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Braneworld, black holes, and the fourth dimension

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If you looked at my other post today about the Living Reviews in Relativity, you may have felt that the subject matter (general relativity) was a bit esoteric. However -- and this shouldn't be a surprise -- it's just this sort of arcana that may come in handy for discussing other mysterious ideas, like the "fourth dimension".

In particular, there's this survey article: Brane-World Gravity.

What's interesting is that a couple of theorists have recently come up with a way that the "braneworld" idea can be tested observationally, and in such a way that would confirm the existence of at least one extra spatial dimension beyond the familiar three:

Scientists Predict How to Detect a Fourth Dimension of Space
Scientists at Duke and Rutgers universities have developed a mathematical framework they say will enable astronomers to test a new five-dimensional theory of gravity that competes with Einstein's General Theory of Relativity.

Charles R. Keeton of Rutgers and Arlie O. Petters of Duke base their work on a recent theory called the type II Randall-Sundrum braneworld gravity model. The theory holds that the visible universe is a membrane (hence "braneworld") embedded within a larger universe, much like a strand of filmy seaweed floating in the ocean. The "braneworld universe" has five dimensions -- four spatial dimensions plus time -- compared with the four dimensions -- three spatial, plus time -- laid out in the General Theory of Relativity.

The framework Keeton and Petters developed predicts certain cosmological effects that, if observed, should help scientists validate the braneworld theory. The observations, they said, should be possible with satellites scheduled to launch in the next few years.

I'm not going to tackle at this moment trying to explain the details, but I will give a few references, in addition to the survey article already mentioned.

I'll just make a few comments. First, the gravitational theory this work is based on (the Randall-Sundrum braneworld gravity model) is not exactly a rival that would completely replace Einstein's general relativity. It does not invalidate Einstein's theory. Rather, it's a generalization that assumes the existence of additional spatial dimensions beyond the familiar three. (These are spatial dimensions, not to be confused with the idea of time as a fourth dimension.) Since these additional dimensions are "compact", i. e. very small, their effect is to contribute small corrections to general relativity which are significant only under special circumstances. Additionally, this kind of model may explain facts that general relativity doesn't even address, such as why gravity is so weak compared to the other three main physical forces (electromagnetism and the "weak" and "strong" nuclear forces).

Second, what Keeton and Petters have done is to use the braneworld theory to predict that the universe could contain a significant number of small primordial black holes, i. e. black holes which were created in the original big bang. Under classical general relativity, as Stephen Hawking demonstrated, all such small black holes must have "evaporated" long ago. Under the new theory, these black holes could make up several percent of the mass of the universe. And they would be numerous enough that some could even exist in our solar system. They would also be numerous enough to affect the properties of gamma rays emitted in distant gamma ray bursts. And such effects are how the existence of these black holes may be observed within a few years.

Third, evidence for the existence of at least one additional spatial dimension would also be evidence for a major prediction of superstring theory and/or its generalization, M-theory. This is not a small deal, since these theories have been criticized as untestable. Of course, finding extra dimensions does not come close to "proving" these theories, but it certainly would be some evidence for one of their rather striking predictions.

Fourth, one of the originators of the Randall-Sundrum braneworld gravity model is Lisa Randall, who recently published a popular book, Warped Passages, that (among other things) explains a bit about the model, and a lot of the background ideas about "branes", superstring theory, and M-theory. (Refernces on Randall: home page, Wikipedia article. References on the book: home page, Wikipedia article, Amazon page.)

Other references:


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It's all relative

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In the previous message we mentioned an interesting article on relativity: Relativity at the centenary, which pointed out that gravitational physics has become an experimental science. There's a lot going on with studies of black holes, the search for gravitational waves, detailed tests of special and general relativity. And on the theoretical side, there's a lot of activity in the (as yet still unsuccessful) quest for a theory of quantum gravity.

Of course, the theory of relativity is intimidating to a lot of people, but it doesn't need to be. Special relativity actually involves little more that basic physics (ideas like mass, velocity, force, energy) and high school algebra with rudimentary calculus. The mathematics needed for general relativity is somewhat more sophisticated. But the basic concept described by the math is pretty simple: concentrations of matter cause space to curve, and the motion of two (or more) massive objects that interact gravitationally can be understood as "straight lines" in the curved space.

Once you've read some overviews, like the Wikipedia articles just referred to, you will have the basic ideas needed to learn more about relativity. Fortunately, there's an excellent online reference with a large number of review articles that explain in more detail many of the most interesting topics in the contemporary theory of relativity. It's called Living Reviews in Relativity -- and it's all free.

The journal, which covers both theory and experiment, is now in its 9th year. Some of the articles are valuable for understanding basic topics in cosmology. Here are some of the more generally accessible articles:


Note that there have been updated versions of some articles. In each case, only the most recent update (as of this writing) and its corresponding date are listed.

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Miscellaneous links, 2006-06-09

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Once again, a link fest...


Probing the antiworld
A free access article from the October 2005 issue of Physics World. (There's at least one in each issue, and it's usually worth the read.) This article describes how antimatter (antihydrogen) is actuall made in the lab. The ultimate goal is to allow a test of the CPT theorem by measuring the spectrum of antihydrogen.

Universe reveals its dark side
Another article from Physics World, the May 2005 issue. It provides a quick overview of what dark matter might be, and more detail on various experimental approaches to detecting dark matter. (Additional evidence is discussed here.)

Relativity at the centenary
One more article from Physics World, January 2005. Most physicists do not seriously doubt either special relativity or general relativity -- at least, not in almost all circumstances. But since relativity does not comport well with the other foundation of modern physics, quantum mechanics, there's plenty of reason to search for minuscule effects where relativity is not quite correct. The hope is that understanding such effets -- if they exist -- may give clues to how relativity needs to change in order to fit better with quantum mechanics. For instance, tests for violations of "Lorentz invariance", a key feature of special relativity, could tell us something about possible theories of quantum gravity, such as string theory. In another direction, tests of the inverse square law of gravitational attraction at small distances (under 10 mm) could indicate the existence of more than 3 spatial dimensions.

Brain's Darwin Machine
April 11, 2006 article from the Los Angeles Times. It describes a fascinating recent research finding. Every human brain contains roughly 100 billion (1011) neurons. Brains of 3-year-olds actually contain twice as many neurons, and half of them die by adulthood. But even in adulthood, thousands of new neurons form every day and play a role in building new memories. Amazingly, most neurons are genetically different from all others. The new discovery is that this difference seems to be due to the insertion of a short sequence of DNA, called a "long interspersed nuclear element", at random locations in the genome. The distinctness of most neurons may help account for the vast amount of information and experience that brains are capable of encoding.

Regrow Your Own
April 11, 2006 article from the New York Times. Regenerative medicine is about discovering how to induce human bodies to regrow damaged parts, just as some amphibians and fish can. Experimental stem cell therapies receive the most attention in this field, but there's an alternative: employing the same (or similar) genetic programs that the amphibians and fish use. This involves mature cells at the location of damage reverting to a more immature state, forming a cell mass called a "blastema", which in turn develops into the appropriate body part. Humans may have the necessary genes to do this, but for unknown reasons they are not switched on. There is reason to hope for a way around this, because in fact there is one organ that actually can regenerate after substantial damage: the liver.

Fighting Cancer Through the Study of Sarcomas
Here's an article from the September-October 2005 issus of American Scientist. A sarcoma is a cancerous tumor of bone, fat, or muscle tissue. Sarcomas are less common than cancers of the blood or internal organs, but exhibit the same abnormalities. It's often noted that cancer is really a large number of different diseases, each reflecting some different combination of genetic damage. Such genetic problems tend to be studied in more detail in sarcomas, leading to diverse kinds of experimental treatments. The article gives a number of examples, which illustrate a variety of the different abnormalities that lead to cancer.

BioCarta
This is the only link in this collection to a whole site rather than a single article. BioCarta is a commercial enterprise that supplies "uniquely sourced and characterized reagents and assays for biopharmaceutical and academic research." Their customers are generally engaged in the field of "proteomics", which is the study of how proteins interact with each other and thereby control all processes in and between living cells. But BioCarta's site is less about their business than it is a fascinating catalog of known "pathways", which are biological programs made up of sequences of protein interactions that drive a particular cellular function, such as "apoptosis" (cell death) or regulation of the "cell cycle". Each pathway is described with both text and high-quality graphics. For example: the p53 signalling pathway -- p53 is a protein that has a key role in regulating the cell cycle, in order to prevent cell division if the cell's DNA has been damaged. Mutations in the p53 gene so that its protein malfunctions are very common in many types of cancer.
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