Gravitational Waves are Not the Ultimate Test of General Relativity

Last year’s ground breaking gravitational wave detections generated some of the widest media coverage of a scientific discovery to date. Many articles and reports described the detection as the “ultimate” test of general relativity, the “final” test of general relativity, or confirmation of Einstein’s “last” prediction. For a theory that is 100 years old, that was developed by one of the most celebrated physicists of all time, and has survived every experiment thrown at it, you could be forgiven for thinking that general relativity is done and dusted. However, general relativity is far from facing its “ultimate” test.

Gravitational Waves — another success for General Relativity, but not the final test.

Gravitational Waves — another success for General Relativity, but not the final test.

Over the past century, general relativity has been put through its paces with some of the brightest minds on the planet developing new ways to test it. General relativity explains anomalies in the planet Mercury’s orbit around the Sun, why the universe cannot be static, and the way star-light is deflected by massive bodies like the Sun and distant galaxies. Predictions of general relativity have been tested using high-precision gyroscopes on satellites orbiting the earth and atomic clocks, by timing the orbits of distant neutron stars, and now, by the direct detection of gravitational waves. General relativity has survived every test, passing with flying colours. Even the fact that GPS functions properly is evidence of the success of general relativity.

But the nature of the scientific method means that no test that general relativity survives will be its ultimate test. Scientists will never stop testing general relativity until it fails. We know general relativity is not the final solution to a theory of everything, and we are reasonably certain that general relativity must fail at some point. While general relativity has been spectacularly successful in describing the large-scale workings of the cosmos (such as solar systems, galaxies, and the entire visible universe), it does not work on the small scale (atoms, nuclei, and fundamental particles), for which we need quantum mechanics. General relativity does not tell us what goes on inside a black hole, or at the moment of the Big Bang, because, in those situations, we are dealing with much smaller scales where quantum mechanics comes in to play. While we are very aware of general relativity’s ultimate limitations, this does not give us a very convenient starting point for developing a replacement theory. Like an engineer investigating the collapse of a bridge, if we can find the exact point of failure, we are better able to develop a solution. Because of this, many tests of general relativity today seek to determine the exact point at which general relativity breaks.

Most of the current approaches to breaking general relativity assume the devil is in the detail, and aim to make more precise measurements of previously tested phenomena. As more and more gravitational wave events are detected and studied, they will provide ever more stringent tests of the predictions made by general relativity. One research group wants to measure the precise orbits of different types of metal around the Earth, while another wants to time the fall of different types of atoms, things that general relativity says should show no difference between the different materials and atoms. The ESA’s ACES mission aims to make the most precise measurements ever of gravitational time dilation and gravitational redshift. If general relativity passes these tests, physicists will celebrate another success for a brilliant theory and for human intellect, and will then set about designing the next test. However, if, for any of these experiments, the predictions of general relativity do not match the data, physicists will celebrate the discovery of the breaking point, and the dawning of a new era in our understanding of the nature of the universe.

In order to get to space, throw yourself at the planet, and miss.

High precision space- and ground-based experiments aim to test General Relativity in minute detail

Rest assured, articles claiming “[Some experiment] is the ultimate/final test of Einstein’s greatest theory” are not yet a thing of the past.

What the Detection of Gravitational Waves Means

Unless you live under a rock, the announcement in February of this year of the detection of Gravitational Waves by LIGO cannot have escaped your attention. Scientists around the world celebrated the achievement, and public curiosity about what all the scientists were yelling about was high enough that the world’s media ran the story for several days. Physicists and science communicators, whether they had anything to do with the discovery or not, were called upon to explain to the public what all the fuss was about.

If two Black Holes crash in deep space, and there is no detector to hear them, do they still make a Gravitational Wave?

The enormous release of energy from the collision of two Black Holes, detected for the first time

For a public that had never heard of Gravitational Waves before the announcement of their discovery, a key question that needed answering was, “So what does it mean?” Many scientists and science communicators did an outstanding job of answering this question, but their responses often left out an important element, that is, they did not answer the implied question, “what does it mean for me?”

Following the announcement earlier this week of a second Gravitational Wave detection (and another detection candidate), I want to take the opportunity to outline some of the ways in which the discovery of Gravitational Waves directly affects the average member of the public, but in the interest of providing a complete answer, I will first recap the importance of the discovery from a scientific perspective.

1) Amazing Achievement

First of all, the detection of Gravitational Waves is significant because it is direct confirmation of a prediction made 100 years ago by Albert Einstein. Einstein’s General Theory of Relativity is the best model physicists have for describing the action of gravity and the behaviour of the solar system, the galaxy, and the universe. Scientists constantly try to invent new experiments to push our understanding of the universe to the breaking point. It is by knowing the point at which our understanding of the universe fails that we are able to make the greatest progress. General Relativity has survived every test it has been subjected to for 100 years. Even the fact that GPS functions properly is a demonstration that Einstein’s General Theory of Relativity is correct.

In 1916, Einstein used General Relativity to predict that bodies orbiting each other, such as the Sun and Earth or two black holes, warp spacetime such that energy from the two bodies is carried away as ripples in the very fabric of the cosmos. These are Gravitational Waves, and their detection is not only another success for Einstein and General Relativity, but for the power of human ingenuity.

Insert fat joke here.

Gravitational Waves are ripples in spacetime caused by massive bodies in orbit

The LIGO detectors are the most sensitive instruments ever created and represent the culmination of 50 years of work by thousands of scientists. Although the first of the detected Gravitational Wave events represents an explosion 50 times more powerful than the power output of all the stars in the universe combined, the energy of the event has spread out and weakened during the 1.4 billion light-year trip to Earth, and so the LIGO detection represents the smallest amount of energy ever successfully detected. The very fact that humanity has the ability to detect Gravitational Waves is something we, as a species, can be proud of.

Couldn't think of funny alt-text.

The Advanced LIGO detectors in Livingston and Hanford

2) A New Spectrum, a New Science

The two confirmed and one candidate Gravitational Wave detections represent the beginning of a new era in studying the universe, a new way of doing astronomy, Gravitational Wave Astronomy.

All of astronomy, and everything we have discovered about our universe through astronomy, has been done using light. Whether we use radio telescopes, optical telescopes, or gamma-ray telescopes, all of these devices detect energy from the electro-magnetic spectrum, they all detect some form of light. Gravitational Waves are not a type of light. They exist in a completely different spectrum.

The Gravitational Wave spectrum gives astronomers a completely new way of studying the universe because the properties of Gravitational Waves are very different to those of electro-magnetic waves. Unlike light, Gravitational Waves are not absorbed by matter. They pass unhindered through the Earth, interstellar gas clouds, and entire galaxies. Unlike light, they cannot be blocked by stuff getting in the way. Gravitational Waves allow us to see past the bright glare of galaxies, and through vast interstellar dust clouds to corners of the universe our other telescopes will never be able to see.

We can also use Gravitational Waves to study things that don’t emit light. As far as our current theoretical understanding tells us, the collision of two black holes, like the Gravitational Waves events seen so far, should not emit any light. These events, and events like them, can only be studied using the Gravitational Wave spectrum.

Since the end of its first, and now famous, observing run, LIGO has been undergoing upgrades that will roughly double the sensitivity of the detector. With this boost in sensitivity, and based on the three events detected so far, Gravitational Wave physicists expect LIGO to detect around one event per week when it is switched back on later this year. With that amount of data, Gravitational Wave Astronomy will become a field in its own right, and one that will revolutionize our understanding of the universe by studying regions of the cosmos previously invisible to us.

woooooOOOOP!

Gravitational Waves open up a new spectrum with which to study the universe

3) What it means for me

The detection is an amazing technological achievement and Gravitational Wave detectors are some of the most advanced pieces of equipment in the world. To be able to build their detector, scientists and engineers have had to invent new technologies along the way, and these new technologies have a huge range of spin-offs and applications that will impact on our every-day lives.

To build the LIGO detectors, super-reflective mirror coatings and polishing techniques had to be developed, vibration isolators that guard against everything from minor earthquakes to people coughing had to be built, hyper-precise lasers and sensitive read-out systems had to be invented, and all of this had to operate in hard vacuum, even special super-hard glues had to be formulated. To actually detect the signal, advanced analysis software had to be written. This is only a taste of the work involved. The complete list of innovations by Gravitational Wave scientists would take dozens of pages to list like this, and I don’t even know most of them!

With this type and variety of advanced technology in the works, it is almost inevitable that other applications would be found for the scientists’ innovations. Just one example from the University of Western Australia, where I am studying for my PhD, is the Rio Tinto Gravity Gradiometer. This new technology, which will detect ore bodies from an aeroplane by measuring tiny changes in the Earth’s gravitational field, spun-off from Gravitational Wave research at UWA.

This thing sits in the lab across the hall from me.

The VK1 airborne Gravity Gradiometer will detect ore bodies below the ground, and spun-out from Gravitational Wave research

Over the coming decades, technology originally developed for Gravitational Wave detectors will be worth billions of dollars, create many thousands of jobs, and will enrich our every-day lives for generations to come.

 

If you’d like to learn more, you can go here to watch PhD comics explain Gravitational Waves.