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John Conway talks about this over at Cosmic Variance: http://cosmicvariance.com/2008/11/02/cdf-ghost-muons/

You haven't heard then?

There was a recent post on the physics group blog Cosmic Variance about potential job applicants having webpages and getting Googled during the course of hiring for academic positions- postdocs and faculty. So it's not just the students, it's faculty as well.

Sean has good reason to have an opinion on the subject, by the way.

There was a recent post on the physics group blog Cosmic Variance about potential job applicants having webpages and getting Googled during the course of hiring for academic positions- postdocs and faculty. So it's not just the students, it's faculty as well.

Sean has good reason to have an opinion on the subject, by the way.

There was a recent post on the physics group blog Cosmic Variance about potential job applicants having webpages and getting Googled during the course of hiring for academic positions- postdocs and faculty. So it's not just the students, it's faculty as well.

There are lots of questions you can't have on a job application (sexual orientation, religion, etc.) but if an applicant volunteers that information, that is permitted. And the attitude seems to be that if information is on a webpage, it is "volunteered" to the world.

Say with MOND, why are we so scared to think that perhaps Newtonian mechanics aren't quite enough to calculate with on galactic scales? Why do they think MOND is for cranks and crackpots?

Who's this "they"? There is a really unpleasant meme running around Slashdot that reactionary scientists scorn and mock anything that isn't mainstream. In reality, MOND is not a mainstream theory (and for good reason), but it's still discussed seriously. Google around for Sean Carroll's presentation a year or two ago on MOND vs. dark matter (can't remember when that was when he was at U. Chicago or Caltech). This is a good summary of his for why dark matter is likely to exist.

What of a static non-expanding universe and alternate redshift paradigms? Are they not just as feasible as exotic matter that only interacts gravitationally?

No, not even remotely. They are vastly less plausible than even MOND, which has problems of its own. But since this is about dark matter and its competitors, I'll stick to those.

I'm just curious as to why dark matter is so widely supported, is it merely because breaking the standard model makes physicists too uncomfortable?

Yeah, it's because theoretical physicists hate new theories. No theoretical physicist ever got fame and tenure by coming up with a new theory. They gotta stick to the old ones to survive.

Seriously, tone down the paranoia. Dark matter also breaks the Standard Model by introducing new kinds of particles. (Well, unless you think it's axions, which are arguably part of the Standard Model). It's not like nobody has ever thought of an alternative gravity theory before. You can carpet a small moon with all the alternative gravity theories out there; scalar-tensor gravity, vector-tensor gravity, conformal gravity, chiral gravity, supergravity, and so on.

The simple facts are that it's really hard to muck around with gravity in a way that simultaneously agrees with observations on tabletop, stellar, planetary system, galactic, and cosmological scales. With MOND it's easy to reproduce galactic rotation curves, but not much else. MOND also contradicts relativity; it's strictly Newtonian. There has been an attempt to correct that in the form of TeVeS (tensor-vector-scalar gravity). But TeVeS requires you to introduce two new gravitational fields, plus couple them together in just the right way, and even then it's far from certain whether it can explain dark matter evidence on all different scales (galaxies, clusters, cosmology, etc.). Furthermore MOND has difficulty explaining rather direct evidence of dark matter like the Bullet Cluster. Even if MOND is correct, it seems likely that you still need dark matter to explain everything.

After all that, MOND looks far more ad hoc than just postulating the existence of a new kind of particle, especially since most of the new particle theories out there predict some kind of dark-matter like particle anyway for completely independent reasons. It's not like weakly interacting particles are terribly bizarre in the first place; neutrinos are dark matter, although they're too light to be most of the dark matter. The main difference between most of the dark matter and neutrinos is mass, and what's so odd something weak like a neutrino, only heavier? Such a particle, predicted by many theories, can (unlike MOND) simultaneously explain all the astrophysical phenomena which point towards dark matter.

Not that inflation factors into taxation anyway. 17% of $20 is $3.40 whether a loaf of bread is $1 or $20. Inflation, however, does affect spending.

17% of $INCOME changes as $INCOME changes, and $INCOME changes as inflation progresses. They're inherently tied together in a very straightforward way.

PS - Check out the Laffer Curve [wikipedia.org]. There is a spot of optimum taxation where you'll get the maximum revenues.

Personally, I wonder where we are along the neo-Laffer curve. I can't tell you how much of a hoot it was to see that chart drawn up in the Wall Street Journal in an editorial from the AEI. Apparently, Norway is at exactly the optimum tax rate. Who new?

Sean Carroll explains things in more detail at his blog. http://cosmicvariance.com/

how do two particles on different time scales stay connected?

So now drop one particle of the pair into a black hole.

If they remain entangled, then you clearly have a way to pass information out of the black hole

What a co-incidence? I was just reading On Choosing a Graduate School: A Dialogue....

A: Hey, what's up? You're looking a little anxious these days.

B: I know. We're getting close to the romance deadline.

A: The romance deadline?

B: Yeah, in a couple of days I have to decide who I'll be going out with for the next five years or so.

A: Oh, right, I forgot. Have you decided between boyfriend and girlfriend?

B: I've thought about it a lot, and I definitely want a girlfriend.

A: That's cool. But don't you worry that the standards are higher if you say you want a girlfriend? I've heard that boyfriends are much easier.

B: I heard that, too. But girls are what I'm really passionate about.

A: Couldn't you just get a boyfriend first, and then switch if you don't like it?

B: Some people try that, but it can be awkward. Better to just be honest about your intentions from the start.

A: Fair enough. So did you get any acceptances?

B: Yeah, two different women have agreed to date me. Cindy and Alyssa. But I have to choose one.

A: Hey, that's great that you go two offers. Have you made a choice yet?

B: Well, I had coffee with Alyssa, and we really hit it off â" she's beautiful, and charming, and laughed at my jokes. I definitely think we would get along well over the next few years. I met Cindy, too; she's a knockout, and clearly very talented, but there wasn't as much of a spark there.

A: That can happen. So are you going to choose Alyssa?

B: I'm tempted, but the thing is â" Cindy's US News ranking is much higher.

A: Her what?

B: Every year, US News puts out rankings of boyfriends and girlfriends. Now, Alyssa is a solid top-20 girlfriend, but Cindy is top five! I'm really worried I'd be making a mistake by passing up the opportunity to go out with Cindy. Everyone has heard of her.

A: That sounds a little weird to me. How do they come up with these rankings?

B: Nobody knows, really. But everyone takes them very seriously. Still, I keep hoping that the NRC will update their boyfriend/girlfriend rankings soon. Those are supposed to be much more scientific.

A: NRC?

B: The National Romance Council.

A: But look, you seem to have really hit it off with Alyssa. Who cares that US News ranks Cindy higher? The concept of a âoeboyfriend/girlfriend rankingâ just doesn't make sense â" what matters is how well you personally get along with them, not some pseudo-objective measure of excellence.

B: It's easy to

A: Hey, what's up? You're looking a little anxious these days.

B: I know. We're getting close to the romance deadline.

A: The romance deadline?

B: Yeah, in a couple of days I have to decide who I'll be going out with for the next five years or so.

A: Oh, right, I forgot. Have you decided between boyfriend and girlfriend?

B: I've thought about it a lot, and I definitely want a girlfriend.

A: That's cool. But don't you worry that the standards are higher if you say you want a girlfriend? I've heard that boyfriends are much easier.

B: I heard that, too. But girls are what I'm really passionate about.

A: Couldn't you just get a boyfriend first, and then switch if you don't like it?

B: Some people try that, but it can be awkward. Better to just be honest about your intentions from the start.

A: Fair enough. So did you get any acceptances?

B: Yeah, two different women have agreed to date me. Cindy and Alyssa. But I have to choose one.

A: Hey, that's great that you go two offers.

We have failed to notice that gravity comes in two forms: the weak and the strong gravitational force. This paper characterizes these two forces, explains their conformance with regularly repeated experiment and observation, and sets out experimentally and observationally testable parameters which will conform to this two force model better than existing models of gravitation.

John Archibald Wheeler and the Smoky Dragon, by Jonathan Vos Post

There is a nice rememberance of Wheeler from one of his former students at the cosmic variance blog.

No comment...

http://www.physicsforums.com/showthread.php?t=202439

this one got pretty heated:

http://cosmicvariance.com/2007/11/16/garrett-lisis-theory-of-everything/

Cosmic variance (Daniel Holz', JoAnne Hewitt's, John Conway's, Julienne Dalcanton's, Mark Trodden's, Risa Wechsler's, and Sean Carroll's blog) has just been upgraded to support LaTeX! (http://cosmicvariance.com/2008/01/22/succumbing-to-latex/)

Comment 8, by Julienne: "This has to be the most fabulously geeky comment thread, EVAH!"

Then read comment 10, by Sean ;-)

There's some more interesting (scientific) commentary on this on the Cosmic Variance blog:
http://cosmicvariance.com/2008/01/01/what-have-you-changed-your-mind-about/

So they tell the NASA to stop spending their petty cash in Mars research and prioritize it to more significant issues, but of course the government can not for any reason reduce the spending in war. I saw that chart first form this post.

For some reason the link got lost: http://cosmicvariance.com/2006/08/21/dark-matter-exists/

It could have funded a a bit more than that.

There's a nice funding comparison chart that puts some perspective on it here

Sean's also a good friend, and I like people to think I have smart friends! And, while he is very smart, he is also a terrific explainer. Check his blog!

http://woodside.blogs.com/cosmologycuriosity/2006/ 05/queens_39_and_r.html

http://cosmicvariance.com/2006/04/26/relatively-pl easant/

Although I'm not an astrophysicist, I have studied astrophysics as an undergraduate and know some things about dark matter theories and cosmology. You are absolutely correct in saying that dark matter must be non-baryonic under current models. Baryonic dark matter is excluded because big-bang nucleosynthesis models (which take observed primordial elemental abundances as input) show that only ~4% of the mass of the universe can be baryonic matter.

You are, however, incorrect in stating that dark matter shares no properties with ordinary matter besides gravity. All energy, including electromagnetic radiation and dark energy, affect the curvature of spacetime. Dark matter also has the property that it behaves in the same way as matter when the universe expands, i.e. that its density decreases as the cube of the scale factor (which determines the rate of expansion). Ordinary radiation and dark energy each behave differently in this regard, so dark matter is indeed uniquely matter-like in a very important way. Aside from galactic rotation curves, very good data from the WMAP project that studies the cosmic microwave background has determined that ~30% of the universe must be matter-like. Combined with the BBN studies, this means that 26% of the universe, by mass, is dark matter, which thus outnumbers ordinary matter by more than a factor of 6.

You are also incorrect in assuming that we haven't found dark matter. There is actually a very excellent photo of colliding galaxies that shows convincing evidence of dark matter. The caption does a decent job at giving an explanation of the photo's significance. If you want a more thorough explanation, both of the photo and why the result is significant, I recommend this blog maintained by several well-known cosmologists.

sciency details:
http://cosmicvariance.com/2007/04/15/dragging-on/ (4:33 p.m.)

Also of interest if you're into this sort of thing, what Beyond Einstein programs will be cut?
http://scienceblogs.com/catdynamics/2007/04/beyond _einstein_iv_showdown_in.php (April 4)
sad if you compare sticker prices to the $10 billion per month on the Iraq adventure.

Here is an excellent article by Sean Carroll of the California institute of Technology that explains why all the suggestions of the parent post may not be correct.

Basically, what it says is that if two large clusters of galaxies went right through each other, and dark matter was really like the normal matter in the way the parent post suggests, we would get a different result from what would happen if dark matter was for real. Astronomers have discovered one such system and this provides conclusive evidence for the existence of dark matter.

If there were set laws and the answers were all clear, why not shrink it to a page or two so people could actually find them.

It's for the same reason that physics books are hundreds of pages long, and not just one or two pages of equations.

Thanks for the link. It is directly on point with Lee Smolin's conjecture, but it and its responses are "all Greek to me".

Allow me to address those two quotes which supposedly render string theory "fundamentally flawed":

"Describing the self-interaction of gravitons consistently turned out to be a tough nut to crack. We now understand that the failure to solve this problem is a consequence of not taking Einstein's principle of background independence seriously. Once the gravitational waves interact with one another, they can no longer be seen as moving on a fixed background. They change the background as they travel."

String theory has a consistent description of the self-interaction of gravitons. Manifest background independence in the equations is not needed; physical background independence of the theory arises dynamically. (There are also manifestly background independent formulations of string theory, but it is not yet known how they are all related to one another.)

"This is because supersymmetry implies that there is a symmetry in time, the upshot being that a supersymmetric theory cannot be built on a spacetime that is evolving in time."

This is false. You can do superstring theory in time-dependent backgrounds. (You can find discussion of this in this thread, particularly in comments by Moshe and Aaron Bergman.) It also is not true that this would mean you can't study cosmology or black holes, either; you can study both even in time-independent backgrounds.

At full throttle (7 TeV beams), the energy stored in the LHC beam is 700 MegaJoules, or 10 TeraWatts of power while the beam is dumped. How big is that? Well, 10 TeraWatts is about half of the world's total instantaneous power output. No wonder the accelerator folks are a bit jittery! They don't want to dump 10 TeraWatts of beam just anywhere...

Here's a picture (courtesy of Tom LeCompte) to illustrate this point. The kinetic energy of battleship guns is 300 Megajoules, or just less than half that of the full LHC beam. Now we understand why the machine folks want to be a bit cautious...

Only the other way round - I love it when physicists, computer guys and basically anybody with a bit of spare time, think that because they are recognized specialists in their field, they can authoritatively speak about philosophy, arts, literature, etc.

Yeah... Sean Carroll, a guy quoted in the article, was spotting in the act of making these semi-bogus pontifications a few weeks ago.

But what would the blogosphere be without all the half-baked retreads of arguments that were already expressed ten times better when they were first brought up by philosophers who have now been in the grave for several hundred years? Not much, I assure you of that.

Sean Carroll (and some other notable physicists) have a blog which covered this in more detail. See http://cosmicvariance.com/2006/11/16/dark-energy-h as-long-been-dark-energy-like/

He provides a great explanation for the reader without familiarity with advanced physics, but at a level which is still interesting to the technical reader.

What I meant is that String theory is falsiable, even Woit agrees with that. The matter is that with our current technology we can't do it. So it may not be a real theory (they insist on calling it a theory because it predicts gravity, but st is part of a paradigm in wich gravity is an existing theory), but it is scientific. more info in the discution at the bottom of cosmicvariance.com http://cosmicvariance.com/2006/06/19/the-string-th eory-backlash/ btw, Sorry about that, yes it was a semantics issue. Its odd, but in american english it seems that science is meant most of the time only as the natural sciences. I didn't knew that :P

They're just proposing that there is no "exotic", new kind of dark matter.

Incidentally, I'd watch the Cosmic Variance blog in the coming days for a discussion of this point; Sean Carroll's post there on dark matter was linked to in the last Slashdot story.

Responding to other posters: the amount of photons in the universe can be estimated based on how many of them reach us, as well as from theoretical predictions on the emission of light from stars, the Big Bang, etc., and is woefully inadequate to produce the needed gravitational effects — not to mention it is too "hot" to be the kind of dark matter needed to explain early universe structure formation.

An eV, or electron volt, is a measure of energy: the amount of energy acquired when an electron is accelerated through a 1-volt electric potential difference. It is about 1.6 * 10^-19 joules. By E=mc^2, it also corresponds to a mass, about 1.8*10^-36 kilograms. An electron, by comparison, masses about 511,000 electron volts.

The great accomplishment of late-twentieth-century cosmology was putting together a complete inventory of the universe. We can tell a story that fits all the known data, in which ordinary matter (every particle ever detected in any experiment) constitutes only about 5% of the energy of the universe, with 25% being dark matter and 70% being dark energy. The challenge for early-twentyfirst-century cosmology will actually be to understand the nature of these mysterious dark components. A beautiful new result illuminating (if you will) the dark matter in galaxy cluster 1E 0657-56 is an important step in this direction. (Heres the press release, and an article in the Chandra Chronicles.)

A prerequisite to understanding the dark sector is to make sure we are on the right track. Can we be sure that we havent been fooled into believing in dark matter and dark energy? After all, we only infer their existence from detecting their gravitational fields; stronger-than-expected gravity in galaxies and clusters leads us to posit dark matter, while the acceleration of the universe (and the overall geometry of space) leads us to posit dark energy. Could it perhaps be that gravity is modified on the enormous distance scales characteristic of these phenomena? Einsteins general theory of relativity does a great job of accounting for the behavior of gravity in the Solar System and astrophysical systems like the binary pulsar, but might it be breaking down over larger distances?

A departure from general relativity on very large scales isnt what one would expect on general principles. In most physical theories that we know and love, modifications are expected to arise on small scales (higher energies), while larger scales should behave themselves. But, we have to keep an open mind in principle, its absolutely possible that gravity could be modified, and its worth taking seriously.

Furthermore, it would be really cool. Personally, I would prefer to explain cosmological dynamics using modified gravity instead of dark matter and dark energy, just because it would tell us something qualitatively different about how physics works. (And Vera Rubin agrees.) We would all love to out-Einstein Einstein by coming up with a better theory of gravity. But our job isnt to express preferences, its to suggest hypotheses and then go out and test them.

The problem is, how do you test an idea as vague as modifying general relativity? You can imagine testing specific proposals for how gravity should be modified, like Milgroms MOND, but in more general terms we might worry that any observations could be explained by some modification of gravity.

But its not quite so bad there are reasonable features that any respectable modification of general relativity ought to have. Specifically, we expect that the gravitational force should point in the direction of its source, not off at some bizarrely skewed angle. So if we imagine doing away with dark matter, we can safely predict that gravity always be pointing in the direction of the ordinary matter. Thats interesting but not immediately helpful, since its natural to expect that the ordinary matter and dark matter cluster in the same locations; even if there is dark matter, its no surprise to find the gravitational field pointing toward the visible matter as well.

What we really want is to ta

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Forgive me for not being up on the details of this, but why do division algebras stop at 8? Does 16 not work?

See Toby Bartels's argument at the bottom of Week 59 of John Baez's TWF series.

Incidentally, the parent poster's argument is wrong. You can construct consistent field theories in N dimensions. Indeed, the division algebras have relevance to why string theory does work in 10 dimensions. (See, for instance, this paper and this comment.

There's a recent discussion of this topic, including debates between some key players, over at:
http://cosmicvariance.com/2006/06/19/the-string-th eory-backlash/#comments

I am a string theorist. I would write my own rebuttal to Peter Woit, who is well known in the community for being very vocal about his opinions, but it has already been well done (these are blog posts by Sean Carroll at Chicago/Caltech).

I'm all for public education on all topics of physics, including string theory, but this is an unfortunate case of a little bit of knowledge being dangerous for armchair physicists. In order to properly understand string theory requires understanding conformal field theory, supersymmetry and supergravity, Riemann surfaces, Kaluza-Klein theory, and so on, just to name a few of the introductory ideas. I don't think it's too unreasonable to assume that most of Peter Woit's audience has not studied any of these. But without studying string theory, I don't think it's possible to judge whether or not the things string theorists find compelling are in fact sufficiently exciting to warrant the attention it receives from them. For my part, I think they are.

I am a string theorist. I would write my own rebuttal to Peter Woit, who is well known in the community for being very vocal about his opinions, but it has already been well done (these are blog posts by Sean Carroll at Chicago/Caltech).

I'm all for public education on all topics of physics, including string theory, but this is an unfortunate case of a little bit of knowledge being dangerous for armchair physicists. In order to properly understand string theory requires understanding conformal field theory, supersymmetry and supergravity, Riemann surfaces, Kaluza-Klein theory, and so on, just to name a few of the introductory ideas. I don't think it's too unreasonable to assume that most of Peter Woit's audience has not studied any of these. But without studying string theory, I don't think it's possible to judge whether or not the things string theorists find compelling are in fact sufficiently exciting to warrant the attention it receives from them. For my part, I think they are.

If by 'concluded', you mean 'two minor figures raised the vague suggestion that various unidentified and pretty implausible factors may somehow conspire to explain dark matter, in a study whose serious flaws meant most in the physics and astronomy community ignored them', then yes. We 'concluded' dark matter doesn't need to exist.

http://cosmicvariance.com/2005/10/17/escape-from-t he-clutches-of-the-dark-sector/

Quoted from the article there....

Let's turn first to the attempt by Cooperstock and Tieu to do away with dark matter. To be honest, there are a bunch of problems with this paper. For example, equations (1) and (2) seem mutually inconsistent -- they have chosen one coordinate system in which to express the spacetime metric, and another in which to express the spacetime velocity of the particles in the galaxy. Ordinarilly, you have to pick one coordinate system and stick to it. More importantly, Korzynski has analyzed their solution carefully and noticed that they have secretly included not only the mass of the stars, but a completely imaginary thin sheet of infinite density in the galactic plane. So the fact that the rotation curves don't decay as they should is really no surprise.

But the real reason why most astronomers and physicsts didn't take the paper seriously is that it violates everything we know about perturbation theory. In the galaxy, there are two parameters that are very small -- the gravitational potential is about 10-6, and the velocity of the stars (compared to the speed of light) is about 10-3. So it would be surprising indeed if perturbation theory weren't doing a really good job in this situation, even just including the first-order contribution. The real reason why nobody paid much attention to Cooperstock and Tieu is that they didn't even seem to recognize that this was a problem, much less offer some proposed explanation as to why perturbation theory was breaking down. Extraordinary claims require extraordinary evidence, and we would need to be given a compelling reason to think that our perturbative intuition was failing before anyone would put a lot of effort into analyzing this paper.

It seems to be getting stronger.

An analysis: http://cosmicvariance.com/2006/01/11/evolving-dark -energy/

..."But we have nuclear fission, wind power, tides, biomass, hydro, and geothermo, and one day we'll have nuclear fusion...right?" No. First, he estimates that we need 10-30 TeraWatts (TW) of supply by 2050. Fission plants come in at about 1 GigaWatt (GW) of generation capacity (we don't know how to safely, securely and make efficient ones much bigger), and so we'll need to construct one new nuclear fission plant every other day -starting now- to meet the challenge. And then they only last 50 years... The biggest and brightest fusion project right now (in europe) is hoping to get break even several years from now, and then maybe built a working demonstration machine when it is probably already too late! He then continues to work down the list of all the other alternative sources, and you realize that they just won't even come close to what we need if we are truly going to stop dumping greenhouse gases into the atmosphere.

...

But then he reminds us that we have one source left, and it has way more energy than we can possibly need. The Sun. Two hours of sunlight hitting the whole earth's surface gives us the equivalent of the 30 TW for a year we need to be working at. Taking into account practicalities, we can expect about 600 TW or so fairly easily, and at 10% efficiency in recovering it and putting it to good use, we still are way ahead of what we need.

..."But we have nuclear fission, wind power, tides, biomass, hydro, and geothermo, and one day we'll have nuclear fusion...right?" No. First, he estimates that we need 10-30 TeraWatts (TW) of supply by 2050. Fission plants come in at about 1 GigaWatt (GW) of generation capacity (we don't know how to safely, securely and make efficient ones much bigger), and so we'll need to construct one new nuclear fission plant every other day -starting now- to meet the challenge. And then they only last 50 years... The biggest and brightest fusion project right now (in europe) is hoping to get break even several years from now, and then maybe built a working demonstration machine when it is probably already too late! He then continues to work down the list of all the other alternative sources, and you realize that they just won't even come close to what we need if we are truly going to stop dumping greenhouse gases into the atmosphere.

...

But then he reminds us that we have one source left, and it has way more energy than we can possibly need. The Sun. Two hours of sunlight hitting the whole earth's surface gives us the equivalent of the 30 TW for a year we need to be working at. Taking into account practicalities, we can expect about 600 TW or so fairly easily, and at 10% efficiency in recovering it and putting it to good use, we still are way ahead of what we need.