Think of the galaxies as raisins in cookie dough, the galaxies aren't really raisins of course, but if it helps you then think of them that way.
"Logic, my dear Zoe, merely enables one to be wrong with authority." -- The 2nd Doctor.
Math is points, lines, vectors, planes, numbers, ... and nature does not have all of this stuff.
Physics tries to explain reality(?) with stuff without dimensions (points), without thickness (lines, planes), with perfect smoothness (lines, planes, bodies)... how can we expect not to run into contradictions when we take a close enough look?
And: No! I have no better answers than others... I only have questions...
Math is points, lines, vectors, planes, numbers, ... and nature does not have all of this stuff.
And yet math works extremely well to describe nature. A lot of people are trying to figure out how far measured values are from calculated values. For some of these phenomena we've long passed ten digits after the comma and it still matches. It's one of the big mysteries of the universe: Why does math work so well?
All this thought about an expanding universe gives me headache.
We here statements like: It's not that galaxies are moving as such, rather it is space itself that is getting bigger. Hence the raisins in cookie dough analogy. Or dots on a balloon being inflated.
I my younger days I reasoned that if it was space itself getting bigger, how would we ever know? Surely the space between all my electrons and protons and whatever particles would also be getting bigger as well? Surely my meter sticks would be getting bigger? I would have no way to measure that space was expanding because everything in that space is expanding with it. (Except my density would be decreasing and my gravitation attraction to the Earth would drop as the same mass gets spread out to a bigger volume, but never mind)
Some how we are to believe that the space between galaxies expands but our local space does not.
We sort of, maybe, have to assume that because otherwise we have a problem with the speed of light. If it were the galaxies that were moving then far enough out there they would have to be moving away faster than the speed of light. Which is impossible right?
On the other hand. I suspect there is no problem with far enough galaxies running way faster than light. We can never observe them, their light would never get here, so there is no problem with causality here. Everything is OK.
The whole history of this expansion idea is interesting. Einstein realized that his model of the universe would be unstable, it would either explode or collapse. Intuitively he did not like that, he wanted it to be stable, so he stuffed another term in to the equation, the cosmological constant. A fiddle factor.
Later it was found the universe is actually expanding, so Einstein wanted to take it out. He thought it was a big mistake.
Now a days, cosmologists want to put the cosmological constant back again...
How do we know that the universe isn't just expanding from our perspective? ... Think of it in terms of magnetic field lines. At the poles it is highly concentrated, where as you leave a pole, the flux lines become less concentrated. From our perspective, we are closer to one pole than the other but can't "see" either pole.
For my earlier debate that time doesn't exist, do a google search on "Problem of time"
How do we know that the universe isn't just expanding from our perspective?
Perhaps we don't.
But this is like suggesting the entire universe, as I experience it and including me, was created 4000 years ago, or last Wednesday or even just now. With everything in place, including me and my memories, to make it look like the there was a big bang and physics and you as I have observed it.
It's an idea but it's useless.
Instead we make some assumptions:
1) The Cosmological Principle. Properties of the universe are the same for all observers. That is, no matter who or where you are or in which direction you are looking the universe looks about the same. Sounds reasonable. No reason to expect there is anything special about me or we or our location.
2) The observed speed of light is a constant. Again a statement about experiments producing the same results no matter your speed.
3) The Equivalence principle. You can't tell the difference between acceleration and gravity.
4) Conservation of information.
I'm sure there are more base assumptions I am missing here. But you get the idea.
Point is, some assumptions, like the above, lead to measurably correct predictions and are useful. Others, like creationism or the univers as a simulation are a useless dead end.
I like your magnetic field lines analogy. It sounds like the question of: Is the geometry of the universe Euclidean and "flat", or does it have a weird non-euclidean geometry? Hyperbolic or spherical? I'm told the evidence of the microwave background radiation suggests the universe is flat.
There is indeed a problem with whole concept of space and time. I encourage you watch "Spacetime is Doomed" by Professor Nima Arkani-Hamed.
The problem of space expanding on a global but not local scale is actually pretty well handled. Within our bodies or the Earth or even a single galaxy, the expansion of space isn't all that great, and tends to be counteracted by the nuclear, electromagnetic, and gravitational forces. It's only on a truly grand scale -- actually even larger than the Local Group -- that the expansion of space becomes more important than that longest of long-range forces gravity.
It seems like space is expanding very fast because out near the Hubble limit things are flying away from us at nearly the speed of light, but that's also billions of light-years away and back here on Earth that same rate of expansion is so slight it's beneath the threshold of detection of even our most sensitive instruments.
The oft stated suggestion is that it's not that distant galaxies are moving though space but rather space itself is getting bigger. The raisins in cookie dough analogy. The dough (space) gets bigger, the raisins (us and galaxies) do not.
Well, no matter how small that expansion rate of space is I could surmise that everything is getting scaled up by the same about. Including my meter sticks. In that case I have no way to measure the expansion. Or even detect it.
My thought experiment goes like this.
I take my meter stick. Which is not expanding and which I use to determine the expansion of space. I clone that stick billions of times and line them up out to some far away galaxy.
Well, my sticks are not expanding, so the far off galaxy is now racing past the far end of my line of sticks. On the other hand space is expanding therefore my line of sticks
is getting longer and the far off galaxy is stationary wrt the far end.
Clearly I have the wrong picture in mind, that just seems paradoxical.
Currently, excuse the pun, I say forget the raisins analogy, those galaxies are really moving through space. The problem that they end up moving away from me faster than light is not actually a problem because I can never observe that. Hence there is no issue with causality.
Comments
"Logic, my dear Zoe, merely enables one to be wrong with authority." -- The 2nd Doctor.
Math is points, lines, vectors, planes, numbers, ... and nature does not have all of this stuff.
Physics tries to explain reality(?) with stuff without dimensions (points), without thickness (lines, planes), with perfect smoothness (lines, planes, bodies)... how can we expect not to run into contradictions when we take a close enough look?
And: No! I have no better answers than others... I only have questions...
We here statements like: It's not that galaxies are moving as such, rather it is space itself that is getting bigger. Hence the raisins in cookie dough analogy. Or dots on a balloon being inflated.
I my younger days I reasoned that if it was space itself getting bigger, how would we ever know? Surely the space between all my electrons and protons and whatever particles would also be getting bigger as well? Surely my meter sticks would be getting bigger? I would have no way to measure that space was expanding because everything in that space is expanding with it. (Except my density would be decreasing and my gravitation attraction to the Earth would drop as the same mass gets spread out to a bigger volume, but never mind)
Some how we are to believe that the space between galaxies expands but our local space does not.
We sort of, maybe, have to assume that because otherwise we have a problem with the speed of light. If it were the galaxies that were moving then far enough out there they would have to be moving away faster than the speed of light. Which is impossible right?
On the other hand. I suspect there is no problem with far enough galaxies running way faster than light. We can never observe them, their light would never get here, so there is no problem with causality here. Everything is OK.
The whole history of this expansion idea is interesting. Einstein realized that his model of the universe would be unstable, it would either explode or collapse. Intuitively he did not like that, he wanted it to be stable, so he stuffed another term in to the equation, the cosmological constant. A fiddle factor.
Later it was found the universe is actually expanding, so Einstein wanted to take it out. He thought it was a big mistake.
Now a days, cosmologists want to put the cosmological constant back again...
For my earlier debate that time doesn't exist, do a google search on "Problem of time"
But this is like suggesting the entire universe, as I experience it and including me, was created 4000 years ago, or last Wednesday or even just now. With everything in place, including me and my memories, to make it look like the there was a big bang and physics and you as I have observed it.
It's an idea but it's useless.
Instead we make some assumptions:
1) The Cosmological Principle. Properties of the universe are the same for all observers. That is, no matter who or where you are or in which direction you are looking the universe looks about the same. Sounds reasonable. No reason to expect there is anything special about me or we or our location.
2) The observed speed of light is a constant. Again a statement about experiments producing the same results no matter your speed.
3) The Equivalence principle. You can't tell the difference between acceleration and gravity.
4) Conservation of information.
I'm sure there are more base assumptions I am missing here. But you get the idea.
Point is, some assumptions, like the above, lead to measurably correct predictions and are useful. Others, like creationism or the univers as a simulation are a useless dead end.
I like your magnetic field lines analogy. It sounds like the question of: Is the geometry of the universe Euclidean and "flat", or does it have a weird non-euclidean geometry? Hyperbolic or spherical? I'm told the evidence of the microwave background radiation suggests the universe is flat.
There is indeed a problem with whole concept of space and time. I encourage you watch "Spacetime is Doomed" by Professor Nima Arkani-Hamed.
@Heater showed it.
Even whole GALAXIES are moving away from Earth.
What more proof we need?
Enjoy!
Mike
It seems like space is expanding very fast because out near the Hubble limit things are flying away from us at nearly the speed of light, but that's also billions of light-years away and back here on Earth that same rate of expansion is so slight it's beneath the threshold of detection of even our most sensitive instruments.
Well, no matter how small that expansion rate of space is I could surmise that everything is getting scaled up by the same about. Including my meter sticks. In that case I have no way to measure the expansion. Or even detect it.
My thought experiment goes like this.
I take my meter stick. Which is not expanding and which I use to determine the expansion of space. I clone that stick billions of times and line them up out to some far away galaxy.
Well, my sticks are not expanding, so the far off galaxy is now racing past the far end of my line of sticks. On the other hand space is expanding therefore my line of sticks
is getting longer and the far off galaxy is stationary wrt the far end.
Clearly I have the wrong picture in mind, that just seems paradoxical.
Currently, excuse the pun, I say forget the raisins analogy, those galaxies are really moving through space. The problem that they end up moving away from me faster than light is not actually a problem because I can never observe that. Hence there is no issue with causality.