Science Explanations

Ths blog has moved!  Click here to see the whole post.

OK, I’ve been posting everybody else’s YouTube videos, so what about METube?  After all, it’s all about me.

Here is my YouTube debut, talking about infrared light as part of a full-length webcast on climate change.  This was totally fun, I left Paul D. back at the webcast studio and ran off-stage, across the museum to the infrared heat camera exhibit…

Click here to see the whole post.

tt_icon_170In keeping with my previous post on the International Year of Astronomy, this week’s 5-minute  Science Teaching Tips podcast is about our perception and the size of the moon.  What coin would just barely cover the full moon? You may be surprised. TI director (and recovering astrophysicist) Linda Shore explains how our brains distort the actual size of the moon. Listen to the full podcast — When the Moon Hits Your Eye.

I’m cross-posting this from a fun little discussion we just had over at Morning Coffee Physics. (Perhaps ironically, all my posts took place in the wee hours of the night, sans coffee). Jasper wrote a really neat little post about why snow sparkles and I asked him if he knows why snow crunches underfoot when it’s cold.  It’s been really cold (really cold, whimpers this recent transplant from San Francisco) in Boulder lately and my tires and feet have been making cacophonous sounds in the snow.  I always had this sense that maybe it was because of snow crystals rubbing together.

Jasper wrote:

I did a bit of googling. From the few (sketchy) sources I saw, it looks like the crunch sound comes from the sudden release of air from the air pockets in a pile of snow. That I can believe, however, one explanation includes the following:

“When you walk on snow, your boots apply pressure. If the snow is warmer than about 14 degrees F (-10 degrees C), the pressure partly melts the snow, which “flows” under your boot instead of breaking. If the snow is colder, it does not melt, and your boot crushes those innocent ice crystals, accounting for that plaintive scrunching sound.”

As elegant as that explanation sounds, I suspect it won’t really add up… (literally even). It sounds a lot like the physicist’s myth of ice skating being explained by a similar process (Pressure from skate -> melting ice -> sliding). In one of my classes we did this calculation and it turned out that the freezing point of ice under a skate would only change by about 1 degree maximum. I suspect something similar for the preceding claim about crunching snow.

Going on physics intuition alone… I’d probably say the temperature dependence of the squeakyness of snow has more to do with the temperature dependence of the structure of the snowflakes. Maybe the shapes that snow crystals take on at low temperatures are better at making noisy air pockets… * shrug *

I shared his skepticism of the online explanation that he found.   It seems implausible that crushing would create that sound, but maybe my experience misleads me. It just seems like most of the sound is coming from the sides of my shoes in the snow, creating friction, rather than from my shoe coming down.  If I step straight down, rather than grinding my foot sideways into the snow, it is quieter.  But I also don’t think that a “different shaped snow crystal” explanation works for me, since the snow has already fallen to the ground and thus its crystal shape is already determined.  Once it’s on the ground, it crunches when you walk on it if it’s really cold, and doesn’t if it’s not.

After I wrote all that, I found a good link that seems to support what I just wrote (don’t you love it when that happens), and also incorporates the idea of different shaped snow crystals, but not in a temperature dependent way.

There are two — no, actually three — physical factors affecting the crunching / noncrunching of trodden snow. The mechanism behind all three is the same — lubrication, good or bad. When snow does NOT crunch, then the grains / crystals in the snow are well lubricated. When snow DOES crunch, then lubrication is poor. The lubricant is of course water in all cases, coming from two sources, both of which are temperature-dependent:

(1) Ice crystals are always surrounded by a very thin layer of water (a phenomenon already observed by Michael Faraday). The thickness of this layer varies with temperature, ranging from a one molecule thick layer at about -10 oC, to hundreds of monomolecular layers at -1 oC.

(2) Pressure lowers the melting point of water. If you step on snow, then the crystals are pressed against each other. The ice at the contact points may melt and create a thin lubricating layer of water. Unfortunately, the pressure from the soles of your shoes is far to small to melt snow at any temperature, so this factor, interesting as it my seem in itself, is rather irrelevant in this connection.

(3) The third factor is the shape of the ice / snow crystals: crystals with a greater number of pointed edges crunches more readily. An extremely pointed structure of the snow crystals can sometimes offset the other factors, making snow crunch even when it is warmer than -10 oC.
It is difficult to say how these phenomena interact in order to lubricate (or not lubricate) the snow crystals, but in any case something seems to be happening at around -10 oC, enough to make a sharply noticeable difference: if it is colder than about -10 oC, then snow crunches, if it is warmer, then it usually doesn’t.

Ten-degree rule of thumb

These factors, taken together, determine the precise temperature at which snow starts crunching. But the -10 oC rule is a surprisingly good rule of thumb, if you want to predict whether or not you will experience the nice crunching sound of snow when you take a walk at Christmastime.

But if anyone knows something more, please let us know!

Yup, it’s time for those “top 10” lists for 2008.  I don’t generally post other peoples’ lists here, but heck, this is one area where I know that I haven’t been paying close enough attention to know what’s important.  So here is an edited version of the Physics Findings for 2008 from Physics News.  Phil Schewe does such a great job with these, they’re a delight to read.  You can read the whole thing at Physics News Update (and subscribe to their e-newsletter).


The following list was chosen by editors and science
writers at the American Institute of Physics and the American Physical
Society.  It winnows a wealth of discoveries into the following ten
topic areas, which are listed in no particular order.


What’s new-discovery of an unusual class of materials made from iron
and arsenic.   Superconductors don’t lose any energy when electricity
runs through them, providing they’re chilled to very low temperatures.
Superconductors are used in specialty applications where high
electrical currents are needed, such as in MRI scanners at hospitals or
in the magnets used to steer particles at atom smashers.  …

The new iron-arsenic materials are the first relatively
high-temperature materials that remain superconducting above a
temperature of 50 K that don’t contain copper; the copper materials are
brittle.  Researchers hope that the iron-arsenic version might lead to
the more practical manufacture of superconducting wire.   Furthermore,
having a new class of materials to study should help theorists
understand how high-temperature superconductors work in the first
Background: A summary of work in this area can be found at Physics
Today, May 2008
; APS survey of topic.



What’s new—the LHC, the world’s largest scientific instrument,
started operations in September.  At this huge particle accelerator,
located underground near Geneva, Switzerland, two beams of protons, each
traveling at unprecedented speeds will be smashed together.  The goal is
to create exotic new particles that can’t be observed in any other way
except in the tiny fireball created by such violent collisions.  ….

Problems with some of the apparatus forced a premature shutdown
…  General operations should resume in summer 2009.
Background: a summary of the magnet malfunction which brought testing to
a halt in September and a timetable for operations are available here.


What’s new-planets orbiting distant stars have been imaged directly, and a host of interesting results have come back from spacecraft hovering near the planets in our own solar system.  Extrasolar planets, planets orbiting far-away stars, had been detected indirectly by watching what happens to the light coming from the star.  But now the glare of the star has been blocked sufficiently that the extrasolar planet itself could be imaged.  The Gemini, Keck, and Hubble telescopes provided pictures. Background summary here.

In our own solar system, at Mercury, the Messenger spacecraft  made  first-ever maps of large portions of the surface. At Saturn, the Cassini  craft found geysers near the south end of the moon Enceladus.    At Mars, measurements made by several craft strengthened evidence in favor of sub-surface glaciers outside the polar regions. Meanwhile, the Venus Express craft recorded pictures at several wavelengths, facilitating, among other things, a better knowledge of clouds on Venus.


What’s new-unusual combinations of quarks were observed for the first time.  Physicists believe that an atom consists of one or more electrons orbiting a central nucleus.  The nucleus, in turn, is made of protons and neutrons, and these particles are made of something still more elementary-quarks held together by gluons.  … One discovery consists of the sighting of nuclear particles containing rare “bottom” quarks.  Background here.

[See the full article at Physics News Update for more on these experiments   -geekgirl]


What’s new-seeing a flash of light from 7 billion light years away.
One of the brightest of all celestial objects is gamma-ray bursters,
objects that emit immense amounts of gamma radiation, the highest-energy
form of light.  The brightest-ever gamma ray burster was observed by the
Swift satellite.   Since looking out into space is equivalent
to looking back in time, this flash would have been coming from a moment
when the universe was only half its present age.  Publication in Nature.


What’s new-first ever accumulation of molecules in large numbers and
at a temperature near absolute zero.  Using lasers to slow a gas of
particles down to near stillness is by now a standard method for
measuring the subtle properties of atoms.  Steven Chu, nominated to be
the Secretary of Energy, won a Nobel Prize for pioneering this subject.
Cooling molecules in this same way is difficult since molecules, made of
two or more atoms, have complicated internal motions.  But this year
several labs succeeded in first cooling atoms and then, at a temperature
close to absolute zero, getting them to combine into molecules. …
Background at; figure; PRL text and overview at


What’s new-getting little imperfections in diamond to tell us about
how atoms behave like tiny magnets.  Diamond is made of a cross-linking of carbon atoms.  If one
carbon atom is missing from this network, the empty hole, in combination
with a stray nitrogen atom, acts as a sort of strange molecule in the
middle of all those carbon atoms.  This “molecule” can light up like a
little LED when you shine laser light in.  This in turn, can be used to
measure extremely weak magnetism.  Possible applications include data
storage for computers or high-sensitivity detectors. … See news summary at


What’s new-experiments settle one mystery and uncover others.  Cosmic
rays are super-high-energy particles whizzing through the cosmos.  When
they smash into our atmosphere the rays turn out mostly to be ordinary
particles, such as protons or electrons, but with energies thousands or
millions of times higher than particles speeded up at accelerators on
Earth. [See full Physics News Update article for new results — there are many!  -geekgirl]


What’s new—getting light to behave in a new way. When light strikes
an opaque material like milk most of the radiation is scattered; little
of it passes through the sample.  But in an experiment at the University
of Twente in the Netherlands, much more of the light can be made to
traverse the scattering material if beforehand the wavefront of the
incoming light is shaped by special filters. Background summary.


What’s new—Scientists at the AURIGA lab in Padova, Italy have cooled
a one-ton aluminum bar to a temperature below 1 milli-kelvin using
special electrical circuits.  The bar is part of a detector designed to
measure passing gravity waves from space.  Using sensitive magnetic
sensors and feedback coils, the ringing of the bar (which is essentially
a large tuning fork) at one characteristic frequency was cooled from an
equivalent temperature of 4 K (the temperature of the bath of liquid
helium in which the bar sits) to a temperature of about 0.17 mK.  Lower
temperatures than this have been achieved with this feedback cooling
technique but only with much smaller masses.  Background: essay and PRL
article at

Phillip F. Schewe

Here is a question posted to a teachers’ listserv:

In discussing phases of matter, one of my students inquired about plasmas. We briefly discussed the ionized gasses and I told him that plasma TV’s actually contain such gasses. He knew that the temperatures of plasmas is very high and we both wondered if the actual temp. inside a plasma TV is on the order of 1000’s degrees Celsius. He actually wanted to know if he would burn his hand if her were to punch the screen. I told him that even if he did not get burned he would get in a lot of trouble with his dad were he to do so.

Here was the detailed response from another teacher —

While the temperature of the plasma in a neon light, fluorescent lamp, or plasma screen TV may be in the thousands of degrees, the low gas density in these evacuated tubes and screens means that the actual amount of heat energy is very slight.  You won’t burn your hand.

Tesla’s invention of neon/gas discharge illumination was an answer to the inefficient hot filament lamp of Edison.  The Edison lamp heated a resistance element up until it radiated visible light but most energy went into heat.  Tesla figured that if you could excite gas at a low pressure you could achieve radiation without the unwanted heat.  The high temperature of the gas is the result of the small electric current going through the gas but the radiation is the ionization resulting as atoms gain then lose electrons.  So the actual heat energy content in the rarefied gas is quite low while the temperature extremely high.  A cup of hot coffee has more heat energy than a small neon sign having a gas temperature of 2,000ºC.  The glass walls of the sign will melt at a temperature less than that but the low density of the gas has not enough heat to warm the glass which conducts the heat away quickly.  The gas near the tube walls ceases to glow and is much cooler.

A plasma TV does use phosphors for the color but the discharge is through a gas so the electrons hitting the phosphors will give illumination.  You can put your hand on the screen and it will not even be unpleasantly warm.  One neat trick is to put a neodymium magnet near a plasma screen and watch the disruption of the electron flow to the phosphors.

Florissant lamps used mercury and argon as the conducting plasma to create UV which makes the phosphors glow on the inside of the tubes.

And another wrote:

I thought Kevin’s question about plasma TVs was interesting, and it just so happens that I’m dating a TV (and all things electronic) repair man, so I forwarded Kevin’s post to him. Here’s his reply:

“Plasmas are created when enough energy is applied to a gas, ionizing it. This energy doesn’t have to be direct heat. A plasma television is made up of over 2 million little boxes called cells. Each cell (pixel) is lined with phosphor, similar to a conventional CRT, and filled with a mixture of gasses (typically neon & xenon). When excited with a high enough voltage (not heated) the gases ionize (electrons jump orbit), emitting ultraviolet light. The UV light strikes the phosphor and causes it to glow its characteristic color (either red, blue, or green). This action is similar to a fluorescent light where the ionized mercury gas emits UV light striking the phosphor lined glass tube. Most of the energy is converted into light, not “heat” (infrared) so to speak. All this is mounted to a massive heat sink which quickly wicks away any heat (conduction).

Television repair man

A teacher on a teachers’ listserv asked some fine questions about the nature of light. Here are her questions, and my answers.

1) If light is energy that is emitted by accelerating electric charges – often electrons in atoms – how do teachers explain the fact that light moves through a vacuum?

I’m not sure how teachers explain it, but the accelerating electric charges *emit* light. The light then self propagates. Sort of like how a ballplayers arm throws a ball, but once moving, the ball no longer needs the thrower’s arm to keep moving after the initial toss. Except in the case of light, the ball (photon) keeps itself going, and doesn’t stop moving.

And an additional comment from Paul Doherty:

Take two balloons, hang them from the ceiling of the room with strings so they are about waist high and touching each other. Then, rub them with wool.

They move apart repelling each other.

We say that each balloon creates an electric field that exerts a repulsive force on the other balloon which has the same electric charge. The electric field is a straight line between the centers of the balloons.

If you put the balloons in a vacuum they will still repel. The electric field has no problem going through a vacuum.

Move one balloon and the other will move in response. By moving one balloon you change the electric field direction.

The change in the electric field actually propagates down the electric field line as a wave.

To move the balloon to the side you must accelerate it to the side. And this acceleration makes a kink in the electric field line that propagates along the electric field line and is the electric part of an electromagnetic wave.

I find that kids have an easier time with this model, for some reason they accept the existence of elecric fields in a vacuum better than the existence of electromagnetic waves in a vacuum.

And another teacher weighs in with another way to teach this, with a nice advertisement for the simulations created by our education group here at U. Colorado:

The PhET group at U. Colorado has a neat applet that demonstrates this idea very well.

In the applet, you can grab an electron in an antenna and wiggle it up and down. The screen displays a line of force and the the resulting electric field. The behavior of another electron in an another antenna is displayed on the other side of the screen. It is easy to see how the two electrons interact with each other.

If you have trouble moving the electron in the first antenna smoothly, you can set the applet so that it oscillates the electron. It is really easy to see how the other electron’s motion is effected by the first electron. The applet is in Java, so you will need to have it installed on your computer, but you probably already have it.

As an aside, I can’t say enough about the PhET collection of applets. They are really cool and my students find them very helpful

The teacher’s questions continue:

2) What propagates the light/electromagnetic radiation (photons) from the sun to earth through space?

The simple answer — nothing. That is, nothing outside of the electromagnetic wave propagates it, it propagates itself. It does this by electromagnetic induction. Say you shake electric charges, as you mention above. That creates an electric wave (which is an electric field that changes over distance). But what does a changing electric field make? A changing magnetic field (by electromagnetic induction). And what does a changing magnetic field make? A changing electric field. So, the electric and magnetic fields swap energy between each other, as one grows the other diminishes. It’s like the electric wave throws energy to the magnetic wave, which then throws it back, as the two of them run forward. I picture it like two people running and throwing a ball back and forth, but that is an incomplete analogy. They keep each other going. The energy doesn’t diminish so it keeps going.

It can do this even in a vacuum, since nothing is “shaking” — it’s just an electric and magnetic field feeding each other.

3) Light moving through atoms is easier to grasp then vibrating electric charges self propagating … anyway do photon’s self propogate and do the photons or vibrating electric charges move sort of up & down and forward?

Vibrating electric charges creating the electric wave can move up and down (or some other more complicated movement). That creates the electromagnetic wave, which is just photons. The photons do self propagate (since “photons” is just another way of saying “moving electromagnetic wave”). The photons themselves don’t move up and down. Rather, the magnitude of the electric and magnetic fields increase and decrease as the wave moves along. (How rapidly they increase/decrease gives us the color of light, and of course it always moves at speed c).

Another teacher asked:

4) I don’t understand light, photons, light’s momentum, and the bending
of light. Are photons “real”? Do photons have measurable mass when
they are moving? (You told me once that photons have no rest mass).

And Paul Doherty answered:

Photons are “Real” in the sense that they do carry measurable energy and momentum from one place to another.

Mass in relativity is a tricky concept and photons are relativistic.

Do you want inertial mass? When an atom emits a photon the atom does recoil.

The photon has momentum, the Mercury spacecraft Mariner 10 lost its fuel due to a stuck valve, scientists used the force exerted by solar photons bouncing off and being absorbed by the solar panels to propel the spacecraft and change its orbit, so indeed photons have momentum.

Do you want gravitational mass, the photon does fall under gravity, and photons do exert gravity. When a photon goes straight up against gravity it loses energy and so shifts its wavelength to the red, when it goes straight down in a gravity field it blue shifts.

Relativistic mass, E = mc^2 photons have energy, step out in the sunlight and feel the energy in the photons, so m = E/c^2

But there is a definition in relativity of a type of mass called rest mass, electrons have it 9 x 10 ^-31 kg. It is the mass when the electron is at rest. Photons in a vacuum always move at the speed of light, they are never at rest so what could the rest mass mean? We get around that by defining it as 0. Only objects with zero rest mass travel at the speed of light.

Newton predicted light would fall under gravity, Einstein did too, but Einsteins prediction was just twice Newton’s. During solar eclipses the bending of starlight has been measured and confirms Einstein’s prediction.

And here’s a very nice post from Built on Facts about the fact that light can push stuff around (ie., it has momentum), which is how solar sails work.