A Tides of Time web exclusive as the first of two long-awaited articles by James Ashworth is published. The fifth in his series on the biology of Doctor Who‘s aliens considers the Silence…

THE SILENCE ARE AMONG DOCTOR WHO’S MOST ENIGMATIC ALIENS. Very little attempt is made at explaining their origins or their powers. The Time of the Doctor provides some context: they are genetically-engineered priests of the intergalactic Church of the Papal Mainframe. This body is later known as the Church of the Silence when it becomes dedicated to preventing the Doctor speaking his name, allowing the Time Lords to return to this universe and thereby reigniting their war with the Daleks. They had the ability to make people forget them, and could also fire electrical bolts.

What he must know and what he must never know
– Silent, The Impossible Astronaut

The method by which a creature can be genetically engineered to make someone forget meeting them is beyond the scope of this article. However, the discharge of electricity is indeed possible, and the most obvious parallel in the real natural world is the electric eel. Eels have specialised organs, known unimaginatively as the electric organs, which allow them to perform this feat. Within them, stacks of specialised muscle cells, known as electrocytes, are arranged closely together, and they are linked together by nerves. They are adapted from tail muscles, and as muscles contain a large number of mitochondria, which respire to produce chemical energy in the form of adenosine triphosphate (ATP), the power the organ generates is significant. One side is very bumpy, with a larger surface area, whereas the other is smooth. Down the centre of the cell is another membrane, dividing it in two. Both sides pump three sodium ions (Na+) out, and two potassium ions (K+) in, which leads to an overall relative negative charge inside relative to the outside, due to there now being more positive ions outside the cell than inside.

Both diagrams on this page are from M. Kawasaki, What is an electric fish? http://people.virginia.edu/~mk3u/mk_lab/electric_fish_E.htm

The membrane is now polarised. When a nervous signal is received, it stimulates the release of acetylcholine, a neurotransmitter. This binds to receptors that open ion channels only on the smooth side of the electrocyte, allowing the entry of sodium ions into the cell. This causes depolarisation of the membrane, but only on the smooth side, and means that there is a difference of 0.065 volts on the smooth side, to a difference of 0.085 volts on the bumpy side. This now means the outside of the smooth section is negative, the inside positive, and then the inside of the bumpy side is negative and the bumpy outside positive, which allows a current to flow. These act in series, like a battery, so the charges add up, and each cell fires almost simultaneously. This is because neurons to cells further from the central nervous system have better conductivity, or are shorter, than those to closer cells, that take circuitous routes or are more resistant, to allow simultaneous discharge. Each cell only produces around 0.15 volts, but there are thousands, which generate an overall voltage of 500-600 volts and a current of 1 amp for a second. For humans, this is non-fatal, but the eel’s prey is small, and so this can stun or kill it.

You should kill us all on sight – Silent, Day of the Moon

Electrical discharge from an organism is indeed possible. However, conductivity of electricity in water is much greater than in air, as water contains dissolved ions that can conduct charge, whereas air is neutral, and so can only conduct electricity when it breaks down. When the voltage exceeds 3000 volts per millimetre, the air is ripped apart, creating positive ions and negative electrons, which can now conduct electricity. For example, a Tesla coil can arc through the air, and it has voltages of between 50,000 and 7 million volts. The Silence don’t seem to be dismantling the atmosphere, however, for good reason. A Tesla coil’s arc is over 100 times bigger than a eel’s electric charge, even at the lowest value, and is much greater at the very high voltages. The electrical arcs of the Silence are directed, whereas the Tesla coil’s arcs ground themselves on the nearest, most conductive objects. So while the Silence’s arc is impressive, and certainly helps them fight in episodes like Day of the Moon and The Wedding of River Song, they probably couldn’t generate their arcs in the first place, let alone direct them. Overall then, I would say that this is quite unrealistic, and unless we are going to see the Silence on some underwater adventures in the future, will probably remain so.