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Bordering on Impossible

I have mentioned my admiration for the LIGO project before but since then it has actually achieved its goal so now might be a good time to discuss it again.

First, what is it? Well if you haven’t heard the news (if you haven’t you obviously don’t follow science news at all) and haven’t read my previous post on LIGO (titled “Ripples in Space-Time” from 2015-11-10) here’s a brief summary…

LIGO is an experiment designed to detect gravity waves.

It consists of two detectors – one in Washington and one in Louisiana – which consist of two 4 kilometer long tubes, containing a high vacuum, at right angles to each other. A precision laser shines down the each tube and is reflected back to the central point.

If a gravity wave hits the experiment it warps the detectors (to be precise it warps the space-time the detectors occupy) very slightly and that can be measured by changes in the light beam, specifically by how the two beams interact. When there is no gravitational warping the two beams are in phase but if one is warped the beams interfere.

The reason there are two detectors (each with 2 lasers) rather than one is that local effects (traffic, small earthquakes, etc) can affect them far more than gravity, but these will only affect the nearby detector. Gravity waves will affect both (with a tiny interval of time between them).

It sounds simple but the complicating factor is the size of the effect. Imagine trying to measure the size of something to a precision of one part in one hundred million trillion. That precision can never be imagined in relation to normal size objects so let’s compare it to the whole planet Earth.

The Earth is about 13,000 kilometers in diameter so to measure it with the same precision the measurement would need to be accurate to 0.00000000013 of a millimeter. If a single grain of sand interfered with that attempted measurement it would distort the measurement by a factor of 8 billion times too much. In other words, the precision is equivalent to measuring the width of the Earth accurate to one 8 billionth the width of a grain of sand.

A good phrase to describe the staggering difficulty of this task was “bordering on impossible”. In fact, many people thought it really was impossible. But it wasn’t. Because gravity waves were actually discovered at LIGO near the end of last year and officially announced earlier this year.

And there are a few interesting details of the discovery which make it even more incredible. Here’s an overview of some of them…

The gravity waves which were detected were created in an event where two large black holes, each 20 to 30 times the mass of our Sun, merged. This happened 1.3 billion light years away which means it happened a billion years ago and the waves had taken that long to get here. The event was translated into a sound which has been described as a “chirp”. It lasted just 0.2 of a second.

The detectors had been upgraded and had just been switched on again. An scientist in Germany first saw the signal and thought it might just be a test because there had been extensive testing of the new system up until then. But he soon found it wasn’t and the timing of the event in the two facilities clearly showed a real gravity wave which could even be isolated to a line through the sky. The collision happened somewhere along that line.

If a third detector had been available the position could have been deduced by triangulation but unfortunately a third device in Europe which might have been used was being maintained. But hey, you win some and you lose some, and finding the event at all so quickly after an upgrade was a big win in itself. After all, massive black holes don’t collide that often!

So what does it mean?

Well the observation finally confirms a prediction of Einstein’s General Theory of Relativity which was published exactly 100 years prior to the confirming observation. Of course, many other aspects of the theory have already been confirmed but gravity waves were one of the few that hadn’t. Relativity really is a remarkable theory and its predictive ability has never failed.

Gravity waves now allow astronomers to look at the universe in a whole new way. Instead of using electromagnetic radiation (light, radio waves, x-rays, microwaves, gamma waves) some super-energetic events can be observed using their gravity radiation.

And the confirmation of Relativity further strengthens its role as one of the core theories in physics. It is a theory related to the most basic levels of reality so there are few obvious practical benefits, but fundamental theories are what everthing in our modern, technological society are based on, so their importance cannot be overstated.

And like all great technical achievements which push the extreme boundaries of technology (the space program being the most obvious) there will be spin-offs from the actual construction of the facilities which will be used in diverse areas of technology in the future.

So yes, LIGO is an astonishing technological tour de force – on a similar level to the LHC, the Apollo program, and the Hubble Space telescope – something that every human on the planet should be truly proud of.

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