# Massless things

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Are there any other things/objects in our universe that are massless other than Gluons and Photons? I guess energy is massless, but it's not so much a thing, more-so a property.

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The graviton, should it be confirmed to exist, would also be massless.

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Anything that moves in a wave motion (e.g. a rope, steel chain, water etc.) would be considered massless. A wave moves energy but not matter.

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Anything that moves in a wave motion (e.g. a rope, steel chain, water etc.) would be considered massless. A wave moves energy but not matter.

No, not really. Masses are most certainly moving is a wave in a rope or water, etc., even if there is no translation of the center of mass. The wave dynamics will depend on the mass. A wave in water and a wave in mercury will not be identical given the same source.

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No, not really. Masses are most certainly moving is a wave in a rope or water, etc., even if there is no translation of the center of mass.

The wave medium has oscillatory motion that effectively results in the matter having no net displacement. As you said there is no translation of the center of mass, so the matter is not being moved only the energy. Because of the oscillatory motion there is a time each cycle when the wave has no mass.

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It will be far easier if you simply recall the basic definition of mass.

mass=resistance to inertia. Massless particles have no resistance to inertia change

A massive particle cannot ever have less than its invariant (rest mass).

So a wave of massive objects will never become massless at any stage of your wave. You can only increase its resistance to inertia change not decrease it below its rest mass.

Edited by Mordred

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Last time I checked, neutrinos were said to have a mass (tiny) because they seem to 'oscillate'. Do we have additional proofs in meantime? Is there now a firm knowledge that neutrinos have mass?... I find it intriguing that we never observed a slow neutrino directly (or did we?) - can you shortly explain why is it so hard to find a slow neutrino? (I am not sure if this is off topic.)

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Last time I checked, neutrinos were said to have a mass (tiny) because they seem to 'oscillate'. Do we have additional proofs in meantime? Is there now a firm knowledge that neutrinos have mass?... I find it intriguing that we never observed a slow neutrino directly (or did we?) - can you shortly explain why is it so hard to find a slow neutrino? (I am not sure if this is off topic.)

It is not so much that they oscillate in space - it is that they almost seem to oscillate between three different mass-types and three different weak-sorts . It is very complex and pretty weird - but there are three mass-types of neutrino (which really is to do with their mass) and three weak-sorts (which we identify through their weak interaction/production) of neutrino (electron, muon, and tau). Different combinations of mass-type give different sorts of neutrino (one neutrino can be a mix of different mass neutrinos - I still don't really 'get' what that means) and vice versa. A weak-sort is the product of a superposition of three mass-types and at the same time a mass-type is a superposition of three weak-sorts.

A neutrino which starts off - say - as a muon neutrino (we know because it was the decay product of a W boson to an anti-muon and muon-neutrino) will as it travels oscillate between forms; ie it starts a muon with a certain but unknown blend of mass-types but it will change both the blend of mass-types and its weak-sort as it travels. We can only really know either the sort of the neutrino or the mass-type of the neutrino at the same time - not both

The simplistic way of putting it is that neutrino oscillation is the changing between electron-neutrino, muon-neutrino, and tau-neutrino as a neutrino travels. For neutrino oscillation to happen - and we have evidence that it does - then neutrinos must have mass

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No, not really. Masses are most certainly moving is a wave in a rope or water, etc., even if there is no translation of the center of mass. The wave dynamics will depend on the mass. A wave in water and a wave in mercury will not be identical given the same source.

As an educational aid (mine) as the mass of the medium theoretically tends to zero does the speed of propagation of an embedded wave tend to c?

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The wave medium has oscillatory motion that effectively results in the matter having no net displacement. As you said there is no translation of the center of mass, so the matter is not being moved only the energy. Because of the oscillatory motion there is a time each cycle when the wave has no mass.

no net displacement ≠ no motion

As an educational aid (mine) as the mass of the medium theoretically tends to zero does the speed of propagation of an embedded wave tend to c?

Possibly. Wave speed in a medium like a rope is given by sqrt (T/u) where T is the tension and u is the mass per length. That's non non-relativistic; I'm not sure what a relativistic treatment would look like.

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Anything that moves in a wave motion (e.g. a rope, steel chain, water etc.) would be considered massless. A wave moves energy but not matter.

Isn't this false for other massive particles, such as electrons, which also behave as waves?

In other words: electrons are waves that move matter, regardless of how you define "move".

Edited by Bender

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It will be far easier if you simply recall the basic definition of mass.

mass=resistance to inertia. Massless particles have no resistance to inertia change

A massive particle cannot ever have less than its invariant (rest mass).

So a wave of massive objects will never become massless at any stage of your wave. You can only increase its resistance to inertia change not decrease it below its rest mass.

Yes, the medium in which the wave propagates through will certainly have a mass but not the manifestation of wave motion. An object in wave motion will have a mass equal to difference between the mass gained by wave motion minus the undisturbed mass of the medium.

no net displacement ≠ no motion

Localized motion. If you hear a sound the air molecules vibrate at the source with localized motion but they don't travel to you ear only the energy does. By definition a wave motion transfers no matter hence no mass.

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Both the medium and the medium wave will have equivalency of mass in the mass/energy relations.

$E^2=(pc)^2+(m_o c)^2$

Even a gravitational wave will generate its own self gravity via the stress/momentum tensor in the Einstein field equations.

The invarient mass $m_o$ is the minimal. Any object including waves gain inertial mass above its minimal via the stress/tensor $T_{\mu\nu}$

I wonder if the term rest mass is appropriate for what you may be trying to describe? the problem may be just terminology usage

Edited by Mordred

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Last time I checked, neutrinos were said to have a mass (tiny) because they seem to 'oscillate'. Do we have additional proofs in meantime? Is there now a firm knowledge that neutrinos have mass?...

I find it intriguing that we never observed a slow neutrino directly (or did we?) - can you shortly explain why is it so hard to find a slow neutrino?

Neutrinos/antineutrinos are detected when they interact with regular matter.

f.e. in Chlorine-based neutrino detector, there is used reaction:

$^{37}_{17}Cl + v_e + 0.814 MeV \rightarrow ^{37}_{18}Ar + e^-$

Relativistic mass/energy of neutrino capable to trigger this reaction must be equal or higher than 0.814 MeV.

If it's smaller, neutrino remain undetected. So the all neutrinos from proton-proton fusion in star are excluded by this detector!

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The wave medium has oscillatory motion that effectively results in the matter having no net displacement. As you said there is no translation of the center of mass, so the matter is not being moved only the energy. Because of the oscillatory motion there is a time each cycle when the wave has no mass.

Yes, the medium in which the wave propagates through will certainly have a mass but not the manifestation of wave motion. An object in wave motion will have a mass equal to difference between the mass gained by wave motion minus the undisturbed mass of the medium.

Not exactly as stated but agree with the second bolded but not the first. A wave is not just a point in space, and the energy of the wave itself, in total, should be constant (any dissipation aside), should it not?

Edited by J.C.MacSwell

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Both the medium and the medium wave will have equivalency of mass in the mass/energy relations.

$E^2=(pc)^2+(m_o c)^2$

Even a gravitational wave will generate its own self gravity via the stress/momentum tensor in the Einstein field equations.

The invarient mass $m_o$ is the minimal. Any object including waves gain inertial mass above its minimal via the stress/tensor $T_{\mu\nu}$

I wonder if the term rest mass is appropriate for what you may be trying to describe? the problem may be just terminology usage

Einstein's Mass-Energy is equivalence is applicable in only two special cases.

Not exactly as stated but agree with the second bolded but not the first. A wave is not just a point in space, and the energy of the wave itself, in total, should be constant (any dissipation aside), should it not?

Yes, the total energy for entire wave would be constant but those values would vary at a set point. A similar example would be a charge flowing through a medium. However, in that case the flow of electrons would contribute to added mass.

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No the energy/mass equivalence is applicable in any case where mass is being used. Its also applicable to any field or particle. Perhaps you should study GR in more detail.

Einstein's Mass-Energy is equivalence is applicable in only two special cases.

Yes, the total energy for entire wave would be constant but those values would vary at a set point. A similar example would be a charge flowing through a medium. However, in that case the flow of electrons would contribute to added mass.

incorrect on the first, correct on the second but that doesn't make something with mass become massless. As long as your medium has any form of energy it will have an energy/mass equivalence at that point. Fields can have this equivalence just as a Gravity wave can have this equivalence via the Field equations.

The second part of that statement in your quote shows you are applying that equivalence. So why the first part?

Einstein's Mass-Energy is equivalence is applicable in only two special cases.

However, in that case the flow of electrons would contribute to added mass.

Do you not see the conflict in these two lines? Change flow of electrons to simply flow of energy whatever its form

Edited by Mordred

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Einstein's Mass-Energy is equivalence is applicable in only two special cases.

What two special cases are those?

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As long as your medium has any form of energy it will have an energy/mass equivalence at that point. Fields can have this equivalence just as a Gravity wave can have this equivalence via the Field equations.

Isn't Einstein's Mass-Energy equivalence purely theoretical until certain conditions such as object traveling at least 10% of c? Gravity waves meet that condition.

What two special cases are those?

The 100% conversion of mass to energy or energy to mass.

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The 100% conversion of mass to energy or energy to mass.

That doesn't seem right: nuclear fission (or fusion) converts a fraction of the mass to energy.

Isn't Einstein's Mass-Energy equivalence purely theoretical until certain conditions such as object traveling at least 10% of c?

No.

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Isn't Einstein's Mass-Energy equivalence purely theoretical until certain conditions such as object traveling at least 10% of c? Gravity waves meet that condition.

The 100% conversion of mass to energy or energy to mass.

Some examples:

- the nucleus of an atom has less mass than the protons and neutrons separately

- a molecule has less mass than the atoms it consists of separately (at least for stable molecules)

- a charged battery has more mass than a depleted one

The last two differences are, as far as I know, not measurable, which makes it easy to forget it is there. That is also the reason that a nuclear reaction is so much more powerful than chemical reactions, because there the difference is measurable.

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That doesn't seem right: nuclear fission (or fusion) converts a fraction of the mass to energy.

Ok, so It's only the mass change that is involved. Nevertheless, there are only certain conditions where any measurable conversion is going to take place Nuclear fission requires high speed neutrons. Nuclear fusion requires extremely high temperature environment.The mass change involved in energy flowing through medium would undetectable.

Some examples:

- the nucleus of an atom has less mass than the protons and neutrons separately

- a molecule has less mass than the atoms it consists of separately (at least for stable molecules)

- a charged battery has more mass than a depleted one

The last two differences are, as far as I know, not measurable, which makes it easy to forget it is there. That is also the reason that a nuclear reaction is so much more powerful than chemical reactions, because there the difference is measurable.

Since the first one is measurable it is particularly interesting, but strange things happen at the atomic scale.

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Ok, so It's only the mass change that is involved. Nevertheless, there are only certain conditions where any measurable conversion is going to take place Nuclear fission requires high speed neutrons. Nuclear fusion requires extremely high temperature environment.The mass change involved in energy flowing through medium would undetectable.

What?

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What?

If you disagree then explicitly state why but don't ask vague questions.