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> The data still do not rule out the possibility that the mass is zero, says KATRIN member Magnus Schlösser, a particle physicist at the Karlsruhe Institute of

> The data still do not rule out the possibility that the mass is zero, says KATRIN member Magnus Schlösser, a particle physicist at the Karlsruhe Institute of Technology. But other lines of evidence, in particular from cosmological observations, show that the neutrino cannot be massless. I would love if neutrinos were massless, just because it would be so interesting. The only way they would interact with gravity would be through the shape of spacetime itself, which for some reason is a fascinating to me.

whatshisface 5y ago
>The only way they would interact with gravity would be through the shape of spacetime itself, which for some reason is a fascinating to me. Photons are massless particles, but alas, they still gravitate because it's mass-energy that gravitates, not rest mass.
- Koshkin 5y ago
  So… Just like the neutrinos?
  - whatshisface 5y ago
    Neutrinos aren't massless, but quite like the neutrinos because they're light. Err, unheavy.
  - pja 5y ago
    No, because neutrinos are believed to have non zero rest mass.
- 6gvONxR4sf7o 5y ago
  > it's mass-energy that gravitates, not rest mass. TIL! That's a very fun fact. I never learned that, and I have a physics undergrad! Or I forgot it, which is just as likely this far out.
  - wholinator2 5y ago
    Ive been around that too and was never explicitly stated but implied by equations once you can calculate with relativity. I mean, they never even formally introduced us to electron volts, I just figured it out myself cause I assumed I was behind the class.
not2b 5y ago
The reason we know that neutrinos aren't massless is that they oscillate between neutrino types. A massless particle must always travel at c, so it doesn't experience time, so it can't decay or change into another particle.
- pdonis 5y ago
  > A massless particle must always travel at c, so it doesn't experience time, so it can't decay or change into another particle. This is not correct, although it's a common pop science misconception. For example, photons are massless, but they can undergo interactions that, for example, produce particle-antiparticle pairs. If your statement here were true, photons would be unable to undergo any interaction at all. A correct statement would be, heuristically, that if all three neutrino flavors were massless, they would all have the same mass, namely zero, so they would all oscillate exactly the same way, so any neutrino state that started out as one particular mixture of flavors would stay the same mixture forever. For example, neutrinos that were produced in an interaction like those in the Sun, which only produces electron neutrinos, would stay electron neutrinos forever. But this would also be true if the different neutrino flavors all had nonzero mass, but all the same nonzero mass. The only way for the mixture of neutrino flavors to change as the neutrinos travel is for the different flavors to have different masses. One of those masses could in principle be zero, but only one, not all three.
  - 6nf 5y ago
    > photons are massless, but they can undergo interactions Not by themselves. They have to bump into other particles to interact. Photons in motion do not experience time and can not 'change flavour' or whatever without bumping into something else. Photon decay is not an observed phenomenon and can only happen if a photon has a non zero mass.
    - pdonis 5y ago
      > Not by themselves. They have to bump into other particles to interact. Not necessarily. There is a very small, but nonzero, probability for photon scattering (due, heuristically, to the small but nonzero probability for photons to become virtual electron-positron pairs). The probability is small, but it is nonzero, and that is sufficient to invalidate your claim. > Photons in motion do not experience time This is not correct. A correct statement would be that the concept of "proper time" does not apply to massless objects (objects that move on null worldlines) at all (it's mathematically ill-defined). But that does not mean there are not distinct events on a photon's worldline and that things cannot happen at those events even if the photon is moving by itself.
SaberTail 5y ago
Based on what we've observed, and our current standard model of particle physics, only one type of neutrino can be massless. We've observed neutrinos oscillating flavors (for example, an electron neutrino later interacting as a muon neutrino), and the rate of those oscillations suggest nonzero mass differences between the three different types. So even if one of them is zero, the other two cannot be.
- dilippkumar 5y ago
  Not a physicist, but I have questions. > Based on what we've observed, and our current standard model of particle physics, only one type of neutrino can be massless. Is the standard model complete to the point where we can predict how many types of neutrinos exist and what their properties should be? I always thought that the standard model as a set of equations (a model) that fits observed data, without venturing far into “why this model is the governing principle for our universe”. That is, it is not able to explain things like “why an electron comes with two heavier varieties”. Are neutrinos somehow different in a way that we can understand them to the point where we know things like “only one type of neutrino can be massless”?
  - deleted 5y ago
    [deleted]
  - ephimetheus 5y ago
    The oscillation mechanism that we’ve come up with (and that fits data from reactor experiments to astronomical ones pretty well) only works if the mass eigen states of the neutrinos are different from their flavor eigen states, otherwise there is no mixing. We can measure the mass differences between the neutrinos pretty well through these oscillation experiments, but this also doesn’t tell us which the mass hierarchy. It could be bottom up or the other way round. In principle, one neutrino could be massless and the mass differences we’ve measured so far would still be correct. Aside from this, pretty much anything is on the table. Neutrinos being their own anti particles? Maybe. Fourth generation of neutrinos? Could be.
  - SaberTail 5y ago
pdonis 5y ago
> The only way they would interact with gravity would be through the shape of spacetime itself I'm not sure what you mean by this. In General Relativity, gravity is "the shape of spacetime", so any gravitational interaction involves the shape of spacetime.

whatshisface 5y ago

>The only way they would interact with gravity would be through the shape of spacetime itself, which for some reason is a fascinating to me. Photons are massless particles, but alas, they still gravitate because it's mass-energy that gravitates, not rest mass.

Koshkin 5y ago
So… Just like the neutrinos?
- whatshisface 5y ago
  Neutrinos aren't massless, but quite like the neutrinos because they're light. Err, unheavy.
- pja 5y ago
  No, because neutrinos are believed to have non zero rest mass.
6gvONxR4sf7o 5y ago
> it's mass-energy that gravitates, not rest mass. TIL! That's a very fun fact. I never learned that, and I have a physics undergrad! Or I forgot it, which is just as likely this far out.
- wholinator2 5y ago
  Ive been around that too and was never explicitly stated but implied by equations once you can calculate with relativity. I mean, they never even formally introduced us to electron volts, I just figured it out myself cause I assumed I was behind the class.

not2b 5y ago

The reason we know that neutrinos aren't massless is that they oscillate between neutrino types. A massless particle must always travel at c, so it doesn't experience time, so it can't decay or change into another particle.

pdonis 5y ago
> A massless particle must always travel at c, so it doesn't experience time, so it can't decay or change into another particle. This is not correct, although it's a common pop science misconception. For example, photons are massless, but they can undergo interactions that, for example, produce particle-antiparticle pairs. If your statement here were true, photons would be unable to undergo any interaction at all. A correct statement would be, heuristically, that if all three neutrino flavors were massless, they would all have the same mass, namely zero, so they would all oscillate exactly the same way, so any neutrino state that started out as one particular mixture of flavors would stay the same mixture forever. For example, neutrinos that were produced in an interaction like those in the Sun, which only produces electron neutrinos, would stay electron neutrinos forever. But this would also be true if the different neutrino flavors all had nonzero mass, but all the same nonzero mass. The only way for the mixture of neutrino flavors to change as the neutrinos travel is for the different flavors to have different masses. One of those masses could in principle be zero, but only one, not all three.
- 6nf 5y ago
  > photons are massless, but they can undergo interactions Not by themselves. They have to bump into other particles to interact. Photons in motion do not experience time and can not 'change flavour' or whatever without bumping into something else. Photon decay is not an observed phenomenon and can only happen if a photon has a non zero mass.
  - pdonis 5y ago
    > Not by themselves. They have to bump into other particles to interact. Not necessarily. There is a very small, but nonzero, probability for photon scattering (due, heuristically, to the small but nonzero probability for photons to become virtual electron-positron pairs). The probability is small, but it is nonzero, and that is sufficient to invalidate your claim. > Photons in motion do not experience time This is not correct. A correct statement would be that the concept of "proper time" does not apply to massless objects (objects that move on null worldlines) at all (it's mathematically ill-defined). But that does not mean there are not distinct events on a photon's worldline and that things cannot happen at those events even if the photon is moving by itself.

SaberTail 5y ago

Based on what we've observed, and our current standard model of particle physics, only one type of neutrino can be massless. We've observed neutrinos oscillating flavors (for example, an electron neutrino later interacting as a muon neutrino), and the rate of those oscillations suggest nonzero mass differences between the three different types. So even if one of them is zero, the other two cannot be.

dilippkumar 5y ago
Not a physicist, but I have questions. > Based on what we've observed, and our current standard model of particle physics, only one type of neutrino can be massless. Is the standard model complete to the point where we can predict how many types of neutrinos exist and what their properties should be? I always thought that the standard model as a set of equations (a model) that fits observed data, without venturing far into “why this model is the governing principle for our universe”. That is, it is not able to explain things like “why an electron comes with two heavier varieties”. Are neutrinos somehow different in a way that we can understand them to the point where we know things like “only one type of neutrino can be massless”?
- deleted 5y ago
  [deleted]
- ephimetheus 5y ago
  The oscillation mechanism that we’ve come up with (and that fits data from reactor experiments to astronomical ones pretty well) only works if the mass eigen states of the neutrinos are different from their flavor eigen states, otherwise there is no mixing. We can measure the mass differences between the neutrinos pretty well through these oscillation experiments, but this also doesn’t tell us which the mass hierarchy. It could be bottom up or the other way round. In principle, one neutrino could be massless and the mass differences we’ve measured so far would still be correct. Aside from this, pretty much anything is on the table. Neutrinos being their own anti particles? Maybe. Fourth generation of neutrinos? Could be.
- SaberTail 5y ago

pdonis 5y ago

> The only way they would interact with gravity would be through the shape of spacetime itself I'm not sure what you mean by this. In General Relativity, gravity is "the shape of spacetime", so any gravitational interaction involves the shape of spacetime.