= 27d567bc2d48d9f40e9ccc20ea370b15)
komponisto comments on Stupid Questions Open Thread Round 2 - Less Wrong Discussion
You are viewing a comment permalink. View the original post to see all comments and the full post content.
= 783df68a0f980790206b9ea87794c5b6)
Loading…= b715d02e85f7b3b4ae65ca3d3e450868)
Subscribe to RSS Feed
Powered by Reddit
= f037147d6e6c911a85753b9abdedda8d)
You are viewing a comment permalink. View the original post to see all comments and the full post content.
Comments (208)
A couple of embarrassingly basic physics questions, inspired by recent discussions here:
On occasion people will speak of some object "exiting one's future light cone". How is it possible to escape a light cone without traveling in a spacelike direction?
Does any interpretation of quantum mechanics offer a satisfactory derivation of the Born rule? If so, why are interpretations that don't still considered candIdates? If not, why do people speak as if the lack of such a derivation were a point against MWI?
Suppose (just to fix ideas) that you are at rest, in some coordinate system. Call FLC(t) your future light cone from your space-time position at time t.
An object that is with you at t=0 cannot exit FLC(0), no matter how it moves from there on. But it can accelerate in a way that its trajectory is entirely outside FLC(T) from some T>0. Then it makes sense to say that the object has exited your future light cone: nothing you do after time T can affect it.
Usually people do include traveling in a spacelike direction as a component of the 'exiting'. But the alternative is for the objects ('you' and 'the other thing') to be at rest relative to each other but a long distance apart - while living in a universe that does stuff like this.
ie. Imagine ants on the outside of a balloon that is getting blown up at an accelerating rate.
The primary argument in favor of MWI is that it doesn't require you to postulate additional natural laws other than the basic ones we know for quantum evolution. This argument can pretty easily be criticized on the grounds that yes, MWI does require you to know an additional fact about the universe (the Born rule) before you can actually generate correct predictions.
Well, every object is separated from you by a spacelike interval. If some distant object starts accelerating quickly enough, it may become forever inaccessible.
Also, an object distant enough on way-bigger-than-galaxy-superclaster scale can have Hubble speed more than c relative to us.
Are you sure about this? I don't understand relativity much, but I would suspect this to be another case of "by adding speeds classically, it would be greater than c, but by applying proper relativistic calculation it turns out to be always less than c".
It looks like it is even weirder.
Proper relativistic velocity arithmetics you mention is about special relativity theory - i.e. local flat-space case. Hubble runaway speed is supposed to be about global ongoing space distortion, i.e. it is strictly about general relativity. As far as I know, it is actually measured based on impulse change in photons, but it can be theoretically defined using time needed for a lightspeed round-trip.
When this relative speed is small, everything is fine; if I understand correctly, if Hubble constant is constant in the long term and there are large enough distances in the universe, it would take ray of light exponential time (not linear) to cross distances above some threshold.
In the inflationary model of early universe, there is some strange phase where distances grow faster than light could cover them - it is possible as it is not motion of matter in space, but change of the stucture of space. http://en.wikipedia.org/wiki/Inflationary_model
Nobody has derived the Born rule, though I think some have managed to argue that it is the only rule that makes sense? (I'm not sure how successful they were). I think people may count it against mwi because of either simple double standards or because it's more obvious as an assumption since it's the only one MWI needs to make. (In other theories the rule may be hidden in with the other stuff like collapse, so it doesn't seem like a single assumption but a natural part of the theory. Since MWI is so lean, the assumed rule may be more noticeable, especially to people who are seeing it from the other side of the fence.)