Tuesday, March 30, 2010

Today is First Physics Day

The CERN Large Hadron Collider had its first stable event about an hour ago. Watch the live webcast!

The New York Times says:

Rolf Heuer, director general of CERN, speaking from Japan, said the new collider “opens a new window of discovery and it brings, with patience, new knowledge of the universe and the microcosm. It shows what one can do in bringing forward knowledge.” He added: “It will also bring out an army of children and young people who will get into the private sector and academia.”
Yesterday I posted on GeekDad about our visit with Chad Orzel, author of How to Teach Physics to Your Dog, at his lab at Union College. One commenter felt that there is no reason for non-scientists to spend time trying to understand this stuff. But the reason is that physicists need the public to fund their research and understand the significance of their discoveries. It was lack of public interest that led to the end of the US's attempt to build the time of facility that now exists in Europe. According to the Times:

The first modern accelerator was the cyclotron, built by Ernest Lawrence at the University of California, Berkeley, in 1932. It was a foot in diameter and boosted protons to energies of 1.25 million electron volts, the unit of choice for mass and energy in physics. By comparison, an electron, the lightest well-known particle, is about half a million electron volts, and a proton about a billion.

Over the last century, universities and then nations leapfrogged each other, building bigger machines to peer deeper into the origins of the universe. But the end was decreed in 1993, the U.S. Congress canceled the Superconducting Supercollider, a 54-mile 20-trillion-electron-volt machine being built underneath Waxahachie, Texas, after its projected cost ballooned to $11 billion.

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Thursday, March 25, 2010

The Famous Double-Slit Experiment and the DIY Quantum Eraser

In How to Teach Physics to Your Dog, author and Union College Physics Professor Chad Orzel talks about an extension of the Double-Slit Experiment called the Quantum Eraser. According to Orzel -- and before him to physicist and wise guy Richard Feynman -- everything the average person needs to know about Quantum Physics is contained in the Double-Slit Experiment.

When Thomas Young first did the Double-Slit Experiment in 1803, he proved that light travels in a wave. He showed this by aiming a narrow beam of light at a barrier with one or two slits and placing a screen behind it. When the light went through one slit, it hit the screen in a single blob. But when it went through two slits, the light on the screen spread out into many stripes of dark and light -- which is what you would see if two waves were overlapping to create an interference pattern.

When Quantum Physics was introduced, the experiment was done with a stream of photons passing through the slits one photon at a time. Amazingly, over time the individual photons also created an interference pattern on a screen on the other side -- meaning that each single photon was interfering with itself as it passes through both slits at the same time!

The Quantum Eraser experiment just makes this weird result even weirder. First polarizing lenses with different orientations are put in place so that you can tell whether the light went left or right through the slits. "Labeling" the photons in this way makes the light go back to acting like particles -- the interference pattern is erased. And if you add still another polarizing filter, so that you can't tell which way the particles went, the pattern reappears!

When I read in Orzel's book that the May 2007 issue of Scientific American had a Quantum Eraser experiment you could do at home, I knew I had to try it! After a bit of searching, I was able to find the article online. (Actually, what I found is everything but the article, but the sidebars and other content include everything you need to do the experiment.) Like a lot of demonstrations that we try, it was a little hard to tell what, if anything, was happening, and I'm not sure it was completely successful. However, the results we did get were good enough to be worth sharing here. The article includes some trouble-shooting tips that may produce better outcomes if we ever try it again.

The experiment consists of four parts:
  1. Create a double-slit set-up using a cheap laser pointer as a light source.
  2. Add a right/left polarizing filter.
  3. Hold up a polarizing filter on a diagonal, which allows some "left" and some "right" particles to pass through.
  4. Make a polarizing lens which filters light on one diagonal on the top and the other on the bottom and add that to the set-up.
Obviously, since we were using a cheap laser pointer and weren't sending light through one photon at a time, this experiment doesn't prove that a single particle will go both ways at once, but it does give you a good approximation of what happens on a quantum level. Below is a description of what we did:

  • laser pointer pen (from the supermarket)
  • polarized film (we used the lenses from cardboard 3D movie glasses)
  • thick rubber band
  • white foam-core board (for projection screen)
  • Styrofoam cups
  • unused twist ties
  • tape

  1. First we made a stand for the laser pointer pen by pushing it through an upside-down Styrofoam cup.
  2. Instead of a barrier with a slit, this version uses a vertical piece of wire to divide the light into "right" and "left." We cut the paper off of a twist tie and removed the wire without bending it. Then we made a stand for the wire by cutting around the top of a foam cup to make it shorter than the laser stand. We turned the cup upside down and poked the wire through the bottom so that it was standing straight up.
  3. We wrapped a rubber band around the laser's ON button so that it would stay on.
  4. The laser was put in its holder and placed on the seat of a chair. The foamcore projection screen was set up by leaning it against a chair about 6 feet away. We could see a small dot of laser light on the screen. (See directly above.)
  5. Then the wire in its holder was set up a few inches away from the laser. We moved it until it was in the path of the laser light. An interference pattern appeared! (Photo at top of post.)
  6. To make the labeler, we took the polarized glasses, and marked the lenses "right" and "left."  Then we cut them out, leaving the cardboard frame around everywhere but the inside edge (towards the nose piece). The two lenses were taped together so that the inside edges were just touching (no overlap or gap). Another twist-tie wire was taped along the join and trimmed.
  7. A holder was made by cutting off the top of another foam cup, then slicing a slot across the bottom. The labeler was set into the slot so that the wire was vertical in the center.
  8. The labeler was put in place of the plain wire. The light hitting the screen returned to blob form.
  9. Taking another pair of polarized lenses, we held up the "left" and "right" lens at a 45 degree angle between the labeler and the screen. At this point the light projected on the screen was hard to make out, but it did seem to spread out again like an interference pattern.
  10. Finally, we took a left and right lens, cut them on a diagonal, and taped them together so that one was on top and one on the bottom. According to the SciAm directions, we should have seen an interference pattern split so that the top was off to one side and the bottom to the other, like misaligned teeth. All we could see was misaligned blobs, though. (See below.)
As I said, if we try this again we will try moving some of the parts around to get better results. Just for the record, the glasses we used had lenses which were tilted at 45 degree angles, rather than the traditional horizontal and vertical. However, they were still perpendicular to each other, and we rotated each the proper amount from its starting point, so I don't think it mattered.

In my opinion, we achieved some interesting effects, for a living-room physics lab.
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Monday, March 22, 2010

What Every Dog Should Know About Quantum Physics

Union College Physics Professor Chad Orzel was kind enough to give a talk based on his new book, How to Teach Physics to Your Dog to a group of local homeschoolers I organized. Even better, he posted the video and slides he showed us in the talk on his blog! The presentation included a look at helium and neon lights using diffraction grating and a demonstration of the double-slit experiment using a laser beam. I'm adding the books he recommended -- some for a popular audience, some aimed at freshman physics students -- to my Amazon store as well.

After the talk, Dr. Orzel brought in his famous dog and co-author Emmy for a photo op. Then we got a tour of his laser cooling lab, the school's own basement particle accelerator, and the astronomy department's observatory. One interesting fact about Union is that, because there are no graduate students to compete with, undergraduates get to use the fancy equipment right from the start.

The talk was entertaining and informative. As you can see, the kids were as interested as the parents. Thanks to Dr. Orzel for such a great program!

UPDATE: Listen to an interview with Chad Orzel from WAMC Northeast Public Radio.
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Friday, March 12, 2010

The Many Worlds of Hugh Everett

We recently watched the PBS NOVA show Parallel Worlds, Parallel Lives about the late physicist Hugh Everett. In 1957, Everett came up with a scenario that would eliminate the Schrödinger's cat -- which said that light didn't take shape as wave or particle until someone was observing it. He called his theory "many worlds," and it proposed the idea that where two states are possible, each splits off into its own universe. Science fiction, especially Star Trek, later adopted the idea for stories involving parallel universes. But at the time, Everett's theory was dismissed by the big guns of physics, like Niels Bohr. Rejected, Everett left academia and went to work for private firms, never developing his theory any further.

Parallel Worlds, Parallel Lives explores the physics of Hugh Everett through his son Mark Oliver Everett. Mark Everett, also known as "E," is a member of the indie rock band EELS and author of Things the Grandchildren Should Know. Mark grew up with his father but had very little contact with him. As an adult, he decides to investigate his father's life and work, meeting with physicists who are trying to further his theories, and visiting with his old colleagues and friends at Princeton and elsewhere. He also uncovers boxes of papers taken from his father's home after the death of his sister and mother and turns them over to his father's biographer. As he says in the documentary, he has become the ambassador from the Everett family to the world.

I really love the NOVA videos we have watched so far this school year, because they both bring in a human perspective and make the most of today's video effects to illustrate difficult physics concepts. This one is no exception, and it has the added plus of being told from the point of view of someone who, like us, has no scientific background. The video is only an hour long and well worth borrowing from your library or adding to your physics teaching materials. There is, as always, clips and lots of supplementary material at the PBS website. My only complaint is that the classroom "activity" doesn't include an actual double-slit experiment, but used a computer simulation instead.