Explaination - Blog Posts

Can you explain in a simple (???) way gravitational waves? please? do you know any books about it?

Sure!

According to Einstein’s General Theory of Relativity, what we think of as "empty space” isn’t nothing. Instead, space is more like a fabric that can be stretched, squashed, bent and shaped, and all matter and energy cause space to bend around them. The more mass or energy something has, the greater the bending of space around it it causes, a bit like heavier and lighter balls on a rubber sheet:

(Source - Note that this picture is a 2D analogy, and space is actually 3D! The bending of space isn’t something we can easily visualise, so we have to use analogies like the “balls on a rubber sheet” analogy - as long as we recognise their shortcomings!)

Let’s imagine the Sun is a bowling ball dropped onto a rubber sheet, creating a huge dent in space. And now let’s roll a marble - Earth - onto that sheet too. IF the marble is rolling too slowly, it will fall into the dent and roll around a few times, spiralling in and eventually colliding with the ball. If the marble is rolling too quickly, its path will be bent, but it will escape. If it’s rolling at a certain speed, however, the marble will roll around the bowling ball and go into orbit around it. (Here’s another shortcoming of the rubber sheet analogy - real rubber sheets have friction, so the marble would eventually slow down and roll in towards the bowling ball. Space, however, has no friction, so the Earth can stay in orbit around the Sun for a long time.) In other words, this bending of space is what we refer to as gravity!

In Newton’s view of gravity, Earth would naturally follow a straight line through space, but its path would be bent towards the Sun by a mysterious pulling force. That force holds the planets in orbit around the Sun and pulls apples to Earth, but Newton couldn’t explain why - a mysterious influence that spread out through space, called the gravitational field, somehow caused bodies to attract one another. Einstein explained that massive objects curve the space around them. Earth would also naturally follow a straight line through space, but the space itself is curved, forcing Earth to follow a curved path - it’s a bit like trying to walk in a straight line along a hill. Try as you might, your path will have to bend to follow the contours of the landscape. According to Einstein, gravity isn’t really a “force” as such but an effect of this bending of space. Matter and energy tell space how to bend; space tells matter and energy how to move. That’s all gravity is. The gravitational field isn’t some mysterious entity in space - the gravitational field is the space itself! Here’s a nice little video to help you visualise all this:

(I’m oversimplifying a little, btw, saying that gravity is the bending of “space.” In Einstein’s theory, the three dimensions of space are unified with time into one four-dimensional fabric, the space-time continuum. So gravity isn’t just the bending of space, but the warping of time too - you can’t change one without changing the other! Gravity actually slows time down, so you would age slightly faster in space than you do at Earth’s surface. The difference is incredibly tiny, but measurable - time passes more quickly for the GPS satellites than it does for us here on Earth, and what the clock of a GPS satellite would measure as “one day” is about 38 microseconds shorter than what we measure as “one day.” That doesn’t sound like a big difference, but engineers have to take it into account when designing GPS systems - if they didn’t account for this, your GPS location would drift by as much as 10 kilometres per day! So this isn’t just some abstract theory - this is a real effect that’s already important for technology you probably use every day.)

General Relativity has now been through many, many tests and has passed every one with flying colours, and all of its predictions had been verified by the beginning of 2016 except one - gravitational waves.

What would happen if we could somehow destroy the Sun? Newton believed that there was a mysterious gravitational connection between the Sun and Earth, holding Earth in its orbit, that would instantly be broken if the Sun was destroyed. Earth would instantly fly out of its orbit in a straight line. Einstein, however, didn’t like this - his Special Theory of Relativity (which he put out 10 years before the General Theory) says that no information could ever travel faster than light. It takes about 8 minutes for the Sun’s light to reach us, so how could Earth fly out of its orbit instantly? That would let us know the Sun had been destroyed 8 minutes before the light from the Sun’s destruction reached us. Einstein wasn’t comfortable with this.

Thankfully, General Relativity resolves the paradox - if you got rid of the Sun, Earth would still stay in its orbit for a while, because the space-time around the Sun would still be curved. Meanwhile, at the place where the Sun was, space-time would spring back to its original flat state, and that would ripple through the surrounding space-time as everything adjusted back to where it was. That ripple - a gravitational wave - would spread out through space at the speed of light, so the space around Earth would stay curved and Earth would remain in its orbit until the same time the light from the Sun’s destruction passed us - at which point the gravitational wave would ripple through the space around Earth and restore it back to its original flat state, and Earth would finally leave its orbit.

Of course, in reality, stars don’t just disappear. But the gravitational environment does change. Stars move around, and the fabric of space-time also moves with them. Stars explode. Black holes and neutron stars form, putting huge dents in space-time, and sometimes they collide. All these events are a bit like changing the environment in a still pond - stars and planets gently orbiting are like ducks gently gliding through the pond, creating gentle ripples as they disturb its surface - and black hole collisions are more like throwing a rock into the pond and sending out massive waves. Almost everything in our universe produces gravitational waves, but most of the time, they’re too tiny to detect. (That’s why I said in real space the Earth can orbit the Sun for “a long time,” and not “forever.” Earth is constantly sending out very faint gravitational waves as it rolls around the Sun and moves through the fabric of space-time. Those waves are too small to detect, but they very, very slowly sap Earth’s energy and cause it to very, very slowly spiral in to the Sun. In reality, that would take unimaginable trillions upon trillions of years, and Earth will probably be destroyed by the dying Sun long before that! Even if Earth manages to survive that, it’s more likely to be pulled out of orbit by an incredibly rare passing star or knocked out by unpredictable gravitational tugs from the other planets or something before it spirals into the Sun. Orbits are stable for a very, very, very, very long time.) More intense sources of gravity than our puny Earth and Sun, however - things like neutron stars and black holes - can generate detectable gravitational waves.

Our first indirect evidence of gravitational waves came in 1984, when the American astronomers Russel A. Hulse and Joseph A. Taylor discovered a binary neutron star system - two intense sources of gravity orbiting each other very rapidly. As they orbited each other, they sent out huge gravitational ripples - a bit like stirring up that duck pond with two oars whirling round and round - and lost energy by a detectable amount. Hulse and Taylor found that their orbital period slowed down by about 75 milliseconds per year - short, but detectable! That slowing exactly matched the predictions of gravitational wave theory and got its discoverers the Nobel Prize for Physics in 1993.

(Source)

But gravitational waves weren’t directly observed until 2015 (and confirmed until this year) by a detector named LIGO (the Laser Interferometer Gravitational wave Observatory). All LIGO is is basically two beams of laser light travelling between two pairs of mirrors oriented at right angles to each other, like this, so you can measure how space-time is stretched in one direction and squashed in the other by a passing gravitational wave by recording how long it takes the light to travel from one mirror to the other*:

(Source for both images: http://phys.org/news/2016-02-ligo.html)

LIGO’s two “arms” (the two beams of light) are each 4 kilometres long, and a gravitational wave passing through the detector stretches or squashes each of the “arms” by a ridiculously small amount - the ones LIGO actually found stretched and about 1/10,000th the width of a proton. As you can imagine, the LIGO physicists had to account for many, many different effects that shook the detector too. But gravitational waves distort the two beams in a predictable way that would make that distortion stand out from ordinary passing trucks or distant earthquakes, and by February 11th, 2016, the LIGO physicists were confident enough that they really had detected a faint ripple in space-time passing through their detector. The signal was consistent with a gravitational wave from two black holes in orbit around each other, spiralling in to one another.

(Source)

This is exciting for two reasons:1) It confirms the last outstanding prediction of General Relativity, and2) It opens up a whole new field of astronomy! Every so often astronomy is revolutionised by the discovery of new things we can look at from space. Originally all we could detect was the visible light that we could see with our eyes and telescopes. But soon we learned to build radio telescopes, and that opened up a whole new world to us - we could see phenomena that were invisible in ordinary light. With space telescopes we could see the sky in gamma rays, x-rays, ultraviolet and infra-red light as well. Now we could see the explosions of distant stars halfway across the observable Universe, look at clouds of gas and dust too cool to shine in visible light, and peer through other dark clouds to see stars forming inside. We also found particles we could see coming from space, too - neutrinos from the Sun and from supernovae, and cosmic rays. These opened up other windows on the Universe. And now we have gravitational waves - yet another new way of “seeing.” Gravitational wave astronomy will let us study some of the most puzzling events in the Universe, like colliding neutron stars, or black holes falling into other black holes - events we’ve never been able to see before. 

So I hope that helps, Anon! 

As for books, the problem is gravitational waves were detected so recently I don’t know of any books that have come out since then on the subject, so everything will be out of date. However, the basic physics has stayed the same since Einstein first predicted them, so any good popular book on general relativity (Spacewarps by John Gribbin, The Fabric of the Cosmos by Brian Greene and Travelling At the Speed of Thought: Einstein and the Quest for Gravitational Waves by Daniel Kennefick are good examples) should give you some good insight - just replace phrases like “if we detect gravitational waves” with “when we detected gravitational waves!”

(*Yes, yes, I know LIGO isn’t actually measuring the time taken for light to travel down each “arm,” but the interference of the laser beams. Still, that interference allows us to infer the travel time for the light, so I’m simplifying.)