#### A confession from a former “keV” junkie (2. Meet Ms. Electron)

- So, there is a state of matter other than solid, liquid and gas?
= Of course, are you thinking what I am thinking?
- ….
= Yes, it’s time for a jello-shot.
- ….

We cannot deny the arbitrary nature of units we use, but there is also a useful feature: a linkability to other arbitrary units.

Going back to vlk’s comment, it is “statistically probable” (and so natural) to see 1 keV photons in T= 10^7 K Thermal Bremsstrahlung (TB) spectra, actually more often than you might expect. But it’s not so in, say, T=300 K spectra.  Then, wouldn’t it be too strange to call 10^7 Kelvin degrees 1 keV?  In other words, T=1 keV is a temperature that describes an environment where you expect 1 keV photons (X-rays).

Hmm, confusing, but interesting.  Ok, then since T=10^10K ~ 1 MeV, it means we should expect 1 MeV photons (Gamma-ray) at T=10^10K?  Then ~1 eV photons (almost UV) from T=10^4 K?  I know X-rays are dangerous, UV is good for suntan, and Gamma-rays will make me stronger? You know the incredible hulk.  Yeah, what’s a big deal? What’s this got to do with anything or the state of matter for that matter?

But wait, I can get a suntan from the sun light, so Sun must be T~10^4 K? There must be some connection with UV light and sun’s temperature?

Please, meet Ms. electron. She dreams finding her life time partner Mr. position one day, and having a pair of beautiful photons.  She only weighs 511 keV. How will she know she find the right one when she does?  Oh, she will know when she meet him.  He will also weigh exactly 511 keV, and he will immediately snatch her with the irresistibly attractive electromagnetic force.

Now mass is in keV?  Yes, by one and only the one equation, E=mc^2. Here you call it electron’s rest energy. Yeah, as if electrons rest in one place ever.  Who remembers the electron mass is 9.1e-31 kg? Well, some freaks do.  But almost all keV junkies know the electron mass is 511 keV.

When an electron meet a positron, two can annihilate and generate a pair of photons, the total energy of which is equal to 1.022 MeV (=2×511 keV) in the center of mass frame.  Don’t worry about the frame thing since it simply tells the energy is so ‘subjective’. Some calls this ‘relative’. Hmm, where did I hear that before?

A funny thing here is that these guys can’t tell the time or the direction of the time. The phenomena can be reversed. A photon with energy of 1.022 MeV or more can suddenly disappear to pull an electron and a positron out of literally thin air.  In other words, in an environment of T=1 MeV, the matter (electrons and positrons) starts popping in and out right and left.

Hmm, the matter at T~1 MeV seems to have a harsh life, and Gamma-rays seem as dangerous as X-rays if not more.  The  enviroment at T~1 MeV is hostile enough to create and destroy electrons and positrons, since their mass is less than 1 MeV. Once we put both mass and temperature in keV, there may be something, something connecting how the matter behaves and what kind of lights we expect to see especially in this high energy environment.

- So is this a new state of the matter?
= Perhaps, although it seems that we may have skipped some.
- Some? There’s more than one besides the three “normal” states?
= Have you heard something called plasma?
- Oh, yeah. How did you know I have a plasma TV?