If an unstable particle decays into other particles, did the resulting particles exist 'inside' the original particle?

If you break an object and water comes out of it, in everyday life, we think that the water was originally inside that object. However, in the world of elementary particles, such common-sense thinking does not hold. For example, a particle called a neutron is unstable in a vacuum and, over time, decays into three particles: a proton, an electron, and an antineutrino. This is a phenomenon called beta decay. However, based on this fact, physicists do not say that 'a neutron contains a proton, an electron, and an antineutrino'.

Inside a neutron, there are not a proton, an electron, and an antineutrino, but three quarks.

The fact that these tiny elementary particles called quarks are inside nucleons such as protons and neutrons was demonstrated in experiments called 'deep inelastic scattering' conducted in the last century. For example, when high-energy electrons are collided with protons, most of the electrons pass straight through, but there is a phenomenon where, with a certain probability, the electrons recoil at a large angle different from their direction of travel. This is interpreted as meaning that the inside of a proton is mostly empty, but contains hard, small particles, and the recoil occurs when an electron happens to collide with one of those particles. These small particles are quarks.

Currently, the internal structure of these protons and neutrons is described with high precision by the Standard Model of elementary particles, which is described by quantum fields. The same is true for beta decay, where a neutron decays in a vacuum. It is a process in which a quark called a 'down quark' inside the neutron emits something called a 'W boson' as radiation and changes into a quark called an 'up quark'. The emitted W boson then decays into an electron and an antineutrino.

What is important here is that the down quark did not contain an up quark and a W boson. Also, the W boson does not contain an electron and an antineutrino. This is the same as not thinking that when an accelerating electron emits a photon, that photon was inside the original electron from the beginning. An electron is an excitation that occurs in the Dirac field, and the excitation of that field, by coupling with the electromagnetic field, creates a photon as a wave inside the electromagnetic field. In other words, it is the image of one wave creating another wave. The new wave that emerged was not inside the original wave. The same thing is happening with the down quarks and W bosons in beta decay.

If a neutron only contained the three stable particles of an electron, a proton, and an antineutrino, there would be phenomena that could not be understood. This is called 'electron capture by a proton'.

A proton can capture an electron and emit a neutron and a neutrino. If only the stable particles of an electron, a proton, and an antineutrino were inside the neutron, where was the new neutrino that came out in this electron capture process hiding at the beginning?

This puzzle has already been solved in elementary particle physics. An up quark in a proton emits an anti-W boson and becomes a down quark, which as a result turns the proton into a neutron. And the electron absorbs the emitted anti-W boson to become a neutrino. In the end, it is not the case that a neutron contains a proton, an electron, and an antineutrino.

The question of what things are made of is not as simple as it seems at first glance. In April 2026, a report was published by an experimental facility called CERN in Switzerland regarding whether there is further internal structure within quarks or if there are smaller constituent particles.

According to that, there is currently no evidence that quarks are made of smaller elementary particles. From the last century to the present, several theories of 'subquarks', which are smaller elementary particles that make up quarks, have been proposed, but current experimental data does not show the existence of such subquarks at all.

On the other hand, another perspective on the origin of things is attracting attention in theoretical physics. It is the idea that what constitutes everything is not microscopic unknown elementary particles, but 'quantum information' that has no weight, shape, or color. This idea, that spacetime and matter emerge from this quantum information, is called 'It From Qbit' and is currently being researched by many physicists around the world.