Origin of Universe - Hole with no bottom?

[Government$]

So i have jut finish watching BBC Horizon Documentary about What happen before big bang. Documentary raises more question then answers. So i suppose main theory goes like this: there was nothing and then at some moment, for what ever reasons, there is a huge explosion and universe comes to existence and continues expanding. Am i right? Anyway, then comes along some scientis and says "No, No that is impossible how can something be created from nothing". So they think of brane, universes form black holes and so on. So let's assume that they are right about brane and that big bang was actually collision of two branes. Ok but this raises question where are branes located and how they come to existence. So for example :

my house-->city-->country-->continent-->earth-->solar system-->our galaxy-->universe-->brane-->>x-->y-->z--> and so on and so on.

So my question on is is origin of universe(branes or what ever) hole with no bottom?
Any thoughts?

 

[marcus]

I watched the BBC program "What happened before the BB?"

It presented various alternative ideas, some with more or less clear models, about what was happening before the start of expansion.

The Colliding "Branes" idea presented by Neil Turok was only one of the alternatives, and not the most convincing one I think. Nor the simplest, nor the easiest to check by astro observation. It was the only one that mentioned "branes".

Most of the ideas presented there are already fairly old. Turok's "branes" one goes back 10 years. Some go back to the 1990s.

The main thing is they are alternatives. We have to do some work to decide which if any of them fits the real universe we see.

===================

I think you are right about bottomless hole, as I understand your idea.
I would say that human explanations are an INFINITE REGRESS. Behind every explanation no matter how good and clear and checkable, there must be an explanation of how that one got set up.

I do not see how there can be an end to explanation. Which is fine. It is a great adventure. There will never be an end to the excitment.
Just my two cents.

 

[DaveC426913]

The question of what started everything is distinct from what started the universe.

The colliding branes hypothesis is a good attempt to answer the latter, though not the former, but it's still nothing to sneeze at.

 

[Chalnoth]

The question of what started everything is distinct from what started the universe.

Well, that sort of depends upon what you mean by "universe". Unfortunately, many people, even cosmologists, rather sloppily exchange different meanings for the word. I'm pretty sure you mean our observable universe here, but you could just as well mean everything when you use the word universe.

Anyway, my favorite idea for why anything at all exists is Tegmark's mathiverse. My favorite idea for why our particular region of space-time exists is something akin to vacuum fluctuations producing new regions of space-time.

 

[Chronos]

Caveats, there are always caveats. I like the quantum fluctuation thing, but, assuming there was no 'spacetime' for the fluctuation to occur within makes it sound like 'turtles all the way down'.

 

[Chalnoth]

Caveats, there are always caveats. I like the quantum fluctuation thing, but, assuming there was no 'spacetime' for the fluctuation to occur within makes it sound like 'turtles all the way down'.

Well, this model is that of a sort of equilibrium universe out of which the occasional fluctuation produces a new region, some fraction of which are habitable. I don't see that there is a problem with this proposal (I think the "turtles all the way down" description is overly-simplistic).

 

[DaveC426913]

Well, that sort of depends upon what you mean by "universe". Unfortunately, many people, even cosmologists, rather sloppily exchange different meanings for the word. I'm pretty sure you mean our observable universe here, but you could just as well mean everything when you use the word universe.

What I mean by the universe is everything that came out of the Big Bang. That includes both the observable universe and the unobservable universe.

We do not have a commonly accepted name for what occurred before the Big Bang.

For this reason, I use the word universe without need of qualification.

 

[dilletante]

The idea of a quantum fluctuation creating a universe seems farfetched to me. Some have asked the question, "a quantum fluctuation of what?" which seems a reasonable thing to ask. On the other hand, if it is a quantum fluctuation within an existing universe -- well, we are IN an existing universe and I haven't heard of any fluctuations creating more than a virtual particle. That's a long way from creating a universe.

So I suppose some questions might be:

1) Do scientists believe a quantum fluctuation can appear out of nothing?
2) If fluctuations have to appear from something, should we be seeing them in our universe?
3) Or is it theorized that we would be unaware if another universe fluctuated from ours?
4) Or perhaps that we would be aware, but it is such a rare event that it hasn't happened recently in our observable universe?

 

[phinds]

So my question on is is origin of universe(branes or what ever) hole with no bottom?
Any thoughts?

Just FYI, this concept is generally referred to as "turtles all the way down"

 

[Chalnoth]

The idea of a quantum fluctuation creating a universe seems farfetched to me. Some have asked the question, "a quantum fluctuation of what?" which seems a reasonable thing to ask. On the other hand, if it is a quantum fluctuation within an existing universe -- well, we are IN an existing universe and I haven't heard of any fluctuations creating more than a virtual particle. That's a long way from creating a universe.

Well, there are some very simple, well-known cases where quantum fluctuations do indeed produce real particles. Specifically, quantum vacuum fluctuations near the horizon of a black hole produce real particles that make up Hawking radiation.

But in this case, the fluctuation would look, to us, like a microscopic black hole which pops into existence and then rapidly evaporates. It would behave, to us, very much like a virtual particle. But it could easily spawn a whole new universe.

One sort of visual representation of this would be to imagine our space-time as being a sheet, and vacuum fluctuations as being ripples on the sheet. These ripples are randomized, with some larger and some smaller. Every once in a great while, one ripple gets so big that a bubble pinches off from the sheet. This bubble becomes its own universe, while the original sheet continues on as if nothing ever happened.

1) Do scientists believe a quantum fluctuation can appear out of nothing?

I'm not entirely sure that something appearing out of nothing is sensible. Certainly nobody knows how such a thing would be described mathematically. But just because we don't know how to describe it now doesn't necessarily mean it isn't possible: perhaps the correct theory of quantum gravity will explain how this can happen.

2) If fluctuations have to appear from something, should we be seeing them in our universe?

I'm pretty sure that the fluctuations that would produce new universes would be exceedingly hard to detect. The exception would be a fluctuation into a lower-energy vacuum state, which would destroy our entire universe.

4) Or perhaps that we would be aware, but it is such a rare event that it hasn't happened recently in our observable universe?

This is also possible.

 

[ViewsofMars]

From the National Science Teachers Association (NSTA):

Universe: The Origin and Evolution of the Universe [page updated 2011]

Description Course:
Science Objects are two hour on-line interactive inquiry-based content modules that help teachers better understand the science content they teach. This Science Object is the fifth of five Science Objects in the Universe SciPack. It provides understanding of how the universe formed, how it has changed over time, and how it continues to change today. The ‘big bang’ theory of universe formation is supported by recent observations of the motion of galaxies, as well as observations of the energy left over from the formation of the universe. This evidence suggests that the origin of the universe occurred approximately 13.6 billion years ago, during a point in time when the state of the universe was much hotter and more dense. The fact that light seen from almost all distant galaxies has longer wavelengths than comparable light here on Earth provides evidence that the whole universe has been expanding ever since the big bang (and continues to expand today).

Please read on . . .
http://learningcenter.nsta.org/product_detail.aspx?id=10.2505/7/SCB-UN.5.1

About NSTA
The Executive Summary
Headquartered in Arlington, Virginia, the National Science Teachers Association is a member-driven organization, 60,000-strong. We publish books and journals for science teachers from kindergarten through college. Each year we hold four conferences on science education: three regional events in the fall and a national gathering in the spring. We provide ways for science teachers to connect with one another. We inform Congress and the public on vital questions affecting science literacy and a well-educated workforce. And with your help, we can do even more.
http://www.nsta.org/about/?lid=tnavhp

 

[dilletante]

But in this case, the fluctuation would look, to us, like a microscopic black hole which pops into existence and then rapidly evaporates. It would behave, to us, very much like a virtual particle. But it could easily spawn a whole new universe.

Hi Chalnoth,

Thank you for the replies. These are difficult things to grasp -- that a microscopic black hole can "easily" spawn a new universe -- which we would likely not notice. I did find a paper by Poplawski, "The universe as a black hole in isotropic coordinates", which theorizes that our universe may exist inside an Einstein-Rosen black hole:

http://xxx.lanl.gov/PS_cache/arxiv/pdf/0901/0901.0215v1.pdf

But this seems to be a relatively new idea and I don't know what level of acceptance it has in the physics community. Of course, your comments may be based on a different concept -- if so, I would love to read about it if references exist.

If a white hole results from certain types of black holes, does the resulting universe contain only the amount of matter contained in the original black hole?

 

[Chalnoth]

Hi Chalnoth,

Thank you for the replies. These are difficult things to grasp -- that a microscopic black hole can "easily" spawn a new universe -- which we would likely not notice. I did find a paper by Poplawski, "The universe as a black hole in isotropic coordinates", which theorizes that our universe may exist inside an Einstein-Rosen black hole:

http://xxx.lanl.gov/PS_cache/arxiv/pdf/0901/0901.0215v1.pdf

But this seems to be a relatively new idea and I don't know what level of acceptance it has in the physics community. Of course, your comments may be based on a different concept -- if so, I would love to read about it if references exist.

Nah, that's a somewhat different idea. Here's a rather detailed description of the idea:
http://arxiv.org/pdf/gr-qc/0512034v3
It looks like this is the original work regarding new universes that look like black holes on the outside, but it requires a subscription:
http://www.sciencedirect.com/science/article/pii/055032139090357J

If a white hole results from certain types of black holes, does the resulting universe contain only the amount of matter contained in the original black hole?

Well, a white hole is a time reversal of a black hole, which means it is decreasing in entropy with time, which means that you got the arrow of time wrong. In other words, white holes are unphysical.

 

[Chronos]

This is why math is a dangerous master, it produces many unphysical 'solutions'. I'm not suggesting math is wrong, merely indicting the usual suspects [assumptions].

 

[Multi]

i suppose main theory goes like this: there was nothing and then at some moment, for what ever reasons, there is a huge explosion and universe comes to existence and
continues expanding. Am I right?

Actualy,you aren't,there was no explosion,the universe was once in an extremely hot and dense state that expanded rapidly and that is called Big Bang

 

[ViewsofMars]

I thought it important to bring forth Cosmic Microwave Background Radiation Anisotropies: their Discovery and Utilization, Nobel Lecture, December 8, 2006 by George F. Smoot. I present only two snippets from the his lecture though encourage people to read it in its entirety:

1. "The Cosmic Background Radiation Observations of the Cosmic Microwave Background (CMB) temperature anisotropies have revolutionized and continue to revolutionize our understanding of the Universe. The observation of the CMB anisotropies angular power spectrum with its plateau, acoustic peaks, and high frequency damping tail have established a standard cosmological model consisting of a flat (critical density) geometry, with contents being mainly dark energy and dark matter and a small amount of ordinary matter. In this successful model the dark and ordinary matter formed its structure through gravitational instability acting on the quantum fluctuations generated during the very early Inflationary epoch. Current and future observations will test this model and determine its key cosmological parameters with spectacular precision and confidence."

2. "According to Big Bang theory, our universe began in a nearly perfect thermal equilibrium state with very high temperature. The universe is dynamic and has been ever expanding and cooling since its birth. When the temperature of the universe dropped to 3,000 K there were insufficient energetic CMB photons to keep hydrogen or helium atoms ionized. Thus, the primeval plasma of charged nuclei electrons and photons changed into neutral atoms plus background radiation. The background radiation could then propagate through space freely, though being stretched by the continuing expansion of the universe, while baryonic matter (mostly hydrogen and helium atoms) could cluster by gravitational attraction to form stars, galaxies and even larger structures. For these structures to form there must have been primordial perturbations in the early matter and energy distributions. The primordial fluctuations of matter density that will later form large scale structures leave imprints in the form of temperature anisotropies in the CMB.”


Big Bang: Cosmic Microwave Background Radiation Anisotropies: their Discovery and Utilization, Nobel Lecture, December 8, 2006 by George F. Smoot.

http://nobelprize.org/nobel_prizes/physics/laureates/2006/smoot_lecture.pdf

 

[nickthrop101]

you could ask this question and from different people get a different responces, for example, a priest or religious teacher would say that the universe was nothing and God (Who has been around for all eternity) started the universe from nothing, on ideas like these you could look up aquinas's theory of cause and affect. an agnostic would say that they don't know and are open to all beliefs
Another example is that there is an infinate time or a "cyclic universe" where the universe has always been there and there is no idea of beggining or end. Another idea is that there was no concept of time before that big bang, just a singularity of super dense hot particles fitted in a space that was infinatlely small, that for some reason from a gravitational brane almost borrowed energy from gravity that started the cosmic expansion. Although it is only speculative, i like to think that the our universe started by when two brane universes collided it caused a super expansion of space made from the two branes :D

 

[phinds]

i like to think that the our universe started by when two brane universes collided it caused a super expansion of space made from the two branes :D

Comforting to you perhaps, but doesn't answer anything at all. Where did the branes come from ? That's still turtles all the way down.

 

[mrspeedybob]

Comforting to you perhaps, but doesn't answer anything at all. Where did the branes come from ? That's still turtles all the way down.

What is wrong with turtles all the way down? Sticking with the turtles analogy there are 3 options that I can see...

Turtles all they way down
The turtle is self contained and "under the turtle" has no meaning
Turtle is standing on a hippo, hippo standing on a rock, pick some number of layers beyond which you cannot formulate a theory.

I'm not sure if it's scientifically possible to determine which turtle analogy best fits the universe, I think it lies more in the realm of philosophy and personal preference.

Personally I find turtles all the way down to be the most beautiful and elegant of the 3 possibilities.

 

[bcrowell]

Let's confine the discussion to physics. PF has a separate philosophy forum. If there is nothing left to say in this thread that is actual physics, then it's time to close the thread.

 

[Naty1]

from Marcus Post #2:

"The Colliding "Branes" idea presented by Neil Turok was only one of the alternatives, and not the most convincing one I think. Nor the simplest, nor the easiest to check by astro observation. It was the only one that mentioned "branes"."

Turok and Steinhardt's book on the subject, THE ENDLESS UNIVERSE, is interesting reading..aimed at the general public. I found it potentially convincing...if experimentally verified.

They have hopes that the EU launched Planck satellite will be able to detect distinct CMBR characteristics which will be able to distinguish big bang from colliding branes...

I was going to post here last week, in fact, asking what has been found, but the data is still be acquired and analyzed.

If anyone is interested in what is being sought experimentally, ask...I'll stop back.

I have posted about it before and should be able to find the specifics which are brief.

 

[nickthrop101]

Comforting to you perhaps, but doesn't answer anything at all. Where did the branes come from ? That's still turtles all the way down.

What if the turtles end there, what in infinate space-time stretched infinatly in all the dimensions, meaning no more turtles :D

 

[phinds]

What if the turtles end there, what in infinate space-time stretched infinatly in all the dimensions, meaning no more turtles :D

Certainly a possibility but why is it any more satisfying than the big bang?

I think we need to know more and everything right now about what we call t <= 0 is just speculation.

 

[nickthrop101]

good point, what if the big bang is the last turtle in the list, maybe we have no concept of previous turtles as time didnt exist. All I am trying to give is a reason for the big bang :D

 

[ptalar]

Attached is a paper that describes how something can come from nothing. But beware, as Chronos rightly stated earlier in this thread: Math is a Dangerous Master.


http://arxiv.org/abs/astro-ph/9712344v1

 

[JLA727]

I've been boggling my mind with a something almost like this...they say that from the big bang everything started. Time, space, matter,...everything.And that before that there was nothing...So if Space was created...not the stuff floating around in it but space itself what was there before that...a solid? If there is NOTHING than isn't that empty space? An endless void with Nothing IN it? this hurts your head thinking about it...Sorry NO answer to your question I just wanted to express my complete inability to wrap my head around even part of this question. But like you said if you have these branes floating around in emptiness, before they existed then wouldn't that be empty space,...or nothingness...or empty space? what is the difference!??

 

[Cosmo Novice]

So if Space was created...not the stuff floating around in it but space itself what was there before that...a solid?

I just nearly fell of my chair laughing! )

 

[DaveC426913]

I've been boggling my mind with a something almost like this...they say that from the big bang everything started. Time, space, matter,...everything.And that before that there was nothing...So if Space was created...not the stuff floating around in it but space itself what was there before that...a solid? If there is NOTHING than isn't that empty space? An endless void with Nothing IN it? this hurts your head thinking about it...Sorry NO answer to your question I just wanted to express my complete inability to wrap my head around even part of this question. But like you said if you have these branes floating around in emptiness, before they existed then wouldn't that be empty space,...or nothingness...or empty space? what is the difference!??

It boggles the mind because we have no previous example and nothing to compare non-space to. But also because "space" is an ill-defined and ambiguous term.

Once we give it some hard definitions, such as: it is defined with three space-like dimensions and one time-like dimension, we can begin to define what it means to change or remove these properties. Then we have a model what what might have existed before the BB - but without having to have a mundane example to compare to.(A plains Indian would be boggled trying to imagine of a squid, since it has no comparison to any creature he's ever seen before "if it's not a horse, fish, or buffalo, the only thing left is a bird, right?". But once you start defining animal anatomy, showing convergence of limbs, fins and tentacles, he can see variations even if he's never met an example.)

 

[Chalnoth]

It boggles the mind because we have no previous example and nothing to compare non-space to. But also because "space" is an ill-defined and ambiguous term.

I don't think that's accurate. We most definitely have some very explicit definitions of space. General Relativity defines it as a manifold, for instance. And there the question as to "what happened before" is nonsensical, because there was no "before". General Relativity only describes the space-time itself, and there just isn't any concept of "nothing" in GR.

That said, when we discover a more accurate theory of gravity, one consistent with quantum mechanics, it is at least somewhat conceivable that we will be properly able to describe "nothing" in a sensible manner. It seems a bit unlikely, but at least it is conceivable.

 

[DaveC426913]

I don't think that's accurate. We most definitely have some very explicit definitions of space.

That is the point I'm making. I was using "we" to be polite. I should have been specific and said "when the OP refers to "space" he sees it as ill-defined and ambiguous, whereas, there are some very definable properties about it that take the mystery away."

 

[-Job-]

Origins are difficult to deal with because they establish a boundary, it's in the same line as arguing that space ends or begins somewhere.

We need a balloon analogy that includes time, e.g. that any given state of the universe eventually leads to itself, following a logical path of intermediate states.

 

[zonde]

Special relativity defines spacetime using rulers and clocks. As far as I know General relativity does not replace this approach with anything else.
So if you have no rulers and no clocks (no matter) spacetime of relativity is not defined any more.

If quantum mechanics can somehow define spacetime independently from matter then maybe one can go further in speculations about BigBang. But that is still very big "if".

 

[Chalnoth]

Special relativity defines spacetime using rulers and clocks. As far as I know General relativity does not replace this approach with anything else.
So if you have no rulers and no clocks (no matter) spacetime of relativity is not defined any more.

Well, as I mentioned, General Relativity defines space-time as a manifold, which is a construct in differential geometry. Basically, a manifold is a "something" upon which it is possible to write a coordinate system and where in a given coordinate system there is a definition of length (you don't need to have a physical ruler to be able to define a length). All of this is quite abstract, and most of the more well-known space-times used in General Relativity are nothing but empty space (this includes Schwarzschild, Kerr, de Sitter, and anti-de Sitter space-times).

 

[zonde]

you don't need to have a physical ruler to be able to define a length

Yes, you can define length using light and physical clock. So you still can't get away from matter.

 

[Chalnoth]

Yes, you can define length using light and physical clock. So you still can't get away from matter.

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.

 

[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.