Good news everyone! Merch!
Your whole life has been building up to this. You asked for it, and now you’re going to get it! It’s “User’s Guide to the Universe” shirts! Also, if you are a baby, onesies.
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
Monopole-izing the conversation
As I posted a screed a few weeks ago complaining about the popular revolt against dark energy, I wanted to note how pleased I was with the response to my monopole article. Granted, the overall hit rate was a bit more subdued this time around, due in part no doubt to io9′s excellent scifi 101 series. Seriously, if you haven’t checked that out, I strongly urge you to look at Charlie Jane’s 25 Classic Sci-Fi Movies that Everybody Must Watch (“Inception” makes the list).
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
Gravity at the center of the earth
I’ve been having a very interesting exchange with an io9 “Ask a Physicist” reader named Pat about what would happen if you were to dig a hole through to the center of the earth. Would you get heavier or lighter as you descended? While this wasn’t really appropriate for “Ask a Physicist,” it did remind me of a classic quals problem, and since our own grad students are taking the quals in a few weeks, here’s my tribute to them.
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
In case you missed it the first time…
If you missed me (Dave) talking about Time Travel on Studio 360 in January, fear not! They’re rebroadcasting it next weekend (August 21-22). Also appearing are Janelle Monae, Connie Willis, and more! Check here to find your local station and time.
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
An email worth sharing
I just got a really sweet email from a young reader:
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
Science, Fairies and Trolls – a rant.
I’ve written a fair amount about what we do and don’t know about what makes up 95% of the universe. Besides yesterday’s “Ask a Physicist column” in io9, see here, here, or here. Yesterday’s column on dark energy was, on the whole, pretty well received, but there were an enormous number of comments along the lines of (and I’m paraphrasing here):
- “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:
- There is a field permeating the universe which makes up approximately 70% of the critical density of the universe
- 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:
- The age of the universe (and the expansion factor at any time in the past)
- The brightness of all standard candles at any redshift (distance) from the earth
- The angular size of all standard rulers at any redshift from the earth
- (Along with the dark matter determination) The shape of the universe.
- 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
Ask a Physicist about Dark Energy
In this week’s “Ask a Physicist” at io9, I dish the dirt on Dark Energy. Specifically, I try to explain how we can be so sure about something that makes no freakin’ sense. Also, there are cool, only vaguely relevant pictures such as:
Enjoy!
-Dave
Too many outlets; not enough news
I’ve been thinking a lot lately about where people go to get their science news. Of course, there are some excellent science blogs out there, but between the dedicated science magazines (and their online presences), individual bloggers, and the websites dedicated to (among much else) science reporting/rumor-mongering there is an enormous push for continuous science news. The problem is that there just isn’t that much real news in a given day.
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
Why do it?
I got an email from superfan Caden this morning, asking if I’d read the latest Dennis Overbye column on the premature announcement of earth-like planets from the Kepler Mission. The background, if you missed it, is that one of the project scientists, Dimitar Sasselov, gave a talk at Ted in which he showed a slide and referred to approximately 140 earth-like planets that had thus far been discovered by Kepler.
He mispoke, but in many ways, it was a forgivable error. Yes, the planets are merely candidates, though for my part, I have little doubt that the approximate distribution of masses will prove to be correct. Yes, the planets are earth-mass, not earth-like (since at this point in the survey discovered planets will be too close to their host stars to support liquid water), but this is not any less remarkable. And, of course, his announcement was premature, but this is where I find Overbye’s criticism laughable.
Experimental and observational results are usually confirmed or disappear pretty quickly, which means that somebody announcing a 2-sigma planetary, Dark Matter, or Higgs detection should be prepared to eat their words in short order. But the importance of those discoveries is also what makes them news, and it is the responsibility of science reporters to temper the potential import of the discoveries with the possibility that they may not pan out. It is remarkable that Overbye can be so cavalierly critical of jumping the gun on reporting experimental results, while at the same time so casually report every left-field theory put forth, regardless of how few (if any) of them ever pan out.
But Caden wrote to me not about science by press-release — about which I’ve babbled on many times — but rather about a quote near the end of the piece:
I can’t say what the discovery of dark matter or the final hunting down of the Higgs boson would do for the average person, except to paraphrase Michael Faraday, the 19th-century English chemist who discovered the basic laws of electromagnetism. When asked the same question about electricity, he said that someday it would be taxable.
This got her thinking:
I had an interesting conversation on my last trip abroad regarding how to explain why we do what we do, as scientists. I can’t count the number of times I’ve been asked, “So why should we care about neutrinos?”… How do you explain why basic science research matters?
This is a very good question, and one that we often lose sight of. We often justify basic science to the public (as Faraday did) on the pretext that someday it will lead to technology, and thus to revenue. And there is a truth to that. Quantum mechanics is vital to our electronics. General relativity (as Overbye points out) is crucial to GPS systems. Special relativity is central to all nuclear reactions.
But that’s not why we do it.
Technology is a laudable goal, which is why we have entire colleges of engineering. It’s also why I feel quite at home in a college of arts and sciences. We want to understand why the universe is the way it is, how it will change over time, and how everything fits together. You want to understand neutrinos because there are more of them out there than any other particle in the universe, and it would be ridiculously provincial to ignore that fact. It may seem old-fashioned, but I see what we do as being akin to the work done by philosophers, artists, and writers. Our goal is to try to put some order into the universe. With that comes the responsibility to tell the public about what we’ve learned, and most importantly to try to convey what we really know, rather than try to dazzle with the absurdly speculative.
There may come a time that basic science (and the arts) are seen as some as a luxury, and that the only research worth doing is applied science. To some, that day may already be here. I think this is a foolish position since you never know what basic science will ultimately tell us. But my basic point is that even if all we get out of a particular endeavor is knowledge, that’s more than enough.
-Dave
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