What is Relativity?
Relativity is about how things exist in relation to each other. This means, for example, arriving at measurements in order to describe where things are located. By “things” we mean physical things, such as buildings and planets, and we also mean things we can’t touch, like light and magnetism. Measurements are also the way that we arrive at conclusions about the velocity with which a thing is moving. Ultimately Relativity might help us understand our place in the universe.
How do we measure things anyway?
Einstein begins by discussing what geometry is and how we measure things. We all remember using rulers and compasses at school. They help us describe the location of things on a flat surface that is rigid and stationary. But measurements for flat-and-rigid surfaces only deal with left/right and forwards/backwards; they don’t deal with height. So whilst this method of measuring would help us describe the location of (for example) a footprint, it is not enough to pinpoint the location of a cloud. This tells us that sometimes we need a different geometry to measure different things.
In order to measure anything, including distances, we always use something else as a yardstick. If you roll a marble across your kitchen, you might take your tape measure and you will work out pretty easily that the marble rolled a distance of 3 metres until it stopped. What you are doing is measuring the distance of the marble’s travel by reference to the floor, or because the floor is attached to the ground, you might say by reference to the Earth.
It’s the same thing when you measure the height of a person. You could give the person’s height by reference to the wall. So the only way to measure anything is by reference to something else. Or, you might say, relative to something else.
But let’s think about that marble some more. It was 3 metres away from its start point when it stopped moving, right? Sure about that? Well, yes – if you judge the start and end points relative to your kitchen floor.
Imagine that your Grandad is standing on the Moon, though. (Bear with me..) Your Grandad can see the start point and end point of the marble’s journey. Because he has brilliant eyesight, obviously. But your Grandad can also see that the kitchen floor (across which the marble is moving) is attached to the surface of the Planet Earth which is spinning on its axis at a speed of 1,670 kilometres per hour. Not only that, he can see that the Planet Earth is also travelling through space in its orbit round the Sun at 30 kilometres per second.
And all this moving is going on during the marble’s journey across your kitchen floor.
If you asked your Grandad how far the start point and end points of the marble’s journey are from each other, what would he say from from his vantage point set against the vastness of space? He would say that relative to his position on the Moon that marble has moved probably thousands of miles. My brain is starting to hurt just thinking about how to work that out.
So here we can see that we can only ever say that an object is moving, and what its speed is, if you judge the object’s progress relative to another object.
With me so far?