speed of light
Sending a tiny spaceship to the nearest habitable planet at 20% of the speed of light? No problem, says theoretical physicist Michio Kaku.
Watch what it might look like to travel from the sun to Jupiter at the speed of light.
Scientists produce a value for the cosmic microwave background that will definitely prove or disprove that the speed of light used to be higher.
If someone comes back from the future, they ought to have packed one thing in their carry on: proof.