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Spacetime
The Universe is a strange place. There are many things that are not easy to understand or explain and one of them is Spacetime. This is a way of thinking about how time and space come together. It's an idea to help us make sense of how things really are and not how they appear to be.
The Spacetime picture below shows three dimensions, one of time going up the page, and two normal directions such as North & South and East & West. To draw the diagram properly, the remaining up & down direction should be included, but it's not easy to draw a 4 dimensional object on a 2 dimensional page. In other words, the up & down direction has been excluded on the diagram so that the flow of time could be shown.
Nothing can travel faster than the speed of light which is about 300,000 kilometers per second. But as fast as this is, it still takes light millions of years to reach us from some galaxies. If you're at an Observation Point, anywhere on Earth, and look up at the night sky and manage to spot a galaxy, you see it as it was millions of years ago. It's just not possible to see it as it is "now" because light does not travel infinitely fast.
For someone at the Observation Point the entire Universe, at a single instant, is represented by a horizontal plane or slice through the Observation Point.
Spacetime diagrams should not give you a sense of movement. They represent all that has happened in the past and all that has happened in the future. It is a static representation of all the things that have happened, everywhere, in the Universe over its lifetime from the Big Bang to the Big Death.
The interesting thing about this diagram is that the surface of the bottom cone represents what it's possible to see from our Observation Point. Galaxies outside the cone are too far away to been seen and galaxies inside the cone are too close to be seen. It's the reason why you can't see a younger version of yourself from the past. That may sound strange. It just means that to see galaxies inside the bottom cone, light would have to travel slower that it actually does. Of course, you can still see them, but only when they lie on the surface of the cone. In other words, you see older versions of these galaxies.
The top inverted light cone represents where light from the Observation Point goes when shone out into space, from the Sun for example. In this case, the diagram shows a two dimensional Sun shining out into three dimensional Spacetime.
Things look different from the point of view of a photon (light ray) travelling along the surface of the cone to or from the Observation Point. When a photon travels at the speed of light, time stops completely for that photon. As far as it is concerned, the photon exists for just an instant because it's emission and absorption is a single event. It hardly seems to exist at all and so barely travels any distance. It's as if the particle at the Observation Point is touching all the other particles on the surface of the cone and the photon is everywhere in the Universe at that single instant.
It's actually possible to calculate the distance across space and time between any two points on the Spacetime diagram. Say, between London at 8:00 and New York at 8:01 or London at 8:00 and London at 8:01. This distance is called the interval and it's calculated by multiplying the time difference by the speed of light, squaring, subtracting it from the square of the spacial distance, and finally taking the square root of the result.
For example, the two points, London at 8:00 and London at 8:01 are separated by an interval of about 18,000,000i kilometers. The i indicates that the number is imaginary in a mathematical sense (i is equal to the square root of -1). It also says that one of the points lies in the cone of the other which means it's possible for the earlier point to have an influence on the latter. If the other point is outside the cone, they cannot effect each other.
If you've read this far you've probably guessed that this has been about Einstein's theory of relativity! It also includes some work by Minkowski on the visualization of Spacetime.
Just like the speed of light, a Spacetime interval has an absolute value. An interval between any two points or events is the same for all observers no matter how they are moving. It is interesting to note that the two Newtonian absolutes of space and time have been replaced by two other absolutes of the speed of light and the Spacetime interval.
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