Another interesting thought that just crossed my mind is this:

Suppose you have a magical superluminal laser. Whatever you point it at (with a direct line of sight), no matter how far away, will be lit up with the laser at the moment you push the button.

That laser beam will appear at the location you pointed it however many light years in the past it would have taken for light to travel to its destination and then return over time to the moment you fired the laser.

From the moment your superluminal laser is operational and verified to be working, how many alternate timelines are collapsing out of probability every moment you either are or are not pushing the button?

If you push the button, you instantly change the past, right? The universe suddenly becomes a universe where a laser, that from the local time observers viewpoint, suddenly appeared out of nowhere and then flew off at the speed of light to your location.

The change may be minuscule (a 5mw red laser light touching on the surface of the moon with no observer is such a small variation as to be essentially negligible in the grand scheme of things after all) but it could also be massive.

For instance, lasers have a radius that they spread out from over a distance. No laser is perfectly uniform that we are currently capable of creating. This laser collimation problem is well known and apparently impossible to correct for.

So, there are 2 ways this could go. If it worked the exact same way forwards as backwards, then no big deal, a few photons from 20 light years away seem to appear out of nothing and zip away. But if this magical laser fires and arrives 20 years in the past with its starting intensity, then that creates some problems.

In the first probability, the laser light will then fly back and begin dispersing normally, spreading out across its arc radius as it reverses to its source and then somehow the massive, multi-light year spread of light all converges simultaneously at the moment the button is pushed (this is the good ending), or alternatively, the amount of energy needed to arrive collimated at the destination is enormous, and since nature abhors a vacuum the universe will provide the needed energy, the laser drawing more and more energy in during the return trip to account for the arc difference over the distance.

If the former happens, that will be an interesting thing to see as light moves at truly infinite speeds for the moment immediately before the button is pushed. Poof! Magic, no harm, no foul.

If the latter happens, then things get really funky.

If you fired a 1 watt laser 20 light years away, then the laser will have to travel 300,000,000m/s * 31557600 light-seconds * 20 = 18,934,560,000,000,000,000,000 meters, or roughly 19 sextillion meters (for simplicity). Lasers spread out by a multiplier of roughly 1x per meter of distance. So the laser that arrived in the past at 1 watt would have necessarily required 19 sextillion watts of power.

The most powerful laser in the world hasn't even been built yet but is estimated to be able to draw 100 petawatts of power, or 1^17. Your laser would be 1.89^22, or 5 orders of magnitude more powerful than that.

It would start out arriving harmlessly at its destination, probably bright enough to stun someones eyes, and then begin a light speed return trip through normal time back towards its source, increasing its power with every second on its way back until, at the infinite moment right before the button was pushed arriving at its destination with nearly 2,000,000 times the amount of energy produced by the entire planet's electrical grid all focused on a single point.

Needless to say, some very interesting things might happen should you ever push that trigger. It is difficult to imagine what those things would be but I'm guessing if all you got was a gigantic explosion that might be all for the best. That much energy density in such an infinitely small location might be enough to destroy the city, or the continent, or wipe out all life on the planet, or possibly jump start the second big bang. I don't have enough knowledge to know for certain, but your magical laser might possibly be the death of the universe.

For every moment that you do not push the button, potential timelines of existence flare out of the range of possibility. But the moment you do push the button, you potentially create an entirely new timeline where an insanely powerful and unstoppable force of nature began hurtling at the speed of light towards a very specific destination, ultimately destroying everything in its inexorable path, a tidal wave of energy, a laser tsunami crashing towards a lens operated by the tip of your finger.

All I can say is that thank God you didn't point it at Betelgeuse (642.5 light years away)