UNDER THE MICROSCOPE: When you look up at the night sky, the stars don't seem all that far away. The simple fact is that they are enormous distances from Earth, however, and we shall have to perfect revolutionary technologies before we can realistically attempt to reach the stars.
We must first discover how to travel faster than light, how to make a spacecraft move without propellant and how to power the spacecraft.
Our closest star, Proxima Centauri, is 4.2 light years away, which means that, travelling at the speed of light, a spacecraft would take 4.2 years to reach it from Earth. The nearest galaxy to our Milky Way is the Andromeda Galaxy, two million light years away.
The speed of light - almost 300,000km a second - is the fastest possible; no material object can accelerate to match it, although it should be possible to reach a large fraction of the speed of light.
Let me give a few examples to illustrate how far we have to go in the speed stakes before travel to the stars becomes feasible. It took three days for the Apollo spacecraft to reach the moon. At that speed it would take more than 900,000 years to reach Proxima Centauri. Voyager 1 and 2, which were launched in 1977, will soon leave our solar system, travelling at almost 60,000km an hour. At that speed, it would take them 80,000 years to reach Proxima Centauri. If we want to travel to stars, and return, within realistic time spans, we must learn how to travel faster than light.
If the laws of physics preclude acceleration to the speed of light, how could we travel faster than light? Einstein showed us that time and space do not exist as cleanly separable entities but are fused together in the space-time continuum. It may be possible to get around the speed-of-light limit by distorting the fabric of the space-time continuum, to create short cuts in space-time called wormholes, or by moving segments of space-time using "warp drives". Star Trek fans will be familiar with the terminology.
To help understand the concept of warp drive, think of a moving pavement, or travelator, such as you see at airports. Say you are walking at x kilometres an hour and you move onto a moving pavement that is travelling at y kilometres an hour. Your overall speed increases to x+y kilometres an hour, because the ground beneath your feet is moving.
A warp drive would expand space-time behind the spacecraft and contract space-time in front. The craft would move at less than the speed of light in its own space-time segment but, when the moving-pavement effect was added, its apparent speed would exceed the speed of light.
A car pushes against the road to propel itself forward. An aircraft pushes against the air. But there is no air in space. Current spacecraft use rockets that blast out propellants, pushing them forward in accordance with Newton's third law of motion: to every action there is an equal and opposite reaction.
The faster and farther a rocket must travel, the greater the amount of propellant it must carry. For a spacecraft the size of a bus to travel to Proxima Centauri in 900 years powered by conventional chemical propellant would require more propellant than there is mass in the universe.
Even a nuclear-fusion rocket, which has yet to be developed, would require a propellant tank the size of 1,000 supertankers. And this doesn't take into account the facts that you might like to decelerate and stop at Proxima Centauri, you might like to make the trip in less than 900 years and you might even like to come home. Even the best rockets imaginable wouldn't do the job in a realistic time. What is needed is a form of propulsion that doesn't need propellant.
The third big problem is energy. Even a drive capable of converting energy into motion without using propellant would require enormous energy. To send a craft the size of the space shuttle on a 50-year trip to Proxima Centauri would require energy equivalent to the lifetime output of a conventional nuclear power plant. To supply energy at this level will require a breakthrough in the physics of energy production.
Manned spacecraft will not travel to the stars within the next 200-300 years, but possibly unmanned spacecraft will. One proposed design, known as Starwisp, works like a sailing ship that is blown along by radio waves. It weighs less than an ounce and is like a fishing net made of fine wires.
An enormous transmitter on Earth generates the radio waves, its beam pointing at the star we wish to reach. The "fishing net" must absorb only a small fraction of the radio waves or it will be vaporised. The net must carry instruments to gather information and must signal back to Earth.
There are many technical problems to be solved before such a craft could perform. Some day it may be possible to scale Starwisp up by a factor of a million, so it could carry human passengers.
In the meantime, you can read more at www.grc.nasa.gov/WWW/PAO/html/warp/scales.htm, the site of NASA's Glenn Research Center. William Reville is associate professor of biochemistry and director of microscopy at University College Cork