For the first time in the new series, the TARDIS heads into the viewers’ past. Because of this, there are only two science-fiction concepts in the story: a “space-time rift” and energy beings who can inhabit gas clouds and dead human bodies.
The Cardiff Rift has been described as a wormhole, a gateway, a place where disparate parts of space and time meet, allowing beings and objects to travel from one part to another. While wormholes are solid scientific theory and not seriously challenged, they are also transient, unstable, and not big enough to send a water molecule through – much less a complex construct such as a mind or a body. On the other hand, these are certainly staples of science fiction; there would be no DS9 without them, just to name one example. So it’s realistically bad science, but like FTL drives, we can probably wave that issue away.
The energy beings are another common SF trope, but one where I have trouble suspending belief. Setting aside any question of souls, here on Earth, a person’s mind needs a brain and a body to inhabit. Computer software needs hardware on which to run. A radio signal needs a transmitter to generate it, and a receiver to play it back; none of these ‘information patterns’ interface with our world without using something physical. Otherwise, it’s like trying to pull spaghetti out of the pot using the beam of a keychain laser pointer.
And a cloud of gas makes a poor carrier for information. Gas is random, disorganized, subject to disruption by currents and slight temperature variation. Try to use a gas cloud to store your financial spreadsheets, and you probably won’t be happy with the results for long. But the other option in the episode isn’t much better…
At first glimpse, the idea of the loose minds taking over corpses seems an obvious one – the original owner isn’t using it, right? But centuries of embalming technology has blinded modern humans to one fact: bodes decay. Shortly after death, various chemical processes in a body are no longer inhibited or controlled; it doesn’t take long at all before eyes are useless for seeing, internal organs are useless for digesting food, brain tissue cannot carry electrical charges, and muscles will no longer flex and pull. And embalming only disguises these processes, or sometimes makes them worse! A loose mind somehow settling into a corpse’s body would find itself extremely frustrated in short order, unable to use it for any of the most basic functions.
(Yes, this means that all zombie movies are complete BS. But no one watching a zombie flick cares, so we’re good.)
Now, it could be some sort of telekinetic puppetry, such as the Nestene Consciousness used two episodes ago. But specially designed plastic mannequins still seem to make better vehicles than decaying bodies. And again, what are the Gelth aliens using to generate the telekinetic forces? The Nestene at least had a giant organo-plastic brain to work with.
In summary, we’ve got two almost certain impossibilities. Both of them are common in SF, so not many folks are probably going to get hung up on either, but they are bad science nevertheless. And in an unrelated note, why can’t the aliens ever just ask for help? The Doctor would bend time and space to help out if they just asked nicely. Stupid aliens.