LIGO

Good vibrations at Vagabond, Highbury

black coffee, Vagabond, Highbury

A good start to the day. Coffee at Vagabond.

A long black, flat white (with soya milk) and a tea. Yes, you could say we spent a fair while at Vagabond in Highbury the other week. It was a lovely space to catch up with an old friend again. There were plenty of comfortable seats and the staff were definitely friendly, supplying us with coffee and space to chat for a while. The coffee was good (Vagabond are roasters as well as a café) with batch brew and Aeropress/drip on offer together with the usual selection of coffees and other drinks. Tasting notes were on a black board behind the counter while on the wall, also behind the counter, was a drawing of a tongue taste map. While the science of this has been disputed, it does serve as a reminder for us to sit back and properly appreciate – and taste – what we are drinking.

Above the espresso machine was a long rectangular sign that said “coffee in progress”, suspended by four cables, one at each corner. Coffee orders were placed onto this sign allowing the baristas to keep track of who ordered which drink. Given how busy this café occasionally got (and we weren’t even there for lunch), it seems that this is a very handy system. Each time an order was placed on the sign, the whole sign oscillated, rather like a rigid trampoline. Even if you had not seen the note placed on the sign by the barista, you would get a clue, a piece of evidence, that something had just happened by the vibrations long afterwards. Perhaps you may say that the sign was some sort of “order-detector”.

order detector oscillation espresso machine

The “order-detector”: sign at Vagabond in Highbury

Or at least, that is what you may say if you were thinking about the LIGO (Laser Interferometer Gravitational waves Observatory) detectors that, back in 2015, detected the gravitational waves produced by two merging black holes between 700 million and 1.6 billion light years away. Not only do these detectors have similarities to the order-detector sign at Vagabond, the beauty of the LIGO detector is that you can start to understand how it works by staring into your coffee. The LIGO experiment consists of two detectors. Each LIGO detector is an L shaped vacuum tube (4km long) with a mirror at each ‘end’. A laser beam is split between the two legs and reflected back by mirrors at the end of each L. When the reflected laser beams return back to the detector at the corner of the ‘L’, how they interact with each other is dependent on the exact distance that each laser beam has travelled between the mirror and the detector. Think about the bubbles on the surface of your coffee. These colourful bubbles appear as different colours depending on the thickness of the bubble ‘skin’. You may remember being taught that, exactly as with oil slicks on water, it was about the constructive and destructive interference of the light waves. As each ‘colour’ has a different wavelength, the colours that destructively interfere change with the thickness of the bubble skin. You can determine the thickness of the bubble by the colour it appears.

LIGO photo

An aerial photo of the LIGO detector at Hanford. The mirrors are at the ends of the tubes going away from the main building. Image courtesy of Caltech/MIT/LIGO Laboratory

In the LIGO experiment, there is only one wavelength because the light is coming from a laser. So whether the detector registers an intense laser beam or the absence of one, depends on whether those two beams coming back from the mirrors interfere constructively, or destructively. (A deeper description of the technique of “interferometry” can be found here). As the gravitational waves emanating from the collision of the black holes encountered the mirrors at the ends of the L’s in LIGO, so each mirror wobbled a little. This small wobble was enough to change the intensity of the laser light received by the detector and so reveal that the mirrors had moved just that little bit. In fact, the detectors are so sensitive that they can detect if the mirrors move by less than the diameter of a single proton. Given that this is a sub-atomic distance, I don’t think I can even start to relate it to the size of an espresso grind, even a Turkish coffee grind is millions (billions) of times larger than the amount that these mirrors moved. Yet this is what was detected a couple of years ago in the now famous announcement that gravitational waves had been detected and that Einstein’s predictions had been shown to be true.

Watching the “coffee in progress” sign oscillate at Vagabond, it is clear how much engineering has gone into isolating the mirrors at LIGO enough that they do not move as people walk by. Yet perhaps it is interesting that, nonetheless, one of the final refinements of isolating the mirrors from the vibrations of the earth involved changing the material for the cables that suspended them, just as with the sign at Vagabond. You can learn more about the engineering behind this incredible feat of detection in the video here, or you can go to Vagabond, enjoy a lovely coffee and think about the physics of detection there.

Vagabond (Highbury) can be found at 105 Holloway Road, N7 8LT

If you would like to hear what the collision sounded like, follow the link here.

 

Setting standards at Brill, Exmouth Market

Brill, Exmouth Market, neon, architectural history

The neon lit “Brill” from the back of the cafe. You can also see evidence of an old arch in the brickwork, an old doorway?

Brill on Exmouth Market has quite a history. Originally a record store, it has evolved into a music shop/cafe more recently. On my recent visit, I ordered a very good Americano (beans from Officina Coffee Roasters) and although cakes were on sale, it was a small bar of Green & Blacks chocolate that appealed to me a bit more that day. It is a small cafe and so the few seats that are upstairs were occupied. This turned out to be a good thing though because I noticed a sign indicating that there were more seats downstairs, which actually meant that there was seating in a lovely little courtyard/garden at the back of Brill. Although it was originally locked (it was February and fairly dismal when I visited, who in their right mind would want to sit in the garden?), the friendly staff unlocked it and quickly cleaned one of the tables so that I could enjoy my coffee and chocolate in peace in central London. Indeed, the occasional (inevitable?) sound of sirens in the distance only served to emphasise the tranquility of the courtyard. The courtyard has four tables and a glitter-ball in the corner hanging from a tree. There was a lot to appreciate outside, both in terms of the science and the history of the place: Leaves deposited by vortices in corners of the yard with brickwork that suggested a significant re-build has occurred to this cafe.

But from my vantage point, it was the word ‘Brill’, lit up in neon lighting inside the cafe, that caught my attention. Neon lights are always interesting to me because their colour is so suggestive of the atoms that make up the light. The colour of a neon light is determined by the energy levels of the atoms that make up the light, the gas ‘neon’ shines red, hence neon lights. But if you wanted blue ‘neon’ lights you could use mercury as the vapour in the tube instead of neon, it is all about the energy levels of the atoms in the gas in the tubes.

glitter ball, disco at Brill Exmouth Market

A glitter ball in the corner of the courtyard at Brill

Under certain conditions, cadmium also emits a red light which brings us to the subject of this cafe-physics review: The definition of length. How is it that we can all agree on what ‘one metre’ is, or even one ‘inch’? Perhaps you are wondering how the red light emitted by cadmium, (or neon), relates to the definition of the metre? It’s about standards and definitions. Up until about 1960, the standard unit of length (the metre) was measured with reference to an actual, physical, metal rod kept in Paris with two scratches carved into it, one metre apart. Any arguments about the precise length of a metre could be settled by referring to the metre, this metal bar in Paris. But of course there were problems, the first of which was that the metre was in Paris. Perhaps you would think it easy to make copies? Yet in the nineteenth century this was already becoming a problem, the measurements that were being made were becoming too precise. Anders Ångstrom’s pioneering work with spectroscopy (investigation of elements by the colours that they emit/absorb) revealed a small difference between the metre kept in Uppsala (where Ångstrom was based) and that kept in Paris. Although the difference was tiny, when it was compared with what people had started to measure, it became significant. Then there was the question of the scratches: Would you measure the metre between the furthest two points of the scratch? Or the closest? Then an even worse problem was discovered: The rod was shrinking! If you’re tempted to abandon metric units and hark back to Imperial units, bear in mind that the UK Imperial Yard was shrinking even faster. No, something had to be done and that something involved changing the definition of the metre fundamentally.

neon sign, light emission

Neon signs have characteristic colours due to the electron transitions in the ionised gases

It is here that cadmium comes in to the story. Rather than use a physical length that we could all measure, the people whose job it is to define our base units decided that the definition of the metre would be with reference to the wavelength of the red light of Cadmium. I do not know why they did not want to use the red of neon lights but even with cadmium it quickly became apparent that there was a problem. The problem was that cadmium exists as several isotopes, all having a very slightly different ‘colour’ of red light that they emit. So, rather than cadmium, in 1960 they settled on the orange line of Krypton as the definition of the metre. One metre was then defined as 1650763.73 vacuum wavelengths of Krypton. That was the definition for over twenty years before the definition of the metre was updated again in 1983. It is now defined as “the length travelled by light in a vacuum during a time interval of 1/299792458 of a second”.

Perhaps it is not a definition that you or I could use, we’d probably still refer to our metre rule! Nonetheless this definition does allow people to perform experiments that need very precise and very accurate measurements of lengths. These standards are important for extremely sensitive measurements such as that needed to detect gravitational waves with the LIGO experiment, reported a few weeks ago. The neon lights at ‘Brill’ do indeed suggest a story that goes way back in time, both for the cafe and for the science.

Brill is at 27 Exmouth Market, EC1R 4QL

Spectroscopy information from “Spectrophysics”, by AP Thorne, Chapman and Hall Ltd, 1974