Wednesday, September 23, 2015

Speculation Unbound!

kw: book reviews, nonfiction, scientific miscellany

What would happen to the Earth if the Sun suddenly switched off? Randall Monroe answers that question beginning on page 248 of What If? : Serious Scientific Answers to Absurd Hypothetical Questions. Randall Monroe created the webcomic, which includes a What If? section, in which he answers questions of all kinds sent in by readers of the web site or, more recently, the book.

I can't believe I didn't stumble across this sooner. It is a step beyond the "Fermi Questions", so beloved of the young victims of Science Olympiad. Answering the really absurd questions requires a skill akin to Fermi's, who was famous for taking on a query with no more than a pencil and the back of an envelope. He is also remembered for his method of measuring the yield of the original Trinity atomic bomb. While others did whatever they were doing in their trench a mile or so from Ground Zero, he was seen busily tearing a sheet of notebook paper to small bits. A second or two after the blast was triggered, just before the shock wave hit, he tossed the handful of confetti as high as he could. After the shock hit, and it was deemed safe to exit the trench, he walked around, mapping the outline of the scattering of paper bits, did a calculation or two, and announced how many kilotons the yield had been.

So what would happen to us if the Sun switched off? Randall's take on it is mainly positive. He catalogs nine consequences, including "no need to force your children to wear sunscreen" and "better astronomy" with a quieter (and soon, nonexistent) atmosphere. Of course, his tenth consequence? "We would all freeze and die."

Interestingly, there are two ways to look at the Sun switching off. He chose to work with an immediate cessation of all energy flow from the Sun. One could also consider a sudden cessation of the fusion reactions powering the Sun. That leads to a more drawn-out scenario, because it would take a long time, hundreds of thousands of years, for the outer layers of the Sun to dim appreciably. It would take several tens of millions of years for the Sun to cool to invisibility. Perhaps that would give us the motivation to really ramp up the space program!

There was an interesting short story I read a couple decades ago, in which a young man (or so he seemed) walked into a reporter's office and informed him that the reason so few neutrinos were coming from the Sun was that Jehovah had left the place in a huff a couple thousand years ago, and being a thrifty sort, had turned off the fusion furnace. He said he was the newly-assigned deity and asked the reporter to run a provocative, cagey story that "perhaps" scientists would find a more "normal" level of neutrino activity from the Sun, starting in a few days, and to give no reason other than "informed by someone in the know". He intended to re-start the Sun. Sure enough, a week later the neutrino level rose to what the scientists had calculated it "ought to be". Of course, this was during the period that "neutrino oscillation" was being theorized, and is now the accepted reason that solar neutrino activity is observed to be 30% what was originally expected.

So, what kinds of questions get asked? Things like, "How many laser pointers do you have to point at the Moon so that we could see it?" or, "How much force power did Yoda produce (when lifting the X-wing from the swamp)?" There are also several short sections in which questions are listed but not explicitly answered; they are in the "Weird (and Worrying)" category: "What is the total nutritional value of a human body?" or, "Is there sound in space (There isn't right?)?"

Actually, that last question has an answer (so does the first: the same as a pig of the same weight). Yes, there is sound in space. Sound requires a medium in which to travel. Although the gas density in "outer space" is very low, it is never zero, anywhere. But the frequency of sound that is transmitted with little loss needs to be low enough that the wavelength is longer than the mean free path of the gas molecules as they bounce off one another. So the sounds in space, that go any useful distance, have very low frequencies. For example, in "interplanetary space", the average gas molecule travels a few meters before encountering another. The speed of sound is different at low pressure, but not by a great amount, so we can still use 300 m/s for rough calculations, and we find that a wavelength of 10m occurs at about 30 Hz. The trouble is, the sonic volume would be low, because so little gas is carrying the sound, but a sensitive microphone could detect low hum-type sounds "out there". In interstellar space, the pressure is lower, perhaps a thousand times lower, meaning that the mean free path is a thousand times as long, and frequencies higher than 0.03 Hz would not travel far. So the sounds in interstellar space would be at very low frequencies indeed. But they are there.

Rather than go on about things like using a Gatling Gun to propel a car (watch out, anyone behind!), I suggest you read the book, and check out the web site. Randall Munroe is an entertaining writer and, with a background in robotics, a deft hand at off-the-cuff mathematics (and a stable of helpful scientists' phone numbers in his Rolodex, no doubt). You'll love it.

No comments: