The Nuclear Double Flash

Identification of a nuclear explosion uses a number of different methods. The Comprehensive Test Ban Treaty Organisation (CTBTO) runs a series of networks which listen for infrasound sound waves produced in the atmosphere by above-ground explosions; which monitor the oceans for underwater tests; and which monitor seismic activity to detect underground tests. The CTBTO also run a network of radionuclide sensors that sample the air to detect certain isotopes produced by nuclear explosions.

But if a nuclear weapon is ever used again as a weapon of war, the first notification will come from space-based networks (e.g. the US DSP or the Russian Oko) looking for the characteristic double flash of a nuclear detonation.

Watch the video above of the first two seconds of the Castle Bravo nuclear test. Do you notice anything unusual? Let’s take a look at a few individual frames.


Frame 01


Frame 11


Frame 49

The explosion begins bright, but then dims before becoming bright again: this is the nuclear double flash. It’s a little easier to see in the slowed-down excerpt below.

The variation in the brightness of the light emitted by a nuclear explosion follows a distinct pattern. It is possible to build light sources that are as powerful as nuclear explosions, or to produce light sources that have the same double flash characteristics, but not to produce a source with both characteristics. Thus the nuclear double flash is taken as irrefutable evidence that a nuclear explosion has taken place.


Note the logarithmic scale on both axes.*

As the nuclear explosion begins, the bomb and all of its components are heated to extremely high temperatures of around ten million kelvin. This causes these components to emit low-energy (“soft”) x-rays and high-energy (“hard”) ultraviolet waves. These x-rays and UV waves are absorbed by the air within a few metres of the device and this causes the air to be heated to temperatures of around one million kelvin, causing it to become incandescent and emit light. This is responsible for the initial, very fast (about three hundred millionths of a second after detonation) bright peak.

At the same time, the explosive shock wave itself (the hydrodynamic front) is expanding outwards and quickly compresses the air in front of it like a piston, causing it to become superheated. Inside this shock wave, the temperature is so high that the gas inside it become completely ionised (i.e. the gas becomes a plasma) and this makes the shock wave opaque to light. The brightness minimum is therefore caused by the shock wave “trapping” light behind it as it forms.

Light is still emitted because the shock wave itself is incandescent and is therefore emitting light outwards, ahead of itself, but this light is about one-tenth of the brightness of the preceding and following maxima. As the shock wave expands, it cools rapidly, and as it cools it becomes more transparent, allowing the light previously trapped behind it to escape. This is responsible for the second bright peak, which lasts much longer than the first because the full energy of the weapon is now being fully released, with nothing to block it. As the fireball expands it dissipates, and this is responsible for the gradual decrease in brightness.


As the graph above shows, the time of the first minimum and the time of the second maximum depend on the weapon’s yield. A larger yield means a more powerful initial release of energy, and a more powerful shock wave, and this shock wave then takes more time to “pass through” the initial hot region created by x-ray/UV absorption, and then also takes longer to cool down to the point at which is becomes transparent to the light that it has trapped behind it.

For a one kiloton device, the time between the minimum and the second maximum is only 30 milliseconds, too short a gap for the human eye to perceive, but bhangmeters aboard satellites can spot it (and by measuring the time interval get a rough idea of the weapon’s yield). For larger weapons, such as the 100 kT warheads aboard the UK’s Trident II D-5 missiles, the interval is long enough (0.3 seconds at 100 kT) for human beings to perceive.

* Taken from Guy E. Barasch, “Light Flash Produced by an Atmospheric Nuclear Explosion”, LASL-79-84, Los Alamos National Laboratory, 1979.

A More Logical System of Coinage

(Please note: I didn’t say this was necessarily a better system, just that it is more logical. I still can’t believe I spent this much time on this total non-issue.)

There are many, many currency redesigns on the internet. But everybody always concentrates on redesigning the paper currency; very rarely does someone look at redesigning the coins. I’m far from a  graphic designer, but what I can do is look at the size and shape of the coins.

Here is what current UK coinage looks like:*

Coin Diameter /mm Thickness /mm Mass /g Material
1p 20.3 1.65 3.56 Copper-plated steel
2p 25.9 2.03 7.12 Copper-plated steel
5p 18.0 1.7 3.25 Nickel-plated steel
10p 24.5 1.85 6.5 Nickel-plated steel
20p 21.4 1.7 5.0 Cupro-nickel (84:16)
50p 27.3 1.78 8.0 Cupro-nickel (75:25)
£1 22.5 3.15 9.5 Nickel-Brass
£2 28.4 2.50 12.0 Cupro-nickel and

Clearly this makes no sense at all. The 1p and 2p coins are both bigger than the 5p, and the 10p is bigger than the 20p. The 2p coin is heavier than the 1p, 5p, 10p and 20p coins. There are four different materials used, and even though both the 20p and 50p coins are made out of cupro-nickel, the copper-to-nickel ratio is different, which must make manufacturing them more difficult.

Step 1: Get rid of the 1p and 2p

There are a number of reasons for doing this. The 1p and 2p coins are now essentially useless for actually buying anything (which is why many countries have got rid of their 1p/2p equivalents), and the UK 2p coin is particularly big and heavy. Also, it will help when we want to scale the sizes of coins logically.

Step 2: Make coins from the same material

Since we have a fiat money system, we don’t need coins to be made out of different materials, and visual appearance isn’t important, as people will be able differentiate between our new coins using other factors. We should just use the cheapest material, which is nickel-plated steel.

Step 3: Make the sizes more logical

As demonstrated above, the sizes of UK coins currently makes no sense. We want the size of our coins to be proportional to their value, so that the £2 is larger than the £1, and the £1 is larger than the 50p and so on. (This has the useful side effect of making less “powerful” or less “useful” coins smaller and lighter, so that they take up less space in your wallet.)

However, given that we don’t want our new coins to be significantly smaller or larger, or lighter or heavier than our existing coins, we cannot vary size linearly with value. If we did this then the £2 coin would be forty times bigger than the 5p coin, and one of them would have to be either unmanageably small or unmanageably big.

We will therefore have to use a logarithmic system to calculate the new sizes (this makes sense because people’s cognition of numbers is naturally logarithmic). Thus a £2 coin will be bigger than a £1 coin, but not twice the size; and a 50p coin will be bigger than a 10p coin, but not five times bigger.

We don’t want any coin to be smaller than the 5p (the coin with the lowest diameter) or the 1p (the coin with the lowest thickness), or to be larger than the £2 (the coin with the highest diameter) or the £1 (the coin with the highest thickness). Thus we will take the measurements of these coins to be our limits: our new coins must have diameters between 18.0 and 28.4 mm, and thicknesses between 1.65 and 3.15 mm.

After some relatively simple calculations, we end up with the following:

Coin Diameter /mm Thickness /mm Mass /g
5p 18.0 1.65 3.13
10p 20.0 1.93 4.50
20p 21.9 2.21 6.22
50p 24.5 2.59 9.08
£1 26.4 2.87 11.74
£2 28.4 3.15 14.87


Old £1 coin shown above for scale.


Most of the coins are similar in size (the 5p and £2 have exactly the same diameters), but there are a couple of notable differences.


The change in properties are easier to understand via graphs.

As you can see, the proposed diameter and thickness increase sensibly, whereas the existing diameters and thicknesses do not. The same is also true for the proposed masses.


As you can see, the proposed masses are higher than the existing masses for all but the 5p and 10p coins. However, considering the removal of the 1p and 2p coins and after running some extensive Monte Carlo testing, I can confidently say that your average pocket full of change will now weigh less. (It would not be difficult to use a less dense material than nickel-plated steel if weight proved to be a problem. We could also reduce the thicknesses of the coins, making the 2.5 mm thickness of the £2 coin our maximum.)

The logic of using a logarithmic system is further demonstrated when considering adding the £5 coin (which is really only a collectors’ item) into our system. The existing £5 coin is enormous: 39 mm in diameter and 28 grams in mass; in our new system it is a svelte 31 mm and only 19.8 grams. We could therefore replace some of our paper money, which requires frequent costly replacement, with longer-lasting coins.

Existing £1 and £2 coins shown for scale.

Known Issues

Use by the blind

Unlike the current system, our new proposed system uses only circular coins: we do not use shapes of equal width as with the current 20p and 50p coins. This could make them more difficult for blind users to deal with.

Our new coins have a greater variation in mass than the existing coins, and this should make differentiating between them by feel easier. Also, the diameter:thickness ratio changes more noticeably (and of course, more consistently) than existing coins: coins become “fatter” relative to their diameter as their value increases.


The problem of blind users is easily fixed by using different edges on our coins, as is currently done with Euro coins: the 2¢ coin has a groove around its edge, the 10¢ coin has fine “scallops” on its edge, the 20¢ coin uses a “Spanish Flower” design, the €1 coin uses interrupted milling and the €2 uses a fine-milled edge with lettering. There are more than enough different options for blind users to easily differentiate between coins.


My choice would be for every second coin (i.e. 10p, 50p and £2) to use a scalloped edge, as the remaining coins would then different enough either by size or by feel (the scallops are easier to feel than fine milling) to differentiate between. Obviously, extensive testing with blind users would be necessary to iron-out any problems.

Other Issues

None. This is clearly a brilliant idea.

* The difference in the number of significant figures should correspond to different tolerances, but I wouldn’t be surprised if it’s a mistake by the Royal Mint.

Adjective Order in English

Adjectives in English follow a certain order. This is why “That’s a beautiful white house” sounds correct, but “That’s a white beautiful house” does not.

The order of adjectives begins with opinions: “beautiful”, “nice”, “great”, etc.

It’s a great car.

After opinions comes size: “big”, “small”, “long”, etc.

It’s a great small car.

It’s a small great car.

After opinions and size comes age: “new”, “old”, “ancient”, etc.

It’s a great small old car.

It’s an old great small car.

(Apologies for how clunky the sentences get beyond here. In English you don’t normally describe objects with quite so many objectives!)

After opinions, size and age comes shape: “rectangular”, “circular”, “boxy”, etc.

It’s a great small old curvy car.

It’s a curvy great small old car.

After opinions, size, age and shape comes colour: “red”, “blue”, “green”, etc.

It’s a great small old curvy blue car.

It’s a blue great small old curvy car.

After opinions, size, age and shape come materials: “leather”, “brick”, “wood”, etc.

It’s a great small old curvy blue metal car.

It’s a metal great small old curvy blue car.

After opinions, size, age, shape and material comes (geographical) origin: “British”, “Spanish”, “Roman”, etc.

It’s a great small old curvy blue metal British car.

It’s a British great small old curvy blue metal car.

Finally, after opinions, size, age, shape, material and origin comes purpose:

It’s a great small old curvy blue metal British racing car.

It’s a racing great small old curvy blue metal British car.

Any combination that doesn’t have the adjectives in the correct order ends up looking weird.

It’s a fantastic big new red American house.

It’s a fantastic American big new red house.

It’s a big new American red fantastic house.

It’s a red fantastic American big new house.

It’s an American new red big fantastic house.

Not all languages use an order for adjectives. For example, in Polish it doesn’t matter what order the adjectives are in: “What a wonderful small blue bag!” and “What a blue small wonderful bag!” would sound just as “correct” as each other.

Colour Mixing

or Why do screens and monitors use red, green and blue pixels whereas printers use cyan, magenta and yellow ink?

The human eye contains three types of cones, light-sensitive cells that are sensitive to specific wavelengths (rather than rods, which are sensitive only to brightness/darkness).

The sensitivity of the three types of cones peak at wavelengths of 564-580 nanometres, 534-545 nm and 420-440 nm. These correspond to red, green and blue light, and by comparing the signals from the three types of cones the brain creates colours along red-green and blue-yellow axes (hence why there is no such colour as “greenish-red” or “yellowish-blue”). White light is created by the brain when all three cones are stimulated by the same amount.

For a colour to be perceived light must enter the eye. The big difference between the pixels on a computer screen and the inks used in printing is that light from screens enters the eye directly whereas light from printed materials must be reflected first. Screens use additive colour mixing, and printing inks use subtractive colour mixing.


Source: Kodak test image library

To create the test image shown above on a screen, which emits light, the image is split into red, green and blue components. The darker areas are where less light is emitted.


To create this image in print, the image is split into cyan, magenta, yellow and black components (because the mix of cyan, magenta and yellow alone does not produce a perfect “strong” black, and would require the images to be lined-up perfectly). The lighter areas are where less ink is printed.


If both red and green light enter the eye in equal amounts, the brain creates the colour yellow.* To create the colour yellow on the printed page, both red and green light must be reflected from the page, and the only way to do this is to absorb blue. If yellow text is viewed under a blue light it will appear black, as all the light incident on the ink will be absorbed.

Colour Additive (Light) Subtractive (Inks)
Red Emit red Absorb green and blue
Green Emit green Absorb red and blue
Blue Emit blue Absorb red and green
Cyan Emit green and blue Absorb red
Magenta Emit red and blue Absorb green
Yellow Emit red and green Absorb blue

If cyan and magenta inks are printed on top of each other and illuminated by white light, the cyan will absorb red and the magenta will absorb green. The net effect of this is that only blue light is reflected, so printing cyan and magenta on top of each other creates the perceived colour blue.

If you tried to do this with, for example, red and green ink, the result would be black, as the red would absorb the blue and green and the green would absorb the red and blue. The reason you cannot print using red, green and blue inks is that red, green and blue ink absorbs more than one colour, whereas cyan, magenta and yellow do not. The only way to produce light colours is to start with light inks.

Ink Absorbed Reflected Colour Perceived
Cyan Red Green and blue Cyan
Magenta Green Red and blue Magenta
Yellow Blue Green and red Yellow
Cyan + Magenta Cyan absorbs red Blue Blue
Magenta absorbs green
Magenta + Yellow Magenta absorbs green Red Red
Yellow absorbs blue
Cyan + Yellow Cyan absorbs red Green Green
Yellow absorbs blue

If you’d like proof of the table above, try printing out squares of red, green and blue and looking at them under a microscope.



It is easy to see, in the image of blue ink above, the cyan and magenta “pixels” (drops of ink) that create the blue colour. It’s more difficult, but possible, in the image of red ink below to see the magenta and yellow “pixels”.


* There is no way to guarantee that both you and I perceive the colour that we call yellow in the same way. We might both call it yellow, but we could be seeing vastly different things.

Alternatives to GPS

“GPS” is actually a brand name, and like “hoover” or “kleenex” has become a proprietary eponym, used to refer to a category of products despite just being one product in that category. There are other satellite navigation systems that offer the capabilities of GPS.




GLONASS (Globalnaya Navigatsionnaya Sputnikovaya Sistema) is the most developed of the GPS alternatives, and is operated by Russia. GLONASS uses twenty-nine satellites compared with the thirty-two satellites that make up the GPS constellation, and the GLONASS satellites are in a slightly lower orbit. Due to the orbital position of the satellites, GLONASS is slightly less accurate overall than GPS, but more accurate at high latitudes (both north and south) in locations such as Russia, because it is inclined at an angle of 65° to Earth’s equator as opposed to GPS’s 55°.

Many mobile phones already use a combination of GPS and GLONASS, and the availability of more satellites means that it is easier to get a line-of-sight satellite lock than when using GPS alone.


DORIS (Doppler Orbitography and Radiopositioning Integrated by Satellite) is a French system that operates the other way around to other satellite navigation systems: each DORIS satellite receives signals from between fifty and sixty ground stations, rather than it being the satellites that are broadcasting the signals. There are no dedicated DORIS satellites, but DORIS receivers are fitted to a number of satellites such as TOPEX/Poseidon and Jason.

The accuracy of DORIS is lower than GPS as ground positioning is not its primary purpose; it is designed to measure the position the orbits of the satellites aboard which DORIS receivers are fitted and to measure the height of the oceans.


BeiDou was a test system developed by China that used four satellites and covered China, India, parts of Japan and other countries.


The BeiDou system is being replaced to create BedDou-2 or COMPASS, and now uses sixteen satellites which cover the Asia-Pacific region, including Australia. By 2020 COMPASS should be able to offer global coverage, using thirty-five satellites, including five in geostationary orbit.



Galileo is a European satellite navigation system run by the EU and the European Space Agency; it plans to offer one-metre accuracy (compared with GPS’s fifteen-metre accuracy) and a search-and-rescue function for free, and will use a constellation of thirty satellites. Higher-precision (centimetre-resolution) data will be available to paying users. Five Galileo satellites have been successfully launched and placed, but the Soyuz rocket and Fregat “space-tug” used for the sixth satellite did not place it into the correct orbit.

Because Galileo satellites are placed in a higher orbit (23 200 km) than GPS satellites (20 200 km), there is a higher chance of being able to achieve a line-of-sight satellite lock, particularly in the “urban canyon” environment of modern high-rise cities. Galileo will also offer better coverage at high latitudes (e.g. northern Norway), where GPS coverage varies during the day due to the orbits of the GPS satellites.

It is highly likely that modern devices will use a combination of GPS, GLONASS and Galileo when the Galileo system becomes fully operational.


IRNSS (Indian Regional Navigation Satellite System) is an Indian satellite navigation system, the development of which is at least partially due to a desire to avoid reliance on foreign navigation systems. (During the India-Pakistan Kargil War in 1999, the US denied India access to GPS data.)

The system will consist of seven satellites in geostationary orbits, and will offer accuracy of below twenty metres within a radius of 3000 km of the centre of India, increasing to below ten metres over mainland India.


QZSS (Quasi-Zenith Satellite System) is a Japanese system that is not a navigation system of its own, but rather augments the GPS system (it send signals compatible with standard GPS receivers). It will offer better coverage than GPS within Japan’s “urban canyons”, as the orbits of the four QZSS satellites will ensure that one satellite is always directly overhead of Japan. QZSS is specifically targeted at mobile applications, and will offer data transfer capabilities alongside positioning information.