Gravitational Waves May Create Permanent Distortions in Spacetime

Gravitational Waves
Credit: LIGO/T. Pyle

Gravitational waves have long been a puzzle for scientists, and while Albert Einstein’s theory of relativity predicted their existence, the renowned physicist was on record as saying he wasn’t sure they could exist. This year, however, scientists working at the Laser Interferometer Gravitational-Wave Observatory (LIGO) project were finally able to detect gravitational waves, and furthermore, what they have discovered could shed light on another giant mystery of the universe–the behavior of black holes.

What Are Gravitational Waves?

Before we dig into the discovery, it’s worth reviewing what exactly are gravitational waves. Basically, you can think of them as ripples in space-time.

The waves are made anytime anything with mass moves, whether it’s an entire planet or a microscopic organism, but unlike ripples that are created when you toss a rock into a pond, they were previously undetectable. In fact, scientists long believed it would never be possible to do so, or at the very least that it would take centuries of development for us to be able to pull off such a feat.

Gravitational Waves Sensed

In February of 2016, however, scientists made a major breakthrough and were able to record gravitational waves. This historic discovery was accomplished by the Laser Interferometer Gravitational-Wave Observatory (LIGO), which has two different detectors–one located in Hanford, Washington, and one located in Livingston, Louisiana. The detectors are shaped like the letter L and consist of two arms that are each 2.5 miles long. Laser beams pass through their arms at regular intervals, and if a gravitational wave is defected, the arm’s size expands a tiny amount that can be measured by atomic clocks as a split second difference in time.

In order for gravitational waves to be powerful enough to be sensed by LIGO, the event that caused them needs to be massive. The event that was picked up certainly falls into that category, and is thought to have been the result of one black hole about 29 times the mass of the sun colliding with another about 36 times the mass of the sun around 1.3 billion years ago. Another event was subsequently recorded in June, giving scientists two different sets of data to study.

Gravitational-Wave Memory Effect Uncovered

Physicists analyzing the data recovered by LIGO have made a startling discovery about what happens in the wake of a gravitational wave. It now appears that gravitational waves actually change the fabric of space-time with a phenomenon called the gravitational-wave memory effect. Here is a theoretical example to illustrate what the term means:

1. Imagine two astronauts floating in space near two black holes, with our spacemen positioned exactly 10 meters apart from each other.

2. While the two black holes are moving toward one another, there is a lot of disturbance in space, and so the distance between the astronauts is constantly changing as a result.

3. Eventually the black holes collide and after the oscillations halt the astronauts will stop their tiny movements. One would then expect them to still be 10 meters apart, but in reality, they are now slightly closer or slightly further apart. This is because the gravitational waves have actually expanded or contracted the size of space-time itself, meaning it must also have a permanent memory of the black holes’ collision.

Tiny But Important

The gravitational-wave memory effect is believed to always be taking place, and with every gravitational wave that ripples through space-time, there is a tiny change occurring to the fabric of space-time. We don’t notice it, though, because the change is infinitesimal.

Scientists estimate that the change is only one-hundredth to one-thousandth the size of the already tiny gravitational wave. Even with something explosive like the events captured by LIGO, the change was only about one five-hundredth of a millionth of a billionth of a meter in size. So yes, space-time is constantly shrinking or expanding but not enough for us to notice it. Nevertheless, even though the memory effect may not have a big impact in terms of what it does to space-time, it may have a huge effect on the study of physics.

Black Hole Information Paradox

Stephen Hawking and other scientists have long been puzzled over the behavior of black holes, and by their very definition, a black hole represents an absence of everything. Nothing exists inside of it; however, this is in direct violation of the law of quantum physics that states that information can never be destroyed.

To account for what has come to be called the black hole information paradox, Hawking and his teams at Harvard University have postulated that gravitational wave memory is at play in the behavior of black holes. They have theorized that the so-called soft hairs of black holes hold onto information, and then release it in the form of radiation as the black holes evaporate. Therefore, the information is retained, and not destroyed, and the memory effect of gravitational waves seems to fall in line with the notion that this is possible.

Of course, we still have much more to learn about memory effect and gravitational waves, but scientists are optimistic that we’ll soon know much more. It is predicted that a number of black holes collision events will be detected within the next few years, especially after LIGO combines its research with its European counterpart, the Virgo interferometer in Italy, thus generating more wave measurement data for scientists to analyse. With more data and the continued work of Hawking and other scientists, the tiny ripple of a gravitational wave may yet revolutionize our understanding of one of the universe’s biggest mysteries, and as Bruce Allen, director of the Max Planck Institute for Gravitational Physics in Hannover, Germany, explains:

“It’s going to be a really good ride for the next few years. The more black holes they see whacking into each other, the more fun it will be.”

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