We have been to the moon. We will go to Mars, and eventually elsewhere in the solar system. After that, things become difficult.

The nearest star to our own Sun is Proxima Centauri, about 4.3 light years away. That’s about 26 million million miles. The spacecraft Pioneer 10, which is leaving the solar system at 27,000 MPH, has only gone about 11 light-hours, and won’t pass near one of the stars in the constellation Taurus until two million years from now.

So to get anywhere we have to go much faster. With currently conceivable technologies, we could build ships that would slowly build up to meaningful fractions of the speed of light. These technologies will advance, and small exploratory probes are imaginable in the late-21st century.

Initially, they will be uncrewed. They may have to be very small, only pounds or ounces, as the energy requirements of moving any object at such speeds are immense.

For large ships carrying their own fuel, energy demands seem nearly insurmountable. One alternative is a vessel powered by a light-sail, which would harness the minute energy in photons of light. Physicist Lawrence Krauss calculates that a 1,000-ton spacecraft could achieve 10% of the speed of light in a year with a sail 100 miles across.

Even so, conventional voyages will take decades. Avoiding this will require faster-than-light (FTL) space travel, now standard in science fiction as the warp drive or hyperdrive.

Though FTL space travel is a staple of science fiction and the futures it depicts, it faces a basic problem: the speed of light is an unbreakable barrier according to the laws of physics as we now understand them. (Experiments in 2000 involved only light itself, and do not contradict the apparent prohibition against anything with mass reaching this speed limit.)

Getting around the speed of light might involve wormholes or warping of space. But Krauss argues that these too would require enormous amounts of energy, and may not work with objects larger than atoms. It is an unfortunate possibility that people may never travel easily between the stars.

We cannot know whether an FTL breakthrough is possible. It requires fundamental advances in physics, and these could come in 2040, 2200, or never. In any case, it is unlikely that any FTL ships will leave Earth until the late 21st century at the earliest.