What’s a Gravitational Wave when it’s at home?
In 1916 Albert Einstein predicted the existence of gravitational waves in his general theory of relativity and now 100 years later scientists have proved this theory through the discovery of gravitational waves. But what are gravitational waves? The simplest explanation is that they are ripples in the space-time created by objects of a super heavy mass that influence the surrounding area of space. Those ripples contain gravitational information about the original source of mass.
In order to understand the scientists’ discovery properly, we need to take a closer look at the waves and to do that, we have to travel back to the creation of the universe itself – The Big Bang. In that moment an infinite number of minute particles smaller than an atom were scattered across the universe. Imagine these particles as floating in an unhindered straight line on the wind, with the wind representing space-time. A problem: the particles are suddenly blocked by an object, let’s say a star. The particles can’t go through the star so they have to go around it, helped by the push of the star’s gravity, and thus the once calm space-time has now been altered.
So if gravitational waves have existed since the universe’s beginning why have scientist only now been able to detect them?
Now, let’s imagine space-time is a lake and the star a stone that has been dropped into the centre. Ripples are formed in the water. At the centre of the lake, the ripples are large and clearly defined but at the shore’s edge, the ripples can no longer be seen. Earth is standing on the shore of that lake, guessing that something has just happened but being unable to prove it. What if that star was no ordinary star but a supernova or perhaps even a binary pulsar (two dense stars in orbit around each other)? Drop a boulder into the lake and the large ripples are much more likely to survive the distance to the shore. It’s not so much the size needed but the power and energy the source mass produces that creates gravitational waves capable of reaching and been seen by people on Earth.
The first indirect proof that the waves existed came in 1974 when astronomers at the Arecibo Radio Observatory discovered a binary pulsar. They believed the stars’ weight and close orbit would be enough to affect the gravity of the other and produce gravitational waves. 8 years later they found the stars were now closer with faster orbits and had moved at the rate predicted by Einstein’s general relativity. A further 40 years of observations produced more proof towards Einstein’s theory.
The latest proof comes from the team at Advanced LIGO (Laser Interferometer Gravitational-Wave Observatory). Using the most sophisticated detector in the world, physicists discovered two colliding black holes 1.3 billion light years away from Earth. The physicists believe they have evidence showing the most destructive phenomenon in the universe producing gravitational waves.
And why are scientists so excited by this discovery? Apart from proving that was Einstein correct about gravitational waves? Well, Einstein also stated that nothing is created or destroyed, only changed.
Remember the particles created by The Big Bang? They haven’t been destroyed. They’re still out there riding gravitational waves throughout the universe. The waves that scientists can now detect could contain information about the origin of the universe. Who knows what they will tell us?
What has Einstein predicted, that others proved?
Einstein was a master of Gedankenexperiment. It was what allowed him to formulate his greatest ideas. Though it sounds complicated, or even mysterious, it was actually quite simple; he used his imagination.
Gedankenexperiment, was what Einstein referred to as thought experiments, his novel approach of using his imagination to visualise hypothetical experiments that he would later translate in to mathematical equations. From riding a beam of light that would propel him to his theory of special relativity, to imagining a falling elevator from which he would conveniently alight on the floor that stocks theories of general relativity.
For over a 100 years now scientists have been devising ways to test Einstein’s postulations. In 1919 astronomers Sir Frank Dyson and Arthur Eddington journeyed to the island of Principle, to test Einstein’s general relativity predictions that light would bend around a massive object as a result of its gravity field. They devised an experiment to see if a distant star that was blocked by our Sun could sbe seen on Earth due to this distortion, but as the Sun outshines its distant cousins this prevented distortions from being observed. To overcome this, Dyson planned to conduct his experiment on the next total solar eclipse. In search of the best location to observe the eclipse they got to Principle, an which would be cast in to darkness for almost seven full minutes – providing time to complete their experiment. It rained for the nineteen days leading up to the eclipse, and when the moment finally arrived the spectacle was obscured by cloud cover, for all but ten seconds. It was enough time to the get the photograph they needed. The results corresponded with Einstein’s predictions and revolutionised the world of physics.
Irvin Shapiro noticed in 1964 that light can take more time to travel in space as it passes through gravitational fields. By bouncing radar signals off Venus and Mercury and measuring the time it took for those signal to return to Earth it was proven that the signals slowed if they passed near the sun. Shapiro later confided he’d hoped to prove Einstein wrong, but the Shapiro effect – as its now known – only confirmed more of his theories.
The principle of equivalence was an aspect of Einstein’s theories that gave rise to numerous experiments. NASA launched Gravity Probe-A in to space on 18 June 1976 to test Einstein’s prediction that time progresses at different rates depending on the strength of gravity, with the difference referred to as redshift. Onboard the shuttle was an immensely precise clock. In space for less than two hours, it successfully recorded measurements that showed that the passage of time did speed up whilst in space.
Einstein’s ‘twin paradox’ confounds notions of time once again by using his special theory of relativity to propose a hypothetical time dilation scenario with a set of twins. This particular thought experiment imagined a set of twins, one of which hops on a rocket travelling at almost the speed of light to do a spot of interstellar sightseeing. When they return to Earth the sibling that remained is markedly older than the one that got on the rocket. This is because time is not singular, but depends upon the observer and their motion. In 1971 Joseph Hafele and Richard Keating set out to test this theory in luxurious style, by flying first class around the world with their atomic clocks in tow. They completed two flights, once in either direction around the world, then compared the results. Although they didn’t meet up with an older colleague, their evidence did indicate deviations in time.
Gravitational lens has proved another invaluable prediction by Einstein that has not only been tested but is also allowing scientists to peer over 9 billion years in to the past. As light is deflected from distant objects, it intensifies and can be used as both a looking glass to view far away galaxies and to measure their mass.
Gravity Probe-B, the aptly named older brother of NASA’s earlier probe, was sent in to space on 20 April 2004 to test if the way Earth’s mass is affecting the fabric of space-time corresponding with Einstein’s postulations. It measured the geodetic effect – how the Earth is warping its local space-time – and revealed the presence on frame dragging – a process where the rotation of massive objects drags local space-time along with them – all of which was predicted by Einstein. Reporting the results from NASA headquarters in Washington D.C. physicist Francis Everitt proudly declared that “Einstein survives!”
What’s all the fuss about?
Einstein predicted the existence of gravitational waves almost one hundred years ago. However, until recently they were just a theory.
Research from the LIGO (laser interfermotere gravitational-wave observatory) suggests that they are in fact a reality. But what are gravitational waves and why are they so important? Gravitational waves are distortions in the fabric of space. They are like ripples in a pond that are created by the collision of two massive objects, such as black holes.
Their discovery is important as they provide further evidence for Einstein’s theory of general relativity. They also allow scientists to view the universe in a completely new way. Scientists can use telescopes to view stars in different galaxies many millions of light years away. However, this is only possible if there are no objects between the telescope and the object.
If dust, planets, other stars or even galaxies are in front of the object that the scientists wish to view they cannot detect it. Gravitational waves are a useful tool because unlike light, x-rays or infrared, they can travel through these objects.
The information they contain will be free of any distortions or changes as the waves travel unhindered and do not react with other objects. Because of this gravitational waves may contain information about the nature of space and time and even very start of the universe, the big bang.
Gravitational waves allow scientists to view situations of strong gravity, i.e. information about massive objects.
Previously scientists were only able to make inferences about these from weak gravitational reactions, such as planet’s orbiting stars.
The waves can also be used to detect and inform researchers about some of the extremes of the universe.
Last modified: 29th February 2016