypothesis believing it to belong to science fiction but it does have a number of prominent supporters including Elon Musk
Possibility 3 Our Universe is just one of many in a multiverse
Another explanation is that our universe is only one of very many different universes in a multiverse. Throughout this post you may have noticed that I have used Universe (with an uppercase U) to mean the particular universe that we live in. In some of these other universes the laws of physics might be very different from our Universe – making conditions totally impossible for life to emerge. Although I touched on this in my previous post, it is worth considering again a few of the many universes which could not support life.
A universe with much more matter
In some universes there might be much more matter than ours (or the force of gravity might be for example a million times stronger which would have roughly the same effects). In these universes, a period of time after the big bang, its expansion would stop, the universe would then start contracting and it would eventually collapse into a singularity before life had had the opportunity to emerge.
Universes with more or fewer than four fundamental forces
In other universes there might be more or fewer than four forces, or the nature of the forces might be very different. For example, if the nuclear force which binds protons and neutrons into atomic nuclei, did not exist (or was much weaker) and all the other forces were the same, then the only atomic nucleus which could exist would be the hydrogen nucleus. This consists of a single proton. The only atoms which could exist would be hydrogen atoms having a single electron orbiting a proton.
In such a universe clouds of hydrogen gas would collapse under gravity, but stars would never form from these dense clouds because they could not produce any energy by fusing hydrogen into helium (because helium wouldn’t exist). Such a universe would contain dense objects formed by gravitational collapse consisting of pure hydrogen. These might be similar to brown dwarfs. However, there would be no observers around to observe this ‘brown dwarf universe’.
In our Universe brown dwarfs are dense compact objects which have masses between 1.3% and 8% of the Sun. Because of their low mass the pressure at their core isn’t high enough to sustain nuclear fusion of hydrogen into helium. But even though they can’t burn hydrogen, brown dwarfs can still release energy in two ways. The first of these would still be available in a universe in which hydrogen was the only possible element
- As brown dwarfs form by gravitational collapse, the gravitational energy released causes them to heat up and glow faintly at infrared wavelengths.
- Brown dwarfs generate energy by slowly fusing deuterium (heavy hydrogen) into helium. This does not require such high pressures as fusing ordinary hydrogen.
Not only would there be no stars, but with hydrogen being the only element in such a universe, the only type of molecule which could exist would be the hydrogen molecule which consists of two hydrogen atoms. There would be no chemical compounds and thus no life!
Universes with more or fewer than three spatial dimensions
Some alternative universes might have more (or fewer than) than three spatial dimensions. Mathematically it is possible to construct objects such as the tesseract which is the four-dimensional analogue of the cube. Just as the surface of a cube has six square faces, the hypersurface of the tesseract has eight three dimensional cubical cells. It is not possible to construct a tesseract because we live in world with three spatial dimensions, but one can be represented in two or three dimensions (if a little imagination is applied) in the same way that a plan of a three dimensional object can be drawn in two dimensions.
A two dimensional representation of a tesseract
However, it can be shown mathematically that it is only possible for planets to have stable orbits around a star if there are three spatial dimensions (Barrow and Tipler 1986). If there are more than three dimensions, then orbits become chaotic. A slight perturbation would cause a planet to spiral away from the star or to spiral inwards eventually colliding with it. We must therefore conclude that three dimensions are a requirement for a universe to support life.
To sum up…
So, if the multiverse theory is true, most of these other universes will have conditions in which life can never evolve. We just happen to live in one of the very small fraction of universes which have the right conditions for life, but then of course if the conditions in our universe weren’t right for life we wouldn’t be around to observe it! 😉
Update 23 August 2022. A video of this post is available at
Barrow, J.D. and Tipler, F.J. (1986). The anthropic cosmological principle. Oxford Oxfordshire ; New York: Oxford University Press p275