Missed Opportunity
I'm so out of touch. From this morning's local paper:
Michael Brigmond, Donald Lance and John Massey Jr. believe the end of the world is at hand. [...]
The three ministers are among the featured speakers at a four-day "End Time Prophecy Conference" at the Redemption Church of Christ of the Apostolic Faith in Troy, where Massey is the pastor.
Dammit, why doesn't anyone tell me these things. I mean, here we went and planned to have dinner with my parents tonight in Oneonta, and go see some Shakespeare up in Saratoga tomorrow. We're driving all over New York for entertainment, when we have crazy people right in our own back yard...
Your "Paging Fred Clark" moment:
These Christian ministers follow a particularly literal interpretation of the Bible.
Lose the "-ly", and you'd be closer. I'd probably like to see scare quotes around "literal" as well, but YMMV.
Posted at 7:34 AM | link | follow-ups | 4 comments
When Gravity Fails
In my last post, I described an experiment to measure gravity at very short distances. It's really a tour de force experiment (as I've said numerous times on this blog), but at this point, you might reasonably ask "Who cares?"
The question of the short-distance behavior of gravity is closely tied to one of the great physics mysteries, namely "Why is gravity so weak?" That might not seem an obvious mystery-- gravity feels pretty substantial whenever I try to dunk a basketball, for example-- but gravity is, in fact, extremely weak. If you take two one-kilogram masses, each with an electric charge of one Coulomb, and put them close together, the electrostatic repulsion between them is 100,000,000,000,000,000,000 times greater than the attractive gravitational force between them. The force of gravity attracting you toward the Earth is pretty substantial, but only because the Earth is gigantic; the force of gravity between two people is completely negligible.
To the average person, this is one of those "Well, whatcha gonna do?" kind of issues-- if gravity is much weaker than the other forces, well, that's just the way it is. It's deeply troubling to theoretical physicists, though, because there's no reason why that should be the case. The world would probably be a pretty weird place if gravity were closer in magnitude to the other forces, but in a certain way of looking at the world, it would make a lot more sense.
Lots of people have struggled to find a halfway satisfying explanation for why gravity is so feeble, and one of the more popular explanations comes out of the world of string theory, as a side effect of the extra dimensions required for most string theories. As I understand it, it's not the reason why string theorists introduced extra dimensions in the first place, but having introduced extra dimensions in order to make some of the math work out, someone hit on the idea that extra dimensions can explain the weakness of gravity.
The central idea here is that the world we live in, which seems to have three spatial dimensions (north-south, east-west, and up-down) actually has a much larger number-- ten or eleven is the current consensus, I believe, but there have been suggestions involving something like twenty at various times. These would be spatial axes perpendicular to all three of the normal ones-- some direction you could potentially move in that is at right angles to all the others. It's a tough thing to visualize, with the standard explanatory tricks involving things like cartoon characters popping up out of the confines of their two-dimensional worlds.
The reason we don't usually notice these extra dimension (because you would think that somebody would've noticed a whole extra direction of motion sometime in the last few million years of evolution) is that they're just not very big. One way to think about it would be to view the universe as being kind of like a sheet of paper: from a distance, a sheet of paper is a pretty good approximation of a two-dimensional object. You've got to get pretty close before you can really see that there's a third dimension there at all. If you were inhabiting a sheet of paper, able to move through 8.5 or 11 inches in the plane, but ony a tiny fraction of an inch in or out of the page, odds are, you wouldn't really notice that third dimension at all.
What does this have to do with gravity? The idea is that gravity extends out into the other dimensions-- out of the surface of the paper, as it were. Gravity isn't actually weak, according to this view-- it's roughly the same strength as any of the other forces. It's just that it looks weak to us, because a lot of its strength is being used up in those tiny extra dimensions that we don't see.
(Some theories even have gravity extending out into nearby parallel universes (neighboring sheets of paper in the giant Xerox machine of the multiverse). You can find people who will argue that the gravitational effects of "dark matter" are really due to normal matter in some other universe affecting the motion of things in our universe. I'm not really sure what that means, either.)
If this is true, you would expect to see some effect of these extra dimensions when you start to look at interactions between objects on roughly the same scale as the size of those extra dimensions. In the same way that you have to get pretty close to a piece of paper before you notice its thickness, you need to look on a really small scale before you notice these extra dimensions. The tool you use to "look" at them, in this case, would be gravity. Specifically, if gravity is really strong in those other dimensions, then you should start to see that extra strength when you look at two objects separated by about the same distance as the extent of one of those dimensions.
The clearest explanation I've heard of why you expect to see something happen on a short length scale requires a diagram, so you'll have to forgive some more ASCII art (I try to keep this blog image-free, for politeness). In this case, you want to imagine a nearly one-dimensional universe, which is very long in one direction (across the page), and very short in a second dimension (bounded by "~" characters). If you look at the lines of force emanating from a little NetHack guy stuck in this world, what you see is something like this:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
------------------------------------------
/ |
/ ----------------------------------------
/ ' | +
@--------------------------------------------- $
\ . | +
\ ----------------------------------------
\ |
------------------------------------------
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
This is supposed to represent lines emanating from the "@" in all directions, and then bending away from the edges of the "universe" to travel more-or-less straight down the line (the "'" and "." are place-holders for nonexistent shallower dashes...). A distant observer (the "$" stands for "$tring theorist") will determine the strength of the force he or she sees from the density of lines passing through some region of space (between the "+" signs, say). In this toy model, that number remains more or less constant until you get pretty close-- there's only one line between the "+" signs for most of that length. Somewhere around the vertical line ("|"), there are suddenly three lines, and then five, and our intrepid little $ will start to notice that the lines are diverging from a point, and not just a uniform bundle after all. The distance at which that starts to become apparant is similar to the scale of the small "extra" dimension.
Opinions vary as to the size of those extra dimensions in a better-drawn world, but everyday sizes have more or less been ruled out because gravity looks nicely Newtonian on those scales. There are some theories that put the size of the extra dimensions at tens or hundreds of microns (0.01-0.1 mm), which is a scale that's just barely accessible to the Eot-Wash expriments. And right at the very smallest separations at which they can measure the force of gravity, they see a slight change. It's a very preliminary result, and they're working to confirm it, but the group doing the experiments has a very solid reputation as a cautious bunch of people, so many people tend to think that if they're willing to mention it in public, they must have some confidence that it's a real effect, and not just noise.
Unfortunately for the really simple models of extra dimensions, the sign of the effect is wrong. What they see is a slight weakening of the force of gravity, where the simplest string theory model says extra dimensions ought to make the apparent strength of gravity increase. If the result holds up, it'll be the first real experimental test of a prediction of string theory, and there will presumably have to be some scrambling to accomodate it. That's when things will get interesting.
(I should note that the talk I saw on the experiment back at DAMOP mentioned a different model (not string theory) that would lead to a weakening of gravity at short distances. I don't recall any of the details of the explanation, though. Lubos Motl refers to it as "fat gravitons", and the idea seems to be that for whatever reason, gravity has a characteristic length scale, and just doesn't work at really short distances. It's sort of hard to see why that should be the case, but then, it's hard to see why there should be a whole bunch of tiny little extra dimensions...)
Posted at 8:42 PM | link | follow-ups | 11 comments
Testing String Theory
The string theory post has generated some lively debate, so I'll try to follow it up a bit more with some attempts to describe a couple of experiments that aim to test some of the basic predictions of theories beyond the Standard Model of particle physics.
Of course, my motive in this is not merely a cynical attemt at traffic generation (because, really, there are easier ways to get noticed than struggling to explain the details of very complicated physics experiments to the laity). There's also a certain element of annoyance at string theorists-- specifically, the heavy hitters in the comments at Cosmic Variance, who won't answer a direct question about experimental results. It's been asked three times, and the closest thing to a detailed answer it's received is from a non-native speaker of English, and thus all but incomprehensible.
(If you're wondering why string theorists have a reputation for arrogance and disdain for experiment, well, this really doesn't help. It's almost certainly not the simple yes-or-no question that it appears, but it surely deserves a response, even if the answer is "It's really complicated.")
So, in the absence of an actual string theorist willing to talk about it, I'll attempt to explain what's being asked in this comment:
If, as rumored, experiments show that gravity weakens at small distances, would this be a serious blow to string theory?
The question is a reference (I think) to some recent preliminary results by the "Eot-Wash" group at the University of Washington. These guys are working on making very sensitive measurements of the force of gravity at short length scales.
The basic expression for the gravitational force between two objects was discovered by Isaac Newton, and it says that the force between two objects is given by the gravitational constant (6.7 x 10-11 N m2/kg2, a very small number) multiplied by the masses of the two objects, divided by the square of the distance between them. The forces involved are really tiny, unless one of the two masses is huge, but if you're clever, you can measure them, and the 1/R2 dependence has been verified over a wide range of distances, from centimeters to the size of the Solar System.
There are some people out there who think that the theory may need to be modified on extremely long length scales (distances comparable to the size of entire galaxies or clusters of galaxies). There are also some predictions that gravity might look different on very short lengths scales-- a tenth of a milimeter or below. That's what the EotWash group is testing.
Amazingly, the basic method they're using to measure the influence of gravity hasn't changed since the late 1700's. In 1783, a fellow named Henry Cavendish came up with the idea of a "torsion balance" to measure gravity. The classic Cavendish experiment uses a dumbell-shaped weight hung from a very fine wire. If you bring two test masses close to the ends of the dumbell, on opposite sides, the gravitational force between the test masses and the dumbell will cause the dumbell to twist, with the amount of twist depending on the magnitude of the force.
What they're doing in Washington is an ingenious update of the Cavendish experiment. Instead of a dumbell, they use a disc with a bunch of holes drilled in it-- the spots where the holes aren't are effectively like the masses at the ends of the dumbell, only there are more of them, and they're symmetrically distributed. The disc is still suspended by a very fine wire, but instead of test masses ont the outside, it's suspended over a set of two more discs with holes in them: a thin top disc, and a thicker disc beneath it, with the holes offset from one another.
If you'll forgive a little ASCII art, a side view of the test discs would look something like this:
Thin Top Disc: ~~~~ ~~~~ ~~~~ ~~~~
Thicker **** **** ****
Bottom Disc: **** **** ****
The point of this odd arrangement is that the two discs should exactly cancel one another. The missing force due to the holes in the top disc is supplied by the solid bits of the bottom disc, which are more massive, but farther away. And the extra force due to the solid bits of the top disc are offset by the missing material in the holes in the bottom disc.
If you take either test disc by itself, and rotate it slowly beneath the pendulum, you should see a periodic twisting of the pendulum, as it's tugged back and forth as the holes rotate by. If you stack the two on top of each other, and then rotate them, you should see basically nothing, as the contributions cancel each other out. Provided, that is, that gravity continues to obey Newton's law (which is what's used to calculate the thickness of the discs, and the arrangement of the holes). If gravity gets stronger or weaker as you bring the test discs closer to the surface, you should see a clear signal.
They've done this basic experiment a bunch of times already, moving to smaller and smaller separations between the torsion pendulum and the test discs. Every time, they've had a null result-- gravity still agrees with Newton's three-hundred-year-old result.
Except, at DAMOP, someone from the Washington group presented highly preliminary results that might show an effect-- a slight weakening of the force of gravity at distances of less than a tenth of a millimeter. These are extremely preliminary numbers, and the possible signal comes right at the point where their everything is starting to crap out, but they're a cautious bunch, and for them to report it at all means that they think there's at least a chance it's a real signal, as opposed to just noise.
They're re-tooling now to do an even more sensitive test, at even smaller distances, so we'll see if this result holds up. If it does, that will be big news, though what this all has to do with string theory will wait for a later post...
Posted at 8:44 AM | link | follow-ups | [ hide comments ]
A tenth of a millimeter seems surprisingly macroscopic. That’s right in the range of modern MEMS devices.
morinao, 2005-07-28, 5:24pm [link]
The interesting thing I’ve heard about the deviation observed is that it is of the wrong sign for most large extra dimension scenarios. Now where did I hear this. Oh yeah Lubos:
http://motls.blogspot.com/2005/06/deviations-from-newtons-law-seen.html
Lubos actually has some very interesting things to say about this.
Dave Bacon, 2005-07-28, 6:46pm [link]
Geez, where did that come from? The reason none of us answered the question is that Robert Helling gave a perfectly sensible answer—in naive theories of extra dimensions, you’d expect the force to grow faster on small scales, not more slowly. So it would certainly not be obviously good news for string theory. Could we invent a model, in string theory or otherwise, that would fit the data? Probably, but I don’t know. Would it be a serious blow to string theory? No, not until we understood things better.
Another reason is because I was planning to blog about the UW results when I get a chance (as I have before). Consistent with the fact that we talk about experiments all the time; in less than two weeks of existence for the new blog, we’ve mentioned LIGO, LISA, radar ranging, the LHC, WMAP, the 2dF survey, the Cassini mission, the International Linear Collider…
If not responding to every single comment on our blog is taken as a sign of arrogance and disdain, then I predict we’re going to upset a lot of people. But actually I think you’re just being way too sensitive.
Dave: Lubos actually has some very interesting things to say about this.
Thanks for the link. I don’t normally read his blog because it auto-refreshes every fifteen seconds or so, and that bugs the hell out of me.
Sean: The reason none of us answered the question is that Robert Helling gave a perfectly sensible answer—in naive theories of extra dimensions, you’d expect the force to grow faster on small scales, not more slowly.
He gave a very brief answer three days and thirty-odd comments after the question was originally asked, and then only after another person repeated the question. It’s not like he jumped in there before anyone else got a chance, and pre-empted everything that could possibly be said.
It’s not like I expect a 3,000-word blog post every time somebody asks an interesting question in comments—God knows, I don’t even deliver on all the things I promise to talk about in actual posts. But the fact that nobody responded at all until three days and a repeat of the question had passed, despite several thousand words of high-level discussion about backgrounds and landscapes and non-perturbative whatsis—that doesn’t really suggest an eagerness on the part of the heavy hitters to engage in discussion with the laity. And the fact that a two-sentence comment describing the “naive” view is seen as a complete response doesn’t suggest that people are all that concerned about the result, or even interested in the question.
To some degree, my complaint is just a reflection of a disagreement about the fundamental purpose of blogging—I see explanations to the interested layman as one of the primary purposes of maintaining a public blog in the first place, while other people view it as merely another way to exchange ideas with other experts in the field. But my immediate impression of that comment thread was of a bunch of high-powered theorists completely blowing somebody off, and that irks me.
Chad Orzel, 2005-07-28, 8:38pm [link]
Does “naive” mean something different in string theory than it does in everyday English?
Kate Nepveu, 2005-07-28, 10:07pm [link]
In physics, at least, something is called ‘naive’ when it’s sort of the most obvious or easiest thing. The implication is that more complicated effects might change the conclusion. The usage is usually along the lines of “naively, you might expect blah, but once you take into account blah2, you see blah3 instead.”
An actual (free with extra jargon!) example might be something like, “A naive calculation of the Higgs mass in supersymmetric theories gives something around the Z mass, but loop corrections can raise that significantly.” Or, maybe, “naively, you might think that shooting something with lasers would heat it up, but it turns out that if you tune things just right, it actually can cool stuff down.”
So, in Sean’s usage, he means that in the most straightforward theories of extra dimensions, we expect that gravity grows stronger at short scales. Incidentally, this is easy to see—I don’t know if Chad has blogged about it, though. But that doesn’t mean that someone sufficiently clever could come up with a way to get something else.
And there are theories, I think, that get rid of this effect. What I don’t know of, however, are any explicit theories where gravity gets weaker at that scale. One obvious guess would be that it might involve something like a ‘fat graviton’ where the size of the graviton makes gravity becomes less important on small scales. I’m sort of enamored with this idea, but I haven’t the slightest idea how to implement it. The naive way to achieve this would be to have it be composed of more fundamental particles in a similar manner as the proton which has an actual size unlike its pointlike constituents. Unfortunately, there is a pretty straightforward theorem that a massless graviton can’t arise like that in any normal quantum field theory. So, someone’s going to have to be clever.
Aaron, 2005-07-28, 10:59pm [link]
In physics, at least, something is called ‘naive’ when it’s sort of the most obvious or easiest thing. The implication is that more complicated effects might change the conclusion. The usage is usually along the lines of “naively, you might expect blah, but once you take into account blah2, you see blah3 instead.”
Incidentally, people are aware of the problems this usage presents when it collides with everyday English. The use of “naive” to mean “simple” is one of the favorite targets of people who worry about how physics is hostile to everyone but white males.
So, in Sean’s usage, he means that in the most straightforward theories of extra dimensions, we expect that gravity grows stronger at short scales. Incidentally, this is easy to see—I don’t know if Chad has blogged about it, though.
A little later this morning, once I proofread what I typed up last night.
Chad Orzel, 2005-07-29, 7:33am [link]
To some degree, my complaint is just a reflection of a disagreement about the fundamental purpose of blogging—I see explanations to the interested layman as one of the primary purposes of maintaining a public blog in the first place, while other people view it as merely another way to exchange ideas with other experts in the field.
So, I have no opinion at all on string theory, but I do have a question: Why does it have to be either-or on the purpose of blogging? It’s certainly not that way in print journals.
Novak, 2005-07-29, 10:35am [link]
Novak: Why does it have to be either-or on the purpose of blogging?
It doesn’t have to be either, but given that I see things one particular way, I have a hard time not being annoyed by the other approach. In much the same way that I struggle not be be unduly annoyed by people who write novels that take a different approach to genre conventions than I prefer, or people who take a different view of the purpose of express lanes in the grocery store, or any of a host of other things that people do wrong.
Some of my complaint with the comment thread over at Cosmic Variance is just my “You’re doing it wrong!” annoyance slipping past the mental isolators.
It’s certainly not that way in print journals.
Um… I’m not sure what you’re after, here. If you’re including popular magazines in the set of “print journals,” then you’re right that the whole set contains both expert-only and general-public publications. The set of single journals that span both is vanishingly small, though.
My personal feeling is that the Web is much more akin to a popular magazine than a professional journal, in that it’s broadly accessible to people of all backgrounds. Given that broad audience, I prefer to see more of a general-public approach. It’s not like we’re suffering from a lack of channels by which experts can converse with one another—what the world needs more of is conversations between experts and laypeople.
Other people take a different approach, and they’re entitled to. That doesn’t mean I have to like it.
Chad Orzel, 2005-07-29, 11:10am [link]
If you’re including popular magazines in the set of “print journals,” then you’re right that the whole set contains both expert-only and general-public publications. The set of single journals that span both is vanishingly small, though.
I left the comment intentionally vague because I didn’t feel like explaining too deeply. But even leaving out publications like “Discover” and sticking to the IEEE, there’s a set of hard-core transactions where a good paper will take a half a day to pick through carefully, and there’s a set of somewhat more popular publications, usually called magazines, to communicate to less experienced or more general audiences within the community.
The general point I was going for, though, is that print publishing is a pretty broad venue and we don’t expect each publiaction to serve the same purpose or audience. Why would we expect blogging to be the same?
Hell, I thought the whole point of blogging was to encourage diversity of publication, not replace one big model with another.
Novak, 2005-07-29, 1:26pm [link]
Aaron wrote:
One obvious guess would be that it might involve something like a ‘fat graviton’ where the size of the graviton makes gravity becomes less important on small scales. I’m sort of enamored with this idea, but I haven’t the slightest idea how to implement it.
I think this is a common stance on fat gravitons. I’ve been spending entirely too much time lately staring at Sundrum’s papers and thinking about the idea when I should be doing more worthwhile things.
For those of you reading who don’t know about the idea, you might want to look at http://www.arxiv.org/abs/hep-th/0306106 by Raman Sundrum to see what it’s all about. The basic idea is that a graviton spread over some distance would explain the cosmological constant and lead to weakened gravity at distances of about 20 microns (and this is a prediction, in the sense that if weakened gravity is not seen by that scale then the fat graviton could probably no longer explain the cosmological constant). The trouble is that it’s a vague idea and no concrete realization exists in field or string theory.
Interestingly, Gherghetta and collaborators recently constructed an emergent gravity theory, http://www.arxiv.org/abs/hep-th/0507245, albeit one that isn’t really a “fat graviton” and that has gravity getting stronger at short distance. The usual lore is that you can’t do this sort of thing by the Weinberg-Witten theorem, but Gherghetta et. al. claim you evade the theorem by having gravity arise at the quantum level. The idea is to start with a theory in a 5D AdS bulk and deform it so the graviton wave function lives on the infrared brane, and thus can be viewed as a composite of some interacting nearly-conformal field theory. It’s interesting, but on the other hand this had seemed to me the most promising route to build a composite graviton model, and it doesn’t have the features one really wants.
I’m not a string theorist, so I can’t give a good answer to the question of what a confirmation of the Eot-Wash results would mean for string theory, but it seems difficult to imagine a way to explain it in either string theory or field theory, as far as I can tell. It certainly calls for clever model-building. Further evidence for that is that Raman Sundrum had the fat graviton idea a while back and still has no concrete model, and he’s very good.
Matt Reece, 2005-07-30, 3:59pm [link]
To some degree, my complaint is just a reflection of a disagreement about the fundamental purpose of blogging—I see explanations to the interested layman as one of the primary purposes of maintaining a public blog in the first place, while other people view it as merely another way to exchange ideas with other experts in the field.
Whatever the proprieties of the comment-thread in question, I’m just terribly puzzled by this.
Why is PUS (Public Understanding of Science) as suitable subject for blogging, while scholarly (or semi-scholarly) exchange of ideas is not?
The Web was invented by High Energy Physicists, with a distinctly scholarly purpose. Granted, the user-base of the Web is … umh ... a bit wider than it was in the early days. But why does that entail that everything must be pitched to a lay audience?
Isn’t the beauty of the Web its diversity?
My personal feeling is that the Web is much more akin to a popular magazine than a professional journal, in that it’s broadly accessible to people of all backgrounds. Given that broad audience, I prefer to see more of a general-public approach.
The Web is akin to neither “a popular magazine” nor “a professional journal”. It is akin to a library — a huge library — which, surely, has room for both.
It’s not like we’re suffering from a lack of channels by which experts can converse with one another
Really? Clifford Johnson wrote a post arguing precisely the opposite.
Jacques Distler, 2005-08-06, 2:12am [link]
The usual lore is that you can’t do this sort of thing by the Weinberg-Witten theorem, but…
...but it seems difficult to imagine a way to explain it in either string theory or field theory, as far as I can tell. It certainly calls for clever model-building. Further evidence for that is that Raman Sundrum had the fat graviton idea a while back and still has no concrete model, and he’s very good.
The Weinberg-Witten Theorem assumes Lorentz-invariance and a conserved stress-energy tensor in 4 dimensions (or whatever number of dimensions you’re trying to induce gravity in). If we’re living on a 4D brane inside a higher-dimensional gravitational theory, then the WW theorem does not apply.
The trouble with coming up with a concrete realization of Sundrum’s idea is that, in that case, one does run smack into the WW Theorem.
Coming up with any theoretical framework which could accommodate this kind of behaviour (gravity weakening at short distances, while the rest of particle physics behaving perfectly normally to much shorter distances) is a tall order.
Jacques Distler, 2005-08-06, 10:54am [link]
Jacques, I wrote “The usual lore is that you can’t do this sort of thing by the Weinberg-Witten theorem, but Gherghetta et. al. claim you evade the theorem by having gravity arise at the quantum level.” Are you claiming this is wrong? It might be that my understanding of the WW theorem is not what it should be, but I think my statement was still true, even though, as you say, the fact that these are 5D theories makes the WW theorem seem irrelevant. The assumption the authors make, however, is that these 5D models are dual to some 4D theory, which still must somehow evade the Weinberg-Witten theorem. (Granted, the existence of such a dual is not really firmly established.) Gherghetta et. al. seem to claim that the way this hypothesized 4D theory evades the Weinberg-Witten theorem is that there is a fundamental metric field in the theory, but that the Einstein action for this field is dynamically generated. Does this make sense?
It seems to me that in pursuing a framework for the fat graviton the WW theorem isn’t the major obstacle; you can always imagine Lorentz violation at very short distances. The real puzzle, as you note, is why the graviton is an extended (more or less macroscopic!) object while the photon, for instance, is essentially pointlike.
Matt Reece, 2005-08-07, 6:51pm [link]
Chad,
I am the guy who repeated Levi’s question and
I just asked the same question again on
Peter’s blog [*]. Aaron was kind enough to respond.
The problem is that one knows the “naive result”
as Robert Helling has put it, but nobody can
calculate the effective potential from M-theory itself.
Thus M-theory / string-theory is currently not
falsifiable above the PLanck scale, as Aaron
has pointed out.
[*] Yes, I am aware that I asked a rhetorical question
and knew the answer already …
Matt:
In a gravitation theory, the stress tensor isn’t conserved, it’s covariantly-conserved. The WW Theorem does not apply to gravitational theories.
Moreover, in a 5D theory, the 4D stress tensor (living on some brane) isn’t necessarily conserved (covariantly, or otherwise), if you can exchange energy-momentum with the bulk.
Wolfgang:
That’s not the way I would state the situation.
The experimental result (which is highly likely to go away), is impossible to reconcile with Field Theory. It is also impossible to reconcile with String Theory, as we currently understand it.
If the experimental result hold up, and if someone can find some clever way to reconcile it with String Theory, then that would “save” String Theory from being falsified.
If the experimental result holds up, but it cannot be reconciled with some hitherto undiscovered String Theory construction, then everything we know is wrong. There’s no theoretical framework on the table that can accomodate this result.
I would be very excited to learn of some real experimental result that indicated that all of our current ideas (about quantum gravity) are wrong. But I’m not holding my breath.
Jacques Distler, 2005-08-08, 6:48pm [link]
Jacques,
thank you for the response. I agree with you that it is unlikely that the rumored deviations from Newton will hold up.
I also agree with you that such a deviation would be very interesting and eliminate several proposals.
But my main point is that one cannot calculate the effective potential from first principles/M-theory, at least not yet. As far as I understand it, one does not even know why space-time compactifies to M4 x Y and thus one does not even know why the potential is 1/r and not 1/r^(1+n).
I understand the claim that M-theory is (currently) the only consistent quantum theory of gravitation, but one should be honest and add that we do not even know if and how much this theory deviates from Newton.
As far as I understand it, one does not even know why space-time compactifies to M4 x Y
You don’t need to know why it happens to know that it does.
Anyway, the point is not whether String Theory is capable of producing sub-millimeter deviations from Newton’s Law. The point is whether it is capable of producing them with the right sign.
I don’t see a mechanism to do that. Maybe, if these experimental results hold up, someone clever will figure out how. Right now, my personal feeling is that it’s not worth the time. By the time you’ve succeeded (if you succeed), the results will likely have gone away.
Jacques Distler, 2005-08-08, 10:30pm [link]
COMMENTS ARE CLOSED.
Please visit Uncertain Principles' new location at ScienceBlogs to comment.
The Critical Questions of Our Time
I'm probably one of about eight people on earth with two weblogs who hasn't already posted detailed Harry Potter comments on either of them. This, of course, is a situation that cannot be allowed to continue. hence this post.
You other seven slackers, get with the program.
Posted at 9:08 PM | link | follow-ups | 2 comments
Ah, the Power of Cheese
iTunes has finally finished downloading the songs I bought the other night, and the new batch has been thrown into the "Party Shuffle" mode. I went to the iTunes store to buy a couple of albums by KEXP acts, but got sucked in by the "iTunes Essentials" lists, and wound up buying a bunch of singles off the "One Hit Wonder" lists for the 80's and 90's, along with a bunch of other cheeseball stuff from various 80's subcategories.
Which means that, mixed in with new tracks by Youth Group, Crooked Fingers, the Shout Out Louds, and Devin Davis (the last was a CD purchase, because Lonely People of the World Unite! isn't on iTunes), there are occasional blocks like:
"A Message to You Rudy," the Specials
"A Little Respect," Erasure
"Total Eclipse of the Heart," Bonnie Tyler
(You gotta love Jim Steinman...)
Or, a little farther down the playlist:
"Keep Your Hands to Yourself," Georgia Satellites
"Electric Avenue," Eddy Grant
"Karma Chameleon," Culture Club
"Come Dancing," the Kinks
It's always faintly disturbing to me to discover the degree to which my musical tastes locked into place at about age fifteen. But, hey, synth-pop is coming back into style, so maybe I'm just avant-garde...
Posting this is probably at least as damaging to my tenure prospects (among the Ivan Tribbles of the world, anyway) as anything I could write about internal campus politics. But I can't help myself.
Posted at 7:44 PM | link | follow-ups | 5 comments
Large Breasted Cephalopods
(Because we don't get enough really weird search engine traffic around here...)
Finally, a sci-fi magazine for PZ Myers.
(via Notional Slurry)
Posted at 7:40 PM | link | follow-ups | 1 comment
Memo to iTunes
Regarding the little dialog box reminding me to back up my purchased music: it's a lovely thought, thanks for caring, but would it be too much to ask to have the downloading continue in the background while you prompt me?
You see, much as it may surprise you to hear this, when I purchase a hundred-odd songs from your store, I don't actually intend to sit in front of my computer for the entire download process. Particularly when the process is initiated at 11:00 at night.
It's somewhat irksome to go to bed, and wake up expecting to listen to new music (or, well, the large amount of 80's cheese that I bought last night), and find that only three songs managed to download before the whole thing came to a halt for your little reminder.
Just a thought.
And while I'm at it, what's the deal with songs being "No Longer Available" via iTunes? I understand physical CD's going out of print, because of the production and storage costs. But electronic files? What's the logic of that?
(That one, at least, has all the hallmarks of a record company decision: it's the sort of thing that might be motivated by a desire to squeeze more money out of music buyers, and it's also idiotic. Really, the people running most record companies are just too stupid to be trusted to manage a 7-11. The fact that they stay in business is a wonderful testament to the power of corruption.)
Posted at 7:10 AM | link | follow-ups | 2 comments
Three Links
This is mostly a throwaway post to cover a template update (you're smart people, you can identify the new sidebar links on your own), but here are three links that might be interesting, in order of decreasing gravity.
First, PZ Myers finds some researchers looking to get their funding slashed by the more puritanical members of Congress.
Second, via Arcane Gazebo, an explanation of the physics of waves using Super Mario sprites. Silly, but vaguely educational.
Finally, if you woke up this morning saying "You know, what I need today is a weird little Japanese language Flash game in which you attempt to keep an angry little man from grabbing your mouse pointer," well, have I got a link for you. (Via a mailing list.)
Posted at 9:19 AM | link | follow-ups | 7 comments
Juan R is not a string theorist. The reason I haven’t responded is that I don’t know the answer. It would certainly not make the large extra dimension people happy, I think, but perhaps it could be accomodated in other ways. I’d have to spend a lot more time thinking about it to give any sort of definite answer.
I think it would be really exciting if this held up. I’m a big fan of the unexpected. Unfortunately, as I’m sure you know, these sort of results have a tendency to go away with time.
Aaron, 2005-07-28, 5:06pm [link]