Alex McColgan

Events that are simultaneous from one point of view are not simultaneous from another.

To be clear, this is not just a weird consequence of light taking its time to reach us.

If we were just dealing with an optical illusion caused by light taking its time to move to our eyes, then we'd see the closer event happen first, and the more distant event happen second.

but this is happening the opposite way around.

In this case, and from the point of view of the pole, the more distant event spatially happens first, and the closer event happens second.

In other words, this is not just a trick of the light, but the suggestion that something more fundamental is going on.

And what is going on?

When I first learned about this, it boggled my mind.

It seemed wrong as a matter of principle.

But the more I thought about it, the more it started to make sense, based on my understanding of the universe.

And the more startling implications started to arise.

Don't worry, I've not forgotten about that seeing the future thing.

I'll get to it.

Let's take another look at what's happening here.

but this time let's bring in a little colour coding and our old friend, a 4D space-time diagram.

Hopefully you are familiar with these already, as I've done a few videos that have included them.

If not, here's a great video we did on the subject, which you should have a look at.

To recap though, just imagine that all of space has been flattened down to the one line along the bottom of our graph, while an object's motion in time is tracked along the side of the graph.

From the point of view of the barn, it is not moving in space, so we draw the arrow directly up, it is only moving into the future.

I've painted the shutting and opening of the two doors as two different colours here on the graph.