4 ms·

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

by narag 4y ago

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.

nimish 4y ago
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.
- tines 4y ago
  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-virtual-particles-can-have-real-observable-effects/ 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are wrong, or that I'm reading them wrong, or that the idea that virtual particles are just for calculations is outdated?
  - tadfisher 4y ago
    We discovered the mesons by photographing their tracks on plates exposed to the sky. They're real, just extremely short-lived.
  - marcosdumay 4y ago
    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.)
  - cwillu 4y ago
    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.
    - nine_k 4y ago
      If virtual particles "fit the math" the same as real particles, what makes them not real? What do they not fit?
      - evanb 4y ago
        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 https://en.wikipedia.org/wiki/On_shell_and_off_shell
      - nimish 4y ago
        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.
      - burnished 4y ago
        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
        gizmo686 4y ago
        Electron holes are quasi particles, not virtual particles.
        4y ago
  - antognini 4y ago
    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.
    - Chabsff 4y ago
      Hawking radiation has been observed? When did that happen?
      - antognini 4y ago
        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 https://www.nature.com/articles/nphys3863.epdf
        Chabsff 4y ago
        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.
        antognini 4y ago
        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.
marcosdumay 4y ago
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.
squeaky-clean 4y ago
> 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 https://en.m.wikipedia.org/wiki/Relativistic_speed
- tsimionescu 4y ago
  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".
Melatonic 4y ago
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 ?
tsimionescu 4y ago
> 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).

by narag 4y ago

nimish 4y ago

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.

tines 4y ago
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-virtual-particles-can-have-real-observable-effects/ 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?
- tadfisher 4y ago
  We discovered the mesons by photographing their tracks on plates exposed to the sky. They're real, just extremely short-lived.
- marcosdumay 4y ago
  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.)
- cwillu 4y ago
  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.
  - nine_k 4y ago
    If virtual particles "fit the math" the same as real particles, what makes them not real? What do they not fit?
    - evanb 4y ago
      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 https://en.wikipedia.org/wiki/On_shell_and_off_shell
    - nimish 4y ago
      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.
    - burnished 4y ago
      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
      - gizmo686 4y ago
        Electron holes are quasi particles, not virtual particles.
        4y ago
- antognini 4y ago
  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.
  - Chabsff 4y ago
    Hawking radiation has been observed? When did that happen?
    - antognini 4y ago
      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 https://www.nature.com/articles/nphys3863.epdf
      - Chabsff 4y ago
        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.
        antognini 4y ago
        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.

marcosdumay 4y ago

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.

squeaky-clean 4y ago

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

tsimionescu 4y ago
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".

Melatonic 4y ago

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 ?

tsimionescu 4y ago

> 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).