Both NASA and the European Space Agency are developing solar sails and, although never tested, the concept is quite simple. A solar sail is essentially a giant mirror that reflects photons of sunlight back in the direction they came from.

Although photons do not have mass, they are considered to have momentum, so according to the law of conservation of momentum, the photon loses some of its energy to the sail as it bounces off, giving the sail a shove in the opposite direction.

But Thomas Gold from Cornell University in New York says the proponents of solar sailing have forgotten about thermodynamics, the branch of physics governing heat transfer. Solar sails are designed to be perfect mirrors, meaning that they reflect all the photons that strike them.

Gold argues that when photons are reflected by a perfect mirror, they do not suffer a drop in temperature.

That brings in a thermodynamic law called the Carnot rule, which basically states that you never get something for nothing: if there is no temperature change when the photons are reflected, it is impossible to extract any free energy from them to push the sail along.

"Carnot's rule says there must be a degradation of energy in any machine that turns out free energy," Gold says in the New Scientist report. "A mirror does not have any degradation."

This does not mean sunlight cannot exert a force- comet tails point away from the sun, and are often cited as evidence in favour of solar sails. But Gold says this is because a comet tail is not a perfect mirror: it absorbs some of the light.

In this scenario Carnot's rule says some energy can be extracted, so long as the object absorbing the light remains cooler than the radiation itself. A solar sail that absorbed photons would heat up within seconds, Gold argues.

The claim has been greeted with scepticism. "There may be limits on how much solar radiation can be turned into work, but I do not think these are thermodynamic limits," says Jeffrey Lewins, a thermodynamics expert at the University of Cambridge.

But Gold insists that thermodynamics does have to be taken into account. "It's no good saying, 'I cannot turn heat into free energy, but I can if I turn the heat into radiation first'," he says. "That's obviously nonsense."

Steven Soter, an astronomer at the Hayden Planetarium in New York, is open to Gold's idea. He says applying conservation of momentum to photons could be a mistake.

"Light is very different from matter, and one may wonder if the momentum rules are also different." There may also be evidence to support Gold's theory, in the form of a quirky device called a Crookes radiometer.

It consists of four paddles attached to the arms of a rotor, inside a vacuum jar. Each paddle is silvered on one side and coated with a black absorber on the other.

When placed in sunlight, the rotor spins. If the theory of solar sailing is right, the rotor should spin with the reflecting silver surfaces moving away from the light. But it actually spins the other way, just as Gold predicts.

The dispute could be settled in September, when the Pasadena-based Planetary Society hopes to launch Cosmos 1, the world's first solar sail. The 100-kilogram craft will be sent into orbit around the Earth, before unfurling a set of reflecting blades in an attempt to boost its altitude. Louis Friedman, the project's director, is undaunted by Gold's criticism. "Solar sailing is possible," he insists.

This article is based upon a report by Paul Parsons which appears in the July 5 issue of New Scientist

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