Parallel Universes

koolkakool

So what do yee all think about parallel universes? I read about them in books, here's a bit from one of them:

"Far, far away, in a galaxy with a remarkable resemblance to the Milky Way, sits a star that looks remarkably like the Sun. And on that star's third planet, which looks remarkably like the Earth, lives someone who, for all the world, looks like your identical twin. Not only do they look the same as you but they are reading this exact same book - in fact, they are focused on this very line. Actually, it is weirder than this. A whole lot weirder. There is an infinite number of versions of you whose lives, leading up to until this moment, have been absolutely identical to yours. If you think this is pure science fiction, think again. The existence of your doubles is no fantasy. It is an unavoidable consequence of the standard theory of our Universe...."

I suppose its hard to prove they are or aren't there, but what's your opinion?

barnicles

I think the idea is a heap of shìt

Michael Collins

This results from one possible interpretation of Quantum Mechanics, but at the moment there's no way to prove which interpretation correctly informs our understanding of nature or indeed if there is an underlying reality as we know it.

Kevster

I favour barnicles' view. I just finished reading Brian Greene's The Fabric of the Cosmos and he mentioned them in great detail. My feeling on it all is that many cosmologists dream up these concepts in their heads and then spend years developing maths to 'prove' that they could exist. It's nice, and keeps cosmology relatively interesting to the layman, but it holds no substance with me.

Take care,
Kevin.

bogwalrus

i believe the idea of paralell universes came from the results of experiments such as the double slit experiment showing that photons could detect instantaneously whether an apparatus was measuring it or not, (acting as a wave when not being measured and acting as matter when being "observed").

It was called the many worlds theory but dont think it is called that anymore. i believe the theory that took over from it was transduction theory where instead of there being paralell universes there was instantaneous signalling.

if someone knows more about what i mentioned above i would love to hear it.
i have only heard about transduction theory once about how there is a signal sent from the particle back in time (or in zero time) to contact off another particle and they recipricate with a signal that cancels out all parts of the original signal except the one on the direct path.......and so on.

Prof fink maybe?:D

Professor_Fink

bogwalrus wrote: »

if someone knows more about what i mentioned above i would love to hear it.
i have only heard about transduction theory once about how there is a signal sent from the particle back in time (or in zero time) to contact off another particle and they recipricate with a signal that cancels out all parts of the original signal except the one on the direct path.......and so on.

Prof fink maybe?:D

You called?

Basically what you are talking about is known, as you say, as the many worlds interpretation of quantum mechanics, or more often as the Everett interpretation. This is currently probably the most widespread interpretation of quantum mechanics out there (at least among theorists), followed closely with the Copenhagen interpretation. The transactional interpretation (not transductional) is something of a curiosity, and doesn't have much of a following. It is also questionable whether the transactional interpretation is a valid interpretation of quantum mechanics, as it may offer predictions which differ from standard quantum mechanics.

So what are interpretations, and why do we have them? Well, basically quantum mechanics is odd. It predicts (and we have verified experimentally) superposition states, which are a linear combination of classical states. Now a problem arises: Why do we see the world as classical rather than in superpositions? Well, in the standard formulation of quantum mechanics we have events called "measurements" which collapse superpositions into classical states according to some probability distribution. These measurements are non-unitary (they are not reversable and preserve orthogonality), but quantum mechanics allows for only unitary evolution. Interpretations of quantum mechanics usually arise when considering what measurements really are (known as the measurement problem).

In the Copenhagen interpretation measurement devices are special, and do not obey unitary evolution, allowing for collapse of the wavefunction. The main criticism of this interpretation is that it is not clear what constitutes a measurement device, and so where to draw the line is something of a mystery.

In 1957, Hugh Everett showed that you don't need to draw the line anywhere, and that if we treat everything as quantum mechanical, with no measurements, we observe exactly the same behaviour as in the Copenhagen interpretation. Here the illusion of measurement is caused by us becoming entangled with whatever it is we are trying to measure. The nice thing is that this removes the need for any special rules for measurements, which would sureley please William of Ockham. This does however lead to the conclusion that we experience just one branch of the wave function, with other equally real versions of the universe in other branches which are and will always be inaccessable to us. This has lead to the name "many worlds interpretation", but this is somewhat misleading as these other branches are forever lost to us and do not constitute a different "place". These branches are not places you can go, merely a kind of physical relic of "what could have been". (Sorry to get all wish washy here)

I should point out that all valid interpretations must predict the same results for any experiments. None is in any way more true than the other, they are just different ways of interpretting the maths.

As I said, the transactional theory is a little messy, because it tries to explain away entanglement via time travel. It's gotten a fair amount of media coverage recently, mostly due to claims it may actually differ from quantum mechanics in a testable way, but remains one of the fringe interpretations.

Hope this helps.

bogwalrus

you just saved me weeks of googling. And grats for correction, i knew it had "trans" in the name somewhere.

So in your view professor fink, which interpretation do you think is closest to the what realy happens in the quantum world? Or does it even matter? as you say its the mathematics that are important and the interpretations are just analogeous.

Gurgle

koolkakool wrote: »

"... The existence of your doubles is no fantasy. It is an unavoidable consequence of the standard theory of our Universe...."

I suppose its hard to prove they are or aren't there, but what's your opinion?

Staying away from quantum stuff, heres another way to look at it:

The universe is infinite
=> There are an infinite number of planets, solar systems and galaxies
=> There is another galaxy which is absolutely identical to ours, down to the spin on the innermost atom of the fourth carbon atom to the left of your medula oblongata
=> There is another galaxy which is identical in every way except for the spin on that electron

Actually, a galaxy must exist with every variation & permutation of every possibility.


The only way for the above not to be true would be if the universe wasn't infinite.
Even thinking about that will take you into the realm of 'ahtofùckwithit' science.

Professor_Fink

Gurgle wrote: »

Staying away from quantum stuff, heres another way to look at it:

The universe is infinite

Says who? We have no evidence that this is the case.

Gurgle wrote: »

=> There are an infinite number of planets, solar systems and galaxies
=> There is another galaxy which is absolutely identical to ours, down to the spin on the innermost atom of the fourth carbon atom to the left of your medula oblongata
=> There is another galaxy which is identical in every way except for the spin on that electron

Actually, a galaxy must exist with every variation & permutation of every possibility.


The only way for the above not to be true would be if the universe wasn't infinite.

Not so. Even if the universe was infinite there are many reasons why your statements may would not follow. Here is a sample one: The universe is made of particles, and since these are discrete there is at most a countable infinity of particles (i.e. equal to the number of integers). The phase space in which these particles can be arranged is continuous, and hence uncountable (i.e. equal to the number of reals). Since the number of configurations is greater than the number of systems to which the configuration can be applied there is by no means "every variation & permutation of every possibility."

Professor_Fink

bogwalrus wrote: »

So in your view professor fink, which interpretation do you think is closest to the what realy happens in the quantum world? Or does it even matter?

I find the Everett interpretation to be the most aesthetically pleasing, since it doesn't require extra rules for measurements. Since it seems the most elegant solution part of me feels that this is somehow what nature would do. On the other hand, I realise that these are all indistinguishable and so amount to philosophy rather than physics. I'm not convinced it is meaningful to even ask if any of the interpretations is true.

Gurgle

Professor_Fink wrote: »

Says who? We have no evidence that this is the case.

Because there is no mathematically possible way for the universe to be finite. There is no physical way for a boundary to exist, inside of which is the universe, and outside of which...... isn't

Even if it turns out that the spacetime we know is a 5 dimensional sphere, there is still more universe outside that sphere.

Professor_Fink wrote: »

Not so. Even if the universe was infinite there are many reasons why your statements may would not follow. Here is a sample one: The universe is made of particles, and since these are discrete there is at most a countable infinity of particles (i.e. equal to the number of integers).

There is no room in my concept of existence for the phrase 'countable infinity'. Thats a contradiction.

I know there is a certain trend to use the word 'universe' to refer to a cluster of galaxies, and the word 'infinite' to mean lots. I'm using the words in their original meaning.

koolkakool

correct me if i'm wrong, but isnt the universe expanding. and doesent the universe have to have a finite size if it's getting bigger. if it's infinite it cant really expand because it already is infinite. as i said, correct me if i'm wrong, i dont really know much about this whole area, which is partly why i made this topic

Professor_Fink

koolkakool wrote: »

correct me if i'm wrong, but isnt the universe expanding. and doesent the universe have to have a finite size if it's getting bigger. if it's infinite it cant really expand because it already is infinite. as i said, correct me if i'm wrong, i dont really know much about this whole area, which is partly why i made this topic

Well, every point in space is getting further away from every other space, so we can safely say the universe is expanding. On the other hand, this is still possible for an infinitely sized universe (essentially the Hilbert's hotel paradox). Basically we don't know the real size of the universe. On the other hand, the visible universe (what we can detect) is getting bigger all the time, although this is largely down to the finite speed of light.

Professor_Fink

Gurgle wrote: »

Because there is no mathematically possible way for the universe to be finite. There is no physical way for a boundary to exist, inside of which is the universe, and outside of which...... isn't

I'm afraid that what you have just said is complete nonsense. Most cosmological models are finite. Mathematical spaces can have non-trivial topologies. For example you can think of a one dimensional space with periodic boundaries. It forms a ring, but there is no second dimension into which you can move. Space-time is the same.

Gurgle wrote: »

Even if it turns out that the spacetime we know is a 5 dimensional sphere, there is still more universe outside that sphere.

Except that the 5th dimension in which it is curved doesn't really exist. You can't move into it. The universe is simply the object, and the notion of space is only meaningful within the universe.

Gurgle wrote: »

There is no room in my concept of existence for the phrase 'countable infinity'. Thats a contradiction.

Then you need to update your concept of existence. Try reading this.

Gurgle wrote: »

I know there is a certain trend to use the word 'universe' to refer to a cluster of galaxies, and the word 'infinite' to mean lots. I'm using the words in their original meaning.

I haven't noticed any such trends, and if your implying that I don't know what infinite or universe means then you may want to ask yourself how I knew what a countable infinity was.

Gurgle

Professor_Fink wrote: »

Except that the 5th dimension in which it is curved doesn't really exist. You can't move into it.

When I referred to a 5 dimensional sphere, the 5th dimension is required to join the 4 linear dimensions of spacetime into a sphere. You can't move into it, it joins the boundaries of the others.

Professor_Fink wrote: »

if your implying that I don't know what infinite or universe means then you may want to ask yourself how I knew what a countable infinity was.

Not in the slightest, I'm trying to conceptually separate the maths from the physics. 'Infinity' is easy to use in maths, difficult to translate to a physical scale.

Georg Cantor showed that the number of real numbers is rigorously larger than a countably infinite set

If the number of real numbers is larger than a countably infinite set, then in an infinite universe, the number of particles is also larger than a countably infinite set.

Professor_Fink

Gurgle wrote: »

If the number of real numbers is larger than a countably infinite set, then in an infinite universe, the number of particles is also larger than a countably infinite set.

Nope. You can't have 1.333534235356... particles. They come in integers.

koolkakool

OK so, one more question...

What do you think of string theory and the 10 dimensions?

Professor_Fink

koolkakool wrote: »

OK so, one more question...

What do you think of string theory and the 10 dimensions?

Well, the dimensionality of spacetime just sort of drops out of the maths in string theory. In order for photons to have the properties they do either 11 or 26 dimensions are required, depending on whether it is bosonic or superstring theory. Whether the assumptions of string theory are valid or not is another question.

Note that these extra dimensions would be compactified (i.e. shrunk), so we wouldn't really experience them. It's actually an idea that has some presedence: Kaluza-Klein theory explains electromagnetism using a fifth (very small) dimension.

tatsel24

Just a quote form a recent Q&A with Michio Kaku he states that parallel, or multible universes may be possible. "The latest cosmological data comes from the WMAP satellite currently orbiting the earth. The data is consistent with “inflation,” i.e. the idea that the universe began in a super turbo-charged expansion at the instant of the big bang. However, inflation is also a quantum theory, i.e. there is a finite probability that if inflation happened once, it can happen again, and again. In fact, big bangs may be happening all the time, even as you read this sentence. There may be a continual creation of universes".

I myself, can see how this may be possible, lets look at this from the big bang theory, it sugests a piont of origen and as Dr kaku (and the bulk of the science community) states, inflation will support this, the galaxies are flying apart and so on. but as with any point of origin, there is preasure bearing down on it from an out side force, be it an atmosphere etc, when any point of enegry expands rapidly, that external force is forced back, meaning that the external force, which may have compressed this enegry into a super-dence mass ( creation of a black hole) must be dispersed away from the origin point. so what lies beyond the "edge" of the known universe, inflation is slowing down, instead of accelerating as you would expect enegry to do if it was un-hinderd.

an example would be ripples in a pond, toss a stone into the pond and it causes ripples, which will continue to expand untill the original enegery transferd fades, but if you throw another stone in a second after the first, the ripples will collide, and the ripples will disperse and slow down, a lot faster, so why not the universe, if multiple big bangs were to happen, this could create the "ripple" effect that slows inflation, the universe is hardly contained within a vacum, nature appor's a vacum, but a point of origin suggests that the big bang was contained under a pressure of sorts.

Professor_Fink

tatsel24 wrote: »

I myself, can see how this may be possible, lets look at this from the big bang theory, it sugests a piont of origen and as Dr kaku (and the bulk of the science community) states, inflation will support this, the galaxies are flying apart and so on. but as with any point of origin, there is preasure bearing down on it from an out side force, be it an atmosphere etc, when any point of enegry expands rapidly, that external force is forced back, meaning that the external force, which may have compressed this enegry into a super-dence mass ( creation of a black hole) must be dispersed away from the origin point. so what lies beyond the "edge" of the known universe, inflation is slowing down, instead of accelerating as you would expect enegry to do if it was un-hinderd.

It certainly is not the consesus view that big bangs keep happening and Kaku seems to go for the sensationalist angle. What you have said above captures the spirit of what is expected, but its factually incorrect in essentially all of the details.

Firstly, there is no point of origin. Space keeps getting bigger, essentially expanding the distance between points. It's like being on the surface of a balloon as it is being inflated. Even though there is more surface area when the balloon is full, there isn't an original part and a new part, it has just been stretched. Spacetime is basically the same.

Secondly, inflation is an observed fact. Galaxies are on average moving away from one another. We can measure this, as the colour of light emited from an object is slightly altered depending on its velocity.

Thirdly there are no external forces. What should slow down inflation is gravity: Every galaxy and every star, every planet and even every atom attracts all of the others via its gravitational field. This field is very weak, so it is only noticable on large scales (i.e. moons or planets), but it is still there, and should be slowing inflation.

Fourthly, inflation is not slowing, as you would expect, but rather accelerating. This is really puzzling and has lead a number of people to propose a background curvature on the universe. Even if there were no forces acting we would not expect the growth of the universe to accelerate! F=MA and all that.

tatsel24 wrote: »

the universe is hardly contained within a vacum, nature appor's a vacum, but a point of origin suggests that the big bang was contained under a pressure of sorts.

The universe isn't contained with in anything, as far as we can tell. Space has no meaning outside of the universe, and in particular this negates your vacuum argument.

norrie rugger

Professor_Fink wrote: »

Space has no meaning outside of the universe, and in particular this negates your vacuum argument.

What about if you are using the "space" to measure a distance between our universe and its neighbour?

Professor_Fink

norrie rugger wrote: »

What about if you are using the "space" to measure a distance between our universe and its neighbour?

You can't. There is no meaningful measure of distances between disconnected spacetimes.

corkstudent

Gurgle wrote: »

Staying away from quantum stuff, heres another way to look at it:

The universe is infinite
=> There are an infinite number of planets, solar systems and galaxies
=> There is another galaxy which is absolutely identical to ours, down to the spin on the innermost atom of the fourth carbon atom to the left of your medula oblongata
=> There is another galaxy which is identical in every way except for the spin on that electron

Actually, a galaxy must exist with every variation & permutation of every possibility.


The only way for the above not to be true would be if the universe wasn't infinite.
Even thinking about that will take you into the realm of 'ahtofùckwithit' science.

Probably because it's bollocks.

If the universe was infinite, then the night sky would be white, as every line of sight would end at a star. More than likely it's just curved, like the earth.

Flamed Diving

norrie rugger wrote: »

What about if you are using the "space" to measure a distance between our universe and its neighbour?

norrie rugger (head)?

Professor_Fink

corkstudent wrote: »

Probably because it's bollocks.

If the universe was infinite, then the night sky would be white, as every line of sight would end at a star. More than likely it's just curved, like the earth.

I'm afraid your argument is also incorrect. Because the speed of light is finite and the age of the universe is finite, we only get light from stars within a sphere of about 13.6 billion light years. The universe could be infities, bur the observable part is most definitely finite.

norrie rugger

Professor_Fink wrote: »

I'm afraid your argument is also incorrect. Because the speed of light is finite and the age of the universe is finite, we only get light from stars within a sphere of about 13.6 billion light years. The universe could be infities, bur the observable part is most definitely finite.

I think that we need to define "Universe" here.

If the universe started a "big bang" and started to expand, then it can not be infinite.
Go further out beyond the bounderies of the modern remenants of that starting point, do you "leave" the universe.

Beyond that there could well be infinity


So what I am asking is
Is the universe the material that constituted the big bang and the space that it occupies/exerts influence on today.
Or is it that and anything beyond this boundry.

Professor_Fink

norrie rugger wrote: »

I think that we need to define "Universe" here.

Simply put, the space-time manifold on which we exist.

norrie rugger wrote: »

If the universe started a "big bang" and started to expand, then it can not be infinite.

l'm afraid that doesn't follow. Space-time is not an object that can be easily reasoned about without resorting to mathematics. The universewasn't necessarily point Sized at the big bang, althoughit is true that our visible region occupied a very small region.

norrie rugger wrote: »

Go further out beyond the bounderies of the modern remenants of that starting point, do you "leave" the universe.

Beyond that there could well be infinity

You can't move off the universe. Space simply doesn't exist beyond it.

norrie rugger wrote: »

So what I am asking is
Is the universe the material that constituted the big bang and the space that it occupies/exerts influence on today.
Or is it that and anything beyond this boundry.

A boundary only even exists if you imbed the universe in ahigher dimensional space, which we have no reason to do except when trying to explain it to the public using (inaccurate) analogies.

norrie rugger

Professor_Fink wrote: »

l'm afraid that doesn't follow. Space-time is not an object that can be easily reasoned about without resorting to mathematics. The universewasn't necessarily point Sized at the big bang, althoughit is true that our visible region occupied a very small region.

OK please forgive my constant questions.

OK so our visible region of the universe was point sized, at the start, and this is expanding. Can one, theoretically, move beyond this matter/energy that was at this beginning and still be within our universe?

If so, and taking that our universe is infinite, can there be an infinite number of expanding regions in the universe that match our own?

Professor_Fink

norrie rugger wrote: »

OK please forgive my constant questions.

No Problem.

norrie rugger wrote: »

OK so our visible region of the universe was point sized, at the start, and this is expanding.

It was very small, but there is a limit to how far back we can see. Whether it was actually an infinitely small point is another question, and one which we don't really know the answer to.

norrie rugger wrote: »

Can one, theoretically, move beyond this matter/energy that was at this beginning and still be within our universe?

Well the gaps in between matter still count as part of the universe. There are areas of the universe with very very low amounts of matter, and it is certainly theoretically possible to go to one of these (although due to the finite speed of light it will have changed since we saw it). Moving outside of space-time is impossible though, essentially by definition.

norrie rugger wrote: »

If so, and taking that our universe is infinite, can there be an infinite number of expanding regions in the universe that match our own?

Well, we don't know that it is infinite. Because space is continuous there is an infinite number of points in any space, infinite or not. To get around this, lets break the universe up into boxes of finite size. In that case, if the universe was infinite then yes, there could be an infinite number of these boxes expanding just like the ones near us.

Gurgle

corkstudent wrote: »

If the universe was infinite, then the night sky would be white, as every line of sight would end at a star. More than likely it's just curved, like the earth.

Only if the photon was infinitely small.
Which it isn't.

If the universe is curved, like the earth, then its not The Universe.
In that case there is an inside and an outside.

We're on the inside (our 'universe') on the outside is.... the rest of The Universe.:D

Professor_Fink

Gurgle wrote: »

If the universe is curved, like the earth, then its not The Universe. In that case there is an inside and an outside.

We're on the inside (our 'universe') on the outside is.... the rest of The Universe.:D

I'm afraid not. The surface of the Earth is a 2D space embeded in 3D space (or 2+1D embeded in 3+1 dimensions if you care about time). The universe isn't imbeded in anything, at least not as far as we know, so curved or not there is not "outside". There is just the universe, curved and all as it is.

the GALL

Gurgle...... you the Man

That has to be one of the best and to me logical explanations ive heard in a while

Professor_Fink

the GALL wrote: »

Gurgle...... you the Man

That has to be one of the best and to me logical explanations ive heard in a while

It does however have the drawback of being incorrect.

And if you mean about why the sky isn't white, well you have the finite speed of light and the finite age of the universe to thank for that, not the size of the photon.

Gurgle

Professor_Fink wrote: »

norrie rugger wrote:

I think that we need to define "Universe" here.

Simply put, the space-time manifold on which we exist.

^^ This is pretty much the point where our opinions separate.

I define 'The Universe' as everything, in all directions, forever.
I don't like the use of the word universe to refer to just this little 13.6 billion year old cluster of galaxies we happen to live in.

On the scale of The Universe, our so-called 'universe' is a puff of smoke from big-bang to ultimate entropy.

Professor_Fink wrote: »

inflation is not slowing, as you would expect, but rather accelerating. This is really puzzling and has lead a number of people to propose a background curvature on the universe. Even if there were no forces acting we would not expect the growth of the universe to accelerate! F=MA and all that.

F=MA is for meso-macro scaled stuff.
You need (special?) relativity when your talking about galaxies floating around the place.

Lets drag Mr Einstein into it, simplified to the level at which I can grasp it:
Gravity is not a force, its the shape of the universe. (I've argued the toss with Son Guko on this one, he eventually managed to convince me).
Gravity goes on forever. Really forever.

If The Universe truly goes on forever, with infinite clusters of galaxies (:o lets call them 'universes'), and is infinitely old, then the overall sum of gravitational space curvature at the location of our little 'universe' pulls us towards the gravity of the others, not to the centre of our own 'universe'.

So it will continue to expand for another few billion / trillion / quadrillion / whatever years until an individual galaxies ultimately meets other galaxies from other 'universes'. These form another 'universe' system, with decaying orbits (as they haven't got escape velocity from each other from a big bang)

This new 'universe' collapses until enough matter reaches the centre, forms a black hole, sucks the rest in......

*BANG*

Podge_irl

Gurgle wrote: »

^^ This is pretty much the point where our opinions separate.

I define 'The Universe' as everything, in all directions, forever.
I don't like the use of the word universe to refer to just this little 13.6 billion year old cluster of galaxies we happen to live in.

On the scale of The Universe, our so-called 'universe' is a puff of smoke from big-bang to ultimate entropy.

Why does "our" universe have to be embedded in anything larger? As intuitively nice as it may seem there is no mathematical necessity for it.

Professor_Fink

Podge_irl wrote: »

Why does "our" universe have to be embedded in anything larger? As intuitively nice as it may seem there is no mathematical necessity for it.

Thanks, that's what I've been trying (apparently ineffectually) to get across.

Professor_Fink

Gurgle wrote: »

^^ This is pretty much the point where our opinions separate.

I define 'The Universe' as everything, in all directions, forever.
I don't like the use of the word universe to refer to just this little 13.6 billion year old cluster of galaxies we happen to live in.

Apparently our opinions diverge earlier, since I thought we were having a discussion rooted in the facts, rather than opinions.

Fortunately, however, our definitions coincide. The oinly problem is that there isn't necessarily an infinite amount of space, and what space there is can have a non-trivial curvature without implying a new dimension.

I do realise that this is extremely counter-intuitive, since we only have direct experience of Euclidean space, and so have no intuition about curved or topologically non-trivial spaces.

On the other hand, it happens to be a more accurate discreption of reality.

I'll try to give you an example of such a space. Consider angles 0-360 degrees. Forget about what they actually represent and just consider the numbers as a 1d space. This space has a finite size: there is only a finite range (from 0 to 360). On the other hand you can travel infinitely far in either direction, wrapping around and around the line. Now imagine the questions you are asking here: What's outside of the line? What dimension is the line curved in? None of these make sense in the context of that space. There is nothing beyond the number line, and there is no extra dimension in which it is curved. The same is true of our universe.

Gurgle wrote: »

F=MA is for meso-macro scaled stuff.
You need (special?) relativity when your talking about galaxies floating around the place.

Actually, not if the galaxies are separated by large distances and moving relatively slowly in relation to one another.

Basic rule:
Super small or super cold: use quantum mechanics
Super fast: use special relativity
Super heavy: use general relativity
Otherwise Newtonian mechanics tends to suffice.

Gurgle wrote: »

Lets drag Mr Einstein into it, simplified to the level at which I can grasp it:
Gravity is not a force, its the shape of the universe. (I've argued the toss with Son Guko on this one, he eventually managed to convince me).
Gravity goes on forever. Really forever.

Certainly in general relativity it is regarded as the curvature of the universe, but we don't really know for certain since we don't yet have a proper quantum theory of gravity.

Gurgle wrote: »

If The Universe truly goes on forever, with infinite clusters of galaxies (:o lets call them 'universes'), and is infinitely old, then the overall sum of gravitational space curvature at the location of our little 'universe' pulls us towards the gravity of the others, not to the centre of our own 'universe'.

Wow, wow, wow, wow, wow... A cluster of galaxies is just that. A cluster.

So yes, the gravity from galaxies and clusters attract other galaxies and clusters. But the sum should be zero, since the universe is isotropic on a large scale. It has the effect of pulling 'space' closer together. Imagine letting the air out of a balloon. Every part is pulled towards every other part and the balloon shrinks.

Let me make this perfectly clear - There is no centre of the universe!
But let me be a little kinder and assume you have simply misused universe to refer to the local group, or our galaxy or something. The gravitational force due to other clusters is tiny on the scale of the gravitational force within our galaxy, so if moves the galaxy as a whole, but does not noticably effect the internal dynamics of the galaxy. Our galaxy doesn't get pulled appart, but rather moved en mass.

Gurgle wrote: »

So it will continue to expand for another few billion / trillion / quadrillion / whatever years until an individual galaxies ultimately meets other galaxies from other 'universes'. These form another 'universe' system, with decaying orbits (as they haven't got escape velocity from each other from a big bang)

This new 'universe' collapses until enough matter reaches the centre, forms a black hole, sucks the rest in......

*BANG*

Huh? This is simply incorrect. Your use of 'universe' here is very odd and very incorrect. The galaxies are flying apart from one another, not toward one another.

Certainly supermassive blackholes exist within galaxies and clusters, and they will indeed be followed by a bang (if the universe lives long enough), since we expect them to decay via Hawking radiation which has a pretty violent end.

I think what you are trying to get at though is the idea of a "big crunch", a sort of reversal of the big bang, when all matter is sucked back together by gravity. Our measurements suggest that the universe is not sufficiently massive for this to happen, although it is certainly a valid cosmological model.

But I don't know what the point of bringing this up is. It doesn't imply some kind of exterior to our universe, only a finite time scale. So what?

Gurgle

Professor_Fink wrote: »

Apparently our opinions diverge earlier, since I thought we were having a discussion rooted in the facts, rather than opinions.

Can you honestly put your hand on your heart and tell me that there is conclusive, incontravertible evidence that we live in a finite universe of the nature you describe?

tbh, even 'opinion' is too strong a word; its all theories, concepts, notions, ideas

Professor_Fink wrote: »

The oinly problem is that there isn't necessarily an infinite amount of space, and what space there is can have a non-trivial curvature without implying a new dimension.

Yes, absolutely.
Thats one of the possible shapes of the universe which has not been disproved.

Professor_Fink wrote: »

I'll try to give you an example of such a space. Consider angles 0-360 degrees. Forget about what they actually represent and just consider the numbers as a 1d space.

I'm afraid I can't do that.
Angles are'nt a position, a distance or a vector. They define an artificially discretised relative direction in 2D. My brain can't use your analogy.

Professor_Fink wrote: »

Certainly in general relativity it is regarded as the curvature of the universe, but we don't really know for certain since we don't yet have a proper quantum theory of gravity.

Its my understanding that general relativity rules out quantum gravity, and that there is no evidence of this being wrong.



Wow, wow, wow, wow, wow... A cluster of galaxies is just that. A cluster.

Professor_Fink wrote: »

the gravity from galaxies and clusters attract other galaxies and clusters. But the sum should be zero

Only at the centre....

Professor_Fink wrote: »

Let me make this perfectly clear - There is no centre of the universe!

Professor_Fink wrote: »

The gravitational force...

its not a force

Professor_Fink wrote: »

The gravitational force due to other clusters is tiny on the scale of the gravitational force within our galaxy, so if moves the galaxy as a whole, but does not noticably effect the internal dynamics of the galaxy. Our galaxy doesn't get pulled appart, but rather moved en mass.

yes...

Professor_Fink wrote: »

The galaxies are flying apart from one another, not toward one another.

In an infinite universe, with infinite mass, they are ultimately flying towards other galaxies which are flying towards them.

Basically, what I'm trying to say is that the BANG is not a truly universal bang - its quite localized and small scale really. It happens all the time, but separated by distances measurable

Professor_Fink wrote: »

I think what you are trying to get at though is the idea of a "big crunch", a sort of reversal of the big bang, when all matter is sucked back together by gravity. Our measurements suggest that the universe is not sufficiently massive for this to happen, although it is certainly a valid cosmological model.

Gravity does not propagate instantly, it propagates at the speed of light (checked & measured; look up gravity waves from binary systems).

If an entire finite universe was launched at the speed of light from the Big Bang, then it would continue expanding at the speed of light forever. Gravity would never 'catch up'.

But there is acceleration, which would not be possible if the universe was already expanding at the speed of light. So the big bang was a finite amout of energy applied to finite mass.

Earlier, you attribute this acceleration to space time curvature, but by general relativity the entire shape and curvature of space-time is gravity.

So this leads me to conclude that there is mass outside of the sphere of the big bang.

Professor_Fink

Gurgle I'm really trying, I am, but you are not making this easy.

Gurgle wrote: »

Can you honestly put your hand on your heart and tell me that there is conclusive, incontravertible evidence that we live in a finite universe of the nature you describe?

Reread my posts in this thread. I have maintained that we do not know whether the universe is infinite or finite yet.

Gurgle wrote: »

tbh, even 'opinion' is too strong a word; its all theories, concepts, notions, ideas

Are you implying that scientific theories are somehow less important than your opinion? That tends to be the creationists favorite argument: "But it's only a theory". In science theory essentially means fact, except that we are aware that it might be possible to give an even better description. The theory of general relativity doesn't admit itself to postmodernism. To butcher a Colbert remark, reality has a well factual bias.

Gurgle wrote: »

Thats one of the possible shapes of the universe which has not been disproved.

Cosmologists don't simply enumerate possible shapes for the universe. The scour star catalogues and the CMB for evidence. Seriously, just pick up any modern astrophysics textbook.

Gurgle wrote: »

I'm afraid I can't do that.
Angles are'nt a position, a distance or a vector. They define an artificially discretised relative direction in 2D. My brain can't use your analogy.

Well, that might be the problem. The space I described was perfectly valid. I asked you to forget about any interpretation of what angles actually were, but you insist in including it. The only relevant property is that angles wrap around to 0 at 360 degrees. If you can't imagine such a system, then you haven't much hope of imagining a non-trivial spacetime. That's not meant as an insult, I simply mean that I don't know how to communicate to you the idea of a finite space, particularly if it may have periodic boundary conditions. You'll just keep imagining it embeded in higher dimensions.

Gurgle wrote: »

Its my understanding that general relativity rules out quantum gravity, and that there is no evidence of this being wrong.

General relativity does not quantize in the standard way we quantize a field. So much the worse for general relativity. The very existence of quantum mechanics implies that something fishy is going on in general relativity when you get to small enough scales. And before you start telling me the problem is with quantum mechanics I should mention that QM has been tested to a far higher degree of accuracy than general relativity.

Gurgle wrote: »

Only at the centre....

Eh.....no. There is no centre!

Gurgle wrote: »

its not a force

Stop being pedantic. It depends entirely on your perspective.

Gurgle wrote: »

In an infinite universe, with infinite mass, they are ultimately flying towards other galaxies which are flying towards them.

No! This is your most fundamental mistake. We know for absolute certain that the space between galaxies is getting bigger. The universe is getting bigger. Ever heard of Hubble? A lot of his fame was due to measuring the rate at which these are moving apart. If the universe was shrinking, which it isn't, then they would be becoming closer as the space between them shrinks.

Gurgle wrote: »

Basically, what I'm trying to say is that the BANG is not a truly universal bang - its quite localized and small scale really. It happens all the time, but separated by distances measurable

Your making a fundamental mistake in thinking that we don't understand anything about cosmology. We do. We have a really detailed of everything that has happened to the universe since the CMB decoupled.

Gurgle wrote: »

Gravity does not propagate instantly, it propagates at the speed of light (checked & measured; look up gravity waves from binary systems).

If an entire finite universe was launched at the speed of light from the Big Bang, then it would continue expanding at the speed of light forever. Gravity would never 'catch up'.

But there is acceleration, which would not be possible if the universe was already expanding at the speed of light. So the big bang was a finite amout of energy applied to finite mass.

That's simply nonsense. First of all, while we certainly expect gravitational waves from such systems we haven't yet managed to detect them.

Massive objects simply cannot travel at the speed of light. Weirdly though the speed of light does not impose limits on how fast space can expand or contract (for much the same reason as the speed of light doesn't limit the speed of a shadow).

Further, the galaxies are moving apart from one another, but at less than the speed of light. so there is a gravitational interaction there.

Gurgle wrote: »

Earlier, you attribute this acceleration to space time curvature, but by general relativity the entire shape and curvature of space-time is gravity.

So this leads me to conclude that there is mass outside of the sphere of the big bang.

Well then your conclusion is incorrect.

Tzetze

Very interesting thread. I have a couple of questions, Professor_Fink, if you don't mind?

Professor_Fink wrote:

First of all, while we certainly expect gravitational waves from such systems we haven't yet managed to detect them.

I've read a little about the LIGO experiment. Correct me if I'm wrong here, but from my understanding it measures the time taken for two perpendicular lasers to bounce back at a distance of (a few?) kilometres. When a gravitational wave (from a binary pulsar) hits the laser tunnels, there should be a variation in the time taken for the laser to bounce back.

I imagine that these waves, having originated at huge distances from Earth, would have a very wide area of effect when hitting us? Would the curve of these waves be so slight at this distance that they would be indistinguishable from a straight line? What I'm really trying to ask is; if the entire apparatus is affected by a grav wave is it possible that we can never see a variation in laser travel time?

Professor_Fink wrote:

Massive objects simply cannot travel at the speed of light. Weirdly though the speed of light does not impose limits on how fast space can expand or contract (for much the same reason as the speed of light doesn't limit the speed of a shadow).

Further, the galaxies are moving apart from one another, but at less than the speed of light. so there is a gravitational interaction there.

When you say the galaxies are moving apart from each other at less than the speed of light, does this refer to motion separate from the expansion of space between these galaxies?

I have read about the Hubble constant being equal to ~71 km/sec/megaparsec. In the same article, it goes on to say that galaxies at a distance of 4200 megaparsecs from each other are moving apart from each other at a rate faster than the speed of light.

This seems to counteract what you were saying above. What am I missing here?

Podge_irl

Tzetze wrote: »

I've read a little about the LIGO experiment. Correct me if I'm wrong here, but from my understanding it measures the time taken for two perpendicular lasers to bounce back at a distance of (a few?) kilometres. When a gravitational wave (from a binary pulsar) hits the laser tunnels, there should be a variation in the time taken for the laser to bounce back.

I imagine that these waves, having originated at huge distances from Earth, would have a very wide area of effect when hitting us? Would the curve of these waves be so slight at this distance that they would be indistinguishable from a straight line? What I'm really trying to ask is; if the entire apparatus is affected by a grav wave is it possible that we can never see a variation in laser travel time?

The LIGO lasers are several kilometres apart, though the effective length is increased by bouncing the lasers back and forth a number of times. Its unlikely they would detect gravitational waves from anything below a cataclysmic event though. A more likely source for their detection is LISA (whenever its operational), as that interferometer is located in space and the lasers/detectors are several million kilometres apart. There is also less terrestrial interference.

The distance these events take place from Earth doesn't have any bearing on their area of effect. Gravitational waves tend to be of exceptionally low frequency though. The waves oscillate in a particular plane though and this is how the interferometer works. There are two arms stretching off at right angles to each other and the gravitational wave will affect space differently in these two different directions and its this difference that we detect.

Gurgle

Professor_Fink wrote: »

Are you implying that scientific theories are somehow less important than your opinion?

I'm enjoying the astrophysics discussion, but please don't go out of your way to insult my intelligence.

Professor_Fink wrote: »

Cosmologists don't simply enumerate possible shapes for the universe. The scour star catalogues and the CMB for evidence. Seriously, just pick up any modern astrophysics textbook.

I do, I know and I have.
I get it, I'm not arguing with the evidence or the physics.
My points and comments are all based on what could be, should be or must be further out than anything that has been directly observed.

Professor_Fink wrote: »

I don't know how to communicate to you the idea of a finite space, particularly if it may have periodic boundary conditions. You'll just keep imagining it embeded in higher dimensions.

I'm not going for higher dimensions, I understand periodic boundary conditions.

My suggestion (is that word ok?) is that within our infinite 4D euclidean spatial dimensions, but ridiculous distances away, there is matter and energy that were not even involved in the big bang from which our planet, solar system, galaxy and all the neighbouring galaxies within the nearest ~13.6 billion light years were expelled.

Professor_Fink wrote: »

General relativity does not quantize in the standard way we quantize a field. So much the worse for general relativity.

But this is my point: General relativity describes how mass shapes space without applying a force.

Professor_Fink wrote: »

the problem is with quantum mechanics I should mention that QM has been tested to a far higher degree of accuracy than general relativity.

Quantum mechanics fits in nicely with general relativity.
Energy and matter can be quantised.
Electron spins, molecular vibrations, atoms, electrons, protons, neutrons, photons, radiation, every aspect of energy and matter can be cut down to a smallest possible change in state.
Gravity can't, its like distance; there is no such thing as the smallest possible change in distance between 2 objects.

Professor_Fink wrote: »

Stop being pedantic. It depends entirely on your perspective.

I'm not being pedantic, this is the whole point in general relativity. Gravity is just so completely different to everything else.
Theres thousands of people all over the world working on any number of unified field theories that they hope will tie gravity in with the e-m, weak nuclear and strong nuclear forces.

Professor_Fink wrote: »

We know for absolute certain that the space between galaxies is getting bigger. The universe is getting bigger. Ever heard of Hubble? A lot of his fame was due to measuring the rate at which these are moving apart. If the universe was shrinking, which it isn't, then they would be becoming closer as the space between them shrinks.

Your making a fundamental mistake in thinking that we don't understand anything about cosmology. We do. We have a really detailed of everything that has happened to the universe since the CMB decoupled..

Again, I am not arguing with any of that.
But this evidence does nothing to point to either a finite or infinite universe.
You compared me to a creationist - well have a look at yourself.
What we can see is everything there is?

Professor_Fink wrote: »

First of all, while we certainly expect gravitational waves from such systems we haven't yet managed to detect them.

Honestly... I did not know that.
I read some article last year about how gravitational waves work and somehow it got stuck in my head that they had been observed.

Professor_Fink wrote: »

Well then your conclusion is incorrect.

:P
So how do you explain galaxies accelerating away from each other?

Professor_Fink

Hmmm... I thought I posted a reply, but must have forgotten to hit send. Podge_irl gives a good description though.

Podge_irl

Gurgle wrote: »

My suggestion (is that word ok?) is that within our infinite 4D euclidean spatial dimensions, but ridiculous distances away, there is matter and energy that were not even involved in the big bang from which our planet, solar system, galaxy and all the neighbouring galaxies within the nearest ~13.6 billion light years were expelled.

But the Big Bang and inflationary expansion were expansions of our spacetime, all of it. There is no matter or energy in the universe that was not involved in the Big Bang. The Big Bang was not a localized event in a spacetime, it was an expansion of an entire spacetime.

Professor_Fink

Gurgle wrote: »

I'm enjoying the astrophysics discussion, but please don't go out of your way to insult my intelligence.

Maybe I just misinterpreted what you were saying, but I got the impression that you were implying that certain facts were opinions. If I was wrong, then I apologise.

Gurgle wrote: »

My points and comments are all based on what could be, should be or must be further out than anything that has been directly observed.

Well, fair enough. The problem that arises though is that you are trying to reason about general relativity without the math by relying on analogies. Unfortunately the analogies used to talk about GR are fairly rough, and break down when you push them.

Gurgle wrote: »

My suggestion (is that word ok?) is that within our infinite 4D euclidean spatial dimensions, but ridiculous distances away, there is matter and energy that were not even involved in the big bang from which our planet, solar system, galaxy and all the neighbouring galaxies within the nearest ~13.6 billion light years were expelled.

The problem is that everything is moving away from everything else. The CMB fits remarkably well with quantum fluctuations in a compact space. Everything is spreading out isotropically, not with a prefered direction. Taken together these basically rule out the scenario you describe.

Gurgle wrote: »

Quantum mechanics fits in nicely with general relativity.
Energy and matter can be quantised.
Electron spins, molecular vibrations, atoms, electrons, protons, neutrons, photons, radiation, every aspect of energy and matter can be cut down to a smallest possible change in state.
Gravity can't, its like distance; there is no such thing as the smallest possible change in distance between 2 objects.

It really doesn't fit. You run into all sorts of problems because the evolution operator in quantum mechanics explicitly contains a t. They simply don't fit together on a small scale. A pointlike particle would create a black hole which would leave it causally disconnected from the rest of the universe, time evolution wouldn't make sence, the background spacetime should be a superposition of different manifolds due to spatial superpositions of massive particles. It all just goes horribly horribly wrong.

On large scales though, it is perfectly reasonably to use quantum mechanics on a fixed curved background, but this is well outside the regime where we expect quantum gravity effects to become apparent.

Gurgle wrote: »

I'm not being pedantic, this is the whole point in general relativity. Gravity is just so completely different to everything else.
Theres thousands of people all over the world working on any number of unified field theories that they hope will tie gravity in with the e-m, weak nuclear and strong nuclear forces.

In case it hasn't become apparent already, I am a theoretical physicist, so there isn't really a need to educate me on these things.

The problem is that gravity isn't that different. GR is just one perspective, and a flawed one. Kaluza-Klein theory for example extends GR to describe the electro magentic force by adding yet another dimension, although it is a tiny circular one. But it too is wrong. We know quantum electrodynamics to be the most stunningly accurate theory we have, and it has in away killed off the KK apprach. These are both clearly different faces of the same force and we are gradually learning more and more about what makes them so. We fully expect gravity to have a similar quantum representation.

Gurgle wrote: »

Again, I am not arguing with any of that.
But this evidence does nothing to point to either a finite or infinite universe.
You compared me to a creationist - well have a look at yourself.
What we can see is everything there is?

I know! I keep saying that we simply don't know whether our universe is infinite or not. We don't. What we can see is stuff within our particle horizon. That's a large point of why we don't know. But that doesn't change the fact that your arguments for why there is something outside the universe (or frankly even our particle horizon) are seriously flawed.

And frankly, after taking a look at my self I can say I can not see any resemblance what so ever to a creationist!

Gurgle wrote: »

:P
So how do you explain galaxies accelerating away from each other?

Well by a non zero cosmological constant. Actually this is a large part of why we are trying to figure out what dark energy is.

Gurgle

Professor_Fink wrote: »

In case it hasn't become apparent already, I am a theoretical physicist, so there isn't really a need to educate me on these things.

I did realise you knew what you were talking about, otherwise I wouldn't waste the time to read your posts.

Professor_Fink wrote: »

The problem that arises though is that you are trying to reason about general relativity without the math by relying on analogies.

Yes.

Professor_Fink wrote: »

We fully expect gravity to have a similar quantum representation.

I'll take a sharp right turn and go off on a tangent regarding gravitational quantization:

If a graviton actually existed, either as a wave-particle (pretty much ruled out afaik) or an energy packet, then it must have some fundamental properties: frequency, amplitude and mass (though, as said, mass is probably zero).

This would infer that:
1) The gravitational field would have a finite range outside of which there would be discontinuities at the surface of the sphere of effect.

i.e. there would be 2 points at the same distance from the source where the same particle would not experience the same effect.

2) It would be possible to create a standing wave (at the fundamental frequency, 180° out of phase) which could negate the graviton, effectively to block gravity.

This would go against conservation of energy.(wouldn't it?)

Professor_Fink wrote: »

Well by a non zero cosmological constant. Actually this is a large part of why we are trying to figure out what dark energy is.

I'm going to need a bit more detail to make anything whatsoever of that.

Professor_Fink

Gurgle wrote: »

I did realise you knew what you were talking about, otherwise I wouldn't waste the time to read your posts.

Ah, well I may have been taking your posts the wrong way. I got the impression you were implying that I didn't know what I was talking about. Hence the harsh reaction, that kind of thing really gets to me. Sorry to have misinterpretted you and sorry for being rude.

Gurgle wrote: »

If a graviton actually existed, either as a wave-particle (pretty much ruled out afaik) or an energy packet, then it must have some fundamental properties: frequency, amplitude and mass (though, as said, mass is probably zero).

This would infer that:
1) The gravitational field would have a finite range outside of which there would be discontinuities at the surface of the sphere of effect.

i.e. there would be 2 points at the same distance from the source where the same particle would not experience the same effect.

2) It would be possible to create a standing wave (at the fundamental frequency, 180° out of phase) which could negate the graviton, effectively to block gravity.

Actually we know quite a lot about what we expect the graviton to look like. It should have no mass, and should have spin 2. So point 1) is incorrect. If the particle had mass then we would expect a fall off in range, but as it is massless we expect it to fall of as 1/r^2 just as in GR and Newtonian gravitation. So no matter how far you go, you will still feel some effect of gravity.

2) is also wrong, but for a more complicated reason. In quantum mechanics all operations are unitary. What this means is that it is impossible to construct an arrangement like you suggest. For example, it is impossible to cancel out the beam from a laser by interference. But wait you say, what about destructive?

Well this is a little tricky. Basically at least two optical modes (in this case paths) are required. You can use a beam splitter to combine two modes (i.e. the two beams you want to interfere). The two output modes are the sum and the difference of the two beams, so that if they are perfectly in phase or shifter by 180 degrees, then one of the two modes will have no light. The trick here though, is that the other will have twice as much. The same is true for any quantum particle.

Gurgle wrote: »

This would go against conservation of energy.(wouldn't it?)

Not when you make the operation unitary by adding the extra modes.

Gurgle wrote: »

I'm going to need a bit more detail to make anything whatsoever of that.

Well basically the universe looks like it has a natural curvature that opposes gravity on a cosmological scale. I doubt I can do a good short explanation, so I'll point you to the wikipedia article instead.

norrie rugger

This thread is one of the reasons I am glad that I did not do Physics

Yet it is also one of the reasons why I wish I did do Physics.


Christ... Even my own statment is hurting my head

I'm back off to biology land to relax

Conor108

So in a Parallel universe I am a moderator of After Hours? Suberb

Professor_Fink

Conor108 wrote: »

So in a Parallel universe I am a moderator of After Hours? Suberb

For that to happen there has to have been a non-zero probability of things going that way at some point in the past, otherwise, no. Sorry.

thebaldsoprano

norrie rugger wrote: »

This thread is one of the reasons I am glad that I did not do Physics

This thread is one of the reasons I am glad that I no longer do physics

Maths ain't too bad, but this stuff is *headwrecking*
Interesting thread though.