h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

With reference to 'Just Six Numbers' by Martin Rees, chapter 5 and Escher's illustration that he uses: http://trese.cs.utwente.nl/taosad/SoftwareArchitecture/Images/EscherCubicSpace.jpg

While playing with a 'Lies to Children' fantasy, two points arose:

1. The rods that are set to lengthen - could they be said to be a loose analogy of dark matter?

2. Galactic clusters not expanding within themselves... due to 'local' gravity?

No idea what you're asking, sorry.

Ditto

Yes, I saw this earlier - it's a bit too cryptic I'm afraid.

Ah, sorry. Lost it again.

Recently I've noticed a couple of references to 'Just Six Numbers' by Martin Rees.

In chapter 5, using one of Escher's creations:
http://trese.cs.utwente.nl/taosad/SoftwareArchitecture/Images/EscherCubicSpace.jpg
he sets the rods to lengthening and talks of speeds of recession [of the cubes] being dependent on the number of intervening rods...

That analogy stayed with me. Recently, I had cause to review my 'knowledge' and constructed a sort of 'Lies to children' fantasy - "if I can't write it down I don't know what I'm thinking" and "when I teach, I learn".

Q1: Based on my understanding of what I've read, I have this picture in my mind of matter being continually created & destroyed in space - but of some *sticking* thereby pushing things apart. Fanciful probably - so please disabuse me of my imagining that those rods could be an analogy for dark matter.

Q2: In that few paragraphs, he also notes that galaxies do not themselves expand, neither do galaxy clusters. Why not?

**dammit. The above link doesn't work for me, from Preview (ellipsis in the middle). It should be:

http://trese.cs.utwente.nl/taosad/
SoftwareArchitecture/Images/
EscherCubicSpace.jpg

Which works for me directly from copy/paste.

Thanks
Rod

"I have this picture in my mind of matter being continually created & destroyed in space"
continually but unpredictable occuring as a particle/antiparticle pair which then anihilate each other yes (poss exception being near a black hole where one is drawn into the black hole but the other is repelled and thus does not get destroyed)

"but of some *sticking* thereby pushing things apart."
Some does remain (as mentioned above) and there is a matter/antimatter imbalance in the universe. But pushing things apart? Not as I understand it. Are you thinkking perhaps that when the energy is converted to matter it has to push aside other bits of matter to come into being? If so then not so, as we are talking sub atomic particles as there is plenty of room for them!

" galaxies do not themselves expand, neither do galaxy clusters."
Depends what he is talking about. If it is the unexplained acceleration of the galaxies then this is related to Dark Energy (even less accepted than Dark Matter) having an effect only on a really large scale where it can overcome gravity. At a local level (galaxy, cluster, super cluster) gravity can overcome Dark Energy and thus they do not expand.

Dark Matter (which Im not sure if you are confusing with Dark Energy or not) is the supposed missing 90% of the universe (or whatever the figure is) and is required to explain various physical observations (like rotational speeds and galactic movements) where the observed behaviour does not match with the calculations - their is too little matter for the observed velocities.

to go further with your rods analogy, dark matter can indeed be seen as the scaffold upon which the observable (by us) universe is built. Where you get large concentrations of dark matter, then you are more likely to see galaxies and clusters etc. Where there is little or no dark matter you have a void.

However, the maps we have so far of both dark matter and normal matter do not completely coincide so something more complicated than your 'rods' is going on. But there are filaments and blobs (according to the papers so far - it could all still be proven wrong) out there that galaxies and so forth seem to be connected to somehow.

Thanks, Ictoan.

>Are you thinking perhaps that when the energy is converted to matter it has to push aside other bits of matter to come into being?...<
- Yes, something similar must have been what was in mind.

>...If so then not so, as we are talking sub atomic particles as there is plenty of room for them!<
- That's perhaps my problem. I'm aware of the hazards of comparison with conventional physics, and that even if 99.999...% was dark matter (other consideration aside) there'd still be plenty of room. However, something like Boyle's Law keeps intruding. OK, I'll work on that (those sub-atomic particles obviously don't have elbows).
- Confusing dark matter with dark energy? Not impossible!

" galaxies do not themselves expand, neither do galaxy clusters."
and, your
>Depends what he is talking about ... related to Dark Energy ... having an effect only on a really large scale where it can overcome gravity.<
- Not stated but seems like it (hence my original guess about gravity).


I hadn't included those aspects in my fantasy - I wonder why.

I am generally confused about the question, but one thing I can clarify. Space dominates at subatomic levels. When we see things as solid, it means that they reflect visible light, which has wavelengths of about 500-600nm. If we could see using light that had a wavelength of 1nm, or about the diameter of an atom, then we would see that everything is mostly empty space.

We can't see using light at 1nm (x-rays): its wavelength is so small and its energy is so high that it goes straight through our eyes.

Yeah, Bouncy, hadn't thought of it in that way.

So, dark matter is dark (to us) because

a) We can't (yet) detect short enough wavelengths - or
b) Those wavelengths just don't bounce off those sub-atomic jobbies - or
c) They do, but it happens so rarely that we've no chance of detecting them
[-or ci) except perhaps in the LHC - though that's stretching it a bit perhaps? Um, yes it is]

actually, the reason dark matter is so called is because it does not interact with electromagnetic radiation (so doesn't give off any and doesn't 'reflect' or otherwise modify any passing radiation). They only way to detect it is via large scale gravitational distortion (gravitational lensing). It's not due to size or anything, merely that it is a different 'sort' of matter that doesn't react as known matter does.

Thank you Bouncy - and thank you, especially, Ictoan.

I've certainly had more clarified here than struggling with the books - bite-sized pieces here, maybe (though not all easily digested).

So, what else? Oh yes.

About my pension...

Sorry for causing confusion, I wasn't talking about dark matter, but normal matter being full of holes.

Bouncy: something like, to a first approximation, there's nothing here?

<>

if dark matter was evenly spread throughout the universe it wouldnt cause lensing and be totally undetectable??

- awaits with interest (thinking of why it was posited in the first place) -

"if dark matter was evenly spread throughout the universe it wouldnt cause lensing and be totally undetectable??"

Indeed. But it isn't, so it is.

Why that should be so (if indeed it is so, lets not forget we are talking about a subject on the frontiers of knowledge and this theory could be wrong, but there is evidence for it) is an interesting question. The related question of why normal matter is not evenly spread out throughout the universe could well be related. And the question of why there is any matter at all, since matter and anti matter theoretically should be generated in equal quantities, is another one. It seems the universe is not equal between the two - something very, very early on in (microseconds or less after the big bang, or perhaps fundamental to the makeup of our universe) means there is more matter than anti matter. Why this should be so we do not know. I seem to recall reading something which suggested that for somereason the universe has a 'left handed' bent - left handed referring to spin and sub atomic particles of which there is more than the opposite.

What intrigues me is that if there is matter, anti matter and dark matter - could there also not be anti-dark matter. And if so what properties would it have and what effect would it have on the model of the universe?



btw dark matter was posited in the first place, as with so many things (higgs boson is another example, as is the neutrino and anti matter) because it fits a hole in the observations/theories.
There's just so much out there to know, and we have such a short time (individually) to find out what we can.

Intriguing...
----
>...because it fits a hole in the observations/theories< Yet another hole in the fabric...

Of course, it's the scenery that's new, not the technique. Poirot invented it methinks, while contemplating a trois-pipe mystère. Non?