# B Which fundamental properties of particles can actually vary?

1. Nov 3, 2016

### Hallucinogen

Hi everyone. Very orderly forum you have here. I'd like to ask a few philosophical questions which are very fundamental to QM.
Firstly I assume we all agree that a fundamental particle - imagine an electron or quark - has multiple inherent properties which are all separable from each other, such as its kinetic energy, its mass, its spin direction, its charge, its colour and flavour (quarks), and their frequency (and perhaps others, please point them out if there are any).
It seems to me like very few of these can actually change for any individual particle. Charge never changes. Colour, flavour and spin direction can change within a few very restricted values, due to direct actions performed on the particle by gauge bosons. Mass can only change as the particle approaches the speed of light (or enters very distorted space-time?).
So my first question is: is it really true that the only values here that can continuously change are kinetic energy (and thus velocity) and frequency? And that the things that cause those changes *are* the unchangeable static properties of *other* particles?
Hence the charge and mass of an electron act together upon the body of a different electron to change that electrons velocity?
Another question I have is how frequency and velocity relate to energy: is it the case that a particle has a "pure" energy, aside from its kinetic energy, which solely determines the particles frequency, and similarly, is the velocity of a particle determined by the more fundamental kinetic energy? Or does "pure" energy not exist, such that, when an electron or quark absorbs a photon, the photons energy is portioned into specific categories of energy such as kinetic or entropic/configurational?

2. Nov 3, 2016

### BvU

Hello Hallo,

Not so philosophical, your question is. A lot is known indeed if you google around a little bit, eg. looking for fundamental particles, what you can build with them, etcetera. (for the first link there must be better ones, updated with the Higgs).
First question: for all practical purposes yes.
Second: Electrons interact with other particles exclusively through carriers of the forces: photons, W or Z bosons.
Another question: See the de Broglie relations. Electrons do not have entropic/configurational energy. No portioning.

Last edited: Nov 3, 2016
3. Nov 3, 2016

### Staff: Mentor

The masses of fundamental particles never change; they are invariant masses (rest masses), not relativistic masses. What changes as the particle moves faster or into a different spacetime geometry is the particle's energy.

4. Nov 4, 2016

### houlahound

clearly all sorts of things can change including one fundamental particle into other particles.

5. Nov 4, 2016

### Hallucinogen

Even when colliding with other particles?
So is kinetic energy equivalent to pure energy, it's the same as the energy in the E = hv equation?
Edit: so I looked at the link - is the answer that the "electron energy" and the kinetic energy (in terms of momentum) are simply different parts of the wavefunction?

Last edited: Nov 4, 2016
6. Nov 4, 2016

### houlahound

there is no pure or impure energy - its just energy; the ability to do work.

7. Nov 4, 2016

### Hallucinogen

By pure energy I am referring to what I was taught as "energy" (I think it was in reference to the E = hv relation) as opposed to other forms of energy like kinetic, thermal, gravitational, elastic, configurational etc.

8. Nov 4, 2016

### houlahound

Sorry I don't get pure energy. Its just a concept measured by what it does to stuff.

9. Nov 4, 2016

### Staff: Mentor

All forms of energy are counted in the $E$ in $E=h\nu$.

10. Nov 4, 2016

### houlahound

Feynman gives a good explanation of energy with an accounting type analogy. Energy itself is just a concept IMO.

You can't pick one up in your hand, but your hand uses energy to pick anything else up.

11. Nov 4, 2016

### Staff: Mentor

Yes. The word "collision" is used in particle physics, but it doesn't mean that the particles are smashing into one another like little billiard balls. Instead, we 're bringing them very close together with a very large amount of energy so that interesting and exciting interactions happen.

12. Nov 4, 2016

### Hallucinogen

Thanks -- do you know of the publication or study which proves this? Or is it an assumption which allows models to be explained?
Also it seems like there are a few properties of particles which can be changed, such as energy, spin direction, colour, flavour and velocity, and these are influenced by both the unchangeable quantities like mass and charge along with the changeable quantities of other particles. Do you think that is correct?

13. Nov 4, 2016

### Staff: Mentor

That the subatomic particles we're "colliding" aren't behaving like little billiard balls? Any standard textbook on quantum mechanics will cover this. It's so basic that you might not even find it stated in so many words, just as it might be difficult to find any publication or study on animal husbandry that explicitly shows that horses have two eyes but only one mouth.

14. Nov 4, 2016

### Hallucinogen

I was more wondering as to what the actual evidence is, rather than how essential the fact is to QM. I would imagine gauge bosons mediating all interactions between matter itself would be pretty difficult to prove, in contrast to what a horse looks like. I'm learning that the W and Z bosons were hinted at by CERN and then fully discovered by the Super Proton Synchotron.
But this is adding to my confusion: for instance I understood that when two atoms repel each other, or "nearly collide", this is due to their electron orbitals exerting forces against each other, between which a mechanical equilibrium is found, and that this is all underpinned by the Pauli exclusion principle. I didn't know virtual gauge bosons were behind it all, and that that's the reason all constituents of matter deflect each other upon "impact".
- I can see how Van der Waals forces would be mediated by virtual photons exchanged between the unevenly distributed electrons of each atom, however.

15. Nov 4, 2016

### DennisN

(n.b. my example does not illustrate "collision", it is an example that shows that the electron around the nucleus does not behave like a "billiard ball", i.e. it does not behave like a classical particle)

If the electron in a hydrogen atom was classically orbiting the nucleus, it would emit electromagnetic radiation (since accelerating charges emit electromagnetic radiation) and it would thus lose energy and spiral into the nucleus. The electron clearly does not do that; the hydrogen atom is stable. See e.g. Rutherford- and Bohr model - Origin, Bohr Model, Failures of the Bohr model. The replacement was quantum mechanics.

16. Nov 4, 2016

### Hallucinogen

Understood, thanks. But if electrons around an atom do not behave like billiard balls, how do these wavefunctions emit or swallow gauge bosons? ie how do atoms collide in quantum mechanics? Their particles communicate with the particles of another atom purely through virtual gauge bosons?
As I'm understanding things here currently, it seems particles never hit one another directly but can only emit or absorb the postulated constituents of "exciting interactions".

17. Nov 4, 2016

### houlahound

18. Nov 4, 2016

### bhobba

Energy is defined these days by Noethers Theorem:
http://phys.columbia.edu/~nicolis/NewFiles/Noether_theorem.pdf

Its a big issue for GR but otherwise is rock solid, although different forms is exactly the same thing in each case, and surprising to the point of being shocking.

Thanks
Bill

19. Nov 4, 2016

### Staff: Mentor

The wave function doesn't emit or absorb anything, because it's an abstract mathematical object that appears in our calculations, not anything that physically exists - but that's beside the point because we're talking particle interactions and hence quantum field theory so there's no wave function involved. (The wave function that you've heard about is part of non-relativistic quantum mechanics, the stuff you learn in a first-year college-level QM class. Because it's non-relativistic, it works only when the speeds are small compared with the speed of light and the energies are small compared with the $E=mc^2$ energy of the particles involved, and neither of these apply when we're colliding particles).

But most of the problem here is that you're visualizing particles as small objects that might hit one another, and gauge bosons as particles that move from one to the other when they're at different nearby locations. That's a natural intuitive picture, and it's what the word "particle" suggests - what is a particle if it isn't a small object? - but it is also hopelessly misleading. We use the word "particle" in physics, but the word means something completely different than in ordinary English usage.

As I said above, there may not be any satisfactory B-level explanation, which is why houlahound asks "You're not getting ahead of yourself in learning QM?". The best I can come up with is that in quantum field theories, a particle is not a little object with some position in space (so that two particles will hit one another if they are moving in such a way that they both would end up in the same place at the same time). Instead, a particle is a "quantized excitation of a quantum field", and the simplest example of what that means would be something like http://www.physics.usu.edu/torre/3700_Spring_2015/What_is_a_photon.pdf

20. Nov 4, 2016

### BvU

The actual evidence is that 'we' can describe what is observed with sometimes astonishing accuracy, make predictions that can be verified and not be invalidated by experiment. The how and why may remain somewhat vague. The proof of this pudding is that it works !