4. Spacetime
https://www.youtube.com/watch?v=ScdLqAA_64E Spacetime is a term that will crop up again and again in discussions about physics. This is largely due to the fact that, as a concept, it's pretty all-encompassing. We used to describe the universe as having three dimensions (height, depth and breadth) plus time. However, we now view it as a smooth continuum of four-dimensional spacetime in which time is an intrinsic part of the fabric of the universe (although there are some theories that suggest could be as many as 26 dimensions). The bringing together of space and time does away with the classical model of time as a fixed entity, independent of motion. It is now known that time passes relative to motion in physical space and this is one of the basic principles of special relativity. Basically, you move through time in the same way that you move through space. For example, the Earth's orbit in through space can be described as an ellipse, but its orbit through spacetime is actually a helix. The velocity of an object warps spacetime, and so time will appear to move at a different rate for that object relative to another observer. This is known as time dilation Mass also bends spacetime. In general relativity, this makes gravity an effect of the curvature of spacetime, as opposed to a force such as magnetism. The bending of space essentially creates the effect of gravity (think of it as forming a kind of "well" around the object) but it also bends time, creating the effect of gravitational time dilation It's possibly not quite right to think of spacetime as an actual, physical thing as such, it's more like a mathematical model that we use to describe the universe. But, then again, isn't everything?