ask a physicist

freyar · Oct 5, 2015

Landifarne said:
Can you summarize of our present level of understanding of what's occuring in the nucleus of moderate-sized atom?

There's some useful background information if you search around upthread a bit (or maybe in another thread in the Misc. Geek Talk forum --- these threads have gotten a bit jumbled in my memory). Basically, protons and neutrons (the particles that make up an atomic nucleus) are made up of 3 quarks each held together by virtual gluon particles. Gluons mediate the strong nuclear force in much the same way that photons mediate the electromagnetic force. The analogy ends there, though; the strong force is so strong that a great deal of the structure of protons and neutrons is determined by the virtual gluons (and also virtual quarks) inside them. In the rest of atoms, the virtual photons don't really show up separately from the fact that there is an electric force binding the atoms together.

The difficulty is that the strong force is strong enough that quarks and gluons can't really exist freely in the nucleus. They're stuck in the protons and neutrons. What holds the protons and neutrons together is a remnant of the strong force. The analogy here is the van der Waals force between atoms; atoms are electrically neutral, but, if you put two of them close together, their electrons arrange themselves so the atoms attract each other. That's the short answer. If you want a little more information: The mathematical description is much more complicated for the strong force, though, and it turns out that different approximations to the full equations work better for different sized nuclei (I'm not really an expert on this in detail, but I have heard about it). For medium-sized nuclei, the main way to think about it is that protons and neutrons are a bit "sticky" when they bump into each other, and this "stickiness" helps hold the nucleus together. Another part of this left-over nuclear force for these nuclei is due to virtual pions, which are unstable particles made up of one quark and one anti-quark. The best description is different (and somewhat harder to explain) for really small or large-ish nuclei. This is a notoriously difficult subject, by the way.

Landifarne · Oct 7, 2015

freyar said:
There's some useful background information if you search around upthread a bit (or maybe in another thread in the Misc. Geek Talk forum --- these threads have gotten a bit jumbled in my memory). Basically, protons and neutrons (the particles that make up an atomic nucleus) are made up of 3 quarks each held together by virtual gluon particles. Gluons mediate the strong nuclear force in much the same way that photons mediate the electromagnetic force. The analogy ends there, though; the strong force is so strong that a great deal of the structure of protons and neutrons is determined by the virtual gluons (and also virtual quarks) inside them. In the rest of atoms, the virtual photons don't really show up separately from the fact that there is an electric force binding the atoms together.

The difficulty is that the strong force is strong enough that quarks and gluons can't really exist freely in the nucleus. They're stuck in the protons and neutrons. What holds the protons and neutrons together is a remnant of the strong force. The analogy here is the van der Waals force between atoms; atoms are electrically neutral, but, if you put two of them close together, their electrons arrange themselves so the atoms attract each other. That's the short answer. If you want a little more information: The mathematical description is much more complicated for the strong force, though, and it turns out that different approximations to the full equations work better for different sized nuclei (I'm not really an expert on this in detail, but I have heard about it). For medium-sized nuclei, the main way to think about it is that protons and neutrons are a bit "sticky" when they bump into each other, and this "stickiness" helps hold the nucleus together. Another part of this left-over nuclear force for these nuclei is due to virtual pions, which are unstable particles made up of one quark and one anti-quark. The best description is different (and somewhat harder to explain) for really small or large-ish nuclei. This is a notoriously difficult subject, by the way.

Lol, I know. Sorry about that.

I have a BS in physics ('94), but the nuclear physics course that I took back in '92 was just that...BS. My impression was that my professor (or anyone, really) did not have a very good idea of what was going on. However, I know that new models have been floating around that I am completely ignorant of.

Was looking for a way to visualize it, so that I could speak more clearly about the topic to my HS students. What you've said here has been helpful, given me a few things to contemplate. I'll probably get back to you on it fairly soon.

-Steve

freyar · Oct 7, 2015

Landifarne said:
Lol, I know. Sorry about that.

I have a BS in physics ('94), but the nuclear physics course that I took back in '92 was just that...BS. My impression was that my professor (or anyone, really) did not have a very good idea of what was going on. However, I know that new models have been floating around that I am completely ignorant of.

Was looking for a way to visualize it, so that I could speak more clearly about the topic to my HS students. What you've said here has been helpful, given me a few things to contemplate. I'll probably get back to you on it fairly soon.

-Steve

No problem. If some of that seemed a bit basic, well, I'm also trying to write for everyone on the board. In any case, I'm far from an expert on the details of this stuff, though I've seen a few seminars on it. I'll look around a bit more. The thing about nuclear structure is that there are a number of approaches, all approximate, and which one(s) works well depends on the size of nucleus you're talking about.

tomBitonti · Oct 11, 2015

CaptainGemini said:
Please excuse the double post.

Here's where it gets complicated: We don't know how big the universe is simply because it's beyond our capacity to observe, but we know of around 91 billion light years.

Using the standard theory that the universe is speeding up as it expands, started at the speed of light, and has been expanding for fifteen billion years... At current, we cannot even observe one percent of one percent of the universe. An infinitesimal fraction of the universe's size could easily have a radius of three hundred billion light years. Even if only 1% of the universe obeyed the laws of physics as observable from Earth, you're still talking about an amount of space that we will likely never explore beyond at any point before the heat death of the universe. And that's even accounting for Star Trek-like FTL.

So, even if Noether's Theorum is wrong, we'll likely never know.

I was wondering ... Has it been ruled out that the observable universe is wrapped, with the current size much less than 91b (or even 15b) light years?

Thx!
TomB

Landifarne · Oct 12, 2015

tomBitonti said:
I was wondering ... Has it been ruled out that the observable universe is wrapped, with the current size much less than 91b (or even 15b) light years?

Thx!
TomB

It was confirmed (two decades ago) that we live in an open, ever-expanding universe.

tomBitonti · Oct 12, 2015

Landifarne said:
It was confirmed (two decades ago) that we live in an open, ever-expanding universe.

I'm presuming this is obtained from the measurements of the energy density, which makes for a flat universe.

How do we get from that to the conclusion that there aren't joins? Does the lack of uniformity in directions (diagonals are longer than the perpindiculars) make that physically impossible? Or is it considered too strange?

(From a topologists point of view, a standard construction is to take a unit square and to identify the opposing sides to make a new quotient space of the original unit square. That makes for a flat, unbounded, yet finite, space. The space is not uniform in all respects.)

Thx!
TomB

Umbran · Oct 13, 2015

tomBitonti said:
How do we get from that to the conclusion that there aren't joins? Does the lack of uniformity in directions (diagonals are longer than the perpindiculars) make that physically impossible? Or is it considered too strange?

"Diagonal" and "perpendicular" are products of a human imposed coordinate grid. The universe does not have such a grid naturally. If you work in, say, spherical coordinates, there is no such distinction, and you find there *is* uniformity in directions.

There could be some local joins or curves (wormholes and black holes and such). But, on a broad scale, there is a point to be made: Mass/energy causes spacetime to curve. As far as we know, mass is the *only* thing that makes spacetime curve. Real physical spacetime cannot have arbitrary topological configurations *without* mass to make it so. This is why we say that the low energy density means spacetime is flat, at least within the observable universe.

tomBitonti · Oct 13, 2015

Umbran said:
"Diagonal" and "perpendicular" are products of a human imposed coordinate grid. The universe does not have such a grid naturally. If you work in, say, spherical coordinates, there is no such distinction, and you find there *is* uniformity in directions.

There could be some local joins or curves (wormholes and black holes and such). But, on a broad scale, there is a point to be made: Mass/energy causes spacetime to curve. As far as we know, mass is the *only* thing that makes spacetime curve. Real physical spacetime cannot have arbitrary topological configurations *without* mass to make it so. This is why we say that the low energy density means spacetime is flat, at least within the observable universe.

Yes. But, with a simple join (side to side, top to top), while there is no local non-uniformity, there is a global one, with three axes (left-right, up-down, forward-back) being distinguished from the three diagonal axes. And changing the join to a spherical one, that is, which identifies opposing points on the surface of a sphere, means either introducing curvature, or means there is a discontinuity of a derivative at the join. (That is evident by pushing a curved arc across the join: The arc flips directions as it crosses the join.)

Your statement "cannot have arbitrary topological configurations" (&etc) seems to match my statement describing the joins as being physically impossible or simply too strange. However, is that actually physically proven, or is it taken as being not pursued because there is no evidence to merit the consideration? Ar, do the non-uniform axes create a substantial problem?

In this same topic, do we have a limit on the flatness of space, and a correlation of that flatness to a minimal size of the universe -- assuming a simple spherical topology?

Doing some searches found this, which is a nice read:

PHYS771 Lecture 20: Cosmology and Complexity
Scott Aaronson
http://www.scottaaronson.com/democritus/lec20.html

That references this, which seems to match my question, but I haven't had a chance to look through it yet to tell:

Measuring the shape of the Universe
Neil J. Cornish (DAMTP, Cambridge), Jeffrey R. Weeks (Canton)
(Submitted on 30 Jul 1998 (v1), last revised 5 Aug 1998 (this version, v2))

http://arxiv.org/abs/astro-ph/9807311

Thx!
TomB

Umbran · Oct 13, 2015

tomBitonti said:
But, with a simple join (side to side, top to top), while there is no local non-uniformity, there is a global one, with three axes (left-right, up-down, forward-back) being distinguished from the three diagonal axes.

Yes, but you are still thinking from the point of view of someone used to thinking in rectangular coordinates. Why would we expect a rectangular coordinate join, specifically, of the universe?

And changing the join to a spherical one, that is, which identifies opposing points on the surface of a sphere, means either introducing curvature

All continuous joins introduce curvature. Try to make a cylinder without bending the paper, dude

Your statement "cannot have arbitrary topological configurations" (&etc) seems to match my statement describing the joins as being physically impossible or simply too strange.

Let me put it another way - as mathematicians considering topologies, we can think up all sorts of spaces, without concern for why they are shaped a given way. But, General Relativity tells us that the topology of our real spacetime is *not* independent of the material within the space. You can only have arbitrary topology if you allow arbitrary placement of material in the space. And, if we don't have material, then the space isn't shaped that way.

However, is that actually physically proven, or is it taken as being not pursued because there is no evidence to merit the consideration? Ar, do the non-uniform axes create a substantial problem?

It is proven, insofar as GR seems pretty good, and we've seen no evidence for other curvature of our spacetime. I know of no physical phenomenon currently unexplained that calls for such curvature. Occam's Razor, then, suggests that I not worry about the possibility too much.

I wouldn't state it as you do - it isn't "non-uniform axes create a substantial problem". I would put it as, "curvature of the space that is not attributed to mass likely (if I recall correctly) equates to perpetual motion machines and/or time travel*." Either of those gives us serious issues with causality and the laws of thermodynamics. You need to have a really good reason to suggest those might exist.

*IIRC, time travel implies perpetual motion, but perpetual motion does not necessarily imply time travel.

tomBitonti · Oct 13, 2015

Umbran said:
All continuous joins introduce curvature. Try to make a cylinder without bending the paper, dude

The curvature is an artifact of the embedding, not of the join.

GR tells us about local topologies, not about global ones. To make conclusions about the global topology, additional evidence is needed.

In the second link posted above, several experiments are described, with the result being, at that time, "we don't know". However, there may be more recent results, as newer experimental data (satellite measurements) should now be available, compared with what was available to the authors of that paper.

Thx!
TomB