# Energy, time and length corresponding to mass of electron.

1. Mar 10, 2012

### H_Psi

Hey,
I am starting to learn the basics of QED and I am trying to get acquainted with the units and scales that are used.

2. Relevant equations
The exercise is to estimate the energy, time and length that correspond to the mass of an electron.

3. The attempt at a solution

That’s what I did:
The mass of an electron is 9.11E-31 kg.

Energy:
E=mc^2 gives me an energy of 0.511MeV.

Length:
Here I used the equation: c * hbar = 197 MeV * fm
-> 386fm = c * hbar / (0.511MeV)
-> In natural units: 386fm = 1 / (0.511MeV)

Time:
Here I used the equation: hbar = 6.58E-22 MeV * s
-> 1.288E-21 s = hbar / (0.511MeV)
-> In natural units: 1.288E-21 s = 1 / (0.511MeV)

Is this calculation correct? Is that the way one can calculate the length and time corresponding to a mass?
Thanks

2. Mar 12, 2012

### gomboc

Makes sense, but I'd take a look at the equation you used to find the length. Do you really want to be using the reduced planck constant (hbar) there, or simply the planck constant (2*pi*hbar)? You can look up the de Broglie wavelength of an electron, and you'll see your value is a bit small.

3. Mar 12, 2012

### turin

That depends on the convention, right? For instance, in Peskin, the convention is to use hbar as the base unit (of action). I don't recall seeing anyone use just plane h as the base unit, but it is just a convention.