Reply #38: Is the message SENT when ship is 20 light minutes away?

[Ready4Change]

Or, does the message reach the ship when it is 20 light minutes away?

Your message lasers will need to be aimed like a shotgun trying to hit a flying clay pigeon. In otherwords, they have to lead the target.

So the message the ship recieves when it is 20 light minutes away was actually sent 20 minutes AGO. ANnd it will be red shifted due to the ships velocity.

If you send a message to a ship which is already 20 light minutes out, you will have to lead the target, by aiming the message to the spot the ship will be, once the message gets out there. If the ship is travelling .5 lightspeed you'll need to aim MORE than 20, or even 30 light minutes ahead. When the message has travelled 20 lm, the ship has moved another 10 lm. 10 minutes later the message still isn't there, because the ship has gone another 5 lm. The message DOES eventually reach the ship, but since the ship is so fast, it takes quite a while.

It's more difficult though. You don't really know where the ship is NOW. When it's 20 lm out, you only know where it was 20 minutes ago, because that's how long it took for that light to reach you. If you 'see' the ship 20 lm out, and it's travelling .5 lightspeed, you can estimate that it's really 30 lm away now. BUt that estimate is only as good as the odds that the ship hasn't changed course since 20 minutes ago and won't change course during the 30+ minutes that the message will be playing catch up.

You'll need someone with stronger math than mine to figure out how long/far that game of catchup takes.