The term “spacetime” and the concept of time being the “fourth dimension” appear to contradict each other. Is time actually a 4th dimension, or are all dimensions captured in time?
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Time is a dimension like the three spatial ones. It’s easier to understand if you think of it as nothing more than another axis on a graph. If I ask where a spot on a wall is, you can tell me using two dimensions, two coordinates, two axes. X,Y. It’s 3 feet up and 2 feet from the left.
For a spot in the middle of a room, like the location of your phone sitting on a table, though, you’ll need a 3rd coordinate for depth. It’s now 3 feet up, 2 feet from the left, and 7 feet away from the wall. Still easy to visualize.
But now we can be more precise with another dimension of time. If I check that spot (3, 2, 7), but I check it *yesterday,* will I still find your phone there? How about tomorrow? A hundred years from now? We can just add a 4th coordinate for time, no differently than we did when we added one to move out into the room away from the wall. It’s 3 feet up, 2 feet from the left, 7 feet away from the wall, and at 10:00 on August 27, 2023.
And while it’s tricky to graph all 4 of these at once in a way you can visualize, you can arbitrarily drop one or two spacial axes and now time fits nicely into a 2 to 3 dimensional graph you can see. And that’s useful beyond being a mathematical trick. Your travel on one axis directly affects your location and travel on the other axes.
For instance, just like it would be impossible to find your phone in your house if the vertical coordinate was 10000 feet, since it would be high in the sky, its also impossible to find it in your house if the time coordinate is 65 million BC, because your house likely didn’t coexist with the extinction of the dinosaurs.
You can also graph time against spacial dimensions to determine cause and effect. Actually demonstrating a Penrose diagram is a little beyond what I can do here, but basically, you can easily see on as simple as a 2-D graph whether an event is possible or not to ever interact with you by making specific assumptions on the graph (i.e. that the speed of light is at 45 degree angles to the corners of the graph and that the edges compress to infinity).
It sounds complicated, but it’s super easy to *see* how time works just like space when drawing cause and effect “light cones.” Basically, if it’s outside the cone, you can never, ever interact with it, since that would mean traveling faster than light, i.e. time traveling.
In this example, your phone being on the table *could* have been influenced by the dinosaurs, since their coordinates were close enough, appearing inside the cone (even if there’s no way for us to actually show a real life connection), but space-dinosaurs from the edge of the universe couldn’t have influenced it because they’re too far away in either space or time, and they’d appear outside the cone you’d draw.
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