Archive for March, 2008

Will physics destroy the world?

Sunday, March 30th, 2008

Apparently a couple of guys are suing to stop the Large Hadron Collider, the new particle accelerator being built at CERN.  They’re worried about the possibility that the collisions will produce something like miniature black holes or other exotic objects that would then destroy the Earth.

This sort of worry has come up a bunch of times before.  Sometimes the worry is about the possibility that the state of matter that we know and love is only a metastable state, not the most stable state.  The idea then would be that, if you produce a single nugget of the true stable state, everything else would collapse into that new state.  It’d be like having a supersaturated sugar solution: as soon as you give it a nucleation point, everything crystallizes out.  Think Vonnegut’s ice-nine.

So should we be worried about the LHC destroying the world?  The short answer is no.  This sort of thing is logically possible, so it’s certainly worth considering the possibility, given the enormous downside of destroying the world.  But people have considered it very carefully and have shown quite convincingly that there is no risk.  There’s a short overview here, with links to the technical reports.

There’s one argument that dispenses with a lot of the various doomsday scenarios.  The sorts of collisions that will happen in the LHC happen regularly in the Earth’s upper atmosphere, as ultra-high-energy cosmic rays strike the Earth.  You can work out that, over the Earth’s 5-billion-year history, the number of times these events have occurred naturally is many times larger than the number of times they will occur at the LHC.   So the fact that the Earth is still around is very strong evidence that this sort of catastrophic scenario is impossible.

As the NY Times article points out, there’s a loophole in this argument.  The collisions in the upper atmosphere are fast-moving particles colliding with particles that are essentially at rest.  Because of conservation of momentum, anything produced in such a collision would be moving at close to the speed of light, so it wouldn’t stick around long enough to do any damage.  In contrast, the collisions in the collider will be of particles moving in opposite directions with essentially equal speeds, so the resulting detritus will be produced nearly at rest.  There is a big difference between a micro-black hole whizzing through the Earth at nearly the speed of light, which would have essentially no effect, and one that’s moving slow enough to stick around.   To see why you still shouldn’t worry, you have to read the technical reports.

Animal magnetism

Wednesday, March 26th, 2008

I really liked this article in Physics Today about how animals sense magnetic fields. I had no idea that (a) so many different kinds of animals can do it, or (b) so little is known about how they do it.

A yard of snow

Wednesday, March 26th, 2008

This post on the nominal illusion, in which the units of measure we use affect our psychological perception of a quantity, reminded me of something else interesting about the way we perceive units.

When I was in college, my roommate told me about a skiing trip he’d been on, where the snow was “a yard deep.” He’s European, so naturally he was thinking “a meter deep” and converting it for my American ears. To any native speaker of American English, of course, that sounds all wrong: we would say “three feet deep.” The question is why?

As far as I can tell, the answer is that yards are units of horizontal measure, not vertical measure. It’s a bit funny that we have units of length that are only used in certain directions, but once you start looking out for them, there are actually a bunch of them. Miles are horizontal (the exceptions are “the mile-high city” and “the mile-high club,” but I think that in both cases the “incorrect” unit is being deliberately used to make the phrase sound funny or memorable). In aviation, feet are vertical. In the ocean, fathoms are vertical and leagues are horizontal (before saying that “20,000 Leagues Under the Sea” proves this wrong, check out what the title actually means: it’s not how far down they went; it’s how far across.)

Of course, in our everyday lives, we experience horizontal distances quite differently from vertical distances, so maybe we shouldn’t be too surprised that there are different units of measure for them.

There’s a nice analogy here to the theory of relativity. In relativity, we learn to think about spacetime, as opposed to thinking of space and time separately. In doing this, it’s much easier to use a system of units in which distance and time are equivalent (and the speed of light has the value 1). Maybe some day in the future, when we’re all zipping around at close to the speed of light in our personal spacecraft, we’ll all have a strong intuitive grasp of relativity. It’ll seem perfectly natural to us to use the same units for distance and time, and the fact that people used to use different units for the two will seem quaint and archaic, like fathoms and leagues.

Maybe.

Update: I thought of one more exception to the statement that miles are horizontal: the Byrds song “Eight Miles High.”  But I think that’s in the same category as the others.  Anyway, they were a bunch of hippie stoners, so who cares what they think?

Eating my brain

Friday, March 7th, 2008

I got the phone call today telling me that I officially have tenure. Now I can make like a sea squirt.

Five years of WMAP

Wednesday, March 5th, 2008

 Update: The New York Times has a short piece about the data release.  Like me, they emphasize the increased precision of estimates of cosmological parameters such as the age of the Universe, and don’t cite any surprises in the data.

The WMAP five-year data have been released. The WMAP maps of the microwave background radiation are one of the most important sets of data in cosmology. A lot of what we know about dark matter, dark energy, the expansion rate of the Universe, inflation, and things like that come from this data set. In a quick glance at the abstracts of the papers and at the tables of parameters, I don’t see any big surprises: the error bars on parameters have gotten smaller, but nothing has radically changed. That’s pretty much what one would expect, of course.

It’ll take a while to chew through all of the results, so maybe there are big surprises that I didn’t notice.

The smallness of the errors on a lot of the parameters are amazing. To take just one example, the Hubble constant (that is, the expansion rate of the Universe) is 72 +/- 3 km/(s Mpc) according to this data. Cosmologists have been trying to measure this number for nearly a century, and as recently as the 1990s, it was uncertain by nearly a factor of 2. Now we know it (and a bunch of other things) with uncertainties of only a few percent.

Here’s the temperature power spectrum from the new data, along with some other experiments. (All plots in this post are from this paper.)
WMAP 5 temperature spectrum

It continues to amaze me how well the data match theoretical predictions.

The next frontier in the microwave background is measurements of the polarization, which is a much harder prospect. The easiest thing to measure about polarization is its cross-correlation with temperature, and WMAP has nailed that very well:

WMAP 5 TE

But the community is hoping to do better than that. The next challenge is to measure the polarization directly, without cross-correlating with the temperature. WMAP and other experiments have done that, but still with very large error bars:

WMAP 5 EE

But even that’s not the whole story. This data shows the E component of the polarization, but there’s another polarization signal called the B component, which is an order of magnitude or more smaller. That component is predicted to contain information about inflation that’s hard to get any other way, so a bunch of people are trying to figure out whether it can be measured. The MBI experiment I’m working on is a technology pathfinder for this effort. Looking at how hard WMAP had to work to get any information at all about the larger E signal, you can see that we have our work cut out for us!

Anybody out there?

Tuesday, March 4th, 2008

Since hardly anybody knows I have a blog, I assume that hardly anyone’s reading this. If you’re reading this, and I don’t know it, post a comment to let me know you’re out there. I’ll be much more motivated to post things if I think anyone’s watching.

Also, in case you’re wondering, I really enjoy playing Dr. Science, so if you have any science questions you think I might be able to answer, drop me an e-mail. I’ll answer here on the blog if I can. My main areas of expertise are big-bang cosmology and relativity, but you can try me on other topics in astrophysics and physics too.

The End of Cosmology?

Monday, March 3rd, 2008

According to Scientific American, it’s coming. The fact that the Universe’s expansion is accelerating means that we gradually have less and less access to information about the Big Bang. This is true in principle, but it’ll take billions of years before there’s a significant loss of information.

I guess we should increase cosmology funding now: We need to find out all we can before the information goes away!