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Comment by narag | Hacker News Reader

The proton is a collection of quarks and gluons moving at relativistic speeds around a central point.

What? Moving at relativistic speeds? I had never heard that.

"Each of these particles, or “nucleons,” is composed of a dense, frothing mess of other particles: quarks, which have mass, and gluons, which do not. Yet the quark masses only add up to a mere 1% of a proton or neutron’s mass, with the bulk of the proton mass coming purely from the motion and confinement of quarks and gluons."

https://physics.aps.org/articles/v11/118

So the interior of baryons is analogous to the interior of atoms, with gluons instead of electrons dancing frantically... weird.

Also the part about mass being generated by motion and how it seems to be an established fact.

Not really. The "roiling sea of particles" is a metaphor for perturbation series, which very specifically does not work for the strong force at low energies.

Take anything involving virtual particles as just that, virtual. They're an aid for computation and cannot be observed directly. They aren't necessary either; lattice gauge theory is always applicable if not practical.

The mass(-energy) being from the strong interactions is still true. And the residual bit of the strong force between protons and neutrons works with the virtual particle/perturbation theory approach pretty well, using pions.

I'm glad you brought this up, because I've been reading some stuff lately that makes it seem like virtual particles actually have visible effects, like this one:

https://www.forbes.com/sites/startswithabang/2019/07/12/yes-...

So is it that these articles are wrong, or that I'm reading them wrong, or that the idea that virtual particles are just for calculations is outdated?

Incomplete analogy that's probably a better starting point: if the tone of a bell is a particle, then the other movements of the bell that don't resonate are virtual particles. Now, imagine we called the thermal motions of that bell “virtual tones”, and you have an idea why physicists always sigh and emphasize that virtual. particles. are. not. particles.

If virtual particles "fit the math" the same as real particles, what makes them not real? What do they not fit?

The more correct idea is that particles aren't fundamental. There are fields, and a disturbance in one field will cause a disturbance in the fields that the first couples with.

Most chains of disturbances die out really fast (“virtual particles”), but some combinations will resonate for a significant amount of time before they die out (“unstable particles”) and others don't die out at all (“stable particles”).

And if the lifetime of a disturbance is fairly short compared to the frequency of the disturbance itself, it becomes hard to even make a solid distinction between those types, but again, that's a flaw caused by imposing a categorization scheme (“particles”) based on something that isn't fundamental.

What makes particles travel long distances as lumps is that they are on-shell. Virtual particles are not on shell.

https://en.wikipedia.org/wiki/On_shell_and_off_shell

An example that comes up in semiconductor is the 'hole' virtual particle. It is the absence of an electron. It is not a real and independent phenomenon. But you can treat it like a particle just fine

Electron holes are quasi particles, not virtual particles.

Ah hell, guess I have to start treating the stuff I learned in undergrad into the unreliable narrator category. Thanks for the correction!

Virtual particles are not particles.

Virtual particles are all the disturbances in a field that don't behave like particles do.

It's really a terrible name to have entered the lay vocabulary: “virtual memory is something that behaves largely like real memory” is _exactly_ wrong, it's more like “virtual memory is all the circuitry that doesn't perform any memory function, but is still made of silicon”.

You don't _need_ them, and IIRC _can't_ (in general) use them if you deal with non-perturbative effects, unless you do stuff like re-summing infinite series of diagrams and that really makes interpreting the ultimately observable effect in terms of virtual particles interacting difficult.

My impression is that the notion of "virtual particles" is a bit of an outdated concept. There isn't really as much of a physical distinction between "real" and "virtual" particles as there is sometimes made out to be. All particles are excitations of some underlying field. Generally speaking, these excitations can have some resonance that allows them to persist for long durations. This resonance occurs when the particle is "on the mass shell" in the jargon --- that is, it has a rest mass that is equal (or at least extremely close) to the observed mass of the particle. Excitations that are "off the mass shell" decay exponentially. But these other excitations do have real observable effects. The Casimir effect is the most famous, but they're also responsible for the Lamb shift and Hawking radiation which have also been observed.

No, it's a pretty well-defined concept when you stick to its technical definition as an aid in interpreting terms of a perturbation series as Feynman diagrams.

> these other excitations do have real observable effects

Yeah, that's the major thing: virtual particles explain observable effects in a sort-of intuitive way.

But you could (to my knowledge) get the exact same results without involving any virtual particles, via lattice gauge theory. Since you get the same observable results without them, virtual particles, IMO, shouldn't be considered fundamental to any effect, even if they make the explanation a lot easier.

Anything involving complicated interactions with relativity like Hawking-Unruh stuff has an even bigger issue since the notion of a particle/vacuum is observer dependent.

All post-classical effects in quantum field theory arise from various "corrections" that come from what you could define as virtual particles, whether it's from the perturbative treatment or a numerical lattice gauge calculation. In the latter, you need to (formally) sum over all possible field configurations, the majority of which will contain half-witted weird off-shell "particles" and the entire spectrum between stuff you'd never see as a particle and the classical resonances.

It is more clearly visualized in a perturbative expansion, for sure, but it's a bit disingenous I think to argue that there are no virtual particles in a lattice calculation.

Hawking radiation has been observed? When did that happen?

It's been observed in sonic black holes, which are mathematical analogs of gravitational black holes in fluids (though to be fair the experiment has been disputed, so the evidence is not absolutely unambiguous):

https://www.nature.com/articles/nphys3863.epdf

Thanks for the clarification.

I don't have the background to be confident about this, but aren't the predicates on which Hawking Radiation is based on part of the equivalency framework between sonic and "real" black holes?

If so, then while the observation of Hawking radiation in the model is certainly interesting, calling it an observation of Hawking radiation with regards to real black holes sounds like a stretch.

I agree that observations from analog black holes don't rise to the level of definitive proof of the existence of Hawking radiation around gravitational black holes. But what has been observed demonstrates that a system whose behavior is described by the equations that we believe to be correct will exhibit Hawking radiation.

Yeah, the 'Standard model of particle physics' actually states that particles don't exist and there are only fields around.

What a bad piece of journalism, intent on confusing instead of explaining.

Virtual particles were invented because they have measurable effects. Physicists don't go around inventing invisible things for no reason. What they are not is "particles". The particle facade is only there because it fits the math.

(The article seems to be describing an experiment that measured energy-time uncertainty.)

We discovered the mesons by photographing their tracks on plates exposed to the sky. They're real, just extremely short-lived.

> Also the part about mass being generated by motion and how it seems to be an established fact.

That's what they mean by relativistic speed. When effects from special relativity become large enough that you need to account for them in your math and measurements. There is a difference between invariant mass (aka rest mass) and relativistic mass, which depends on the object's velocity relative to the observer.

https://en.m.wikipedia.org/wiki/Relativistic_speed

While your description of what relativistic speeds means is accurate, this has nothing to do with the mass.

Instead, composite "particles" have mass mostly because of the energy of their components. This is a famous observation (the most famous by far) in general relativity: E=mc². E here can be the kinetic energy of the constituent particles, or some other kind of energy (for example, a polar molecule like water owes some of its mass to the electrical energy of the bond; and its mass will increase or decrease if placed in a strong electric field, depending on the orientation of the field).

This has nothing to do with the concept of "relativistic mass", which is anyway not commonly used in modern physics anymore. Mass almost always refers to "rest mass", and observer-dependent "relativistic mass" is accounted for only through differences in observer-dependent time and position measurements. That is, instead of saying "as an objects speed approaches the speed of light, its relativistic mass approaches infinity", the preferred interpretation is "as an object approaches the speed of light, time passes more slowly for it, so it takes longer for it to accelerate even if pushed with a constant force".

> Also the part about mass being generated by motion and how it seems to be an established fact.

It's not motion per se, it's energy of any kind. And it's also probably the most famous equation in all of physics: E=mc² (so, m = E/c²).

You meant gluons instead of photons? Because the things analogous to electrons and the nucleus are the quarks.

Anyway, the similarity is only on the level of "it's a bunch of moving things locked together by a force". Those things are about as similar to themselves as they are to planetary motion.

I have no good background in this but could it not be that from an outside observer view the motion appears as mass but when thought of modeled within it is instead energy/motion ?

The Strong Force and Strong Nuclear Force are so utterly fascinating to read about. Gluon Flux Tubes! Seems like pure science fiction and imagination.

This article was around recently: "Inside the Proton, the ‘Most Complicated Thing You Could Possibly Imagine’"

https://www.quantamagazine.org/inside-the-proton-the-most-co...

https://news.ycombinator.com/item?id=33262637

I'm not an expert, but I'll try to give a friendly explanation.

Consider a photon. We're all pretty familiar with how these work. Photons are light, and move at c. Photons also crucially don't have any mass, which is why they can move at the speed of light-- nothing with mass can move at c.

Photons are energy carriers for the electromagnetic field. They transfer electromagnetic energy from one particle to another. There's several quantum fields permeating the universe, and each has its own energy carrier particle[0]. These are known as bosons, or sometimes force mediators.

Atoms can influence each other through the electromagnetic field. An electron in one atom can drop to a lower energy state (or orbital), and releases a photon with all the energy that the electron "lost". That photon can bump into another atom, which causes one of its electrons to jump up by the same amount of energy.

Protons are made of quarks, which are held together with the strong nuclear force. The energy carrier for the strong nuclear force is called a gluon. Gluons bounce back and forth between the quarks, transferring energy for the strong nuclear force. This energy is used to pull the quarks together[1]. Again, because the gluons don't have mass, they can (and must) move at light speed. The quarks themselves do have mass, and they vibrate and wiggle around, but only at sub-light speeds.

Particle physics is really weird at first glance, but it makes a certain kind of sense once you learn a bit about it. It also makes less sense the more you learn. You've been warned.

[0] Except gravity. Maybe. We've theorized, but haven't observed the graviton particle mediating the gravitational force [1] gluons keep quarks a certain distance apart, sort of like a spring

I think that just means "moving fast enough that relativity is significant and has to be accounted for". It doesn't mean they are right at the speed of light (although they could be, I don't know). You could call a rocket "moving a relativistic speeds" because relativity has to be included in the calculations for accurate missions within the solar system.

Gluons are massless so they must move at the speed of light. However they are strongly interacting with quarks and themselves so their worldlines will be a crazy mess of intersections.

I wonder what the cut off speed is for relativistic speeds. If you think about it, it's not only a matter of speed but also how much precision you want in your calculations. Even if something was moving very slowly, you need to account for relatively if you want very accurate calculations.

You also get some relativity with some elements. Gold is the one that I recall most readily.

Relativistic Effects and the Chemistry of Gold - https://link.springer.com/content/pdf/10.1007/BF03215471.pdf

> In atoms of high nuclear charge (Z), as a consequence of a relativistic effect, the s electrons of an atom become more bound and their orbitals smaller than if this effect were absent. Simultaneously, the d (and f) electrons are less bound because of this effect, which scales roughly as Z2. Gold exhibits a large relativistic effect. This accounts for gold being more resistant to oxidation than silver. It also accounts for higher oxidation states being more accessible in gold than in silver. These effects are illustrated by some fluorine chemistry of gold and silver.

https://en.wikipedia.org/wiki/Relativistic_quantum_chemistry

> Relativistic quantum chemistry combines relativistic mechanics with quantum chemistry to calculate elemental properties and structure, especially for the heavier elements of the periodic table. A prominent example is an explanation for the color of gold: due to relativistic effects, it is not silvery like most other metals.

A rule of thumb in classical mechanics is that 0.1c is when you should start taking (special) relativity into account, I think it’s 0.5% contribution then.

For general relativity I don’t know such a cutoff rule of thumb. Astronomy-wise, Mercury is the only planet that is obviously general-relativistic (its orbit is not an ellipse because it’s so close to the sun). On Earth, we don’t have strong/inhomogeneous enough gravity, so unless you’re synchronizing satellites or atomic clocks, GR is not something to worry about.

Mercury's orbit is absolutely an ellipse. It is incredibly elliptical, with an apogee of 69.8 million km, and a perigee of 46 million km.

The relativistic effects on Mercury concern its precession - the way that elliptical orbit rotates [1] around the sun. And it's not caused by Mercury's speed (Which is only ~59 km/s at its maximum, compared to the Earth's 30 km/s). It's caused by spacetime being curved by the immense gravitational field of the sun.

If Mercury had a circular orbit, it would have no precession.

[1] Precession is akin to spinning a hula hoop around your body - with the hula hoop representing an orbit. https://en.wikipedia.org/wiki/Apsidal_precession

Some of the most precise clocks can actually measure precisely enough that a few feet of elevation change makes a measurable difference, which I expect also means that what you've placed under the table it's on also could.

I think a good rule of thumb is 'check what accuracy you need your answer to, and then include all effects relevant to that precision'. If you needed an answer accurate to four significant figures, then you'd include relativistic effects for v < 0.1 c

I don't think that fits the definition of rule of thumb.

As a rule of thumb, I like to define my definitions first. /s

Even if something isn't moving at all (relative to your reference frame of choice), you might still need to care about local gravity's effect on time.

The specific composition of baryons is fairly niche physics. Assuming you're not educated in quantum physics, there's as much reason to expect to know about that as there is to expect a non-programmer to know about the minutia of different manual memory management strategies.

For a back of the envelope calculation we consider the uncertainty principle

Δx*Δp >= ℏ/2

Δp = Δ(mv) >= ℏ/(2Δx)

m = 9.1 * 10 ^ -31 kg (mass of electron)

x ~= 1 * 10^-15 m (radius of proton)

ℏ = 6.6 * 10 ^ -34 kg m^2 / s (plancks)

Δv >= 31 524 512 m /s

Which is about 1/10 the speed of light. There isn't a true cutoff for "relativistic speeds" but in general, 1/10th counts.

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