Physicist: Classically (according to Newton) kinetic energy is given by 
The difficulty comes from the fact that Newton’s laws paint an incomplete (and ultimately incorrect) picture. When relativity came along it was revealed that there’s a fundamental difference in the physics of the massive and the massless. Relativity makes the (experimentally backed) assumptions that: #1) it doesn’t matter whether, or how fast, you’re moving (all physical laws stay the same) and #2) the speed of light is invariant (always the same to everyone).
Any object with mass travels slower than light and so may as well be stationary (#1).
Anything with zero mass always travels at the speed of light. But since the speed-of-light is always the speed-of-light to everyone (#2) there’s no way for these objects to ever be stationary (unlike massive stuff). Vive la différence des lois! It’s not important here, but things (like light) that travel at the speed of light never experience the passage of time. Isn’t that awesome?
The point is: light and ordinary matter are very different, and the laws that govern them are just as different.
Light and Matter: different
That being said, in 1905 Einstein managed to write a law that works whenever: 
For light m=0, so E=Pc (energy and momentum are proportional). Notice that you can never have zero momentum, since something with zero mass and zero energy isn’t something, it’s nothing. This is just another way of saying that light can never be stationary.
Also! Say you have an object with mass m, that isn’t moving (P=0). Then you get: E=mc2 (awesome)!
Unrelated tangent: It took a little while, but the laws governing the massive and the massless are even more inter-related than the ‘Stein originally thought. He figured out that the energy of a photon is related to it’s frequency (E=hf), but why are photons so special? Why do they get to have frequencies? They’re not special. Years later (1924) de Broglie drew the most natural line from Einstein’s various equations from light to matter. 
On the off chance that anyone out there got unduly excited about that last statement: the frequencies never go out of wack, you can’t tune them, more importantly they are utterly unimportant on the Human scale, or even the single-cell scale, and don’t ever buy a bracelet or anything else with “quantum” in the name.
No, no, no, no, no, no, no, no, no.
Hi,
An interesting article, indeed! What I’d like to ask in relation to photon mass, m=0, is how does this get explained in terms of the bending of light, through gravitational lensing? How can photons not have a mass as they have a dual nature–particle and wave? How can gravitational fields have any effect upon photons not having any mass? A useful link to look into where mass is defined according to its seven attributes mentioned therein: http://en.wikipedia.org/wiki/Mass.
Thank you.
Gravity doesn’t only affect particles with mass.
GR version (a): Matter/energy (same thing) changes the distance between events.* Since light always always always takes the shortest distance possible, gravity changes the trajectory light follows.
*Events are locations + times.
GR version (b): There’s no difference at all between accelerating upward and being stationary in a gravitational field pulling downward. Since acceleration will cause you to perceive light to bend, standing in a gravitational field will cause you to perceive light to bend. Therefore, gravity acts on massless as well as massive particles.
Hi,
I have some doubts regarding this topic so, please help ASAP.
(i)Here ‘m’ in energy expression is rest mass or is it gamma*m? And is this expression of total energy of the system i.e. including all types of Potential energies, nuclear energies, etc etc.
(ii)Let a body is in a free fall which was initially moving with velocity 0.4c so, how we will have to find the velocity at distance h below the starting point? And also tell what is the force due to gravity rest mass*g or gamma*rest mass*g and what is th corresponding acceleration of the body.
thanx
(i) rest mass. E here is more of a kinetic/rest-mass energy. All other forms of energy are tied up in the rest mass. But rather than figuring out how much energy is tied up in nuclear and chemical potentials, you can just weight the matter in question. For example Uranium literally weights more than an equivalent number of protons an neutrons, because of its binding energy which (M=E/C^2) actually does all the important stuff that matter does. Specifically, weighing more than nothing.
(ii) “gamma*rest mass*g”. The acceleration is a bit trickier. It kinda depends on who’s asking: the thing falling, or something standing on the planet that it’s falling toward.
But,shouldn’t the force of gravity be just rest mass*g(as that’s what the actual matter is) and to find acceleration we must equate it to gamma*rest mass*a. Please tell regarding both the frames- body itself and the planet. So, what about the energy conservation to find velocity at the bottom?? Would that also be different, please tell by that method also.
thanx a lot for such a spontaneous response!
In the mass’ own frame it thinks it’s gravity is dependent only on its rest mass. However, once it gets moving that’s no longer the case.
Energy conservation is maintained, and is probably the best way to calculate how much speed increases.
However, I’ve never done the calculation myself, and I’d feel nervous directing someone else.
do you can calculate virtual photon mass in C.M frame?we have the c.m energy of e-e+ collision in e-e+—>phton-photon process
There are many pot holes which occur in todays science, and you have just stepped into just one of them.
Here is another which may explain why we are oten confused.
It seems strange that Universities and schools are still teaching our new physics students who are travelling at incredible velocities on a moving planet, the wonders of a stationary 3 dimensional quantum atomic model.
A question for The Physicist:
Frank Close, in “The New Cosmic Onion” says (Table 4.3 Yukawa and the Pion):
“A photon has no mass, so it is possible for the photon to carry no energy at all. In this case it could voyage for infinite time and transmit the electromagnetic force over infinite distance.”
How is it possible to reconcile the contradiction that a photon “has energy” but can “carry no energy at all”?
The amount of energy that a photon carries is proportional to the frequency of that photon. The photon’s frequency can be arbitrarily small. However, the smaller the frequency, the more uncertain the photon’s position. At zero frequency, the photon has no definable position, and even worse, no definable emission time.
In general, nothing has a definite frequency. It’s better to think of every photon as having a “frequency envelope”; a set of frequencies simultaneously. This is actually a requirement of the uncertainty principle. While one of those frequencies can be zero, there are a lot of problems with there being only one frequency (especially if it’s zero).
I’m told a photon contains no mass. However all photons examined, I think, are said to contain no mass because the scale is not large enough. Most photons seem to be lab created or created on Earth for the purpose of X-rays.
If one examined a photon on a much larger scale…say…the Sun’s corona or the photons released in a nuclear explosion….then could a photon demonstrate that is has mass? And with the new scientific information about the expansion of the universe being at a faster rate than once thought couldn’t the photon now be seen differently?
Especially if factoring in a value for the speed of light, C, which may be subject to change due to the faster accelerating universe?
Thanks for any information or thoughts.
The weight of anything has two contributions, a) the rest mass and b) the contribution from binding energy. So if we weigh a hydrogen molecule we get the weights of the particles and a weight contribution from the stored electromagnetic and nuclear energies. Similarly as light passes by a planet, the planet is effectively “weighing” the light. The photon rest mass is zero and so the weight contribution is from effective mass. (Gravitational mass and inertial mass being equivalent). It carries momentum is the same “effective” way, so although it has no rest mass, it has both weight and momentum. When an excited atom emits a photon, the atom recoils. Photons also scatter free electrons which means they exchange momentum with them.
Is Keith Allpress a mathematician or a physicist?
Thanks
Don’t know.
So….If the photon has “effective mass” then it should be considered to hold
mass in the broad sense of the word mass. My point in this endeavor is to get
the world to stop saying photons are “massless.” You may have proven my point,
effective mass as you say……….conjures up a sense of mass…therefore the photon isn’t
mass-less. Am I right here? Thanks for any support on this…the photon certainly behaves as a mass-holding object. Thanks again.
Wait!
Photons definitely don’t behave like “mass-holding objects”, although it’s hard to tell the difference between massive and massless things outside of the language of relativity. Specifically, there’s a universal law called the “Energy-Momentum relation“: E2=m2c4+p2c2, where m is the “rest mass”. For light we find that E=pc, always. That is, we find that m=0 in every experiment concerned with light, regardless of source.
More than that (again, this is relativity), objects with mass can have a variety of velocities, including zero, and light never travels any slower than exactly 299,792,458 meters per second (light speed!).
The physicist “light never travels slower than…..” Do you mean faster?
Or if you did mean slower, should you add ‘in a vacuum’?
Nope!
Light speed is always the same, even in materials. Rather than just slowing light down, a material absorbs and re-emits light over and over, which leads to a lower average speed. But in between those absorptions, the light is screaming along at the usual speed.
“For light we find that E=pc…”
Wait…isn’t momentum = mass*velocity? So for light, E=(mc)c=mc^2? If something has no mass, how can it have momentum?
That’s the incomplete picture that Newton’s laws paint; momentum is p=mv for objects with mass, but for massless objects (like light) that equation is invalid. All you can say is that E=pc.