What is gravity?
That might sound silly.
THAT'S gravity.
The Moon orbiting the Earth, orbiting the Sun - that’s gravity too.
But these only explain how gravity behaves,
not really what it is, or how it functions,
and - small spoiler -
this video won’t explain those things either. Because...
No one really knows.
In fact, given how central gravity is to the structure of the entire universe,
we know relatively little about it,
which makes any gravity-related discovery extremely exciting.
One such source of excitement occurred in September 2015,
when the Laser Interferometer Gravitational-wave Observatory,
LIGO, if you don’t like tongue twisters,
made the first ever detection of a gravitational wave.
But to understand the excitement surrounding that,
we might need to backtrack a bit.
Imagine a bedsheet, being pulled tight by four people - one at each corner.
If you were to place a basketball in the centre of that sheet,
it would create a dip in the fabric,
and any smaller objects you place close enough on the sheet
will all roll towards the basketball.
This is essentially how gravity works -
in this analogy the sheet is space, the basketball is a planet,
and the dip in the sheet is the planet’s gravitational pull.
And the analogy goes further:
The more distance between objects and the planet,
the less they’re affected by its gravity.
And if you were to add a much heavier object to the sheet
- say a bowling ball -
that might be analogous to a much denser planet, or a black hole,
wreaking gravitational havoc on anything nearby.
...and probably ruining your bedsheet.
The point is, by observing the way objects on the sheet
affect the fabric around them,
we can visualise how objects in space interact with gravity.
And how, as they move they create ripples,
which will pass through the “fabric” of space and time.
These ripples are gravitational waves.
They were first theorised by Albert Einstein in 1915 -
a hundred years before we actually observed one,
which explains some of the excitement.
Their detection confirmed what had been
almost entirely theoretical for a full century.
But the other exciting thing about this discovery was that it represented
a staggering scientific achievement.
You see, detecting gravitational waves is... tricky.
When one travels through space,
it bunches up and stretches out everything it passes through.
Everything. Including our detection tools - rendering them kind of useless.
So what’s the solution?
Well, as with all the best questions in science, the answer is...
lasers.
At LIGO, a laser is fired at a beam splitter,
and the two beams are reflected back and forth between two mirrors 4km apart,
travelling more than 1000km each in total.
That’s around the distance from New York to Chicago,
which might seem excessive,
but the further the lasers travel, the more accurate the experiment.
Eventually, on arriving back at the beamsplitter,
the two lasers are combined,
and here, something very cool happens.
You see, lasers emit lightwaves,
and if you combine lightwaves that have the exact same frequency
but are perfectly out of phase
- meaning the peaks and troughs of the two waves are exact opposites -
they cancel each other out.
Essentially, they vanish -
which is exactly what happens at LIGO.
Once the beams are recombined
if everything is working correctly, you can’t detect any light at all.
And why is that good?
Well, the crucial thing is the level of accuracy required for this to work.
If either beam is disrupted by the most microscopic amount,
they’ll no longer be perfectly aligned, and light will be visible.
And this is what happens
when a gravitational wave passes through the Earth -
it knocks the beams out of step by a minuscule amount,
and a tiny flicker of light is visible.
Voila, an observable gravitational wave.
If this wasn’t impressive enough already, there’s a twist -
the sensitivity required to measure such tiny disruptions
makes LIGO’s greatest strength also its greatest weakness.
The disturbance caused by a gravitational wave
can be around 1/10,000th the width of a proton,
and any system that sensitive is going to suffer from background noise.
As such, LIGO is equipped with
an astonishingly complex series of damping systems
to isolate it from outside vibrations.
These include an internal seismic isolation system,
which senses and then counteracts ground movements,
a complex mirror suspension system,
and one of the largest and purest sustained vacuums on Earth.
And if all of that wasn’t enough, they built two of them,
three thousand kilometres apart,
in order to be able to compare two isolated sets of data.
This whole thing is incredible,
but it’s not just an exercise in scientific showing off -
the work being done here is ground breaking and important.
Detecting gravitational waves gives astronomers
a new way to observe the universe.
And with a developing network of detectors around the world,
including VIRGO in Italy, GEO600 in Germany
and KAGRA in Japan,
this new wealth of data can be combined
with more traditional observation techniques
to gain new insight into things we’ve already been studying for a long time.
They’ll allow us to observe events difficult to see,
such as black hole collisions, and potentially other phenomena
that right now, we know nothing about.
Plus they open up a path to a better understanding of gravity itself,
one of the fundamental building blocks of the universe.
And while it’s anybody’s guess where that might take us,
it’s hard to overstate
what it might mean for our scientific and technological potential -
both out there in the cosmos, and right here on Earth.