The bullet thing.

[kpax]

From what I gather the Universe is a certain size and a certain age. People always ask me what would happen if they fired a bullet straight up and the bullet kept going, would it reach the edge of the Universe then stop. This is an age old question which I've heard many times over. I don't know the answer to the question and have always wanted to know. So I'll rephrase it slightly to make it more sensible. Is the Universe shaped like a balloon with the skin of the balloon representing the edge of the Universe and all the air molecules inside representing the matter in it? If so and I were right near to the skin or the edge and I fired a beam of light towards it, what would happen? Would the light make the Universe bigger?

Thanks...kpax

[Jeff RV]

NO, POP!!

I was always lead to believe there is no end, edge. Pass.

Jeff.

[Paxo]

Easy answer is there is no edge to the universe. People who follow Einsteins general theory of relativity talk about a curved space-time continuum.

Cue philosophy debate..........................is it a multi-verse, or multi-dimensional universe, ahhhhh string theory and all that.

Steve

[Baskii]

This analogy is a good way of visualising what happens to objects in an expanding Universe, but it does have limitations.

In this analogy, the three physical dimensions of the Universe are represented by the two-dimensional skin of the balloon. The inside of the balloon doesn't represent anything in the real Universe.

If you choose any point on the balloon's surface (i.e. any point in the Universe) and draw a line in any direction, it never reaches an "edge" - it just keeps going until it ends up back where it started.

There are in fact several different possibilities for the overall "shape" of the Universe. In one of them, like the balloon, if you go far enough in any direction you will eventually return to the place you started from. Another possibility is that the Universe is truly infinite in extent.

Either way though, there is no "edge" to the Universe.

[narrowbandpaul]

The question you have posed, is a question of the geometry of the universe. The geometry, or curvature, is created by the amount of matter per unit volume...the density.

In cosmology, the term, Critical Density is used. It is the density required to produce a spacetime (think of a strecthed sheet of rubber) that is flat. In this case where the density of the universe is equal to the critical density, a projectile fired will travel for infinity and never reach an edge.

If the density is larger than the critical density, then gravity forces the spacetime to curl up like a tennis ball. This universe is called spherical. The universe is not infinite, as it has a finite radius, but a projectile will travel for an infinite amount of time, as it runs round the edge of the tennis ball.

If the density is smaller than the critical density, the universe is said to be open. Its geometry is called Hyperbolic, like the shape of a saddle. Not a lot is known about hyperbolic geometries. There are more than one type of Hyperbolic shapes.

Measurements from WMAP (Wilkinson Microwave Anisotropy Probe) reveal that our universe is very close to being flat. The mathematics of a flat spacetime (Euclidean Geometry...learned at school) hold in our universe.

If your question was really addressing the boundary problem, i.e. does the universe have an edge, then the answer is tricky.

[kpax]

Easy answer is there is no edge to the universe. People who follow Einsteins general theory of relativity talk about a curved space-time continuum.

Cue philosophy debate..........................is it a multi-verse, or multi-dimensional universe, ahhhhh string theory and all that.

Steve

I don't understand. When I look through my scope I see a Universe that has three dimensions in which I can plot stars and other celestrial objects, like looking into a room with objects placed at different points. I see the Sun and other objects placed within the Solar system in a three dimensional space, is the rest of the Universe not like this?

[Baskii]

To borrow a description from Brian Greene...

Imagine an ant walking around on the surface of the balloon. From the ant's perspective, it can go in two independent directions: forward and backward, or left and right. In other words it inhabits a two-dimensional Universe.

Now from our perspective, watching the ant, we can see that the balloon actually has three dimensions, but the ant has no way of visualising this. It's the same for us - we can only visualise our familiar three dimensions, but for the curvature of the Universe to be apparent we would have to somehow stand "outside" it and observe it in higher dimensions.

[Jeff RV]

Easy answer is there is no edge to the universe. People who follow Einsteins general theory of relativity talk about a curved space-time continuum.

Cue philosophy debate..........................is it a multi-verse, or multi-dimensional universe, ahhhhh string theory and all that.

Steve

I don't understand. When I look through my scope I see a Universe that has three dimensions in which I can plot stars and other celestrial objects, like looking into a room with objects placed at different points. I see the Sun and other objects placed within the Solar system in a three dimensional space, is the rest of the Universe not like this?

I would have thought it was the same as what we see, just unchartered space.

Jeff.

[Paxo]

Easy answer is there is no edge to the universe. People who follow Einsteins general theory of relativity talk about a curved space-time continuum.

Cue philosophy debate..........................is it a multi-verse, or multi-dimensional universe, ahhhhh string theory and all that.

Steve

I don't understand. When I look through my scope I see a Universe that has three dimensions in which I can plot stars and other celestrial objects, like looking into a room with objects placed at different points. I see the Sun and other objects placed within the Solar system in a three dimensional space, is the rest of the Universe not like this?

I was about to formulate an answer based on what you see through your telescope being an infinitely small region of the universe but then I read Baskii's post so thanks to him (and Brian Greene) it has been explained very well.

Steve..

[kpax]

To borrow a description from Brian Greene...

Imagine an ant walking around on the surface of the balloon. From the ant's perspective, it can go in two independent directions: forward and backward, or left and right. In other words it inhabits a two-dimensional Universe.

Now from our perspective, watching the ant, we can see that the balloon actually has three dimensions, but the ant has no way of visualising this. It's the same for us - we can only visualise our familiar three dimensions, but for the curvature of the Universe to be apparent we would have to somehow stand "outside" it and observe it in higher dimensions.

So we do have a three dimensional prospective ourselves? That's kind of comforting seeing as though I can actually see it and travel it. So my understanding is that we have a three dimensional sphere in which we all sit with the rest of the matter but it is bent in a way I havn't yet imagined - yes?

[narrowbandpaul]

Hubble (edwin, not the telescope, although it has aswell) showed in the 1920's that the distance between galaxies was increasing. And the rate of increase was proportional to the distance that the galaxy was away.

What is strange however is, if you moved your body to a distant galaxy, say a quasar, that is very distant, and therefore moving away from us very fast, what would you see?

The answer to the question, is that you would see all other galaxies expanding away from you, in exact accordance with Hubbles Law. This is explained by the expanding balloon mentioned earlier. all points expand from all other points in a way governed by hubble's Law.

From this quite remarkable property we can deduce that wherever you are you carry your very own universe, 13.6 Billion light years in extent, centered exactly on you.

This means that if you were to go to the edge of the visible universe, you would still have this 13.6 Billion light year universe surrounding you. For this reason, the universe has no edge, even though the universe came from a point, 13.6 Billion years ago. It has a radius of 13.6 Billion light years but no edge!

Using the same argument, you can show that the universe has no centre, even though it came from a point

hope this answers your question, but its bound to raise a billion more

Paul

[Jeff RV]

It makes sense in a way Paul.

Jeff.

[kpax]

Pardon me for being a bit slow but I'm not picking this up very well. I'm sat here in a three dimensional world where everything seems a huge volume of space and I seem to have the notion naturally that I am inside a ball, like a molecule trapped inside a balloon or a fly inside a huge indoor arena. Where does it change for me visibly for me to deduce that the Universe is a different shape from what I can see?

[narrowbandpaul]

I dont try to understand cosmology, it starts to hurt the brain after a while. Its full of wierd and wonderful, counter intuitive notions

[Paxo]

The anologies you keep raising just don't realise the vast dimensions of the universe and your place in it. There is a very finite relationship between the size of a molecule and a balloon, indeed, a fly and a huge arena, but these are very fixed 3D structures, when we are talking about universal dimensions it is difficult to comprehend the vastness that we are talking about. You could read a dozen texts on the subject and probably get 13 answers to your question, there is still much to debate and understand, there are theories which fit the small amounts of evidence we have, but thats all, theories that fit the evidence we can measure. I'm sure that in time we will come to realise the structure of the universe (probably when we come to understand gravity), in the meantime don't get too worried about not picking these theories up, there are many great minds superior to any of ours who struggle with these notions.

Steve..

[MikeP]

You are not being slow kpax. In my humble opinion, none of us are capable of viewing the universe any differently from the way you describe and by us, I mean all of humanity.

There is a tiny minority who through the use of mathematics are able to speculate on its true nature. They end up with conclusions that make perfect sense in the world of mathematics, but are not capable of being related in non-mathematical terms, which is why we end up with balloons and heavy balls on rubber sheets.

Just my opinion.

Mike

[narrowbandpaul]

the world you are sat in may look 3 dimensional, but thats because the brain has chosen to recognise 3. This is probably due to the fact that the other dimensions are huge, universal in size, or minisculely tiny. Either way, the other dimensions if they exist dont influence us. A flat universe is flat in 3 dimensions...which is hard to get your head around. On the Grand scheme of things the geometry of the universe, its shape, is flat. Perhaps think of the universe as an infinite cube. It is natural to feel that you are at the centre of the universe. but like the balloon analogy, we exist on the surface, but that surface is 3 dimensional. So I suppose then that there must be at least one more dimensional to enclose us

I hope this helps...although the human brain cannot visualise more than the 3 dimensions of common understanding

Paul

[MikeP]

we exist on the surface, but that surface is 3 dimensional. So I suppose then that there must be at least one more dimensional to enclose us

Using that analogy, why do I think that if I were a 4 dimensional being, I would be postulating a 5th dimension. :D

Mike

[narrowbandpaul]

if string theory (the theory of Quantum Gravity) really is true, then our universe has not 4 dimensions (3 in space and one time) but 12...some of which, if not all, are tiny. Tiny looped and interwarped balls of 6 or higher dimensions (from Brian Greenes 1st book?) called calabi-yau spaces...or something like that. Very few minds can comprehend this. I certainly cant...this really is cutting edge theoretical physics.

Even quantum theory requires 11 dimensions...I think

The universe really is a screwed up thing.

But it may be that string theory is wrong, or that quantum thoery is wrong. But there seems consensus that the universe is not showing us all it can.

I believe experiments are being carried out to test the theory of gravity on small scales. They hope to see gravity stop obeying newtons famous inverse square law, and see it fall as some other power of R. We live in a 3d world where gravity falls with R^2. If scientists see gravity falling with R^3, then we have moved into a 4 dimensional world. The way gravity propogates is dirctly related to the number of dimensions through which it propogates

paul

[narrowbandpaul]

Mike that arguement can be extended ad-infinitum...and perhaps it does. perhaps if we could see on large enough scales, we would see dimension after dimension enclosing other dimensions

[Paxo]

If it's any consellation KPax, I carry out research at the other extreme of nature (down at the quantum level i.e structures less than a nanometre ) and it gets just as crazy at that end. IMHO we are best keeping within our comfort zone in the middle somewhere if only to keep our sanity.

Steve..

[MikeP]

I'm sure most of us on here know this one, but for those that don't ....

Big fleas have little fleas on their backs to bite em.

Little fleas have smaller fleas and so on ad infinitum.

Mike

[narrowbandpaul]

what do you research steve? i am intrigued to know

best wishes

Paul

[Baskii]

I'm sure most of us on here know this one, but for those that don't ....

Big fleas have little fleas on their backs to bite em.

Little fleas have smaller fleas and so on ad infinitum.

Mike

...until you reach the Planck scale and the fleas go a bit wibbly.

[narrowbandpaul]

dont want to go there...it all breaks down