[Harri (OP)]

The mass of objects increases with increasing speed. This is one reason why we can't get an object with mass up to light speed? If a photon has mass does it too increase when it travels at light speed?

[Bill S]

I’m just trying to get my head round the idea of proper mass, inertia etc, in another thread; so I’m going to have a go at this, to see how I’m doing.

I may be confusing “rest mass” with “proper mass” (possibly they are the same), but let’s start with rest mass.

This is the mass of an object as measured by an observer who sees that object as stationary. We can never observe a photon as being stationary, so we cannot assign it a rest mass. 

However, the photon has inertia, so it adds to the inertia of any system of which it is a part.  Because of the relationship between inertia and mass, the photon must, therefore, contribute mass to that system.

[Halc]

If a photon has mass does it too increase when it travels at light speed?

It doesn't increase with speed since it always moves at c.  It increases with frequency, which makes the photon mass still frame dependent.

I’m just trying to get my head round the idea of proper mass, inertia etc, in another thread; so I’m going to have a go at this, to see how I’m doing.

I may be confusing “rest mass” with “proper mass” (possibly they are the same), but let’s start with rest mass.

They're the same, yes.

This is the mass of an object as measured by an observer who sees that object as stationary. We can never observe a photon as being stationary, so we cannot assign it a rest mass.

However, the photon has inertia, so it adds to the inertia of any system of which it is a part.  Because of the relationship between inertia and mass, the photon must, therefore, contribute mass to that system.

All sounds pretty good to me.

[Colin2B]

If a photon has mass does it too increase when it travels at light speed?

It doesn't increase with speed since it always moves at c.  It increases with frequency, which makes the photon mass still frame dependent.

The photon mass is always zero, the momentum varies with frequency ie energy.

However, the photon has inertia, so it adds to the inertia of any system of which it is a part.  Because of the relationship between inertia and mass, the photon must, therefore, contribute mass to that system.

Again, momentum. You can’t say a photon has inertia because it doesn’t accelerate.
Much better to work in momentum, in fact Newton first described his 2nd law as force is the rate of change of momentum, i can’t remember who reformulated it later to F=ma.

[yor_on]

Not really Halc. A photon don't change frequency or momentum as far as I know. What we have is a result of measuring by different frames of reference.. So when a 'photon' is said to 'blue shift' then that is a result of from where the measurment is taken, for example inside a 'gravity well' as earth measuring a incoming 'photon'. And if you send one out of that gravity well it's 'red shifting' losing energy as measured by you at rest with that gravity well. It has two modes, its creation and its annihilation. The creation is normally defined by a 'photon recoil', I'm not sure how that fits a 'spontaneously created photon' as by Lunds university (squid experiment) though? It's the annihilation event we measure and that one is observer dependent.

[yor_on]

Actually a photon has two things, that I know of. A momentum and a energy. The energy defined to it can be seen as a equivalence to mass, possibly :) If all mass are 'energy' that is. Turning it around you just made a case for photons having no mass, as I don't expect any theorist to be able to avoid the conclusion that it doesn't matter what real proper mass you want to give a photon. If it has one it's not 'c' as we define it any more.


okay,  it also has 'polarization,' aka spin too.

[yor_on]

Actually if you look at my first comment about red and blue shifts, it would make the 'photon' vary its speed relative the observer. And as all observers will have their own frame of reference it would then give the 'photon' 'simultaneously different speeds', depending on observer. How about letting it be 'c'?

Think of uniform motion and relative speeds to see what I mean.

[PmbPhy]

They're the same, yes.

There is a difference between them. E.g. rest mass is different from proper mass when its in a gravitational field and one measures it from far away from the source.

Inertial mass is defined by p  = mv. Since a photon has momentum it also has (inertial aka relativistic) mass.