By Benjamin Vermette
Two Colliding Neutron Stars Create Gravitational Waves
For the first time in humanity’s history, and by using the unprecedented technique of measuring both the light and the gravitational waves emitted into space, scientists were able to detect a collision between two neutron stars. The news was made public on October 16, 2017.
Interestingly, it can easily be said that a neutron star collision is a once-in-a-lifetime discovery: “Our background analysis showed an event of this strength happens less than once in 80,000 years by random coincidence, so we recognized this right away as a very confident detection and a remarkably nearby source [of 130 million light-years away],” said Laura Cadonati, professor of physics at Georgia Tech and deputy spokesperson for the Laser Interferometer Gravitational-wave Observatory (LIGO), one of the observatories that measured the gravitational waves emitted during the collision. “This detection has genuinely opened the doors to a new way of doing astrophysics. I expect it will be remembered as one of the most studied astrophysical events in history.”
But first, what is a neutron star? And what about a gravitational wave?
You’ve probably heard of the term ‘gravitational wave’ sometime in the not-so-distant past. In fact, they’re new to us! Even though Einstein predicted them on paper in 1915 alongside his general relativity theory, gravitational waves were only detected two years ago, on September 14, 2015. (For more on this subject, go to http://espritdecorps.ca/from-sky-to-space/2016/3/23/from-sky-to-space?rq=LIGO)
In order to better understand the core concepts of gravitational waves, we need to build around the fact that space itself is something. Even though you may rightfully think of empty space as dark, cold, and empty, it’s still something. Consider this: empty space can’t be nothing, since we can name it; true, nothingness wouldn’t bear any other name than ‘nothing.’ Now, putting aside philosophy and letting place to cosmology, if empty space is something, then what is it?
For the sake of being gentile with our intuition, let’s imagine space as being a kind of fabric — something that bends and distorts. For instance, you can imagine it as being a big mattress, where the galaxies, stars and planets represent balls of different masses. What happens if you put a big bowling ball on a mattress? Well, obviously, the mattress is distorted; the same happens with space and massive objects — that’s what we call gravity.
Try and picture it in your mind: a bowling ball on a mattress creates a deeper area around the ball. Then, if you slide, say, a golf ball near the bowling ball, the golf ball’s path will be curved towards the bowling ball, creating the illusion that the two are mysteriously attracted by an unknown force. Furthermore, you could theoretically succeed in putting the golf ball in ‘orbit’ for a couple of seconds — that is, making the golf ball turn in circles around the bowling ball — if you throw the golf ball on a particular path with a precise speed.
In the end, Einstein’s theory on general relativity is simple (yeah, right — perhaps if you put the mathematics aside); just reread the last paragraph by replacing the word ‘mattress’ by ‘space’, the word ‘bowling ball’ by ‘star’ and the word ‘golf ball’ by ‘planet.’ So, it’s not entirely true that there is a mysterious attractive force between two objects (sorry, you’ve probably been taught so your entire childhood, but that’s the way it is!); rather, objects are attracted to each other because they follow their natural motions, or, in more technical terms, they follow their geodesics.
With that in mind, the concept of a gravitational wave can even be intuitive! What happens when two bowling balls collapse on a mattress? It creates waves, both in the air (where we perceive them as sounds) and on the mattress — it might be hard to detect, but the mattress will jiggle like the surface of a pond after you’ve dropped a rock in it.
Thus, gravitational waves can be caused by a loud cosmic event (like a neutron-star collision or the merger of two black holes) before being propagated through the entire fabric of space. Then, just like regular water waves distort the surface of a pond, gravitational waves distort space, and, since you are a part of space, it distorts you as well — by a tiny amount, but it’s still mind-blowing!
Now, let’s get back to the collision of the two neutron stars scientists discovered by first learning about neutron stars themselves. As you may know, everything is made out of atoms, but the atoms are also made out of something, but what? The atom structure can be separated into two parts: the electronic cloud and the nucleus. The electronic cloud surrounds the nucleus and is made of particles called electrons, while the nucleus is a little package of protons and neutrons. The interesting thing is that if we could bring an atom to the scale of, say, a football stadium (the electronic cloud representing the perimeter of the stadium), then the nucleus would only be the size of a pea! That means that everything is essentially made of … emptiness!
Cool, but how is this pertinent to understanding neutron stars? Well, it turns out that neutron stars are one of the only celestial objects to make exception to this rule: they are not made of atoms therefore they are not made of emptiness. Rather, neutron stars are only made of neutrons, so they are a kind of gigantic atom nucleus! This characteristic makes them darn dense: a single cubic centimetre of neutron-star matter would weigh about 400 million tons. In order to create such denseness, you would have to squeeze over 3,000 CN Towers into the size of a die! However, playing with that die wouldn’t be something I would recommend, as just putting it down on the table would make a hole through the table, through the floor, and through the earth.
Even though neutron stars are itsy bitsy in a cosmic sense (around 20 kilometres in diameter), their unique density and mass make them capable of generating gravitational waves. However, we need to bear in mind that this time the collision occurred 130 million light-years away — which is a relatively small distance on a cosmic scale, but still significant — so the gravitational waves detected here on Earth were not as powerful and destructive as you might think; in fact, the ones detected disrupted space (and everything in it including you, me and the Earth) by only a fraction of an atom’s width. So, the question now is: How is it even possible to measure and detect distortions on the nanometric scale?
Well, the LIGO facilities and the other gravitational-wave observatories were arranged in such a way that two long cylinders, each measuring precisely the same length, are placed perpendicularly to each other. In those long tubes shine two lasers that are being reflected at both ends. The arms being of the same length, it makes the lasers come back at exactly the same time, cancelling each other out. The only way for the lasers not to come back at the same time and to not cancel each other out is if the tubes change in length. But how can you do that? Just ask … gravitational waves!
If a gravitational wave passes through the Earth, and subsequently through a LIGO facility, the tubes will change in length, one being longer and the other being shorter. This way, the lasers won’t come back at the same time and won’t cancel each other out; this creates a variation in light intensity that scientists are able to measure and quantify. Then, they try to deduce the origins of the variation in light intensity. In this case, characteristic gamma rays emitted during any neutron-star collision were also detected, helping scientists arrive to the conclusion that a neutron-star collision was indeed at the origin of this precise measurement.
Why should we care?
Gravitational-wave detection is now opening up a whole new sphere of astronomy. As we saw, scientists can now detect cosmic events that would otherwise be invisible. Gravitational waves open up our eyes … or our ears, rather. We have been trying to understand the universe by looking at it for centuries, but we can now listen to its sounds. Humanity has officially recovered its sense of hearing: just imagine the possibilities that can emerge from this.
But in the end, what does it all mean?
Discovering such an improbable event as the collision of two neutron stars in a remote region of our universe by measuring the quasi-insignificant distortions in space caused by the gravitational waves emitted from that event reminds me of what I believe is the true nature of the human mind: Exploration.
Imagine a world where findings in physics had not been applied. This means no more cell phones, computers, or GPS. In such a world, I think physics still wouldn’t be irrelevant. Humankind would still try to understand the universe, because it fills our innate sense of wonder and curiosity. These are the precise states of mind that we have been exploiting since our ancestors started walking on two feet, exploring Africa. These are the precise states of mind that made us an “intelligent” civilization.
Few domains nourish wonder and curiosity as science does. We know the universe has more complexity, beauty and art than we can currently comprehend, so what better way to satisfy our instincts than to study it?
Science and cosmology are in fact the only links to our true origin, the cosmos. So, do we really want to cut space-exploration funding to focus on “true human affairs”? Do we really want to cut our only source of genuine emancipation by putting an opaque box around the Earth? Obviously, no, we don’t want that as this would create an obscure intellectual environment.
The healthiest civilizations have always stayed connected to the stars, whereas the most destructive and cowardly ones operated within an opaque box surrounding their territory, allowing them to apply their dogmatic, unscientific and often despotic worldviews.
Science isn’t only a blind method; it’s a mind-opening philosophy that has a word to say on human affairs because it clearly settles humankind’s place in this convoluted cosmic ocean. No, the Earth is not the centre of the Universe, nor is our sun or our galaxy. This tells us that humans shall not act as if it were so; we are just a temporary coincidence in the history of time and we must cherish and protect that privilege.
In the end, science is moral.