83 votes
Accepted

Why are protons heavier than electrons?

There are multiple reasons why protons are heavier than electrons. As you suggested, there are empirical and theoretical evidence behind this. I'll begin with the empirical, since they have important ...
KareemElashmawy's user avatar
73 votes
Accepted

Is the Higgs boson an elementary particle? If so, why does it decay?

Most fundamental particles in the standard model decay: muons, tau leptons, the heavy quarks, W and Z bosons. There’s nothing problematic about that, nor about Higgs decays. Your question may come ...
Bob Jacobsen's user avatar
  • 14.5k
69 votes

Why are protons heavier than electrons?

As noted "why" is a tricky question but we may ask what is the most fundamental view known concerning this question. Electrons and protons are very different beasts. Electrons as far as we can tell ...
OON's user avatar
  • 8,334
67 votes
Accepted

Resolving Conflicting Reports on Fermilab $g-2$ Results

This seems rather incredible that these two seemingly conflicting announcements come on the same day. The pre-print for the Nature paper by the BMW group was placed on arXiv in 2020 around the same ...
ohwilleke's user avatar
  • 3,903
64 votes

Why are muons considered to be elementary particles in the Standard Model?

That a particle decays into other particles is completely disjoint from it having substructure/being fundamental or composite. Some examples: A highly energetic photon may "decay" into an electron ...
ACuriousMind's user avatar
  • 125k
57 votes
Accepted

Why are muons considered to be elementary particles in the Standard Model?

Addressing misconceptions First, I address some misconceptions in your question. the decay indicates that the muon may be just a composite particle The fact that the muon decays at all is not ...
knzhou's user avatar
  • 102k
57 votes

Theoretically, could there be different types of protons and electrons?

Your friend is correct: there's only one type of proton. The proton is the lightest baryon. It has charge $+1$, spin $1/2$, and baryon number $+1$. These three quantum numbers are so fundamental ...
knzhou's user avatar
  • 102k
47 votes
Accepted

Theoretically, could there be different types of protons and electrons?

It is an experimental fact that all electrons and also all protons (but this often applies also to nuclei, atoms and even molecules) are indistinguishable from one another, i.e. they both are ...
valerio's user avatar
  • 16.2k
47 votes

Why doesn't an electron rip itself apart?

As far as we know, electrons are fundamental particles and have no internal structure or components. Also, an electron cannot decay into other particles (unless it has a very high kinetic energy) ...
gandalf61's user avatar
  • 53.1k
46 votes

Why was the neutrino thought to be massless?

I thought the particle was hypothesised in order to maintain the conservation of momentum in a beta-decay. If it was massless, this would have no effect, right? This is where you are confused. Having ...
StephenG - Help Ukraine's user avatar
46 votes

How seriously can we take the success of the Standard Model when it has so many input parameters?

It is inaccurate to think that all of the standard model of particle physics was determined through experiment. This is far from true. Most of the time, the theoretical predictions of particle physics ...
joseph h's user avatar
  • 29.4k
42 votes

What is the evidence for 'billions of neutrinos pass through your body every second'?

The existence of the neutrinos was established using energy and momentum conservation in neutron decays. There have been experiments with neutrino and antineutrino beams both at Cern and Brookhaven ...
anna v's user avatar
  • 234k
42 votes
Accepted

If the Higgs field gives particles mass, and is present everywhere, then why are there massless particles?

The usual pop-sci explanation of "the Higgs field exerting a drag force on particles that move through it, sapping their kinetic energy" is unfortunately not very accurate. In technical ...
Nihar Karve's user avatar
  • 8,445
40 votes
Accepted

Why are real photons so much less efficient in carrying momentum than virtual photons?

Within the usual handwavy accounts of virtual particles, the answer is "simple": Virtual particles aren't required to obey on-shell mass-energy relations (in this case $E=pc$), so there can ...
ACuriousMind's user avatar
  • 125k
39 votes
Accepted

Can neutrinos "hit" electrons?

Yes, neutrinos "hit" electrons all the time inside the sun, on their way to getting out, which results into the resonant conversion of their flavor, predicated on the changing effective index of ...
Cosmas Zachos's user avatar
35 votes
Accepted

How (or when) do gluons change the color of a quark?

The idea that baryons contain three quarks is a significant oversimplification wrong. It works for some purposes, but in this case it causes way more confusion than it's worth. So you should stop ...
David Z's user avatar
  • 76.4k
35 votes

Why doesn't an electron rip itself apart?

An electron is an elementary particle in the standard model of particle physics. . The table axiomatically assumes that elementary particles are point particles in the QFT of the model, i.e. have no ...
anna v's user avatar
  • 234k
33 votes

Why are protons heavier than electrons?

It's just an empirical value. According to our current knowledge, the masses actually come from some more fundamental quantities - the electron yukawa coupling and the Higgs field vev, in the case of ...
Mitchell Porter's user avatar
33 votes
Accepted

If electrons were positive and protons were negative, would life be different?

The point is that whether we call it 'positive' charge or 'negative' charge makes no difference, as long as we are consistent. If we decided to label the charge of a proton as 'negative' then, to be ...
lemon's user avatar
  • 13.2k
33 votes
Accepted

Elementary Particle of Magnetic Field

The gauge boson associated with the magnetic field is the photon. Electric and magnetic fields are in effect different views of the same thing, i.e. the electromagnetic field, and the gauge boson for ...
John Rennie's user avatar
33 votes

Do standard model particles actually exist or merely usefully describe behaviors of a medium?

Here is a bubble chamber picture of an event It is interpreted as particles: $$ {\newcommand{Subreaction}[2]{{\rlap{\hspace{0.38em} \lower{25px}{{\rlap{\rule{1px}{20px}}} {\lower{0.5ex}{\hspace{-...
anna v's user avatar
  • 234k
32 votes

How seriously can we take the success of the Standard Model when it has so many input parameters?

The Standard Model may have many parameters, but it also talks about many things, each typically only involving a very limited number of parameters. For example, the muon lifetime$^\dagger$ $$\tau_\mu=...
J.G.'s user avatar
  • 24.8k
31 votes

Why was the neutrino thought to be massless?

An interesting aspect not touched on by existing answers at the time of writing. What was observed about beta decay is that for the same change in the atomic nucleus, the emitted electron (which, ...
Andrew Steane's user avatar
30 votes

Why is Standard Model + Loop Quantum Gravity usually not listed as a theory of everything

One can pinpoint the technical error in LQG explicitly: To recall, the starting point of LQG is to encode the Riemannian metric in terms of the parallel transport of the affine connection that it ...
Urs Schreiber's user avatar
30 votes
Accepted

What is the decay width and why is it given in energy units?

Like @DavidZ I found this a very good question but unlike him I am not a professional Physicist and so will try and answer the question on a simplistic level which may not suit @Martin as I do not ...
Farcher's user avatar
  • 96.1k
30 votes

What is $U(1)$ symmetry?

Let us first refer to symmetry generically. When we say a theory is symmetric under $G$ ($G$ some group) we mean that the elements of $G$ transform the states, and the operators of a theory, (in the ...
ohneVal's user avatar
  • 4,408
28 votes

Where is the evidence that the electron is pointlike?

One who is familiar with the history of particle physics, and physics in general, knows that physics is about observations fitted with mathematical models. This review examines the limits on size we ...
anna v's user avatar
  • 234k
28 votes

Is the Higgs boson an elementary particle? If so, why does it decay?

Another way to answer this question is that particles are not "elementary," not even in a given quantum field theory. Quantum field theories (like the Standard Model) are expressed in terms of fields, ...
Chiral Anomaly's user avatar
28 votes

Why was the neutrino thought to be massless?

Why did he figure it couldn't be detected? Because the neutrinos interact so weakly that it was for some time believed that it will not be possible to detect. E.g. in the 1934 paper by Bethe and ...
OON's user avatar
  • 8,334

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