[This is the seventh post in a series that begins here.]
In the last post in this series, I pointed out that there’s a lot about quantum field theory [the general case] that we don’t understand. In particular there are many specific quantum field theories whose behavior we cannot calculate, and others whose existence we’re only partly sure of, since we can’t even write down equations for them. And I concluded with the remark that part of the reason we know about this last case is due to “supersymmetry”.
What’s the role of supersymmetry here? Most of the time you read about supersymmetry in the press, and on this website, it’s about the possible role of supersymmetry in addressing the naturalness problem of the Standard Model [which overlaps with and is almost identical to the hierarchy problem.] But actually (and I speak from personal experience here) one of the most powerful uses of supersymmetry has nothing to do with the naturalness problem at all.
The point is that quantum field theories that have supersymmetry are mathematically simpler than those that don’t. For certain physical questions — not all questions, by any means, but for some of the most interesting ones — it is sometimes possible to solve their equations exactly. And this makes it possible to learn far more about these quantum field theories than about their non-supersymmetric cousins.
Who cares? you might ask. Since supersymmetry isn’t part of the real world in our experiments, it seems of no use to study supersymmetric quantum field theories.
But that view would be deeply naive. It’s naive for three reasons. (more…)
Heads up! Literally! If you live between Georgia and Maine, and as far inland as Ohio, you’ve got a good chance of seeing the launch of a rocket off Maryland’s Wallops Island, currently scheduled for 8:15 Eastern time. In the New York area, look to the south and east.
Ever since the horrific earthquake, tsunami and ensuing nuclear accidents hit north-eastern Japan in March of 2011, the world has been keeping an eye on Fukushima, where the Fukushima Daiichi nuclear power plant suffered extraordinary amounts of damage. Initially the news out of the power plant, operated by the company TEPCO, was awful, but gradually the situation seemed to be increasingly under some control. But that control has not been convincingly secured, and has even perhaps been slipping of late. And the worries about a variety of possible risks from the plant have been growing, especially because the clean-up at the plant is still run by TEPCO, which has engaged in repeated cover-ups and poor decisions… not to mention the fact that it’s a power company, not a nuclear accident site cleanup company. I find it extraordinary that the situation hasn’t been put into the hands of a blue-ribbon international panel of nuclear scientists and engineers, with full power to make decisions and with full transparency for all to see as to what is going on. It’s taken the Japanese government far too long to step in.
I’m bringing this topic up now because TEPCO is finally ready to address one of the major issues that they face in the clean-up. In addition to finding ways to deal with the melted-down nuclear fuel at Reactors 1, 2 and 3, which will take years, they have to deal with the stored and mostly undamaged fuel rods that are sitting outside of Reactor 4, in a water-filled pool. The water keeps the fuel cool, and right now there’s nothing wrong with the pool or the cooling. The problem is that this pool is on the 3rd floor of the Reactor 4 building, which was damaged in a (chemical, not nuclear) hydrogen explosion shortly after the earthquake… and it would be better to get the fuel rods into a safer pool, at ground level, outside of the compromised building. This is not easy for many reasons, and apparently there is some risk involved — not risk of a nuclear explosion, which is physically impossible in these circumstances, but of some amount of radioactive gas being produced and released into the atmosphere if the fuel rods are not kept submerged in water or are otherwise damaged. However, I’m not precisely clear on the nature of this risk.
Just the same as anyone else who might be affected if fish from the Pacific become unsafe to eat (which, as far as I can tell so far, remains the main risk to areas outside Japan), I want to know what is happening at Fukushima and what exactly the risks are. But I’m not an expert on this subject. Just because I’m a scientist doesn’t mean that it’s that much easier for me to figure out what’s really going on. It’s just perhaps easier for me, compared to the general reader, to recognize misinformation for what it is. And when I look around the web, I am seeing huge amounts of it. (For instance, starfish on U.S. coastlines are being afflicted by some sort of disease; around the web you will see suggestions that this has something to do with Fukushima, which, given that the amount of Fukushima-related radioactivity currently in the Pacific is small, is manifestly ridiculous.)
There are good reasons to be concerned that things are at risk of getting out of hand on many different fronts, both in terms of actions on the ground and in terms of public understanding. On the one hand, I’m reading more and more scare-mongering: irresponsible statements made by non-experts, such as the ones about starfish, that are starting to frighten my friends and neighbors unnecessarily, especially on the west coast of the United States. (Here’s a response by a deep-sea biologist to one of the most egregious; I can’t directly verify all of the points he makes, but many of them were obvious to me even before I found his website.) On the other hand, I’m not at all convinced, given their terrible track record, that TEPCO is capable of dealing with the extreme technical difficulty and considerable danger of putting their nuclear plant back into a safe condition without there being additional significant releases of radioactive material. And meanwhile, media reporting is just not sufficiently reliable; the journalists aren’t experts and often don’t understand the issues well enough to get it all straight or put it in proper context.
If there were ever a time when level-headed scientific discussion, careful calculation and thoughtful consideration were needed in a public setting, this would be it.
I haven’t yet found a sensible, trust-inspiring blog that does for nuclear engineering and radiation safety what I try to do for particle physics (though this one looks somewhat promising.) Consequently, I don’t really have a way to understand the whole story and to gauge it properly. So I’d like to find a way to use my website and its readers, some of whom surely know more about nuclear engineering and radioactivity risks than I do, and some of whom are perhaps getting more information than I am, to assemble a clearer understanding of what the risks and dangers really are and are not.
Fair warning: In contrast to my usual policy, I am going to be strictly editing the comments on this post, and all similar posts on Fukushima. I will accept thoughtful scientifically-based discussion, and links to such discussion, only. I want neither my own mind nor my readers’ cluttered with unscientific chatter from non-experts. Polemical diatribes will be deleted; activism for or against nuclear power is inappropriate here [I happen to oppose nuclear power in its current form, but that’s beside the point right now]; and unscientific assertions without any support from replicated studies will be marked as such, and if sufficiently egregious, deleted. My goal is the same as that of most people: to get a better grasp of the situation, and to get a clearer sense of what to worry about and what not to worry about, both for now and looking into the future.
So: do I have any readers with expertise in this area? If you’re one of them, can you help us establish a baseline of solid science that we can build on? Does anyone know of particularly even-handed and sensible blogs by experts that we can draw on? Websites with data or resources that are run by people without an obvious big axe to grind? One of the big problems I find is that there are plenty of scientific studies quoted on blogs, but few guides to the non-expert reader to help us put the results in precise perspective.
By the way, here’s one site that shows the radioactivity levels in and around Berkeley, California; as far as I can see, nothing above normal levels has been measured for well over a year, and never were levels high even in 2011. http://www.nuc.berkeley.edu/UCBAirSampling
I have two very different presentations to give this week, on two very similar topics. First I’m going to the LHC Physics Center [LPC], located at the Fermilab National Accelerator Laboratory, host of the now-defunct Tevatron accelerator, the predecessor to the Large Hadron Collider [LHC]. The LPC is the local hub for the United States wing of the CMS experiment, one of the two general-purpose experiments at the LHC. [CMS, along with ATLAS, is where the Higgs particle was discovered.] The meeting I’m attending is about supersymmetry, although that’s just its title, really; many of the talks will have implications that go well beyond that specific subject, exploring more generally what we have and still could search for in the LHC’s existing and future data. I’ll be giving a talk for experts on what we do and don’t know currently about one class of supersymmetry variants, and what we should be perhaps be trying to do next to cover cases that aren’t yet well-explored.
Second, I’ll be going to Argonne National Laboratory, to give a talk for the scientists there, most of whom are not particle physicists, about what we have learned so far about nature from the LHC’s current data, and what the big puzzles and challenges are for the future. So that will be a talk for non-expert scientists, which requires a completely different approach.
Both presentations are more-or-less new and will require quite a bit of work on my part, so don’t be surprised if posts and replies to comments are a little short on details this week…
For Non-Experts Who've Read a Bit About Particle Physics
I spent yesterday, and am spending today, at Princeton University, participating in a workshop that brings together a group of experts from the CMS experiment, one of the two general purpose experiments at the Large Hadron Collider (where the Higgs particle was discovered.) They’ve invited me, along with a few other theoretical physicists, to speak to them about additional strategies they might use in searching for phenomena that are not expected to occur within the Standard Model (the equations we use to describe the known elementary particles and forces.) This sort of “consulting” is one of the roles of theorists like me. It involves combining a broad knowledge of the surprises nature might have in store for us with a comprehensive understanding of what CMS and its competitor ATLAS (as well as other experiments at and outside the LHC) have and have not searched for already.
Yesterday afternoon’s back-and-forth between the theorists and the experimentalists was focused on signals that are very hard to detect directly, such as (still hypothetical) dark matter particles. These could perhaps be produced in the LHC’s proton-proton collisions, but could then go undetected, because (like neutrinos) they pass without hitting anything inside of CMS. But even though we can’t detect these particles directly, we can sometimes tell indirectly that they’re present, if the collision simultaneously makes something else that recoils sharply away from them. That sometime else could be a photon (i.e. a particle of light) or a jet (the spray of particles that tells you that a high-energy gluon or quark was produced) or perhaps something else. There was a lot of interesting discussion about the various possible approaches to searching for such signals more effectively, and about how the trigger strategy might need to be adjusted in 2015, when the LHC starts taking data again at higher energy per collision, so that CMS remains maximally sensitive to their presence. Clearly there is much more work to do on this problem.
[Apologies: due to a computer glitch, the figure in the original version of this post was not the most up-to-date, and had typos, now fixed.]
On Tuesday, the New York Times Editorial page ran an Op-Ed about dark matter… and although it could have been worse, it could certainly have been better. I do wonder why these folks don’t just call up an expert and confirm that they’ve actually got it right, before they mislead the public and give scientists a combination of a few giggles and a headache.
Here is the last paragraph from the Times:
“This experiment is probing a major hole in the way we understand the cosmos. Roughly speaking, the force of gravity in the universe can be explained only by a corresponding amount of mass, or matter. Some undiscovered mass — dark matter — must exist in order to explain gravity, but no one has seen any traces of it. Those traces, when they are finally found, will be exotic particles left over from the Big Bang. In the tale we tell about everything we know, scientists have now brought us to the edge of the deep, dark woods. They, and we, are waiting eagerly to see how the rest of the story goes.”
Ok, out comes the professorial red pen.
First, a relatively minor point of order. “…the force of gravity in the universe can be explained only by a corresponding amount of mass, or matter…” This isn’t great writing, because mass and matter are not the same thing. Matter is a type of substance. Mass is a property that substance (including ordinary matter, such as tables and planets) can have. Mass and matter are as different as apples and applets. You can read about these distinctions here, if you like. The author is trying to evade this distinction to keep things simple: the more correct statement is that gravity (in simple circumstances) is a force exerted by things (including ordinary matter) that have mass.
But here’s the real offending remark: “Some undiscovered mass — dark matter — must exist in order to explain gravity, but no one has seen any traces of it.” Dark matter is most certainly not needed to “explain gravity” in some general way; there’s not one bit of truth in that remark. For instance, the gravitational pull of the sun on the earth (and vice versa), and the pull of the earth on you and me (and vice versa), has absolutely nothing whatsoever to do with dark matter, nor is dark matter needed to explain it.
What the author should have said is: since the 1960s we have known that gravitational forces on large astronomical scales seem to be stronger than we can account for, and so either our equations for gravity are wrong or there is matter out there, pulling on things gravitationally, that we cannot see with any type of telescope. The reason the latter possibility is taken more seriously than the former by most experts is that attempts to modify gravity have not led to a convincing case, while the evidence for additional “dark” matter has grown very strong over recent decades.
Here’s one of the several arguments that suggest the possibility of dark matter… the simplest to explain. Experts study the motions of the stars in our own galaxy — the star city known as the Milky Way — and also study the motions of stars in other galaxies. [The overall motions of galaxies themselves, inside giant clusters of galaxies which can be found in deep space, are also studied.] Now what we ask is this; see Figure 1. Supposing all of the matter that is out there in the universe is of a type that we can see in one way or another: stars, gas, dust of various types. Then we can figure out, just by looking with a telescope and doing simple calculations, roughly how much measurable matter is in each galaxy, how much mass that matter has, and where it is distributed inside the galaxy. We can next use that information to figure out how hard that matter pulls on other matter, via the force of gravity. And finally — crucially! — we can calculate how fast that pull will make the matter move, on average. And what do we find when we measure how fast the stars are moving? Our calculations based on the matter that we can see are wrong. We find that the stars in the outer edges of a galaxy, and the galaxies inside clusters, are moving much, much faster than our calculation predicts. (This was discovered in the 1960s by Vera Rubin and Kent Ford.) It’s as though they’re being pulled on by something unseen — as though the gravity on the stars due to the rest of the galaxy is stronger than we’ve guessed. Why is this happening?
Fig. 1: One of several lines of evidence in favor of the hypothesis of dark matter is that stars in the outer regions of galaxies move much faster than would be the case if the galaxy was made only from what we can see.
One possibility is that there is matter out there that we can’t see, a lot of it, and that matter is inside galaxies and inside clusters of galaxies, exerting a pull that we haven’t accounted for properly. A huge “halo” of dark matter, in this view, surrounds every galaxy (Figure 2).
Clearly, this isn’t the only logical possibility. Another option is that there could be something wrong with our understanding of gravity. Or there could be some other new force that we don’t know about yet that has nothing to do with gravity. Or maybe there’s something wrong with the very laws of motion that we use. But all attempts to make sensible suggestions along these lines have gradually run into conflicts with astronomical observations over the recent decades.
Fig. 2: The visible part of every galaxy is believed to lie roughly at the center of a much larger halo of dark matter.
Meanwhile, during those last few decades, a simple version of the “dark matter” hypothesis has passed test after test, some of these tests being very complex and subtle. For example, in Einstein’s theory of gravity, gravity pulls on light, and can bend it much the same way that the lenses in eyeglasses bend light. A galaxy or galaxy cluster can serve to magnify objects behind it, and by studying these lensing effects, we again conclude there’s far more matter in galaxies and in clusters than we can see. And there are other arguments too, which I won’t cover now.
So while an explanation for the fast motion of stars inside galaxies, and galaxies inside clusters, isn’t 100% sure to be dark matter, it’s now, after many years of study, in the high 90%s. Don’t let anyone tell you that scientists rushed to judgment about this; it has been studied for decades, and I can tell you from experience that there’s a lot more consensus now than there was when I was an beginning undergraduate 30 years ago.
“Those traces, when they are finally found, will be exotic particles left over from the Big Bang.” Will they? Will the dark matter turn out to be particles from the Big Bang? Not necessarily. We know that’s one possibility, but it’s not the only one. Since I explained this point last week, I’ll just refer you to that post.
Now here come the big meta-questions: should the New York Times be more careful about what it puts on its editorial page? Should its editors, who are not scientists, talk broadly about a subtle scientific topic without fact-checking with an expert? What are the costs and benefits when they put out oversimplified, and in some ways actually false, information about science on their editorial page?
