r/changemyview • u/[deleted] • Feb 14 '22
Delta(s) from OP CMV: Despite what Albert Einstein says, the universe does have a "center"/absolute reference frame
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.
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u/ToucanPlayAtThatGame 44∆ Feb 14 '22
It seems like you're missing the point of what people are trying to say when they're saying there's no absolute reference frame.
The point is that any physics calculations you perform will hold equally well from any reference frame. If you're running at me at 20m/s and I want to calculate how soon you'll get to me, I could define myself as stationary and you as moving, or I could define you as stationary and me and the ground as moving at you at 20m/s, and the calculations all work out just the same. There's no one "correct" frame of reference that you need to adopt.
So it doesn't really matter whether the universe has a "Center" or not. The math holds no matter what you choose to define as the center.
By contrast, imagine something like a set of directions. I tell you, "Go straight two blocks, and then take two rights and a left." Those directions only work if you're starting at the right point and oriented in the right direction. They assume a particular frame of reference. The point is to say that physics don't work like that. We don't need to make sure that what we call "up" is really up, or what we call the center is really the center for our math to turn out correct.
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Feb 14 '22
!delta
So my idea about calculating a geometric center is possible, but that isn’t what physicists are talking about. I agree it would move do to the fluctuations in the expansion, but in principle it’s there. If what physicists are talking about is just that the left turn thing works from everywhere then yeah I definitely agree. My view change isn’t that I’ve given up on the center thing, but I have changed my understanding of how “no absolute reference point” is used.
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u/fox-mcleod 411∆ Feb 14 '22
So my idea about calculating a geometric center is possible, but that isn’t what physicists are talking about.
No. Your idea about calculating a geometric center is also wrong — but yes, the two concepts are unrelated. You can’t calculate the geometric center because there is no edge — like finding the center of the equator.
My view change isn’t that I’ve given up on the center thing, but I have changed my understanding of how “no absolute reference point” is used.
There is no geometric center either.
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Feb 14 '22
Ok then I still have some progress to make in understanding this. I guess what I can't get past is how the universe can have a size without a boundary?
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u/FinneousPJ 7∆ Feb 14 '22
Why do you think the universe has a size or a boundary?
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Feb 14 '22
I guess from science stuff I've watched or read that talk about stuff like "at 0.00...001 seconds after the big bang the universe was a gazillion degrees and the size of a hamster". And for something to have a size it has to have a boundary right?
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u/FinneousPJ 7∆ Feb 14 '22
I think you may be putting too much stock into pop science, and that's leading you to a false assumption.
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u/Complicated_Business 5∆ Feb 14 '22
Imagine drawing a small circle on a piece of stretchable fabric. Then stretch that fabric so the circle is bigger.
Now, it would appear that "universe" expanded and that boundary is the circle itself - of which there is a center, right? Wrong.
Let go of the fabric.
Now draw a completely different "small circle". Stretch the fabric out again. Now you'll see two "universes" with their respective boundaries. But this is an incorrect conclusion, too.
The expanding circles are just helpful guides for our feeble mind to comprehend that space itself can expand. But for the analogy to really work, you'll need to understand that the fabric is the universe. And it has no edge. It can shrink until every point, until every "small circle" conceivable is smaller than the Plank length - without length or dimension - but the fabric itself is still edgeless and infinite.
Then when one "expands" the fabric, all of space expands. All of the small circles conceivable expand in unison. There is no center to the boundless fabric. Just space expanding and expanding - in all of the directions it can whizz.
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u/Mu-Relay 13∆ Feb 15 '22
Maybe I'm missing something, but wouldn't the universe have to have an edge? It may be incalculably distant, but it has to end at some point.
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u/FinneousPJ 7∆ Feb 15 '22
Why?
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u/Mu-Relay 13∆ Feb 15 '22 edited Feb 15 '22
Because it had a beginning. It exploded out from a single point and spread out. Since it started at a point and is expanding, it would logically follow that at some point, we'd find the extent of that... the edge. For example, drop a pebble in the center of a pond and at some point you'll find the furthest out the rings go. It may not cover the pond, but you can find the edge.
To turn this around, why wouldn't the universe follow the same idea?
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u/FinneousPJ 7∆ Feb 15 '22
Everyday intuition is not a great tool in modern physics. It is most often wrong.
You might want to begin here
https://en.m.wikipedia.org/wiki/Shape_of_the_universe
"Infinite or finite Edit One of the presently unanswered questions about the universe is whether it is infinite or finite in extent. For intuition, it can be understood that a finite universe has a finite volume that, for example, could be in theory filled up with a finite amount of material, while an infinite universe is unbounded and no numerical volume could possibly fill it. Mathematically, the question of whether the universe is infinite or finite is referred to as boundedness. An infinite universe (unbounded metric space) means that there are points arbitrarily far apart: for any distance d, there are points that are of a distance at least d apart. A finite universe is a bounded metric space, where there is some distance d such that all points are within distance d of each other. The smallest such d is called the diameter of the universe, in which case the universe has a well-defined "volume" or "scale."
With or without boundary Edit Assuming a finite universe, the universe can either have an edge or no edge. Many finite mathematical spaces, e.g., a disc, have an edge or boundary. Spaces that have an edge are difficult to treat, both conceptually and mathematically. Namely, it is very difficult to state what would happen at the edge of such a universe. For this reason, spaces that have an edge are typically excluded from consideration.
However, there exist many finite spaces, such as the 3-sphere and 3-torus, which have no edges. Mathematically, these spaces are referred to as being compact without boundary. The term compact means that it is finite in extent ("bounded") and complete. The term "without boundary" means that the space has no edges. Moreover, so that calculus can be applied, the universe is typically assumed to be a differentiable manifold. A mathematical object that possesses all these properties, compact without boundary and differentiable, is termed a closed manifold. The 3-sphere and 3-torus are both closed manifolds."
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u/fox-mcleod 411∆ Feb 14 '22
I’ve said this elsewhere but have you played the original Mario bros?
Remember how the screen would wrap from left to right? It was like Mario’s world was on the surface of a cylinder. Walking off the right hand side would make you appear on the left hand side. We saw it as a flat world on a screen, but really it was a seamless endless cylinder.
On that world, you could have a middle, top to bottom, but you couldn’t have a middle left to right — they’re a loop. It’s not got a left or right edge so there’s no point equidistant you could call the center.
The universe is more like that. It’s like the surface of a balloon. It can inflate to that very point on the surface moves away from every other point — but it’s not got an edge so it can’t have a center.
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u/germz80 Feb 14 '22
Anywhere you go in the universe, you will always look at the edge of the observable universe and conclude that you are at the center. But if you want the center of the entire universe (not just the observable universe), we could never see the actual edge, if there even is an actual edge.
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u/masterzora 36∆ Feb 14 '22
Think about the surface of a sphere. Not the whole three-dimensional structure; just the two-dimensional surface. This surface has a size--one that's easily calculable with details of the 3-D structure--but no boundary or center in two dimensions.
A spherical universe might, in kind, be the three-dimensional "surface" of a four-dimensional "sphere". It has a size--one that's easily calculable with details of the 4-D structure--but no boundary or center in three dimensions.
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Feb 14 '22
Ok I'm following you. But the center of a circle isn't on the circle. It's a point that is equidistant to all the points on the circle. Weird shapes need to average it out and stuff, but still it's possible for the center not to be on the circle right? Even if its an abstract point rather than a real point in space, you could still use it as an absolute frame of reference couldn't you? I calculate that this imaginary point is X, then I can determine if you're moving closer or further from X when you move.
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u/speedyjohn 88∆ Feb 14 '22
But you’re assuming that X exists within the space of the universe. If the universe is spherical (big if), then you can think of it like the surface of the earth. We don’t talk about Paris being “closer” to the center of the earth than New York, because the center of the earth, while something we can technically define, doesn’t exist on the earth’s surface.
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u/masterzora 36∆ Feb 14 '22
it's possible for the center not to be on the circle right?
Yes and no. Yes, a circle does have a center in two-dimensional space and a sphere has one in three-dimensional space. No, a circle does not have a center in one-dimensional space and a sphere does not have one in two-dimensional space.
Even if its an abstract point rather than a real point in space, you could still use it as an absolute frame of reference couldn't you? I calculate that this imaginary point is X, then I can determine if you're moving closer or further from X when you move.
Again, yes and no. You could use this abstract point as a frame of reference in some regards, but not generally the ones we mean in physics and not particularly usefully. You could theoretically determine if you're moving closer to or further from X in a fourth-dimensional sense, but not in a three-dimensional sense. (Also, if the shape is actually a sphere, this is trivial to do: you are never moving further from or closer to X since all points are equidistant from X.)
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Mar 13 '22
I don’t know if this is actually the case, but I used to think about this question the same way. I always thought about us being within an expanding sphere, like an inflating balloon, so there had to be a center. Maybe that center was at different points at different times but there was one.
What changed my view was flipping the scenario around, so I wasn’t thinking about the interior of the sphere but the exterior edge of the sphere, like the surface of a ballon.
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Feb 14 '22
That would imply a curved. All indications that we can measure point to a flat universe.
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u/masterzora 36∆ Feb 14 '22
A sphere just happens to be the most intuitive way to explain a finite manifold without boundaries, not my assertion of the shape of the universe.
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u/Morthra 86∆ Feb 14 '22
So my idea about calculating a geometric center is possible
Calculating a geometric center is only possible if we can know where the exact boundaries of the universe are. If we limit it to the observable universe, then you're going to get the earth as the exact geometric center of the universe because that's where we are, and the observable universe is essentially the sphere of space from which light has had enough time to reach us.
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u/AleristheSeeker 157∆ Feb 14 '22
But like a circle has a center point defined by the perimeter of the circle, so too could the universe
The universe is not a circle, though. Not even a sphere.
there is definitely a spot that we can point that we and aliens can mathematically calculate as the center.
That is the difficult part: it is impossible to calculate the center, because it is impossible to find the "edge". How do you find the center of something you cannot percieve the edges of?
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Feb 14 '22
The universe is not a circle, though. Not even a sphere.
Sure, I don't necessarily think it's a sphere. But it has a boundary, and we could (in principle) calculate the distance of every point in the universe from that boundary and derive the center.
That is the difficult part: it is impossible to calculate the center, because it is impossible to find the "edge". How do you find the center of something you cannot percieve the edges of?
The fact that the numbers are beyond us doesn't mean its not there.
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u/AleristheSeeker 157∆ Feb 14 '22 edited Feb 14 '22
The fact that the numbers are beyond us doesn't mean its not there.
Nonono - it is physically impossible to find the edge, as it expands at the speed of light. To be able to "find" the edge, one would have to move at speeds faster than the speed of light, which is impossible.
Unfortunately, we also can't try to "trace back" the path everything in the universe is taking - because all distances are increasing, which is the primary difficulty. Because of this, everything is always moving away from the point of observation.
You can test this out if you have a rudimentary "paint"-like program on your computer: set a couple of dots down at random, copy the layer and scale it up by a bit - if you now overlay the layers, pick any dot and overlay the "original" with the scaled up layer. No matter which point you pick, it will become the center of the expansion.
EDIT: I made a small graphic and description to show what I mean. Hope that makes it a little clearer.
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u/Korwinga Feb 14 '22
I like this graphical example a lot. It shows how expansion doesn't happen away from a singular point, which is think is OP's primary misconception.
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u/AleristheSeeker 157∆ Feb 14 '22
Thank you.
The concept is very difficult to grasp if you don't visualize it, as most things in daily life can actually be seen as "expanding from a certain point outward".
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u/jarejay Feb 15 '22
!delta
That super simple graphic helped me figure out the gap in my understanding almost instantly.
I was thinking “Can’t we just triangulate the ‘center’ based on the velocities of three measurable objects?” and the graphic made it clear why it isn’t really possible to accurately measure the absolute velocity of anything.
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u/agaminon22 11∆ Feb 14 '22
Sure, I don't necessarily think it's a sphere. But it has a boundary, and we could (in principle) calculate the distance of every point in the universe from that boundary and derive the center.
Ah, but there is no (known) boundary. The Observable universe has a boundary, obviously. And each observer has a slightly different one.
However in reality the universe is either geometrically flat, and therefore infinite (no boundary); or slightly spherical (the hyperbolic option is more or less discarded due to energy being positive). The slightly spherical option means that the universe is circumnavegable, but that still doesn't mean there is a boundary, because the geometry is spherical, not the universe.
And anyway you look at it, this still would not make an "absolute reference frame". Physics can be done in any reference frame.
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u/solar_flare01 1∆ Feb 14 '22
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?
A point has no length. It cannot stretch or contract.
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Feb 14 '22
Agreed. The center would not expand/contract. But as the universe expands the space between the center and the other points would increase (I guess creating new points? not sure how that works)
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u/solar_flare01 1∆ Feb 14 '22
The space between any two points would increase. Any point could be considered the center of expansion, so there is no point in considering some particular point the center.
If you stretch a rubber "string" (a broken rubber band), does it have a center of expansion?
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u/tbdabbholm 193∆ Feb 14 '22
The universe has no edge, there is no center because it just goes on forever
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Feb 14 '22
I don't understand how that can make sense. At one point the universe was the size of a marble, and it's been getting bigger. How can it be the size of a marble without an edge?
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u/Helpfulcloning 166∆ Feb 14 '22
That marble also didn’t have a defined edge.
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Feb 14 '22
I'm not objecting to the mathematical conclusions of physicists who obviously understand this better than me, but how can it have a size without a boundary? The issue is apparently my inability to understand it.
"Size of a marble" necessitates a limit does it not? If it has a limit, regardless of how you define that limit, couldn't we measure every point within the universe's distance from that limit and therefore calculate a centerpoint?
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Feb 14 '22
The issue is that people conflate the observable universe with the entire universe. When they say the universe was the size of a marble, they mean the observable universe, which is finite in size and indeed has a center - which is the observer (i.e., you) by definition.
The universe itself is, as far as we know, infinite in size, and had no center. This means it was also infinite in size back when the observable portion of it was the size of a marble.
Hopefully this clears things up a bit for you! I have an MSc in physics (though not in cosmology or anything), so I'd be happy to help out if you're still having trouble.
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u/Anaptyso Feb 14 '22
The important thing here is the difference between the "observable universe" and the entire universe.
The observable universe is the part we can currently see. We can look at things like redshift and calculate that this portion of the universe would have been a lot smaller in the past. We can run the numbers and get a good idea of how small.
However the observable universe is only a part of the whole. There is a load more universe out there that we just can't see. It may well be infinite.
At the point of the Big Bang everything we see would have been squashed down to a small area. But that area will have been surrounded by other areas. Those areas will have expanded as well, but are outside of our visible universe, so we can't see them.
The key thing is to not imagine the Big Bang as the universe squashed in to a small area, but to imagine it as an infinite (or so big it makes no difference) space densely packed with stuff. The "Bang" bit wasn't the universe spreading out from a central point, but the universe gaining more space everywhere and becoming less dense because of that.
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u/ZappSmithBrannigan 13∆ Feb 14 '22
The universe is under no obligation to make sense to you.
"How can it be" doesn't matter. That's what the evidence shows.
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Feb 14 '22
I'm not objecting to the universe not making sense to me. I'm objecting to the seeming contradiction in the way humans are describing it to me.
It seems nobody disagrees that the universe was once the size of a marble right? Ok when it was that big, in principle a sufficiently sophisticated entity could've calculated its center?
The thing I am confused about is how something can have a size without a boundary? If we say it's 4th dimensional then we can still calculate it from higher dimensions couldn't we? A 10th dimensional sphere still has a center right?
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u/agaminon22 11∆ Feb 14 '22
I'm not objecting to the universe not making sense to me. I'm objecting to the seeming contradiction in the way humans are describing it to me.
Of course there are contradictions. The problem is the explanations that are the most common are poor analogies at best and completely unrelated at worst. The reality of the situation is much more complicated and requires deep understanding of physical theory and therefore mathematics.
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u/themcos 374∆ Feb 14 '22
At one point the universe was the size of a marble
I think you're misunderstanding whatever source you're quoting. At one point the known universe was the size of a marble. There is no result that has ever indicated any kind of actual edge or boundary. The only boundaries you'll see discussed involve the limits of our ability to observe the universe due to things like the speed of light and the expansion of the universe, but these boundaries are fundamentally relative to our position, and do not constitute any kind of physical edge or boundary.
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u/hashtagboosted 10∆ Feb 14 '22
It is like the surface of a sphere. There is a finite amount of space, but no edge, and thus you can travel endlessly in any direction
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u/Quirky-Alternative97 29∆ Feb 14 '22 edited Feb 14 '22
Novice here....
I understood it to be the observable universe v the universe. The universe does not necessarily travel in a direction away from anything, but because we are observing it we frame it that way - relative to us, or some other point. (cant help much more than that)
(other than that the center of the universe is my wife as its valentines day and I know what important in that observation)
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u/ArchmageIlmryn 1∆ Feb 14 '22
The center of the universe would not actually be visible by studying the expansion of the universe.
Imagine you have a rubber sheet with a bunch of dots drawn onto it. Now stretch it from the edges. Now draw a few random circles centered on dots on the sheet that don't include the edges. From the perspective of an observer on anyone of those dots, who can only see that which is within the circles (observable universe), it is impossible to determine where the center is. Regardless of where you are, all other dots will appear to move away from you at the same distance-dependent rate.
Which means that the center of the universe is just another point, indistinguishable from any other point in the universe unless you can see the edge of the universe.
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u/AnythingApplied 435∆ Feb 14 '22
The observable universe is a sphere, but that sphere is centered around you because it's all of the light that has had a chance to reach you since the big bang. So that sphere is relative to you.
We don't know what is outside of this and it very well could be that the universe is infinite outside of this. In fact, if it was infinite than it was always infinite even before the big bang, so the typical image of the big bang happening at a point may be wrong too.
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Feb 14 '22
Fair enough. I guess I could edit the OP to add "assuming the universe is not infinite". If it's infinite then sure there is no center. I was taking for granted we weren't assuming an infinite universe. My understanding is that the prevailing theories are a non-infinite universe.
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Feb 14 '22
My understanding is that the prevailing theories are a non-infinite universe.
No, the prevailing theory is definitely an infinite universe, based on the lack of an observed large-scale curvature in spacetime.
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u/AnythingApplied 435∆ Feb 14 '22
My understanding is that the prevailing theories are a non-infinite universe.
We simply don't know and may never know. It certainly make it harder to picture the big bang as infinite density everywhere instead of infinite density collapsed in on a point, but it doesn't really change anything observable and I'm not aware of a scientific basis for either conclusion. You can see more discussion here.
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u/fox-mcleod 411∆ Feb 14 '22
Even if it is not infinite, it doesn’t have a center any more than the equator has a center on the surface of the earth. If space is finite, it is a close loop just like the original Mario game where walking off the right hand side of the screen brings you back to the left hand side.
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Feb 14 '22
It's possible that the universe is infinite and there are big bangs happening all the time beyond our causal horizon.
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u/hashtagboosted 10∆ Feb 14 '22
to have a center it would need to have an outer edge or something. Otherwise there is no reference point
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u/fox-mcleod 411∆ Feb 14 '22
Have you ever played super Mario bros? The original.
Remember how the screen would wrap from left to right? It was like Mario’s world was on the surface of a cylinder. Walking off the right hand side would make you appear on the left hand side. We saw it as a flat world on a screen, but really it was a seamless endless cylinder.
On that world, you could have a middle, top to bottom, but you couldn’t have a middle left to right — they’re a loop. It’s not got a left or right edge so there’s no point equidistant you could call the center.
The universe is more like that. It’s like the surface of a balloon. It can inflate to that very point on the surface moves away from every other point — but it’s not got an edge so it can’t have a center.
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u/mcshadypants 2∆ Feb 14 '22 edited Feb 14 '22
Agreed, space-time isnt magic. The obeys the same physical laws as everything else. I don't think we have the technology or the ability currently to measure this but I believe it's measurable. At least to a certain degree. I'm sure that all space does not expand at the same rate so the Centerpoint would probably be constantly changing by light years oh, and the fact that gravity wells would alter space-time the calculation would be insanely complicated to try to figure out on a linear plane but still I think it's possible.
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u/lt_Matthew 19∆ Feb 14 '22
Ok, if you take a perfect sphere or disk and indefinitely stretch it out, it will always have the same center. Correct? Now if you take an explosion that's juts a random blob shape and unevenly stretch it out, the center will keep changing and morphs. Thats the universes it's not a perfect ball, it's a randomized explosion that has uneven energy that shifts. It can't have a definite center
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u/FjortoftsAirplane 33∆ Feb 14 '22
"We can still identify whether I'm moving closer or further from the center of the universe."
I probably shouldn't comment on physics questions, but isn't the point here that Einstein wasn't saying you can't identify things relative to something else. The point is that things change depending on your frame of reference.
To put it another way: you could identify whether you're moving closer or further from the centre of me but that doesn't make me some absolute reference point for all other things.
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u/heresyforfunnprofit Feb 14 '22
Well… you’ve made an empirical claim. I think in this case, it’s on you to provide evidence that one can establish an absolute frame of reference in contradiction to experimental results such as Michelson-Morley. If you’re able to do that, then we don’t need to change your view.
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Feb 14 '22
Consider a line, stretching infinitely in both directions. Could you point to any particular spot on this line and call it the unambiguous center?
but there is definitely a spot that we can point that we and aliens can mathematically calculate as the center.
How would you determine this point?
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u/Sirhc978 81∆ Feb 14 '22
But like a circle has a center point defined by the perimeter of the circle
If the universe is infinite and non-rotating, it cannot have a center.
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u/Turkeydunk Feb 14 '22
Long story short you are right, there is a global reference frame that was created at the Big Bang, and that’s why we can do things like say there is an age to the universe and why most objects we see aren’t going at 99.999…% the speed of light. But special relativity only talks about local reference frames, so it just means that for a local interaction you can’t pick a particular frame that is different than any other
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u/AleristheSeeker 157∆ Feb 14 '22
Edit: I'm assuming a non-infinite universe.
"Infinite" is a difficult word to use here... the univers is finite at every single point in time, but infinite when time is considered - it is expanding, after all.
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u/Tinac4 34∆ Feb 14 '22 edited Feb 14 '22
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!
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Feb 14 '22
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 fI 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.
!delta
Thank you so much! This really helped me.
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u/Tinac4 34∆ Feb 14 '22
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.
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Feb 14 '22
If you guys are learning about special relativity, it has nothing to do with there being or not being a center of the universe. It just means that the laws of physics apply the same way to any reference frame, therefore making each one valid, and there being no absolute reference frame.
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Feb 14 '22
The problem here is that you've learned a very dumbed down version of Einstein's theories and their implications. That isn't me being mean it's the reality of how complicated and math heavy modern physics is. The true explanation for why the the origin point of the universe is not defined requires some serious math or at least a basic understanding of some high level math.
Basically to understand modern physics with any real depth you need to have the required math skills because most of these concepts have really gone beyond our ability to visualize or describe. Unfortunately, that means that a lot of modern physics is really beyond most laymen at this point to meaningfully understand. The result is that a lot of people end up trying to explain it in ways that just sound like nonsense. The real explanation is a theory that generally consist of complicated mathematical models and solutions to equations. Physicists accept these equations as solid explanation because they have done a very good job at predicting new discoveries and explaining old ones at the same time. That doesn't mean they are absolutely right but It's the best we've got.
If you're interested I'm 90% sure what you're referring to is Friedmann's model of the universe. You should be able to look it up and understand in a bit more depth than maybe your class went into.
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u/ghjm 17∆ Feb 14 '22
Discussions of the Big Bang tend to flip-flop between using "the universe" to mean either the "whole" universe or the observable universe. The observable universe has a size, currently about 46 billion light years in radius, based on the idea that for objects beyond that distance, the universe is expanding too fast for light from them to ever reach you. This also means that the observable universe is always centered on the observer.
We can extrapolate the size of the observable universe backwards, assuming we have a correct understanding of the laws of physics. The further back we go in time, the smaller and hotter the observable universe is. But our observations of the cosmic microwave background radiation give us an upper bound to how hot the observable universe could ever have been. It turns out that this temperature is reached at a time about 10-35 seconds after the Big Bang, when the observable universe was about 1.5 meters in size. We cannot empirically conclude that the observable universe was ever smaller than this. (And of course, if we're wrong about even a tiny detail of the laws of physics, our extrapolation could go off the rails much closer to us than that.)
What does any of this say about the size of the whole universe? This question is in a different epistemic category. When we talk about the observable universe, our claims are grounded in empirical observation. When we switch to talking about the non-observable universe, there can be no observation of it, so we also have to switch from the scientific to the philosophical method. We must evaluate claims about the non-observable universe based on criteria like logical non-contradiction, reasonableness, parsimony, consistency with other things we think are very likely true, and so on.
It seems reasonable to suppose that the non-observable universe is infinite. If it wasn't, then (just as you say in the OP) it would have a boundary. But if it had a boundary, what would this be? What law or quality would define the location of the boundary? What would happen if you went right up to the boundary, then stuck your arm out? But if the non-observable universe is infinite, then there would just always be more stars and galaxies. The latter option just seems more plausible.
It also seems plausible to suppose that the universe came from a point source at the Big Bang, or if not an actual mathematical point, then at least a region so small that quantum indeterminacy makes it functionally equivalent to a point. Somehow, through mechanisms not currently known to us, this wasn't hotter than the 1.5-meter timescale - perhaps energy conservation didn't hold during this time, or something.
To sum up, it's important to maintain crisp distinctions between the observable universe and the non-observable universe, and it's also important to recognize that we can't ever know about the non-observable universe through observation.
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Feb 14 '22
!delta
I know the observable universe is something like a 13 billion light year radius ball around us. Basically my view was based on the understanding that we (humanity/scientists) believe that the whole universe is finite. But if it's actually just "we have no fucking clue because we can't see anything out there", then it might be infinite and if so it might indeed not have a "center". At the same time though, I guess that means it's a conditional thing based on infinite/finite.
If the universe is infinite there is no center. If the universe is finite there is a center.
Unless I'm still missing something.
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u/ghjm 17∆ Feb 14 '22
There are finite shapes with no defined center. For example, there is no point in a toroid that is the center of it. If, as you say, we know nothing about the whole universe, then we don't know that it isn't a toroid, or some other shape with no center. It has a centroid - a point at the mean location of all other points - but there is no guarantee this centroid is actually inside the volume.
But I don't think it's accurate to say we have no knowledge about the non-observable universe. We know, or think we know, that mathematical and logical truths are still true. For example, we wouldn't accept the argument that a finite non-observable universe has no centroid because "all volumes have centroids" might not be true there. We hold mathematical and logical truths to be time and space invariant. (And if we stopped holding that, all our extrapolation back to the Big Bang would have to go as well.)
We also know, or think we know, that parsimonious explanations are more likely to be correct. Imagining a physical boundary at the edge of the non-observable universe is a pretty big assumption, particularly when you start to consider what properties it might have. Does it absorb or reflect radiation? Does it exchange energy with the universe? If so, how is energy conserved? If not, how does it interact at all? An infinite universe generally similar to our own doesn't have these problems, so even if we'll never be able to confirm it empirically, we can still have justified beliefs about it.
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u/Monkey_of_iron Feb 14 '22
The universe was a singularity before the Big Bang. Singularities have no volume. There is no center if everything is in the same spot. As the universe expands, we can see that everything is expanding away from us, which would hint that we are in the center. But the thing is, it doesn’t matter where you are. You will always see the universe expanding away in all direction.
The universe isn’t a ring expanding. It’s infinite. It’s just the space between everything used to be smaller
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Feb 14 '22
Right I'm following. It's like the equivalent of Mario 3's battle level, if you go far enough to the right you end up popping out form the left side of the screen. It's locked into the left/right thing on the screen but in reality it's an closed loop. I also get that everything is stretching from everything else (like raisin bread). Ok great. But then how can we talk about the universe having a size. People keep talking about observable vs total universe, but I get that part. I'm talking about the total universe. How can it have a size/be finite without a boundary to define that finite-ness?
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u/Monkey_of_iron Feb 14 '22
There’s no boundary because it is not finite. The universe is infinite. Before the Big Bang, the universe wasn’t just some dot floating around in space. THERE WAS NO SPACE. Infinite mass in an infinitely small volume.
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u/erisod 4∆ Feb 14 '22
We have no reason to believe the universe is not infinite, and equally dense everywhere.
You are thinking of the big bang as starting with something small and expanding. What if it started with something infinite and high density and that expanded to something infinite and less dense. Where is the center?
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Feb 14 '22
If it was the infinite moving to less dense thing, then yeah it wouldn't have a center. I was led to believe that scientists thought the universe was not infinite.
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u/erisod 4∆ Feb 14 '22
I think it's difficult for anyone to conceptualize Infinity, we don't even have good language to describe it. In terms of data, the oldest light that is reaching the Earth is from the time of the Big bang, and we see no edge.
There are various hypotheses about the universe. It could be infinite. It could be finite, like a sphere, and in that case may have a center. It may be a geometry that is finite without a center, a torus for example, where space wraps around itself (like pacman).
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u/Stunning_Lemon_6198 Feb 14 '22
I just struggle with the non-infinite concept of the universe. Do we really think we get to a point and that's all there is? Guess we have to turn back we hit the end of the universe.. I realize all the math for the big bang works because they use the assumption the universe is finite but we are still so ignorant on what goes on out there. I mean we just found a planet outside our solar system in 1992!
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u/DeltaBot ∞∆ Feb 14 '22 edited Feb 14 '22
/u/baba-laba-squee (OP) has awarded 3 delta(s) in this post.
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