... into what exactly? It doesn't expand into anything. Internal distances between points in space merely increase.
This may be possible to imagine if you think of an infinite sheet of rubber. The sheet is unbounded, but you can still imagine it stretching in all directions.
If the universe is finite, like the surface of a sphere, you can also think of it as stretching without imagining a higher dimensional space "into which" it is stretching; it is perfectly possible to talk about distances between points in the universe changing without referring to points outside of the universe. It's just not possible to visualize this scenario with a finite space, because we always visualize 2D surfaces as embedded into a 3D space. But it's logically and physically possible.
Another way you could sort of visualize a finite unbounded space expanding, without it expanding "into" something, is to imagine the spherical surface just sitting there, and everything inside of it shrinking.
All of these visualizations, however, are limited, as I said. (Especially since space is not a two dimensional rubber sheet...)
How can something with no boundary expand? We often think of expansion as the movement of matter through space; the expansion is then delineated by a boundary between points that matter has visited and points that it hasn't yet.
In the Big Bang picture, space is uniformly filled with matter. Instead of the matter moving to different parts of space it hasn't been before, matter starts out everywhere filling space, and space itself expands. By this I mean, space has a natural geometry (e.g., Euclidean, or non-Euclidean, or whatever). A geometry defines how far apart different points are. It is this geometry itself that changes: the matter doesn't do anything in particular, but the distances between all points changes. This geometry can be defined completely intrinsic to the universe, without appealing to any "outside" "into which" the universe expands.
Yes, but let's not confuse the visable universe with the Universe, Hawking's "Breif History of Time" includes a description of the visable universe as a black hole residing in the Universe. The funny thing about universe size black holes is that you could fall through the event horizon of one and not even notice. There is a difference between "the universe" and "the visible universe". The visible universe is delineated by a "cosmological horizon" which is in some ways similar to an event horizon of a black hole, but there are significant differences. The visible universe is not really "a black hole residing in the universe". See here for more.
If space is not expanding into anything except itself, then there is no absolute frame of reference on which to determine whether or not it really is expanding. That's not true; it would only be true if our "meter sticks" (whatever we use to measure distances) were expanding at the same rate as the universe itself. However, they do not.
So it wouldn't look like a black hole AT ALL. I call bullshit on the whole article. I think you may want to take a more informed look at their claims before making such strong statements.
The authors propose a wormhole constructed such that light takes so long to escape from its mouth, it's effectively indistinguishable from a black hole, because nobody can realistically wait long enough to see anything come out of it.
They write,
An immediate consequence of the metric (2.1) is that time in the throat is extremely slow from the point of view of a distant observer. Indeed, they are related by lambda, [...] The throat thus mimics what happens at the event horizon of a black hole where time is "frozen" [we recall that the old name (especially in Russia) for a black hole was a "frozen star"]. The only difference from an actual horizon is that time does not completely stop in the throat: if an observer makes observations during a time of order GM/lambda he or she will resolve the processes happening in the throat and thus be able to distinguish a wormhole from a black hole. Reciprocally, this preliminary remark suggests that if an observer only looks at a wormhole during a finite time he or she might not be able to distinguish it from a black hole. We shall see below, in several examples, that this is indeed the case, even for phenomena that are usually considered as characteristically linked to the presence of an horizon (such as no-hair properties, or dissipative properties). However, we shall see that the observing time span needed to distinguish a wormhole from a black hole is not GM/lambda, as suggested by the above naive argument, but rather GM/ln(1/lambda).
Which is actually something that's been bothering me since I thought of it: I feel like there's a tendency in cosmology to forget that time is also a dimension, and that the big bang is an expansion not of SPACE, but of SPACE-TIME. No. "Expansion" refers to space, specifically an increase of spatial distances over time.
However, it is true that spacetime is curved.
So if space and time is expanding, how can it be something that is taking time? How can time be expanding along a timeline? That's one of many reasons why nobody speaks of "spacetime" expanding. It doesn't have a meaningful definition.
This is an issue of semantics, not of physics.
I get the feeling, in all these many multi-dimensional theories of our universe, that it's a mistake to think about "time" as being somehow distinct and "special" as a dimension. Geometrically, it is distinct and special. It's because the geometry of spacetime is described by a modified Pythagorean theorem (a Lorentzian metric), in which the sign of a (squared) timelike displacement is opposite to that of a spacelike displacement.
Space and time are unified into spacetime, but that doesn't mean that space and time are the same thing. Rather, it means that what is "space" to one observer may be a mix of "space and time" to another. However, all observers agree on whether a direction is overall timelike or overall spacelike.
But in that case, does time play some special role in the big bang? In general relativity, time isn't even defined at the Big Bang; the geometry of spacetime breaks down. In a replacement theory of quantum gravity, who knows...
There is a cosmological event horizon beyond which no events will be able to influence us, as they expand away from us so much that light from them will never reach us. This is quite different from the event horizon of a black hole, however. The location of a cosmological event horizon is observer-dependent: different observers can receive information from different places. The location of a black hole is observer independnt, because it is defined as a region from which light cannot escape to infinity: a concept that does not depend where you are observing the black hole from.
Hang on, I thought it was matter/energy that carried space(-time) with it, not the other way around? It's metaphorical: it is difficult to make that statement precise and physically meaningful. I phrased it that way to get away from the incorrect idea of the Big Bang as an explosion of a point of matter inside an otherwise empty space.
Once can say that the expansion of the universe is due to the expansion of space, which means that the distances between spatial points change with time.
As John A. Wheeler said, space tells matter how to move; matter tells space how to curve.
New Scientist has a link to the paper, which is small and off to the side and easily overlooked (and does not make clear that the whole paper can be accessed, not just the abstract). The paper is here for anyone who may have missed it.
Well, if everything has gravity, then the universe itself has a gravitational pull. Eventually the mass of the universe would be such that any light trying to escape it would be pulled back inside, which would make the universe appear to be black hole from anyone on the outside looking in... It sounds like your teacher may have had the misconception that the universe is an expanding sphere, with stars and galaxies on the inside, and a void outside into which the matter expands.
That's not how Big Bang cosmology works, however. In that theory, all of space is filled with matter, and space itself expands, carrying the matter with it. There is no "edge".
Consequently, it doesn't make much sense to speak of light trying to "escape" the universe, since the universe has no boundary. That's why it's problematic to speak of the whole universe as a "black hole".
I'm not talking about the economic cost of removing carbon from the air. I'm talking about doing the carbon accounting. That is to say, how much carbon do you have to produce in power plants to build and power these one million extractors, and what are the carbon costs of all the support devices and facilities? That's part of the economic analysis of sequestration: the ultimate question they attempt to answer is not what are the costs of sequestration, but rather what are the costs of stabilizing atmospheric CO2 concentrations at a desired level.
The main upshot: it's cheap to store the carbon. It's not so cheap to capture it in the first place, and doing so requires ramping up power generation at coal/gas plants. (Most of the carbon is captured from power plants that emit it, not sucked generically out of the air.) However, even with ramped up power production, net CO2 emissions still drop substantially: to maybe 10%. The side effects: greater amounts of air pollutants, and greater fuel consumption. The increased fuel consumption leads to greater carbon emissions from activities related to coal mining, but that is minimal compared to the current carbon emissions of the power plants themselves — the emissions which would largely be eliminated with capture and sequestration.
I've seen a number of comments here arguing that we should reduce emissions (abatement) instead of capturing CO2 (sequestration). After all, it's better to prevent the disease than have to cure it, and relying on "quick fixes" just encourages people not to face up to the real problem.
Well, that's nice in principle, but not fully realistic. It takes a great deal of economic and social change as well as political will to substantially reduce emissions. Even with full support from government and industry, it takes time to alter basic infrastructure. A "quick fix" buys time to put those slower changes into place. A lot of studies are showing that there are modest economic benefits to sequester early while abatement is ramping up, than not to sequester at all and try to do more abatement all at once: with a carefully planned combination of sequestration and abatement, overall CO2 concentrations may level out sooner and at less cost.
Most likely, after a detailed energy accounting is done, this system of 1 million co2 removing machines and its associated systems will be seen to be drawing out of the atmosphere slightly less than the amount of carbon that powering it puts into the atmosphere. Come on now scientists. Stop this one dimensional thinking. Do the balancing. Yeah, because you're so much smarter than those dumb so-called "scientists", nobody has ever thought of that before.
This whole situation reminds me of Bruce Schneier's observation that when deep quality metrics are unavailable, customers will base their decisions on shallow metrics instead. "Since it is generally impossible to measure what is important, bureaucrats instead turn their energies toward making important what is measurable."
Now, what state is the rock in? Is it in one, or the other? The answer is both, of course, as per Schrodinger's Cat, until it is observed. But then what state is whatever it is that observed the rock? That's pretty much the issue that the consistent histories interpretation is designed to address. Of course, you may not like that interpretation.
Humans don't have anything special to do with "observing" ("collapse of the wavefunction" or "state reduction"). A particle can be "observed" by a rock, or by any other "classical" macroscopic system with which it can entangle. Quantum decoherence in the consistent histories interpretation, IMHO, comes closest to explaining this process.
Damn those fuckers to hell. You play nice, you're a "primadonna" because you had a nervous breakdown when your parents split. You play rough, and you're a lowlife scumfuck without the sophistication to breed. Fuck'em all and their social games. They'll see. You'll wake them up and they'll see. They'll see themselves for the compassionless, stupid fucks they are. Yeah, it'll be sweet. Somebody better call the cops on this "MeanderingMind" guy. He sounds unbalanced.
The article mentions that a new round of global-warming may be taking place on Mars - does this lend any credence to the theory that global warming is an unavoidable solar event? No.
There are good reasons to believe that the warming on Mars is not due to the Sun (here and here). There are even better reasons to believe that most of the warming on Earth is not due to the Sun (e.g., here and a bunch of essays here).
By 'nope' I mean that QM and GR _do_ work together, because we've tested both of them and found them to be correct. We just don't know how to use both of them at the same time:) If we can't use them both at the same time, then they don't really "work together". In fact, they are mathematically inconsistent with each other. But you can, as I mentioned, work in the fixed-background spacetime approximation and do quantum theory in gravitational fields.
And quantum mechanics and special relativity (i.e. quantum mechanics in flat Minkowski space) were successfully united back in 30-s. It's GR which causes problems. That's why I wrote "(general) relativity".
String theory has exactly the same problems with renormalization. What "problems"? String theory is UV finite, unlike, e.g., quantum electrodynamics.
I don't know what the "nope" is supposed to disagree with. The statement that QM and GR don't get together? You appear to agree with that. The statement that you'd get a Nobel prize for combining QM and (general) relativistic effects? You would, if you could provide strong support for such a theory.
As for your own statement, quantum theory works fine in curved spacetime, as long as you ignore the backreaction of the quantum fields on spacetime geometry. If you don't ignore it, that's the putative theory of quantum gravity the parent poster was talking about. There are various candidates for such a theory, e.g. string theory (which uses both renormalization and perturbation theory, by the way).
Whether you agree with Lindzen or his skeptics, one thing you must conclude from the article is that global climate is still not understood well enough for anyone to make accurate predictions of what will happen in 1 year, 10 years, 100 years. And why "must" we conclude that from the article?
It is clear from the article that the role of clouds (which is only one component of many in climate change) is still being seriously debated, for instance. The role of clouds is uncertain, but that doesn't mean that we can't predict anything. Uncertainty in the cloud feedback means that we are uncertain about the climate sensitivity to CO2 emissions, which summarizes the contributions of all the various climate feedbacks. But when you assimilate the observational data, the probable range of climate sensitivity allowed by the data still leads to substantial warming over the next century.
And those predictions are always based on models which includes assumptions about how different components of climate change interact. This is non-insightful. All predictions in all fields of science are based on assumptions, but that does not mean that all predictions are worthless — not even when some of the assumptions are inaccurate! As George Box once said, "all models are wrong, but some are useful". Every model makes approximations, and there are always known inaccuracies in any model. The issue is whether current climate models are accurate enough to be useful. On the basis of the physical tests of the assumptions which go into them, and on cross-validation and intermodel comparison studies, I would argue that they are. If you want to argue the opposite, go ahead, but you can't just dismiss them with a trite "they make assumptions".
It's much easier to believe information about Mars because the readings are extremely accurate and only come from modern instruments, and we know there is no human influence on temperature. Actually, the temperatures on Mars are known less well than the current temperatures on Earth; it is only as you go further back into the past that the terrestrial uncertainties become greater.
More to the point, however, is that less accurate data on Earth is still better than no data on Mars, which gets back to the 6-year criticism. And most to the point is that the Mars data does not actually establish a causal link between Martian and Earth climate, and there are very good reasons to believe that there is no such link. The only physically possible link is solar output, and changes in solar output are implausible sources of warming both on Mars (here, here) and on Earth (here, here).
The scientific community generally regards Milankovitch cycles as being in large part responsible for non-industrial era warming. No, but many regard Milankovitch cycles as being largely responsible for the ice age cycles. Milanokovitch cycles can't explain shorter-period climate changes (even those in the non-industrial era), because the orbital forcing doesn't take place on such short time scales.
Yet, when it comes to industrial era warming, proponents of human-caused global climate change say that CO2 emissions are driving temperature. This is a logical departure from the previous theory because it readjusts causality. It doesn't "readjust causality". Orbital variations cause climate change. Greenhouse gas emissions also cause climate change. Climate isn't driven by just one thing.
If from that above graph you believe that in ancient eras radiation drove temperature which drives CO2, then why the switch? Because the modern era isn't like the ancient era: it has additional large amounts of greenhouse gas emissions which were not present before.
Note several points:
1. In ice age cycles, temperature drives CO2, but CO2 also drives temperature: there is feedback. 2. Today we have large additional amounts of anthropogenic CO2 which are not driven by temperature. 3. Orbital variations are not responsible for the current increase in temperature, due to total mismatch in timescales. 4. Variations in solar output are also not responsible for the current increase in temperature: the solar variation over the last 150 years, and in particular over the last 40 years when the warming has accelerated, is wrong in timing, rate, and magnitude when compared with the climate trend.
The sun is a massive fusion reactor 330,000 times the mass of earth. Even small fluctuations matter. That obviously depends on just how small they are. As it turns out, fluctuations in power received by the Earth are too small (and of the wrong timing and rate) to explain the warming we observe. That is what you conclude when you go beyond handwaving and sit down and calculate. There have by now been a number of studies of this; the best review is by Foukal et al. last year.
Slashdot Mirror
... into what exactly? It doesn't expand into anything. Internal distances between points in space merely increase.This may be possible to imagine if you think of an infinite sheet of rubber. The sheet is unbounded, but you can still imagine it stretching in all directions.
If the universe is finite, like the surface of a sphere, you can also think of it as stretching without imagining a higher dimensional space "into which" it is stretching; it is perfectly possible to talk about distances between points in the universe changing without referring to points outside of the universe. It's just not possible to visualize this scenario with a finite space, because we always visualize 2D surfaces as embedded into a 3D space. But it's logically and physically possible.
Another way you could sort of visualize a finite unbounded space expanding, without it expanding "into" something, is to imagine the spherical surface just sitting there, and everything inside of it shrinking.
All of these visualizations, however, are limited, as I said. (Especially since space is not a two dimensional rubber sheet
In the Big Bang picture, space is uniformly filled with matter. Instead of the matter moving to different parts of space it hasn't been before, matter starts out everywhere filling space, and space itself expands. By this I mean, space has a natural geometry (e.g., Euclidean, or non-Euclidean, or whatever). A geometry defines how far apart different points are. It is this geometry itself that changes: the matter doesn't do anything in particular, but the distances between all points changes. This geometry can be defined completely intrinsic to the universe, without appealing to any "outside" "into which" the universe expands.
The authors propose a wormhole constructed such that light takes so long to escape from its mouth, it's effectively indistinguishable from a black hole, because nobody can realistically wait long enough to see anything come out of it.
They write,
However, it is true that spacetime is curved. So if space and time is expanding, how can it be something that is taking time? How can time be expanding along a timeline? That's one of many reasons why nobody speaks of "spacetime" expanding. It doesn't have a meaningful definition.
This is an issue of semantics, not of physics. I get the feeling, in all these many multi-dimensional theories of our universe, that it's a mistake to think about "time" as being somehow distinct and "special" as a dimension. Geometrically, it is distinct and special. It's because the geometry of spacetime is described by a modified Pythagorean theorem (a Lorentzian metric), in which the sign of a (squared) timelike displacement is opposite to that of a spacelike displacement.
Space and time are unified into spacetime, but that doesn't mean that space and time are the same thing. Rather, it means that what is "space" to one observer may be a mix of "space and time" to another. However, all observers agree on whether a direction is overall timelike or overall spacelike. But in that case, does time play some special role in the big bang? In general relativity, time isn't even defined at the Big Bang; the geometry of spacetime breaks down. In a replacement theory of quantum gravity, who knows
There is a cosmological event horizon beyond which no events will be able to influence us, as they expand away from us so much that light from them will never reach us. This is quite different from the event horizon of a black hole, however. The location of a cosmological event horizon is observer-dependent: different observers can receive information from different places. The location of a black hole is observer independnt, because it is defined as a region from which light cannot escape to infinity: a concept that does not depend where you are observing the black hole from.
This article clarifies some of these concepts.
Once can say that the expansion of the universe is due to the expansion of space, which means that the distances between spatial points change with time.
As John A. Wheeler said, space tells matter how to move; matter tells space how to curve.
New Scientist has a link to the paper, which is small and off to the side and easily overlooked (and does not make clear that the whole paper can be accessed, not just the abstract). The paper is here for anyone who may have missed it.
That's not how Big Bang cosmology works, however. In that theory, all of space is filled with matter, and space itself expands, carrying the matter with it. There is no "edge".
Consequently, it doesn't make much sense to speak of light trying to "escape" the universe, since the universe has no boundary. That's why it's problematic to speak of the whole universe as a "black hole".
For a related FAQ, see here.
The main upshot: it's cheap to store the carbon. It's not so cheap to capture it in the first place, and doing so requires ramping up power generation at coal/gas plants. (Most of the carbon is captured from power plants that emit it, not sucked generically out of the air.) However, even with ramped up power production, net CO2 emissions still drop substantially: to maybe 10%. The side effects: greater amounts of air pollutants, and greater fuel consumption. The increased fuel consumption leads to greater carbon emissions from activities related to coal mining, but that is minimal compared to the current carbon emissions of the power plants themselves — the emissions which would largely be eliminated with capture and sequestration.
The article lacked a photo of the Z Machine in operation. Amazing!
I've seen a number of comments here arguing that we should reduce emissions (abatement) instead of capturing CO2 (sequestration). After all, it's better to prevent the disease than have to cure it, and relying on "quick fixes" just encourages people not to face up to the real problem.
Well, that's nice in principle, but not fully realistic. It takes a great deal of economic and social change as well as political will to substantially reduce emissions. Even with full support from government and industry, it takes time to alter basic infrastructure. A "quick fix" buys time to put those slower changes into place. A lot of studies are showing that there are modest economic benefits to sequester early while abatement is ramping up, than not to sequester at all and try to do more abatement all at once: with a carefully planned combination of sequestration and abatement, overall CO2 concentrations may level out sooner and at less cost.
— J.M.W. Slack, Egg and Ego
Humans don't have anything special to do with "observing" ("collapse of the wavefunction" or "state reduction"). A particle can be "observed" by a rock, or by any other "classical" macroscopic system with which it can entangle. Quantum decoherence in the consistent histories interpretation, IMHO, comes closest to explaining this process.
The jets produced by a black hole originate outside of it, not inside of it. The black hole does not lose mass by producing jets.
There are good reasons to believe that the warming on Mars is not due to the Sun (here and here). There are even better reasons to believe that most of the warming on Earth is not due to the Sun (e.g., here and a bunch of essays here).
You're thinking of the work of Adrian Thomspson.
You mean perturbation theory, I assume.
I don't know what the "nope" is supposed to disagree with. The statement that QM and GR don't get together? You appear to agree with that. The statement that you'd get a Nobel prize for combining QM and (general) relativistic effects? You would, if you could provide strong support for such a theory.
As for your own statement, quantum theory works fine in curved spacetime, as long as you ignore the backreaction of the quantum fields on spacetime geometry. If you don't ignore it, that's the putative theory of quantum gravity the parent poster was talking about. There are various candidates for such a theory, e.g. string theory (which uses both renormalization and perturbation theory, by the way).
More to the point, however, is that less accurate data on Earth is still better than no data on Mars, which gets back to the 6-year criticism. And most to the point is that the Mars data does not actually establish a causal link between Martian and Earth climate, and there are very good reasons to believe that there is no such link. The only physically possible link is solar output, and changes in solar output are implausible sources of warming both on Mars (here, here) and on Earth (here, here).
Note several points:
1. In ice age cycles, temperature drives CO2, but CO2 also drives temperature: there is feedback.
2. Today we have large additional amounts of anthropogenic CO2 which are not driven by temperature.
3. Orbital variations are not responsible for the current increase in temperature, due to total mismatch in timescales.
4. Variations in solar output are also not responsible for the current increase in temperature: the solar variation over the last 150 years, and in particular over the last 40 years when the warming has accelerated, is wrong in timing, rate, and magnitude when compared with the climate trend. The sun is a massive fusion reactor 330,000 times the mass of earth. Even small fluctuations matter. That obviously depends on just how small they are. As it turns out, fluctuations in power received by the Earth are too small (and of the wrong timing and rate) to explain the warming we observe. That is what you conclude when you go beyond handwaving and sit down and calculate. There have by now been a number of studies of this; the best review is by Foukal et al. last year.