Gravitational Waves explained

Birmingham researchers have, as part of a global collaboration, confirmed a major prediction of Albert Einstein’s 1915 theory of general relativity through the detection of gravitational waves.

Professor Alberto Vecchio and Professor Andreas Freise have been at the forefront of developing a new field of gravitational wave astronomy. Alongside their colleagues at Cardiff and Glasgow Universities, they have developed and built instrumentation for Advanced LIGO, and pioneered the techniques that have allowed them to extract the properties of the sources from gravitational wave signatures.

Dr Kat Grover (University of Birmingham Outreach Officer), who recently received her doctorate in gravitational waves, met with Professors Andreas Freise and Alberto Vecchio to discuss gravitational waves and what this means for the future of astronomy and astrophysics.

Gravitational Waves Detected

Kat GroverKat: What has been discovered?

“LIGO observed gravitational waves from two black holes that orbited each other and then merged to form a bigger black hole. The final black hole has a mass about 60 times that of our Sun. This event occurred around a billion light years away from Earth. The merger was extremely energetic (for a fraction of a seconds the event released 50 times more energy in gravitational waves than the all the stars in the entire Universe in light), but by the time the waves reached us, they were so weak that the change in the length of LIGO's arms was less than a 1000th of the diameter of the core of an atom.”

Andreas FreiseAlberto VecchioAnswers by Alberto Vecchio, Professor of Astrophysics
and Andreas Freise, Professor of Experimental Physics 


Kat: What does this mean for general relativity?

“The measured signal matched the waveform predictions of Einstein’s theory; we've never tested the theory in such extreme conditions before, so it has passed its toughest test!”

Kat: What does it mean for astrophysics?

“This tells us that binary black holes do exist. It also tells us that they form, evolve and die during a period shorter than the age of the Universe. We’ve never seen binary black holes before. We've never found black holes of this mass before. It looks like these mergers should be common enough that we will see more in future observations with LIGO. Then we can start to understand exactly what is out there and how these binaries are made.”

Kat: What are “binary black holes”?

“Most stars have a companion and orbit around each other as the Earth orbits around the Sun. A binary black hole is a system in which two black holes orbit around each other.”

Background to gravitational wave astronomy

Kat: What is Einstein's theory of general relativity?

“General relativity is our best theory of gravity. In general relativity, gravity can be thought of as the effect of the curvature of spacetime. Massive objects bend space and time; the curvature in spacetime changes how things move.”

Kat: What are gravitational waves?

“Gravitational waves are ripples in spacetime. When objects move, the curvature of spacetime changes and these changes move outwards (like ripples on a pond) as gravitational waves. A gravitational wave is a stretch and squash of space and so can be found by measuring the change in length between two objects.”

Kat: What is spacetime?

“In our everyday lives we think of three-dimensional space (up/down, left/right, forward/back) and time as completely separate things. But Einstein’s theory of special relativity showed that the three spacial dimensions plus time are actually just part of the same thing: the four dimesions of spacetime.

“In general relativity, Einstein went further. Not only are space and time part of the same thing, but they are both warped by mass or energy, causing a curved spacetime.  Things like to move along the shortest route available; when spacetime is flat, this looks like a straight line. But when spacetime is warped, the shortest route might not look straight anymore. For example, when you are flying over the curved earth, your aeroplane’s flight path will look curved, even if you are going “straight” from A to B. We can see and measure the effect of curved spacetime; for example, the sun’s mass curves spacetime so the Earth moves in a circular orbit around the sun.”

Kat: What does curvature of spacetime mean?

“It is hard to imagine a four-dimensional spacetime, let alone what a curved version of this looks like, so we often simplify this by thinking of an example in two dimensions. We can imagine a two-dimensional spacetime as a rubber sheet; dropping a heavy object on the sheet will bend and distort the sheet. In a similar way, mass or energy distorts spacetime around it.”

Kat: What are black holes?

“Black holes are the regions of strongest gravity in the Universe. They are where the curvature of spacetime is so steep that all paths lead inwards. Eventually nothing can climb up the curvature no matter how fast it goes; even light, the fastest thing in the universe, can’t escape if it gets too close to a black hole.”

Kat: What does detecting gravitational waves mean?

“Einstein first predicted gravitational waves 100 years ago. We have some good evidence they exist from watching binary pulsars (which won the 1993 Nobel Prize). We see the orbit of the binary shrink by the amount predicted by gravitational waves emission, but we don't see the waves themselves. Measuring the waves themselves would be the final piece of evidence for the predictions of Einstein's general relativity.”

Kat: What are “binary pulsars”?

“Neutron stars are old, dead stars that collapsed down to an extremely dense object. Roughly the mass of our sun compressed into about the size of a city. Pulsars are rotating neutron stars which emit a beam of radiation. As the pulsar rotates, the beam of radiation sweeps across the Earth like a cosmic lighthouse. A binary pulsar is where a pulsar orbits another star or sometimes another pulsar.”

Kat: What can we learn from gravitational waves?

“Gravitational waves are a new way of observing the Universe. Astronomy traditionally uses light to explore the cosmos, but there are lots of things you can miss because a lot of the universe is dark, including black holes. One source of gravitational waves is two dense objects (like black holes or neutron stars) in orbit around each other.”

Kat: What is LIGO?

“The Laser Interferometer Gravitational-Wave Observatory (LIGO) is made up of two gravitational wave detectors in the USA designed and operated by Caltech and MIT. In addition the LIGO Scientific Collaboration with a 1000 scientists from around the world provides crucial support for the LIGO science from instrument development to data analysis and astronomy. One LIGO observatory is located in Livingston, Louisiana and the other in Hanford, Washington. Each observatory contains an enormous, extremely sensitive laser ruler. We bounce lasers along two 4-kilometre long paths, or “arms”, which are at right angles to each, and then compare the length of each path.  A gravitational wave can change the length of the arms, but the effect is extremely small (one part in 1,000,000,000,000,000,000,000 for the strongest waves), so the instruments need to be extremely sensitive, which became possible using completely new technologies and a new interferometer concept.”

Kat: What is the future for gravitational-wave science?

“LIGO has just finished its first observations using its new “advanced” sensitivity. It will slowly be improved over the next five years, making it even more sensitive. Next year it should also be joined by Virgo, a detector in Italy. There is also another detector being built underground in Japan called KAGRA. There is also a plan for putting a LIGO detector in India. Plans for a network of third generation of observatories such as the Einstein Telescope are under way. Improving the worldwide network of detectors will help us to measure properties of the signals, especially helping us figure out the position in the sky of the source of the waves. At the same time, Pulsar Timing Arrays are taking data to observe giant black holes at the centre of galaxies.

“Further in the future, there will be a space-based mission called eLISA. This will be much bigger (100 times the size of the Earth) and look for gravitational waves from much more massive objects.”

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