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Geraint's Profile

Geraint's Avatar Joined almost 7 years ago
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Go to: This used to be me

Geraint's Avatar Jump to comment 7 by Geraint

Can someone explain why this guy would have something against Minneapolis?

Thu, 08 Mar 2012 19:55:18 UTC | #925452

Go to: Stupid and clever questions for people who understand the physics

Geraint's Avatar Jump to comment 116 by Geraint

Comment 114 by Jollyroger : Geraint - are you now saying that space is fixed and objects, such as galaxies, move apart because they received an initial push from the big bang (possibly augmented by dark matter etc)? That is the only conclusion I can reach from your balls and gravity analogy.And I think it is absolutely wrong.

Great. So if you already know the answer, why are you bothering to ask questions about it on a web forum?

Now Greyman says it is the metric of space that is expanding. By metric I assume he means the unit of measurement, the yardstick that we use to define lengths, et.al.

The metric is a tensor describing the structure of spacetime in general relativity.

I don't disagree with Greyman. I'm trying to give you a good mental picture of why atoms don't expand but distant galaxies move apart, without just saying 'go away and learn general relativity'. The dynamics of particles work out as I've said. The reason they work out that way is because of how the metric expansion of space works in GR. No, it's not the same as Newtonian dynamics in Euclidean space, but in certain restricted situations, viewed in certain ways, it works out similarly and so it can be used as an analogy.

I'm not going to bother replying to any more sarcasm, if you don't like the way dynamics works in an expanding Universe in GR, take it up with the Universe, not with me.

Thu, 03 Nov 2011 03:04:45 UTC | #886687

Go to: Stupid and clever questions for people who understand the physics

Geraint's Avatar Jump to comment 112 by Geraint

Comment 111 by Greyman :

Comment 109 by Geraint I'm saying that the expansion of the Universe consists of the contents of the Universe moving apart. They only continue to move apart because there is no force between them strong enough to pull them back together

No, it is the metric of space itself that is expanding.  That's why it appears that distant galaxies are moving directly away, at a rate proportional to their distance, from ours.

Yes, this is why I've kept on hedging my explanations. I think I've been pretty careful to do that. But talking about the expansion of space easily gives rise to misconceptions about what this means for bound objects.

If you consider the dynamics of a couple of bodies in the FLRW metric, the way I've described things in terms of an initial velocity gives a more accurate mental picture than thinking of space pushing things apart. There's a good discussion of this in Peacock's book, for example.

Thu, 03 Nov 2011 02:07:14 UTC | #886672

Go to: Stupid and clever questions for people who understand the physics

Geraint's Avatar Jump to comment 110 by Geraint

Comment 106 by raytoman : They postulated that at one time the increasingly seperating galaxies must have been together at a point (a singluarity which must have projected our universe. The math did not work so that had to invent inflation to explain the difference?

It's a fair bit more interesting than that. The math worked, but our Universe seemed to have fairly special properties that weren't required by the math. They weren't ruled out either, however: there was no discrepancy as such. Inflation was postulated as an extra process which explained those properties, and turned out to make some predictions that have been borne out quite well. The main problem is that further tests of inflation from this point require some very difficult observations.

A hot Big Bang is supported by a lot more evidence than some regression analysis on galaxies, but there are a lot of good resources out there which explain that much more thoroughly than anyone can manage on a web forum (for example the WIkipedia articles on cosmology are pretty good, in general).

Thu, 03 Nov 2011 01:46:46 UTC | #886667

Go to: Stupid and clever questions for people who understand the physics

Geraint's Avatar Jump to comment 109 by Geraint

Comment 103 by Jollyroger : Not being a physicist, I can't quite see your connection with throwing balls up in the air.

I'm saying that the expansion of the Universe consists of the contents of the Universe moving apart. They only continue to move apart because there is no force between them strong enough to pull them back together. Usually, if you throw a ball up in the air, the force of the Earth's gravity is enough to pull it back to Earth. But if you throw it hard enough, it will be going fast enough that it never gets pulled back to Earth: it will keep on moving away from the Earth forever. That doesn't mean that it has a 'tendency' to move away from the Earth, just that the Earth's gravity was insufficiently strong to stop it moving away.

Now, instead of the Earth and a ball, think of two galaxies given a large relative velocity at the Big Bang. The situation is the same, they keep on moving apart not because of some 'tendency', but because no force was strong enough to overcome the initial velocity. But, if they started off close enough together at the Big Bang, their mutual gravitational pull would be enough to pull them back together and they would merge. They have now forgotten about their initial velocities moving them apart. They don't know they 'should' be moving apart. There is no 'tendency' for them to separate from each other and move apart again.

(Note that I've used 'galaxy' here as a shorthand for ' dense region that will eventually become a galaxy').

What I don't understand is your 'explanation' - which explains precisely nothing, IMO.If the size of particles is fixed with respect to the remainder of the universe, as you say it does, (and I don't disagree with this) then it follows that any and every such fixed region could be taken as a fixed frame from which velocity - in particular the velocity of expansion - can be referred.That is my conundrum, because it contradicts just about all the other known laws of physics.

It doesn't contradict anything. Relativity doesn't say that no frames of reference exist. If I want to perform a calculation, I have to choose a frame of reference in which to make that calculation. What it says is that there is no 'preferred' frame of reference. I can't say that the rest frame of an atom in the Milky Way is a special, unique, more correct frame of reference from which to measure the expansion than any other, and I can't do it for any atom in some distant galaxy which is moving away from us either. The rest frame of either atom would be a valid frame. Neither is privileged. I could also use the rest frame of the centre of an expanding void in the galaxy distribution to be a valid rest frame. The void is expanding, but so what, I can still do it. Whether an object is expanding is neither here now there when it comes to choosing a reference frame.

Why should the space within a particle be exempt from otherwise universal expansion? Does the strong force or whatever exactly balance the tendency to expand?

What I'm saying is that there is no tendency to expand.

What makes that inner space so special?

Nothing.

And if we place such spaces end-to-end to form a rod - at what point does the agglomeration exhibit expansionist properties?

Never. The expansion is a property of the initial state, not space. (Or at least, I'm saying you have to think of it that way to have the correct picture of expansion in mind in this respect, unless you want to deal with general relativity and/or dark energy).

Thu, 03 Nov 2011 01:32:15 UTC | #886660

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