S03E23: The Lunar Excitation

The Moon is being pulled into the Earth.   Both are being pulled into the Sun.

But don’t duck and cover. That’s what orbits are all about.    I find a common misconception among students about why astronauts float around the Space Station.   They sometimes think that this is because the astronauts are out the reach of Earth’s gravity.  That, or because they drink Tang.

Scale drawing shows that the Space Station astronauts are not far enough from Earth to ignore its gravitational pull.

Neither is true.   Orbiting astronauts,  Space Stations and satellites  are being pulled by gravity toward the center of the Earth just as we are.   At the Space Station’s altitude, about 185 miles  above the Earth’s surface, the astronauts experience a pull of gravity that is still about 90% as strong as down here.   The astronauts you see floating still have nearly their entire weight.

The astronauts are falling, but so is the floor of the station.  Just like when you go on the free fall ride at an amusement park you experience weightlessness, not because you have no weight, but because the floor falls away from you at the same rate you are falling.  You weigh the same.  On Earth, the ride can can last only a couple of seconds.  The astronauts and the Space Station fall for years and years.    The astronauts still have 90% of their weight, but you just can’t tell by putting a bathroom scale on the floor, since the floor is falling away.   The astronauts don’t drink Tang any more either.

The reason the astronauts don’t hit the ground is that they are moving fast, over 17,000 miles per hour, at a right angle to the downward direction.    In the absence of any other force, they would move in  a straight line forever,  disappearing from our solar system.  The pull of the Earth changes their direction, not enough to pull them to the ground, but to keep the astronauts and the station in a  circular orbit that takes them around the earth every hour and a half.   (Physicist nitpickers would probably want to comment that the orbit is not perfectly circular, so in the spirit of full disclosure:  the orbits can be ellipses rather than circles.)  It is just as if you were to swing a cat over your head by its tail.  You pull inward but the body of the cat stays at the same distance from you, moving perpendicularly to the direction you are pulling.  When the speed and distance are just right, the astronauts and station stay forever at the same altitude above the Earth.

The same thing happens for the Earth orbiting the Sun.   It is why we haven’t yet fallen into the Sun even though the Earth feels a large gravitational force towards it.  The Moon orbits the Sun too.  If  it didn’t we’d have lost  it by now.

So the Earth and Moon are falling into the Sun.   Like Galileo dropping one-pound and ten-pound objects from the Tower of Pisa we can ask, “Do they fall at the same rate?”  This is where a “lunar ranging” experiment such as performed by the boys comes into play.   They can bounce a laser off of mirrors left by the Apollo astronauts on the Moon.  The mirrors are the corner of a cube and any light ray that hits them bounces of all three mirrors at just the right angles so that it returns from the direction it came.

Retroreflectors left by the Apollo 11 astronauts on the Moon will reflect lasers back in the direction from which they came.

It takes about 2.5 seconds for the light to travel to the Moon and back.  By measuring the exact timing to better than a hundred billionth of a second, these laser lunar ranging experiments have measured the distance to the Moon to better than a millimeter.   Now astrophysicists can check that the Moon is behaving exactly as it should.

The central principle of Einstein’s theory of general relativity is the “equivalence principle”, that objects should fall at the same rate regardless of their mass or chemical composition.   This gives a testable prediction.  The Moon are Earth are significantly different materials and size.  Yet the lunar ranging experiments show the Moon and the Earth fall together towards the Sun.   Actually this is only just the “weak” version of the equivalence principle.   There is more to mass than just the composition of the objects.  Since the energy of assembling the Earth and Moon is so much different, according to Einstein’s  famous m = E/c² , they have a different amount of  this source of mass as well.    Yet still, lunar rangers measure that we and the Moon fall at the same rate.  The best test of this “strong equivalence principle” comes from this lunar ranging.

The lunar ranging experiments are the best tests of  many other aspects of Einstein’s theory of relativity.  In addition their close monitoring of the Moon have told us that it actually has a liquid core.  The lunar ranging experiment is one of the longest running experiments in physics.  In its 35 year history it has marked that not only is the Moon not in danger of actually hitting the Earth, but it is moving away from us at about 1.5 inches per year, due to energy lost as it creates high and low tides for surfers.   In about five hundred million years the Moon will be so far away, there will never again be a total eclipse of the Sun.   So go out and enjoy one while you still can.

Laser ranging to the Moon. (From the Apache Point Observatory Lunar Laser-Ranging Operation). The Moon is overexposed to take the photo.

Last (and certainly least), as Leonard explained to Zack, the presence of reflections from the retro-reflectors  often are used to rebut claims that humans did not really go to the Moon.   Actually Leonard’s argument is specious.   After all, unmanned missions could have left the reflectors, just as the Russians did.    I’m still waiting for the producers to invite me to see NASA’s soundstage on the backlot.

While preparing the set, a few BBT crew members asked a question I never thought of.  Two and a half  seconds later, the apartment, and detecting apparatus has moved since the Earth is rotating.   So why doesn’t the laser spot miss their detector?   The experiment still works because the laser’s spot  spreads out as it travels.  The spot size when it returns to Earth is over 10km, much bigger than a laser pointer, and smaller than the distance Pasadena moves in 2.5 seconds due to the Earth’s rotation.

Tonight was the season three finale.  Thanks to those who followed this blog after each show this year.  Tune in next season, when  ***SPOILER ALERT*** the Moon will be about an inch farther away.

S03E22: The Staircase Implementation

Fans will no doubt complain about a scientific inaccuracy tonight.  The production crew wouldn’t let me bring real rocket fuel for the episode and instead used water. Apparently Warner Brothers has some rule against bringing hydrazine and nitrogen-5 onto their sets.

Hydrazine has a long history in the rocket world as a propellant.  Its first use was for the German rocket-propelled military aircraft, the Messerschmitt Me 163 Komet.  To date, the only such aircraft ever in regular operation.

A hydrazine-fueled aircraft in WWII

When mixing the hydrazine with solvents, the Germans called the fuel “B-Stoff”.   Today hydrazine is used for more peaceful purposes, such as adjusting the orbits of satellites and as auxiliary power for the International Space Station.

The hydrazine reaction was on tonight’s boards so it was a kind of spoiler for those paying attention.

The boards in an early scene show how hydrazine works as a rocket fuel... and foreshadows what happens next.

The concept the writers explained to me was that Leonard’s mistake was that something didn’t scale.   They wanted that what would work for a real rocket, would not scale to the small amount of fuel he brought.   The hydrazine reactions happen faster by exposure to the element iridium.   The word chemists would use, is to say the reaction is “catalyzed” by iridium.  A catalyst accelerates a reaction but is not used up.   This is what the platinum does in a car’s catalytic converter and was the reason for the ‘iridium flask’.

By what is now the season’s third application of  the square-cube law, the full amount of hydrazine would be exposed to a relatively small surface area of iridium.  In Leonard’s small container, a far greater fraction of hydrazine is exposed to iridium, and as Sheldon realizes, becomes highly explosive as shown on the boards above.

We added some “Nitrogen-5”, or pentanitrogen, to sweeten the mixture.  That was a fuel that was being developed in 2003, and would likely have some secret aspects Leonard should not discuss.

Not everything on the boards relates to rocket fuel.  Recall it is 2003.   Drs. Abrikosov, Ginzburg and Leggett had just received the Nobel Prize in physics

for pioneering contributions to the theory of superconductors and superfluids.

Their theoretical work is laid out on the boards as well, as something Sheldon would have been thinking about.

The whiteboards star the show once again.   I don’t know why the director keeps letting the actors walk around and upstage them.

S03E21: The Plimpton Situation

In the music world, the death of a star is  precipitated by sex, drugs and rock-n-roll.  In the Universe, the death of a star is precipitated by extinguishing the nuclear fusion reactions in its stellar furnace.  The end result is often one of the most fascinating objects in the universe,  a pulsating neutron star,  “pulsar” for short.

Shortly after discovering pulsars (while still a graduate student in 1967) Jocelyn Bell was told "Miss Bell, you have made the greatest astronomical discovery of the twentieth century".

Neutron stars hold the key to what Dr. Elizabeth Plimpton had written on her hand in this episode.

Dr. Plimpton has the coordinates to what?

The life of a star is a constant tug-of-war.  The force of gravity never ceases pulling all the star’s material inward, attempting to make it smaller and denser.  But a denser star would accelerate the nuclear fusion reactions, raising the star’s temperature.  Like heating a pan of jiffy-pop, the heat causes an outward pressure, trying to making the star larger.   For much of a star’s life the forces strike a balance and stars such as our own stay pretty much the same size for billions of years.

But gravity always wins.  Fo now,our own Sun shines largely by turning hydrogen into helium.  By making a more tightly bound nucleus in this reaction, energy is left over and produces the light and heat of the Sun.   About 5 billion years from now, after its hydrogen is used up, our Sun will turn to alternative energy, fusing  helium into carbon and oxygen, becoming much larger and growing briefly into a Red Giant star (while engulfing Mercury and Venus, and incinerating the Earth) in the process.   But that’s all folks.   Our  Sun will finally run out of energy, and puff off all the excess material.  The Sun’s now naked core will quietly cool.   Gravity will pull it tighter and tighter until the electrons in the Sun resist being pushed any closer together.  The remaining ember of carbon and oxygen is incredibly dense.  A white dwarf  with the mass of the Sun will be the size of only the Earth.  It really isn’t even much of  star any more since it is no longer producing its own energy.  It only glows by  radiating the energy from its former life, a cosmic Zsa Zsa Gabor.

But a star more massive than our Sun face a different fate.    A larger star does not go quietly but often blows off material in a dramatic supernova explosion followed by a gravitational collapse of the remaining core.  The inward pressure due to gravity is so great that the electrons that hold up a White Dwarf are “pushed into” the remaining protons to form neutrons.   The stellar material now moves even further inward under the force of gravity to make an object as dense as an atomic nucleus.  Since it is mostly made of neutrons, it is called a neutron star.   A neutron star 3 times the mass of Sun is so small and dense, it is smaller than Los Angeles.

A typical neutron star is smaller than Los Angeles and more massive than the Sun.

It is so dense, that one teaspoonful of neutron star material here on Earth would weigh as much as a mountain.   As Dr. Plimpton says in the episode, if you went even close to its surface you’d be crushed by its strong gravity.   That is if not first ripped limb-from-limb first by the differences in its strong force of gravity on different sides of your body.

But a neutron star’s useful life is far from over.  While a graduate student Jocelyn Bell and her thesis advisor Anthony Hewish discovered regular bursts, from seconds to fractions of a second, of radio static from specific points in the galaxy.  These turned out to be the fast spinning remnant neutron stars.  Just as when a slowly rotating Olympic skater pulls his or her arms inward to speed up, the small neutron star remnant of a star that probably rotated about once per few weeks, now  rotates every few seconds or even faster.   When the poles of the neutron star point at us on Earth we see a burst of radio and other light.   Just as the spinning lamp in a lighthouse produces a flash of light to those at sea, we on Earth see a bursts of energy from the pulsar as it rotates.  For this and her career’s work, Dr. Bell-Burnell was awarded the highest rank an British citizen can attain, Dame of the Britsh Empire.

And just as the spinning lamps in a lighthouse  produce a regular flash of light for ships, cosmic voyagers would see these pulsars, spinning neutron stars,  as regular and bright beacons from afar.   The Pioneer spacecrafts launched in the 1970s are now leaving the solar system.  With them they take our calling card on a gold-plated plaque.    To instruct whoever or whatever discovers them how to find us, we show them the Earth relative to pulsars, cosmic beacons that will be visible throughout our portion of the Milky Way.

The plaques carried on the Pioneer spacecrafts out of our solar system shows our location relative to 14 neutron stars (pulsars)

I’m told Carl Sagan caught hell for putting naked pictures in space.  So for the subsequent Voyager spacecrafts, now the farthest spaceprobe from Earth, we instead  sent “the golden record”…

The "Golden Record" (click to hear) riding on the Voyager spacecrafts includes "The Sounds of Earth". The cover still includes the location of Earth relative to nearby pulsars...but without the naughty pictures.

…which sends our regards with more puritanical messages.  The record is our ultimate mix tape to our alien friends.   If we humans were to launch such a space probe now, I fear it would only have a golden MP3.  Voyager 1 has left the Solar System and is about 110 times further than the Earth is from the Sun and is our most distant space probe.  It will leave our solar system around 2015 and carry out message into interstellar space.

But as Steven Hawking points out, this might not have been such a good idea.  Just ask the ancient Aztecs how much they benefited from the visits of the Spanish explorers.   Even if we are visited by a species that is not violent, visitors  may inadvertently bring microbes we’ve never been exposed to before that wipe us out.    This may even be an inevitable by-product of all such contacts.   If you haven’t read it, I highly recommend the masterpiece Guns Germs & Steel written by Jared Diamond (also at UCLA) who gives a scientific basis to the unfolding of such historical events.   But the sword cuts both ways…  In H.G. Wells’s War of the Worlds, it is our own microbes that ultimately killed the Martian visitors.

So it was the location of a new pulsar, a new neutron star, that Elizabeth Plimpton had written on her hand for tonight’s episode.  A brand-new one  (a “Soft Gamma-ray Repeater” at RA 4h40m, Dec 55⁰35′, which is effectively its longitude and latitude on the sky) was announced on the  The Astronomer’s Telegram before the episode was taped.   In another easter-egg to the High-Def enabled,  note the shout-out to Brian Greene’s excellent popular science book, in the title of Dr. Plimpton’s book:  The Effervescent Universe.

No matter how much we hide, our radio and television transmissions are already giving us away, at the speed of light.   Even tonight’s episode is already beyond Mars.   The extra-terrestrials can easily find us, and may already be on their way.

S03E20: The Spaghetti Catalyst

The weather tonight (May 3, 2010)  is a cool 10,000 degrees Fahrenheit.  In some regions temperatures may increase over the next few days to over 15,0000,000 degrees.    Wind-speeds will be in excess of one million miles per hour.  That’s the actual forecast.  If you live on the surface of the Sun.

Keeping the science correct  for an episode typically involves just a little research and maybe a few notes on a napkin.  But  tonight’s episode involved serious prognostication and luck.    Tonight Raj says there are no solar flares.  All I needed to know, back in early March when the writers sent me that line, was that Raj would be right, i.e. that tonight, when the episode would air, that there would be no particles arriving at Earth produced by solar flares.

(Click to activate.) Image of the Sun with a solar flare by NASA's SOHO satellite with ultraviolet light

So I’ve been anxiously reading the space weather every day for the last few weeks and hoping for the best:

April 15:

“Solar activity is very low. No significant flare events are expected.”

Whew.  April 16, the same!  And so on every day April 17 and onward.   Right down for the rest of the month:

“Solar activity is very low. No significant flare events are expected.”

But then, with a small addition on April 30:

“There is a small active region in the NE disk.”

Uh oh.  Then I saw this for May 1:

Two C-class flares were observed yesterday….Unnumbered region, N24 E68 (X=-800,Y= 400). Alpha region. C-class flare possible. Position approximate.

My luck had run out.

What am I worrying about?  The weather report at the top of this entry really does describe the active Sun.    Lines of magnetic field on the Sun can burst open releasing enormous amounts of energy, comparable to millions of atomic bombs within minutes.  Light of all types reach Earth: from radio waves to gamma-rays.  ( My undergraduate senior thesis work was a cyclotron experiment to predict the rate of gamma from solar flares.)   Worse still, trapped charged particles such as electrons protons and atomic nuclei, previously tied to the Sun by magnetic fields, pour out of the opening and some make their way to Earth.   It is as if the Sun produced a giant fart.

X-ray image of the Sun from the Japanese satellite Yohkoh. Unlike X-ray pictures from your dentist, this image is not made by absorbing X-rays. Rather million-degree hot gas in solar flares directly emits the pattern of X-rays that are photographed.

Yohkoh webpage

Ejected particles arrive at Earth a day or so later and can wreak havoc.  Usually we are protected from charged particle radiation by the Earth’s magnetic field which deflects them.  But these can stream into openings in our own Earth’s magnetic field near the poles, releasing bursts of energy that at their worst can disrupt all sorts of radio communications and the power grid.   The largest storms can eject matter and magnetic fields together.  Those magnetic fields can cancel that at the Earth and create an opening for the energetic, and damaging, ionizing particles deep into the Earth’s atmosphere, even reaching the ground.

One among many famous solar events, on Halloween 2003, such a solar storm interrupted satellite communications and even destroyed a research satellite.  Astronauts on the Space Station hid as best they could deep in the station but still saw flashing lights in their closed eyes, due to cosmic rays crossing and ionizing their eyeballs’ vitreous fluid.  In March 1989, a solar storm disrupted power transmission and knocked out power to 6 million Québécois.  In 1958,  radio communication from  the U.S.  to Europe was cut off.   But perhaps the most tragic event was the complete interruption of a critical four minutes of the 1941 radio broadcast of the  baseball playoffs of the Pittsburgh Pirates against the Brooklyn Dodgers.

On a positive note, these charged particles produce beautiful displays of light as they ionize the air in the upper atmosphere creating Auroras, curtains of shimmering light typically only seen near the polar regions, but on the night of Feb 11, 1958 could be seen as far south as Los Angeles.

Aurora ("Northern Lights") produced by charged particles from the Sun hitting the Earth.

(From Astronomy Picture of the Day)

Radiation damage caused by the particles in storms is one of the major hazards to astronauts in space.  Personally I worry more for them about memory leaks from C++ programs.

Raj’s report of no solar flares had been a pretty good bet.  Astrophysicists worldwide have marveled at the lack of solar activity over the last few years.  NASA reports the Sun has not been this quiet in nearly a century.  Fear not though.  The solar activity follows a reliable 11 year cycle.  Every 11 years the orientation of the Sun’s magnetic field changes direction completely.  During the flip, the magnetic field becomes unstable and the number of solar storms increases dramatically.  In 2012 and 2013 solar astrophysicists predict a large number of storms that will disrupt GPS service and perhaps even broadcast of the 2012 Olympics.  Dish Network subscribers may as well order seasons 5 and 6 of The Big Bang Theory (TBBT) on DVD now.

Time to check the latest report for solar activity on May 2, which would let us know if Raj will be  right tonight:

An unstable nest of magnetic fields emerged over the sun’s northeastern horizon yesterday, and it is crackling with C-class solar flares.

Image taken on May 1 this year of the current active region on the Sun that I am worrying about.

Image by P. Lawrence at Spaceweather.com

Sounds grim. But C-class are among the weakest solar flares we have.  They are fun for amateur astronomers to look at, but shouldn’t disrupt Sheldon’s GPS nor, more importantly, your reception of tonight’s episode of TBBT.