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.

Perfect mirrors

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.

ā€œCarnotā€™s rule says there must be a degradation of energy in any machine that turns out free energy,ā€ Gold says. ā€œA mirror does not have any degradation.ā€

This does not mean sunlight cannot exert a force ā€“ 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 Carnotā€™s rule says some energy can be extracted, so long as the object absorbing the light remains cooler than the radiation itself.

Heating up

A solar sail that absorbed photons would heat up within seconds, Gold argues. The claim has been greeted with scepticism. ā€œThere may be limits on how much solar radiation can be turned into work, but I do not think these are thermodynamic limits,ā€ says Jeffrey Lewins, a thermodynamics expert at the University of Cambridge.

But Gold insists that thermodynamics does have to be taken into account. ā€œItā€™s no good saying, ā€˜I cannot turn heat into free energy, but I can if I turn the heat into radiation firstā€™,ā€ he says. ā€œThatā€™s obviously nonsense.ā€

Steven Soter, an astronomer at the Hayden Planetarium in New York, is open to Goldā€™s idea. He says applying conservation of momentum to photons could be a mistake. ā€œLight is very different from matter, and one may wonder if the momentum rules are also different.ā€

There may also be evidence to support Goldā€™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 worldā€™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 projectā€™s director, is undaunted by Goldā€™s criticism. ā€œSolar sailing is possible,ā€ he insists.

Gamma-ray bursters are enormously intense blasts of gamma rays lasting only a
couple of minutes. NASAā€™s Compton Gamma-Ray Observatory showed that they come in
two types. One rapidly reaches a peak of intensity then gradually fades away,
and is known as ā€œFREDā€ (Fast Rise, Exponential Decay). A second type (anti-FRED)
peaks slowly then disappears fast.

One possible reason for the bursts was gravitational lensing of so-called
active galaxies, powerful galaxies up to billions of light years away. Gamma
rays from active galaxies could be magnified by the gravity of objects such as
ordinary galaxies passing in front. But this theory was dismissed because it
predicted that instead of giving FREDs and anti-FREDs, a burstā€™s intensity
should rise and fall uniformly.

Now Torres and his colleagues suggest that gravitational lensing by wormholes
might explain some bursts. Wormholes would have negative mass, and exert a
repulsive gravitational force. John Cramer of the University of Washington in
Seattle and his colleagues had already pointed out that they should therefore
deflect light away from them to converge on a bright curve called a caustic.

So as a wormhole moved across the line of sight to an active galaxy, says
Torres, it would drag the caustic along with it, producing two burstsā€”an
anti-FRED followed by a FRED, a couple of years apart. Given the number of
gamma-ray bursts that have been spotted in the same region twice, Torresā€™s team
suggests that wormholes with a negative mass of about a tenth of the Sunā€™s mass
could explain up to 5 per cent of gamma-ray bursts. His team has submitted their
results to Physical Review D.

ā€œI view this as a nice suggestion for placing a limit on the amount of
negative mass in the Universe,ā€ says Matt Visser of Washington University in St
Louis, Missouri. The number of FRED/anti-FRED pairs observed would give an upper
limit on how much negative matter exists.

But Torresā€™s work is being greeted with some scepticism. ā€œIt would obviously
be fascinating if gamma-ray bursts involved some ultra-exotic speculation like
wormholes,ā€ says astronomer Martin Rees of Cambridge University. ā€œIā€™d
nonetheless bet quite high odds in favour of a relatively conventional
±š³ę±č±ō²¹²Ō²¹³Ł¾±“Ē²Ō.ā€

ā€œMost gamma-ray bursters must have a simpler astrophysical origin like
fireballs,ā€ Torres admits. But he argues that itā€™s worth exploring the wormhole
alternative. ā€œThis would definitively change our vision of the Universe.ā€

IF THE rate of advancement of a field of science were proportional to the
number of popular books written on it, the origin of the Universe would have
been sorted out years ago. So itā€™s refreshing to see a relatively accessible
title about the other great mystery of modern cosmology: how the galaxies
formed.

Since the 1930s, astronomers have known that much of the Universe is
invisible. Now, by measuring the internal movements of galaxies and working out
how much gravity is needed to produce them, they know that as much as 90 per
cent of the Universe falls into this category. Many candidates have been
suggested to play the role of this elusive ā€œdark matterā€, from neutrinos to a
plethora of hypothetical subatomic particles. Each candidate offers a different
scenario for the formation of galaxies, clusters of galaxies and superclusters,
and has had its own merits and drawbacks. So which is it?

The bottom line is that we still donā€™t know. But advances in observational
astronomy and computing technology are bringing us ever nearer. A renowned
cosmologist who has contributed much to the technical literature of his field,
Thanu Padmanabhan offers an enjoyable account of our current understanding of
galaxy formation and evolution, as well as a comprehensive tour of the necessary
physics in After the First Three Minutes.

The general reader should be wary though. Although the text is free from
mathematics, the author presents his material in a no-nonsense,
no-glossy-pictures format. While itā€™s nice to have a book that gets down to the
nitty-gritty, the facts are delivered at speed and there is little by way of
history or personal anecdotes to reduce the pace by a notch or two. Of course,
that doesnā€™t make it a bad book. Quite the reverseā€”despite some annoying
typographical errors, I enjoyed it. But itā€™s certainly reserved for those who
are seeking weighty explanation and prepared to go the distance to get it.