The big bang


by FizixFreak
Tags: big bang
Passionflower
Passionflower is offline
#37
Aug15-10, 04:03 AM
P: 1,555
Quote Quote by Chalnoth View Post
I don't see how this distinction is any different from any of the other dimensions.
What do you mean by "any of the other dimensions"? The manifold is 4-dimensional, but no singe dimension is a spatial or temporal. Only a coordinate chart maps (a region of) this manifold, with or without off-diagonal components, onto 4 dimensions of which one is temporal and three are spatial.
finiter
finiter is offline
#38
Aug15-10, 05:37 AM
P: 26
Dimensions are strictly mathematical. It may or may not represent the physical reality. The real world is just three dimensional. However, to analyze it, we can use one-dimensional or four-dimensional frames.

An expanding system requires a four dimensional frame. As time moves forward, the three space dimensions increase. The spherical surface of the expanding system, or the Gauzian surface described by the three space dimensions, encloses the spacetime. The spacetime can be regarded as the volume at a given time; it is the product of a volume factor and a time factor, ie, it is four dimensional. When the system contracts, the time factor decreases. Mathematically it is time moving back. But in real terms, the direction of time does not change, but the directions of the space dimensions are reversed.
Chalnoth
Chalnoth is offline
#39
Aug15-10, 06:28 AM
Sci Advisor
P: 4,721
Quote Quote by Passionflower View Post
What do you mean by "any of the other dimensions"? The manifold is 4-dimensional, but no singe dimension is a spatial or temporal. Only a coordinate chart maps (a region of) this manifold, with or without off-diagonal components, onto 4 dimensions of which one is temporal and three are spatial.
Right. But your point about time being the metric distance between two space-time points for an observer is important, because the choice of dimensions is not completely arbitrary: the motion of an observer picks out a specific set of them. This indicates, for instance, that while no particular direction on the manifold can be identified uniquely as time, one cannot pick any direction as being time: there are some directions on the manifold which no observer can traverse (because it would mean moving faster than the speed of light).
Chalnoth
Chalnoth is offline
#40
Aug15-10, 06:30 AM
Sci Advisor
P: 4,721
Quote Quote by finiter View Post
Dimensions are strictly mathematical. It may or may not represent the physical reality. The real world is just three dimensional. However, to analyze it, we can use one-dimensional or four-dimensional frames.
While true, the empirical evidence for four dimensional space-time is exceedingly robust.

Quote Quote by finiter View Post
The spacetime can be regarded as the volume at a given time; it is the product of a volume factor and a time factor, ie, it is four dimensional. When the system contracts, the time factor decreases. Mathematically it is time moving back. But in real terms, the direction of time does not change, but the directions of the space dimensions are reversed.
That would merely indicate that you chose a poor proxy for "increasing time", as increasing time should always be identified with increasing entropy.
AC130Nav
AC130Nav is offline
#41
Aug15-10, 04:14 PM
P: 154
[QUOTE=Chalnoth;2839984]While true, the empirical evidence for four dimensional space-time is exceedingly robust.[QUOTE]

Is it really evidence for FOUR-dimensional space-time? I think quantum mechanics indicates time must have at least two dimensions. After it's absolute nonsense that the observer determines and outcome.

The original evidence for the Big Bang was a uniform background radiation and seeming uniform expansion in all directions. The first was seen as making the formation of galaxies impossible and quickly non-uniformity was found. The second is based on the part of the universe we can observe. Is the world really flat?
Chalnoth
Chalnoth is offline
#42
Aug15-10, 05:55 PM
Sci Advisor
P: 4,721
Quote Quote by AC130Nav View Post
Is it really evidence for FOUR-dimensional space-time?
Yes.

Quote Quote by AC130Nav View Post
I think quantum mechanics indicates time must have at least two dimensions.
Huh? No, not at all. All of quantum mechanics is based around a single dimension of time. And when people try to add a second dimension of time, they end up with closed timelike loops, which many people consider to be contradictory.

Quote Quote by AC130Nav View Post
After it's absolute nonsense that the observer determines and outcome.
Yes, but that doesn't require more than one dimension of time. Everett explained how this works back in the 50's.

Quote Quote by AC130Nav View Post
The original evidence for the Big Bang was a uniform background radiation and seeming uniform expansion in all directions. The first was seen as making the formation of galaxies impossible and quickly non-uniformity was found. The second is based on the part of the universe we can observe. Is the world really flat?
Er, it wasn't that quickly. The non-uniformity of the CMB wasn't observed until the early 90's (with the COBE satellite), about 40 years after it was first observed. That isn't very fast in my book. But it was pretty obvious that such non-uniformity had to exist, it was just too small to detect until that time.
finiter
finiter is offline
#43
Aug16-10, 08:21 AM
P: 26
Quote Quote by Chalnoth View Post
That would merely indicate that you chose a poor proxy for "increasing time", as increasing time should always be identified with increasing entropy.
I agree. In the case of universe that is the accepted opinion. But in the case of a theoretical system, is it not possible that entropy decreases with time? Here, I am tempted to question the concept of entropy itself. Is it not logical to take that the entropy of a contracting star decreases, while the entropy of the universe increases? Both are related and there would be symmetry. Matter contracts while universe expands and vice-versa, and expansion would thus be self limited. Of course, this would go against the existing concepts.
Chalnoth
Chalnoth is offline
#44
Aug16-10, 08:32 AM
Sci Advisor
P: 4,721
Quote Quote by finiter View Post
I agree. In the case of universe that is the accepted opinion. But in the case of a theoretical system, is it not possible that entropy decreases with time?
The direction in which time increases is defined as the direction where entropy increases, so the answer is no. This is, by the way, the only way in which you have an arrow of time at all: if the entropy is constant (which would mean the system is at equilibrium), then there is no way to distinguish the past from the future, and there is no arrow of time.

Quote Quote by finiter View Post
Here, I am tempted to question the concept of entropy itself. Is it not logical to take that the entropy of a contracting star decreases, while the entropy of the universe increases? Both are related and there would be symmetry. Matter contracts while universe expands and vice-versa, and expansion would thus be self limited. Of course, this would go against the existing concepts.
Obviously one has to be careful when describing what one means by "entropy increasing". In general the direction of increasing time is the direction of increasing entropy only for a closed system. If it's an open system, we can still make the same identification, but it requires we take into account anything flowing into/out of the system, so the full statement becomes more complicated.

In general we actually have a rather poor understanding of exactly how entropy relates to gravitational systems, so we don't actually know how to write down the entropy of a contracting star. But we can write down the entropy of a diffuse gas, and we can write down the entropy of a black hole. The entropy of the black hole (which can be seen as a far extreme of the contraction of th star) vastly exceeds the entropy of the diffuse gas from which it came. From arguments like this we understand that the universe becoming more clumpy with time is a manifestation of increasing entropy. In fact, it is this fact, the clumpiness increasing with time, and not the expansion, that is the primary increase in entropy since the end of inflation.

If it were to be the case that our universe were to recollapse (which today seems manifestly unlikely), then we would still expect our universe to become more and more clumpy as it did so.
finiter
finiter is offline
#45
Aug16-10, 08:53 AM
P: 26
Quote Quote by Chalnoth View Post
In general we actually have a rather poor understanding of exactly how entropy relates to gravitational systems, so we don't actually know how to write down the entropy of a contracting star. But we can write down the entropy of a diffuse gas, and we can write down the entropy of a black hole.
Is it not that the entropy of a black hole thus obtained is more speculative than factual?
Chalnoth
Chalnoth is offline
#46
Aug16-10, 09:08 AM
Sci Advisor
P: 4,721
Quote Quote by finiter View Post
Is it not that the entropy of a black hole thus obtained is more speculative than factual?
No. Basically, a black hole is a much simpler system than, say, a star, and the entropy can be derived through a variety of independent methods, all arriving at the same result: the entropy of a black hole is proportional to the area of its horizon.

Ultimately, understanding the entropy of most systems where gravity is a significant factor (e.g. stars, galaxies) is likely to require knowledge of quantum gravity. But a black hole is just one of those special cases that is mathematically simple enough that we can be quite sure about its entropy already.
finiter
finiter is offline
#47
Aug17-10, 04:06 AM
P: 26
Quote Quote by Chalnoth View Post
But a black hole is just one of those special cases that is mathematically simple enough that we can be quite sure about its entropy already.
Then, what about the black holes themselves? Are these not just a theortical stuff, that too more mathematical than physical?

One should be sceptical about mathematical models. Mathematics is a tool, in fact, an excellent tool, for analyzing. But of late, it has changed its role, it appears, and has become a shaping tool.

Coming back to black holes, does the scientific community accommodate the Doubting Toms even now, or do the Doubting Toms outnumber the others?
Chalnoth
Chalnoth is offline
#48
Aug17-10, 05:58 AM
Sci Advisor
P: 4,721
Quote Quote by finiter View Post
Then, what about the black holes themselves? Are these not just a theortical stuff, that too more mathematical than physical?
There are different degrees of skepticism where black holes are concerned. Most today are largely convinced that they are the objects at the centers of galaxies, and also make up one member of certain binary systems. We have some observational tests in the works using extremely large baseline interferometry to actually observe the shape of the event horizon, so in any case we'll be quite sure whether or not these objects are black holes in a few years' time.

Quote Quote by finiter View Post
Coming back to black holes, does the scientific community accommodate the Doubting Toms even now, or do the Doubting Toms outnumber the others?
The scientific community doesn't kick anybody out. It's just that nobody listens to "Doubting Toms" that don't bring evidence to the table, or worse refuse to pay attention to the evidence we already have. This is the way it works in science: slowly more and more people become convinced of an idea as more and more evidence mounts in support of it. There typically remain some holdouts who continue to seek alternative explanations, and often even if we don't agree with them, the rest of the scientific community recognizes that they provide essential value to the scientific enterprise as a whole because there is always the possibility that we are wrong.

But at the moment my impression of people who do research in the area of black holes has been that the number of people who seriously doubt that black holes are real (or at least are not a very good approximation to reality) is vanishingly small.
AC130Nav
AC130Nav is offline
#49
Aug17-10, 07:37 AM
P: 154
Quote Quote by Chalnoth View Post
But at the moment my impression of people who do research in the area of black holes has been that the number of people who seriously doubt that black holes are real (or at least are not a very good approximation to reality) is vanishingly small.
I would expect most who spend time and money looking for black holes to believe in them. But how many have the earlier Hawking belief they are forever (might even be little universes), or adopt the Hawking revision which allows them to dissipate? Hopefully, there are yet other positions.

Obviously, something happens when too much mass gets in one place, ergo some kind of black hole. But math belongs in the experiment phase of the scientific method, not in theory.
Chalnoth
Chalnoth is offline
#50
Aug17-10, 09:16 AM
Sci Advisor
P: 4,721
Quote Quote by AC130Nav View Post
I would expect most who spend time and money looking for black holes to believe in them.
Well, no, this is a false characterization, because we're talking about people who are studying the most compact objects. Whether or not they are black holes is a crucial question that must be answered when studying these objects.

Quote Quote by AC130Nav View Post
But how many have the earlier Hawking belief they are forever (might even be little universes), or adopt the Hawking revision which allows them to dissipate?
There is no question that black holes evaporate. If they don't evaporate, they're not black holes. It's intimately connected with the entropy calculation I mentioned above.

Quote Quote by AC130Nav View Post
Hopefully, there are yet other positions.
There have not yet been any compelling alternatives to black holes presented.

Quote Quote by AC130Nav View Post
Obviously, something happens when too much mass gets in one place, ergo some kind of black hole. But math belongs in the experiment phase of the scientific method, not in theory.
I hope you realize that this has been tested? That people haven't merely taken this on faith?
DaveC426913
DaveC426913 is offline
#51
Aug17-10, 10:38 AM
DaveC426913's Avatar
P: 15,325
Quote Quote by Chalnoth View Post
I hope you realize that this has been tested? That people haven't merely taken this on faith?
I don't know what you mean by 'tested'. We can't test the physics of BHs, but our mathematical models of them do explain what we see via (albeit rather remote) observation.
Chalnoth
Chalnoth is offline
#52
Aug17-10, 11:13 AM
Sci Advisor
P: 4,721
Quote Quote by DaveC426913 View Post
I don't know what you mean by 'tested'. We can't test the physics of BHs, but our mathematical models of them do explain what we see via (albeit rather remote) observation.
Well, theories are only really tested in science through the collection of a variety of independent sources of evidence. When the evidence from many directions all agrees with a given theory, and no compelling alternatives are presented, most scientists become convinced that the theory is at least approximately true. From what I gather from those who study this sort of thing, black holes passed that point some time ago.

Now, we may speculate wildly as to whether or not the objects we currently call black holes may be something that we haven't thought of yet. And they may be. But there isn't really a good way to actually go out and investigate the vague notion of, "something we haven't thought of yet," so it only makes sense to provisionally accept the theory we have until such time as evidence is presented that contradicts this.

Edit: And I'd like to point out that I purposely don't often express this level of skepticism, because it turns out that once a majority of scientists become reasonably convinced something is true within their own field of study, that is almost never overturned. In fact, I can't think of a single situation where this has occurred since the onset of modern science (say, about 150 years ago for physics, give or take). The possibility always remains, of course, but it's just unlikely.
AWA
AWA is offline
#53
Aug17-10, 11:50 AM
P: 134
Quote Quote by Chalnoth View Post
I'd like to point out that I purposely don't often express this level of skepticism, because it turns out that once a majority of scientists become reasonably convinced something is true within their own field of study, that is almost never overturned. In fact, I can't think of a single situation where this has occurred since the onset of modern science (say, about 150 years ago for physics, give or take). The possibility always remains, of course, but it's just unlikely.
How about the aether, energetic versus atomic theory of matter (Boltzmann suicide included), blackbody radiation continuous emission, static universe... just to name a few that come to mind.
You haven't given your assertion much thought, have you?
Chalnoth
Chalnoth is offline
#54
Aug17-10, 12:23 PM
Sci Advisor
P: 4,721
Quote Quote by AWA View Post
How about the aether, energetic versus atomic theory of matter (Boltzmann suicide included), blackbody radiation continuous emission, static universe... just to name a few that come to mind.
You haven't given your assertion much thought, have you?
Demonstrate that a majority of scientists were convinced of any of these things at one time.

Yes, there have been many ideas in science that turned out to be false. But it seems to me that the majority of them never passed the level of, "Well, maybe this explains things, let's test it!" Many others stem from pre-science concepts, and weren't overturned until we developed the ability to actually investigate them (thus they couldn't be rightly considered conclusions of science, and instead just suppositions).


Register to reply

Related Discussions
big bang, schmig bang: everything's just shrinking General Astronomy 43
Big Bang Cosmology 16
Big bang and small bang black holes Cosmology 5
Big Bang and the first cause General Discussion 37
Round One: Big Bang vs. Little Bang... General Astronomy 4