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

Thankyou for this well explained simple introduction to alternate geometry and how it is actully applicable. well done.

I agree very nice...

However is it not possible, that its a cycle...the universe expands, then contracts, at a certain point in the contraction, forces slow it, and stop its contraction, forces of density pushing outwards, AND SO THE UNIVERSE expands again,....then contracts...then expands....Ad infinitum..continually....So...its here for every....just fuxtuating but never comeing to an end?

I dunno? Just threw that out there?

Best to all.

I agree very nice...

However is it not possible, that its a cycle...the universe expands, then contracts, at a certain point in the contraction, forces slow it, and stop its contraction, forces of density pushing outwards, AND SO THE UNIVERSE expands again,....then contracts...then expands....Ad infinitum..continually....So...its here for every....just fluctuating but never coming to an end? I dunno? Just threw that out there?

I'm not a scientist, just an armchair science lover.

Best to all.

OOPS...accidental double post

Good idea, but have you any idea what sort of an "outward" force would stop it collapsing? And what would the effect of this force be on normal matter? Would it cause it to explode?

Excellent entry; readable, enticing and enlightening. Well done!

As a philoshphy head I baulk slightly at "It's often said that a massive star will bend the light rays around it due to its gravity, but in reality the light continues in straight lines, following the shortest distance between two points. It is the space itself that is curved around the star."

Space is a hard thing to say anything descriptive about; it is most elusive and un-thinglike. I think "in reality" is too stark; I would prefer "you could say" . . . but never mind, I'm willing to accept your terms as a convenient shorthand.

Brilliant at explaining complicated concepts to non-mathematicians.

But... need to keep the maths and physics seperate! The various ideas of curved space are completely independant of physcial theories. Why the physical world behaves as it does, why space might be curved, are all questions for the physicists. Certainly at the very deepest level the justification is that it seems to match what we see, i.e. experiments work. General relativity is a whole lot more complicated than just curved space, and the more exotic gauge theories, string theories, supersymmetry theories up the ante considerably.

Quite apart from general relativity et al, modern mathematicians investigate geometries in quite abstract ways, and many ideas can be restated as geometric ideas - think about playing with the shapes of graphs, etc.

As a final word, a lot of confusion comes from the notion of higher than 3 dimensions - can anyone think of a good way of explaining them without falling back on calling time a fourth dimension? What about 5, 6, 7, dimensions?

I make the claim to have inveted 4D noughts and crosses. the main thing about more dimensions is the potential to have more things beside each other. so in regular 3 by 3 2D Xs & Os, a corner square has 3 connecting points, including diagonals. in 3D (think a 3 by 3 by 3 cube) it has 7, and in 4D it has 16 I think. It realy is a great game; you need about five to play or it's too easy; there's so many options that there's no way you could block each other with just two playing.

I seem to remember Martin Gardner talking about 4-d noughts and crosses back in the sixties.

NO no no it was me!
well, whatever, I was working independetly of Martin Gardner. & the 3D version is much simpler & good for 3 to play, as we did on trains all around Europe.....

Ha, super, but it must be a headache to visualize! Do you always notice when you've won?
BTW I think there are fifteen connections to the corners in four dimensions (2^4 -1), and the centre connects every one of the other 80 points! No wonders it's hard to block.
I

The 3D is fine to visualise as a cube. the 4D is a bit of a problem. can be done by having as many cubes as there are lines of winning on the regular 2D version. these overlap as much as those lines of winning do, but each of these cubes considered as a 3D version will show up any potential wins. I kinda stopped doing that, & just looked at the patterns on the page instead. we drew the 3D one simply as 3 2D versions next to each other, on the understanding that they are actually stacked. we drew the 4D as when you draw a 2D one(but much bigger), & put another 2D one in each space. thus you can connect the lines of winning as I was saying. If you get used to it you can just notice the patterns developing & see a line of win. That's why I brought it up in the forst place, it's a way of explaining 4D without resorting to time.

If the universe contracted to an infinitely small point, would exclusion principles, the statistical thing about similar particles not occupying the same point, mean it would explode again?

My physics education gets very hazy once we leave Newton behind.

Ermmm, actually everybody's physics education gets hazy when looking at infinitely small points, thats the problem! As to what infinity, or infinitely small actually means in physics...
Actually the exclusion principle only applies to things like electrons, protons,... - photons quite happily pass through each other. Its actually quite a subtle idea involving waves, rather than rubber balls bouncing off each other.

Did you notice that you used the word "actually" in every sentence in that last post?

Well done Gnomon

I don't think we have to worry about infinitely small points. Long before that time an imploding universe would start containing more and more black holes, and ending with all mass in one black hole with a finite size.
After that the process will be something completely different.

Yes, good work Gnomon.

Perhaps the problem doesn't lie in the 2-d model "not working". Perhaps a 2-d model can't work in our universe due to the other dimensions - we do have depth and time. A "straight line" cannot exist as a constant in a constantly moving universe - of course it would be bent or at least moved and if you try to freeze the model you are again attempting to take away one of our dimensions, consequently this model cannot work in our universe. You could say that there is limited possibility of a constant straight line.

The irony is that we are theoretically travelling through time in a straight line - in a relative manner that is!

Several points:
A good wat of explaining the extra dimensions in string theory, is to imagine a telephone wire. From a distance it looks 1D. Look closer you can see that it is infact a cylindrical 2D shape - you can go clockwise/anticlockwise and forwards/backwards. For more on string theory I recomend "The elegant univese" by Brian Greene.

If the universe contracts, then in theory it will become one huge black hole - all of matter compressed in to an infinitely small point (sound familiar).

Although do black holes not eventually decay?

"We" don't have to worry about the hot or cold death of the universe, or even of our own solar system. It is due to take place longer ahead than homo sapiens has been around so far, which implies that our descendants (if any) are likely to have evolved into something quite unrecognisable to "us". And perhaps by that time we/they will have got over our present addiction to survival, which is only reasonable to the extent that we are driven by our genes, that is to say not entirely reasonable.

Our genes "want" us to survive and reproduce, and they can make us feel bad about the prospect of not surviving for ever, but that doesn't mean we have to agree or buy into that feeling.

"He lives eternally who lives in the present" --Ludwig Wittgenstein A1024156