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.
The grapple gun is a staple of the action movie genre: simply point the hook skywards, fire and have the gun lift you onto the roof of your local Abandoned Warehouse™ or Deserted Chemical Plant™. The Batman is particularly fond of the Grapple Gun, making it a staple of his famous Utility Belt.
So let’s look at the physics:
Assuming The Batman is a well-built adult male and that he’s wearing a substantial amount of body armour and equipment, a mass of 150kg is probably reasonable. Raising a 150kg weight to the top of a ten storey (30m) building requires about 45,000 joules of energy (45 kJ). If The Batman takes thirty seconds to do the journey then that is equivalent to a power of 1500 W. A motor capable of lifting The Batman’s 150 kg weight is probably about 75% efficient, meaning the motor has to develop about 2000 W.
If we assume that the grapple gun’s motor is no more than 5 kg in mass (for ease of wielding) that gives a power-to-weight ratio* of 400 W/kg. This is within the capabilities of modern electric motors, but only just. Finding a battery that can provide 45 kJ is not difficult; lithium ion batteries can provide about 600 kJ per kilogram. However, they can’t supply that electricity quickly enough, managing only about 300 W/kg which means that the Grapple Gun’s 5 kg motor is going to come with a substantial 5kg battery to match. Then there’s the weight of the gun itself and the super-strong cable to consider …
Whilst devices for firing grappling hooks do exist (I’m told the Battelle Tactical Air Initiated Launch system is good) and powerful electric motors are fairly common, merging the two to create a useful handheld device is beyond the capabilities of physics at the moment. A real grapple gun would be far too bulky, heavy and unwieldy to be of any practical use.
* Of course this should really be power-to-mass ratio, but I’m going to stick with the more commonly used term.
The problem with bombs is getting them to their target. Dropping them from the air has always been the standard approach (Austrians used air-dropped bombs during the siege of Vienna in 1849), but this is beset by problems. Even stealth aircraft like the B-2 Spirit stealth bomber and the F-22 Raptor stealth fighter are notperfect.
The Rods from God idea did away with aircraft, and with explosives altogether. The concept is very simple: a series of satellites orbit Earth armed with “tungsten telephone poles”. Once a target has been selected and the satellite is overhead, the “pole” is released and pushed towards Earth by a small rocket motor. As the pole falls to Earth from space it requires no fuel; it is powered by gravity alone.
With such a large distance to fall the rod has plenty of time to accelerate, even when the effect of air resistance is taken into account. Hitting the ground at more than ten thousand metres per second (more than 24000 mph) the rod would be able to strike targets buried deep underground.
With a mass of more than eight and a half tonnes, a rod travelling at that speed would have an energy density of fifty million joules per kilogram, far more than TNT (4.6 million joules per kilogram) or nitroglycerin (6.4 MJ/kg). With very little warning of incoming attacks, the weapon’s speed would make it almost impossible to defend against.
Fortunately (or unfortunately, depending on your viewpoint) the Rods from God (or “kinetic bombardment”) system has yet to be deployed. The most recent mention was in a 2003 US Air Force report (PDF) that classified “hypervelocity rod bundles” as a “Post-2015” technology.
When people think of nuclear fuel they tend to think of uranium and plutnonium, or more specifically their fissile isotopes: uranium-235, plutonium-239 and plutonium-241. But there is another fissile isotope that doesn’t get the attention it deserves: uranium-233.
A fissile isotope is one that can sustain a nuclear chain reaction. There is only one naturally-occurring fissile isotope: U-235 which makes up 0.7% of mined uranium (the other 99.3% being non-fissile U-238). Plutonium-239 and -241 are both “bred”, created artificially in a reactor: Pu-239 from the inert U-238 and Pu-241 from Pu-240 which is itself bred from Pu-239.
Making plutonium-239:
In the first stage U-238 is bombarded with neutrons (n) to create U-239. This U-239 then undergoes beta decay* to form neptunium-239:
This neptunium then undergoes a second beta decay to form Pu-239:
Making plutonium-241:
To create plutonium-241 the plutonium-239 from the previous step is bombarded with neutrons to form first Pu-240 and then Pu-241.
Breeding uranium-233 from thorium:
Uranium-233 is produced by bombarding thorium-232 with neutrons to create Th-233 which then undergoes two beta decays to form U-233. This can all be done inside the reactor itself.
Using thorium as a nuclear fuel has a number of significant advantages: it is made up of only one isotope which means that no costly (in both financial and energy terms) enrichment processes are necessary and thorium is at least four to five times more abundant in Earth’s crust than uranium.
Thorium can be used in a molten salt reactor, where it is dissolved into uranium fluoride to form a fluid that is both fuel and coolant (the full name of this reactor is the liquid fluoride thorium reactor). The advantage of using molten thorium as both fuel and coolant is that the reactor then has passive (“fail-safe”) safety: if the fuel begins to overheat then the reaction rate decreases, making a meltdown impossible.
The reaction takes place at a pressure of one atmosphere, meaning no pressure containment vessels are needed. Thorium reactors produce far less waste than uranium reactors do, and the waste produced is far safer: after 10 years 83% of the waste can be sold to recyclers and reused.
* Completists will notice that I’ve missed out the electron antineutrinos produced in beta decay; I’ve removed them for simplicity since they don’t really play a role here.
In my opinion it’s an outstanding piece of advertising. First I’ll explain how they did it, and then I’ll explain why I like it so much.
In order to make the advert work the balloon has to be neutrally buoyant. A neutrally buoyant object has a mass that is equal to the mass of air (or another fluid) that it displaces. This means that the weight of the object is cancelled out by the buoyant force upon it. There is no overall force on the balloon and it floats neither up nor down.
With no buoyancy or weight forces acting, the balloon responds only to the forces exerted upon it by the surrounding air. Because of the way that the Dyson Air Multiplier works the sucking effect of the “fan” draws the balloon in, and the expelled air then drives it forward. The absence of any blades means that the balloon is able to pass through the fan without harm.
But why is it such a brilliant advert? First, the soundtrack: there’s no voiceover, no actors or actresses, just the sound of the fans themselves. And they are quiet and smooth. One of the things people like least about conventional fans is the noise they make – here Dyson show off one of their fan’s unique selling points without you even realising it.
Then there’s the idea itself: a balloon passing through a fan? The unique selling point of the Air Multiplier is the absence of any blades; a fragile balloon is the perfect test object. If a balloon can pass through a Dyson fan safely then so can your children’s fingers and your pets’ noses.
And lastly there’s the setting. The way that the fans are arranged (carefully set up to show the Dyson’s ability to tilt and rotate) around the offices and manufacturing plant puts you inside Dyson and shows you the sort of company you’re buying from.
I should also point out that this doesn’t seem to be the work of an advertising agency. The disclaimer on the video even goes as far to specify that the advert was “created and executed by trained Dyson engineers”.
