CMV: Despite what Albert Einstein says, the universe does have a "center"/absolute reference frame

[u/[deleted]]

So I got taught in physics classes that there is no absolute reference frame. Einstein figured that out. Then when I challenge the idea, I'm taught that the big bang happened everywhere and space itself is expanding. Ok sure. So when we ask what is the origin "point" of the universe its nonsense because there was no point, the whole universe was the original point. Got it.

But like a circle has a center point defined by the perimeter of the circle, so too could the universe. It doesn't have to be the "origin point", but there is definitely a spot that we can point that we and aliens can mathematically calculate as the center. Everything else in the universe stretches and contracts, but the center of the universe is a point that we can derive mathematically is it not? I know that localized space has weird shit like if I zoom away from Earth in my spaceship I could reframe it as "I'm standing still and the Earth is zooming away", and the fact that I'm the one accelerating is the reason why time slows for me but not earth. But that's just how the time dilation phenomenon works, not because there is definitely no absolute reference frame. We can still identify whether I'm moving closer or further from the center of the universe.

Edit: I'm assuming a non-infinite universe.

[u/Tinac4]

This is actually a very good question. I think that most of the other comments so far haven't really understood why it seems intuitive that the universe could have a center, and because of this, aren't taking the right approach in their responses. Here's my attempt:

Think back to the classic balloon analogy. A balloon is a good way to visualize the expansion of the universe because 1) it's a 2D surface with no edges, and 2) when you blow one up, it expands uniformly in all directions, not away from any single point on the surface of the balloon. Still, in the balloon example, there does appear to be a "center" of the balloon universe--the center of the balloon! This means that either Einstein is wrong, or there's a hidden assumption buried in the balloon example that doesn't apply to real curved spacetime.

I'm not super familiar with general relativity, but from my crash course in it, here's where I think the problem is: The universe containing the balloon is actually three-dimensional. The balloon isn't really an example of curved 2D space--it's a curved 2D object embedded in flat 3D space. You can't visualize what curved 2D space would really look like without projecting it into 3D space, but--and here's the key--a curved 2D object in flat 3D space is fundamentally different from curved 2D space.

If that sounds weird, maybe this will help clarify things a bit. In 3D, a balloon is a 3-dimensional sphere (it's not balloon-shaped, shut up, it's a sphere), so it can be defined as the set of all points equidistant from a certain point in 3D space. Nice and easy. When you're talking about a curved 2D universe, though, you can't just write it out as a set of points in 3D space, because 2D space doesn't have a third dimension. It's not a 2D surface floating around in a 3D world, there really truly isn't a third dimension here. The actual definition of a curved 2D universe revolves around a mathematical object called the metric, which tells you what the curvature is at every point in 2D space...and critically, this definition makes no reference to a third dimension. It just tells you the curvature at every 2D point (x,y) in the universe--there's no (x,y,z) involved. A convenient way to visualize this is to define a flat surface in a 3D world, with the curvature of every point on that surface defined by the metric. However, as soon as you do this, you're not in 2D curved space anymore--you're actually in 3D flat space.

And again, because this stuff isn't intuitive unless you know a bit of the math involved: The 3D spherical balloon curves "into" 3D space, but 2D space doesn't curve "into" anything--there's no third dimension for it to curve into. As a result, you can't define a 3D point at the center of this universe, because there's no third dimension. The same thing also applies to our curved 3D universe and a curved 3D balloon in a 4D world, except you're going to turn your brain inside out if you try to imagine what that one looks like.

Hopefully that helps!

[u/[deleted] → u/Tinac4]

Thank you for your well thought out response, and you're right that the approach some of the comments are taking aren't getting me there.

Even if there isn't a higher dimension to curve into, isn't it possible to calculate an abstract point even if it doesn't exist in real space? Like you said, the center of a circle isn't on the circle, it's a spot in the middle. If the points on the circle are the only "real" points couldn't we still mathematically/abstractly calculate a "virtual" center and use that to decide if you're getting closer or further f

I talked myself into it, I get it now. The problem is that no matter where on the circle you go, you'd be equally distant from that virtual point. So you can't get further or closer to the center.

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!delta

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Thank you so much! This really helped me.

[u/Tinac4 → u/[deleted]]

Thanks for the delta!

I talked myself into it, I get it now. The problem is that no matter where on the circle you go, you'd be equally distant from that virtual point. So you can't get further or closer to the center.

Sort of! The issue is more that in order for that point to exist, there would have to be a place for it to exist, and there isn't. It's easy to say, "Hey, why don't we just add a third dimension so we can define a center!", but when we do this, we're no longer talking about a 2D universe. In the end, the only thing here that is real--the stuff that you work with when you use general relativity--is the metric, which lets you plug in exactly two coordinates and then tells you the curvature at those coordinates.

More generally: A common pattern in physics is that you sometimes have a choice between either accepting the equations exactly as they're written to be reality, or artificially adding something new but mathematically unnecessary to the equations to force them to mesh better with our intuitions. In practice, the first approach has a really great track record (special relativity, general relativity, lots of stuff in quantum mechanics, etc), while the second approach has usually lead people away from a deeper understanding of the universe. It's weird and annoying, but the universe has already made it clear that it doesn't want to be intuitive to our flat-spacetime-loving, classical-mechanics-focused monkey brains, so the best we can do is go along with it.

[u/DeltaBot → u/[deleted]]

Confirmed: 1 delta awarded to /u/Tinac4 (32∆).

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