Why are lightsabers large and round?


There are many problems with the physics of lightsabers [1]. Here I’ll investigate an under-explored one: “why are they round?” If you want to cut something, you want a sharp edge so you apply higher pressure to a smaller area. This is why we cut things with knives, not rolling pins.

“But,” cry Star Wars fans, “it’s not cutting with pressure. It vaporises whatever it comes into contact with.” This makes the problem one of energy rather than force. In order to get the human body to decompose, you need to apply a very high heat for a long time – crematoria use temperatures of about 800-1000°C and have to keep it up for several hours. This is enough to burn the flesh, but bones can still retain some integrity and have to be mechanically crushed [2]. To cut through a bone, you’d need to apply enough heat to completely char it and probably then melt it.

The melting point of pure bone is surprisingly well-known, as bone ash is used in bone china. It’s 1670°C. That’s not to say the blade only needs to be that hot – it needs to heat up an object it’s touching for a fraction of a second to at least that temperature. It is, however, a good lower limit. The steady-state heat diffusion equation is ∇2T = 0, with boundary conditions of T = T0 a large distance away and T = T1 on the cylinder, length ~ 1m. Bunging this in a differential equation solver, we find the temperature half a meter away from the center is about 5% of (T1T0). For T1 in the thousands, this will be a few hundred degrees, so the jedi will bake themselves pretty quickly. They’re also going to all need mechanical hands.

Let’s say that air somehow doesn’t interact with the lightsaber (they’re also canonically usable underwater, so maybe transparent things are immune?). They’re still called lightsabers for a reason, and radiation from such a large, hot object is also problematic. It’s difficult to be sure of the precise luminance, because we don’t know what exactly causes the light. For most non-lasery objects in our universe, the light energy they emit can be approximated by what’s known as ‘black-body radiation’. This means that temperature is the only thing that affects the colour. Using this relationship, the users of blue-bladed lightsabers (6000°C+) would be very dead, whereas red lightsabers are a relatively practical 1000-2000°C. Alternatively, the colours may be from some specific particle interactions, in which case it’s harder to calculate. Looking at the black body radiation of objects around our lower limit of 1670°C, we’re still looking at tens of thousands of watts incident on the wielder’s body, and I certainly wouldn’t recommend holding it near your face. That’s about 10 times the brightness of sunlight. Not necessarily fatal, but you’d at least want sunglasses.

There’s one final problem with the actual cut, and this brings us back again to the sharpness of the blade. A thick blade has to vaporise a larger quantity of material, and needs a lot of energy to do this. Cutting through an arm with a blade a couple of centimeters wide – something that happens a lot in the series – needs you to vaporise a volume of about 10-5 m3 of both bone and other stuff, which we’ll say is all water [4]. The total energy needed to do all this, according to some gross calculations, is around 140000 J. This is equivalent to about 30 g of TNT exploding, resulting in an overpressure of 10-100s of psi for a few tenths of a millisecond. This is probably not enough to prove fatal in an open environment, but could easily cause lung damage, and would usually result in ear drum rupture [5]. The possibility that half the people in star wars are deaf would go some way towards explaining the terrible dialogue, and why everyone pretends they can understand Chewbacca, but is probably not intended. Cutting through larger amounts of a body, e.g. cutting a person in half, would quite likely be fatal (to both parties).

How can we solve these problems? Make the blades smaller and thinner, and also put a guard on them. Or give up on making the space-wizards physically plausible and focus on the real question the series raises: how does a long-ago, lightspeed-surpassing multi-species civilisation collapse to leave us in the world we see today?


[1] See, for example, https://arxiv.org/pdf/1906.02575.pdf

[2] There’s a paper about the degradation of the body during cremation at http://www.sciencedirect.com/science/article/pii/S0379073898000760 ,

doi:10.1016/S0379-0738(98)00076-0 , featuring some of the most heavy-metal pictures of burning bones in any science paper ever.

[3] Unfortunately you can’t treat this as a 2D problem because an infinitely long source with 3D radiation give a divergent series unless the external temperature is pinned to a finite limit closer than infinity. Standard solutions of the Laplace equation are at the end of http://planetmath.org/laplaceequationincylindricalcoordinates

[4] There’s a bizarrely good list of studies on the heat capacity of human body parts at http://www.itis.ethz.ch/itis-for-health/tissue-properties/database/heat-capacity/ , however really this is unlikely to remain constant over the whole range. I have taken into account the enthalpy of vaporisation for water but not for bone, which does not seem to be readily available.

[5] Details of the damage done by different peak overpressures (approximate) https://www.ncbi.nlm.nih.gov/books/NBK202251/

Madrid: Poetry festival and COP25

Last week in Madrid was full of the most action-packed, exciting and exhausting things I expect to do for a long time, so I guess I’ll write a blog about it. It included the Madrid Poetry Festival, followed by the climate change conference COP25.

Friday 29th November:

Traditional Spanish poetry

I meet the other international poets. Most of these people are friends from the world cup earlier in the year: Beau is representing Ireland again, Martje the Netherlands and Christoforos Greece, although Alejandra of Spain is an organiser rather than a competitor (she has two replacements because of a draw in the national slam). It’s good to see each other again. There’s also new people (the Spaniards, Claudia and Carmen, Francesca from Italy and Jim representing the USA). The festival starts with a crazy mix of semi-improvised Spanish poetry, rap/rock music and beatboxing. We then go out for pizza together. Finally we arrive an hour late to a two-man poetry show in a small theatre, scheduled to run for an hour and a quarter. I don’t know how much we miss, but such is their mastery of poetry time it continues for another hour and a half. It’s all in Spanish, although there is a lot of body language, cheap beer and linguistically universal nudity.



The day of the international slam starts late, with a traditional Spanish breakfast in the early afternoon (body clocks all being somewhat squidgy from travelling, and poetry time being squidgier still).

The structure of the slam is rather interesting. The first round is fairly normal – 5 scoreboards are handed to whoever catches balls thrown at the audience, scores are the sum of the middle 3 scores. The first unusual feature is that all scores are whole numbers, hence they often reintroduce the extreme scores to act as tiebreakers. Three of the eight then qualify for the final round, which is bullfight-themed. The audience make noise and wave yellow hankies to signify their approval of each poet after all have performed, with the loudness/number of hankies of the response determining the winner (going overtime means instant disqualification). All poets who perform are awarded plastic ears, having won the ears of the audience. Fortunately for me, my academic supervisor was present and has an extraordinarily loud whistle. This meant I was judged the overall winner, so I won a second ear, along with a book and a poster signed by the other competitors. I alone then get to read out my third poem – I’m glad to win, but I also feel very sorry for the translators, who have had to translate all the other potential-victory poems, which will never get seen.

We all go out to a small theatre again, where there’s food and drink, and a collection of Spanish poet-musicians. The rest of the evening consists of a talent show of international poetry, pop songs played on an acoustic guitar and regional Spanish dancing. Effectively, it’s an open mic, but because of the mix of skills and each performer being nominated by the previous performer/general acclaim, it feels much more fun and organic. I’m sure it helps that the performers are much better than at most open mics. Some of the non-English poetry is given a gloss beforehand, but in one case Alejandra’s poem received a wonderful spontaneous and theatrical live translation.



Mostly a day of rest, museums and goodbyes as most of the other poets leave today or early the next day. I go to IFEMA to pick up my security badge for COP25. The Metro service is comfortable and frequent but the signposting is a little odd and the displays onboard it are run on Windows XP. You can tell from the logo it displays every time it reboots.

Security is taken VERY SERIOUSLY at COP – you need to be metal-detected and have your bag x-rayed to get in. Upon getting in, I pick up my badge, then am instructed that there’s nothing for me to see today.



The start of COP25 proper. I pop into a random room, which appears to be a meeting of indigenous peoples. There’s a lot of powerful things said, I’m not sure who the intended audience is. A large proportion of the room is in native dress and they take a picture at the end of the meeting, which I decide I’m not supposed to be in.

I find other people from Imperial and we go to the official opening ceremony. There’s an interesting reference to Eunice Foote from the Spanish Prime Minister, the woman who originally suggested the carbon dioxide heats the earth. She’s generally left out of the histories, because no one listened to her. In general the gender balance of panels at COP is pretty good, but this will be the first of two events where the importance of diversity is stressed by a group that includes only one woman. We are then off to a talk about Article 6, the prospect of setting up international carbon markets, which is framed as the only practical outcome to expect from COP25.

I attempt to find food and am physically sickened by the 7 euro price tag on a small sandwich. Somehow they have already run out of their only non-meat sandwich and are charging 8 euros for literally just some couscous. I don’t think catering got the message about plant-based diets.

In the afternoon we float between different rooms. We discover we are not allowed in to see the American delegation press conference. In one of the rooms there is a range of pavilions – I note that the American pavilion is nondescript, closed off from the general public and basically ashamed of itself. Many other countries, including the UK, have large, open expanses and put on a stream of talks. Some countries really play up their stereotypes in these pavilions – Indonesia has elaborate bamboo-work, the Italian pavilion has coffee, Japan has an environmentalist anime playing on its wall-mounted TV, and the Indian pavilion is covered in quotes by Gandhi. The UK pavilion has talks and TV screens, along with heavy use of red, white and blue everywhere except on our flags (depicted in white-on-white). It’s a good thing no other countries use this colour combination. The signs all read ‘Green is great’.

At the end of the day it’s the CONSTRAIN presentation at the EU pavilion (hard to find the entrance to but nice inside). The two take-home messages from this are that everything is going worse than predicted, and that the recent turmoil in Chile that resulted in the COP being moved to Spain had a lot to do with a freak drought that can be directly attributed to global warming. On the plus side, the worse predictions of how sensitive the climate is and how terrible the weather is already don’t necessarily affect the net zero year. The other major theme of this discussion (and elsewhere) is that politicians want predictions of where exactly will be harmed by climate change both to know how to adapt and to access the money that is theoretically available for adaptation, but in practice requires lots of paperwork. This is unfortunate because it’s a hard problem, and because local simulations tend to focus on the country that pays for them, i.e. not the countries that need money.



I begin by investigating the UN meeting rooms. They turn out to spend about a quarter of their time thanking each other and half the remainder saying that there isn’t enough time to get everything done. They occasionally mention delegating tasks to subcommittees, which is presumably where everything actually happens, and where observers like me are not allowed.

I’m acutely aware of the opportunity cost of wasting time – there’s so much to see, but so hard to judge what will be interesting – so give up and visit the green area. This has a lower security clearance level, and a lot of it is in Spanish and aimed at schools. It has a stall that mentions a prototype hydrogen-powered ship and a self-sustaining house that stores heat in its swimming pool. A very excited man with little English is selling (vegan) Impossible Burgers here for only 10 euros, which after seeing a side portion of edamame beans for 8 euros yesterday, feels like an economic miracle. I make the mistake of going through the pollution pods on my way out – plastic domes (think mini-Eden projects) filled with air designed to mimic the types of pollution in major cities. I start coughing as soon as I leave the safety of Trondheim. As someone who regularly shoves filters up my nose on the underground, I know that London has terrible air quality, but this is must be exaggerated. I feel nauseous for several hours afterwards.

I go back to more academic talks. For once, this means more practically useful. There’s a good talk on what happens if we miss the various targets. That isn’t really news (it’s bad), but it had a discussion about the difficulty of understanding who will be most severely affected within a country – traditionally, the expectation is that richer people have more flexibility to adapt, but here they also discuss how many years of savings are required to invest in career change. It also makes the point that nominally richer areas may have higher costs of living, hence be hit just as hard . My EA-efficiency inner voice points out that they’re also more expensive to help, and if there’s anything we’ve learned so far it’s that there isn’t enough money in the system.

I meet up with an old Cambridge friend, who is apparently setting up self-sustaining homes powered by old batteries now. We and an Imperial friend go to one talk that still hasn’t actually started a quarter of an hour in, so we change rooms to the one lots of people are standing outside of. My bag is checked and I’m asked if I’m carrying fruit, which I’m not. It’s given out for free in the morning but I’ve eaten it by now. It’s only when I’m inside that I realise this means someone Really Important is coming. If you want to throw fruit at members of the UN, there have been plenty of opportunities already. The talk is about health and climate change, from a selection of high-ranking UN and Spanish politicians, but it’s interrupted. It’s partway through a talk that the Queen of Spain makes an appearance. Dozens of cameras spark into life, take a shot of her entering, then the press all leave. Apparently her interest in the event does not spread to her photographers.

There’s also a video communication from Arnold Schwarzenegger, which a lot of the remaining people film. There’s a lot of discussion of the co-benefits of removing pollution – the enormous amounts of money that can be saved from healthcare and days off work. Schwarzenegger even claims this motivated the recent legal push towards net 0 from California. I understand that emphasising the benefits of climate action is great and clearly a popular approach (it’s great from a selfish perspective), but this rhetoric focuses on local forms of pollution that only motivates the conversion from coal (locally fatal, globally catastrophic) to gas (locally unfortunate, globally ‘cooks us slower’). The host, WHO director Dr Neira, says there should be a corresponding calculation of healthcare savings to go with the national emissions targets.



My last day here. Two groups are giving away milk chocolate, both at the entrance and at the WWF pavilion. This is actually better for the environment than dark chocolate because cocoa is unfortunately worse for the environment than milk, but I’m surprised that vegans haven’t vetoed it. I start at a talk on an index described as the ‘Global Climate Risk Index’, although in practice it seems to be the ‘Global Climate Already Happened Index’, since it exclusively uses data from previous climate-linked disasters. It lists Japan, the Philippines and Germany as worst-affected in 2018. I’m assured that results were not influenced by fact that it’s constructed by a German organisation, called Germanwatch. I feel like the nations who will soon be underwater would have something to say about not making the top 10, but probably their whole GDP is less than the damage a few heatwaves can do to Germany. Then there’s an update on the carbon budgets, which received a pretty detailed writeup on the BBC website – we emitted less than expected last year, in part due to a slowdown by India and China. Coal use is also really tanking, possibly the message about it killing everyone who works near it is finally putting people off? No, apparently the (climate-linked) excess flooding in India flooded the coalmines. At least the irony worked out in the Earth’s favour for once.

How to make wind power cheaper – “Innovation”.

I go to a talk at the UK pavilion. It’s about the role of the sea in climate change. I haven’t been following progress on offshore wind, but it’s been going pretty well. The trick is that every year we just make the windmills bigger, which produces more power (and more consistent power) for a similar price. It’s very much an engineer’s solution but it’s working. They are already larger than the Gherkin tower in London and are predicted to reach price equality with fossil fuels around 2024. Tidal power is still not kicking off, which is a shame because it’s predictable. Other parts of the talk also mentions the fact that large parts of our oceans are now seriously lacking in oxygen, and that burying CO2 under the ocean is legally a bit dodgy under the London convention, which is the international agreement that says you can’t chuck whatever you want in the sea. Most countries haven’t signed it anyway.

That’s me done for one COP. The general message seems to be: politics isn’t going anywhere fast and hasn’t delivered on old promises to those in need but renewable energy is doing OK anyway.

Raptor Rap Tour… in Spanish?

In order to perform at Poetry Slam Madrid, Antonio Díez had to translate some of my poems into Spanish. Apparently this one took the longest (3 days), and I’m always interested in how people translate puns, so here it is!

Gira rap de rapaces

Se acerca la hora de una gira rap de rapaces
Empieza con águilas que levantan el vuelo en éxtasis,
Llevando en nuestras garras la presa capturada,
Con una gorra en las cabezas con una corona de oro porque somos los amos.

El “calva” en águila calva significa “parche blanco”.

Soy un águila calva, no tengo igual
Lleno hasta la bota como si fuera una gaviota
Porque robo del comedero de pájaritos
Y saquearé la basura como un pitbull (pa mí)
Lo siento chica, soy tan real
Crees que mi dieta es legal
Los dólares me deben su fulgor así que
¿Cómo dices que mi estilo no es verdad?
Daré de comer a mis polluelos

… Tengo, tengo, tengo, tengo
Patas de rey
Por comida grasa porque siendo pescadora
Mis garras se agarran a los peces
Y uno puede girar para dar la vuelta
Recolectores de huevos antaño
Nos extinguieron en el Reino Unido
Reconolizamos las Islas Británicas
Desde Noruega de vuelta al nido
De instinto depredatorio
Mi territorio es mi solo anidamiento
Crecimiento efectivo en colectivo
Y construyo mis nidos anualmente

… Ningún cóndor andino no
Pingún cóndor andino no
Ningún cóndor andino
Daña al ganado hasta su último suspiro
Encontrar carne fresca,
Siguiendo al buitre que la detecta
Mutua coexistencia
Porque también de nuestros picos se aprovechan
De un mordisco atravesamos
Incluso los huesos quebramos
Y cada día volamos
Maratones sobre nuestros anidados
¿Qué veo? ¿Es posible?
¿Alguien me dejó un cadáver visible?
Así que desgarro el cuerpo yo
Pero el difunto píldoras y pociones tomó
Así caemos como efecto dominó
Cuando la sobredosis comenzó
Oh Dios mío, mira ese pico Míralo –

Hey, pollos.
Fa1 demasiado 2mesticada
Para escapar su vida DIN a-4
Tomáis polluelos de 5 días
Que saben sumar y restar
Cuando a niños de 6 meses
Eso mismo les suele costar
Cuando a niños de 6 meses
Eso mismo les suele costar
Y piando con sus polluelos
Antes de levantar el vuelo
19 billones de nosotros sobrevivieron
Para ser aplastados al nacer luego
Un génerocidio de huevos machos
Cuya bajo valor para productos de maquillados
Significa que ni nos usan para cubicaldos
¿Al final seremos nosotros los gilipollos?
Es la econo-pía


Raptor rap tour

This poem involves different birds performing rap music. It begins with Eminem’s Rap God as performed by a bald eagle, then Kendrick Lamar’s DNA as performed by an osprey, Nikki Minage’s Anaconda as performed by a condor, and finally Mos Def’s Mathematics as performed by a chicken.

(Rap god)
It’s beginning to sound like a raptor rap tour
Start with eagles who can rise up like the rapture, rapped our
talons around the prey we capture, capped our
Heads with a gold crown cos we’re the masters

The bald in bald eagle means white patch

I got no equals, all love bald eagles
Yet I will keep full like I’m a seagull
Cause I steal small bird’s food haul
And I’ll ransack a bin like a Pit-bull-finch
Sorry girl, I’m so regal you believe my diet’s legal,
Dollar bills owe their glow to me, so
How you gonna call my lifestyle deceitful?
I’ll feed my eaglets eagrets


I got
I got
royal feet for oiled eats cause being an ospray
Talons tear through fish, and one can twist to hold the other way
egg collectors one time wiped us out in the UK
we recolonised the British isles by flying from Norway
And though predatory my territory is only my nest-eyrie
Breed effectively collectively and build my nest up yearly
I don’t …

No Andean condors don’t
No Andean condors don’t
Harm livestock until their lives stop

To find fresh flesh, follow vultures who can smell
Mutualistic coexistence cos they need our beaks as well
we can bite through hide
And even crush up bone
Each day we fly
Marathons around our home

Ohh, what I see?
Could it be
Someone left a fresh kill for me
So I tear that body open
But the corpse ate pills and potions
That make us fall like dominos when we start overdosing.

Oh my God. Look at her beak.
Look at her


bukka bukka bukka bukka
Haha: chicken.

Onedering jungle foul, too tame to break
Free from our A four-page lives, you take
Five day old chicks, they can add and subtract
When six-month old kids may struggle with that, you
Sever ties with mum who Mos’ Def-hen-eight-ly
Counted and chirped with her chicks before they hatched
Nineteen billion of us survived being crushed alive at birth
In gendercide of non-egg providing males whose lower worth
As bodily commodities means you won’t even make us into stocks.
All this and somehow we’re the bunch of cocks?
It’s chickenomics.

Eating insects


Recently I wrote a poem with insect- and anthropology-expert Charlotte Payne (@LibertyRuth on twitter), which explored human attitudes to insect consumption throughout the world. Her research feeds into a growing notion of insects as better for the environment than larger animals. It differs from the mainstream presentation in recognising that cultural value and delicate ecological balance may be lost if the insects are factory-farmed unwisely. Here’s a video of the poem, recorded by MuddyFeet at the Genesis Slam Final.

London open mics spreadsheet

Having been to quite a lot of London open mics recently and been asked which I recommend, I thought I would answer this question in the most poetic way possible: with a spreadsheet. All entries are what I vaguely recall at the time of writing. Feel free to rehost, and contact me for the power to edit it if you want to add events or change anything in it.



Who knew that infinity could evaporate?

That those bright-clouded darknesses

May one day spin everything into light?

That Beginning and perhaps End are words we

Must teach the universe,

While it in turn thrums out the mystery

Of inward-turned space

And the dreams inside its scars.



Notes:  Hawking radiation refers to the loss of mass of a black hole via a quantum effect, predicted to eventually empty black holes, which radiate more as they deplete themselves. Eventually these holes are predicted to predominate in the universe, then gradually lose mass to this radiation.
Hawking was also involved in the Hartle-Hawking hypothesis, a proposed explanation for the Big Bang, which denies the meaningfulness of the question ‘what came before the Big Bang.’ If the universe ends with a Big Crunch, a similar answer is possible, though we currently think there will be black holes and heat death, as above. Even so, it’s not clear what would constitute the ‘end’ of such a universe, as it gradually empties of all matter (with the remainder possibly decaying spontaneously).
He was responsible for a partial proof of the ‘no-hair  theorem’, which says that you lose most information about the contents of a black hole and can’t know about what goes on inside. As with all physical proofs, it relies on uncertain assumptions, so can be considered a proof  but isn’t definitely correct, and is still debated today – Hawking changed his opinions on the matter within his lifetime.

He was an inspiration to me and many physicists and a colossal figure in both the discovery and popularisation of science. We will miss him.