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#19
Aug510, 11:22 AM

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Basically, in some models, such as in Stephen Hawkings' no boundary proposal, there simply isn't any time before the big bang. Asking "what came before the big bang" is analogous to asking "what lies north of the north pole." This is because in his no boundary proposal, the spacetime manifold doesn't actually have any sort of edge, just like there is no end to the surface of the Earth (in the sense of people who thought the Earth was flat thought of an edge). It is, however, finite, wrapping back on itself in a very specific way. Thus what we see of as "time" has a beginning of sorts, but there is nothing "before" it (just as the Earth has a point that is furthest north, but with nothing north of that point). 


#20
Aug610, 02:12 AM

P: 188

cant i say that the big bang actually triggered the creation of the ''things'' that could experience time rather than saying that big bang caused the creation of time (as before it there was nothing or none that could measure or evaluate time)???????????? 


#21
Aug610, 02:19 AM

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#22
Aug1010, 12:03 AM

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#23
Aug1010, 04:17 AM

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#24
Aug1210, 12:39 PM

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#25
Aug1210, 02:59 PM

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#26
Aug1210, 04:40 PM

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i didn't quite understood that. 


#27
Aug1210, 08:35 PM

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#28
Aug1310, 06:36 PM

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and does the time represents all known direction or just one specific direction. 


#29
Aug1310, 11:40 PM

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#30
Aug1410, 03:02 AM

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PF Gold
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Science tries to avoid the 'God' hypothesis. Not because scientists hate 'God', but, because they wish to explain as much as possible about the universe without resorting to 'miracles' [which history has proven to be a bad idea]. I have no problem accomodating a 'God' in my universe, just understanding the role s/he plays.



#31
Aug1410, 03:40 AM

P: 1,555

To address a common misunderstanding: time is not an actual dimension on the manifold.
The confusion arises because often a coordinate chart is used where an observer's x_{0} (or sometimes denoted as t) is identical to his proper time. For instance a rest frame in Minkowski spacetime using Cartesian coordinates or Fermi normal coordinates in curved spacetimes. Curved spacetime is a four dimensional manifold but no single dimension is explicitly time. So what is time? Well for any timelike observer time is the metric distance between two events on his worldline. In GR worldlines can simply end (at a singularity), by time symmetry (and GR is time symmetric) that implies that worldlines can simply begin as well. Hence according to GR it is possible that for a given observer time can have a begin and an end. 


#32
Aug1410, 06:38 AM

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#33
Aug1410, 10:40 AM

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or may be that time exists within the three dimensions of space occupying some part of all three dimensions????? 


#34
Aug1410, 11:21 AM

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Mathematically, time is exactly the same as the other dimensions, except that the sign of a metric component associated with time is opposite from the spatial dimensions. For example, if the spatial dimensions have positive metric components, then time has a negative metric component. With this convention, if you find the metric distance between two different times for a particular observer, you get a negative number (a timelike distance...this is the actual time that the observer sees on their clock). If you find the metric distance between two simultaneous events, by contrast, you get a positive number (a spacelike distance: this is the distance you would measure between these events in a reference frame where they occur simultaneously). Finally, if you find the metric distance between two points in the travel of a light ray (for example, from when a light ray is emitted to when it is absorbed), you always get zero. So light rays themselves act as a boundary between timelike distances and spacelike distances. 


#35
Aug1510, 02:26 AM

P: 1,555

"So what is time? Well for any timelike observer time is the metric distance between two events on his worldline." You have the answer as to what time is for an observer. Time is observer dependent in GR. Now if you use for instance a Fermi normal coordinate chart in curved spacetime or simply a rest frame in Cartesian coordinates in flat space you can use time (which is then proper time) on one axis so it looks like it is a separate dimension. But just by using such a charts does not make it a dimension. There is a distinction between the manifold and a choordinate chart and it is a mistake to assume that any of the dimensions of the manifold is time. 


#36
Aug1510, 03:36 AM

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