Comment on wat
plutopos@lemmy.zip 2 weeks agoMoving is just putting more space between you and something. If it walks like a horse, talks like a horse, bites like a horse… you get the idea. It’s not clear to me why the increase of inbetween space in cosmic inflation “gets a pass” whereas the increase of inbetween of space from movement doesn’t
starman2112@sh.itjust.works 2 weeks ago
Apologies for the wall of text. I spent an hour and a half trying to find the most concise way to explain this, and there kind of isn’t one…
An attempt at an analogy
A submarine increases its depth by sinking lower in the water, right? Imagine a tub of water 1 foot deep. Put a toy submarine 6 inches from the bottom, and it’s 6 inches underwater. Now add another six inches of water to the tub. Your sub hasn’t moved through the water at all, but now it’s 1 foot deep. The submarine can increase its depth without sinking at all. In fact, if you let the submarine rise slower than you add water, then it can rise upward through the water as it continues to get deeper below the surface.
There are two ways for the distance between two objects to increase
There are two ways for the distance between two objects to increase over time. One of them involves displacement through a medium, and the other involves that medium itself expanding. From our perspective, a galaxy 5000 megaparsecs away is “moving” away from us by about 350,000 km every second, but that’s only because for every megaparsec between us, the space itself is expanding by about 70 km every second. If you ask a guy smack in between us, he’ll say we’re both moving away from him at 175,000 km every second. That galaxy isn’t experiencing any acceleration in that direction; it isn’t moving through space at that speed.
The speed of light is a constant
The speed of light (in a vacuum) is a constant. That is to say, every observer in every reference frame measures every photon (in a vacuum) as moving at 299,792,458 meters per second. This fact supersedes all others. It supersedes time itself. Imagine I’m on a train going 50 mph, and I throw a ball forward at 50 mph. In my reference frame, that ball is going 50 mph. To an outside observer, that ball is going 100 mph. How fast the ball is going depends on your frame of reference. This is not true of light. If I’m traveling on a train going half the speed of light and I shine a flashlight forward, the train’s speed doesn’t add to the light’s speed. You and I will both agree that those photons are moving 299,792,458 meters every second, in both our reference frames. This happens because we aren’t experiencing the passage of time at the same rate. The photons coming out of that galaxy 5000 megaparsecs away are also going 299,792,458 meters per second in our reference frame, even as the space between us grows by more than that in that amount of time. That galaxy isn’t moving faster than light, space is just expanding.