The Search for the Graviton

What holds the world together? It sounds like a simple question - but it isn’t.We know of four fundamental forces of nature that seem to be doing the job, but have only found three exchange carrier particles (particles which give rise to the forces between other particles). Physicists like symmetry, and we like to think that the universe does too. Faced with the scenario of more forces than carriers, it is natural for a physicist to assume we’re missing one.

So far, we have found photons, which carry the electromagnetic force (which describes the interactions between charged particles); W and Z bosons, which carry the weak nuclear force (responsible for radioactive decay); and gluons, which carry the strong nuclear force (which holds the nuclei of atoms together). But what about gravity?

Not all types of elementary particles interact with all other types, and only some fundamental particles interact with particles of the same type. For example, photons do not (directly) interact with other photons, or with gluons. Image credit: Public domain

This idea is summarised nicely in an article by science writer Brian Koberlein: in quantum field theory … [you] start with a wave form and then ‘quantize’ it, you break the wave down into the smallest amounts of it that can exist (for example, photons are quanta of light). This has been used with photons, and doing so with gravitational waves leads to the idea of gravitons.

However, this approach encounters some problems when tackling gravity. General relativity tells us that that gravity and the curvature of spacetime are intricately linked. As matter travels through spacetime it causes spacetime to curve around it, and as spacetime curves, this tells matter where to move. It is this motion, caused by the curvature of spacetime, that can be considered ‘gravitation’ ^[8].

How were Mars’ moons formed?

The formation of Deimos and Phobos, the moons of Mars, is still somewhat of a mystery. They were discovered by Asaph Hall in 1877, and observed in 1971 by Mariner 9, a NASA spacecraft orbiting Mars^[1]. Although not the smallest moons, they are much smaller in comparison to Mars than Earth’s moon is to Earth.

Deimos and Phobos have mean diameters 280 and 154 times smaller than Earth’s moon respectively. Images courtesy NASA (Phobos) and NASA/JPL-Caltech/University of Arizona (Deimos), composite by TWDK

Possible Theories of Formation: Asteroid Capture

Despite being known for so long, there is no accepted theory regarding their creation. They appear to be made of “...carbon-rich rock mixed with ice”^[1], and are oddly shaped, which led to the idea that they are captured asteroids. This would also explain their heavy cratering and small size.

An asteroid is captured when it passes a larger mass (in this case, a planet), and is “caught” by the planet’s gravitational field and is forced into orbit. This means that the orbits of captured asteroids are expected to be very eccentric ellipses, meaning that the asteroids pass close by before swinging out further away. The orbits of Phobos and Deimos, however, are almost circular. Because of this, we can’t consider asteroid capture to be the definitive theory of the formation of Mars’ moons.

Introducing Alice

Hello all, I’m Alice and I’m the new SEPnet intern at Things We Don’t Know. I’ve just finished my third year studying physics at Royal Holloway, University of London, so I’ve just got my masters year left to go.

My interest in physics started in secondary school when I was taught about fundamental particles and forces. At that time, science had found neither the Higgs boson nor the Graviton, and I decided then that I would study physics and contribute toward the search. We’ve now found the Higgs boson, but as the Graviton still eludes us, I am writing my Research Review on the work that has been done so far at CERN to find it, or at least, to find where it isn’t.