A major piece of Britain's new one has arrived at a dockyard, China is testing one, but 100 years after the concept was invented, does anybody still need aircraft carriers?

They are floating airfields that can deploy a nation's military might across the world's oceans.

In May 1912, the first plane took off from a moving warship, HMS Hibernia, temporarily adapted for the purpose. The idea of dedicated floating platforms had been mooted in 1909, but it wasn't until 1918 that HMS Argus became the first proper carrier.

Today, ownership of a carrier for fixed-wing aircraft admits a nation to an elite club, but the US has more than everyone else in the world put together.

The UK is currently not a member, but this month has seen a significant milestone on the country's journey to retaking its place.

Giant jigsaw puzzle Graphic on A mind-boggling construction job Pieces built at six shipyards around the UK and slotted together at Rosyth in Fife

10,000 workers employed on the £5bn project Turbulent times for carrier aircraft

Part of the hull of Queen Elizabeth has arrived at Rosyth where it is to enter the dry dock for the rest of the ship's construction. The ship, which left BAE Systems' Portsmouth yard, is due to be finished in 2017 and will be followed by sister ship Prince of Wales.

The Ministry of Defence says the ships and jets will cost £7bn, leading some newspaper columnists to argue that they are both an unnecessary expense and no longer strategically necessary for the UK.

For the Guardian's Simon Jenkins it is the "greatest waste of public money" of any government programme. The Times's Matthew Parris argued that the action in Libya had not shown the need for a UK carrier.

There have been sceptics for some time. In 1981, David Howarth wrote in Famous Sea Battles that "the only practical value of carriers in the future will be in simply existing, not in fighting". To use them in anger would be to trigger a nuclear war, he argued.

But just a year later, the UK's carriers ensured that the Falkland Islands were regained.

Different nations take different approaches to carriers. The US owns 11, or 20 if plane-carrying amphibious assault ships are counted.

Image caption Part of HMS Queen Elizabeth en route to Rosyth

Its 10 Nimitz class carriers are floating cities - the size of four football pitches - each housing 5,000 crew and 80 strike jets.

By contrast, Spain and Italy have miniature carriers with around a dozen planes, while China has only one ex-Soviet model, preferring to put its defence budget into missile technology.

After the US, France has the only serious naval aviation, according to IHS Jane's Fighting Ships.

Thailand has a very small carrier, which is not thought to have launched aircraft for some years. India is using the UK's former HMS Hermes and an ex-Russian carrier, and Brazil's is an ex-French vessel.

But does any country need aircraft carriers?

The US doesn't need so many, says Prof Andrew Lambert, a naval historian at King's College London.

Image caption The US has amphibious landing ships larger than some nations' aircraft carriers

But for the Americans it is about projecting power around the globe. And they see the aircraft carrier as the best equipment for their global role, Lambert says.

The logic for carriers is very simple. It allows a nation to take air power around the globe without having to worry about countries in between who might refuse the use of ground bases or airspace.

Bosnia was such a case, says former Royal Marine Major-General Julian Thompson.

"The Italians said you aren't going to fly from our airfields or over Italy." And he points out that the only enemy planes shot down by British aircraft since WWII have been by the Fleet Air Arm based on carriers.

Key naval battles 31BC : Actium - galleys

: Actium - galleys 1571: Lepanto - galleys with guns

Lepanto - galleys with guns 1588 : Spanish Armada - sailing ships

: Spanish Armada - sailing ships 1905: Tsushima - ironclads

Tsushima - ironclads 1916: Jutland - dreadnoughts

Jutland - dreadnoughts 1942: Midway - aircraft carriers

But have things changed in the age of the nuclear submarine, precision missiles and the unmanned drone?

Unlike "pointless" frigates, carriers are still as relevant as ever, says Lewis Page, a former naval officer and author of Lions, Donkeys and Dinosaurs: Waste and Blundering in the Military. The drone might be all the rage but you still need somewhere to launch it from.

Nuclear submarines are "excellent" at many things. Their Tomahawk cruise missiles flew hundreds of miles to knock out Colonel Gaddafi's air force.

"But a submarine can't tell you where the targets are. And they can't be easily rearmed apart from at a naval base."

Image caption HMS Argus was the first dedicated carrier

Nuclear weapons give a nation "cachet" says Lambert. But carriers give a nation "capability", he argues.

Take a scenario where Iran decides to close the Straits of Hormuz, Lambert suggests. Any such closure would have an immediate effect on world energy supplies.

Flat-top ships of 10,000+ tonnes displacement

Iran has about 1,000 fast patrol boats that can offer a new kind of asymmetrical warfare. By operating as a swarm, a frigate or destroyer would be overwhelmed by the sheer number of attackers. Whereas a carrier some way from the threat could pick off the attackers by scrambling its jets.

So it's easy to agree with the naval experts who say you need carriers to be a power on the seas. But does the UK have that need?

As an island, there is a particular case for the UK to have carriers, says Lambert.

"If we're not secure at sea we risk starvation. Out of a population of more than 60 million we can probably feed 25 million ourselves," Lambert says. And 95% of the nation's imports arrive by sea, including much of its energy needs from the Middle East.

But there will be those who would suggest the UK's diminished military power makes carriers unnecessary.

"Any major conflict in the Straits of Hormuz or the Gulf won't be decided by Britain," argues Parris. "It will be decided by the US."

But the UK does take part in coalition actions, and for the advocates its carriers would be one of the things giving it a say.

For a nation with a proud maritime history there's also a question of image. Lambert says naval power is "a quintessential expression of what it means to be British."

Their sheer size as they cut through the ocean, the scream of planes taking off and coming into land, makes them an exciting, visceral experience, says Thompson.

Image caption The US Nimitz-class carriers are like floating cities

"The deck when launching planes is frenetic and extremely noisy. It's like a ballet - a triumph of everyone knowing their job. If you screw up you can have your head cut off by a rotor blade."

And the powerful imagery and symbolism of carriers will continue to play a part in the debate.