Dr. SkySkull in Rome: Walking the city

My recent trip to Europe was officially work-related, as I was an “opponent” in a PhD defense for a student of my former postdoc advisor in Amsterdam.   We decided some time ago, however, to add a trip to Rome after the defense, and spent a lovely 4 days in the Italian city together with the graduating student.  Between my cellphone and my 35mm camera, I took some 1000 photos, and of course I wanted to share a bunch of them here!

I’ll break up my tour of Rome into several posts.  Of course I’ll dedicate entire posts to major sights such as the Colosseum, the Vatican Museum, and the Appian Way, but in this first post I thought I’d share some of the sights as seen just walking around the city on the first day.

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Posted in Personal, Travel | 3 Comments

Dr. SkySkull in Amsterdam: Optics in the Rijksmuseum

The lower level of the Rijksmuseum, an area relatively few time-strapped visitors manage to visit, is reserved for more practical forms of art: musical instruments, ceramics, ship figureheads, weapons, and the like.  I explored this whole area on my recent whirlwind tour of the Rijksmuseum and was delighted to find something I wasn’t expecting: optics-based art!  The museum contains an excellent and diverse collection of magic lantern slides, as well as an optical art form that I had never heard about before, called a diaphanorama.  I spent a good 10 minutes photographing everything I could in the exhibit, probably looking like a weirdo in the process, and would like to share those photographs and a few words about the optics and history here.

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Posted in Entertainment, History of science, Optics, Travel | 2 Comments

Dr. SkySkull in Amsterdam: A silly tour of the Rijksmuseum

I’ve been rather quiet lately because of work and travel!  A bit over a week ago, I flew to Amsterdam to participate in a PhD defense, and then traveled to Rome to give a talk and tour the city, which kept me quite busy.  As I’ve done in the past, I thought I would do a series of photo essays on my travels.

I flew to Amsterdam on the 18th of June, arriving on the 19th, and pretty much had the entire day to myself, as my former postdoc advisor and his PhD student were taking care of last-minute preparations for her thesis defense.  So, after a quick lunch with them, I was set free on the town.

Ready for some culture! I think I always look my most handsome on two occasions: right after a skydive, and right before a museum.

I was staying at the Hotel Piet Hein, which is within a short walking distance to Amsterdam’s magnificent art museum, the Rijksmuseum.  When I lived in Amsterdam back around 2003, the museum was mostly closed for major renovations, and so I had never had the opportunity to see it in its full glory.  This trip was a nice opportunity to do so!

Panorama of the Rijksmuseum. Note the weird ugly mouth-like sculpture in the pond center-right. No, I have no idea what it’s supposed to be.

The Rijksmuseum was originally founded in 1800 and moved to its current building in 1885.  It houses works of art from around the world, but has a particular focus on the Dutch masters like Rembrandt and van Gogh (though obviously most of van Gogh’s work is in the nearby van Gogh museum).

Times have changed in art museums.  When I was growing up, photographs were strictly forbidden, but in the cell phone era, only flash photography is prohibited.  I was taking photos throughout the museum and, realizing that I had wifi through a cool university system called eduroam, I started posting photos on twitter. The captions of these photos started off sincere, but quickly evolved into being largely irreverent and silly.  Heck, if you want a serious tour of a museum, I’m really not the person you should be following!

So, without further ado, let’s begin “A (Largely) Silly Tour of the Rijksmuseum.”

(Note added: all the photos are high-resolution. If you want to see detail, click on a photo and hunt for the “view full size” button.)

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Posted in Travel | 2 Comments

Agents of Dreamland, by Caitlin R. Kiernan

Taking a brief break from posts on quantum entanglement — will be back with more on that subject soon! Meanwhile…

Caitlin R. Kiernan is a brilliant writer. This is an indisputable fact.  Several years ago, I blogged about her beautiful, dark and haunting novel The Drowning Girl, a ghost story that I rank as not only a classic of the genre but a classic of literature.  I also was awed by her 2009 novel The Red Tree (which I read but never blogged about, for some reason).

When I found out that a new Kiernan novella came out recently, Agents of Dreamland (2017), I immediately snapped it up.

This is a short book — 123 pages — but it provides a disturbing and haunting peek at the beginning of the end of the world.

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What is quantum entanglement? Part 5: Making it happen

This is part 5 in a lengthy series of posts attempting to explain the idea of quantum entanglement to a non-physics audience.  Part 1 can be read here,  Part 2 can be read herePart 3 here, and Part 4 here.

So at this point we have an idea of what entanglement is, and some reassurance that it doesn’t ruin all of physics with its existence!  Now we turn to a very important question: how, in practice, do we produce entangled quantum particles?

In our discussions so far, we have imagined entanglement arising through the hypothetical decay of a neutral pion into an electron/positron pair, as illustrated below.

This is a fine idea in principle, but it is an utterly impractical method to reliably create entangled particles in a laboratory setting.  First, let’s talk about the pion: a pion is a high-energy particle that we typically¹ only see as a product in high-energy particle physics experiments, such as those done at places like CERN. When produced, they are usually moving at relativistic speeds, and in a direction that is more or less random.  They decay very quickly, on the order of 10 billionths of a billionth of a second when at rest, which means you can’t store them for future use. Even if you could store them, the direction of their decay products is random as well, which means we should revise our image above to appear as shown below.

In short: even if we managed to get a pion to sit still in one place, we wouldn’t know where to put our pair of detectors to spot the particles.  We would miss the vast majority of entangled pairs.

So using pions as a source of entangled particles to do experiments is not practical.  Fortunately, it turns out that we have a great source of particles available to us that are relatively easy to produce in an entangled state: photons, i.e. particles of light!  By the use of a process known as spontaneous parameteric down conversion, we can reliably and easily produce photons with entangled polarization that appear in very predictable locations.  Let’s see how!

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Posted in Optics, Physics | 3 Comments

What is quantum entanglement? Part 4: relativity and entanglement

This is part 4 in a lengthy series of posts attempting to explain the idea of quantum entanglement to a non-physics audience.  Part 1 can be read here,  Part 2 can be read here, and Part 3 here

In the last post, we finally introduced the concept of quantum entanglement.  An example of an entangled state between two quantum particles is given by the decay of a spin-zero pion into a spin-1/2 positron and a spin-1/2 electron, as illustrated below.

This results in a combined quantum spin state for the electron and positron that may be written as:

We may read this as “the two spin-1/2 particles end up in a quantum state which is an equal superposition of the positron being spin-up and the electron being spin-down with the positron being spin-down and the electron being spin-up.”

This suggests that the electron and positron, when produced in the decay, might be considered to exist simultaneously in a state where the electron (-) is up and the positron (+) is down, and vice-versa — their fates are “entangled.”  When we measure the state of one of the particles, say the electron, it is 50% likely to “choose” the spin-up state and 50% likely to choose the spin-down state.  When it does, the positron, no matter how far away, must instantly take on the opposite spin state — at least according to the original Copenhagen interpretation of quantum physics. In short, after measurement, the combined state of the electron and positron is either:

But, note the use of the word “instantly.” Because angular momentum is conserved, if the electron is measured spin-down, the positron must be in a spin-up state.  This collapse of the wavefunction must happen as soon as the electron is measured, otherwise there would be the possibility of measuring the positron also in a spin-down state, which would violate angular momentum conservation.

This would seem to suggest that the electron must send a “message” to the positron, and this message arrives instantaneously, regardless of the distance between them.  However, according to Einstein’s special theory of relativity, nothing is supposed to be able to move faster than the vacuum speed of light.

This raises the question: does entanglement violate special relativity?  And, if it does, can we use it to communicate over vast distances at superluminal speeds?

As it turns out, the correct answer is “neither.”  Entanglement, when considered carefully in the context of the full quantum theory, turns out to be perfectly consistent with relativity.  But, as we will see, it is quite a theoretical adventure to come to that conclusion!

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What is quantum entanglement? Part 3: Entanglement, at last

This is part 3 in a lengthy series of posts attempting to explain the idea of quantum entanglement to a non-physics audience.  Part 1 can be read here, and Part 2 can be read here.

Here, in part 3, we will at long last introduce entanglement! But, before we do, we need to be sure we really understand what the wave properties of a quantum particle imply about its behavior.

So, by the late 1920s, physicists knew that discrete bits of matter — electrons, for example — sometimes act like a wave and sometimes act like a particle.  This seemingly contradictory nature is often referred to as wave-particle duality.  It was not immediately obvious how to interpret the wave properties of matter, but physicists finally settled on what is referred to as the “Copenhagen interpretation,” after the city in which it was more or less developed.  If we consider the motion of a single electron, the Copenhagen description of quantum behavior could be summarized as follows:

  • While freely propagating through space, the electron and all of its properties evolve as a wave.
  • The wave may be described as a “wave of possibilities”: the amplitude of the wave only describes the probability of the electron to be found in a certain position or configuration.
  • While evolving as a wave, the electron in general has no definite position or configuration — it is, roughly speaking, existing in all possible configurations simultaneously.  In Young’s double slit experiment, for instance, it is often said that the electron goes through both slits.
  • When a property of the electron is measured in an experiment, where the property must take on a definite value, the electron “chooses” an outcome, based on the wave probabilities mentioned above.  The wave “collapses” into that single outcome.  For instance, if we have a detector to measure the position of the electron, we will see that electron at a single definite location on the detector.
  • If the particle still is free to continue moving, the process repeats itself, the collapsed wave evolving again, usually resulting in a wave of many possible outcomes again.

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Posted in History of science, Physics | 13 Comments