Writing a post to do with Time Lord heartbeats and reading through a section of Time Zero for research, and it's fascinating after the relevant bit to see Eight fully explain the principles of gravitational wave detection (accurately, I'll add, as someone who worked on this stuff during my Master's project):
The Doctor slumped down in one of the folding chairs. He raised his two hands, index fingers extended as if there was a string between them. ‘Gravity waves,’ he said. ‘Not easy things to detect because they affect the world around them. So what do you do? You dig a tunnel. A very long tunnel. Long as you can, in fact. Let’s not worry about the curvature of the Earth for the moment, or how you focus the monitor to detect the source of the waves. . . But just imagine if you will that you have a completely flat tunnel perhaps, what, fifty miles long?’ ‘Fifty miles?’ Nesbitt was wondering where this was leading. ‘More if you can manage it. Then you set up a big laser and shine it along your tunnel. The clever thing now is that you’ve put a mirror at the other end. A perfect mirror, of course – flat, no blemishes, atomically accurate. What happens?’ He snapped his fingers and pointed at Lansing. The Corporal blinked. ‘The laser gets reflected back.’ ‘Exactly. Along the self-same line. And of course you’ve set it up so that the waves of light exactly match. The length of the tunnel from laser-tip to mirror is a perfect multiple of the wavelength of the laser light. Which means?’ The Doctor rippled his fingers, as if drawing the answer out of Lansing. ‘Well, it means the laser exactly matches up with itself.’ ‘Good. Excellent.’ The Doctor leaned forward. ‘Now along comes a gravity wave. They’re very small. And because they change the size of the ruler as well as the thing you’re measuring, you can’t detect them. But. . . ’ He paused and raised his finger, lecture-style. ‘But what happens to the tunnel?’ He waited for a couple of seconds before his expression betrayed his disappointment at the lack of an answer. ‘The tunnel changes length,’ he told them. ‘Just very slightly. The greater the gravity wave, the greater the distortion. And that means?’ ‘Well. . . ’ Lansing looked as Nesbitt, who shrugged. ‘Think about the laser,’ the Doctor prompted. ‘Its waves exactly match, remember. But if the tunnel changes length. . . ?’ ‘They no longer match!’ ‘Exactly right. Light’s wavelength isn’t affected by the gravity wave, so the two waves – the original and the reflected light – get ever so slightly out of synch. Bounce the light up and down the tunnel a few times, or even a few hundred times, and you’ll see the difference even more clearly. Or rather you won’t as it’s still so small. But you can detect it.’
Keep in mind, this novel came out in 2002. The first direct gravitational wave detections were made in 2015! Obviously the theory behind GW interferometers existed much longer and there was a long construction process before LIGO or VIRGO came online, but still interesting to see.
Some funny things to note:
1. Eight suggests building a tunnel 50 miles long, which isn't wrong of course, but is much longer than LIGO ended up needing, only being 4 km long.
2. It's mentioned a bit after that gravitational waves of the scale of nm are detected.
‘So how big are the gravity waves your chums in Whitehall have detected?’ ‘They’re talking in nanometres,’ Lansing said. ‘Except. . . ’ ‘That’s quite big,’ the Doctor admitted. ‘Except,’ Lansing went on, ‘they said the last one was off the scale.’
While the Doctor does say this is quite big, this is kinda an understatement! LIGO, which has an effective length (due to reflections) of 1120 km typically detects changes of length on the scale of 10^-19 m - about 10 billion times smaller! I don't think it's said how long (or effectively long) the detector in the book is, but it must be huge. If the "nm" signal was comparable to those typical signals we see (tbf it's obviously meant to be on the large side, but explicitly normal compared to a much larger detection in the book), that would seemingly correspond to a laser path length a good fraction of a light year!
TBF, the waves are artificial, so they could very well be very powerful, but it's still pretty extreme, and funny the Doctor makes such an understatement after discussing very realistic detection methods. Let's just say that if there was something producing waves that powerful, you wouldn't need miles-long interferometer arms to detect it.
The detection is also made by "chums in Whitehall" which makes it sound like it's from a pretty small lab instrument, not a full observatory like LIGO. Maybe it's using alien technology from UNIT or Torchwood or something? Maybe something bigger on the inside so it can have such a large and powerful interferometer, and that's how they're reading "nanometres"? That's the only way I can think to justify anything, though it's possible I'm missing something from the rest of the book, since I'm only reading part of it.
