Do photons have mass?

[King E]

Do photons have mass?

[MigL]

Nope.
But they do have momentum.

[Ghideon]

11 minutes ago, King E said:

Do photons have mass?

An individual photon have zero invariant mass and is referred to as massless.
 

Edited May 29, 2020 by Ghideon
x-post with MigL, no need to add what I intended to write :-)

[joigus]

Photons do not have mass, although you can define mass for a composite system of two photons that are getting farther and farther away from each other.

[King E]

6 minutes ago, MigL said:

Nope.
But they do have momentum.

Doesn't momentum depend on velocity and mass?

[joigus]

Just now, King E said:

Doesn't momentum depend on velocity and mass?

Not for photons.

[King E]

Just now, joigus said:

Not for photons.

So photon only works one way, 'Its way'. 

[Sensei]

18 minutes ago, King E said:

Do photons have mass?

"Having mass" is shortcut from "having rest-mass" these days in scientific circles..

Rest-mass (aka "invariant-mass") is strictly tied to rest-frame.

https://en.wikipedia.org/wiki/Rest_frame

https://en.wikipedia.org/wiki/Invariant_mass

 

 

[King E]

9 minutes ago, Ghideon said:

An individual photon have zero invariant mass and is referred to as massless.
 

How do we know that?

Edited May 29, 2020 by King E

[joigus]

1 minute ago, King E said:

So photon only works one way, 'Its way'. 

I guess you could say that.

[Ghideon]

1 minute ago, King E said:

How do we know that?

Experimental observations matching a model with zero invariant mass. Example: If the photon is not a strictly massless particle, it would not move at the exact speed of light, c, in vacuum. Its speed would be lower and I guess it would also depend on its frequency.

[King E]

1 minute ago, joigus said:

I guess you could say that.

Does the same happen for electron?

[joigus]

Just now, King E said:

Does the same happen for electron?

No, electrons are peculiar in an entirely different way: their spin.

[King E]

Just now, joigus said:

No, electrons are peculiar in an entirely different way: their spin.

So each quantum particle has a unique set of characteristics?

[joigus]

Here's a simplified scheme:

Gauge bosons (spin 1 or 2) --> photon, graviton, electroweak bosons, gluons

Fermions: Leptons (electric charge) and quarks (spin 1/2) (fractional electric charge plus chromodynamic charge)

Fermions are weird in that they distinguish left and right also

and quarks (nuclear particles) are weird in that they can't escape to long distances because of confinement due to chromodynamic charge, similar to electric charge but far more complicated

There are more peculiarities...

Edited May 29, 2020 by joigus
addition

[random_soldier1337]

As far as my knowledge goes, photons have a zero rest mass. They can have an effective mass based on their frequency/wavelength/energy/momentum.

[King E]

Just now, joigus said:

Here's a simplified scheme:

Gauge bosons (spin 1 or 2)

Fermions: Leptons (electric charge) and quarks (spin 1/2) (fractional electric charge plus chromodynamic charge)

Fermions are weird in that they distinguish left and right also

and quarks (nuclear particles) are weird in that they can't escape to long distances because of confinement due to chromodynamic charge, similar to electric charge but far more complicated

There are more peculiarities...

Therefore I can say that, ''Quantum Particles are weird. They only work in one way, 'their way'. 

[Sensei]

5 minutes ago, joigus said:

Fermions are weird in that they distinguish left and right also

Photons also have left-hand and right-hand polarization.

https://en.wikipedia.org/wiki/Circular_polarization

 

[swansont]

20 minutes ago, King E said:

How do we know that?

The short answer is: Theory says this, and experiment confirms the theory is correct. 

Either/both aspects of this can be discussed, but it's best if you already have familiarity with the subject.

4 minutes ago, Sensei said:

Photons also have left-hand and right-hand polarization.

https://en.wikipedia.org/wiki/Circular_polarization

 

But can fermions have a left- or right-handed nature. In the standard model, neutrinos are left-handed.

10 minutes ago, random_soldier1337 said:

As far as my knowledge goes, photons have a zero rest mass. They can have an effective mass based on their frequency/wavelength/energy/momentum.

One must be careful. Mass generally refers to rest mass or invariant mass. To say that photons have mass requires one to use a different definition of mass (e.g. effective mass, or relativistic mass). When one does not pay close attention to the details, it can cause confusion.

[King E]

10 minutes ago, joigus said:

Here's a simplified scheme:

Gauge bosons (spin 1 or 2) --> photon, graviton, electroweak bosons, gluons

Fermions: Leptons (electric charge) and quarks (spin 1/2) (fractional electric charge plus chromodynamic charge)

Fermions are weird in that they distinguish left and right also

and quarks (nuclear particles) are weird in that they can't escape to long distances because of confinement due to chromodynamic charge, similar to electric charge but far more complicated

There are more peculiarities...

Fermions are weird in that they distinguish left and right also. What do you mean by that?

[swansont]

2 minutes ago, King E said:

Fermions are weird in that they distinguish left and right also. What do you mean by that?

https://neutrinos.fnal.gov/mysteries/handedness/

[random_soldier1337]

4 minutes ago, swansont said:

One must be careful. Mass generally refers to rest mass or invariant mass. To say that photons have mass requires one to use a different definition of mass. When one does not pay close attention to the details, it can cause confusion.

I don't follow. I did say photons have zero rest mass.

[swansont]

2 minutes ago, random_soldier1337 said:

I don't follow. I did say photons have zero rest mass.

And you also used the term effective mass. I was issuing a caution that, for example, if one says "mass" it usually means rest mass, unless one is being sloppy.

IOW there are plenty of posts here and web pages elsewhere, where people insist that photons have mass, because they aren't making a very necessary distinction.

[random_soldier1337]

Got it.

[joigus]

1 hour ago, Sensei said:

Photons also have left-hand and right-hand polarization.

https://en.wikipedia.org/wiki/Circular_polarization

 

Yes, sorry. I was neither clear nor thorough. What I meant is: Once a photon is "born" left-handed, it's left-handed forever, as long as it doesn't interact, and conversely. But on the other hand photons can be produced with equal probability as left or right handed. The electron that is "born" in a beta decay, on the other hand, is always left-handed. Once it's in free flight, though, you can see it as right-handed by just changing to a different inertial system, which you can't do to a photon. AAMOF, handedness of photons and electrons must be defined in different ways. For photons it's helicity, while for electrons it's quirality. Only in v --> c limit do they coincide in some sense. Please, feel free to check me for mistakes, @Sensei. +1.

Linearly polarized photons have no definite handedness, for example. Photons and fermions have very different handedness properties.

Neutrons will always "spit" lefty electrons when they decay. Electrons, on the other hand, "spit" photons every which handed way. No bias.