Sylickon
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Could i predict what everything in the universe will look like in 10 million years using formulas, or is that impossible due to probability?
Well, that sort of depends.Sylickon said:Could i predict what everything in the universe will look like in 10 million years using formulas, or is that impossible due to probability?
Chalnoth said:If you want to take the bird's-eye view of the universe, then all physical laws that we know so far are perfectly predictable: as long as you have the full state of the universe at one time, you can, in principle, compute the full state of the universe at any other time.
But this isn't what we can do, because we cannot view the universe from the outside. Instead, we view it from inside.
bcrowell said:Well, all you need to know about is the initial conditions in the past light-cone of the event you're trying to predict. You don't need to know initial conditions for the whole universe.
That's actually not true. In fact, in the classical big bang, the exact opposite happens: the closer you are to the big bang, the less of the universe is in the past light cone. This has to do with the fact that in the classical big bang, the expansion early-on is slowing down. When you have an expansion rate that is slowing down, the past light cone tends to include more and more of the universe as time progresses. This is known as the horizon problem, and is one of the problems that inflation was proposed to address.Shovel said:If you extrapolate backwards in time to the big bang, as you approach t=0, any two points will become arbitrarily close together and thus be in each other's light cone.
Sylickon said:Could i predict what everything in the universe will look like in 10 million years using formulas, or is that impossible due to probability?
Shovel said:If you extrapolate backwards in time to the big bang, as you approach t=0, any two points will become arbitrarily close together and thus be in each other's light cone.
Right, so, in that sense it's a somewhat confusing name. The point is that if you take a slice of the universe in time (for your favorite choice of time coordinate, whatever that is, and were able to know perfectly the state of the universe across that entire slice of time that lies within your past light cone, then you could in principle calculate everything else that happened since then that you can potentially observe.bcrowell said:"Initial" doesn't mean the Big Bang. (It can't mean that, because the Big Bang is a singularity, so it's not an event in spacetime at all.) "Initial" just means any spacelike surface cutting across the past light-cone.
Chalnoth said:Right, so, in that sense it's a somewhat confusing name. The point is that if you take a slice of the universe in time (for your favorite choice of time coordinate, whatever that is, and were able to know perfectly the state of the universe across that entire slice of time that lies within your past light cone, then you could in principle calculate everything else that happened since then that you can potentially observe.