[Johann Mahne]

Johann Mahne  asked the Naked Scientists:
   
If a photon has no mass but has energy and momentum .....
Then how can we trust e=mc^2  , or mom = mv ?

Cheers
Johann
What do you think?

[yor_on]

Well, what is mass?

[PmbPhy]

Johann Mahne  asked the Naked Scientists:
   
If a photon has no mass but has energy and momentum .....
Then how can we trust e=mc^2  , or mom = mv ?

Cheers
Johann
What do you think?

It seems that the terminology confuses a lot of people on this point. A photon actually does have mass. This mass is referred to as inertial mass, aka relativistic mass. The "mass" that you have in mind, the one which is zero, is referred to as proper mass aka rest mass.

[zx16]

I thought  energy and mass were supposed to be "equivalent".

So if a particle gains more kinetic energy - by moving faster - then it also gains the mass-equivalent of the energy.
Thus, as the particle goes faster, its mass increases.  Is that right?

Now, photons are particles that always move at the speed of light.  Which is the ultimate speed.  Nothing can go faster than a photon.

The photons must have colossal kinetic energy.  Caused by their speed.  So why don't the photons have the colossal mass-equivalent of their kinetic energy?

I mean, when you go out and look at things every day, do your eyes get bombarded and blasted by massive photons?

[alancalverd]

The energy of a visible photon is about 2 electron volts. The mass of an electron is 10^-30 kg or  511,000 eV. Words like "colossal" and "massive" have no place in physics.

[zx16]

If the mass of an electron is 10^-30 kg, as you say, why doesn't this mass increase to infinity as the electron reaches light -speed?

Surely it ought to.

Or are electrons different from photons.  Can a photon go at light-speed without increasing mass.  But an electron can't reach light-speed?

[agyejy]

A very non-rigorous and not at all complete explanation from the point of view of field theory goes something like this:

All particles are excitations of one of a large number of fields. In certain cases these fields couple strongly enough that we can detect  the field itself. For example the photon is an excitation of the electromagnetic field and we can detect the field by how it impacts other things. Electrons on the other hand are excitations of an "electron" field and while electrons couple to the electromagnetic field nothing couples strongly to the "electron field" in a way that allows us to measure it (other than to say that electrons exist). When an excitation in a field propagates without any interference caused by interactions with other fields it follows a very simple propagation rule. This very simple rule is the one that photons follow in free space (i.e. photons travel at one speed which is the speed of light). Now if the excitation of the field is interacting with other fields that changes the rule. It turns out that there is this field called the Higgs and when excitations (i.e. particles) interact with this field the rule that governs their motion changes and suddenly the rule gets a term sometimes called rest mass or invariant mass. The particles no longer travel at one speed but many different speeds and can no longer travel at the speed of light.
 
Basically the interaction with the Higgs field adds an extra energy term to the equation of motion for the excitation. The equation of motion without the Higgs interaction doesn't have this term. Now energy and mass are equivalent so even without the extra Higgs term there is still mass. However the mass without the Higgs term behaves differently than the mass with the Higgs term and so does the energy. In short since the electron interacts with the Higgs field and the photon doesn't the motion of the electron is different and not everything that happens to an electron in terms of energy and motion has an exact analog for a photon.   

[evan_au]

If the mass of an electron is 10^-30 kg, as you say, why doesn't this mass increase to infinity as the electron reaches light -speed?

Before the LHC was buillt, the tunnel was occupied by the Large Electron-Positron collider (LEP). This collided electrons & positrons with energies up to 200GeV.
Half of this energy would have come from the electron, and half from the positron, so let's call the energy of the electron 100GeV (around 10^-25 kg).

alancalverd mentioned that the rest-mass of the electron is 0.0005 GeV (500,000eV).
So as these electrons were accelerated close to the speed of light, their mass increased by a factor of 200,000, which is certainly heading towards infinity.

But you can never accelerate an electron (or any particle with non-zero rest mass) to actually reach the speed of light in a vacuum, as this would require infinite energy.
So electrons are radically different than photons, in that photons in a vacuum can't be slowed down below c (as measured in your laboratory).
See: https://en.wikipedia.org/wiki/Large_Electron%E2%80%93Positron_Collider

[jeffreyH]

So interaction with the Higgs field indirectly prevents acceleration to light speed.

This is via the acquired rest mass term. What if the rest mass term could be reduced?

[jeffreyH]

What if extreme gravitational potential did this?

[alancalverd]

If the mass of an electron is 10^-30 kg, as you say, why doesn't this mass increase to infinity as the electron reaches light -speed?

It does. Or at least it appears to, up to the highest energies we have achieved so far.


[

Or are electrons different from photons.  Can a photon go at light-speed without increasing mass.  But an electron can't reach light-speed?

Yes. Electrons are particles with mass and charge, so cannot be accelerated to c. Photons are quanta of electromagnetic energy, which can only travel at c.

[yor_on]

If the mass of an electron is 10^-30 kg, as you say, why doesn't this mass increase to infinity as the electron reaches light -speed?

Surely it ought to.

Or are electrons different from photons.  Can a photon go at light-speed without increasing mass.  But an electron can't reach light-speed?

Not really. The definition of a 'photon' or light is that it always is at a same propagation (speed).
There is no 'rest frame'  for it, and you can't be  'at rest' with it.
And yes, electrons contain a 'rest mass'. And it goes to 'infinity' if you try to accelerate it to the speed of light, just as you expected btw.

The 'mass' defined by Pete is not 'proper mass' per se, but a equivalence where light becomes a origin, well, as I think of it.

[PmbPhy]

I thought  energy and mass were supposed to be "equivalent".

They are under certain conditions such as the mass and energy of a photon. Equivalence means that the two are related by E =mc² where m is inertial mass and E is the sum of kinetic energy and rest energy.

So if a particle gains more kinetic energy - by moving faster - then it also gains the mass-equivalent of the energy.
Thus, as the particle goes faster, its mass increases.  Is that right?

No. The definition of the inertial mass of a free particle is the m in p = mv. If the particle is a tardyon (i.e. always moves slower than the speed of light) then it can  be shown that both E and m vary with speed. If the particle is a luxon (i.e. always moves at the speed of light) then the rest energy is zero and E =mc² still holds.

Now, photons are particles that always move at the speed of light.  Which is the ultimate speed.  Nothing can go faster than a photon.

Relativity allows for the existence of tardyons, i.e. particles which always move faster than the speed of light.

The photons must have colossal kinetic energy.  Caused by their speed.  So why don't the photons have the colossal mass-equivalent of their kinetic energy?

Not at all. See above.