15:30 03 April 2008

NewScientist.com news service

Michael Marshall

What is truly impossible? To accompany Michio Kaku's article on the physics of impossibility, we have rounded up 10 things that were once thought scientifically impossible. Some were disproved centuries ago but others have only recently begun to enter the realm of possibility.

1. Analysing stars

In his 1842 book The Positive Philosophy, the French philosopher Auguste Comte wrote of the stars: "We can never learn their internal constitution, nor, in regard to some of them, how heat is absorbed by their atmosphere." In a similar vein, he said of the planets: "We can never know anything of their chemical or mineralogical structure; and, much less, that of organized beings living on their surface."

Comte's argument was that the stars and planets are so far away as to be beyond the limits of our sense of sight and geometry. He reasoned that, while we could work out their distance, their motion and their mass, nothing more could realistically be discerned. There was certainly no way to chemically analyse them.

2. Meteorites come from space

Astronomers look away now. Throughout the Renaissance and the early development of modern science, astronomers refused to accept the existence of meteorites. The idea that stones could fall from space was regarded as superstitious and possibly heretical - surely God would not have created such an untidy universe?

The French Academy of Sciences famously stated that "rocks don't fall from the sky". Reports of fireballs and stones crashing to the ground were dismissed as hearsay and folklore, and the stones were sometimes explained away as "thunderstones" – the result of lightning strikes.

3. Heavier-than-air flight

The number of scientists and engineers who confidently stated that heavier-than-air flight was impossible in the run-up to the Wright brothers' flight is too large to count. Lord Kelvin is probably the best-known. In 1895 he stated that "heavier-than-air flying machines are impossible", only to be proved definitively wrong just eight years later.

The problem was set out in 1716 by the scientist and theologian Emanuel Swedenborg in an article describing a design for a flying machine. Swedenborg wrote: "It seems easier to talk of such a machine than to put it into actuality, for it requires greater force and less weight than exists in a human body."

4. Space flight

From atmospheric flight, to space flight. The idea that we might one day send any object into space, let alone put men into orbit, was long regarded as preposterous.

The scepticism was well-founded, since the correct technologies were simply not available. To travel in space, a craft must reach escape velocity – for vehicles leaving Earth, this is 11.2 kilometres per second. To put this figure into perspective, the sound barrier is a mere 1,238 kilometres per hour, yet it was only broken in 1947.

5. Harnessing nuclear energy

On 29 December 1934, Albert Einstein was quoted in the Pittsburgh Post-Gazette as saying, "There is not the slightest indication that [nuclear energy] will ever be obtainable. It would mean that the atom would have to be shattered at will." This followed the discovery that year by Enrico Fermi that if you bombard uranium with neutrons, the uranium atoms split up into lighter elements, releasing energy.

6. Warm superconductors

This is a strange case: a phenomenon can be observed and measured, but should not be happening. According to the best theories of superconductivity, the phenomenon of superconductivity should not be possible above 30 Kelvin. And yet some superconductors work perfectly well at 77 K.

Superconductors – materials that conduct electricity with no resistance – were first discovered in 1911. To see the effect, a material normally has to be cooled to within a few degrees of absolute zero.

7. Black holes

People who think of black holes as a futuristic or modern idea may be surprised to learn that the basic concept was first mooted in 1783, in a letter to the Royal Society penned by the geologist John Michell. He argued that if a star were massive enough, "a body falling from an infinite height towards it would have acquired at its surface greater velocity than that of light... all light emitted from such a body would be made to return towards it by its own proper gravity."

However, throughout the 19th century the idea was rejected as outright ridiculous. This was because physicists thought of light as a wave in the ether – it was assumed to have no mass, and therefore to be immune to gravity.

It was not until Einstein published his theory of general relativity in 1915 that this view had to be seriously revised. One of the key predictions of Einstein's theory was that light rays would indeed be deflected by gravity.

8. Creating force fields

This classic of science fiction went from wild speculation to verifiable fact in 1995 with the invention of the "plasma window".

Devised by Ady Hershcovitch from the Brookhaven National Laboratory, the plasma window uses a magnetic field to fill a small region of space with plasma or ionised gas. The devices, developed by Hershcovitch and the company Acceleron, are used to reduce the energy demands of electron beam welding.

The plasma window has most of the properties we associate with force fields.

9. Invisibility

Invisibility is another staple of fantasy fiction, appearing in everything from Richard Wagner's opera Das Rheingold to H. G. Wells' The Invisible Man, and of course Harry Potter.

There is nothing in the laws of physics to say invisibility is impossible, and recent advances mean certain types of cloaking device are already feasible.

10. Teleportation

This is a word with a long and rather dubious history. It was coined by the paranormalist writer Charles Fort in his book Lo! and was subsequently seized on by legions of science fiction writers; most famously as the "transporter" in Star Trek.

Despite its fantastical origins, physicists have achieved a kind of teleportation thanks to a bizarre quantum phenomenon called entanglement.