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Author Topic: Why Did Time Have to Exist At The Commencement Of The Big Bang ?  (Read 27723 times)

Offline LeeE

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...but having three spatial dimensions is important for a long lived universe

But isn't that just saying that a universe with a different number of spatial dimensions has to have different laws?

Also, don't just think down to < 3 spatial dimensions - you've got to think up to > 3 as well.
 

Offline Soul Surfer

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Yes but changes from the number of extended dimensions of three space and one of time in either direction result in gravitational orbits not being stable because three space dimensions result in an inverse square law in which a two body orbit can be stable against quite large disturbances. If you change the inverse square law by even a tiny bit you can produce an orbit in a two body system but the slightest disturbance will cause that orbit to become unstable and not last for a long time.
 
You can however have a many extra compacted dimensions you like
 

Offline LeeE

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I think you're still applying 4D space-time laws to non-4D space-time environments and the point is that spacetime environments with other than four dimensions would need different laws.  Saying that less than four-dimensional space cannot exist because it doesn't conform to 4D-spacetime rules is like saying apples can't exist because they're not oranges.  Saying that greater than 4D spacetime can exist only if the other dimensions are treated differently is like saying apples can only exist if you buy them at a particular store.  In either case, there needs to be a reason why different solutions, to what is really a simple hierarchic system, are treated so differently.  Claiming that only one level in the hierarchy is possible because only that level works with that level's rules doesn't really seem like a valid arguement to me.
 

Offline JP

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Doesn't the definition of spatial and temporal dimensions depend on their geometric properties?  In other words, space dimensions behave one way, and time another, and their interaction is defined in a geometric way by general relativity.  If you assume these same relations hold and change the number of dimensions, the universe as we know it isn't stable. 

I believe that lot of extensions of GR (string theory, for example) include extra dimensions by assuming they don't have the properties of spatial/time dimensions.
 

Offline LeeE

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I think it depends on who you ask  :)

This comes back to saying that we appear to be able to move freely through the spatial dimensions but not through the temporal dimension.  However, it's not as simple as that.  While it seems that we cannot move freely back and forth along the temporal dimension this must be qualified - that is to say we cannot move backwards in time while our personal time-frame continues moving forward, which is what most people think of when talking about going back in time.  In absolute terms, what you did last week is still happening last week, and if you were to go back to last week you wouldn't be aware of the fact unless your personal time-frame continued going forward while you were going backwards i.e. you'd actually end up being a week older than you were a week ago.

Yes, the time dimension appears to be different, from our point of view, to the spatial dimensions but saying exactly how and why is very difficult to put your finger on, although I personally think that phenomenon like time dilation give an insight to how the spatial and temporal dimensions are essentially equivalent and their appearance and behaviour depends upon your point of view (not only from what you're doing but also from how many dimensions you're observing from).  It's a bit like not being able to see the forest because of the trees - we can only see it from inside, whereas we really need to see it from outside to definitively answer this.

Purely personally, I don't like solutions where anything other than three spatial dimensions has to be treated differently without a clear reason for doing so, even when those solutions provide some good answers.  Sure, those solutions are worth studying and refining, and we learn and gain new insights from them, but they hardly seem to be unified if they add new abstracts - new clauses that cannot be expressed in related lower order solutions.
 

Offline Soul Surfer

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The simple point is that less than three dimensions and there isn't room for anything to develop.  Two dimensions gives a simple inverse law of power fall of with distance.  Three dimensions of space gives an inverse square law of power fall off and four dimensions of space an inverse cube law.
 

Offline DoctorBeaver

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The simple point is that less than three dimensions and there isn't room for anything to develop.  Two dimensions gives a simple inverse law of power fall of with distance.  Three dimensions of space gives an inverse square law of power fall off and four dimensions of space an inverse cube law.

Isn't that only true if the dimensions are the same size?
 

Offline Soul Surfer

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Same size?  please explain what you mean.  dimensions can either be large and extended to infinity (or the size of the universe) or curled up incredibly tiny and cyclic I am not sure that a halfway position makes any sense.
 

Offline DoctorBeaver

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There are theories of warped extra dimensions that allow them to be up to 0.1mm in diameter. That is something that is to be tested with the LHC.

If 1 dimension is that size and the others are compactified to, say, the Planck scale, there would not be a consistent dilution of gravity through all dimensions. It would dilute more in the large dimension.
« Last Edit: 03/11/2008 01:56:38 by DoctorBeaver »
 

Offline LeeE

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SoulSurfer - you still seem to be doggedly applying 4D laws to non-4D situations, and what is more, some of these 4D laws don't always hold to be true in our 4D space-time.  For example, the angles of a triangle always add up to 180, yeah?  Well this only works in flat space-time - plot a triangle in the curved space-time near a black-hole, equivalent to plotting it on a sphere, and the sum of the angles will always be greater than 180.  There is no flat space-time in our universe and any laws that depend on it being so will always be very slightly wrong.
 

Offline neilep

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INTERLUDE ANNOUNCEMENT

Thanks for the fascinating posts all !

END OF INTERLUDE
 

Offline DoctorBeaver

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INTERLUDE ANNOUNCEMENT

Thanks for the fascinating posts all !

END OF INTERLUDE


OI... I didn't have time to get an ice-cream!  [:(!]
 

Offline neilep

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INTERLUDE ANNOUNCEMENT

Thanks for the fascinating posts all !

END OF INTERLUDE


OI... I didn't have time to get an ice-cream!  [:(!]

Sorry for the short interlude !

I didn't want to upset the flow of the thread...here..have this on me !





 

Offline DoctorBeaver

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Have it on you? You think I'm depraved?
 

Offline neilep

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Have it on you? You think I'm depraved?

LOL..I kind of walked into that one....would I be more attractive ....if it ....was hummus  ?

 

Offline DoctorBeaver

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No.
 

Offline LeeE

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Ice-cream - yummy ;D
 

Offline Soul Surfer

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LeeE you seem to be accidentally or deliberately misunderstanding what I am saying.  Your original question concerned how the fundamental laws of an understandable universe would vary as a function of the number of extended dimensions that the universe occupied.  I have been trying to explain to you the answer to this question.
« Last Edit: 03/11/2008 23:14:53 by Soul Surfer »
 

Offline LeeE

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Any misunderstandings on my part are accidental - I'm certainly not trying to wind anyone up.  Umm... I haven't asked any questions, except rhetorical ones - I think?  Made quite a few statements, but no questions that I recall.

NeilP asked the original question, regarding the need for time to exist before the Big-Bang.

I think our discussion started when you said:

Interesting points and I see they could be very relevant but having three spatial dimensions is important for a long lived universe.  The reason for this that long range energy fields follow an inverse square law and the only law that allows long term stable orbits to form is an inverse square law.

...and that's when I said:

Quote
...but having three spatial dimensions is important for a long lived universe

But isn't that just saying that a universe with a different number of spatial dimensions has to have different laws?

Also, don't just think down to < 3 spatial dimensions - you've got to think up to > 3 as well.

The problem I have with some of what you say is that, for example, citing the need for inverse-square laws presupposes that energy has the same structure and form in less than or greater than 4D space-times, which seems impossible to me.  If the energy is different to energy as we know it, it will be unlikely to be governed by the same laws that we know.

Let's say that energy in 5D space-time looks like a 3D solid to us (entirely debatable, of course) - how then, does the inverse-square law work with a solid?  Is a 4D solid in 5D space-time still convertible to energy according to e=mc^2?
 

Offline DoctorBeaver

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LeeE - it is possible that elementary particles exist in more than 3 spatial dimensions (string theory, for instance). Therefore, your question "Is a 4D solid in 5D space-time still convertible to energy according to e=mc^2?" is irrelevant.
 

Offline Soul Surfer

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One of the fundamental requirements of an understandable universe is to have physical laws that are not dependant on (time, see below) your position or your orientation (in an empty universe).  This implies that the universe obeys the conservation of energy and the conservation of angular momentum. This in turn implies that as the energy spreads out in three dimensions the surface area depends on the square of the distance form the source. therefore fields follow an inverse square law.  In fact in a general understandable universe with n dimensions the energy law is an inverse n-1 law.

These are totally fundamental requirements and independent of most of the detailed physical laws.
« Last Edit: 04/11/2008 23:09:42 by Soul Surfer »
 

Offline JP

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Just to correct you slightly, Soul Surfer, independence of physical laws on time gets you conservation of energy, while independence on position gets you conservation of momentum. 
 

Offline Soul Surfer

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Obviously. I forgot to state that. Thanks for the correction.
 

Offline LeeE

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DoctorBeaver:  only possibly irrelevant, I think  :)
« Last Edit: 05/11/2008 05:48:50 by LeeE »
 

Offline LeeE

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SoulSurfer:  Key word
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understandable

You still seem to be arguing that the only valid laws are the ones that apply to 4D space-time, which naturally apply to 4D space-time, and which don't apply to non-4D space-times.

Otherwise, I surrender.
 

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