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this post was submitted on 25 May 2026
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Wrong, the expansion doesn't have a speed because it isn't motion. But you have to think about it longer than you'll probably want to before hitting the up or down arrow and/or scrolling.
Best intuition I've heard for this is that "things" can't move faster than light, but not everything is a "thing".
Imagine doing shadow puppets on the wall with a flashlight. You move the bunny left, shadow moves left. The further away the wall is, the faster the apparent speed of the shadow bunny. You might think that, far enough away and with a strong enough light, your shadow bunny would be racing across the sky faster than the speed of light -- and the crazy thing is, you'd be correct! The shadow (absence of light) can move arbitrarily fast. But the light itself is moving at its normal constant speed from the flashlight out into space, perpendicular to the travel of the bunny, like a garden hose spraying water. The time it takes for the shadow to even begin to move is governed by the speed of light. No information can be communicated faster than light because the light travels at the speed of light to illuminate the places where the shadow isn't.
Very eloquent explanation. The one glitch I must point out is that the shadow (or absence of light) can't move faster than light, because the shadow is information and information can't travel faster than light. If it could, you could use a sequence of shadows, coded by length and spacing, for FTL communication.
The shadow moving is more akin to bandwidth of transmission rather than speed of transmission. You still have to wait for the photons (so speed of light) for the information to arrive, even if the "speed" of the shadow appears FTL.
The shadow has no FTL information here. Watch https://youtu.be/JTvcpdfGUtQ
Ok I watched the video and I get that no information is travelling FTL. "Speed of darkness" is misleading tho, because the light at the shadow's edge has the same "speed" as the shadow.
The edge of the shadow can move across the surface of an object an essentially infinite speed. That's the speed of dark
And the edge of a ball of light created by a powerful spotlight could do exactly the same thing. That video is nothing but sleight of hand style tricks dressed up as the speed of dark. You can recreate every scenario using light to argue light is faster than the speed of light. What you're doing isn't movement at all, period.
Imagine earth has a ring around it the distance of the moon. You have a powerful laser pointer and rotate 360 degrees while pointing at it. The dot of light "travels" around it at insane speeds. This is what the video argues "dark" or the shadow is doing. But if you think about it for a moment you realise each "dot", whether it's the shadow or the laser pointer, is actually a unique thing, and nothing is actually "moving" at all. Shadows don't move. Ever. They aren't even something. They are just light being blocked.
Congrats you get it
Yeah I get it. Do you get that the light around the shadow, that isn't part of the shadow, is moving at the same "speed" - so it's not the speed of "dark". It could be dark, it could be red light, blue light, full white, any color or lack of color.
That's the point yes
Best analogy I heard for it is if you put a load of dots on a balloon, then inflate it. Are the dots getting further away? Yes. Is there just the same amount of rubber between each dot as when you put the dots on? Yes. Can you measure the relative speed of the dots? Yes! But have they actually gone anywhere? No...ish?
Yeah, but in this case is the universe just the dots on the surface of the balloon or is it the whole balloon with its entire volume? Intuitively I think it's the latter (although there's probably no "hard" edge that's bounding the ends of the universe like the rubber of the balloon), and if that's true, you could measure the speed of one wall getting away from the centre or the speed of two opposite walls getting away from each other.
I could be wrong of course, I'd be happy if someone points out what I might be missing.
Yep I think you have to imagine dots suspended in space inside the balloon to better get what's going on, and you're right, the "edge" of the universe is definitely nothing like the surface if the balloon. Probably?
No, you completely understand quantum physics, you are one of the elite.
But in the analogy I don’t think we know what the air in the balloon is, we call it expansion. But I don’t know enough to say anymore
We are the dots on the surface of the balloon. Things really far away seem to be moving away from us. Hopefully we can figure out what gravity is because that would have a lot of gravitas I dunno whatever
I’m fairly certain that in the balloon example metaphor, we are two dimensional creatures on the surface of the balloon
Space time itself is expanding which means I will now blow up soon. Yay!
Yeah, when you put the dimensions implication in it it starts making a bit more sense - implying that we're two-dimensional and the third dimension inside the balloon is the things that we don't fully comprehend (yet).
I don’t know how accurate the analogy is though I’m not a physicist
Hola, my other response was not meant to belittle you. I was just trying to explain how much I personally don’t understand about this topic and can only repeat what other scientists have told me. I do like listening to weird scientific mit lectures and stuff to fall asleep sometimes though. But im only an osmosis physicist myself. If I am average knowledge on this topic I would be happy but maybe I know slightly more, this is a dumb post. But I was reading through my past comments and I can see how my other response to this message may seem like I am belittling you which was not my intention at all
No offense taken. I'm not a scientist either, but I believe in the idea that anything can be taken as an explanation if it makes sense at the time, and later be taken apart, discarded and replaced with a better explanation when one comes along.
Great analogy, and yeah the "ish" is the fun part!
The fun of justifying a reference frame outside the universe. Sorry I'm getting a headache.
Understanding this is the easy part imo
You can explain but don't but criticize you are idoltarer and are in degenerate dharma like all Christians nowadays