“Interference between different photons never occurs:” Not! (1963)

Posted on September 12, 2008 by skullsinthestars

Note: This post is my contribution to the third edition of The Giant’s Shoulders, a carnival of blog posts on classic science papers.

One of the most famous statements concerning quantum mechanics, as it relates to the light particles known as photons, was made by the brilliant scientist Paul Dirac in his Quantum Mechanics book1:

“each photon then interferes only with itself.  Interference between different photons never occurs.”

This statement is bold and unambiguous: in Dirac’s view, a photon only creates interference patterns by virtue of its own wave function, and wave functions of different photons do not interact.

The statement is bold, unambiguous, often quoted — and wrong!  In 1963, Leonard Mandel and G. Magyar of Imperial College disproved this statement2 with a clever and simple experiment and a two-page paper in Nature.  I was reminded of this work by a question on my recent post on coherence, and it seemed worth reexploring.  Follow me below…

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Posted in History of science, Optics, Physics | 26 Comments

4 days until The Giant’s Shoulders #3!

Posted on September 11, 2008 by skullsinthestars

This is a reminder that there’s 4 days until The Giant’s Shoulders #3, to be held at Entertaining Research.  Entries can be submitted, as always, through blogcarnival.com.

Posted in General science, Science news | Leave a comment

LHC fires up!

Posted on September 10, 2008 by skullsinthestars

From various sources in the science blogotopia, we learn that the LHC has been started for the first time!   For those unfamiliar, the LHC (Large Hadron Collider) is designed to be the world’s largest particle accelerator, which will probe the frontiers of known physics in search of a greater understanding of the fundamental forces of nature.

I don’t have a lot to say on the start up of the LHC itself, other than the fact that it’s very exciting (I did undergraduate and graduate work in high-energy physics, so I have a bias).  I can’t resist, however, pointing out CNN’s initial lede on the subject:

Scientists applauded as one of the most ambitious experiments ever conceived began today. The Large Hadron Collider — designed to simulate conditions of the Big Bang — was switched on this morning. Skeptics claim the experiment could create a black hole capable of swallowing the Earth.

Emphasis mine.  This is highly misleading, as the ‘skeptics’ in question to the best of my knowledge have no scientific experience and are contradicted by every reputable scientist on the planet.  The structure of the story has changed since I first read it, but it still highlights the concerns of the ‘skeptics’.  Just for fun, let’s look at what some other CNN ledes might look like if given similar skeptical counterbalance:

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Posted in Science news | 5 Comments

Busy with my book…

Posted on September 9, 2008 by skullsinthestars

If anyone’s noticed a slowdown in my blog output recently, the reasons are twofold: 1.  The beginning of the academic semester, and 2.  I’m finally trying to make progress on my textbook.  I will probably continue to post at a slower pace for a few weeks, but definitely won’t stop completely.

As a consolation, here’s a cute video I took the other day of two of our cats, Simon and Sabrina, getting acquainted with a new motorized water dish:

Posted in Animals, Personal | 4 Comments

Richard Marsh’s “Philip Bennion’s Death”

Posted on September 7, 2008 by skullsinthestars

As promised, here’s the first discussion of some classic Richard Marsh, in celebration of the release of Valancourt’s edition of The Beetle.  I start with a brief discussion of another Valancourt edition, Richard Marsh’s Philip Bennion’s Death (1897).

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Posted in Mystery/thriller | Leave a comment

Peanuts animator, and voice of Snoopy, Bill Melendez, dies at 91

Posted on September 5, 2008 by skullsinthestars

Sadly, the producer/animator who brought us such classics as “A Charlie Brown Christmas” and “A Charlie Brown Thanksgiving” has passed away at 91, of natural causes.

He was also the very familiar voice of Snoopy in many of these specials, though the “voice” was a series of barks, growls, and squeaks.  He also did the voice of Woodstock in many of the specials.

He will be missed!

If you’re not familiar with Melendez’s voice acting as Snoopy, this classic clip from “A Charlie Brown Thanksgiving” is the best introduction:

Posted in Entertainment | Leave a comment

Optics basics: Coherence

Posted on September 3, 2008 by skullsinthestars

In previous optics basics posts, the interference of waves has played a major role.  When two or more monochromatic (single-color) waves are combined, they form a pattern of light and dark regions, in which the combined light fields have constructively or destructively interfered, respectively.  The simplest of these patterns can be created by the interference of two plane or spherical waves, and would appear as shown below:

One way of producing such a pattern is by Young’s two-pinhole (or double-slit) experiment, which we will have cause to discuss in more detail below.  The actual size of the interference pattern depends on the experimental setup, including the wavelength of the light, but can be easily made visible to the naked eye.

An astute observer of nature, however, will find something fishy about this whole discussion of interference: it does not seem to manifest itself in everyday experiences with light.  Sunlight streaming through a window, for instance, doesn’t interfere with the light emanating from a lamp inside the room.  Something is missing from our basic discussion of interference which explains why some light fields, such as those produced from a single laser source, produce interference patterns and others, such as sunlight, seemingly produce no interference.  The missing ingredient is what is known as optical coherence, and we discuss the basic principles of coherence theory and its relationship to interference below the fold.

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Posted in Optics, Optics basics | 71 Comments

15 days until The Giant’s Shoulders #3!

Posted on September 1, 2008 by skullsinthestars

Labor Day is a day for the working citizen of the U.S. to relax… but not science bloggers!  This is a reminder that there’s 15 days until The Giant’s Shoulders #3, to be held at Entertaining Research.  Entries can be submitted, as always, through blogcarnival.com.

Hmm… that reminds me that I’ve got to get cracking on my entry!

Posted in General science, Science news | Leave a comment

Roald Dahl, sex spy?

Posted on September 1, 2008 by skullsinthestars

This is entertaining.  Roald Dahl, famous author of children’s stories such as Charlie and the Chocolate Factory and nasty little shockers such as Man From the South, was apparently a spy for the British government during World War II.  His job?  He was part of a group tasked with getting U.S. involvement in the war, and his role was gathering intelligence from wealthy women.

This is not necessarily surprising, in hindsight; Dahl wrote a number of very randy stories, including a very nasty one called The Visitor, about a playboy named Oswald Cornelius who has one sexual adventure too many.  Dahl’s stories contain a significant amount of cynicism, which one might expect from a man whose task was to exploit women romantically and sexually for information.

I wrote a ‘horror masters’ post on Dahl some time ago, and it can be found here.  This is the second surprise spy revelation within a month; thank goodness Julia Child was apparently tasked with a different sort of information gathering.

Posted in ... the Hell?, Horror | Leave a comment

Valancourt’s definitive edition of Marsh’s “The Beetle” released!

Posted on September 1, 2008 by skullsinthestars

I just received in the mail the other day my copy of Valancourt Books’ edition of Richard Marsh’s The Beetle: A Mystery.  The Valancourt edition is the definitive edition for the collector, with an introduction by Richard Marsh scholar Minna Vuohelainen and detailed notes putting the novel in its proper historical and literary context.  As with other Valancourt Marsh books, it also reproduces the original cover art:

It’s quite a lovely volume!  To celebrate the release, I’ll be posting over the next week about other works of Richard Marsh that I’ve been reading.

Posted in Horror | 4 Comments