Archive for October, 2010

S04E06: The Irish Pub Formulation

October 28, 2010

The first force discovered by the first physicists was gravity.    Over four hundred years ago our understanding that gravity was a force at work throughout the Universe, was explained by Sir Isaac Newton.  Nearly 100 years ago we modernized our understanding with Albert Einstein’s General Theory of Relativity.   And despite all this homework, gravity is still the force least understood by physics.

Physicists understand gravity well enough to get us to the Moon. So why is Sheldon still working on gravity?

Over the centuries three other basic classes of forces were discovered:

Electromagnetism is familiar to us and pushes currents into our houses and electrons through our electronic devices.  Lucky for us, it holds atoms and molecules together.

The “strong” force holds nuclei together.  The atomic nucleus is made of neutrons (no electric charge) and protons (positive electric charge).   Since same-sign charges repel, the atomic nuclei should all blow apart.  They are trying to, but another force, the strong force, holds them together.  The strong force is stronger than the electromagnetic force in the nucleus.  But only at short distances.  That’s why we don’t experience the strong force on our human scale.  Its range determines the size of the biggest nuclei we can make.    We have never made a nucleus larger than about 300 protons and neutrons.   Since they are packed in a sphere and protons and neutrons are about the same size, you can calculate the radius using simple geometry:  Volume =4/3πr3.  

So the radius of the largest nucleus is about 4 protons and neutrons in radius (8 in diameter).  That’s about the range of the strong force, about 4 protons’ width.   That is 0.000000000000004 meters.   (We’ve since learned that this “strong” force is generated by a more elementary force at work inside the protons and neutrons themselves, called “quantum chromodynamics”, but that’s a story for another day.)

The final force on our list  is the so-called “weak” force.  The weak force is so weak it doesn’t hold anything together (that I know of).  But it still operates at the subatomic level.   Its strength would be comparable to the electromagnetic force but its range is even smaller than the strong nuclear force.  It’s range is almost 1000 times shorter.  It’s important though because it is the only force that can change protons into neutrons and other transitions. Without it, the Sun could not fuse 4 hydrogen atoms (4 proton) into helium (2 protons and 2 neutrons) and would not have an energy source.

But these last three, while discovered later, are the best understood.   These three play nicely with the over-arching framework of quantum mechanics.   You may often hear that quantum mechanics is the domain of processes that are very small.  That isn’t really the right story.   Quantum mechanical effects have been seen to operate over distances the size of the Earth as one type of neutrino morphs into another.   And that’s not the limit.   I once co-authored a paper about a possible quantum mechanical effect that takes place over a distance of 1 billion light years.

So it is an over-simplification to say quantum mechanics is the physics of very small distances.   It misses a key feature.  Quantum mechanics actually uncovers a special relationship between distance and momentum.  So it is really when the product of the characteristic distances and momenta are small that quantum mechanics is important.

Just as quantum mechanics reveals a special relationship between distance and momentum, the theory of relativity, which describes gravity tells us about a special relationship about time and space.  And that’s not all.  Quantum mechanics also describes a special relationship between energy and time.   Meanwhile relativity is concerned with how momentum and energy are related.  To summarize…

Relativity and Quantum Mechanics tie together different physics quantities at a deep level.

The two theories are about different relationships. It is when we try to deal with all these relationships at once, that gravity gets into trouble.

One of the biggest dramas in physics of the last 50 years of physics has been to reconcile the relationships of quantum mechanics with the theory of relativity.  And three out of four times we were ultimately successful.  There are good working theories of the quantum mechanical behavior of the electromagnetic, strong and weak forces.  They are tested every day in the laboratory to high precision.

But gravity has not yielded.  Much of the theoretical work in physics works on forming a consistent formulation of quantum mechanics and gravity.  These are the goals of string theory and loop quantum gravity that Leonard and Leslie once argued about.    Other attempts are underway.  So far though, none has passed an experimental test, most don’t even make a distinguishing experimental prediction.

And that, finally, brings us to the science in today’s show.  Sheldon’s seminar is not just about “thermodynamic fluctuations”, an elementary part of our theories of heat and energy.    Rather, it is how gravity might emerge from a process that looks a lot like them.  A new theory arguing this point came out recently from a respected theorist in the Netherlands, Erik Verlinde.   The relevant equations were on tonight’s apartment whiteboards.

(Meanwhile, the whiteboards in Leonard’s lab were about something different…the search for hypothetical particles called axions.   I promise that had nothing to do with Sheldon’s editorializing with what he wrote on the board above it.)

So it is a great embarrassment of physics, that the first force we discovered, gravity, is still the least understood.   But I have hope.  After all, Sheldon is on the job.

S04E05: The Desperation Emanation

October 21, 2010

Bob on Sesame Street taught us to know the people in our neighborhood.

(Song starts at 0:50)

But Sheldon in The Big Bang Theory taught us to know the stars in our neighborhood, too.

Our stellar neighborhood is a bit larger than your own neighborhood.  As we discussed before, the nearest star to our own solar system is Proxima Centauri.   Suppose you lived in a typical suburban house with a 50 foot driveway.  If your driveway were like the distance from the Earth to the Sun, then Proxima Centauri would be about 2500 miles away.   Even survivalists can’t get this far from their neighbors.

When the writers asked me to find the names of the stars, in order of proximity to us, I figured that would be easy.  But it was a case where the internet fails.  Nearly all the lists on the web are in disagreement with each other.   And the writers needed an answer…fast.

Luckily one of my friends at  UCLA, a professor over on the Astronomy floor bailed me out.  He told me about RECONS, the Research Consortium on Nearby Stars.   They maintain a definitive list on the stars in our neighborhood.   (And for the record, Wikipedia had it right.)

These are the stars in your neighborhood. In your neighborhood. In your neighborhood.

So we heard the list from Sheldon. Special thanks to none other than “The Bad Astronomer” for helping out with the pronunciation of the star names.

(Of course the closest star to Sheldon is not Proxima Centauri at all.  It is Sol, our own Sun.   If you were thinking that during Sheldon’s song, good for you!  You may stay after class and clean the erasers.)

What about those crazy names?  These stars were discovered over thousands of years.  Some are visible to the naked eye.  “Sirius”, the brightest of the stars, was named by the Ancient Greeks after their word for scorcher.  Others are named for the constellation they are in.  “Alpha Centari A” is the brightest of the stars making up the constellation Centaurus.   “Epsilon Eridani”, named after the constellation Eridanus and the fifth greek letter, is the fifth brightest star in that constellation.  But closest need not mean the brightest.  Many of these nearby stars were not discovered until modern times and are named after their discoverers: Jérôme Lalande discovered “Lalande 21185” in 1801 and “Ross 154” was only found in 1925.

And to this day, astronomers still are finding nearby stars.  Teegarden’s star was missed until 2003.  It is so close that it moves across our sky faster than almost any other star.   Surveys find nearby stars because over the years their position on  the sky can change slightly, just thousandths of a degree per year.  But Teegarden’s star, a modest little red dwarf,  moved so fast across the sky, it was always overlooked.   It is humbling to think that the 23rd closest star closest to our own solar system was missed until this very decade.   And there may be more…

Some of the stars are close together:  Proxima Centauri and Alpha Centauri are a pair, forming a binary star system.  So are Sirius A and B.   About half the stars closest to us are pair-bonded.   Our star appears to be alone.  Or is it?  Some people have proposed we have a distant and dim partner, called Nemesis.  So-named because when its orbit brings it back close to Earth, its gravity would disrupt the comets and asteroids causing them to rain down on us.  It has been proposed to explain a possible periodicity, about 27 million years, of mass extinctions found by paleontologists.  The periodicity of these extinctions is not universally accepted.  And the explanation of periodic extinctions being induced by a companion star even less so.  Nevertheless, I named the first electronics board I build as a graduate student “Nemesis”.

If there is such a “Nemesis” star orbiting our own, a new survey will find it.  The WISE satellite, the Wide-Field Infrared Survey Explorer (led by my same friend at UCLA) is looking.  Infrared light is redder than the reddest light you can see.  Really hot objects, thousands of degrees, glow in the visible light such as a lightbulb filament or the Sun.  The reason you can see your friends’ faces it that visible light reflects off of their faces to your eyes.  But if you had infrared eyes, your friends, cooler than the Sun but still hot, would glow but in the infrared.   (Compared to absolute zero, all your friends are “hot”.  Compared to the Sun, they are “cool”.  Feel free to compliment them on this.)   So infrared is the go-to color for astronomers to find small, cool, faint stars, that might have been missed by all astronomers until now.

The human body glows with the infrared light due to the heat it generates. Astronomers look for dim, cool stars with infrared telescopes.

Such dim stars could have their own planets orbiting them, and if close enough, could sustain life, maybe even intelligent life.   There may even be one closer than Proxima Centauri.  When I mentioned that to one of the co-creators and writers of the Big Bang Theory- when he was visiting UCLA to give the Physics and Astronomy Department commencement address — he told me, “The Federation may be sooner than we think.”

Update:  Since the time this espisode aired, the measurements of the distances to the Procyon stellar system and 61 Cygni system have changed slightly, so their order according to RECONS is now different than the order in the song.   Thanks to eagle-eared viewer Åingeal S. for asking me why they were “wrong” which allowed me to locate the difference using the internet archive of the RECONS webpage.

S04E04: The Hot Troll Deviation

October 14, 2010

Sometimes you need a secret decoder ring.  We had a few shout-outs to the world of physics and chemistry tonight.

Starting with the very first line of the episode:

KOOTHRAPPALI:   (TO SHELDON) I’m telling you, if xenon emits ultraviolet light, then those dark matter discoveries must be wrong.

And now you are in on the most controversial discussions in physics today.    We’ve discussed here before that about two-thirds of the matter in the galaxy is a dark, unknown substance: the aptly named “dark matter”.   Meanwhile teams of physicists are working hard to be the first to prove dark matter exists, by capturing one of its interactions in a particle detector.  For whoever detects it first, there is no end to the fame.


Sensitive detectors look for dark matter. A dark matter particle may kick a nucleus in the detector leaving behind detectable energy, such as ultraviolet light.


The race is on.   Many detectors are running.    Each is gambling on different techniques.  But what almost all have in common is they are looking for extraordinarily weak and rare events.   So physicists build their detectors from materials with extremely low radioactivity and place them deep under ground to keep them as quiet as possible.   Two of the running experiments have a signal the authors have claimed is consistent with dark matter.  The first is called  “Dama-Libra” (the Italian group who Leonard talked about to his mother in season two) and the other is called CoGeNT (some physicists need their shift key taken away from them.)

But a new type of detector started working recently.  Xenon is a gas in every breath you take, but being a noble gas just goes along for the ride, never interacting in your lungs.  But xenon can be refrigerated to below -162 degrees F where it becomes a liquid.   When a dark matter particle passes through it, it occasionally will give a single xenon atom a small kick.   That small kick causes the xenon’s atomic nucleus to move a short distance through the liquid—producing free electrons, heat, and light.   The highest frequency light your eyes can see is violet.  But energy deposited in xenon produces light with a  color a little bluer than violet, called ultra-violet light.  You can’t see it but particle detectors can.   The xenon detectors is enormous, 100 kilograms, hence its name XENON-100.  But XENON-100 doesn’t see the tell-tale ultra-violet light from dark matter collisions.   Is it because the others’ dark matter discoveries were wrong?  Or is there just not enough ultra-violet light being produced in the liquid xenon?  That’s what Sheldon and Koothrappali are arguing about.  And so. are. the physicists.

But the whiteboards today had nothing to do with this science.  Today’s whiteboards honored a special guest.  Once while talking to a Big Bang Theory writer, he recommended I watch the film Real Genius (1985).   I didn’t know what to expect but put it in my Netflix queue nevertheless.   When I saw it I was blown away….not necessarily by the story or the characters (which were fine), but by the important part: the scientific sets and dialogue.    It turns out that Real Genius had a scientific consultant,  Martin Gundersen, a professor of physics from across town, the University of Southern California (USC).    Now that I know how much goes into getting sets and stories right, I was in awe of what a great job they had done, from the sets to weaving physics right into the plot line.   So I sent Prof. Gundersen a fan letter.   He  responded and eventually was able to visit the set of The Big Bang Theory during the taping of this episode.


Prof. Martin Gundersen, the science consultant for Real Genius (1985). He recognized the whiteboard in Leonard and Sheldon's apartment during the taping of this episode.


So those of you that are whiteboard fans AND have a good memory know what was on the whiteboards.  It was identical to one of the boards used in Real Genius 25 years ago….



Chris Knight (Val Kilmer) steps out of the way so we can see the original whiteboard in Real Genius (1985).


I don’t want to spoil the plot of Real Genius by explaining how excimer lasers work.  It’s only been 25 years and not everybody has had a chance to see it yet.

Finally, we saw Sheldon make a smell of hydrogen sulfide and ammonia gas.  Hydrogen sulfide smells like putrefying eggs.  And ammonia smells like ammonia.   We were careful not to tell how hydrogen sulfide could really be made since it’s been in the news that people have been hurting themselves and others when making it with household chemicals.   We at The Big Bang Theory are nothing if not socially conscious.  So instead I imagined Sheldon made it with something only available around the lab,  an aqueous solution of hydrogen sulfide.   That immediately produces:

(NH4)2S →H2S + 2 NH3

By now I expect you are running out of the room.

S04E03: The Zazzy Substitution

October 7, 2010

In tonight’s episode we heard the names of many physicists who took part in the Manhattan Project, the U.S. program that built the first nuclear bombs.  We were  introduced first to the name of one of the most famous physicists of the twentieth century, the chief physicist in charge of building the so-called “gadgets”, Dr. J. Robert Oppenheimer.


J. Robert Oppenheimer, theoretical physicist and leader of the Manhattan Project

Unlike Sheldon (and many others),  I prefer to say “nuclear” not “atomic”.   “Atomic” tells us nothing special.  All chemical reactions use atoms, and you’d be justified in calling even T.N.T. an atomic bomb.  What is special about nuclear power is that it uses the forces in the nucleus, which are about a million times stronger than the forces holding the rest of the atom together.  It is specifically nuclear reactions, not chemical reactions, that are responsible for the extraordinary power of a nuclear bomb.

Oppenheimer was a theoretical physicist, who was reported to be extraordinary clumsy around laboratory equipment. “Oppie”, as he was called, was a fan of languages and even taught himself Sanskrit.    Those who knew him described him as somewhere between aloof and pretentious.   Either way, he had trouble dealing with people.   His brother Frank, also a physicist, reports him having said:

“I need physics more than friends.”  – J. Robert Oppenheimer

At this point I wonder, does he sounds similar to any of the fictional physicists we know?

But at the same time, Oppenheimer and our fictional hero could not be more different.  Oppenheimer had a driving ambition to be close to the political powers in Washington.  So much so, Oppenheimer even lied and falsely implicated his friend, Haakon Chevalier, as being linked to Communist espionage, ultimately causing  grave damage to his friend’s career, while furthering his own.   Like a Greek tragedy, this misstep ultimately led to Oppenheimer’s own fall from political grace, ultimately even having his security clearance revoked — a stunning blow to the man who had been the scientific leader of what was  perhaps the largest secret military project ever undertaken.

Oppenheimer also had a strong affinity toward Eastern religion, specifically Hinduism.  When the first test atomic bomb was dropped at the Trinity Site on July 16, 1945, he famously recalled pondering several phrases from the Bhagavad-Gita:

If the radiance of a thousand suns were to burst at once into the sky, that would be like the splendor of the mighty one.


Now I am become death, the destroyer of worlds.

I never understood the strange grammar of that second quote, since he was speaking in translation.   Perhaps a Sanskrit-reading reader of this blog could explain below if a similar construction exists in the original.  (Updated: see comments.)

As it happens, I visited the Trinity Site last weekend.   I had given a seminar last Friday nearby, at the National Radio Astronomy Observatory, home of the Very Large Array in Socorro, New Mexico.  (That’s the same array of telescopes Jodie Foster used in the movie Contact.  And yes, she really went there; they still have pictures of her visit on the walls.)  Twice per year, the Trinity is open to the public.  You can combine that with a trip to the VLA.


Your science consultant at the Trinity Site.

After a short drive through the White Sands Missile Range we arrived at the site.  You might worry about the the wisdom of  walking around unprotected where a 20 kiloton nuclear weapon was detonated.  What about the radioactivity?  After the atomic bomb test, the heat of the blast melted the sand and plutonium fallout into a glass, forming a unique  mineral called trinitite.    Small bits of the green glass are underfoot nearly everywhere you walk.


During the nuclear explosion at the Trinity Site, desert sand fused with nuclear fallout to produce a new mineral, trinitite.

For the hour I walked around,  I was exposed to radiation dose of 0.5 “millirem”.   A millirem is one thousandth of a “Roentgen Equivalent Man”, an outdated but well-known unit for measuring radiation exposure.

That may sound scary but 0.5 millirem is very small compared the natural sources of radiation which are everywhere.   The average person in the U.S. receives over a 350 millirem dose every year, mostly from radon.  Even if you try to escape radon,  the potassium-40 in your bones are constantly undergoing radioactive decay.   For my trip to the Trinity site, I received by far most of my dose from the two-hour airplane flight each way from Los Angeles to Albuquerque.  In a commercial jet you are above much of the atmosphere that normally protects you from radiation due to cosmic rays, particles from space striking the earth.   (Extra for experts:  it is not just the dose, but the duration of the dose that matters.  Doses received slowly, over the course of a year, give your DNA more chance to repair itself before possibly forming tumors than if you receive it all at once.)   It takes a 100,000 millirem dose before it starts to have measurable effects on   your blood.  At twice that, you start feeling radiation sickness.

In many other cases radiation is  outright helpful.  X-rays help doctors diagnose broken bones and the positrons emitted in PET scans allow doctors to find cancer.   Gamma-ray and other beams are often used to destroy tumors once they are found.  Biologists use radioactive markers to understand all sorts of processes important to life.   Smoke detectors rely on the decays of americium  to light a phosphor.    Nuclear power reactors provide an enormous supply of electricity while producing essentially no greenhouse gases.

Now disregarding my earlier complaint about “atomic” versus “nuclear”, let us now in all seriousness consult the Doomsday Clock of the Bulletin of the Atomic Scientists:


The Doomsday Clock of the Bulletin of the Atomic Scientists

It is six minutes to midnight, folks.

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