# Why do wormholes necessarily alter time? Why don't they just make two places causally linked?

I've heard many-an-explanation of wormholes. From as a kid with the hole punch through paper, to the sci-fi something something magic, to popular YouTube science channels talking about how it could be used for time travel.

I don't buy the time travel bit. It seems as though the two places that are connected by a wormhole would just be causally linked. Is this not the case? Why?

Suppose in the year 3000 the planet Splot passes the Solar System at a speed of $c/\sqrt{2}$ (I chose this value because it makes the time dilation a factor of 2). The Splotians like our calendar so they decide to reset their calendar to also be the year 3000 as they pass, which means we all agree on the dates. At the moment Splot passes the Earth we create a wormhole between the two planets.
It turns out that Splot is a bit of a dump, so you very quickly get bored and want to come home. But here's the problem. In Splot's rest frame it was the Earth that zoomed past at $c/\sqrt{2}$ so it's the time on Earth that is running slowly. That means when the year is 3050 on Splot it is only the year 3025 on Earth. So you jump into the wormhole and get back to Earth in 3025, which is 75 years before you left. This is when you decide you always really hated your grandfather :-)