Negative Temperature

I'm not entirely sure how to feel about this one.  You see, in a recent issue of Science magazine, a group of physicists have claimed to have achieved negative temperatures.  That is to say, a temperature of below absolute zero.

I'll back up a step and review, so you can understand why I can't help but approach this idea with some trepidation.  You see, temperature is basically a measurement of the amount of energy in a system.  At the level of particles, that basically translates to movement-- both positional and vibrational.  The lower the temperature, the less energy in the system, and the less movement.  At absolute zero-- about 273 degrees below zero centigrade-- there is no more movement.  That's why it's absolute-- you can't have less than no movement.  So how could these scientists have achieved such a feat?

As far as I can tell, by redefining "temperature."  Although what I gave is the simple, basic definition of temperature, at some point physicists decided that it would be more useful to refine this definition in a variety of ways.  In particular, in terms of the average energy of the system rather than the specific energy at any given instance.  For the most part, this doesn't make any difference-- the average energy still can't go below zero.

However, with enough control over the particles, you can create a situation where instead of having a normal distribution of particles (that is, a bunch of particles moving at close to the average and a few zipping around as outliers), you can flip that, creating a situation where almost all of the particles become "outliers", being held artificially in extreme states.  This allows you to create a pattern of behavior for the particles that is opposite to how they behave at "positive" temperature.  And if it's opposite to the positive temperature, why, then it must be negative temperature, right?  Right?

I'm not denying that it's an impressive achievement, but I really feel like there's an aspect of... semantics?  Philosophy, perhaps?  Lurking in the shadows.  On the one hand, if you can look at a system and say, "Potassium atoms in a particular arrangement at positive temperature behave in a certain way.  At zero temperature, they have these other characteristics.  In this new third situation, they have a set of characteristics that are not described by either previous category, and that in fact look completely backwards from the atoms at positive temperature, so it makes sense to call it negative temperature."  Classic "If it looks like a duck" territory.  On the other hand, the fact that your highly technical definition makes it possible to achieve a situation that was previously considered a theoretical impossibility makes me wonder if, just perhaps... you need to rethink your definition instead.

Laser Class

No, not where you sign up to learn how to emit coherently.

Commercial lasers are categorized into 4 basic classes (with some further subdivisions), on the basis of wavelength and power (directly related concepts in EM radiation).  I'm not going to give the actual power ratings, though, because they can apparently vary depending on the construction and type of the laser.  Most classification is done instead by "accessible emission limits," a formula depending on power, exposure time, and distance from source.

Class 1 lasers are the least dangerous-- there's still no good reason to look into them, but they're unlikely to do any real damage if you do.   Class 1M lasers are safe for all use except under magnification, which tends to increase the power over the safety limit.

Class 2 lasers are more powerful but still basically safe, since the eye under normal conditions will blink to protect itself if you accidentally look into the beam, and this will cut off the exposure.  Deliberately staring into it will cause damage.  Laser pointers are often Class 2 lasers.  Class 2M is the next half-step up-- the same rules apply, except (like 1M) in the case of magnification.

Class 3R lasers are dangerous if you look directly into them, but safe if the beam is at all diffused (by reflection off of a matte surface, for example).  Class 3B is slightly higher again, and lasers of this class require protective eye wear when using, and are equipped with key switches and safety locks.

Class 4 is the most powerful class of lasers-- the beam can not only cause damage to the unprotected eye, even indirectly, but it's also powerful enough to burn skin, or light flammable materials.  Most lasers of this class have industrial, medical, or military applications.

Class 5 lasers don't exists, but if they did, the one from Real Genius would probably qualify.

Polar Orientation

This is one for the "duh" file-- those things that are more or less obvious in retrospect, but unknown because you never have any reason to stop and think about them.

To start with, let's have a quick refresher on magnets.  All magnets have two poles, conventionally called "north" and "south."  Opposite poles attract, and like poles repel, so the north pole of one magnet will push away the north pole of another, or be instead pulled toward its south pole.

Now we shift gears, and refresh our geology a bit.  The earth has a magnetic field, generated [oversimplification] by its spinning molten iron core.  The North Magnetic Pole is in the north, although NOT at the geographic north pole.  It's actually in Canada, up north of Baffin Bay, and currently wandering in a vaguely Russian direction.  See above re: molten core; fluctuations and eddies and whatnot cause movement etc etc and that's beside the point.

Now put them together.  Look at a compass.  The needle is a magnet, with a north and south pole.  The north end points north-- that's the whole point of the compass.  But wait-- the north end of a magnet is attracted to south poles of other magnets.  Which means, logically, that the magnetic pole in the north is actually a south pole.  The North Magnetic Pole is a south magnetic pole.

I thought that was rather cool, actually.