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?

Beside the obvious benefit of confusing the public and colleagues in other fields, the apparent chaotic use of physical units like keV and Kevin has an addictive convenience beyond a simple matter of convention. Yes, I said “convenience”.

All roads to Rome, All quantities to Energy

In fact, mixing up the units of the physical quantities doesn’t end with energy and temperature.  If I am the one breaking this to you, then I am terribly sorry, but the plague has already spread to pretty much all the physical quantities.

Energy = Temperature = Mass = Length = Time = …

So, there you have it: Energy costs Money since Time is Money.

At the center of this pandemic, you find “energy”, which seems to be the culprit, linking all of them together. Although, once they are all linked, it doesn’t really matter. This appears a gross misuse or misunderstanding of these quantities at best. Now what drives “normal” physicists to become “keV” junkies if there is such a thing called normal physicist? Well, the answer is already in vlk’s slog about “keV vs keV”.

Ice, Water or Vapor

Do you know what it feels like at T=200 K, T=300 K or T=400 K?

Of course, it’s a matter of being frozen to death, being comfortable under the sun shine or being burnt to death.

Ok, then, now how about T=10,000 K, 10,000,000 K, or 10,000,000,000 K?

Hmm, hmm, interesting. Very hot, extremely hot, and hellish hot? I know I will be dead either way.

A shear number of zeros is already annoying and begging for another unit or a shorter form, but more importantly, it seems that we, … I mean, normal human beings don’t really have a good “physical” feel of Kelvin, Celsius or Fahrenheit at these high temperatures. Why should we? After all, we use Celsius or Fahrenheit for the daily use, and it’s not like that we need to burn and destroy T1000 to save the humanity everyday. Even so a couple of 1000 K heat might do the job.

Ok, ok, Let me think for a moment for change.  Judging from a factor of 1000 differences which is a lot more than 100, there’s got to be equally significant differences in the matter at these temperatures. Perhaps changing-their-physical-state kind of differences?

Hmm, only if we have an easy way to express the state change or a unit to convey the essence of their differences. A-ha, yes, yes, I knew it’s not our fault, our ignorance or lack of our knowledge.  It’s the useless unit here that keeps us in the dark. The unit is good at around, say, less than 1000 C.  Just like T=0 C means ice to water and T=100 C means water to vapor.  More than that, this K, C, or F is so useless.

De Nile isn’t just a river in Egypt, and it flows in our mind too, quite beautifully I suppose.

Wait a minute, there are more states than solid, liquid and gas?