Less Wrong is a community blog devoted to refining the art of human rationality. Please visit our About page for more information.

Phil_Goetz2 comments on Belief in the Implied Invisible - Less Wrong

30 Post author: Eliezer_Yudkowsky 08 April 2008 07:40AM

You are viewing a comment permalink. View the original post to see all comments and the full post content.

Comments (32)

Sort By: Old

You are viewing a single comment's thread.

Comment author: Phil_Goetz2 11 April 2008 04:00:42AM 0 points [-]

To make it clear why you would sometimes want to think about implied invisibles, suppose you're going to launch a spaceship, at nearly the speed of light, toward a faraway supercluster. By the time the spaceship gets there and sets up a colony, the universe's expansion will have accelerated too much for them to ever send a message back. Do you deem it worth the purely altruistic effort to set up this colony, for the sake of all the people who will live there and be happy? Or do you think the spaceship blips out of existence before it gets there? This could be a very real question at some point.

I don't see any difference between deciding to send the spaceship even though the colonists will be outside my lightcone when they get there, and deciding to send the spaceship even though I will be dead when they get there.

I don't think it's possible to get outside Earth's light cone by travelling less than the speed of light, is it? I'm not well-educated about such things, but I thought that leaving a light-cone was possible only during the very early stages (eg., the first several seconds) after the big bang. Of course, that was said back when people believed the universe's expansion was slowing down. But unless the universe's expansion allows things to move out of Earth's light-cone - and I suspect that allowing that possibility would allow violation of causality, because it seems it would require a perceived velocity wrt Earth past the speed of light - then the entire exercise may be moot; the notion of invisibles may be as incoherent as the atomically-identical zombies.

Comment author: rocurley 21 August 2011 04:20:12AM 4 points [-]

I'm pretty sure it is possible to escape Earth's light cone at sublight speeds. You can go arbitrarily far from earth (if you're patient). Eventually, you will get to a point where your distance from Earth*the Hubble constant is greater than the speed of light (you are now a Hubble length from Earth). At this point, a photon you shoot straight towards Earth will not approach Earth, because the distance in between is expanding at the speed of light.