I wanted to alert you to a very nice paper that my former student, Adrienne Leonard (who has since gotten her Ph.D. at Cambridge and is now at Saclay), her former thesis adviser, Lindsay King, and I have written and submitted to Monthly Notices. The title is “New Constraints on the Complex Mass Substructure in Abell 1689 from Gravitational Flexion,” and you can read it here on the arXiv.

Those of you having trouble parsing the title, gravitional flexion is an area of gravitational lensing that I worked on about 5 years ago (along with Priya Natarajan, David Bacon, Richard Massey and others), which basically describes the “banananess” or “arciness” of lensed images. I have a very cheesy (and outdated) webpage describing it and giving references to a lot of the early work. The advantage to using flexion (instead of, or even better, in addition to shear) is that it is a far more sensitive probe to local density perturbations, and thus a potentially can tell you a lot about substructure.

To date, there have been a ton of theoretical papers on flexion (a quick arXiv search yields 30 hits), but only a handful of observational ones, including Adrienne’s work. Another standout is Yuki Okura’s HOLICs measurements (also on A1689). The cool thing about our current paper is that we are able to detect three distinct, and very complex mass peaks within the cluster, yielding a total mass of about 7×10¹⁴ h^-1 M_sol. This is a very strong evidence that the cluster has experienced a recent merger.

Be sure to check it out!

-Dave

They’re all available at our Cafe Press shop. Act now and order limited addition shirts and whatnot without the title of the book written on it. We’re changing them later tonight, so you can get the limited edition version if you act quickly!

-Dave

The other issue was that unlike Dark Matter, Dark Energy, and Black Holes, Monopoles get relatively little detailed discussion in the popular press (beyond the name), so it’s not clear what people expected. I was very touched by the comments this week, in large part because there were a fair number of people who clearly enjoyed learning about something genuinely new. It also didn’t hurt that apparently the Big Bang Theory did a bit about searching for monopoles last week.

So if folks out there have more questions on really under-represented or under-explained physical phenomena, just let me know.

-Dave

This week on io9′s “Ask a Physicist,” I figure out
whatever happened to magnetic monopoles.

Don’t forget to send me your questions about the universe!

-Dave

And yes, I’m assuming the earth is a sphere, instead of an oblate spheroid. Not that that would make too much of a difference.

You calculate the gravitational acceleration at any height, r, using:

a(r)=GM(r)/r²

where M(r) is the mass interior to your radius.

Incidentally, I’m deeply sorry about the equations; that’s why this one is labeled “technical.” However, everybody’s life would be a whole lot clearer if there were a wordpress version of LaTeX.

Anyway, back to the central thrust. For a uniform density earth (which it’s not), the solution is quite simple:

a(r)=g*r/R

Which is kind of cool. You become weightless at the center, and since the gravitational force is proportional to the distance, you would bounce around to the other side of the earth and back with a period of: 2*pi*sqrt(R/g). In case you’re curious, that’s about 1.4 hours, exactly the amount of time it would take to send a rocket around the earth that just barely skimmed the surface.

But Pat wanted to know if with the real earth there wasn’t any situation in which you might gain weight on the way down. This got me thinking about a much more general relationship. It’s fairly straightforward to show that:

da(r)/dr=(3*rho/<rho>-2) a(r)/r

Where <rho> is the average density interior to a particular radius. So for the gravity to increase inwards, we need:

rho/<rho> < 2/3

For the earth, the average density is about 5.5 g/cm³, about 5 times that of water, which means that if the surface has a lower density than about 3.6 g/cm³, you’re in luck.

What happens as you move through the crust? Well, as Pat points out, the density there is only about 2.5 g/cm³, which more than satisfies our criterion. In fact, it looks like your weight wouldn’t start to actually decrease until you got near to the bottom of the Mantle or to the top of the outer core. The density there is about 5.6 g/cm³, which is still less than 2/3 of the average density of the core (closer to 10-13 g/cm³ ).

For what it’s worth, the additional gravity wouldn’t amount to all that much. Even in the outer core, you’d only weigh about 10% more than surface normal.

Fun question.

-Dave

And if you want to read all about the initial excitement, click here, here or here.

And if you want to skip the hassle of listening to the radio, and want it beamed directly to your brain or mobile device, you can download the episode here (and see a video of Connie Willis and me talking time travel with Kurt Andersen).

-Dave

Dr. Goldberg:

I’ve read a lot of cosmology books (and I’m 13, so a lot is somewhere around four…) and yours is definitely my favourite! I love humor, who doesn’t right? So mixing fact, humor, and some movie trivia was a great idea. You guys made dark matter, time travel, etcetera so much easier to understand! Also, who wouldn’t love a squid-like scientist named Dr. Snuggle from a distant galaxy?
Nobody, that’s who.

Thanks!
-Hopefully a future cosmologist

• “People believed in the Aether once, too.”
• “In a hundred years, scientists are going to think you guys are idiots.”
• “How is this any different than explaining things by invoking gods?”
• “I think they made a mistake somewhere.. Or they have the wrong premises. They need to re-evaluate.” (which is an actual comment)

and so on.

I have a strict “Don’t feed the trolls” policy, since it rarely does any good, and more often simply puts ridiculous arguments on equal footing with well-grounded ones. I’d also like to point out that the title of my column was, “Are physicists just making up dark energy?”, replacing the slightly more antagonistic first draft version, “When it comes to dark energy, are physicists dumb or just lazy?” The point is that I know that introducing dark energy (and dark matter, for that matter) seems like a kludge. I realize that the simplest theoretical models are off by a factor of a googol from the observed densities. In fact, I made those very points from the outset.

But what people (okay, some people) don’t seem to understand is that when physicists introduce concepts like dark matter or dark energy, it’s at a much more precise level than simply saying, “Oh, the universe is accelerating? There must be a mysterious substance making that happen. Next question!”

We instead hypothesize the following, with the specific numbers determined from observation:

1. There is a field permeating the universe which makes up approximately 70% of the critical density of the universe
2. It has an equation of state (pressure/energy density) of -1.

From this, we make a huge number of very concrete and testable predictions, including:

1. The age of the universe (and the expansion factor at any time in the past)
2. The brightness of all standard candles at any redshift (distance) from the earth
3. The angular size of all standard rulers at any redshift from the earth
4. (Along with the dark matter determination) The shape of the universe.
5. The linear growth rate of structure at all points in the past.

This means that we’re able to correctly predict all sorts of quantifiable things, from gravitational lensing tomography to the angular scale of the first acoustic peak in the CMB to the supernova redshift-magnitude relation to the number of high redshift clusters to the SZ effect and on and on.

The standard Lambda-CDM cosmology explains all of these things pretty much perfectly. When people make asinine statements like, “Off the top of my head, I can think of 10 different things as dumb as dark energy to explain the perceived acceleration of the universe,” it makes me deeply sad. They clearly don’t get that we’re actually doing predictive science here.

Now, what is dark energy? I don’t know. That isn’t to say that I don’t care. I do, deeply. But the fact remains that we can make all sorts of predictions about our universe without knowing exactly what DE is, and ditto with dark matter. When dark energy is ultimately explained, I personally don’t care whether it turns out to be a cosmological constant, some sort of vacuum energy, or some other sort of field. If all of them make the same predictions, then they are identical from an observational perspective. However to imply that not knowing what, exactly, dark energy is means that it has no more validity than attributing nature to gods or fairies is, I’m afraid, quite ignorant.

It is, of course, the case that something like variable physical “constants” may turn out to up-end our current Lambda-CDM model. But such theories as they are now are completely ad hoc. If you have a problem with an arbitrary (but very simple) field permeating the universe, why, exactly don’t you have a problem with the speed of light arbitrarily knowing how to change throughout the age of the universe?

And what’s more both dark matter and dark energy have strong historical/scientific precedents. Would you have believed in the existence of neutrinos based only on the recoil of the electron from a neutron decay? Would you have believed in anti-matter based on Dirac’s attempts to linearize relativistic quantum mechanics?  And dark energy does have laboratory analogs.  Vacuum energy has all of the properties (albeit at a much higher energy density) of dark energy; why is it so strange to imagine that the universes is pervaded by a fluid of the same equation of state?

I was chatting with some of the grad students in my department, and it occurred to me that if members of the public were really cognizant of what constitutes a “detection” of a top quark or the Higgs (should it be found) they’d be astonished. We don’t observe these particles directly — we infer them based on recoils and decay detritus. Indirect measures can still tell us a hell of a lot.

I get particularly riled because io9, slashdot, physorg and others report on almost every crazy theory that someone comes up with, and as a result people have become so jaded that they think these theories have the same stature and support as things like dark matter and dark energy.  They don’t.

And yes, I normally like to take the high road, but sometimes ignorance about how science really progresses is just too much to take.

-Dave

Enjoy!

-Dave

I commented on a similar issue with regards to journalism generally some time ago.  Essentially, I said, being a paid journalist is not the same thing as doing it for fun:

There comes a point where people who truly have something to say will want to be compensated in a manner commensurate with the amount of time and knowledge they’re putting into it, and if the market doesn’t bear that cost, then the market gets amateurish reporting in every sense of the term.

This doesn’t mean that the writing at these websites isn’t fun and engaging and even informed.  The problem is that in an effort to continuously put out product, emphasis is given to half-formed theories, or ideas known to be wrong, or even experimental results which have not only not been verified, but haven’t even actually been published (or written into an actual paper).  The problem is that people aren’t so much getting science news as B.S.

Every now and again, when I’m feeling particularly energetic, I try to hunt down a few of the very speculative theory papers that are making their way across the pages of the science blogs with an effort toward debunking them.  But the torrent is relentless; who has the time?   To their credit (I suppose) most of the blogs report on papers featuring (for example) variable speed of light and gravitational constants with skepticism, but they report on them nevertheless.  They have to fill their pages.  Yes, I’m being cynical today, but I simply don’t see the point in reporting on things that you know are wrong.

Sorry to be such a Debbie Downer, but I’m afraid scientific revolutions just don’t happen that often.

- Dave