Albert Einstein predicted the existence of gravitational waves as part of his general theory of relativity in 1916. Long undetected, they were said to be distortions or ‘ripples’ in the fabric of space-time caused by some of the most violent and energetic processes in the universe. The most exciting thing about gravitational waves is the potential for us to investigate the events that caused them. These are phenomena on a truly cataclysmic scale, for example colliding black holes, the collapse of stellar cores (supernovae), coalescing neutron stars or white dwarf stars, and the remnants of gravitational radiation created by the birth of the universe itself. This year the Laser Interferometer Gravitational Wave Observatory (LIGO) announced it had physically sensed distortions in space-time caused by passing gravitational waves generated by two colliding black holes nearly 1.3 billion light years away! LIGO and its discovery will go down in history as one of the greatest human scientific achievements.
These activities can be adapted for a wide range of abilities across KS3 and KS4 and focus on games and competitive activities designed by Caltech and its collaborators from the LIGO team. As well as learning about some of the specific challenges facing the scientists who design the real gravitational wave interferometers, your students will also learn about spending a large budget wisely, and about making trade-offs between different interlinked subsystem parameters.
WHY TEACH THIS?
The detection of gravitational waves proves Einstein’s theories surrounding their existence and is a potentially Nobel prizewinning discovery. The aim of this lesson is to make gravitational waves and LIGO accessible to students from KS3 upwards, of all abilities. STEM club activities are also a brilliant way to bring LIGO to life in school with build projects including a spectroscope and laser interferometer.
Black Hole Hunter
The search for gravitational waves is difficult because the signals are buried in large amounts of noise. To simulate this the University of Cardiff has designed a listening challenge [AR1] called Black Hole Hunter. Students will need access to a computer and some headphones.
The objective is to listen to a distinctive signal, and then determine which of the four subsequent data files contains the signal you heard. To begin with, the signals are relatively loud. As the challenge progresses the signals will get quieter. The activity is competitive as students start with three lives and each time they select an incorrect data file they lose a life. Use the activity to discover which students in the class are the best listeners.
1 – Black Hole Pong
Continue this computer-based lesson’s activities with a game that helps students to understand some of the intricacies of gravity and black holes; Black Hole Pong [AR2] from GW Optics. In this remake of the classic computer game, each player now controls a black hole rather than a paddle! The objective is to position their black hole on their side of the screen and utilise its gravitational potential to fling an approaching star back. Whoever lets a star slip past loses!
Original Pong relies on students’ intuitive knowledge of Newtonian elastic collisions between a ball and paddle. Success in Black Hole Pong, by contrast, requires an understanding of how gravitational potentials behave. Each black hole has an attractive force on the star, which drops rapidly with separation distance according to an inverse square law (1/r2). This is similar to how a star would be affected, at a distance, by the gravitational field of an object much more massive than itself. In order to fling a star back, students should try to position the black hole so that the star passes just off centre. The closer the star and black hole pass each other, the stronger the force and the greater the deflection. If the distance of closest approach is increased the deflection will appear more and more gentle until hardly noticeable. It is also possible to capture stars by approaching them from behind. This causes the star to decelerate to the point at which it is travelling below escape velocity, entering a bound, elliptical, orbit around the black hole. Timing is essential when removing the black hole from the orbit or you may just hurl the star towards your goal!
As an interesting aside, your most perceptive students might notice a distortion of the background image around the black holes, this is actually a realistic phenomenon called Gravitational Lensing! The intense strength of the gravitational field of a black hole is sufficient to bend light around it, in a similar manner to a lens. This allows an observer to view what is behind a black hole as it appears as a stretched image around its edge.
2 – Space-Time Quest
This activity, called Space-Time Quest, also from GW Optics [AR3] places students in control of a budget of £100 million. They need to design and optimise an interferometric gravitational wave detector like the LIGO interferometers, which has recently detected gravitational waves. The better the combination of factors the more sensitive the detector will be. This in turn affects different sources of gravitational waves you are able to detect which earn points. Students can enter their highest scores onto a hall of fame.
Within the game play students can modify many different features of the detector, the power of the laser, the vibration isolation equipment, cryogenic cooling and the location of the detector just to name a few. The best outcomes are generated when these are realistically balanced and within the budget constraints that are inherent in every large scientific project.
The main part of the game is controlled through what is known as the Principal Investigator’s (PI) office. From the PI’s chair the students adjust their detector in order to improve its sensitivity. Example choices they can make include the environment they place the sensor in. Will they choose city, desert, island or forest and how do man-made noise, seismic activity and cost affect their decision? The sensitivity of the instrument your students will have managed to create is determined by sum of various noise contributions. Students are able to compute and check all the noise curves in the Noise Model screen.
Higher ability students should be directed to the ebook on gravitational wave detection [AR4] or the American Museum of Natural History’s Interactive: Operate LIGO! Animation [AR5]. These describe how the instrument works and the importance of each of the subsystems in contributing to its functionality. Once back in the game students are able to put this knowledge into action in their own detector design by changing its parameters from the PI’s office.
1) Environment Subsystem: Here you can experiment with various values for the depth of the detector, vacuum and cryogenic cooling.
2) Vibration Isolation: Isolating the experiment’s equipment from seismic noise is very important, here you need to design the pendulum system to reduce the noise as much as possible.
3) Optics Subsystem: The detector relies on high quality optical equipment. For this subsystem you must experiment with different laser and mirror properties to get the best result.
Catch a Wave from Space
Enlist your students to participate in the detection of gravitational waves through the Einstein@Home project [AR6], which uses a computer’s idle time to search for weak astrophysical signals from spinning neutron stars (often called pulsars) using data from the LIGO gravitational-wave detectors, the Arecibo radio telescope, and the Fermi gamma-ray satellite. Einstein@Home volunteers have already discovered about 50 new neutron stars; could you or your school contribute to finding many more? Einstein@Home volunteers can participate in teams. Sadly there isn’t a secondary school team from the UK in the current top 20. Could your school change that?
[AR1] BLACK HOLE HUNTER OW.LY/ZQTWM
[AR2] BLACK HOLE PONG OW.LY/ZQTZS
[AR3] SPACE TIME QUEST OW.LY/ZQU5T
[AR4] GRAVITATIONAL WAVE EBOOK OW.LY/ZQU86
[AR5] INTERACTIVE: OPERATE LIGO! / OW.LY/ZQUBX
ABOUT OUR EXPERT
Dr Joanna L. Rhodes M.Chem, D.Phil, MRSC is a teacher of science at Shelley College, Huddersfield.