Monthly Archives: August 2010

Liquid metal telescope

As previously mentioned, size matters when it comes to telescopes: the bigger the mirror, the better the telescope (i.e. the greater the resolution and light-gathering capability). The world’s largest single telescope mirrors are the 8.4-metre telescopes used by the Large Binocular Telescope.

Telescope mirrors must be perfectly polished and accurate in shape and size to within a billionth of a metre. This means that making telescope mirrors is extremely difficult and therefore time consuming and very expensive, costing millions of dollars.

But there is an alternative to polished metal and silvered glass: mercury. Simply filling a pan with liquid mercury will create a perfectly flat* reflective surface; the surface can then be made curved by spinning the pan. No expensive manufacturing is required and this keeps costs low: a liquid mirror costs about 1% of the cost of a similar-sized conventional mirror. (They do have the disadvantage that they can only point upwards, however.)

The University of British Columbia’s six-metre Large Zenith Telescope in Vancouver is the world’s largest liquid mirror telescope.

The simplicity of constructing a liquid mirror telescope has even led to suggestions that one should be built on the Moon.

* To give you some idea of how flat a liquid mirror is: if you built a mirror the size of the Earth, the largest bump would be less than a millimetre in height.

Things From Movies That Cannot Exist Number 2: The Magical Shotgun

The Magical Shotgun (and it’s close cousin, The Magical Pistol) is a staple of the over-the-top action movie. The Magical Shotgun will be familiar to anyone who’ve ever watched a John Woo film: a character hit by a shotgun blast is thrown backwards at great speed through the air, usually into a plate glass window.

Unfortunately this just isn’t possible and the Law of the Conservation of Momentum explains why: in any collision, whether it’s a car striking a bus, or buckshot striking our leading man, momentum must be conserved. The total momentum before the collision must equal the total momentum after the collision.

Momentum is the product of mass and speed and can be loosely thought of as indicating how difficult it would be to change the motion of something. The graph below shows how momentum changes – a darker background indicates greater momentum.

The momentum before the collision is the mass of the shot multiplied by its speed: using typical values of 30 grams of shot travelling at 350 metres per second we have a momentum of 10.5 kgm/s. After the collision the momentum is the combined mass of the target plus the shot, multiplied by the speed of the target moving backwards.

If we assume the target is an average-sized man with a mass of 85 kg and that he’s standing still before he gets shot then the combined mass is 85.03 kg, which, with a momentum of 10.5 kgm/s gives us a final speed of 0.12 m/s or twelve centimetres per second (0.27 mph); this bears no relation to what’s seen on film.

How dangerous is it to ride a motorbike?

Statistics from the Office for National Statistics (via the Department of Transport) and the Motor Cycle Industry Association show that (for the year ending December 2009) motorcyclists make up 2.6% of road users, and 21.2% of road deaths.* So how dangerous is it to ride a motorbike?

This is really a question of quantifying risk, and that’s not something we’re very good at. But let’s take a look at the statistics:

There were 2222 “all road user” deaths in 2009, of which 472 were motorcyclists. If motorcyclists were killed at the same rate as they are present on the road then we would expect only 58 (well, 57.7) of the 2222 dead to be motorcyclists. Can we therefore say that 414 motorcyclists died who “shouldn’t” have died? Can we say that the rate of motorcyclist deaths is 8.14 times what it “should” be?

Looking at death figures says nothing about the ability or skill of motorcyclists. Some would argue that the majority of motorcyclists are killed by the poor driving of car users and not by their own poor driving; but this does not alter the fact that it is the motorcyclists who die.

How likely are you to die on the road?

85.9 people are killed or seriously injured on the UK’s roads per billion motor vehicle miles. That means that if you drove a billion miles (more than eleven times the distance from Earth to the Sun) in one vehicle you would expect 85.9 deaths (or serious injuries) to occur. To put that in more manageable terms, if you drove the UK average of 8000 miles per year you could expect to kill or injure 0.000687 people (per year). To kill or seriously injure one person you would either have to drive 8000 miles per year for 1455 years, three months, five days, twenty hours, forty minutes and thirty-one seconds; or drive 11 641 444 miles per year.

If you started driving at midnight on the morning of 1st January of zero AD you could expect to kill or seriously injure someone (or yourself) by twenty to nine on the evening of 5th March 1455, a month or so before Pope Calixtus III takes over from Pope Nicholas V as the 209th pope (the current pope, Benedict XVI, is the 265th). If you decided to drive the eleven million miles in one year that would require an average speed of 1329 miles per hour, nearly two and a half times the speed of sound.

Whichever way you look at it, you’re pretty unlikely to die on the roads.

But more likely if you’re on a motorbike.

* Doing research for this post I also discovered that in 2008 the DVLA licensed 319 “lifeboat haulage vehicles”, nineteen “mine rescue vehicles” and three “digging machines”.

Naming element 114

Element 114 was first created at the Joint Institute for Nuclear Research in Dubna, 120km north of Moscow, by bombarding plutonium-244 with ions of calcium-48. This created an unstable atom of element 114 (indicated by the asterisk) which then decayed into a different isotope of element 114 and three neutrons:


But what is that symbol – “Unq”? Unq is the current chemical symbol for element 114, known at the moment by its systematic name, ununquadium (“un” – one, “quad” four).

Now that the work by the JINR has been verified by work at the US Berkeley Lab and German GSI laboratory, the International Union of Pure and Applied Chemistry (IUPAC) will invite the researchers from Dubna to submit a “proper” name; this name will then be scrutinised for six months before being approved or disapproved.

Scientists at the Dubna laboratory are already responsible for naming element 102 “nobelium” and element 105 “dubnium” (there was some controversy over this). According to the rules, the discover may not submit a name that has already been proposed for another element so both “kurchatovium” (which Dubna proposed for element 104, after Igor Kurchatov) and “nielsbohrium” (which they proposed for element 105) are out. (Niels Bohr was later honoured by the naming of element 107 “bohrium”.)

Of the ten heaviest named elements, seven are named after people (copernicium, roentgenium, meitnerium, bohrium, seaborgium, rutherfordium and lawrencium) and three are named after places (darmstadtium, hassium and dubnium). Seaborgium is unique in that it is the only element to have been named after someone who was alive at the time of naming.*

Readers at The Guardian have suggested atlantium and salubrium as names, whilst commenters on a post at Physics World (a better crowd, of course) have suggested fibonaccium, darwinium and diracium. What do you think? What should element 114 be called?

* The discovery of einsteinium and fermium (by a team that included Seaborg) was kept secret during the Cold War and thus their names did not become known to the public until after both Einstein and Fermi had died.

My favourite “proof”

I have a favourite question in physics:

“Why do things get darker when they get wet?”

This is my favourite question because the proof is so brilliantly simple, and easy to demonstrate.

Objects appear darker when wet because more light passes through them. Brightness is a measure of how much light is reflected to your eyes, and if less light is reflected then more light must be being transmitted through the material (or absorbed).

When wet, water fills in the “gaps” in the material, “channeling” light through it to the other side.

You can prove this is the case by holding a wet piece of material up to the light – it appears brighter than the surrounding material because more light passes through the material to your eyes.