What are gravitational waves – and why are they a new frontier in astronomy?

What are gravitational waves
Image courtesy: NSF/LIGO/Sonoma State University/A. Simonnet

Einstein was right. And an academic at Charles Sturt University (CSU) has helped to confirm it. Dr Philip Charlton works at the forefront of astronomy. His work as part of an international consortium has contributed to the confirmation of Einstein’s theory of relativity – and opened up a whole new world (or universe, if you like) of scientific possibility.

So what are gravitational waves?

Dr Charlton explains.

Dr Charlton“Gravitational waves are ripples in space-time itself. Space-time is Einstein’s conception that combines space and time into a single four-dimensional structure. Relativity tells us that massive bodies cause that structure to curve. An analogy would be a ball sitting on a mattress. The presence of the ball causes the mattress to curve. And if you shake or agitate that mattress, waves move across it. Movements of large masses in the universe have the same effect on space-time. Large objects moving around each other disturb the structure of space-time locally, and then those waves radiate out through space.

“Whenever you have large masses moving rapidly, they radiate gravitational waves. If the masses are moving in a predictable way, the waves have a pattern that we can detect and measure. The bodies need to be large scale – the size of stars or bigger – and close together for this to happen.

“The prime example – and the one that was used to prove the existence of the gravitational waves – is two neutron stars orbiting very closely together. Neutron stars are the smallest, densest stars known to exist and are formed when massive stars explode in supernovas.

“While they are orbiting, they produce regular wave-like emissions – gravitational waves. However, eventually the neutron stars get so close together that they collide and collapse into a black hole. This releases a large burst of both electromagnetic radiation and gravitational waves. The gravitational wave component is what LIGO detected.”

So what is LIGO?

Dr Charlton is part of an international consortium of physicists who operate in the science and mathematics fields and work on the LIGO project. LIGO stands for the Laser Interferometer Gravitational-Wave Observatory, which consists of two sites in the United States. Dr Charlton explained how the observatories actually work.

“They are comprised of long evacuated tubes, with lasers bouncing back and forth between mirrors at each end. We detect changes in the distance between the mirrors, which can be due to the stretching and squeezing of space caused by the passage of a gravitational wave. The distance between the mirrors at either end of the tubes is changed very minutely by the passage of the waves. We are talking very small indeed: 1019 metres, about 1/1000th the width of a proton.

“On 17 August 2017, LIGO made the first detection of gravitational waves emitted by the merger of a binary neutron star system. As the neutron stars spiralled together, they emitted a regular pattern of gravitational waves, which we could detect (having travelled through space for millions of years). Then when the stars exploded (or rather when we could first detect that explosion, which happened millions of years ago), LIGO and our optical astronomy collaborators detected both gravitational waves and light in the form of gamma rays and X-rays.

“We can’t observe the collision directly. But we are able to restrict the source of the signal to a patch of the sky and associate it with a simultaneous gamma-ray burst in the same region to deduce what has happened.”

What led to this discovery?

It’s this interaction or contemporaneity of observable (light) and non-observable (gravitational) waves that is at the heart of the recent discovery. The LIGO project meant that for the first time, scientists have directly detected gravitational waves in addition to light from the spectacular collision of two neutron stars. This marks the first time that a cosmic event has been viewed in both gravitational waves and light. And this discovery led to three of the founding members of the LIGO team being awarded the Nobel Prize. However, as Dr Charlton outlined, it built on previous discoveries.

“We knew that gravitational waves existed, as they had already been detected indirectly. In the 1970s, the scientists Russell Hulse and Joseph Taylor discovered the so-called ‘binary pulsar’ using a radio telescope. This is a binary system containing a neutron star and a pulsar. They analysed the orbital decay of the binary system and showed it was happening exactly as predicted if the system was emitting gravitational waves in accordance with the theory of relativity. For this discovery they won the 1993 Nobel Prize.

“The next step was to try and detect the waves directly. This would, for instance, enable us to determine the shape of the waves. If you know what the wave looks like, you can make deductions about what is happening at the source.”

How significant is the observation of gravitational waves for astronomy?

For Dr Charlton, the implications of the LIGO discovery are immense.

“This discovery opens up an entirely new spectrum to observe what’s going on in the universe. First we had optical observation, using eyes and telescopes. Then we had radio telescopes that revealed things out of the visible spectrum, such as pulsars which radiate in X-rays and radio waves. With the discovery of gravitational waves, we are doubling the amount of spectrum we have to look at the universe in. It is a new channel of information with which to understand the universe.

“Furthermore, gravitational waves are not impeded by dust or astronomical bodies. So it makes the universe transparent in a new way.

“I’m now part of a team using the data we have from LIGO to understand the parameters of the background noise arising from unresolved sources such as supernovae. These are exploding all the time, and LIGO is detecting these as part of the background noise of the universe. However, we have yet to be able to distinguish these individual events from the rest of that noise. Hopefully, studying the gravitational wave background will help us pin down characteristics of the population of different types of bodies in the universe.”

After gravitational waves, what’s next?

The exploration of space doesn’t stop, and as Dr Charlton outlined, there is still so much to be discovered through research.

“In the long term, one goal is to test quantum gravity using gravitational waves. What has been achieved at LIGO is just the very first step towards that. The exploration of quantum gravity will be done over many generations.

“Future observations will help to put limits on or perhaps even measure the mass of the graviton, a hypothetical particle that mediates quantum gravity. During the binary neutron star merger, we observed the gravitational waves as well as the light, the gamma ray burst, at almost the same time. So we have high confidence they both came from the same event. However, the photons from the light arrived very slightly behind the gravitational waves. Some delay is expected due to the light passing through the interstellar medium. But with more accuracy, we might be able to detect a real difference between the speed of light and the speed of gravitational waves, which could tell us if the graviton has mass.”

Will you boldly go?

Do you want to work at the vanguard of space exploration? The Bachelor of Science at CSU is the perfect start. You can specialise in physics and get introduced to astronomy, relativity and cosmology. Who knows, maybe you’ll be the next Einstein!

Contact us to find out more.