Spooky sighting

In the modern day, science moves quickly. RadioactivityThe release of energy or particles spontaneously by an unstable atomic nucleus. was discovered only in 1896.¹ From there it took less than 50 years to make the atom bomb.

Special subatomic particlesThe minuscule pieces of matter that make up atoms. Scientists have discovered 36 so far. known as "ghost particles" were first theorised in the 1930s.² And now we might be about to see them for the very first time.

Ghost particles are believed to be particles that behave like no other. They are not made up of ordinary matter and so they can pass through other things. That is why they have proved impossible to detect.

So how do you spot something that cannot be touched? Scientists think that in some rare conditions, these ghost particles turn into ordinary particles.

To observe this, researchers at the European particle research centre CernThe European Organisation for Nuclear Research is the scientific body in charge of the Large Hadron Collider. will smash particles into a hard surface and see if the instrument can detect the ghosts as they turn into ordinary particles.

The discovery of ghost particles could get scientists closer to one of the biggest prizes in physics: the fifth force of nature.

In our current understanding of physics, there are four forces of nature. They are gravity, electromagnetism, the strong force and the weak force. Between them they make up the Standard Model of physics.

The problem is that the universe is full of things that we cannot explain using just these four forces. For example, we would expect that after the Big BangThe moment the universe came into being. It is thought that this took place in a kind of explosion from a dimensionless singularity., the expansion of the universe would start to slow down. Instead, it is actually speeding up.

And researchers have found these effects closer to home too. Last summer, a team near ChicagoOne of the biggest cities in the USA. observed a kind of particle called a muon behaving in ways that the Standard Model cannot predict.³

The best explanation scientists have for this is that there are forms of being that we cannot currently detect, known as "dark matterA hypothetical substance that can not be detected by scientific instruments. Its existence is inferred from evidence that the universe consists of more mass than we can perceive." and "dark energyAn unknown form of energy causing the expansion of the universe to accelerate.". They could make up as much as 95% of the universe.⁴

With this new experiment we might finally be able to track it down.

And that would get us closer to explaining how the universe really works. Some physicists believe dark matter and dark energy cause a force of nature that we do not fully understand known as "quintessenceA hypothetical form of dark energy that could explain the accelerating expansion of the universe. ".⁵

So we might finally be on the brink of discovering the force that is causing the effects that we do not understand.

Is there a fifth force of nature?

Yes: The evidence is piling up that we only know a small fraction of what goes on in the universe. There must be an additional force that can help us explain all this.

No: The Standard Model is not necessarily wrong just because it cannot explain some things. The four forces we do know about might just work in ways we have not yet discovered.

Or... The Standard Model is only that, a model. It might be that as we make more discoveries we have to abandon thinking about a fixed number of "forces" in the universe entirely.