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:

Materials:

• 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.

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.

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.

Measuring Microwaves with Chocolate

I wrote up our latest lab as a post for GeekDad, and it ended up going popular on Digg! (For those who care.) To see how we measured the speed of microwaves with a chocolate bar, follow the link.

However, we did several trials, so here are some photos from our earlier attempts. And yes, the scale did go up in the last few days...

 

We only got only hot spot with this one ... and the paper plate started to burn (note lower right corner).

We tried multiple bars to get broader coverage. This worked a little better.

 

A dish full of chips provided the best coverage of all, but was too hard to pinpoint the hot spots. After several minutes of microwaving, we got one fused, hard point of chocolate (indicated by spoon) but not a second spot to measure. 

The results:

Best holder: glass baking dish

Best stand (to cover the rotating thing in the microwave): small plate

Best chocolate: Valentine's Day cherry cordials

This experiment has also been done with marshmallows and by kids on YouTube. 

Big Blog Theory

If you are a fan of The Big Bang Theory -- and I don't watch near as often as I should -- then you will enjoy the blog of UCLA particle physicist David Saltzberg, who is the show's consultant. The Big Blog Theory explains the science behind the episodes.

The show can be seen on CBS Monday nights at 9:30 EST.

Hat tip to a post by fellow GeekDad writer John Booth.And for other interesting physics blogs, check the list way down in the sidebar.

Wave Lab Part 2

After watching some cool videos on YouTube, I decided it would be fun to make patterns with sound waves. These patterns are caused by the same kind of waves, and wave interference, that we saw with our pseudo-ripple tank experiment. Again, our setup was crude: we took a recycled container and set it over a tiny set of speakers and an mp3 player loaded with video game soundtracks. Then we sprinkled some salt on a plate and put it on top. We also tried sprinkling salt directly on the metal top, and then tried it with some water.

We didn't always get fancy patterns, but we did see some nice movement. Watch!

In the videos on YouTube done with real lab equipment, you can see cool Chladni patterns.

Here's an explanation from Teacher's Domain:

When an object vibrates at one of its natural frequencies (a rate of vibration at which it naturally tends to move), standing wave patterns are formed within the object. These patterns are the result of wave interference, which occurs at the meeting of two waves traveling within the same medium in different directions. The resulting disturbance within the material at the point where the waves meet is the net effect of the two waves. At certain points in the material, the waves cancel each other out through destructive interference and there is no net disturbance. These points are called nodes, or nodal points. Around the nodes, the waves constructively interfere; the points with the greatest disturbance are called antinodes, or anti-nodal points.

And here's an explanation of their origin and use from Robert Krampf:

These patterns are called Chladni patterns, named after Ernest Florens Friedrich Chladni of Saxony, who has been called the father of acoustics. He sprinkled sand onto metal plates and studied the way that they vibrated.

Besides being fun to play with, these patterns are useful. These patterns are used in designing musical instruments. If a part is attached to a place where the instrument vibrates, the sound will be dampened. By attaching parts at nodes, the instrument makes a full, rich sound. These patterns make the difference between an average instrument and a quality one.

Here are the rest of our videos:

Book Review: How to Teach Physics to Your Dog and The Macroscope

(I wrote this article for the Albany, NY Times Union newspaper. It originally appeared, in edited form, on January 24, 2010.)

When you think about it, “modern physics” isn’t really all that modern anymore. Einstein began drafting his theory of relativity in 1905, and quantum mechanics – which describes how things work at the sub-atomic level – was described by Max Planck in 1900. Today quantum mechanics is at the core of everything from bar code scanners to computer chips. It’s the most accurately tested theory in the history of science.
And yet very few people are aware of even its most basic concepts. Ideas like particle-wave duality (the fact that light and matter has both wave and particle nature) are rarely covered in college physics classes, let alone high school. So when Internet rumors claim that the CERN Large Hadron Collider, which smashes atoms together to see what pops out, is about to suck the Earth into a black hole, or when the latest DaVinci Code book features a physicist who uses “thought particles” to transform matter, most people don’t know what to believe.

That’s a gap two new books by local educators are hoping to bridge. In “How to Teach Physics to Your Dog,” (Scribner, 2009) author Chad Orzel explains quantum mechanics to Emmy, his German Shepard mix, in language so down-to-Earth and entertaining that even humans can understand. Why a dog? As Orzel, an associate professor in the department of physics and astronomy at Union College in Schenectady, explained recently, dogs have no preconceptions about where things come from. That makes it much easier for them to accept the idea of virtual particles and parallel universes.

“As bizarre as it seems to a human, as far as a dog is concerned dog treats appear out of the air,” Orzel said. “She will sit there staring, hackling at evil squirrels from another dimension.”

For readers, following Orzel as he discusses the probability of bunnies made of cheese suddenly appearing in the backyard, or whether dogs can use their wave nature to pass around both sides of a tree at the same time, makes modern physics easier to understand.

“As scientists,” Orzel said, “we speak about it in math. I wanted to find ways to get around that, to show how fascinatingly weird the world is without forcing them to go through three years of physics.”

At the same time, Orzel added, “There is some heavy stuff in the book -- decoherence, ‘many worlds’ theories – that you don’t often encounter in popular treatments of the subject. The nice thing about writing with the dog is that whenever things get a bit thick, I can have her break in.” At those times Emmy pipes up to remind Orzel, “I don’t want to describe the universe, I want to catch squirrels.”
The goal for Orzel is to help readers understand that although the universe is a really strange place, it still has rules, and physicists have been sucessful so far in understanding them.

“You can’t will yourself into another universe where you’re wealthy,” he said. “I hope the dog is cute enough to carry people past some of the need for it to be magic.”

While Orzel’s book was written for adults whose schooldays are behind them, “The Macroscope,” the first in the Adventures in Atomville series, aims to inspire kids who have yet to set foot in a physics classroom. It’s a fantasy story in which all the characters are atoms which behave in ways that reflect the properties of their particular elements. They eat (and emit) photons, and swat away pesky electron gnats. But the physics is hinted at, not explained outright. (A website explaining the science behind Atomville is under development.) Co-authors Jill Linz, a senior physics teaching associate at Skidmore College in Saratoga Springs, and Cindy Schwarz, a professor of physics at Vassar College in Poughkeepsie, both said that the plan was to pique kids’ interest, not lecture to them.

“We don’t necessarily want these kids to walk away knowing what’s going on with subatomic particles,” explained Schwarz. “We want them to keep the words in the back of their heads and feel more comfortable when they hear them again.”

Linz first developed Atomville as a way to reach non-science majors, and later went on to produce physics videos for elementary schools. Schwarz uses creative writing and music in her physics classes for non-majors, and has published a book of her students’ physics poems and stories called Tales from the Subatomic Zoo.
Linz and Schwarz are hoping schools will invite them in to talk about their book and about physics. Last spring Schwarz showed students in Poughkeepsie how atoms emit photons and letting them look through diffraction glasses to see the spectrum created by an element. She was happy to find that, months later, they still remembered the concepts they learned.

“They really got something out of this,” she said.