4 ms·
I think I see what you mean, but isn't it true that if there is light shining on the object, then the lamp must be in the light cone? It is literally impossible
by delluminatus 13y ago
I think I see what you mean, but isn't it true that if there is light shining on the object, then the lamp must be in the light cone? It is literally impossible for something outside the light cone to have any physical impact on the object at all.
We do know the sphere of causality already, no computation needed: it is the sphere defined by the distance light has had time to travel. You don't need to simulate anything outside that zone to know that it is impossible for matter outside it to physically impact the center.
Think of it this way: if you want to simulate Earth through the year 2000, you know you don't need to simulate Alpha Centauri after 1996, since it is more than 4 light years away. You can know this without doing any computation at all.
- nitrogen 13y agoI know this is an old thread, but I wanted to point out that you do need to simulate reality beyond an object's light cone so that you know what's there when the light cone expands to reach it.
- TophWells 13y agoI don't think that's right. Nothing outside Earth(2014)'s past light-cone can affect anything inside it, nor can anything outside Earth(2015)'s past light-cone affect anything inside it. Expanding the light-cone into the intervening year doesn't require you to simulate anything further out - well, you need to go outside Earth(2014)'s light-cone to simulate Earth(2015), but that's hardly worth mentioning. I think you're picturing a sphere of 13Gly radius (or whatever) centred on present-day Earth, expanding at lightspeed to encompass new stars and galaxies. But while new matter enters our light-cone, it is not doing so as stars and galaxies. Those all have pasts within the light-cone - you don't need to go outside our past light-cone to find all the things that can affect them. Any matter that only entered our observable universe in the last year doesn't have a past, because it only just came through the Big Bang. It's primordial chaos, not fully-formed stars; you don't need to work out millenia of its past to know what's there. Unless for some reason the simulation needs to calculate pre-Big Bang conditions, which is possible, but then the definition of "light-cone" needs to be amended.