That’s what einstein said. There is no fixed reference frame, but only relative ones. Every “inertial”(meaning, motion without any external force) frame of reference is equally valid as any other inertial frame movibg with respect to it.
But for sure we can tell earth’s orbit is not inertial since circular motion occur, which is due to external force of gravity.
Shouldn’t it be (at least theoretically) possible to find some sort of geometric center where - on average - the rest of the universe is expanding away from?
That could sort of explain why it’s inherently impossible to determine the center - but that doesn’t rule out the existence of a geometric center of the universe, right?
I’m not sure if I follow the balloon analogy. Sure, you can’t find the center on it’s surface. But somewhere within the balloon, there is a center. It might be virtually impossible to determine the center while actively inflating the balloon, but that doesn’t mean there isn’t any center? What makes the rest of the universe fundamentally different from an inflating balloon?
I’m genuinely curious.
OK, so what we discovered was that if we look far into the universe, all stuff is moving away from us (we can measure that using the famous red shift). Additionally, the farther something is, the faster it is moving away from us.
Now the simplest explanation would be that the Earth just happens to be exactly in the middle of this expansion. However, it is much more reasonable to assume our location in the universe is not special in any way and that you’d see things moving away from you if you did these measurements anywhere else. If that is so, the universe is expanding, everywhere. There’s more space in the universe every second and there’s still the same amout of matter, hence it is becoming larger, emptier and colder.
The next step is to look back and think of how the universe looked in the past. Since it’s getting bigger and bigger, it must have been smaller before and if you go back in time enough you’ll find a tiny universe that still has the same total amount of matter in it, just densely packed into possibly just a single point. Hard to say what preceded that moment, but we can predict a universe which started as an incredibly energetic singularity which exploded out and has been growing ever since. We call that moment the Big Bang and if anything, that is the centre from which everything is expanding. Not found somewhere in space but in time.
Back to the balloon analogy, the centre would be the deflated little thing you start with. Maybe a better analogy would be the little clump of molten glass at the end of a glassblower’s pipe. He begins to blow, the big bang happens, expansion starts. Fast forward 13.7 billion years (which happens to be today) - the glass has expanded into a large spherical object and there’s some tiny people living on it.
They only live on the surface of the glass and the sphere is so huge (or they are so tiny) that they can’t even tell that their world is spherical. They measure distances to some other specs littered across the glass and find that they are all moving away from them, faster the farther they are. Their universe is expanding, but where is the centre of expansion? They cannot point to it, because they only live in two dimensions, fully defined by the sufrace of the sphere. But if they could point in a direction perpendicular to all the spacial dimensions they know, they could point to the point where the sphere started, long in their past.
So the right question is not form “where” the universe expands but “whence”. The Big Bang. The very start. Somewhere far in time, which is just another direction, perpendicular to up, left, forward.
I very much appreciate the effort to write your repsonse, and if you’re out of time or energy I completely understand.
So if I’m understanding you right, we’re 3-dimensional creatures living in a 4-dimensional universe, with the 4th dimension being… time? And time behaves completely different from the other 3 dimensions, which is why we can’t just disregard it when trying to determine a center?
That’s what einstein said. There is no fixed reference frame, but only relative ones. Every “inertial”(meaning, motion without any external force) frame of reference is equally valid as any other inertial frame movibg with respect to it.
But for sure we can tell earth’s orbit is not inertial since circular motion occur, which is due to external force of gravity.
Edit:typo
Shouldn’t it be (at least theoretically) possible to find some sort of geometric center where - on average - the rest of the universe is expanding away from?
Turns out, no; every point is expanding away from every other point, so every point sees itself as the center of expansion.
That could sort of explain why it’s inherently impossible to determine the center - but that doesn’t rule out the existence of a geometric center of the universe, right?
No, for the same reason you can’t find a point a balloon is expanding from on its surface. Everything is expanding everywhere.
I’m not sure if I follow the balloon analogy. Sure, you can’t find the center on it’s surface. But somewhere within the balloon, there is a center. It might be virtually impossible to determine the center while actively inflating the balloon, but that doesn’t mean there isn’t any center? What makes the rest of the universe fundamentally different from an inflating balloon? I’m genuinely curious.
OK, so what we discovered was that if we look far into the universe, all stuff is moving away from us (we can measure that using the famous red shift). Additionally, the farther something is, the faster it is moving away from us.
Now the simplest explanation would be that the Earth just happens to be exactly in the middle of this expansion. However, it is much more reasonable to assume our location in the universe is not special in any way and that you’d see things moving away from you if you did these measurements anywhere else. If that is so, the universe is expanding, everywhere. There’s more space in the universe every second and there’s still the same amout of matter, hence it is becoming larger, emptier and colder.
The next step is to look back and think of how the universe looked in the past. Since it’s getting bigger and bigger, it must have been smaller before and if you go back in time enough you’ll find a tiny universe that still has the same total amount of matter in it, just densely packed into possibly just a single point. Hard to say what preceded that moment, but we can predict a universe which started as an incredibly energetic singularity which exploded out and has been growing ever since. We call that moment the Big Bang and if anything, that is the centre from which everything is expanding. Not found somewhere in space but in time.
Back to the balloon analogy, the centre would be the deflated little thing you start with. Maybe a better analogy would be the little clump of molten glass at the end of a glassblower’s pipe. He begins to blow, the big bang happens, expansion starts. Fast forward 13.7 billion years (which happens to be today) - the glass has expanded into a large spherical object and there’s some tiny people living on it.
They only live on the surface of the glass and the sphere is so huge (or they are so tiny) that they can’t even tell that their world is spherical. They measure distances to some other specs littered across the glass and find that they are all moving away from them, faster the farther they are. Their universe is expanding, but where is the centre of expansion? They cannot point to it, because they only live in two dimensions, fully defined by the sufrace of the sphere. But if they could point in a direction perpendicular to all the spacial dimensions they know, they could point to the point where the sphere started, long in their past.
So the right question is not form “where” the universe expands but “whence”. The Big Bang. The very start. Somewhere far in time, which is just another direction, perpendicular to up, left, forward.
I very much appreciate the effort to write your repsonse, and if you’re out of time or energy I completely understand.
So if I’m understanding you right, we’re 3-dimensional creatures living in a 4-dimensional universe, with the 4th dimension being… time? And time behaves completely different from the other 3 dimensions, which is why we can’t just disregard it when trying to determine a center?
It’s not, because the space itself is expanding