Dear Dr B: Is it possible that there is a universe in every particle?

“Is it possible that our ‘elementary’ particles are actually large scale aggregations of a different set of something much smaller? Then, from a mathematical point of view, there could be an infinite sequence of smaller (and larger) building blocks and universes.”

                                                                      ~Peter Letts

Dear Peter,

I love the idea that there is a universe in every elementary particle! Unfortunately, it is really hard to make this hypothesis compatible with what we already know about particle physics.

Simply conjecturing that the known particles are made up of smaller particles doesn’t work well. The reason is that the masses of the constituent particles must be smaller than the mass of the composite particle, and the lighter a particle, the easier it is to produce in particle accelerators. So why then haven’t we seen these constituents already?

One way to get around this problem is to make the new particles strongly bound, so that it takes a lot of energy to break the bond even though the particles themselves are light. This is how it works for the strong nuclear force which holds quarks together inside protons. The quarks are light but still difficult to produce because you need a high energy to tear them apart from each other.

There isn’t presently any evidence that any of the known elementary particles are made up of new strongly-bound smaller particles (usually referred to as preons), and many of the models which have been proposed for this have run into conflict with data. Some are still viable, but with such strongly bound particles you cannot create something remotely resembling our universe. To get structures similar to what we observe you need an interplay of both long-distance forces (like gravity) and short-distance forces (like the strong nuclear force).

The other thing you could try is to make the constituent particles really weakly interacting with the particles we know already, so that producing them in particle colliders would be unlikely. This, however, causes several other problems, one of which is that even the very weakly interacting particles carry energy and hence have a gravitational pull. If they are produced at any substantial rates at any time in the history of the universe, we should see evidence for their presence but we don’t. Another problem is that by Heisenberg’s uncertainty principle, particles with small masses are difficult to keep inside small regions of space, like inside another elementary particle.

You can circumvent the latter problem by conjecturing that the inside of a particle actually has a large volume, kinda like Mary Poppins’ magical bag, if anyone recalls this.


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Sounds crazy, I know, but you can make this work in general relativity because space can be strongly curved. Such cases are known as “baby universes”: They look small from the outside but can be huge on the inside. You then need to sprinkle a little quantum gravity magic over them for stability. You also need to add some kind of strange fluid, not unlike dark energy, to make sure that even though there are lots of massive particles inside, from the outside the mass is small.

I hope you notice that this was already a lot of hand-waving, but the problems don’t stop there. If you want every elementary particle to each have a universe inside, you need to explain why we only know 25 different elementary particles. Why aren’t there billions of them? An even bigger problem is that elementary particles are quantum objects: They get constantly created and destroyed and they can be in several places at once. How would structure formation ever work in such a universe? It is also a generally the case in quantum theories that the more variants there are of a particle, the more of them you produce. So why don’t we produce humongous amounts of elementary particles if they’re all different inside?

The problems that I listed do not of course rule out the idea. You can try to come up with explanations for all of this so that the model does what you want and is compatible with all observations. But what you then end up with is a complicated theory that has no evidence speaking for it, designed merely because someone likes the idea. It’s not necessarily wrong. I would even say it’s interesting to speculate about (as you can tell, I have done my share of speculation). But it’s not science.

Thanks for an interesting question!