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Sue Wright posted:

Does anyone on the list understand the M theory?

The topic of last nights Compass [must stop watching this] was a 'big bang/string theory/M theory/where did we all come from type thing [well it is Compass after all].

I tried to get my head around what was being discussed "strings and membranes and multiverses" but it was difficult because I couldn't focus after being bombarded with images of an ice skating US physicist dressed in black.

Some day I would like to understand this (not the ice dancing thing ... that's just plainly disturbed :-) and I'll obviously go away and read a hell of a lot more, but can anyone help with the following:

- there are 11 dimensions - what are the hell are they. It sounded as if people just thought 11 would be a good idea as it would make their theory work!

- the universe is not a 'uni' verse at all it's suggested to be a multi-verse. Why isn't this still just a universe - just one that's bigger and more complex than previously thought. To me it makes no sense at all to have multiple universes as the 'branes' would just be a part of the universe.

- how can 'branes' bump up against each other, in order to create a big bang or bump. I.e. It was suggested that I could create a universe in my basement (shades of the science of discworld) which would then expand infinitely but wouldn't displace anything already there. If this is so and nothing is displaced - how the hell can things 'bump'?

- for those who watched why did the physicists (mathematicians) simply introduce an externality to explain everything. I.e. If we simply add another dimension (that we don't know exists) then our theory will work. Sounds to me a bit like the Government saying "we're not going to acknowledge that the welfare/justice system is entirely stuffed in the first place so we'll just keep adding bits to it in order to try and manage it and make it all balance"?

The way branes were explained made it all sound like phlogiston.

Does anyone know how to get ice dancing images out of ones head?

Bruce Kellett replied:
 

It was an interesting program; a courageous attempt to explain the
inexplicable. But I agree with you that the Beeb "Horizon" team did
slightly worse than their usual standard with the very repetitive graphics
on this program.

>Some day I would like to understand this (not the ice dancing
>thing ... that's just plainly disturbed :-) and I'll obviously go away
>and read a hell of a lot more, but can anyone help with the following:
>
>- there are 11 dimensions - what are the hell are they. It sounded
>as if people just thought 11 would be a good idea as it would make
>their theory work!

That is more ar less correct. As the program said, string theory
(theories) require 10 dimensions to work. By "work", the physicists mean
things like consistency, absence of anomalies, no ghosts (negative
probabilities) and such like nasties. In fewer dimensions, these
unpleasant things tend to wreck the theory, so the bold souls just add
enough dimensions to make it all work. Originally, the idea was that this
would just be a temporary working strategy, and that once the theory was
better understood, we would see how to do without the extra dimensions. To
date, this hope has not been realized, and people are becoming comfortable
with the extra dimensions. The 11th dimension was added with M-theory,
which was supposed to show how the five alternative string theories were
actually just different facets of the same thing.

Brian Greene's book "The Elegant Universe" is a quite readable
introduction to a lot of these ideas by a practitioner.

>- the universe is not a 'uni' verse at all it's suggested to be a
>multi-verse. Why isn't this still just a universe - just one that's
>bigger and more complex than previously thought. To me it
>makes no sense at all to have multiple universes as the 'branes'
>would just be a part of the universe.

The multiple universe idea comes about because the separate universes may
well have different physical laws, and are out of causal contact with each
other. Note that "multiple universes" in this cosmological context is a
different thing from the "many worlds" interpretation of quantum
mechanics.

>- how can 'branes' bump up against each other, in order to create a
>big bang or bump. I.e. It was suggested that I could create a universe
>in my basement (shades of the science of discworld) which would
>then expand infinitely but wouldn't displace anything already there.
>If this is so and nothing is displaced - how the hell can things 'bump'?

Very good questions, indeed. The Compass program was late last night--I
had had a bottle of good red, and fell asleep through it, so I don't
remember all of these points. The "bumping brane" idea I have heard
elsewhere, and it makes little sense to me now. I daresay one could give
it a meaning, but I don't know any details.

>- for those who watched why did the physicists (mathematicians)
>simply introduce an externality to explain everything. I.e. If we
>simply add another dimension (that we don't know exists) then
>our theory will work. Sounds to me a bit like the Government
>saying "we're not going to acknowledge that the welfare/justice
>system is entirely stuffed in the first place so we'll just keep
>adding bits to it in order to try and manage it and make it
>all balance"?

I hope it is not as bad as government work. ;-)
I think the reason for these speculative excursions is that there are many
unresolved problems with existing theories, and physicists hope that by
pursuing such ideas they might come to some deeper understanding. I think
that all--in their more sober moments--agree that it is very speculative.
But when you have reached an impasse in standard theory, it makes sense to
try out lots of ideas--knowing full well that most will prove to be dead
ends. It is like the process of adding extra dimensions. You don't
necessarily believe in the "objective reality" of these things, you just
hope that they will help you to gain some greater understanding.

>The way branes were explained made it all sound like phlogiston.

It can do so, but there is actually some deep mathemetics at work here.
I do not know enough differential geometry or topology to understand
it either, but I am assured that it is all very profound.

>Does anyone know how to get ice dancing images out of
>ones head?

Take a cold shower?

> New Scientist did a feature article on Branes a few months ago,
> Try their archives
> Bill

Would I get away with calling the subject of this thread a 'no-braner'?

No? Oh well, at least I tried.

Chris Forbes-Ewan

Big bang   -  Our Universe exploded into action after being hit by another, say physicists

Exclusive from New Scientist magazine

It was mighty quiet in our Universe: devoid of all matter and energy. Then another universe collided with it. Suddenly space became a searing soup of particles and radiation, far hotter and denser than the centre of the Sun.

This, says a team of physicists, is how the big bang happened. The approach of the parallel universe caused the expansion of space and the collision itself produced all energy and matter. A comprehensive description of this new theory has been submitted to the journal Physical Review D.

The model is a viable alternative to the theory of cosmic inflation, which was cooked up to solve some of the problems of classical big bang theory.

According to inflation theory, the Universe underwent a brief period of exponential expansion in the first split second of its existence. "Our new model solves the same problems," says co-author Paul Steinhardt of Princeton University, who was one of the founders of inflation some 20 years ago.

Brane storm

So how might this primordial collision have come about? According to the theory, our three-dimensional Universe, known as a "3-brane" to cosmologists, is just one of the two boundary surfaces of a thin four-dimensional "bulk space", rather like one of the two surfaces of a CD.

But there was another universe, or bulk brane, inside the bulk space which encroached on our boundary brane. When this "brane storm" led to a collision, the energy released resulted in the big bang. The authors of the paper dub the model the "ekpyrotic universe", related to the Greek idea of "cosmic fire".

Unlike inflation, the brane-storm model fits in neatly with the popular string theory of particle physics. "That's one of the exciting things about it," says Neil Turok of Cambridge University, another co-author.

Big bang II

"I'm delighted to see an alternative picture for the early Universe," says cosmologist Jim Peebles, also of Princeton. "It has been frustrating to me not to have any alternatives to inflation, which I feel has been accepted by many cosmologists too easily."

As for the credibility of the new model, he says: "I haven't heard my string theory colleagues complain. That's a positive sign."

But if there are other branes floating around in 4-D bulk space, might we be in for another big bang? Turok says it can't be ruled out. Luckily, the fact that Newton's gravitational constant doesn't appear to be changing implies we're safe from a collision for many billions of years.

More at: The Ekpyrotic Universe (abstract)

1743 GMT, 10 April 2001

Note from Barbara Sloan

See also
A Brief Introduction to the Ekpyrotic Universe   - Paul J. Steinhardt
'Brane-Storm' Challenges Part of Big Bang Theory
and for a dissenting view
Ekpyrotic Universe  - (c) Robert Neil Boyd