Einstein was right…again. Gravitational waves rule!

About 1.3 billion years ago, two black holes collided – and the whole fabric of space-time rang like a bell. Last September, researchers in the US detected the gravitational ripples. They announced it this week – and THAT, my friends, is awesome news.

Representation of gravitational waves produced by two orbiting neutron stars. NASA, public domain, via Wikipedia.

It’s not just the fact that Albert Einstein predicted them 100 years ago. The discovery is going to have ripple effects through physics, from informing speculations about quantum gravity to giving us a new way of ‘listening’ to the universe.

Until now, everything physicists have detected about the universe has been via the electromagnetic spectrum – the one that runs from gamma rays down through x-rays, visible light, heat and then radio. We give its parts different names, depending on frequency, but it’s all the one thing.

Now, with the Laser Interferometer Gravitational-Wave Observatory (LIGO) detector built by CalTech and MIT – largely the brain-child of MIT’s Rainer Weiss and CalTech’s Kip Thorne and Ronald W. Drever, we can look out at the universe through different means. And there are other detectors under construction.

So what are gravitational waves? It’s important to call them that – not ‘gravity waves’, which are an atmospheric or oceanic phenomenon.

Albert Einstein predicted gravitational waves in 1916 with his theory of General Relativity. His thinking went like this. Popularly, we call ‘time’ an illusion. Actually, according to Einstein, gravity is the illusion. Space and time are a ‘fabric’ comprising the three dimensions of space, plus time – often called the ‘fourth’ dimension.

Albert Einstein lecturing in 1921 – after he’d published both the Special and General Theories of Relativity. Public domain, via Wikimedia Commons.

That fabric can be bent by energy or mass – which, according to Einstein’s theory of Special Relativity, are the same thing (you KNOW the equation…). Velocity is entwined because time travels at the speed of light. Thus, a concentration of mass/energy bends space-time.

According to Einstein, gravity is a product of space-time being bent. Particles travel in straight lines. Bent space-time forces them to move in curved paths, and we feel that as gravity. The more mass/energy, the more space-time is bent, and the more ‘gravity’ there is.

The thing is, in a dynamic system, space-time is changed dynamically. The result is a series of ripples in the fabric of space-time, spreading  from a source like ripples from a pond.

Everything creates gravitational waves. However, because gravity (as a force) is exponentially weaker than every other force, those waves are miniscule. If you pick something up with a magnet, it’s out-pulling the Earth. Specifically, gravity is 0.000000000000000000000000000000000001 times as strong as electromagnetic energy (1 x 10 exp -36, in case I miscounted a zero there…) Einstein predicted that gravitational waves probably existed (he wasn’t totally sure), but if they did, they could be used as a detector, just as we use electromagnetism as a detector (visible light bounces off things, radar bounces off things).

Enter the biggest source of gravitational disturbance there is – black holes, the ultimate distorters of space-time. The fact that black holes spin – making them dynamic – was proven by New Zealand physicist Roy Kerr in 1963, when he found a specific mathematical solution for General Relativity that described space-time around a black hole. Now imagine two black holes, orbiting each other on a collision course. They whirl around each other faster and faster – up to 75 times a second – before crashing in a monolithic blast that sends gravitational waves rippling across the universe.

That’s what LIGO picked up on 14 September last year. It’s taken until now to confirm the discovery. Two black holes, of 29 and 36 solar masses respectively, collided around 1.3 billion light years away (yup, in a galaxy far, far away and a long time ago).  Wham! Three solar masses were annihilated in the process – driving the gravitational waves and, for 20 milliseconds, generating fifty times the energy of every star in the visible universe. Yup – that’s what E = MC<exp>2 means. A one-gram paperclip represents more energy than the Hiroshima bomb. Now imagine three solar masses turning into energy. Yah…^

The gravitational wave data picked up from the collision also proved a theorem about black holes devised by Stephen Hawking in 1970.

LIGO schematic. Public domain, via Wikimedia Commons.

What is LIGO? It’s a laser interferometer capable of measuring changes of distance down to atomic level. That’s the sensitivity needed to pick up gravitational waves. The system is 4 km long and needs hard vacuum. A laser beam is split, sent down two known distances, bounced off a mirror, and reflected to a detector. They should cancel each other out. If they don’t, it means one leg of the journey has been stretched or compressed – implying that space-time has been distorted and a gravitational wave just ran through.

Unfortunately, and more usually, it also implies a truck just rolled along a road nearby or there was an earthquake somewhere. To eliminate error the system includes deliberate ‘false positive’ data. And it’s been duplicated – there’s a LIGO installation in Livingston, Louisiana and another at Hanford in Washington state. To be genuine, a gravitational wave has to be detected by both – and in this case, the signal arrived 7 milliseconds later at Livingston, allowing the teams to determine that gravity (like time) moves at the speed of light.

Apart from proving Einstein was right (again) the discovery has all kinds of ramifications. It offers a new way of looking at the universe. But it also gives us data that can be fed back into theoretical physics. One of the questions has been reconciling ‘quantum gravity’ with General Relativity.

The new proof that gravitational waves exist in space-time, among other things, gives an upper limit for the mass of the proposed quantum gravity particle, the graviton. The maximum mass is trivial; the implication is huge.  And, in case you were wondering, gravitational waves can be rendered as sound. I always knew the universe was drippy:

This is the new frontier, and we’ll learn more as the instruments are developed, and more LIGO-style detectors are built.

Meanwhile, I think I know where the next Nobel Prize in physics will be awarded. That’s how big this discovery is.

Copyright © Matthew Wright 2016

^ OK, I’m a geek, I had to calculate it – three solar masses is the energy equivalent of  1.281467216 x 10 <exp> 38 tons of TNT.