# Higgs - interactions and forces; distinction between these?

1. Mar 16, 2013

### Paulibus

Higgs --- interactions and forces; distinction between these?

At the moment I'm reading Lisa Randall's Knocking at Heavens Door, and I have a quick question that can no doubt be easily answered.

I've often read and accept that there are four forces of Nature: Strong, Electromagnetic (EM), Weak and gravity. Forces are things that cause motion, and arise when charged (colour, ?, EM and massive) particles interact. Forces are a label for "interactions". Yet in discussing the interactions of matter with the Higgs field the "force" label seems not to be used; instead an unlabelled interaction is talked about, as in explaining the origin of mass. Why is the label "force" not used for this interaction, which in a sense is just an opposite sort of thing that prevents or obstructs motion by conferring inertia on matter?

Last edited: Mar 16, 2013
2. Mar 16, 2013

The concept of force as in classical physics is not so prominent at quantum level as it is only really meaningful when discussing stable or metastable particles that are involved in "elastic" collisions, ie where the particles coming out are the same as those that went in. Obviously a particle must survive for a certain length of time to enable measurements of changes in its momentum, such as typically result from acceleration under a force, to be possible.

Leaving aside gravity, the other three traditional "forces" correspond, at quantum level, with particle interactions mediated by vector bosons, eg the photons of the electromagnetic force. Acceleration under a force becomes changes to a particle's "probability current" in QM.

But, even then, the weak force in particular manifests itself mainly in ways that are not analogous to the actions of classical forces. It's most common effects are actually particle decays (or collision outcomes) that transform one type of particle into another or others - these are termed "inelastic" processes. For example a classic example of the weak force in action is the decay of a free neutron:
$$n → p^+ + e^- + \bar{\nu_e}$$
The outgoing antineutrino is equivalent to a (non-anti) neutrino coming in, so the above is equivalent to the neutron changing to a proton and a neutrino becoming the electron. Rather than being deflected, as it would be by a force, the proton is destroyed and replaced by a new particle.

It doesn't therefore really make much sense to use the word "force" to describe these kinds of interactions.

In the case of the Higgs field, the main effects of its interactions are to give masses to other particles. Again, this is not in any way analogous to classical forces.

The Higgs field also differs from the other interactions in that it is a direct interaction between a scalar field and other particles, rather than between particles and vector bosons.

3. Mar 18, 2013

### Paulibus

Thanks for the clear, informative reply. I suppose that in the usual classical sense the word "force" is concept that emerges from particle physics, as does for example "temperature", from statistical mechanics. As you wrote:
.

I have one further question. It seems to be accepted (justified by special relativity?) that any particle without (rest) mass must travel at light-speed, like the photon. Now the Higgs field is, as I understand it, the result of spontaneous symmetry breaking. It seems to me that such symmetry breaking must be a process which bridges the passage from a phase of physics in which all fundamental particles move at light speed, and where, as a consequence, the concept of an observable time has no meaning, to our familiar phase of physics where time appears as a fundamental dimensional concept. Put simply: no symmetry-breaking Higgs mechanism, no time as an observable concept?

Do particle physics folk ever discuss such mad nonsense, which could have implications for cosmogeny?

4. Mar 18, 2013

### suprised

Time exists independently of any matter fields, purely in GR. In fact there is no deep relation between GR and the Higgs mechanism. It is not true that the mass of "all" matter is due to the latter; in fact, 95% of the observed mass is not generated by the Higgs mechanism, but rather by the strong interactions. This point is confused and misrepresented in the media almost everywhere.

5. Mar 19, 2013

### Paulibus

Yes, I'm aware of this, but thanks, Surprised.

Perhaps some of this confusion arises from the rather sloppy use of "mass" to mean "rest mass" in physics. A better generic term may be mass/energy. If (and I stand to be corrected) the Higgs mechanism is responsible for rest mass, and rest mass only, the point I was making is, I think, still valid: in the absence of the Higgs mechanism, particles have no rest mass and must travel at c, whatever their energy. Outside superconductors, for instance, photons do this; inside they acquire mass and don't, as Phil Anderson pointed out long ago. Hence the Meissner effect.

Travel at c relative to an observer stops the flow of any "time" she observes in the moving frame, and I find it difficult to understand how observed time could exist as a useful parameter for gauging process in physics, in a universe built out of particles that have no rest masses. This is where I come unstuck.

I can only speculate that lacking the Higgs mechanism, observed time might be a consequence of such light-speed particles being bound together in complexes (at rest or slowly moving) in an observer's frame, like nuclei, atoms, molecules and galaxies, iff these are part of the universe. Here kinetic and potential energy are ruled by the virial theorem and masquerade as observed mass? Mad nonsense.

Last edited: Mar 19, 2013
6. Mar 19, 2013

### ofirg

The proton and neutron mass is rest mass, which is defined as the energy of the particle in its rest frame. It would be almost the same even without the higgs mechanism ( in that case the quarks building the proton and neutron would be massless)

All known elementary particles except the higgs itself get there mass from the higgs mechanism. The higgs itself would be massive regardless.

You must remember that the higgs mechanism is way for particles to get mass when they cannot get mass otherwise, but realy has nothing to do to the fundamental concepts of mass and time in relativity.

7. Mar 20, 2013

### Paulibus

Now I'm getting really confused. Your statement sounds as if the Higgs mechanism is a charity for destitute massless particles! It would help me if you could explain what you mean by "otherwise".

I agree that once-upon-a-time one could simply define the measured mass of these hadrons as their rest mass. But was this not just an oversimplified definition from a time when physicists didn't know they are made of quarks whose mass/energy consists mostly of "binding" energy associated with the strong interaction --- presumably a balance of kinetic and potential energies struck by some averaging process, like the virial theorem?

The "rest mass" of composite objects like nuclei, solar systems and galaxies is their mass measured (somehow) when their centre of mass is at rest in the measurer's frame and is a lower bound on the sum of the rest masses of their constituents, when these masses are similarly measured (somehow). I guess the Higgs mechanism applies only to "truly fundamental " non-composite particles (if any such exist)?

Last edited: Mar 20, 2013
8. Mar 20, 2013

### ofirg

No, The definition of rest mass has nothing to do with if a particle is elementary or bound. It has always been the energy in the particle`s rest frame ( the frame where the particle is at rest, i.e. its momentum is zero), regardless of the physics responsible for this rest mass. It is usually simply called mass.

One of the defining properties of a particle, is that it has (all replicas of it) a specific value for its mass.

I think what your basically saying is that in a bound system the total energy in its rest frame
(mass) is smaller than the energy when all its constituents are seperated to infinity (which is simply the sum of the masses of the constituents if they are at rest at infinity).
This is difficult to define in the case of quarks and hadrons, since quarks can't be seperated to infinity.

A fundamental symmetry in the standard model (SM) (gauge symmetry) forbids most particles
(all except the higgs) to have mass. One, however can write a potential with a vacuum (ground state) where the symmetry is spontaneously broken and the particles have mass.

There are many references on this (you can start with http://en.wikipedia.org/wiki/Higgs_mechanism), But the point is that it is needed to reconcile that these particle have mass with the symmetry present in the SM.

But has nothing to do with the fundamental concept of mass

9. Mar 20, 2013

The mass of a composite system of particles is defined as
$$\sqrt{(\sum E_i)^2 - (\sum \textbf{p}_i)^2}$$where $E_i$ and $\textbf{p}_i$ are the energy and momentum, respectively, of the ith particle.

It is not true that this gives a lower bound on the sum of the masses of the constituents. Consider a system of two photons of the same energy (frequency) passing in diametrically opposite directions. The two photons' momenta, being opposite, sum to zero, so the mass of this system is $\sqrt{2}E$ where E is the energy of either one. Yet neither photon, individually, has any mass.

Often the mass of a system of massive particles is less that the sum of their individual masses as a result of a "mass defect" resulting from the negative potential binding energy of the constituents. For example the mass of a Hydrogen atom is less than the sum of a proton and electron mass; the mass of an alpha particle is less than that of two protons and two (free) neutrons; and similarly for almost all atoms, molecules and atomic nuclei.

10. Mar 21, 2013

### Paulibus

I seem to have been more confused than I thought. Thanks for the clear clarification, AdrianTheRock. In mitigation, I can only claim to have been exercising gauge freedom by not choosing the conventional zero for energy, and not qualifying "sum" as algebraic. Perhaps I was influenced by the gauge freedom enjoyed by hordes of swallows here, which are presently perching with impunity on wires at various high potentials.

(but the trick of the Higgs mechanism of course rides to the rescue).
The Standard model treats 18 particles (including the Higgs; Randall's Fig.23), of which 17 (excepting the photon) have rest mass/energy (I believe). Which of these masses are attributable to the Higgs mechanism? All of them, all leptons and bosons ? Or just the W and Z gauge bosons?

11. Mar 21, 2013

### ofirg

The gluon is also massless, and within this framework the neutrinos are also massless. (other ingriedients have to added for their masses)

All these masses except the mass of the higgs itself are attributable to the Higgs mechanism.

12. Mar 22, 2013

### Paulibus

Thanks, ofirg, for confirming for me that in the framework of the Standard Model of Particle Physics all particle rest masses (excepting the Higgs mass) are attributable to the Higgs mechanism.

To return to a situation I mentioned earlier (post #3). Here is a quote regarding the role the Higgs mechanism may play in the history of the universe, taken from Lisa Randall's book, Knocking on Heaven's Door, p.117 (The Bodley Head.London):
The idea of an early universe without the Higgs field, occupied only by particles that have no rest mass and travel at light speed and have energy, seems to have taken root in cosmology. But, as far as I am aware, the implications for the dimension of time that stem from this strange scenario of a light-speed universe, so to speak, have not been discussed, although the very idea of a phase transition seems to me to imply the passage of time over the time-span of a process.

In particular, I cannot accept suprised's rather dismissive and didactic comment concerning this strange "earlier" phase of the universe:

.

Perhaps this was the beginning of the universe's history of evolution? History is a time-based story by definition. There may be no need for a distasteful singularity to start things rolling, after all.

13. Mar 22, 2013

### Bill_K

Nevertheless, it's quite correct. And your idea that masslessness implies "the concept of an observable time has no meaning" is not!

Last edited: Mar 22, 2013
14. Mar 22, 2013

### tiny-tim

of course it has meaning!!

whyever not?

15. Mar 22, 2013

### Paulibus

I seem to have stirred a bit of a hornet's nest here!

By masslessness do you mean mass/energy-lessness, or rest-mass-lessness? The use of "mass" in physics as a catch-all for the concept defined by Newton's second law is sloppy. Not taking care about the exact meaning of the words you use leads to needless confusion.

How would you (if you were there, hopefully somehow alive as an observer, but sans any feasible clock) measure, or even describe, time in a universe that consisted only of particles associated with fields, no bound objects; particles that zip around at light-speed? What is the meaning of a parameter that can never be measured?

I can only think that by "correct" you mean a mathematical construct without, in this case, any physical foundation - a pure and meaningless abstraction.

This two-liner doesn't resolve anything. It's for you to amplify your claim "of course". Please.

Last edited: Mar 22, 2013
16. Mar 22, 2013

### tiny-tim

amplify? ok … it has the same meaning as when everything doesn't move at the same speed

17. Mar 22, 2013

### ofirg

As mentioned before, by the term mass means what you call rest mass, or its lorentz invariant generalization $sqrt{((\sum E_i)^2 - (\sum \textbf{p}_i)^2)}$ mentioned before.

I think you're confusing time in general with proper time. the proper time of a particle is the time measured by a clock which is "attached" to that particle. That is ill defined for a massless particle. However, time in general can be measured with massless particles.
Just take a ruler and mark when the particle has arrived to the begining and when he has arrived to the end. That can be done wether or not the particle moves at the spped of light.

18. Mar 22, 2013

### Paulibus

Despite the fact that it can be carefully defined, as written above, "mass" is still a word often used sloppily in physics. It can lead to confusion, especially now that we're so remote from Einstein's milieu. Clarity demands care. But this is just a prejudice I have. I guess I shouldn't question fashion. Particularly not that which is approved en haute !

No, I'm not confused about proper time. You say "Just take a ruler.....". Where do you get one from? Rulers and observers almost certainly didn't exist in the early universe. It has been postulated that here, before the appropriate symmetry broke and enabled the Higgs mechanism, there were no bound structures, or stuff like rulers and observing folk. Only things around were individual particles zipping along at c in a very hot, dense and energetic universe; too much so to even be a candidate heaven for particle physicists!

How do you define time in such a situation, even in principle and with the best of intentions, when no conceivable clocks can exist? For that matter, how does one measure distance without being able to measure light travel time? A very strange situation, which I certainly don't understand; I just wonder if the whole schtick was once a frozen geometrical point with no dimensions at all.

But maybe there was a starter key?

Have a look at proposals made about the early universe. Think about a universe sans the Higgs mechanism. Maybe our tame well-described universe wasn't always around. You could start with Lisa Randall's book, or check the Cosmology Forum at this site.