# B Are massless particles truly massless?

#### Christofer Br

Most of the mass of matter comes from energy of strong force interactions between quarks. However the quarks still have intrinsic mass. Other particles have no intrinsic mass but still have energy. So according to mass-energy equivalence, these particles should still have effectively mass, to my understanding. If so, how can we label a particle as massless or not if they all have mass due to energy they carry? Doesn't mass of quarks come from energy too?

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#### phinds

Gold Member
Most of the mass of matter comes from energy of strong force interactions between quarks. However the quarks still have intrinsic mass.
Exactly. They are not massless particles.

Other particles have no intrinsic mass but still have energy.
Yes, a photon is massless but is subject to gravity

So according to mass-energy equivalence, these particles should still have effectively mass, to my understanding. If so, how can we label a particle as massless or not if they all have mass due to energy they carry?
Because they don't HAVE mass, they have energy.

Doesn't mass of quarks come from energy too?
Quarks are not massless.

#### Christofer Br

So what it boils down to is gravity acts on both energy and mass, but they're not the same thing, theres no "duality" there? Does energy resist acceleration in the same sense that mass resists acceleration (F=m*a)?

#### phinds

Gold Member
So what it boils down to is gravity acts on both energy and mass, but they're not the same thing, theres no "duality" there?
That is correct, sort of. Gravity is the curvature of space-time and that "curvature**" is a result of the stress-energy tensor of an object.

Does energy resist acceleration in the same sense that mass resists acceleration, giving it a finite value (F=m*a)?
Yes, energy "acts like mass" in that sense. If you weigh a box of gas and then heat it up, it will weigh more.

** "Curvature" is a common term for this, but really it's straight lines (formally geodesics) in space-time geometry, which is pseudo-Riemann, not Euclidian.

#### mathman

Simple distinction: Anything moving at the speed of light (in a vacuum) is massless. Otherwise it has mass.

#### mfb

Mentor
Calculate the invariant mass of a photon and you'll get zero. It has energy but not mass.
Calculate the invariant mass of an electron and you'll get 511 keV - in every reference frame. We call this the mass of the electron.

#### Meir Achuz

Homework Helper
Gold Member
"according to mass-energy equivalence, these particles should still have effectively mass,"
That is wrong, which is the basis for your confusion.

"Are massless particles truly massless?"

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