S03E04: The Pirate Solution

In tonight’s episode Raj works alongside Sheldon on “The Dark Matter Problem”.   In my opinion this is the biggest problem in physics today that we have a hope of solving.

Vera Rubin discovers the dark matter in the galaxy.

Vera Rubin discovers galactic dark matter.

Physicists love it when theories have problems. Like pulling a stray thread on a sweater, it might give you just a tuft of yarn, or it might unravel the whole thing.  The best thing that can happen to a scientist is to ruin somebody’s sweater.

One of the biggest problems of the 19th century was the age of the Earth appeared much larger than the age of the Sun.  Geologists argued (correctly) that the Earth’s age was measured in billions of years based on sedimentation rates.  Meanwhile physicists calculated that for any known energy source, the Sun would have burned out after at most 20 million years.  The physicists’ arguments were convincing and screwed up geology for a century.  Ultimately the problem was resolved by a major change in the way we understand energy.   The advent of nuclear physics in the 1930’s explained that sunshine is powered by nuclear reactions.  By converting mass into energy, a previously unknown energy source, nuclear reactions are nearly a million times more powerful than chemical reactions.  They allow us a 4.5 billion-year old sun.

Maybe that is a bad example to encourage pursuing problems.  Although the Sun-versus-Earth’s age problem signaled a misunderstanding of utmost importance, it was solved only because of work in a completely different subject than astronomy and geology.  This story is typical.

Often physicists just find things out by having a lucky break while toiling away on some other problem.  This happened to physicists in Japan working underground with a big tank of water they called “Kamiokande”.  I remember as a kid reading an essay by Isaac Asimov about their experiment: “After Many a Summer Dies the Proton”, describing their search for a decaying proton.  The theorists said they should find it, since it would solve some of their theoretical problems. Asimov’s title was premature—now, more than twenty-five years later, neither the Japanese nor any other experiment has ever seen a single proton decay.  Meanwhile, the Kamiokande physicists had to study particles called neutrinos crossing their detector since they were a source of noise.  They found while studying this noise an amazing effect called “neutrino oscillations”, which revealed essential properties of neutrinos.  The Japanese physicists had made the biggest scientific discovery in particle physics in decades.  (During that time I was in  Geneva also looking for “neutrino oscillations” with parameters the theorists said were more likely.  We found nothing.)  Had the Kamiokande experiment not been built to chase down this wrong proton-decay prediction by theorists, we wouldn’t have this important discovery.

(Asimov’s essay was just  one of many he wrote about science for the monthly “Fantasy and Science Fiction Magazine”. While I was in junior high school, Asimov’s science article was always the first thing I read when the magazine arrived, not the science fiction stories.  Perhaps that foretold why I am now only a science consultant instead of a writer.)

Our century’s problem, the dark matter problem, has many facets, but the most glaring is the speed of our solar system.  Just as Earth and other planets in our solar system revolve around the Sun, our whole Solar system orbits  the center of the Milky Way galaxy.  While every year the Earth goes around the Sun, every “galactic year” (250,000,000  Earth years, or  nearly 2 billion dog years) the whole solar system makes a full galactic orbit.   Every planet that goes around the Sun does so as described by Newton’s laws of mechanics.  The farther out a planet is from the Sun, the slower it should move, given fairly precisely by the square root of the distance.    For example, Saturn is about 9.5 times farther from Sun than the Earth is from the Sun, and so moves square-root(9.5)=3.1  times as slowly as Earth.    This works because the gravitational pull of the Sun keeps the planets moving in near-circles.   By adding up all the objects that astronomers see, the core of the galaxy should cause the stars in the rest of the galaxy to undergo orbits analogous to the planets’ motion around the Sun.  However, when astrophysicist Vera Rubin made the measurements,  she measured no drop in speed at all. Since astronomers can only count what they can see, what is light, we suspect there is dark matter filling the galaxy that pulls stronger on our solar system and other stars.   So 250,000,000 years may be a long time, but without dark matter it would be much longer.

Speed of stars vs distance from the center of the Milky Way.  The Earth and Sun are located at about 8 on the x-axis.

Speed of stars vs distance from the center of the Milky Way. The Earth and Sun are located at about 8 on the x-axis.

The discovery of dark matter has told us that we don’t even know what 90% of the matter is in the universe.  While we may all be hoping Sheldon gets a Nobel Prize, let’s hope Dr. Rubin is honored as well.

Physicists would love other proof of dark matter, but we don’t even know what it is.  That is what Sheldon and Raj were working on.  Some physicists try to find it in space.  If the dark matter is made of particles that can collide and annihilate, they will give up very energetic light called gamma-rays.  This light is more powerful than even X-rays.  Gamma-ray telescopes around the world are looking for evidence of these dark matter collisions. If you look carefully at the white board, you will see the name of one gamma-ray telescope friends of mine built called “VERITAS”.  You’ll also see a sketch of how it works:  gamma rays hit the upper atmosphere and produce small amounts of light detected by big curved mirrors on the ground.   Meanwhile other physicists are competing to be the first to find the dark matter by observing directly the extremely small amount of energy a dark matter particle might deposit in a detector as they pass through Earth.   Some experiments use Sodium (which has an atomic mass 23) and other use Xenon (with atomic mass 131).  Now you know why Raj crosses out 131 and changes it to 23.  Sheldon was calculating the rate for the wrong target material, xenon not sodium.

Tune in next week in two weeks to watch  the apartment’s whiteboards for Sheldon catching up by studying sodium.

16 Responses to “S03E04: The Pirate Solution”

  1. shellorz Says:

    Hey, first off, kuddos to you for being involved in such a great show. Must be thrilling to know they rely on you.
    I wanted to point out that there’s no such thing as the “discovery of dark matter” :). There’s the discovery of the unexpected rotational speed of galaxies (according to the usual gravitation law). Dark matter is one (say the most debated) of the possible explanations. But, its name only says it all. It’s invisible (dark) and weighted (matter -although some particles are weighted and not considered as regular matter -neutrinos e.g.) to be able to have an influence according to Newton’s laws and GR. So it’s more a “lack of something’. I read a lot about alternative theories like a change (more like a better definition) of the gravitational laws that would imply a change of behavior at high scales (this already exists : the gravitational force is so weak that every other force prevails but at large scales it rules the whole cosmos -or so it seems). I once read about the bi-metric universe (the universe having a parallel twin that has negative masses). Also the most oustandingly elegant theory about this concerns anti-matter. What if antimatter would antigravitate ? It would have a mass (and so influence matter through gravitation) but would be repelled out. This would also explain the apparent lack of anti-matter in the universe.
    Also, I’m surprised that Sheldon, being a haughty string theorist, isn’t reluctant to do experimental things. He’s more the guy that will predict the course of the thrown apple than throwing it a thousand times to check if it always lands in the same place :) I’ve known about a lot of kinda arrogant physicists and their goals is to be the new Einstein by especially not following the rest of the herd. (for instance, this crazy dude had interesting theories about cosmology : http://en.wikipedia.org/wiki/Jean-Pierre_Petit not that I agree with them, but it adds up to the argument)

    Anyways, thanx for the 23/131 explanation and thanx for keep on with the good work/blog

  2. BigBongTheory Says:

    Why is xenon the wrong target material?

    • David Saltzberg Says:

      There are many different experiments that look for direct hits by dark matter particles. Some use either sodium-iodide, xenon, or germanium and silicon. Raj and Sheldon were discussing one that happens to use sodium (iodide) and Sheldon had it confused with a different experiment that uses xenon.

  3. DiscoDave Says:

    I used to read Asimov’s column first thing whenever I got an issue of F&SF Magazine, too. I try to model my own technical writing after his style; his simple and clear language dotted with historical notes and engaging asides made even the driest material seem interesting.

    Thanks for clearing up the “23-101″ confusion. I thought those numbers were some sort of matrix operators for high energy physics equations. All that math gave me the heebie-jeebies; that’s why I’m a crystal grower!

  4. EBD forever Says:

    For a cool sounding nerd show, they sure aren’t marketing it well – it’s nowhere online legally. Not Hulu, not cbs.com.
    I’m not sure if my interest is piqued enough to go on the bittorrent hunt. Oh well.

  5. Thomas Says:

    Funnily enough, only a few days after the show aired, a paper was published, in which two physicists claim to have found
    “Possible Evidence For Dark Matter Annihilation In The Inner Milky Way From The Fermi Gamma Ray Space Telescope”

    This “discovery” is still debated in the physics community, and it will need more time to be sure, that they really see decaying anti-matter. The particle proposed by Hooper e.a weighs about 30 GeV, so Sheldons upgrade to a detector for 500 GeV would be a move in the wrong direction, if Hooper is right.

    Also, I’m not sure, if Kamiokande only treated the neutrinos as background, since the Solar Neutrino problem needed to be solved somehow, so I’m rather sure, that quite a few physicists used Kamiokande fore neutrinophysics right from the beginning (although the funding arguments might have been proton decay). It’s like saying, LHC was searching for a wrongly predicted higgs, and only by accident found Leptoquarks or a Z’.

    Nontheless your blog is very nice to read, even for a physicist ;)

    Cheers,
    Thomas

    • David Saltzberg Says:

      The neutrinos that were a background to the proton decay search were not the solar neutrinos but neutrinos from the atmosphere. These are created by cosmic rays (charged particles) hitting the upper atmosphere. Those “atmospheric” neutrinos turned out to give evidence of one type of neutrino oscillation. The solar neutrino sample gave evidence for another type of neutrino oscillation. Kamiokande and its follow-on Super-Kamiokande has been a very fruitful experiment for particle physics.

  6. Simon Says:

    When I took Phys. 1C, I learned that almost 90% of the matter in the universe is dark matter because they does not emit any electromagnetic waves. I think the joke that Raj made is true, we need a flashlight. The show is really fun. LOL

    • shellorz Says:

      Simon, dark matter and dark “energy” shouldn’t be confused. Dark matter is “invisible” mass that could explain the way galaxies spin and represents about 25% of the universe (matter and energy being equivalent it means 25% of the matter or energy of the universe). About 5% is ordinary matter and energy (baryonic matter and photons and other bosons). The other 70% is what is called dark energy and would be responsible for the accelerating expansion of the universe. As for what it is actually, it seems there are different theories. Cosmological constant, vaccum energy, quintessence or flawed gravitational laws (including General Relativity)

  7. Clex Sipsoxard Says:

    “For example, Saturn is about 9.5 times farther from Earth than [from] the Sun, and so moves square-root(9.5)=3.1 times as slowly as Earth”

    I think it should be “9.5 times farther from Sun than Earth”, am I correct?

  8. The Big Blog Theory en Español Says:

    [...] Artículo original por David Saltzberg [...]

  9. Tradução: “S03E04: The Pirate Solution (A Solução Pirata)” « The Big Blog Theory (em Português!) Says:

    [...] ver Sheldon tirando o atraso e estudando o sódio. Tradução feita a partir de texto extraído de The Big Blog Theory, de autoria de David Saltzberg, originalmente publicado em 12 de Outubro de [...]

  10. N Says:

    Hi Mr. Saltzberg. I wonder if you were interested in an theory that explains the EPR paradox, which says that the c velocity limit is not exceded (so the paradox is not, and can be understood in a Relativist System), and also show us why light is percepted sometimes like a spot and sometimes like a wave.
    No joking.
    If interested send me a mail, I will show you. (If you think I am some kind of crazy or stupid, you always can make a “sandbox mail” a new account only for contact me).
    Sorry for my English.

    Yours N.

Comments are closed.


Follow

Get every new post delivered to your Inbox.

Join 374 other followers

%d bloggers like this: