At this rate, a journey to Proxima Centauri would take 72,000 years. Of course, this mission was not specifically intended to travel fast to the stars, and current technology could do much better. The travel time could be reduced to a few millennia using solar sails, or to a century or less using nuclear pulse propulsion. Special relativity offers the possibility of shortening the travel time: if a starship with sufficiently advanced engines could reach velocities near the speed of light, relativistic time dilation would make the voyage much shorter for the traveller. However, it would still take many years of elapsed time as viewed by the people remaining on Earth. On returning to Earth, the travellers would find that far more time had elapsed on Earth than had for them (twin paradox). Many problems would be solved if wormholes existed. General relativity does not rule them out, but so far as we know at present, they do not exist. Communications The round-trip delay time is the minimum time between a probe signal reaching Earth, and the probe getting instructions from Earth.