Rhian Holvey takes us on a tour through physics and fiction, dark matter and dust.
Philip Pullman’s ‘His Dark Materials’ trilogy is undoubtedly one of the most popular and controversial fantasy series on the current market. It is a story about a girl, Lyra, from a parallel universe and a boy, Will, from our own. The series is wide-ranging and covers many themes, including religion, love, humanity and even concepts from modern scientific research. Perhaps the most obvious of these theories is that of parallel worlds; but in fact the core concept of the trilogy, even its title, points to another: Dust, shadows or dark matter.
Dust
The concept of Dust is introduced almost from the very beginning of ‘His Dark Materials’. It is first mentioned when Lyra hides in the retiring room and hears Lord Asriel giving a presentation to the Jordan College scholars. In it he shows a ‘photogram’ (slide) in which a special solution has been used to show up a glowing collection of particles falling down upon a man and it is these he identifies as Dust. An explanation of this is given later on. When Lyra enquires as to what these particles are, she is told:
"Dust is what makes the alethiometer [Lyra’s truth telling device] work…. You’ve heard of electrons, photons, neutrinos, and the rest? They’re called elementary particles because you can’t break them down any further…"
This idea is built upon in the second book (‘The Subtle Knife’) when Lyra visits Dr. Mary Malone, a physicist researching what she calls ‘shadow particles’ or ‘dark matter’ in the University of Oxford. Mary has been using a machine to research the strange phenomenon and is surprised to find that Lyra’s visit proves conclusively that the particles are conscious.
Although there is no evidence that any such conscious particles exist, dark matter is a real field of research and interesting parallels can be seen between the books’ description and reality.
Missing Masses
Dark matter was first postulated by a Swiss scientist, Fritz Zwicky, in 1933 due to his studies of the velocities of galaxy clusters, but no further evidence was found until the late 1960’s, when Vera Rubin was studying spiral galaxies. The force acting on such a galaxy can either be described by Newtonian gravity or centrifugally; the equations for both are shown below:
G.M(r).m mv2
Newtonian gravity: F= r2 Centrifugal force: F= mrw2 = r
Where M(r) is a function of mass with distance from the galaxy centre (r), m the mass of a single star, w the angular velocity and v the linear velocity.
As the forces are equal, these equations can be combined and, after rearrangement, the linear velocity can be given as a function of the gravitational force and M(r):
Thus, as r increases, a point is reached where the mass outside this radius is negligible, i.e. M(r) ~ MT (or the total mass of the galaxy) so the velocity is expected to taper out as it approximates to (a constant/ r)1/2. This does not occur since most stars orbit at the same speed. The simplest solution to this problem is to assume that there is a significant proportion of mass outside the containment area. In some cases, more than 50% of the mass of the galaxy must be within the relatively dark galactic halo.
These masses cannot be accounted for by observed bodies and are only detectable through observation of galactic gravitational forces. Hence the name ‘dark matter’ as, since it does not emit or reflect electromagnetic radiation, it cannot be observed directly. This gives the first parallel with the Dust of Pullman’s novels, for it is invisible to the naked eye and can only be detected or observed by use of special equipment such as Mary Malone’s machine in Oxford or, later, the amber spyglass.
So how does dark matter interact? There are four known forces: gravity, electromagnetism, strong (that which holds nuclei together) and weak (that which governs radioactive decay). We have already ruled out electromagnetism and dark matter’s gravitational effects have already been discussed, leaving strong and weak forces. Strong forces, however, have too short a range: 10-15 m. Thus, weak forces give the best chance of detecting dark matter.
The Nature of Dust
The exact composition of dark matter, even the question of its existence, is still being debated. Many suggestions as to its nature have been proposed, from ideas wildly different to Dust to models where the leap is not hard to achieve.
To begin with models unlike Dust, it has been suggested that astronomical bodies that do not shine could be responsible for the undetected masses in galaxies. The main entities in this group are brown dwarfs: stars which never got large enough to achieve hydrogen nuclear fusion. These “stars” can have up to 80% of the mass of our sun but are also very dense, having powerful quantum effects which make them a good candidate for dark matter. Other bodies include normal planets (which are difficult to detect even when eclipsing their star), clouds of non-luminous gas and black holes. These are collectively called MACHOs -MAssive Compact Halo Objects – an acronym invented mainly to match the similar WIMPs discussed below!
These entities seem to fit the bill for dark matter, as they are suitably massive whilst being undetectable by electromagnetic radiation. However, planets and asteroids are not favoured as a model for the missing matter since planets like Earth are not heavy enough to account for it. Black holes are and though they remain undetectable by light, as this cannot escape, large ones can be detected by the tumultuous activity at their limits, thus rendering them an unlikely explanation.
Another theoretical candidate for dark matter is a new type of elementary particles called WIMPs (Weakly Interacting Massive Particles) which, as the name suggests, have mass but interact primarily by weak forces and not electromagnetism. Dust is frequently described as an elementary particle in the trilogy and certain of their properties (their smallness and presence throughout the universe, even their penetration of matter) suggest WIMPs as the “real” counterpart of Dust. The smallness of WIMPs is overcome by proposing that many of them exist, and there is often talk of “clouds” of such particles which normal matter moves through unimpeded.
Neutrinos in particular have been proposed as a candidate for dark matter as they are uncharged and found in b-decay (i.e. have weak force interactions). The issue with this theory is that for a long time neutrinos were thought to have no mass and thus could not be responsible for the gravitational effects on galaxies. Relatively recent studies on the neutrinos released from the b-decay occurring during fusion of hydrogen to helium in the Sun indicate otherwise. It is possible, though difficult, to detect the neutrinos reaching Earth and it is found that the quantity is a third of that expected. There are in fact three types of neutrino: electron, muon and tau neutrinos of which the Sun produces the first, and these are the ones detected. If the neutrinos have mass, however, it is possible for them to undergo neutrino oscillation, converting them into the different forms. This is thought to be the reason for the neutrino deficit, as two thirds are converted into the undetected types and thus neutrinos must have mass.
However, the theory of neutrinos being the missing mass in galaxies is not widely accepted as they cannot pack closely enough at galactic centres due to their repulsiveness. They also move too fast and thus would not clump together to encourage the formation of galaxies.
Other forms of WIMP have not been ruled out. In the process of trying to construct a unified theory of electromagnetism and weak forces, a postulate known as supersymmetry (one first used in 1973 for early string theory and subsequently applied elsewhere) was used. This theory suggests that for each boson there is a “superpartner” which is a fermion, e.g. photons and their partner photinos. This theoretically doubles the number of particles known, but it is thought that partners may be much heavier than their counterparts and thus would require much higher energies to be created. This heaviness makes them favourable candidates for dark matter; a particular example of such a postulate is the neutralino which would be heavy and long-lived as well as being weakly interacting. These particles would be able to pass through normal matter and thus would be hard to detect. Experiments would require high sensitivity as information from the detection of dark matter must be removed from background signals due to normal matter. This draws another parallel with ‘His Dark Materials’, as the amber spyglass made by Mary Malone can still see normal matter while making the eye sensitive to Dust, converting its image somehow into light (an unlikely candidate for a detector outside of fiction
ue to the seeming dependence on electromagnetic radiation!). Current experiments on the detection of WIMPs show conflicting results and thus the question of whether this is dark matter remains unresolved.
The Future?
Despite the fiction of Mary Malone, in recent years a new theory of dark matter has been proposed by researchers in the University of Oxford. They propose a dark matter particle too light to detect or clump together but which can collide with denser dark matter in galaxy centres. Such a collision would, in theory, produce pairs of electrons and their antiparticles, positrons, which would move off at high speed until annihilation and production of a high energy photon beam (gamma rays). The interest of this theory is that such beams have already been detected coming from the centres of galaxies, but research is too early to draw conclusions yet.
The fact remains that dark matter is still a mystery and that any or all of the theories could prove in some way to be correct. It may even be possible that future research will disprove the existence of dark matter entirely, attributing the “missing mass” to modified gravity or quantum effects. Or perhaps, however unlikely, it may be revealed that dark matter is conscious, as in the case of Philip Pullman’s elusive Dust.
