[the8472]

You receive a message from an an alien civilization 1000ly away that they have started a deterministic process that will shoot a death ray at earth in 500 years. Should you act on the information that there's doom heading for earth right now or not?

[542354234235]

Nothing can travel faster than light. If the Death ray travels at the speed of light, then the 1,000 years it takes the message to get to us, and the 1,000 years it takes the Death Ray to get to us are exactly the same, so cancel each other out. So it would still be 500 years between message and Death Ray.

If someone dropped a ball from a tall building with a message that said “in 5 minutes, I am dropping a ball filled with explosives”, you would still have the same 5 minutes if the building was 5 stories tall or 200 stories. The message and the danger have to travel the same amount of spacetime to get to you.

[the8472]

> So it would still be 500 years between message and Death Ray.

Yes of course, but you'd still act as if the death-ray is on its way, i.e. firing it already happened 500 years before you received the message. The systems may be causally separated but as long as each follows near-deterministic processes we can still calculate its state forward.

In the case of betelgeuse we could hypothetically, if we had sufficiently accurate models, derive from observations that it'll explode in 100 years (relative to the observed state) which given the separation means the light of that event would only be 100ly away which in a sense does mean the event already happened.

We can't be certain, perhaps a rogue black hole might swallow it in the meantime, but for casual conversation "predictable thing effectively already happened and its results are on the way" is good enough, and if one one had to worry about GRBs aimed at earth even prudent.

[542354234235]

I see what you are saying but I think I still disagree. We can make predictions about what will happen in the future but we don’t know. Adding the separation of lightyears seems to make people talk as if they do know what will happen in the future, which we don’t.

We don’t know the Death Ray is on its way. Maybe a new alien was elected Supreme Gbectravic and canceled the project. We are guessing what will happen in 500 years, whether we are looking at our sun or a star 1,000 light years away.

And since light is the universal speed limit for all things, including information, then for casual conversation when we observe something is when it is effectively happening.

[the8472]

> Adding the separation of lightyears seems to make people talk as if they do know what will happen in the future, which we don’t.

Perhaps a distinction is that we like to think that at least in principle we can influence almost any future events that are causally downstream of previous events on Earth. Even someone with very lethal radiation poisoning (alive but predictably dead) might just be one hypothetical stem cell transplant treatment away from defying the previous odds. So we don't treat those as set in stone.

Something causally separated on the other hand is seen as a mechanistic process. And there are very few things that would stop a star from becoming a supernova.

That reminds me of the short story Schwarzschild Defense https://archive.is/SZbHa#selection-1162.0-1162.1