Saving the world with microscopic gravity sensors Written by Alan Barker, Freelance Writer, British Science Festival Richard Middlemiss is the recipient of the Isambard Kingdom Brunel Award Lecture for Engineering, Technology and Industry. His lecture is called Saving the World with Microscopic Gravity Sensors. Alan Barker felt the pull. Microscopic gravity sensors sound like something in the TARDIS! What are they? A gravity sensor is just that: a machine that can detect gravity. Everything with mass has a gravitational pull. You’re being held on the Earth by gravity. The person next to you also has a gravitational pull, but your attraction to them is about 2 billion times less than the Earth’s. So gravity varies with variations in mass? Yes, but the variations are incredibly small. So, for example, gravity in the sun-moon-earth system creates tides, but it’s not just the seas that move around; the Earth itself expands and contracts over a twelve-hour period, by about 20 centimetres. And that creates a gravity variation. We’re building detectors that can measure variations at that kind of level. How do you detect gravity? Well, Isaac Newton used an apple. The way you measure gravity is by looking at how objects with mass – whether that’s apples or tiny lumps of nanofabricated silicon – move under its influence . Historically, gravity sensors have been big and expensive: about ten kilograms in weight and costing about £100,000, so they’re only available to rich corporations. But you also have a gravity detector in your mobile: the accelerometer that turns the screen round when you move the phone. It’s basically a mass on a spring. When you turn the phone, gravity pulls the mass down and that sends a signal to the electronics to alter the screen aspect. We’re creating accelerometers of the same kind, but with springs ten times thinner than a human hair. So you’re using the same manufacturing process to create a much more sensitive instrument. Yes. It’s called a microelectrical mechanical system, MEMS for short: taking a mechanical structure and miniaturising it. Most of the work on my PhD went into creating the chip that sits at the heart of it. It’s about the size of a postage stamp. Around that you wrap a vacuum tank, and then around that you wrap the electronics to read it out... About a year and a half ago we had all this equipment that filled most of a room. Another PhD student joined the team and helped to shrink it all down. Now I’m working with a fabrication company to shrink some of the components even further... We should have something about smartphone size within a year... And what could you use these sensors for? Traditionally, they’ve been used by oil companies to detect oil and gas. With something this small and cheap, a lot of applications come within reach: detecting activity in volcanoes, or putting them on drones to do environmental surveys. They work best on quite large things, because gravity is so ridiculously weak. Detecting submarines, for example. We also have a PhD student working on using them to control the attitude of a satellite in orbit. What was the biggest challenge in making them? Maybe temperature control. As temperature changes, things expand and contract and that creates a hugely bigger signal than the ones you’re looking for. We have electronics to control the temperature of the chip to about one thousandth of a degree. It’s much easier to control the temperature of a system if you can reduce its size. You know Harrison’s clocks? He started out with quite a big system, with a lot of temperature gradients that affected the clock’s accuracy, but Harrison 4 is about pocket-watch size. There are actually a lot of advantages to getting smaller. Saving the world with microscopic gravity sensors is on Tuesday 5 September at 14.30. Book tickets on the British Science Festival website.