Origin of Universe - Hole with no bottom?

[zonde]

Nope. Not at all necessary. Now, when you actually perform a measurement you have to use some sort of physical system. But there is no such construct within General Relativity itself. As I mentioned, there are neither clocks nor rulers in most space-times commonly used in GR.

General relativity uses concepts of proper time and coordinate time. They are defined using clocks.

Or what do you mean?
Not sure that I understand.

 

[Chalnoth]

General relativity uses concepts of proper time and coordinate time. They are defined using clocks.

No, they aren't. Or, at least, they don't have to be. The idea of a clock only comes into play when you get down to actually determining the result of experiment. You can tell quite a lot about a space-time in General Relativity without ever referencing experimental results.

And furthermore I should point out that for the most part, clocks and rulers are only taken as abstract concepts, and not actually added to the stress-energy tensor.

 

[zonde]

No, they aren't.

From wikipedia http://en.wikipedia.org/wiki/Proper_time" :
"In relativity, proper time is the elapsed time between two events as measured by a clock that passes through both events."

If you know other definition please quote it.

 

[Chalnoth]

From wikipedia http://en.wikipedia.org/wiki/Proper_time" :
"In relativity, proper time is the elapsed time between two events as measured by a clock that passes through both events."

If you know other definition please quote it.

As I've been trying to say, there is no need to reference these things within General Relativity. Yes, you can make use of the idea of proper time within GR. Yes, this is extremely useful for connecting GR to experiment. But it is not part of the definition of the space-time which GR describes.

The space-time is described in GR by the metric, which is a measure of length on the manifold. The idea of "proper time" is something added onto the space-time that is used to determine how you'd go about doing an experiment, and how you'd get the results out in the end.

 

[zonde]

As I've been trying to say, there is no need to reference these things within General Relativity. Yes, you can make use of the idea of proper time within GR. Yes, this is extremely useful for connecting GR to experiment. But it is not part of the definition of the space-time which GR describes.

The space-time is described in GR by the metric, which is a measure of length on the manifold. The idea of "proper time" is something added onto the space-time that is used to determine how you'd go about doing an experiment, and how you'd get the results out in the end.

Wrong. Proper time is used in definition of spacetime metric.

GR does not describe properties of some geometry. Instead it says how certain geometry can be used to describe physical things. Geometry is only a tool used by GR and not GR itself.

 

[Chalnoth]

Wrong. Proper time is used in definition of spacetime metric.

And how is that, precisely?

GR does not describe properties of some geometry. Instead it says how certain geometry can be used to describe physical things. Geometry is only a tool used by GR and not GR itself.

Geometry is the entire description of space-time in General Relativity. This isn't all of GR: GR also adds the relationship between geometry and matter. But geometry is the whole beast when it comes to the description of the space-time itself.

 

[eah2119]

I still don't see the point in considering their are multiple universes and each universe is created from another. Even if there is a mother universe, it would go back to the same problem we faced earlier: how or from what was our universe created?

I still continue to hold that there is only one universe; our own, and that our universe wasn't created. It just always existed, exists, and always will exist. There was no t=0. As you go back in time, space contracts, but there will never be a time when space is infinitely dense. And since space is accelerating in expansion, wouldn't it follow to have a decelerating contraction when time progression is in reverse? Thus, an infinite density will never be reached. It's somewhat similar to the lorentz factor.

I'm probably straying far from the subject of this thread. My point is that the turtle analogy follows. You can't have a never ending generation of universes without a mother universe different from all the rest that was never created.

Maybe it's something we simply can't fathom. How can everything be created out of nothing? Our normal cause and effect thinking doesn't work here.

 

[DaveC426913]

There was no t=0. As you go back in time, space contracts, but there will never be a time when space is infinitely dense. And since space is accelerating in expansion, wouldn't it follow to have a decelerating contraction when time progression is in reverse? Thus, an infinite density will never be reached.

Any basis for any of this? because our current model shows otherwise. We've modeled the universe back to less than attoseconds after the BB.

 

[Chalnoth]

I still don't see the point in considering their are multiple universes and each universe is created from another. Even if there is a mother universe, it would go back to the same problem we faced earlier: how or from what was our universe created?

There isn't any requirement that the universe was created, however. It could just as easily have always existed.

Granted, our own region of space-time had to have a beginning, but this could easily have been from a pre-existing space-time, and the larger universe from which ours was born may well be eternal.

I still continue to hold that there is only one universe; our own, and that our universe wasn't created. It just always existed, exists, and always will exist. There was no t=0. As you go back in time, space contracts, but there will never be a time when space is infinitely dense. And since space is accelerating in expansion, wouldn't it follow to have a decelerating contraction when time progression is in reverse? Thus, an infinite density will never be reached. It's somewhat similar to the lorentz factor.

This view doesn't hold up to scrutiny. The difficulty is that if you have any matter or radiation in the universe whatsoever, you necessarily get a singularity in the finite past if General Relativity is true. This is a very strong argument that whatever our universe may have been born from, it did, in fact, have to be born somehow. Furthermore, thermodynamic arguments prevent there from being any simple explanation for the low-entropy start of our universe.

Finally, there really isn't any reason to believe that the observable universe, whose boundaries are determined by the speed of light and how the universe has expanded in the past, is the only region that exists.

 

[eah2119]

Any basis for any of this? because our current model shows otherwise. We've modeled the universe back to less than attoseconds after the BB.

No, I have no source. It's just a pure idea coming from me. But I would definitely like to see the basis for your information. Does it come from the universal background radiation?

The only education I have is high school physics so don't expect that anything I'm saying is true or backed by evidence. In fact, in the class, we didn't even get to the laws of entropy, so that's a definite fault in my ideas. The only books I read were half my high school physics book and about the first 30 pages of Einstein's theory of general and special relativity.

 

[LookbackDelay]

By "hole with no bottom", do you mean "singularity"?

 

[zonde]

And how is that, precisely?

Metric gives distances on manifold. Space-time is four dimensional manifold where one of dimensions is time and time is defined using clock (and other three dimensions are defined using clock and speed of light).
Output of metric is dimensionfull quantity that is dependent from dimensions of manifold.

Geometry is the entire description of space-time in General Relativity. This isn't all of GR: GR also adds the relationship between geometry and matter. But geometry is the whole beast when it comes to the description of the space-time itself.

No, you can't strip dimensions out of description of space-time. Without dimensions there is no reason to call particular geometry space-time.

 

[Chalnoth]

Metric gives distances on manifold. Space-time is four dimensional manifold where one of dimensions is time and time is defined using clock (and other three dimensions are defined using clock and speed of light).
Output of metric is dimensionfull quantity that is dependent from dimensions of manifold.

This can all be defined in a coordinate-free manner. You don't get space-time distances until you actually use a coordinate system of some sort. The description of the space-time still exists as an abstract quantity before you apply any coordinates, before you connect the mathematical description to experiment.

No, you can't strip dimensions out of description of space-time. Without dimensions there is no reason to call particular geometry space-time.

Huh? What part of what I wrote leads you to think I'm talking about stripping dimensions out of space-time?

 

[zonde]

Huh? What part of what I wrote leads you to think I'm talking about stripping dimensions out of space-time?

Just a sloppy language. I was meaning that you replace physical dimensions with abstract coordinate dimensions without physical meaning i.e. striped of physical units.

This can all be defined in a coordinate-free manner. You don't get space-time distances until you actually use a coordinate system of some sort. The description of the space-time still exists as an abstract quantity before you apply any coordinates, before you connect the mathematical description to experiment.

Distances are coordinate system independent (not free) but metric is coordinate system dependent. For different coordinate systems it is written differently. That's because arguments of metric are expressed in coordinate system dependent manner.

Besides mathematical description without any correspondence to physically measurable things (experiments and observations) is not physics theory i.e. it can not describe physical reality.

 

[Chalnoth]

Just a sloppy language. I was meaning that you replace physical dimensions with abstract coordinate dimensions without physical meaning i.e. striped of physical units.

You can still learn a number of things about a space-time without reference to any particular coordinate system, such as the existence of singularities. In fact, coordinate systems can complicate things here because many coordinate systems will have singularities that are merely artifacts of the coordinates chosen and not intrinsic to the system itself.

Distances are coordinate system independent (not free) but metric is coordinate system dependent. For different coordinate systems it is written differently. That's because arguments of metric are expressed in coordinate system dependent manner.

The metric encodes the geometry of the system, and that geometry is unchanged no matter what coordinates you choose to use.

Besides mathematical description without any correspondence to physically measurable things (experiments and observations) is not physics theory i.e. it can not describe physical reality.

Even when just considering the space-time geometry as an abstract mathematical concept, the connection to experiment is pretty direct. Basically, you just add a measurement apparatus to the space-time, and define your measurement based upon that apparatus. The measurement apparatus is not inherent to the abstract mathematical construct, but can be added to it to obtain measurable values.