# Can the speed of an electromagnetic wave be measured in the absence of neutrinos?

Let me explain better: from what I understand neutrinos are so pervasive they are literally everywhere. And since they have such a tiny electric charge they barely interact with anything and cannot be "removed" or "shielded" from an area in order to take a measurement of light speed in their absence. However, all three neutrino flavors do have some charge (see neutrino oscillation) and do interact to a very small degree.

So, my question is: if we can only ever measure the speed of light in the presence of neutrinos, could it be that the limit of the speed of light is actually the "resistance" of the neutrinos to the energy passing through it?

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Neutrinos are not charged particles... they are electrically neutral. I think you're getting the weak interaction confused with a tiny electric charge. – Kitchi Feb 20 '13 at 11:02
To elaborate on Kitchi's point: neutrinos don't have a tiny charge, they have zero charge. Neutrino oscillation is evidence that they have mass, not charge. So neutrinos and photons do not interact at all in the standard model, except for a process in which they temporarily turn into a virtual charged lepton and W boson. Such interactions are incredibly suppressed by the large mass of the W, so they have no measurable impact on light propagation. – Michael Brown Feb 20 '13 at 12:24
Well, this is straight from the Wikipedia page on neutrinos: "The discovery of neutrino flavor oscillations implies that neutrinos have mass. The existence of a neutrino mass strongly suggests the existence of a tiny neutrino magnetic moment[14] of the order of 10−19 μB, allowing the possibility that neutrinos may interact electromagnetically as well." – Andy Feb 20 '13 at 13:00
@Andy A magnetic moment is not the same thing as an electric charge. Neutral particles can have a magnetic moment. Another example is the neutron, which gets its magnetic moment from its internal structure (quarks). The neutrino would get its moment from the virtual process I mentioned involving the W's, though I didn't actually know the magnitude of it. So thanks for that. :) – Michael Brown Feb 20 '13 at 13:35
The vacuum contains virtual particles of all species. Note that there are several question one could reasonable ask in this realm. Things like "Would the speed of light be different in the absence of field-theoretical fluctuations?" or "Can QFTs explain the permitivity and permeability of free space from first principles?" – dmckee Feb 20 '13 at 14:07

Probably not, because the speed of light is constant everywhere and in all directions, but the neutrino density is essentially larger next to a star.

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Thanks @Vladimir, that's a good point. Is this quantified somewhere? Is it possible to know the density of neutrinos in different areas of space? Seems like a difficult task seeing how hard it is to measure them on earth, let alone near a star... – Andy Feb 20 '13 at 13:09
@Andy: There are estimations of the neutrino flux from the Sun, and it fades as $1/R^2$. I do not remember the numbers and I currently cannot search for them, unfortunately, sorry. – Vladimir Kalitvianski Feb 20 '13 at 13:27
@Andy: Maybe the neutrino density resembles the photon density because both kinds of particles are produced by stars. – Vladimir Kalitvianski Feb 20 '13 at 14:10

Neutrinos don't have any charge, they are neutral particle. Only they have very small amount of mass. So they don't interact with any em field. Also since there mass is very small they practically don't interact with any thing. So it is not possible to accelarate them using any em field.

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This is straight from the Wikipedia page on neutrinos: "The discovery of neutrino flavor oscillations implies that neutrinos have mass. The existence of a neutrino mass strongly suggests the existence of a tiny neutrino magnetic moment[14] of the order of 10−19 μB, allowing the possibility that neutrinos may interact electromagnetically as well."...... Anyway, I'm not talking about accelerating them, I am talking about measuring the speed of an EM wave in the ABSENCE of neutrinos, to see if the neutrinos are responsible for slowing it down. – Andy Feb 20 '13 at 13:26
c.f. my reply to your comment on the original question - magnetic moment is not the same as charge – Michael Brown Feb 20 '13 at 13:37
Nor is the wikipedia a good reference in the event of a controversy. However, one presumes that the footnote the OP keeps cutting a pasting points to something reliable. Hmmm...seems to be an outdated edition of the PDB. – dmckee Feb 20 '13 at 14:18