Did you know that when you look up at the stars that you are seeing back in time? It's true—even the light from the Sun is 8 minutes and 20 seconds old when it reaches Earth. The next closest star is 4.2 light years away which means it takes light from that start 4.2 years to get here. Any even that we observe in the night sky happened in the past, and sometimes a long time in the past. In fact, scientists now think that our universe is approximately 90 billion light years in diameter. If you consider our ability to take quality images from the Hubble Telescope, we can see things that happened billions of years ago.

Consider that space itself has three dimensions: height, breadth, and depth. You can add to that understanding a fourth dimension—time. Every object in the universe can be put on this space-time continuum in terms of position and time in history. To two observers who are stationary and side-by-side, they would agree on where those measurements fall on the continuum; however, if they are moving relative to each other, there will be some discrepancy in those measurements.