What determines a particle's size?

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Discussion Overview

The discussion centers on the concept of particle size, particularly comparing fundamental particles like electrons to composite particles like protons, and exploring the relationship between particle size and mass. Participants examine definitions of size in the context of particle physics, including the implications of mass and interactions with the Higgs field.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants note that while particles differ in size, it is often more useful to discuss their mass, as size can be misleading in particle physics.
  • One participant claims that the electron is expected to be point-like and has no known size, while the proton is a composite particle with a non-zero volume.
  • Another participant emphasizes that the definition of size varies between fundamental particles and composite particles, suggesting that the size of an electron should be considered in terms of its atomic context.
  • There is a discussion about the relationship between energy and distance, with one participant suggesting that larger mass implies smaller distances in terms of particle interactions.
  • Some participants express uncertainty about the use of "size" in the context of quarks, noting that they are also considered point-like, while the proton's size can be determined by measuring the distribution of its constituent quarks and gluons.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the definition of size in relation to particles, with multiple competing views on how to interpret size, mass, and the nature of fundamental versus composite particles.

Contextual Notes

There are limitations in the discussion regarding the assumptions made about particle definitions and the implications of mass versus size. The conversation reflects a range of interpretations and lacks resolution on the definitions used.

Ryan Reed
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I know that particles differ in size such as the electron is smaller than the proton, but why is this?
 
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Its much more useful to talk about why different particles differ in mass. When physicists talk about "sizes" of particles, they are really talking about mass for the most part. A down quark is much "smaller" than a top (or truth) quark, but its smaller in the sense that it has less mass. Its sort of pointless to talk about particles as having a finite radius anyway since they arent even localized objects.

Now, physics does have an answer to why particle masses are different: the more a particle (like a quark) interacts witht the Higgs field, the more mass it acquires.
 
The electron does not have known size, it is expected to be point-like.
The proton is a composite particle, its size is the volume those particles are in - which is non-zero. This is a completely different "size" definition than the size of the electron. If you ask for the volume where an electron is in an atom, for example, you get the size of the atom (approximately), which is much larger than a proton.

DeldotB said:
A down quark is much "smaller" than a top (or truth) quark, but its smaller in the sense that it has less mass.
I have never seen any physicist use "size" in that way.
 
@DeldotB The size in such a way wouldn't fit in the discription for the top/down... The energy is the reciprocal of distance, so a larger mass(larger energy) would imply a smaller distance. And that's true- you need larger energies to see "small distances"

The electrons at least as we know them today, have no size - they are fundamental particles and they are point-like.
The quarks are also considered pointlike.
The proton however is a soup of quarks and gluons. You could try in a similar (to the atom) manner to measure what's the distribution of its constituents and thus determine a size of it. I think the size is ~0.8fm (and so is the Hydrogen nucleus radius), that means that within that range you have the largest possibility to find a quark that is bound to the proton.
 
mfb said:
The electron does not have known size, it is expected to be point-like.
The proton is a composite particle, its size is the volume those particles are in - which is non-zero. This is a completely different "size" definition than the size of the electron. If you ask for the volume where an electron is in an atom, for example, you get the size of the atom (approximately), which is much larger than a proton.

I have never seen any physicist use "size" in that way.
Right. I am being informal about it.
 

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