I’m afraid this post isn’t about the best surf spots out there but a different type of waves, gravitational waves – big news for physicists in the last year. In 1915 they were predicted by Einstein as a product of his theory of general relativity and in 2015, on the theory’s 100th anniversary, their existence was verified – but what is all the fuss about and how did the long awaited detection come about? Let’s recap.
Now if you’ve read some of my previous posts you’ll be familiar with the term ‘spacetime’ the continuum (or manifold to speak mathematically) of the four dimensions – three of space and one of time. As we know, masses create distortion or ‘bending’ of their surrounding spacetime – just like a bowling ball creates a dip if placed on a trampoline. If masses then accelerate the continual changing in the distortion of their surrounding spacetime will cause ripples to occur – just like the ripples produced on the surface of a pond when a pebble is dropped in it or a duck accelerates through the water. These outward ripples act like waves and transporting energy in the form of gravitational radiation. These waves are very similar to electromagnetic waves, which transport a form of radiant energy, for example visible light. These electromagnetic waves travel at a speed of c, colloquially ‘the speed of light’ and hey presto so do gravitational waves. In a nutshell gravitational waves can be thought of as ripples in the curvature of spacetime due to the acceleration of masses.
Now what is the effect of these gravitational waves? Well the ripples in the fabric of spacetime signify the changing of the amount of space between objects. Because it is spacetime itself that is rippling it is spacetime itself that is being distorted by the acceleration of the masses. If two objects A and B had a certain amount of space between them, a gravitational wave could cause this space to expand or contract. But there is a catch. How do we measure such shifting of space? A conventional ruler wouldn’t work! Think about it, if we put a marker on the floor by A and a marker on the floor by B the gravitational wave will cause the space between the markers on the floor to also expand or contract relative to each other and hence we would notice no difference! We need a special ruler and for this we use the speed of light. If the space between two objects expands light will take longer to travel from one object to the other and it is through measurements like these we can detect the waves. Cue the LIGO experiment – Laser Interferometer Gravitational-Wave Observatory.
The LIGO experiment has 4 kilometre long tunnels and uses lasers to measure the changes in the distance between the ends of the tunnels. The tunnels are set up perpendicular to each other so that when a gravitational waves passes through one of the tunnels is contracted and the other is expanded. Now the source masses and precision involved in order to produce a valid detection are very high .Gravitational waves are produced whenever any masses accelerate – for example if two sharks whirl around a circle, gravitational waves will occur (as well as water waves of course!). However the gravitational force is the weakest of all the fundamental forces and seeing as the mass of the sharks is very small on the astronomical scale of things the magnitude of these gravitational waves will be pretty much negligible. We need big masses in order to produce detectable waves, i’m talking black hole heavy.
The waves given off by the cataclysmic final merger of two distant black holes (GW150914), reached Earth after travelling over a billion lightyears, as a ripple in spacetime that changed the length of a 4km LIGO arm by a ten thousandth of the width of a proton – this is a noticying the difference of a few parts in 10^-23 meters! If you can’t visualise the sheer infinitesimal scale of these numbers try this – it is proportionally equivalent to changing the distance from us to the nearest star outside the Solar System (Alpha Centauri) by one hair’s width and being able to measure this! My mind is blown, if yours isn’t you must have inhumanly fat hairs on your head. And on top of all this you need an amazing data analysis capabilities to cancel out all the random noise that could interfere with the signal -like identifying a whisper in a noisy room. All these factors make even the most extreme gravitational waves uncatchable on Earth by any means other than the most sophisticated detectors, like LIGO, who confirmed their findings last year.
Now that we know with certainty that gravitational waves are out there, more advanced versions of LIGO are to be created such as LISA – a space-based gravitational wave detector operating on much larger scales than LIGO which, being in space, won’t have to worry about things such as seismic effects messing up the readings. Our ability to scour the skies for gravitational waves opens up a completely new way to study the universe, almost like discovering a new sense with which to investigate our surroundings. The waves will reveal the cataclysmic explosions and collisions occurring throughout the far reaches of the universe. A plethora of events such as spinning neutrons stars, supernova, black holes emit gravitational waves and now that our ears are opened we can finally tune into the sound of orchestral universe.