For 400 years the memory of Bruno has haunted the historians of science. But Bruno has his revenge every few weeks: about twice a month a new extrasolar planet is discovered orbiting a star: more than 250 such planets have now been documented. Bruno's prediction of extrasolar planets has been vindicated. But one question lingers. Although the Milky Way may be teaming with extrasolar planets, how many of them can support life? And if intelligent life does exist, what can science say about it?

Some people claim that extraterrestrials have already visited Earth in the form of UFOs. Scientists usually dismiss the possibility of UFOs because the distances between stars are so vast. But last year the French government released a report by the French National Centre for Space Studies, which included 1,600 UFO sightings spanning 50 years, including 100,000 pages of eyewitness accounts, films and audiotapes. The French government stated that nine per cent of these sightings could be fully explained, that 33 per cent had likely explanations, but that it was unable to follow up on the rest.

The most credible cases of UFOs involve a) multiple sightings by independent, credible eyewitnesses and b) evidence from multiple sources, such as eyesight and radar. For example, in 1986 there was a sighting of a UFO by JAL flight 1628 over Alaska, which was investigated by the Federal Aviation Administration. The UFO was seen by the passengers of the JAL flight and was also tracked by ground radar. Similarly, there were mass radar sightings of black triangles over Belgium in 1989-90 that were tracked by Nato radar and jet interceptors. In 1976, there was a sighting over Tehran, that resulted in multiple systems failures in an F-4 jet interceptor. But what is frustrating to scientists is that, of the thousands of recorded sightings, none has produced hard physical evidence that can lead to reproducible results in the laboratory. No alien DNA, alien computer chip or physical evidence of a landing has ever been retrieved.

We might ask ourselves what kind of spacecraft they would be. Here are some of the characteristics that have been recorded by observers.

a) They are known to zig-zag in midair;

b) They have been known to stop car ignitions and disrupt electrical power;

c) They hover silently.

None of these characteristics fits the description of the rockets we have developed on Earth. For example, all known rockets depend on Newton's third law of motion (for every action, there is an equal and opposite reaction); yet the UFOs cited do not seem to have any exhaust. And the g-forces created by zig-zagging flying saucers would exceed 100 times the gravitational force on Earth - the g-forces would be enough to flatten any creature on Earth.

Can such UFO characteristics be explained using modern science? In movies it is always assumed that alien beings pilot these craft. More likely, however, if such craft exist, they are unmanned (or are manned by a being that is part organic and part mechanical). This would explain how the craft could execute patterns generating g-forces that would normally crush a living being.

Any alien civilisation advanced enough to send starships throughout the universe has certainly mastered nanotechnology. This would mean that their starships do not have to be very large; they could be sent by the millions to explore inhabited planets. Desolate moons would perhaps be the best bases for such nanoships. If so, then perhaps our own moon has been visited in the past by a civilisation similar to the scenario depicted in the movie 2001: A Space Odyssey, which is perhaps the most realistic depiction of an encounter with an extraterrestrial civilisation.

Some scientists have scoffed at UFOs because they don't fit any of the gigantic propulsion designs being considered by engineers today, such as ramjet fusion engines, huge laser-powered sails and nuclear pulsed engines, which might be miles across. But UFOs can be as small as a jet aeroplane, and can refuel from a nearby moon base. So sightings may correspond to unmanned reconnaissance ships.

Time is one of the great mysteries of the universe. We are all swept up in the river of time against our will. Around AD400, Saint Augustine wrote extensively about the paradoxical nature of time: 'How can the past and future be, when the past no longer is, and the future is not yet? As for the present, if it were always present and never moved on to become the past, it would not be time, but eternity.' If we take Saint Augustine's logic further, we see that time is not possible, since the past is gone, the future does not exist, and the present exists only for an instant.

In 1990, Stephen Hawking read papers of his colleagues proposing their version of a time machine, and he was sceptical. His intuition told him that time travel was not possible because there were no tourists from the future. If time travel were as common as taking a Sunday picnic in the park, then time travellers from the future should be pestering us with their cameras. There ought to be a law, he proclaimed, making time travel impossible. He proposed a 'Chronology Protection Conjecture' to ban time travel from the laws of physics in order to 'make history safe for historians'.

The embarrassing thing, however, was that no matter how hard physicists tried, they could not find a law to prevent time travel. Apparently, time travel seems to be consistent with the known laws of physics. Unable to find any physical law that makes time travel impossible, Hawking recently changed his mind. He made headlines when he said, 'Time travel may be possible, but it is not practical.'

Time travel to the future is possible and has been experimentally verified millions of times. If an astronaut were to travel near the speed of light, it might take him, say, one minute to reach the nearest stars. Four years would have elapsed on Earth, but for him only one minute would have passed, because time would have slowed down inside the rocket ship. Hence he would have travelled four years into the future, as experienced here on Earth. (Our astronauts actually take a short trip into the future every time they go into outer space. As they travel at 18,000 miles per hour above the Earth, their clocks beat a tiny bit slower than clocks on Earth. The world record for travelling into the future is held by the Russian cosmonaut Sergei Avdeyev, who orbited for 748 days and was hence hurled .02 seconds into the future.) So a time machine that can take us into the future is consistent with Einstein's special theory of relativity. But what about going backwards in time?

If we could journey back into the past, history would be impossible to write. As soon as a historian recorded the history of the past, someone could go back into the past and rewrite it. Not only would time machines put historians out of business, but they would enable us to alter the course of time at will. If, for example, we were to go back to the era of the dinosaurs and accidentally step on a mammal that happened to be our ancestor, perhaps we would accidentally wipe out the entire human race. History would become an unending, madcap Monty Python episode, as tourists from the future trampled over historic events while trying to get the best camera angle.

But perhaps the thorniest problems are the logical paradoxes raised by time travel. For example, what happens if we kill our parents before we are born? This is a logical impossibility. It is sometimes called the 'grandfather paradox'.

There are three ways to resolve these paradoxes. First, perhaps you simply repeat past history when you go back in time, therefore fulfilling the past. In this case, you have no free will. You are forced to complete the past as it was written. Thus, if you go back into the past to give the secret of time travel to your younger self, then it was meant to happen that way. The secret of time travel came from the future. It was destiny. (But this does not tell us where the original idea came from.)

Second, you have free will, so you can change the past, but within limits. Your free will is not allowed to create a time paradox. Whenever you try to kill your parents before you are born, a mysterious force prevents you from pulling the trigger. This position has been advocated by the Russian physicist Igor Novikov. He argues that there is a law preventing us from walking on the ceiling, although we might want to. Hence, there might be a law preventing us from killing our parents before we are born.

Third, the universe splits into two. On one timeline the people whom you killed look just like your parents, but they are different, because you are now in a parallel universe. This latter possibility seems to be the one consistent with the quantum theory.

 The film Back to the Future explored the third possibility. Doc Emmett Brown (Christopher Lloyd) invents a plutonium-fired DeLorean car, which is actually a time-machine for travelling to the past. Marty McFly (Michael J. Fox) enters the machine and goes back and meets his teenage mother, who then falls in love with him. This poses a sticky problem. If Marty's teenage mother spurns his future father, then they never would have married, and he would never have been born.

The problem is clarified a bit by Doc Brown. He goes to the blackboard and draws a horizontal line, representing the timeline of our universe. Then he draws a second line, which branches off the first line, representing a parallel universe that opens up when you change the past. Thus, whenever we go back into the river of time, the river forks into two, and one timeline becomes two timelines, or what is called the 'many worlds' approach.

This means that all time-travel paradoxes can be solved. If you have killed your parents before you were born, it simply means you have killed some people who are genetically identical to your parents, with the same memories and personalities, but they are not your true parents.

Extracted from 'Physics of the Impossible' by Michio Kaku