Particles' Intrinsic Properties in QFT

[LarryS]

TL;DR

Intrinsic vs extrinsic properties in QFT?

In single-particle nonrelativistic QM, a particle, say an electron, has intrinsic properties like rest mass, charge and spin magnitude and extrinsic/dynamic properties like momentum and energy. Does this intrinsic/extrinsic dichotomy carry over to, say, the electron field in QFT?

Thanks in advance.

 

[Demystifier]

I don't think that intrinsic and extrinsic are the right words for what you are trying to say. Perhaps better words are invariant and non-invariant. But yes, this dichotomy carries over to QFT.

 

[LarryS]

Are the invariant properties part of the unexcited (vacuum state) state of the field?

 

[Roberto Pavani]

@Demystifier nailed it. Perhaps the simplest way to see why: energy and momentum are frame-dependent, so they can't be intrinsic to the particle.

 

[Matterwave]

Are the invariant properties part of the unexcited (vacuum state) state of the field?

Define "part of"?

As I read the question, I would say "no". The vacuum state itself does not posses charge, mass, spin, etc.

But also QFT is "not my thing" so hopefully someone more knowledgeable will chime in and correct me if I'm wrong. :)

 

[Demystifier]

The vacuum state itself does not posses charge, mass, spin, etc.

I would say it has zero charge, mass, spin, etc.

 

[gentzen]

@Demystifier nailed it. Perhaps the simplest way to see why: energy and momentum are frame-dependent, so they can't be intrinsic to the particle.

So you mean, only a scalar property can be intrinsic to the particle?
I don't see anything wrong with a Lorentz covariant vector like the four-momentum.
Of course a scalar property is frame-invariant, but I see nothing wrong with frame-covariant properties.

 

[Roberto Pavani]

You're right that the 4-momentum is covariant. But its only frame-invariant content is the norm $p_\mu p^\mu = -m^2$, which is just the mass.
The direction of $p^\mu$ (i.e., the velocity) is frame-dependent. So the intrinsic information carried by the 4-vector reduces to a scalar: $m$.

 

[gentzen]

The direction of $p^\mu$ (i.e., the velocity) is frame-dependent. So the intrinsic information carried by the 4-vector reduces to a scalar: $m$.

I see. But then it mostly boils down to how one wants to interpret the word "intrinsic" (including connotations) and how one wants to denote "frame-covariant properties" with another word like "extrinsic" with other appropriate connotations.

 

[Matterwave]

I would say it has zero charge, mass, spin, etc.

Fair enough~