What is a Zero Rest Mass Photon

[NeutrinoWave]

So the mass of a travelling photon may appear to vary by virtue of its energy if it is blue shifted or shifted.

But if a photon travelling at light speed can never be at rest, should we not say that a photon has zero invariant mass?

[JulianO]

A photon has a relativistic mass based on its energy, which is directly related to it's frequency or (inversely) wavelength. Blue shifting is just an effect that changes that frequency.

You're right though - it has no rest mass, which is why it can travel at the speed of light.

[NeutrinoWave]

A photon has a relativistic mass based on its energy, which is directly related to it's frequency or (inversely) wavelength. Blue shifting is just an effect that changes that frequency.

You're right though - it has no rest mass, which is why it can travel at the speed of light.

Agreed, so your are talking about inertial (relativistic)mass that varies with energy.

So to be precise from a physics perspective there is INERTIAL MASS and INVARIANT MASS

Rest Mass can be both misconstrued and misunderstood

[JulianO]

Agreed, so your are talking about inertial (relativistic)mass that varies with energy.

So to be precise from a physics perspective there is INERTIAL MASS and INVARIANT MASS

Rest Mass can be both misconstrued and misunderstood

Yes - and it makes my head hurt

The full eqn being

E^2 = m^2c^4 + p^2c^2

Energy = RestMass(Invariant) component + RelativisticMass(InertialMass) component

For a particle at rest WITH a mass, momentum (p) is 0, so that comes to

E^2 = m^2c^4 which reduces to the familiar E=mc^2

for a Particle without mass, this becomes

E^2 = p^2c^2

which is E=pc for a photon.

For a moving particle with rest mass/invariant mass you need both terms.

[Space Bat]

But the speed of light varies doesn't it...

You can slow it down and speed it up

[NeutrinoWave]

But the speed of light varies doesn't it...

You can slow it down and speed it up

Explain the speeding up part for me as a starter, I'm intrigued given its an absolute unit of physics.

Are you talking about different types of medium in which it travels versus a vacuum?

[smolloy]

The speed of light in a vacuum is a constant, and is the same no matter which reference frame you view it from.

The speed of light in various media can be different, and in fact it is this change in the speed of propagation that allows refraction to work.

If you've ever seen pictures of the strange blue glow coming from the water surrounding cooling nuclear fuel, this is caused by the radiation being emitted from the spent fuel moving faster than the velocity of light in the water. The fact that they move faster than the light means that their electromagnetic fields can't keep up with them, and you get an electromagnetic shockwave exactly like the sonic boom from a Concorde. This causes the pretty (and deadly) blue light.

But the initial point still stands, since all of the relativistic discussions about the speed of light are really referring to the speed of light in a vacuum.

[Space Bat]

The speed of light or shall we say a photon doesn't have a constant speed...in a vacuum.

It has a maximum speed based on......(intentionally left blank)

And the intentionally left blank isn't that hard to grasp if you think outside the box

Faster than light is possible...and for good reason as the speed of light is understood by a few to be quite slow

[Space Bat]

Explain the speeding up part for me as a starter, I'm intrigued given its an absolute unit of physics.

Are you talking about different types of medium in which it travels versus a vacuum?

Who says it's an absolute unit of physics??? Or should I say its an absolute limit of what is man's understanding of physics...?

Please don't be offended I am not trying to provoke an argument on this by in anyway attempting to ridicule people.

It's just that physics has been stunted for good reason...and there is a place - where such rules doesn't apply.

[smolloy]

Who says it's an absolute unit of physics??? Or should I say its an absolute limit of what is man's understanding of physics...?

Well, yes. But then we can only discuss our own understanding of physics! Couldn't you say that about *anything*? Let's just say that that comment applies equally to anything we discuss here, and it doesn't need to be mentioned again.

It's just that physics has been stunted for good reason...and there is a place - where such rules doesn't apply.

I got my PhD in physics 6 years ago, and since then I have worked as a postdoctoral researcher in Stanford University, lectured in physics at the University of London, and now work for one of Europe's largest particle accelerator labs. Through my research, I have had the pleasure to work at labs (CERN, DESY, KEK, SLAC, MaxLab) spread all across the world.

During this time, I have never felt that physics was in any way "stunted". Sure, there are many things that we don't know, and often we discover even more things that we don't understand, but the field is strong, and it is moving forwards. I think I will have an interesting career for many years to come

I cannot imagine what you mean when you say that there are places where physical rules don't apply, or who it is that thinks the speed of light is quite slow. Perhaps you could help me out of my "box"?

Anyway, to answer the original question that started this thread: Physicists never discuss the photon as having mass of any kind. It has momentum (E = hf = (pc)^2), but try to forget the idea of momentum being the product of mass and velocity, since that only works for big, slow-moving, objects.