# Which is the lightest thing in this universe? Is that a photon or neutrino?

I hear a lot of people saying that neutrino is the lightest subatomic particle but according to me a photon must be the lightest as nothing can travel faster than light because it gets heavier and heavier and bends time.

• If you are thinking of the PHOTON as the particle of light, then yes, it is massless (the proton is far heavier than the neutrino and the photon). But it is not a matter particle, it is a force carrier, so people don't usually include this in "subatomic particles". – ACuriousMind Jun 17 '14 at 16:59
• The upper bound on the mass of electron neutrino is around 2 eV. The upper bound on the mass of the photon is about $10^{-17}$ eV. To the best of our current ability, photons appear to be lighter than some neutrinos. – Kyle Kanos Jun 17 '14 at 17:06
• The above two comments are the best answer so far! – innisfree Nov 23 '15 at 11:59

There's a subtle distinction here, which is that neutrinos are matter particles (having spin $\hbar/2$ and obeying Pauli's exclusion principle) while photons are force carriers (having spin $\hbar$ and obeying Bose-Einstein statistics).

There are three flavors of neutrino and they all have different masses. Therefore at least two of them are massive; whether the lightest neutrino is massless is an open question.

Among the force-carrying particles, the photon, gluon, and (hypothetical) graviton are all identically massless.

Light particle is a vague term actually. There are two ways I can think of a particle being "light".

1. If you mean has least mass than photons have zero mass so it is indeed lightest. The exact mass of lightest neutrino is not yet known.
It is actually wrong to talk about "A photon" because it is not a single particle. Photon is a "quanta" of electromagnetic waves(or light) and it not, strictly speaking, correct to call it a"particle" of light thought it has properties similar to particles.
2. If you mean which particles does not interact with other particles then the answer is different. Even though photons have no mass but they have a momentum. So when light bounces off a surface it exerts a force. $$p=\frac{h}{\lambda}$$ It means that light with lower wavelength, $\lambda$, will have higher momentum, $p$.
So momentum of photon depends on it's energy and can be greater than momentum of a neutrino.
• So electrons are not particles because they are quanta of the electron field? – jinawee Nov 23 '15 at 10:46