Issue 5 Understanding Science

Undetectable: On LIGO and the importance of experiment

đź•’ 8 min

The principle of science, the definition, almost, is the following: the test of all knowledge is experiment. Experiment is the sole judge of scientific truth.

Richard Feynman, The Feynman Lectures on Physics
Credit: LIGO.

A new theory of gravity

Modern theoretical physics concerns itself very much with finding a deeply beautiful mathematical model of the universe. Albert Einstein is famous for his contributions to this cause – his theories were known as mathematically beautiful (though notoriously difficult) and robust from the start. So much so, in fact, that some of Einstein’s contemporaries thought it more appropriate to consider them achievements in mathematics rather than physics. Einstein’s theories were, however, heavily based on physical reality. His general theory of relativity, which we now know as a modern theory of gravity, came out of an attempt to explain the inconsistencies of astronomical orbits (specifically, the orbit of Mercury) with Newtonian predictions.

As a result, Einstein’s theory of gravity was so rigorous that it lent itself very nicely to experiment. Its predictions are robust – and testable. Einstein himself already proposed three testable predictions alongside the theory. The verification of its first major prediction, the way massive objects bend light, was completed only a few years after its publication, in 1919. However, knowing what to test for didn’t necessarily point to an easy way how to test it. The difficulty of the task proved a welcome challenge for physicists. General relativity soon became one of the most widely tested theories in physics and its major predictions were verified one by one, each producing a brilliant technological solution. The last major prediction of general relativity left unverified by the 21st century were gravitational waves, and it was an especially difficult one.

Gravitational waves had been discussed long before Einstein’s theory was there to predict them theoretically. They came naturally out of the similarities between classical models of gravitation and electricity. The idea was, if oscillating charges create electromagnetic waves, accelerating masses should analogously produce gravitational waves. And in fact, gravitational waves showed up as a result of the general theory of relativity. However, no amount of beautiful mathematical analogy could definitively prove their existence – we needed an experiment.

The problem was, gravitational waves, as predicted by Einstein’s theory, would be very, very weak. Unlike electromagnetism, which you could observe in your backyard, we would need to observe a violent, almost cataclysmic gravitational event to be able to catch even a glimpse of gravitational waves. The idea of detecting them was simply absurd. Ask a physicist of Einstein’s time, or even much later, and they might think it impossible.

Detecting the undetectable

Another idea that came quite naturally out of general relativity was that of binary pulsars radiating gravitational waves. Pulsars are very dense remnants of dead stars which rotate extremely fast. They are such insane objects that their interactions provide an excellent basis for studying the most extreme boundaries of physics. Twenty years after Einstein’s death, a team of scientists detected one such system and observed it for another eight years. They never directly observed gravitational waves – those emitted by the system were still far too weak – but they watched the two pulsars slowly get closer to each other as they radiated energy in the form of gravitational waves. General relativity predicted the rate at which this happened extremely precisely – gravitational waves were no longer just theoretical. However, many still doubted they would ever be directly observed. We could see their effects, but not them.

This is where LIGO comes in. The Laser Interferometer Gravitational-Wave Observatory, or LIGO, was created with the exact goal of detecting Einstein’s evasive final prediction. After several years with no success, LIGO switched off and began a series of upgrades which eventually turned it into one of the most ambitious scientific projects of the century, as well as one of the most ingenious ones.

Advanced LIGO was built to be the most precise measuring tool on Earth – gravitational waves demanded no less. To get around the problem of gravitational waves being seemingly far too weak to measure, a very specialized interferometer was quite possibly the only option. Interferometers measure the interaction of two or more beams of light, obtaining information from the patterns that form as a result. Two sources of light can amplify each other to a combined maximum or even cancel completely depending on how they are set up – the in-between allows us to observe unimaginably small shifts in their positions as the intensity of the light in the interferometer changes ever so slightly. To get a better idea of what happens, check out this video.

Briefly, LIGO consists of two perpendicular tubes that are 4 km long, with a near-perfect vacuum inside. Powerful lasers beam near-infrared light down the tubes, which get reflected back by highly polished mirrors at the ends and interfere back where the tubes intersect. This means that, if there was a change in position between the two mirrors, the interference pattern observed at the end would change – even for shifts smaller than an atom. However, to avoid false alarms from local disturbances (because we wouldn’t want to mistake a truck for a black hole merger), LIGO built two identical setups, one on either side of the continental USA. They would only consider detections made by both observatories.

LIGO setup. Credit: Johan Jarnestad, The Royal Swedish Academy of Sciences.

Once this setup was achieved, they could expect to detect the effects of merging black holes from up to 5 billion light-years away. All it took was to wait for one to happen –

– and it did. Even sooner than expected, the new setup started returning promising detections almost immediately after switching back on. Eventually, LIGO captured the signal of two black holes 1.3 billion light-years away from Earth spiraling into each other and merging in a collision of gargantuan proportions. This catastrophic merger resulted in a black hole over 60 times as massive as the Sun, but for the researchers at LIGO it was anything but a catastrophe. They had finally detected a gravitational wave – and confirmed the idea that binary black holes collide and merge over time while they were at it.

The signals detected by LIGO observatories in Hanford, Washington and Livingston, Louisiana. The top two plots show the detected signal and the waveforms predicted by general relativity. The bottom plot shows the two signals overlaid on top of each other, proving both detected the same event. Credit: LIGO.

And so, over a century after Einstein originally predicted their existence, gravitational waves were proven directly. This cemented general relativity’s status as one of the most vigorously tested theories in physics, alongside quantum electrodynamics. However, this achievement wasn’t LIGO’s lone contribution – their method for detecting gravitational waves opened the door to an entirely new astronomical method.

The importance of black holes

LIGO allowed us to observe events of such titanic proportions that it is fascinating to think that, up until a few years ago, we were completely blind to them. Gravitational-wave astronomy is helping to shape our understanding of some of the most bizarre phenomena in the universe.

Recently, black holes have been all the rage in scientific circles. This year’s Nobel Prize in Physics, awarded to Roger Penrose, Reinhard Genzel and Andrea Ghez, recognized their incredible discoveries in black hole physics. LIGO made its contribution to the buzz a little sooner – the detection of the greatest black hole merger in history was made public in early September. These black holes were the most distant we had ever detected – the signal from their collision took 7 billion years to reach Earth, more time than the Solar System has been around for. They were the most massive – so massive, in fact, that they bend the boundaries of what we thought possible for stellar-mass black holes. This also made it possibly the most baffling detection of black holes to date.

The question comes up, of course, of why one should care so much about impossibly distant, invisible objects. Well, besides the fact that they are simply interesting to no end, black holes play an important role in modern physics. They are the embodiment of the boundaries of our understanding. As Stephen Hawking and Roger Penrose showed, they fit smoothly into general relativity, but any physics we throw at them breaks down as soon as we go deeper in our attempt to predict their nature. A physicist would be no less surprised to find out there was a quantum firewall beyond the event horizon of a black hole than they would be by there being an infinite number of calico cats. Trying to understand the extreme allows us to patch up the holes in our understanding of the familiar.

As we’ve established, a lot of modern physics does not care as much about the practical value of its discoveries or the usefulness of its predictions – it is much more concerned with constructing an unbreakable, rigorous model of reality, even if it means sacrificing some mathematical beauty. The goal is to be able to place an observer somewhere, anywhere in the universe, from the inside of an atom, to the vacuum of space, to the inside of a neutron star, and know exactly what they will experience. Black holes are some of the weirdest objects there are and, as such, they help us build a strong understanding of the universe’s fundamental properties, all substantiated by the brilliance of experiment.

Have you tried your luck with general relativity? Are you familiar with LIGO’s current experiments or any other gravitational-wave astronomy projects? Let us know in the comments!

Sources and further reading

  1. A century of correct predictions
  2. LIGO, Caltech
  3. A “bang” in LIGO and Virgo detectors signals most massive gravitational-wave source yet
  4. Astronomers detect most powerful black-hole collision yet
  5. On the Anniversary of Two Scientific Revolutions
  6. Observation of Gravitational Waves from a Binary Black Hole Merger
  7. Interferometer Response to Gravitational Wave

By Laura Busak

Laura is a physics student with a love for all things cosmic. She enjoys making and listening to music, reading books that make her think, and generally doing whatever random things she thinks of, often until 2 am.

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