Imagine a photo of the crew on board the International Space Station (ISS). Now think of a still from Star Trek, showing the crew onboard Enterprise or Voyager. Aside from the fact one is reality and one is fiction, what are the other significant differences?

When I carried out this thought experiment, I noticed that the ISS crew was floating around. In contrast, the Enterprise crew are walking around, just as they would back down on Earth.

Contrary to what you might think, this isn’t because there’s no gravity onboard the ISS. It’s more complicated than that. There is gravity – the gravitational field of Earth – but what causes the look of floating is that both the crew and the ISS are in free fall, orbiting around our planet.

The same happens even when it doesn’t seem as if a spacecraft is orbiting a planet. For example, Apollo astronauts on the way to the Moon were also in free fall, first in the Earth’s orbit and then under the gravitational tug of the Moon’s orbit.

So what prevents crew members on Star Trek’s spaceships from floating through the hallways? Why are they not shown to be influenced by the gravitational pull of nearby celestial objects? The answer is artificial gravity.