@[email protected] I see at least one that is, and I know because I remember connecting to it.

this is why the phoenix wright generator has been "in progress" since 2018

And what if you put one end on an enemy planet and the other end in low orbit around Betelgeuse when it finally goes supernova?

How many gamma rays will come through a hole in space about a meter across?

Maybe not one that lets you trivially violate causality but with moving portals or multiple portals, whoops...

Like, the worked example from Atomic Rockets has 7 kilograms of cat litter moving at 90% of lightspeed hitting a stationary target with 195 megatons of kinetic energy.

https://www.projectrho.com/public_html/rocket/spacegunexotic.php

You can also make it bigger by not using a roughly round rock and instead using a long rod of the densest material you can get your hand on.

But mass you pay for, speed you don't.

This isn't a ship-destroying weapon, this is a civilization-ender if not planet-killer.

You've got a projectile moving at a significant fraction of the speed of light. This is a relativistic weapon: it's going to hit harder than if it was a nuke.

BTW, as a variant on the kzinti lesson, the portals are extremely dangerous as a weapon, because of how good they are as a weapon.

Ignoring the obvious ways to fight with them like opening a portal on the enemy's hull, shoving out a nuke and then closing the portal...

You could also just have a rock that you're letting accelerate to arbitrary speeds in a vacuum. That's free unbounded kinetic energy, the only limitation being the "charge" time.

Anyway the lazy, boring way to use a portal and pretend you aren't violating a bunch of physical laws is to just use it for fuel transport.
You have a bunch of fuel on the ground, a tiny tank on your rocket, and you keep topping off the rocket's tank by piping in the fuel.

If you could make portals bigger you could also have a fun setup where you build your spaceship and then just let gravity accelerate it though a portal-loop.
You get going as fast as you want, then just swap the portals so you're now aimed at Mars.

And that's just thinking about a static arrangement of portals. You could also use a dynamic arrangement where you use gravity to accelerate mass to arbitrarily high speeds and then fling it out the back

I'd do the math on how much thrust you'd get out of sticking one portal at the bottom of the Mariana Trench and the other in a ship, but I think it'd maybe be slightly tricky because you've got yourself an inertialess thruster right there, which is slightly illegal according to physics.

The Einstein cops are gonna show up and impound your spaceship