The Department Of Physics And Astronomy.
Newsletter Spring 2016.
It’s been a busy time for us here in Sheffield. We’re particularly excited to share Dr Ed Daw’s work with you: he’s one of the researchers who was involved in the landmark observation of gravitational waves in February. In this newsletter, he shares the inside story on this once-in-a-generation discovery. There’s also news about a prize for our fundamental physicists, and an exciting student project based around dark matter searches. If there’s anything you need in the meantime, feel free to get in touch. You’ll find our contact details on the back. We look forward to seeing you again soon. Best wishes, Nigel Clarke Head of Department
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Physics and Astronomy Newsletter
Gravitational waves SHEFFIELD PHYSICIST PART OF LIGO TEAM THAT ANNOUNCED MAJOR DISCOVERY In February, scientists announced that ripples in the fabric of spacetime called gravitational waves had been observed for the first time – and University of Sheffield physicist Dr Ed Daw was one of the researchers behind this ground-breaking discovery. The new findings confirmed a major prediction of Albert Einstein’s general theory of relativity, made in 1915. Ed, who has been researching gravitational waves as part of the LIGO Scientific Collaboration since 1998, said: “Discoveries of this importance in physics come along about every 30 years. “A measure of its significance is that even the source of the wave – two black holes in close orbit, each tens of times heavier than the sun, which then collide violently, has never been observed before – and could not have been observed by any other method. This is just the beginning.” TURN TO P4 TO READ ED’S EXPLANATION OF HOW AN EXPERIMENT AT LIGO WORKS VISIT WWW.SHEFFIELD.AC.UK/PHYSICS TO WATCH VIDEOS OF ED DISCUSSING THE LIGO EXPERIMENT PICTURED: Dr Ed Daw Flying over the LIGO site in the US
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Physics and Astronomy Newsletter
How does an experiment DR ED DAW, WHO HAS BEEN STUDYING GRAVITATIONAL WAVES AS PART OF THE LIGO SCIENTIFIC COLLABORATION SINCE 1998, COMMENTS ON HOW EXPERIMENTS TO DETECT THEM WORK. Gravitational waves are mysterious ripples in the fabric of space and time that travel across our universe at the speed of light. Predicted by Einstein exactly 100 years ago, a number of experiments have been searching for them. One of these experiments, LIGO, has been the focus of much speculation. But how does it actually work and how reliable is it? Gravitational waves are caused by violent astrophysical events, involving massive, compact objects like neutron stars and black holes, colliding into each other. Even though the events that cause them are cataclysmic, they are so far away that the effects on our local fabric of space and time here on Earth are very subtle. For that reason, scientists have had to build huge optical instruments that are supremely sensitive, called laser interferometers, to search for them. The Laser Interferometric Gravitational-Wave Observatory, or LIGO, brings these efforts together in an experiment with over 1,000 scientists from 86 institutions around the world working with these instruments or the data that they produce. Two light beams, some mirrors and a detector All you need to build a gravitational-wave interferometer is two light beams, travelling between pairs of mirrors down pipes running in different directions, say north and west. The effect of a passing gravitational wave should stretch space in one direction and shrink it in the direction that is at right angles. On Earth, that would cause the mirrors to swing by tiny amounts, so that the distance between one pair of mirrors gets smaller, while the other gets larger. The swinging is actually the mirrors responding to the stretching and compression of space-time, which is just amazing. It is very similar to waves on a pond. Put down a floating object and, as a wave passes through, the object bobs up and down several times. LIGOs mirrors are bobbing in a pond of gravitational waves, which are more complex but still cause the motions to differ from place to place in a characteristic way. It is very similar to waves on a pond. Put down a floating object and, as a wave passes through, the object bobs up and down several times. LIGOs mirrors are bobbing in a pond of gravitational waves, which are more complex but still cause 4
Physics and Astronomy Newsletter
at LIGO actually work? the motions to differ from place to place in a characteristic way. The subtle changes in distance can then be registered by a detector, put in place to monitor the laser light returning from the two interferometer arms. Just to ensure that it wasn’t a fluke, we have two of these machines and position them at opposite ends of the US and require both of them to do the same “dancing mirrors” thing at the same time: one in Livingston, Louisiana and the other in Hanford, Washington. So, how does this work in practice? A key task is “locking” the interferometers, which means stabilising the separations between the mirrors so that the laser light resonates between the mirror surfaces as it was designed to do. Back when I worked on a LIGO prototype at MIT in 1997, locking was done by hand by scientists, using a hand held box with 12 knobs on it. It is now computercontrolled, so that an operator initiates the sequence, and sensors indicate when each of the mirrors has moved into the right position. Mirror positions and angles tend to drift slowly due to temperature changes, mechanical relaxations in the hardware, and even the position of the moon in the sky, so adjusting the mirrors is a daily task. Scientists and engineers on-site also monitor diagnostic information about the detector and the physical environment, so that when the detector isn’t working properly, the cause can be identified and addressed. I’ve spent many hours in the LIGO control rooms and labs; my most recent machine work was making precise measurements of the distances between mirrors during a troubleshooting exercise. In practice, this meant hours wearing clean-room garments and leaning over steel tables in a very large room, often working late into the night. If I make this sound easy, it isn’t. LIGO is oozing with groundbreaking technology developed especially for the detectors. The interferometer arms, each 4km long, had to be constructed with a correction for the curvature of the Earth. Each detector has to be exquisitely isolated from vibrations of the ground and it has to be in a vacuum so that contaminants and gas don’t corrupt the laser light between the mirrors. The two detectors have to take data for months at a time – never missing a single data point and never getting behind. When your detector is distributed over several kilometres, this is a technological challenge in itself. LIGO is an engineering and physics marvel, one of the most sophisticated machines ever constructed and it’s exciting to be part of it. 5
Physics and Astronomy Newsletter
Fundamental physics RESEARCHERS SHARE IN PRIZE AWARDED AT STAR-STUDDED CALIFORNIAN CEREMONY Sheffield academics have shared in a major international prize for fundamental physics. Ten Sheffield researchers, along with other members of the international T2K collaboration, were awarded the Breakthrough Prize for Fundamental Physics for their role in the discovery and study of neutrino oscillation. The Breakthrough Prize was established by Facebook founder Mark Zuckerberg, amongst others. It was given out during a star-studded ceremony at NASA’s Hangar 1 in California, attended by celebrities including Christina Aguilera and Russell Crowe. T2K is an accelerator-based long-baseline neutrino experiment in Japan. The discovery recognised by the Breakthrough Prize sets the stage for the study of differences in the neutrino oscillation process relative to their antiparticles (antineutrinos). This process, CP violation, may help researchers show how the universe came to be dominated by matter. Professor Lee Thompson, head of the Sheffield T2K group, said: “It is a tremendous honour to be awarded a share in the Breakthrough Prize which reflects the important progress that has been made over the past few years in understanding the details of neutrino oscillations.” PICTURED: The inside of the Super-Kamiokande detector used in the T2K experiment in Japan
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Physics and Astronomy Newsletter
Dark matter STUDENT JOINS 100 MILLION DOLLAR SEARCH AS PART OF SUMMER PROJECT Each summer undergraduate students get to experience what university research is really like by taking part in the SURE scheme. Last summer physics undergraduate Nigel Gibbions became part of a $100m project to search for dark matter. Nigel wrote a computer program to calculate the background signal from environmental radioactivity in the LUX-ZEPLIN experiment – a seven tonne liquid xenon dark matter detector to be located deep underground. The experiment will look for flashes of light released by the xenon when a dark matter particle interacts with it. Nigel calculated how many flashes of light are due to gamma rays coming from rock surrounding the detector. He presented his results at a collaboration meeting in Portugal later in the summer.
PICTURED: An illustration of the detector, and Nigel with his supervisor Dr Vitaly Kudryavtsev as he wins the University’s annual award for the best SURE supervisor
“OVER THE SUMMER, I LEARNED A LOT ABOUT C++, LINUX AND OTHER ‘TOOLS OF THE TRADE’ USED BY PARTICLE PHYSICISTS. ABOVE ALL, I LEARNED SOMETHING ABOUT HOW A LARGE PHYSICS EXPERIMENT WORKS, THANKS TO THE DEDICATION OF MANY INDIVIDUAL SCIENTISTS, TECHNICIANS AND ENGINEERS.” Nigel Gibbions 7
Contact us The Department of Physics and Astronomy Hicks Building Hounsfield Road Sheffield S3 7RH United Kingdom T: 0114 222 4362 E: physics.ucas@sheffield.ac.uk W: www.sheffield.ac.uk/physics @UoSPHY
‘How does an experiment at LIGO actually work?’ (p4-5) originally published by The Conversation (theconversation.com) Super-Kamiokande detector image credit (p6): T2K Experiment, t2k-experiment.org Dark matter detector illustration credit (p7): The LZ Dark Matter Experiment, lz.lbl.gov