Are particle masses commonly normalized against a natural standard?

[rbj]

in HEP research and lit, is it common to think and write about the masses of the various leptons and quarks as relative to the invariant mass of some chosen standard particle? such as the electron? or something else?

 

[Hypersphere]

I think the normal way is to express the mass in units of eV/c^2.

 

[mfb]

eV is the common unit. The speed of light is usually set to 1, so there are no factors of c hanging around. meV, keV, MeV and GeV give the prefactors needed for particle masses.

In theory, you could express all masses as fraction of the Planck mass, but that would give really unhandy numbers.

 

[Meir Achuz]

It usually depends on what you are calculating.
In pion interactions, it is convenient to take the pion mass =1, etc.
The MeV is widely used, but is an arbitrary unit that depends on how the volt is defined.
It would be more appropriate to set the proton mass=1, and that may eventually be used (if I am elected).
Then we could compare our masses to those of alien civilizations.

 

[mfb]

In pion interactions, it is convenient to take the pion mass =1, etc.

Can you show where this is done? I don't remember seeing that.

The MeV is widely used, but is an arbitrary unit that depends on how the volt is defined.

Right

It would be more appropriate to set the proton mass=1, and that may eventually be used (if I am elected).

?

Then we could compare our masses to those of alien civilizations.

We can compare ratios, that's fine.

 

[OmCheeto]

I thought Carbon-12 was the "natural" standard for mass?

 

[mfb]

Carbon-12 is used for the definition of the atomic mass unit. This is used in chemistry and maybe nuclear physics, but not in particle physics.

 

[QuantumPion]

Mass has no fundamental way of being defined, unlike time and distance which are defined by the speed of light. The kilogram is defined as the weight of the international prototype kilogram (a platinum bar made to be about the same weight of 1 liter of water at room temperature).

 

[mfb]

Mass has no fundamental way of being defined, unlike time and distance which are defined by the speed of light.

The second has a completely arbitrary definition as multiples of a period of electromagnetic waves emitted in a transition in Cs-atoms. Meters are derived from that and the speed of light, indeed.

The kilogram is defined as the weight of the international prototype kilogram (a platinum bar made to be about the same weight of 1 liter of water at room temperature).

I expect this to change in the next 2-3 years.

 

[OmCheeto]

...
I expect this to change in the next 2-3 years.

Cool...

Do you think it will involve Carbon-12?

 

[QuantumPion]

Cool...

Do you think it will involve Carbon-12?

I think it has something to do with precise measurements of the mass of silicon crystal atoms, I saw a video about it somewhere recently.

 

[Bill_K]

Cool...Do you think it will involve Carbon-12?

Carbon-12 atoms could be used, but we are still looking for a volunteer to count them.

 

[OmCheeto]

Carbon-12 atoms could be used, but we are still looking for a volunteer to count them.

Here's an interesting article from about 6 years back:

Official Kilogram Losing Mass: Scientists Propose Redefining It As A Precise Number Of Carbon Atoms
Sep. 21, 2007

How much is a kilogram?

It turns out that nobody can say for sure, at least not in a way that won't change ever so slightly over time. The official kilogram -- a cylinder cast 118 years ago from platinum and iridium and known as the International Prototype Kilogram or "Le Gran K" -- has been losing mass, about 50 micrograms at last check. The change is occurring despite careful storage at a facility near Paris.

That's not so good for a standard the world depends on to define mass.

Now, two U.S. professors -- a physicist and mathematician -- say it's time to define the kilogram in a new and more elegant way that will be the same today, tomorrow and 118 years from now. They've launched a campaign aimed at redefining the kilogram as the mass of a very large -- but precisely-specified -- number of carbon-12 atoms.

Obviously brilliant scientists...

Oh! And there's the silicon sphere:

At least two other proposals for redefining the kilogram are under discussion. They include replacing the platinum-iridium cylinder with a sphere of pure silicon atoms, and using a device known as the "watt balance" to define the kilogram using electromagnetic energy.

And if I knew the following, I'd forgotten it:

The kilogram is the last major standard defined by a physical artifact rather than a fundamental physical property. In 1983, for instance, the distance represented by a meter was redefined by how far light travels in 1/299,792,458 seconds -- replacing a metal stick with two marks on it.

And you are apparently correct about finding someone to count out these carbon atoms:

"You could imagine having a lump of matter that actually had exactly the right number of atoms in it," Fox noted. "If you could build it by some kind of self-assembly process -- as opposed to building it atom-by-atom, which would take a few billion years -- you could have new kilogram artifact made of carbon.

Where's mfb? I suspect he knows about some secret government project.

hmm... google google google

ah ha!

Redefining the Kilogram
Scientists have crafted two new definitions for the common unit of mass. The fight to pick the best one is getting nasty.
Bob Grant | July 5, 2012

There's a secret war going on, and the future of science is at stake. The battle rages over the definition of "kilogram."

Just as I suspected.

So one is the carbon-12 cube, and the other is:

Defining the Planck kilogram is much trickier, requiring a working knowledge of quantum mechanics and special relativity, a massive and expensive piece of machinery, and considerable skill in mathematics.

A quantum kilogram? hmmm... several lame jokes just popped into my head...

 

[mfb]

Defining the Planck kilogram is much trickier, requiring a working knowledge of quantum mechanics and special relativity, a massive and expensive piece of machinery, and considerable skill in mathematics.

That sounds like a weird description of the Watt balance project.

 

[lpetrich]

Let's see what's happened to the other metric-system units.

Length is now defined from time by fixing c, the speed of light in a vacuum. Strictly speaking, that's the speed of something that travels on a null geodesic. That's general-relativity-speak for a straight-line-like path with zero space-time distance along it in space-time.

Time is now defined in terms of the frequency of the hyperfine transition of cesium-133. It's to be improved by making it the ground-state one at 0 K temperature.

Proposed redefinition of SI base units - Wikipedia

Planck's constant, the elementary charge, Avogadro's number, and Boltzmann's constant are to be fixed, as c has been.

Planck's constant will give mass in terms of length and time.

The fixing of the elementary charge will make the electric permittivity of the vacuum a measured quantity instead of a defined one, as it currently is. The magnetic permeability of the vacuum and the impedance of free space will follow along with it.

The fixing of Avogadro's number will force one of two possibilities. Currently, a carbon-12 atom is defined as 12 daltons / atomic mass units, and a mole of carbon atoms is defined as 12 grams. So either (1) carbon-12 atoms will continue to weigh exactly 12 daltons and the mass of a mole of carbon-12 atoms will become a measured quantity, or (2) a mole of carbon-12 atoms will continue to weigh exactly 12 grams, and the mass of a carbon-12 atom will become a measured quantity.

The fixing of Boltzmann's constant will make the temperature of the triple point of water a measured quantity instead of a defined one. That's where solid, liquid, and gas phases coexist.


What does this mean for the electron volt? It's defined as
((elementary charge)/(1 coulomb)) * (1 joule).

It'll change from a measured quantity to a defined quantity, being given as an exact though non-integer multiple of the energy of a Cs-133 hyperfine-line photon. That energy is about 3.80*10^(-5) eV, and the multiplication factor is about 26300.