The space-time continuum is a physics model that describes space and time as connected. All things exist on the space-time continuum, and their locations always exist as a set of four coordinates: the three dimensions of space plus time. Space-time is a continuum because it covers the entire universe. There are no breaks in space or time.
Einstein’s Theory of Relativity
Albert Einstein found that energy equals mass times the speed of light squared. This is represented by the equation E=mc². What this means is that the faster an object goes, the more mass it gains and the harder it is to move. This is why reaching the speed of light is so hard. Eventually, an object’s mass becomes so great that it can’t be moved.
Time and Space
While that sounds simple, it affects time as well. Einstein realized that the speed of light is constant regardless of how fast other objects move. He also found that nothing can move faster than light. Because of this, two people moving at different speeds will see light moving at the same rate, yet their experience of time will shift. To put it more simply, as an object’s speed increases compared to another object, it moves through time slower. For example, if a person were to board a spaceship and travel at half the speed of light, they would age much slower than a person back on Earth.
Even lower speeds cause this effect. For instance, by using special clocks, scientists determined that a satellite orbiting the Earth gained 38 microseconds every day. That means it passed through time just a bit slower than the scientists on Earth. This shows that objects move through time as well as space relative to each other.
Space-Time and Gravity
As an object moves faster, time slows down for it and mass goes up. However, mass also acts on space-time. Imagine space-time as a mattress and a planet as a bowling ball. If you place the bowling ball on the mattress, it curves the fabric around itself. If you place a golf ball next to that bowling ball, it will fall into the dent caused by the larger object.
Huge objects do the same thing to space-time. The larger an object is, the more it bends space-time around itself, leading other nearby objects to move through space (and time) toward it. This is what we call gravity. Our planet, for example, is so massive that it bends space-time to keep you on its surface.
The time slowdown caused by movement is the only form of time travel all scientists agree is possible. However, Einstein’s theory of relativity does allow for wormholes, or direct passages between separate points in space-time. While the limit on moving faster than light might normally make fast travel between those two points impossible, a wormhole between these two points would let an object move from A to B instantly without going faster than light. While wormholes remain an abstract idea, scientists might one day find out how to make use of them.
Where We Get Space-Time Wrong
There are a few things that Einstein’s theory of relativity and our current knowledge of the space-time continuum can’t explain. At the atomic level, for instance, particles don’t always move the way they should according to relativity. Likewise, we don’t understand what happens to matter when it falls into a black hole, or where the matter goes when a black hole dies. New ideas such as string theory attempt to make relativity work in these situations, while the concept of phase space suggests that even the time-space continuum itself might be relative. As scientists learn more, questions such as these will hopefully be resolved.