• # Question: Why exactly are particles' masses given as giga electron volts or tera electron volts - aren't these units of energy? Is it just easier to refer to them as their equivalent energy from E=mc^2?

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Asked by strangeness to Arttu, Ceri, James_M, Monica, Philip on 15 Jun 2011.
• Ceri Brenner answered on 15 Jun 2011:

Yes, you’re exactly right. Mass and energy can be interchangable using the constant c^2 in einstein’s equation. In accelerator physics, one way in which we can describe how accelerated a particle has become is to say what it’s energy is, in MeV, GeV or TeV. Just to be keep things easier for comparison, we also describe a particle’s rest mass in eV units also. Physicists tend to try and keep to the same units when comparing the qualities of a particle (it’s speed, energy, mass etc) so as not to get confused and conform to a standard notation.

• Arttu Rajantie answered on 15 Jun 2011:

Ceri is right, it is easier to use electron volts for mass so that you don’t need to be constantly multiplying or dividing by 300000000. One way to think about it is that we express speed in units of c (the speed of light) rather than m/s, because the speeds involved tend to be to closer to c. Therefore in our units c=1. Similarly, we usually set Planck’s constant to 1 as well, because then any mechanical quantity can be expressed in electron volts. In cosmology and quantum gravity it is even common to set the gravitational constant to 1, and then everything is a dimensionless number. This simplifies calculations, but it is risky because then you can no longer spot simple mistakes in calculations by checking that your answer has the correct units.

• James M Monk answered on 15 Jun 2011:

yup, it’s nice when you start doing high energy physics and you redefine as many physical constants as possible to be 1 – the equations become so much easier to remember and manipulate. Not Pi though – that would make things go weird!