"If you travel close to the speed of light, so if you had a spacecraft traveling close to the speed of light, then distances shrink from your perspective. The one number I always have in my mind is at the Large Hadron Collider at CERN, the protons go around the ring, which is 27 kilometers in circumference, and they go around at 99.999999% the speed of light, so close to the speed of light. At that speed, the distance is shrink by a factor of 7,000, and so that ring is something like four meters in diameter to the protons. According to laws of physics, if you can build a spacecraft that goes very close to the speed of light, you can shrink the distance to the Andromeda galaxy, and therefore the time it takes to get there. You could traverse across that distance in principle, in a minute, according to physics. However, if you came back to the Earth at that speed to tell everybody what you'd found, at least four million years would have passed on the Earth. We could, in principle, explore the galaxy and beyond, but getting to chat to everybody about what you found is forbidden by the structure of the universe, which is the way that relativity works."