Gravity Wave Detector#2

GEO600 One arm

GEO600

After presenting a paper at the European Society of Precision Engineering and Nanotechnology (EUSPEN) in Hannover back in May, I was offered the chance to visit a Gravity Wave Detector. Wow! I jumped at the opportunity!

The visiting delegation were driven in a three-minibus convoy for about 30 minutes, ending up in the middle of a field of cabbages.

After artfully turning around and re-tracing our steps, we found a long, straight, gated track running off the cabbage-field track.

Near the gate was a shed, and alongside the road ran some corrugated sheet covering what looked like a drainage ditch.

These were the only clues that we were approaching one of the most sensitive devices that human beings have ever built: the GEO600 gravity-wave detector(Wikipedia or GEO600 home page)

Even as we drove down the road, the device in ‘the ditch’ was looking for length changes in the 600 metre road of less than one thousandth the diameter of a single proton.

Nothing about how to achieve such sensitivity is obvious. And as my previous article made clear, there have been many false steps along the way.

But even the phenomenal sensitivity of this detector turns out be not quite good enough to detect the gravity waves from colliding black holes.

In order to detect recent events GEO600 would have to have been between 3 and 10 times more sensitive.

The measuring principle

The GEO600 device as it appears above ground is illustrated in the drone movie above.

It consists of a series of huts and an underground laboratory at the intersection of two 600 metre long ‘arms’.

In the central laboratory, a powerful (30 watt) laser shines light of a single wavelength onto a beam-splitter: a piece of glass with a thin metal coating.

The beam-splitter reflects half the light and transmits the other other half, creating two beams which travel at 90° to each other along the two arms of the device.

At the end of the arms, a mirror reflects the light back to the beam-splitter and onto a light detector where the beams re-combine.

Aside from the laser, all the optical components are suspended from anti-vibration mountings inside vacuum tubes about 50 cm in diameter.

When set up optimally, the light traversing the two arms interferes destructively, giving almost zero light signal at the detector.

But a motion of one mirror by half of a wavelength of light (~0.0005 millimetres), will result in a signal going from nearly zero watts (when there is destructive interference) to roughly 30 watts (when there is constructive interference).

So this device – which is called a Michelson Interferometer – senses tiny differences in the path of light in the two arms. These differences might be due to the motion of one of the mirrors, or due to light in one arm being delayed with respect to light in the other arm.

Sensitivity

The basic sensitivity to motion can be calculated (roughly) as follows.

Shifting one mirror by one half a wavelength (roughly 0.0005 millimetres) results in an optical signal increasing from near zero to roughly 30 watts, a sensitivity of around 60,000 watts per millimetre.

Modern silicon detectors can detect perhaps a pico-watt (10-12 watt) of light.

So the device can detect a motion of just

10-12 watts ÷ 60000 watts per millimetre

or roughly 2 x 10-17 mm which is 10-20 metres. Or one hundred thousandth the diameter of a proton!

If the beam paths are each 600 metres long then the ability to detect displacements is equivalent to a fractional strain of roughly 10-23 in one beam path over the other.

So GEO600 could, in principle, detect a change in length of one arm compared to the other by a fraction:

0.000 000 000 000 000 000 000 01

There are lots of reasons why this sensitivity is not fully realised, but that is the basic operating principle of the interferometer.

The ‘trick’ is isolation

The scientists running the experiment think that a gravity wave passing through the detector will cause tiny, fluctuating changes in the length of one arm of GEO600 compared with the other arm.

The changes they expect are tiny which is why they made GEO600 so sensitive.

But in the same way that a super-sensitive microphone in a noisy room would just makes the noise appear louder, so GEO600 is useless unless it can be isolated from noise and vibrations.

So the ‘trick’ is to place this extraordinarily sensitive ‘microphone’ into an extraordinarily ‘quiet’ environment. This is very difficult.

If one sits in a quiet room, one can slowly become aware of all kinds of noises which were previously present, but of which one was unaware:

  • the sound of the flow of blood in our ears:
  • the sound of the house ‘creaking’
  • other ‘hums’ of indeterminate origin.

Similarly GEO600, can ‘hear’ previously unimaginably ‘quiet’ sounds:

  • the ground vibrations of Atlantic waves crashing on the shores of Europe:
  • the atom-by-atom ‘creeping’ of the suspension holding the mirrors

Results

So during an experiment, the components of GEO600 sit in a vacuum and the mirrors and optical components are suspended from silica (glass) fibres, which are themselves suspended from the end of a spring-on-a-spring-on-a-spring!

In the photograph below, the stainless steel vacuum vessels containing the key components can be seen in the underground ‘hub’ at the intersection of the two arms.

GEO600 Beam Splitter

They are as isolated from the ‘local’ environment as possible.

The output of the detector – the brightness of the light on the detector is shown live on one of the many screens in the control ‘hut’.

GEO 600 Control Centre

But instead of a graph of ‘brightness versus time, the signal is shown as a graph of the frequencies of vibration detected by the silicon detector.

Results

The picture below shows a graph of the strain – the difference in length of the two arms – detected at different frequencies.

[Please note the graph is what scientists call ‘logarithmic’. This means that a given distance on either axis corresponds to a constant multiplier. So the each group of horizontal lines corresponds to a change in strain by a factor 10, and the maximum strain shown on the vertical 10,000 times larger than the smallest strain shown.]

Sensitivity Curve

The picture above shows two traces, which both have three key features:

  • The blue curve showed the signal being detected as we watched. The red curve was the best performance of the detector. So the detector was performing close to its optimal performance.
  • Both curves are large at low frequencies, have a minimum close to 600 Hz, and then rise slowly. This is the background noise of the detector. Ideally they would like this to be about 10 times lower, particularly at low frequencies.
  • Close to the minimum is a large cluster of spikes: these are the natural frequencies of vibration of the mirror suspensions and the other optical components.
  • There are lots of spikes caused by specific noise sources in the environment.

If a gravity wave passed by…

…it would appear as a sudden spike at a particular frequency, and this frequency would then increase, and finally the spike would disappear.

It would be over in less than a second.

And how could they tell it was a gravity wave and not just random noise? Well that’s the second trick: gravity wave detectors hunt in pairs.

The signal from this detector is analysed alongside signals from other gravity wave detectors located thousands of kilometres away.

If the signal came from a gravity wave, then they would expect to see a similar signal in the second detector either just before or just afterwards – within a ‘time window’ consistent with a wave travelling at the speed of light.

Reflections

Because powerful lasers were in use, visitors were obliged to wear laser google!

Because powerful lasers were in use, visitors were obliged to wear laser goggles!

This was the second gravity wave detector I have seen that has never detected a gravity wave.

But I have seen this in the new era where we now know these waves exist.

People have been actively searching for these waves for roughly 50 years and I am filled with admiration for the nobility of the researchers who spent their careers fruitlessly searching and failing to find gravity waves.

But the collective effect of these decades of ‘failure’ is a collective success: we now know how to the ‘listen’ to the Universe in a new way which will probably revolutionise how we look at the Universe in the coming centuries.

A 12-minute Documentary

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