Comment by Hendrikto | Hacker News Reader

Comment by Hendrikto | Hacker News Reader

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Your interpretation of the expansion is incorrect. Basically, it looks like all points are moving away from each other over large distances.

It is easier to understand in 2D: Imagine a deflated balloon with two dots drawn on it. When you inflate the balloon, its 2D surface increases, which results in the two points "moving away" from each other, without either of them actually moving. It‘s just that the surface on which they lie grows. This is also the reason why distant galaxies are getting more distant faster than the speed of light: They are not actually moving, but the space in between us and them grows.

Notice that the farther two points on the surface of the balloon are apart, the faster they will appear to be moving away from each other, which is why the effect is unnoticeable over small (in astronomical terms) distances. Gravity pulls things together much faster than space expansion pulls them apart.

Bringing this back to your example: Imagine "blowing up" the box. Things inside it are growing a little bit (but gravity pulls them back together), but everything seems to be moving away from everything else.

What's still not clear is how I know the other point on the surface of the ballon is moving away from me? I measure it as 9 tick marks away. After inflation it's still nine tick marks.

That is to say, what is my ruler that is untethered to the surface of the balloon?

Your ruler is absolute distances, like the speed of light. You can inflate or deflate the balloon and the number of marks on its surface won't change. But the time required for the light to travel between them will.

Stupid question - How do you know for certain it isn’t just that light is slowing down?

Relativity probably doesn’t work this way at all, but if light did actually slow down, would it be perceived by us as light moving at the same speed across a greater distance?

I asked Ann Nelson (look her up) this when I audited her cosmology class in grad school, thinking I'd come up with a great new idea. She said yes, it's equivalent to the speed of light changing, and then moved on.

Bear in mind that time is not universal. For the speed of light to change over time, it would change at different rates for observers in different reference frames, and this difference would be measurable.

As to the previous question about relative size. If the universe doubled in size, the objects within it such as atoms don’t also double in size. Gravity and the atomic forces don’t change, so eg the size of stable electron orbits or planetary orbits don’t double in size.

https://en.m.wikipedia.org/wiki/Variable_speed_of_light

Not exactly. Remember, we're not timing the light's transit. We're observing a redshift of it's frequency which could not be caused by c simply decreasing over time.

I think physical forces (electromagnetism, gravity, strong/weak nuclear) probably become weaker even though your metric (9 ticks) stays the same. I'm just guessing here, that's the only consequence I can think of.

It's incorrect to think with this analogy. There is no ruler reaching across the universe, growing with it.

It is not the meter that is growing, that's impossible. Instead you must see it as it is, which is that there are more meters continuously appearing in all of space.

> looks like all points are moving away from each other ...

Do you mean all particles are moving away from each other? I think the question here was what would it mean for "points" to move? What are points anyway?

The analogy was dots on a balloon, and the next sentence used "points" instead of "dots" but meant the same analogy.

A point (in space) is something that could be occupied by an object.

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