Anti Matter Discovery

[rov1200]

http://news.bbc.co.uk/1/hi/sci/tech/291299.stm

What is the difference between dark matter and anti-matter?

[jake-the-peg]

Anti-matter is just the same as normal matter but in a way is mirror imaged in all senses.apart from mass.

So in the 1928 Paul Dirac first predictied anti-matter, specifically an anti-electron or positron. This would have a charge of +1 and equal and opposite spin and the same mass. It was discovered 4 years later.

There are anti- forms of all particles but if an particle and anti-particle interact they anihilate each other releasing their combined mass as energy.

Because of this we see very little anti-matter - there was a sucessful creation of an anti-hydrogen atom but you're not going to be able to pick up a rock of anti-matter.

Dark matter is a name we give to whatever it is that is adding additional gravitational mass to the Universe. We see the dynamics of gravity in the motions of galaxies but there's not enough visible mass to explain it.

It's not clear what is doing this the term "Dark matter" is kind of a place marker for this.

This may be much less exotic than it sounds. One candidate was "Machos" (Massive astrophysical compact halo objects) these are simply things like burn out stars and other normal bodies that we just can't see.

I don't think these are now a strong candidate.

the term was a bit of a joke compared to WIMPs (these are weakly interacting massive particles) These are a theoretical form of matter that have mass but don't interact. Recently it has been discovered that neutrinos do have some mass so might be the dark matter themselves or might be related to these wimps in some way.

The advantage that neutrinos have is that they are already in the standard model and don't need to be conjured up.

So really they are quite different

[jake-the-peg]

So the underlying rule of thumb is when you see the word "Dark" don't automatically think of something weird and exotic thing " something we don't know"

A bit like the way Roentgen used the term "X rays" where X meant something unknown

[R1Geezer]

What Jake said!

As this is a subject of interest I had hoped to be able to add something here but jake has explained it perfectly.

[jake-the-peg]

Personally I'd like to put the cosmologists and particle physicists in a boxing ring.

The particle physicists have just about nailed their standard model - if they find the Higgs it'll be there - beautiful and symetric - a new periodic table.

The idea of a bunch of cosmologists coming along and saying "You know we really think we need a new particle" is a bit like wandering into the Louvre with a paintbrush and saying - "you now Leonardo just didn't use enough red!"

[R1Geezer]

You mean Dark matter jake, as the new particle? As you say above that's just a place holder to make the equations work. I think most agree that there is not enough detactable matter for cohesion in galaxies. I shouldn't be too harsh they are only emulating Einstien who used the term "cosmological constant" to make his equations work. Will the discovery of the Higgs somehow explain the missing matter?

[jake-the-peg]

Not really

Higgs is about explaining where particles get their mass from. Why some have mass and some do not.

As for the Cosmological constant that was Einstein dithering - he claimed putting it in was his greatest mistake - he had no better reason for including it than excluding it. Remember all this was pre-Hubble when the Universe was thought to be much smaller and very much more simple. But that's to do with Dark Energy rather than Dark matter

Neutrinos are still my favorite option. There are uncountable numbers of them. at least one for every photon. It was thought that they were massless for a very long time.

However we knew that there were about 1/3 as many of them as there ought to be coming from the sun

Now there are 3 types (electron neutrinos, muon neutrinos and Tau neutrinos) and our experiments could only detect the first. However *IF* neutinos had mass, even an ever so tiny amount they could "switch" constantly oscillate between the three types.

We've recently discovered that they can switch and must have mass but it's way to small for us to measure.

But remember how many of these there are - that is a lot of matter out there!

There are reasons that people don't think neutrinos are the dark matter but are related to these WIMPS.

Personally I look at it and think is it more likely that it is neutrinos and some of this logic is wrong or is it more likely that there's a whole new particle and I have to go with the first.

But that is simply down to aesthetics and not wanting to mess up the standard model - It's a very Einsteinian view point

[rov1200]

Its all getting rather confusing to the lay observer. There is so much terminology wrapped up in the subject. You would think the processes could be easily explained from the primordial soup condensing followed by a big bang. With all the conflicting arguments I am convinced the big bang never existed at all.

[jake-the-peg]

Why do you think that something as fundamental as the birth of the entire Universe should be easy to understand?

If I had to find something that I'd have expected to be difficult to understand I'd have selected that!

The evidence for the big bang is utterly overwhelming. The discovery of galaxies rushing away from us in all directions was one thing but when it was calculated what the background temperature ought to be and that was found to be the case to within a few percent by a couple of researchers looking for something totally different.

Well that pretty much sealed it.

It also sounds like you've not yet grasped the really mind numbing point about the big bang.

It was not a huge star that exploded

It was the very creation of space and time

There was no physical space "before" it and no time either - the very notion of "before the big bang" is therefore as meaningless as the notion of what you were doing before you were born

[rov1200]

///It was the very creation of space and time///

Your statement Jake is where your theory about the universe falls down.

I thought you never believed in creation but you are suggesting just that! Maybe you believe nothingness is something?

[beso]

Jake said:
There are uncountable numbers of [neutrinos]. at least one for every photon.

Could you explain the reason for this or post a link to an explanation?

[jno]

rov1200.... yes, it is rather odd, but scientists' views about the creation of time and space are now rather similar to those in the Bible: no god, but otherwise everything did indeed start all at once. ('Start' might be a simpler than 'was created', since it doesn't hint that somebody was doing the creating.) In my innocent youth we all believed the universe had always been there, which was very different from biblical teaching, but this theory was replaced by the Big Bang theory in the 60s or thereabouts.