Could the neutrino be the most interesting particle in physics?

Flaka Tahiri – Year 12 Student

Flaka Tahiri (Year 12) asks whether the neutrino is the most interesting particle in physics, in response to the recent Write a Science Blog competition advertised in The GSAL Journal. This excellent essay demonstrates Flaka’s passion and curiosity in a complex field of physics. CPD

As the science of the universe, physics aims to understand the events of the world on several different scales. Consider the extremes of this scale: there is research into particle physics, belonging to the smallest end of the scale. This type of scientific study investigates particles as tiny as 10^-18m (quarks); whereas, on the much more massive scale, the largest galaxy we currently know about (IC 1101) is estimated to be millions of light years wide, with one light year being 9.461×10^15m for comparison. Whilst subatomic particles, and planets and stars belong to opposite ends of the scale, with their own disciplines dedicated to developing understanding of each independently, in order to truly understand the extremes of the universe scientists must apply their knowledge of the tiniest elements of matter to a much grander scale.

One of the greatest mysteries within physics today is dark matter. Dark matter was postulated in the 1900s, and observations made by astrophysicists supported the theory that there must be more within the universe than just the normal matter that can be seen and detected. This is because it was theorised that, in spinning galaxies, they must spin faster in densely packed areas where the mass is concentrated. However, as you go towards the edges of the galaxy, where the mass decreases because of the more sparsely spread out matter, the velocity of the spinning galaxy must decrease. This is because where mass is concentrated, there is a stronger gravitational field strength, resulting in a greater velocity due to the greater centripetal force acting on the mass. This can be seen using the equation: Fc = (mv^2)/r, where Fc is centripetal force, m is mass, v is velocity and r is the radius of the galaxy.

Galaxy clusters surrounded by hot gas (pink) and dark matter (blue). (Smithsonian Institute)

Rather problematically, however, the observations made were that the galaxies spun with a uniform velocity throughout, regardless of how densely/sparsely packed the matter was. Instead of assuming laws of physics to be wrong, it was thought that this proved that there must be something else in the universe affecting the gravitational field strength of these galaxies, with enough mass that this mystery something’s gravitational pull could cause a galaxy to spin with the same velocity at each point.

Whilst we don’t know what constitutes dark matter yet, there are some things it cannot be. For example, the particles that make up dark matter cannot be charged, as the ‘dark’ in dark matter reflects its physical characteristic of being invisible to the eye – meaning that it doesn’t interact with photons of light in any way. As (virtual) photons are the exchange particle of the electromagnetic force, which affects all particles that are charged, then if dark matter is unable to reflect light then it cannot be made of a charged particle. Furthermore, dark matter cannot be baryonic, as all baryons by nature are unstable apart from the proton; therefore, all baryons will eventually decay into protons, which have a positive charge, and so would be affected by the electromagnetic force.

The first use of a hydrogen bubble chamber to detect neutrinos. (Argonne National Laboratory)

One possible candidate for the particle that makes up dark matter are neutrinos: neutrinos have no charge, they are not baryons, and they rarely interact with matter. This last point is important because, whilst dark matter’s effect on the universe can be observed, dark matter itself rarely interacts with normal matter – making it incredibly difficult to detect directly. However, in order to exert the needed gravitational pull, dark matter must have a specific mass. Whilst it was thought previously that neutrinos had a mass of zero, it was recently discovered that neutrinos have the ability to change flavours, for example from an electron neutrino into a tau or a muon neutrino during its flight – something that could only be considered possible if the neutrino had a non-zero mass.

To conclude, if the neutrino’s mass works out to be the mass needed to account for the dark matter in the universe, it could result in the neutrino serving a much more important function than just conserving energy, as originally thought, instead being responsible for making up approximately 25% of our universe. Flaka Tahiri (Year 12)

References

Ethan Siegal 2019, How much of the dark matter could neutrinos be? Forbes, viewed Tuesday 30/04 2019.

Ian G. Mccarthy 2018, Study suggests the elusive neutrino could make up a significant part of dark matter, The Conversation, viewed Tuesday 30/04 2019.

National Geographic 2019, Dark matter and dark energy? National Geographic viewed Tuesday 30/04 2019.

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