A Higgs/Relativity Question

A friend recently wondered why the discovery of the Higgs boson and thereby the verification of the existence of the Higgs field didn't help unify the quantum realm and relativity. What I think my reading was telling me is that the Higgs field and the mass it provides to particles is an intrinsic property of particles, while the mass of relative motion is an observer dependent quantity.So what the Higgs field is conferring upon particles is a resistance to change in velocity that we experience as mass, and the velocity itself doesn't matter. But isn't the observer dependent mass of relativistic motion experienced in the same way? In other words, whether a mass is intrinsic or caused by relativistic motion, doesn't any attendant gravity have the same effect?I also read that while the Higgs field is responsible for mass, it is not responsible for gravity. I would have thought that since mass is responsible for gravity and the Higgs field is responsible for mass that the Higgs field and gravity would be intimately related, but apparently not. My reading says that the Higgs is a quantum field while gravitational fields might be but haven't been shown to be as of yet. Discovery of a "graviton" would imply the quantum nature of gravity.My efforts to take what I'm reading and put it into terms I understand have high potential for resulting in gobbledygook, so apologies. Clarifying thoughts appreciated. Obviously I'm a layperson and need to have things simplified and laid out clearly for me.--Percy

Excellent question.As I understand these thing:Again we come up against the need for an effective theory of quantum gravity (QG). Other than some speculations based on string theory we have no idea what a graviton would look like. If we have nothing but speculations then we are at a dead end.We need to know what a graviton (via QFT eigenstates) would look like, act like, what energy levels correspond to what gravitational effects. We have none of this.I don't think we can answer your questions, yet. Good analogy. Appears there may be more than one Higgs boson with separate higgs mechanisms. We aren't completely sure yet. We have more to find first.A QG theory, a QG theory! My Universe for a QG theory!

First of all more than 99.9% of the mass around you is unrelated to the Higgs. It comes from the nuclear binding energy within neutrons and protons which the Higg's doesn't really contribute to. In a typical atom the Higgs is only giving the electrons mass, which are less than 0.06% of an atom's mass.Secondly, quantum mechanics and relativity function perfectly fine together, that's what quantum field theory is about. Regardless of whether field one is considering the Higgs or not quantum field theory takes care of combining the principles of quantum mechanics and relativity, both special and general. So there is no problem with the quantum particles having relativistic mass or causing spacetime curvature. For a long time we've had the ability to write down theories where quantum matter causes gravity/spacetime curvature. It doesn't really matter if that mass comes from the Higgs or nuclear binding energy or whatever.The issue with quantum gravity is a seperate one and has two parts.(i) How to make gravity/spacetime curvature itself quantum. Even this is not a major issue, there do exist quantum treatments of gravity. It's more a problem of getting experimental evidence to compare them with. So for example there is no real issue with treating the Sun's gravitational field quantum mechanically, but the differences this makes from classical gravity are minute.(ii) How quantum theory is affected by event horizons, either those of black holes or cosmological ones. Horizons restrict ones ability to make measurements, e.g. somebody outside a black hole cannot measure what is inside a black hole. Since quantum mechanics is all about what one can measure and what probabilities those measurements have, we know this affects QM in some way, but we don't know exactly how.Probably in the full theory questions like "What is the probability there are X quasars in the Virgo supercluster?" will not be well-defined since no observer could actual measure this experimentally due to cosmological expansion and the theory will instead return "undefined" as the answer. This is similar to how in quantum electrodynamics the theory returns "undefined" for the question "what is the probability an electron will be in this region" and only returns a well defined answer for "what is the probability a detector in this region will activate".Strictly speaking this would mean even natural questions one might ruminate on like "What is the chance there is at least one habitable world per galaxy in the Virgo Supercluster?" would be banned/nonsensical according to theory.

Percy gave an analogy for the Higgs mechanism as resistance to change in velocity that we experience as mass. Other analogies involve the progress of guests across the room or sheets of molasses.In your experience what analogies for the Higgs do you think are most appropriate?

This makes the unification of relativity and QM seem like a solved problem, yet the general layperson perception reinforced by layperson articles from SciAm and American Scientist and so forth is that it isn't solved, and the Internet is filled with comments like this from Why QM and general relativity cannot be unified:

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The fact that there is no mathematically obvious way to combine GRT and Quantum Field theory is only the smaller problem: the main one is that there is no experimental fact that depends on the way these could be united. Everything happens at a spatio-temporal scale which is completely inaccessible so far. So any solution conceivably discovered would probably remain pure hypothesis for at least a century.

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Confused.--Percy

It's just not well explained in most sources. "Unify" unfortunately is a vague buzzword that physicists have overused in popular accounts. I'm just going to avoid it.1. There is no problem with having quantum matter interact with classical gravity/spacetime.
Thus we have a formalism that mathematically combines QFT and GR. That quote is simply not correct literally.2. There is no theoretical problem with treating gravity quantum mechanically, there is however an experimental issue of detecting which one of these treatments is right.3. There is a deep theoretical question concerning how horizons restructure measurements and thus quantum theory.That's the current state of things. "Unify" can mean 1, 2 or 3 above depending on the writer, so you simply will get confused information online. And then add that some of those commenting are physicists who've no real knowledge of this area.

The issues between quantum theory and relativity are in the extremes. Like Newtonian motion is indistinguishable from relativity on most practical scales, so quantum theory and relativity play nice together over the vast range of actions. The disconnect in calculated outcomes is in the most extreme events. As with most things in science, the extremes of physical events is where the new physics is discovered.Yes, we can shut up and calculate but we don’t have any idea what we are doing. The hope is if we can devise a quantum theory of spacetime that explains the extremes, we can better understand why this universe has the attributes it has.

It's quite difficult. Despite the name "Quantum Field Theory", most of the fields are not physical entities you can actually measure or interact with. They're simply mathematical conveniences that could be eliminated from the theory if one so wished. They're used to make calculations easier. The Higgs field is an example. Strictly speaking there's no Higgs field in the real world.Rather Quantum Field Theory is a way of treating relativistic quantum systems. In this framework physical systems behave quite differently from classical systems and even non-relativistic quantum systems. A physical system can have particle-like properties, wave-like properties, force like properties and so on. Though in most situations it isn't clearly any of these. Particle, wave, force and so on turn out to be very simple limit situations where systems behave in a neat way. This is why in truth it would be strictly wrong to say you are "made of" atoms and particles. Your body isn't in the particle limit and so there is no well-defined notion of particle for your body. In this sense particles aren't really fundamental.In the universe's early history space acquired a charge called weak isospin. "Weak Isospin" is a physical property like electric charge. So this would be like if all of space became positively polarised, but the quantity of weak isospin is more abstract than electric charge. When space acquires a charge like this the particle limit (the limit in which systems act like particles) changes. Before space acquired a charge the limit involved massless species of particles that don't exist today. After it acquired the charge the particle limit involved today's particles, most of which are massive. This change in particles is just a consequence of the behaviour of all systems changing in response to space suddenly being charged.

Well there's no real disconnect. If you right down quantum field theory in curved spacetime coupled to classical gravity, the two are then part of a single theory that gives a single predicition for any scenario. There's no real scope for them giving different predictions.It's more that we suspect this theory will be wrong since gravity is not classical. However I should say that some people, for example Freeman Dyson, have thought and many still think gravity is classical and that the above theory is correct. It has survived all experimental checks thus far.