slate – Bean Thinking

Light and gravity at Tab x Tab, Westbourne Grove

drinks in ceramic mugs, Westbourne Grove — A soya hot chocolate and my black coffee at Tab x Tab

Earlier this summer, a new café opening on Westbourne Grove attracted a lot of attention. “Tab x Tab” quickly received reviews from Brian’s Coffee Spot (who noted the unusual espresso machine), Bean There at and Doubleskinnymacchiato. Bean There at also suggested that there should be plenty to ponder at Tab x Tab when I finally got the chance to get there. And so, a trip to this café had been on the agenda for a fair while.

Wandering into the café, it seemed exactly as described by the reviews: clean, sharp interiors in a modern building. It was fairly crowded when we arrived just after lunch and so we ordered before taking a seat at the bar (two of the few seats left). I had a long black while my fellow imbiber had a soya hot chocolate. The drinks arrived in those distinctive mugs mentioned by doubleskinnymacchiato (and pictured above). As we had just had lunch, on this occasion we didn’t check the edibles on offer but with plenty of other reviews of the coffee and the cake, I’m sure that you’ll find recommendations there (I understand the avocado on toast with cashew nut is well worth trying).

Graphite, double layer graphene, stacked hexagons — Plant on two slates at Tab x Tab, Westbourne Grove

Sitting down to enjoy our drinks, the first thing to notice was that Bean There at was absolutely right. Despite the slightly minimal and elegant decoration, there were plenty of things dotted around that were slightly quirky. Firstly there was the plant that had been placed on two hexagons of slate that had been ever so slightly displaced from each other, presumably for aesthetic effect. Could this link to graphene and graphite with their strong intra-layer bonding and weak interlayer bonding (so the hexagons of carbon in graphite slide over each other)?

Then there was the selection of items for sale that also provided food for thought. Books and other items from the School of Life, something to think about as you stop with your coffee perhaps. In the other direction, on the counter top, a couple of Venus Fly Traps were waiting for their lunch. There is so much we have yet to learn about the symbiotic relationships between plants and animals and especially between plants and fungi. As we looked further around the café, there was something else a little odd. Just as the name “Tab” was written both the correct way and upside down in the window, so the plants in the hanging baskets were hanging upside down.

which will win, gravity or light — Plants hanging upside down in the window at Tab x Tab

This seemed a bit strange in itself. Plants have a tendency to move upwards towards the light. This behaviour of plants (and trees in particular) provides one way to identify which way is south when walking in the country without a compass¹. It is odd to see a plant growing downwards and suggests that the plants in the window are regularly rotated so that they don’t try to reach up. As Simone Weil wrote “Two forces rule the universe: light and gravity”². Which would win in the end? To be fair, Weil was not referring to the light that streamed through the windows in Tab x Tab giving the plants the force they need to move upwards. Nonetheless, whether one is thinking literally or analogously, it is an interesting question what pulls us down, what brings us up?

There is a story that Newton arrived upon his idea of universal gravitation by contemplating a falling apple. Considering that the plants were approximately 2m above the floor level, and using the fact that the acceleration due to gravity, g, is 10 m/s², if the plants were to fall from their hanging position, they would take:

s = ½gt²

t = 0.6 seconds

to fall and smash to the ground*. While this brings to mind Newton’s experiments dropping pigs bladders filled with liquid mercury from the dome of St Paul’s Cathedral, it is worth instead thinking more about the universal nature of the gravitational force. This is of course what made Newton’s idea of gravity different from the theories that had preceded it. People had known that if an apple fell from a tree (or a plant fell from its hanging basket) it would fall to Earth. What was key to Newton’s idea was that what applied to the apple, applied to all other masses too. The same maths that could be used to calculate how fast a plant dropped, could be applied to the Moon. So, if this was the case, could we calculate the orbital distance of the Moon in the time it took us to enjoy a coffee at Tab x Tab? We know that the Moon’s orbital period is τ = 27.3 days (2.36 x 10^6 seconds) so assuming that the gravitational force acting on the Moon is balanced by the centripetal force, we can equate the two:

Gravity: F = GMm/r²

Centripetal: F = mv²/r

Where, G is the gravitational constant (6.67 x 10^11 Nm²/kg²), M is the mass of the Earth (5.97 x 10^24 Kg), m is the mass of the Moon and r the moon’s orbital distance (which is what we want to calculate). If we assume that the Moon travels in a circular orbit (not quite true but not a bad first approximation), then we know the speed, v, of the moon in terms of the orbit period, it is just:

v = 2πr/τ

A bit of re-arrangement and some plugging in of values leads to a back-of-the-envelope value for the Moon’s orbital distance of 383 000 Km. A value that does not compare badly at all with the average distance of the Moon given by NASA as 384 400 Km.

Perhaps if we’d stayed for an additional flat white we could have refined the calculation somewhat and so obtained a value closer to reality. Nevertheless, the fact that the force that is pulling the plant down at Tab x Tab is the same as is pulling the Moon around the Earth, and that we can quickly check this (and get an approximately correct answer to our calculation), is one of those ‘wow’ moments in physics. Realising the universality, and elegance, of certain mathematical relations. So perhaps it is entirely appropriate that this thought train of mathematical elegance was prompted by the quirky but aesthetic elegance you will find at Tab x Tab.

Tab Tab can be found at 14-16 Westbourne Grove, W2 5RH

¹ The Walker’s Guide to Outdoor Clues & Signs, Tristan Gooley, Hodder & Stoughton, 2014

² Gravity and Grace, Simone Weil, Routledge (1995 vsn)

*Although you could use a more accurate value for g, the error on the estimate of the height of the plants makes such precision potentially misleading. The value 0.6 seconds is absolutely a back-of-the-envelope, calculation.

Wonders of the World at Espresso Base, Bloomsbury

Hasten coffee, long black, black coffee, espresso base — ‘Has Bean’ coffee at Espresso Base

Espresso Base is exactly the sort of café that you want to make sure that you know about, but part of you is selfishly quite happy if not too many others do. It is not that the the place is small, far from it. There is plenty of space in the courtyard at Espresso Base, beside St George’s Church, to sit and enjoy your coffee. The thing is, it is great to have the place almost entirely to yourself. With few others around, the oasis-like quality of the place is emphasised, astonishing as it is so close to the busy Bloomsbury Way. Only this oasis serves great coffee. Their coffee is roasted by Has Bean, which I admit is the reason that I first dropped into Espresso Base a few weeks ago. The black coffee that I had was certainly very good and the environment in which to enjoy the coffee was thought provoking which, for me, is an important aspect of any café. Cafés need to be places that you can go, slow down and notice things and Espresso Base certainly falls into that group of cafés that I would highly recommend both for the coffee and the café.

stone recycling, slate, slate waterfall, geology — The purple slate waterfall feature in the courtyard area at Espresso Base. You can just see the stone with the rectangular holes carved into it at the bottom of the wall.

On the day that we arrived, it had been raining. For a café with seating outside this may have posed a problem but the chairs had been thoughtfully folded so that they remained dry. The rain had however seeped into some of the paving slabs around the chairs and so that was the first thing to notice, the fact that many objects when wet appear darker, why? Opposite our seating was a rock feature that to me looked like a waterfall made out of slate, the slate had a purple tinge which again, had been made slightly more purple by the rain. Below the slate ‘waterfall’ and forming a wall, were a series of stones that had clearly been taken here from somewhere else. I say ‘clearly’, because the stone at the bottom had two holes that had been carved out of it, one square, one slightly more rectangular. Presumably the stone had been used as part of a gate post in the past and yet there is no evidence of the remains of a gate on the other side of the courtyard (I think that a gate post would have to be deeper than the square indent in the paving slab that is at the other side of the courtyard). It is therefore more likely that the stone had been used somewhere else beforehand and ‘recycled’ for use in this wall. This juxtaposition of slate above and recycled stone below reminded me of the early geologists and how they identified the Great Glen fault that runs through Loch Ness in Scotland. Slate is a metamorphic rock, meaning that it has undergone changes due to the high pressure and temperatures within the Earth. Slate is however quite a low-grade metamorphic rock so, compared with higher grade metamorphic rocks, it has not been subjected to that much pressure or that much temperature. By mapping the lower grade and higher grade metamorphic rocks along the Great Glen, the early geologists noticed a line that sharply separated the metamorphic rock types. This fault would have, in the past, caused earthquakes as the ground slipped along the fault.

Replica of Mausoleum of Halicarnassus — The steeple of St George’s church, Bloomsbury Way. The statue on top is of King George I rather than King Mausolus in his chariot. The statue of Mausolus, his wife/sister Artemisia and a horse from his chariot can be seen in the British Museum.

On leaving Espresso Base I turned and looked up at the church. If you get a chance, take a look at the steeple. Particularly ornate, the stepped steeple is apparently built to the description of the Mausoleum of Halicarnassus by Pliny the Elder. This monument was one of the seven wonders of the Ancient World and was built to be the burial chamber of King Mausolus of Karia. Described as standing approximately 40 m in height, this massive stepped, marble pyramid stood on top of 36 columns surrounded by statues. Topping the pyramid was a statue of King Mausolus himself, in a chariot. This ancient wonder is thought to have been destroyed by an earthquake in the fourteenth century after which the stones were ‘recycled’ by the Knights of Malta to build a fortress. A history that is aptly mirrored in the geology and stone recycling evident in the courtyard of Espresso Base.

Espresso Base can be found in the courtyard of St George’s church, Bloomsbury Way, WC1A 2SE

Artefacts from the Mausoleum of Halicarnassus can be seen in room 21 of the British Museum (conveniently just around the corner from Espresso Base).

Geology help from: “Geology Today, Understanding our planet”, Murck/Skinner, John Wiley & Sons, 1999