Talking about the shape of the universe with physicist/data scientist Nathan Brewer
JnBrymn
Physicist, turned Data Scientist, Nathan Brewer has recently joined our ranks, so on a whim I asked him a physics question that I had been wondering about and soon I found myself in my first "lightening" Penny Chat in a while.
Without getting into the details of my (somewhat misguided) question, it had to do with how space is connected together. Apparently there are points in space that are so disconnected that that light from one point shining toward the other point will never ever reach the other point, even if given an arbitrarily long time.
So I asked my (somewhat misguided) question and Nathan quickly came back with this:
It's a sideways image of a light-cone diagram (we weren't sure why it was sideways). When I saw the "Nathan is typing…" indicator in slack, I thought "Uh oh, he's going to explain this in text. That's going to take forever." So I asked him if he was free, and in 2 minutes I found myself in an impromptu "lightening" Penny Chat. We jumped onto Zoom and started talking about the shape of the universe.
The answer to my (somewhat misguided) question gets interesting and confusing real quick. It turns out that if you assume the universe is "flat" (no time spacetime curvature). Then the only way that two points can be disconnected from one another is if they are flying in opposite directions at exactly the speed of light (which isn't possible anyway). So, everything is connected! You can always shine your flashlight from one planet and know that it will hit the distant, distant planet you were shining it toward.
… But that was assuming the universe was "flat". It's not. (Sorry, Flat EarthersUniversers.) This is where stuff gets real weird. Space is growing. Like… the space between galaxies is getting bigger. And the speed at which space is getting bigger is sorta-kinda faster than the speed of light. "But I thought there was nothing faster than the speed of light?" Well yeah, that's still true. It's just that space isn't moving faster than light; rather, space (spacetime) is the stuff that things move through. So nothing can move through space faster than light, but the fabric of spacetime itself can expand or contract regardless of some notion of speed. Speed is defined within spacetime.
Weird, right?
So, the answer to my (somewhat misguided) question is that, because space is expanding, points in our real universe can actually be disconnected so completely that in an eternity you could never travel from one point to the other, or even shine a laser beam from one to the other. And I think I actually understand why!
Holy Cow, that was awesome. Thank you for your time, and I apologize for facts that I assuredly got wrong here :P
-John