Well, virtual photons can have mass.

This can be verified by calculating the invariant square of a virtual photon's four vector in a suitable electrodynamic process, such as pair annihilation. During that time, since the four-momentum is conserved, the photon will appear to have mass.

of course, all this is cheating, in a sense. Such a particle is virtual by its very nature and could never be observed.
Personal pet peeve: I hate when they claim such things can be "explained" (or even just "understood") if you invoke the uncertainty principle.
Yeah, sure. But I don't feel comfortable arguing physical basic principles on account of that.

~~



A photon at rest?
That doesn't seem possible.
Don't go all "it's own rest frame" on me -- the principle you want to invoke (every observer is equal) is nice, but you can't use it if it conflicts with the principle of seeing light always move at c. Which means, seeing photons always move at c.
So said photon (at rest) would need to see itself moving with c. That's a contradiction.
I don't think there's such a thing as a rest frame for a massless particle (since that'd mean it moves with c).

Now, Joey is still right in saying that photons don't have mass. They also don't have mass if they move (which, uh, they always do). That does not mean photons are unable to have momentum. In fact, things get easier for them. E=p*c implies p = E/c. There you go! No need to invoke mass.
In case you want to use this for your homework, note that E=p*c is only the special case of relativistic energy-momentum in case of mass=0. E=sqrt(m^2*c^4+p^2*c^2) gives you the above if you set m=0.

To see if one is an honest scientist or a charlatan,
you just need to ask him in that panel-speech:

"How could you falsify your theory?"

if the answer is

"It can not be falsified beacuse it is true"

then you quickly make out the charlatan.

Yet we all secretly hope that's the answer with FTL neutrinos

From Wikipedia

quote

In physics, a photon is an elementary particle, the quantum of the electromagnetic interaction and the basic unit of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force. The effects of this force are easily observable at both the microscopic and macroscopic level, because the photon has no rest mass

unquote

Saying that photon does not have rest mass does not mean that you can catch the light
It must be understood that photons are the only entities having only the mass due to its kinetic energy
Material particles, such as electrons ,have both rest and dynamic mass

in classic mechanics momentum = mass x velocity thus the claim photon does not have mass but it has a momentum would not simply make any sense

The definition of momentum and mass have however changed to keep up with the new experimental evidences
in relativity momemtum is

P = k x Mo x V

Where k = 1/sqrt(1-(v/c)^2) and Mo is the rest mass
if a photon is also a particle then Mo = 0 since v = c
While p = 0/0, the equation falls into a singularity
The momentum of the photon can not be therefore calculated

Einstein to explain the photoelectric effecte assumed following Planck , that

E = hf

Where E is the kinetic energy of the photon
, from which you can get P and M

Besides, in his initial formulation of the equation, Eistein wrote

Mo = L / C^2

Where Mo is the rest mass and L the kinetic energy
Einstein did not use the letter E even though it was also popular in Germany at the time ,to indicate a generic form of energy
Letter L stands for LebenKraft which specifically mean kinetic energy i.e a mass related form of energy

if a body absorbs ( emits ) a radiation of energy L then the body absorbs (loses) the mass L / c^2
Thus photons in motion have mass
Even though Einstein never said that an eletromagnetic radiation is "massive" this was implicit in his words

Summing upr the characteristics of a photon were

rest mass Mo = 0
energy E = hf
mass M = hf/c^2
momentum P = hf/c

If highly energetic photons collide , their direction change same as they were material particles
Diffusion of light was definitley excluded by classic Maxwell's equation

Ray of light are deviated by the gravitational field of the sun same as material particles

The scenario changed again with modern quantum physics
Actually some other experimental evidences are not consistent with the assumption of photon havin a mass in motion even though the "dynamic" mass related properties of the phorons can not be ignored thus:

virtul particles popped up and they have... mass

This is what in my opinion is reasonable,the rest sounds as bullshit even though I wonder whether a university may ask a charlatan to hold a conference

First of all matter is not a scientific term let's say mass instead
There are two interpretation of the Einstein's equation which are known as the weak and the strong claim

Einstein wrote
M = L / C^2
Where L is the kinetic energy only
He did not write
E = M * C^2
where E is a generic form of energy

in both cases M is the rest mass

From a math wiewpoint it is the same stuff but from a physical viewpoint it is not
Einstein assumed, at least at the beginning , that if a body absorbs or emits kinetic energy then absorbs loses mass but he did not exclude that a body may have a residual mass regardless of the content of energy
This is the weak claim

The latter equation is the modern form
In this interpretation the whole rest mass can be turned into energy
This is the strong claim which is accepted by most scientists but not all

An other controversial topic is : is a radiation also matter ?

let's assume that matter is a synonimous with mass
This is what Newton thought

Matter annihilation , by definition, means that an entity having a rest mass such as electron or proton turn into entitity which dont have a rest mass i,e a photon
However photons have also dynamic properties similar to material particles
Scientists say that photons do not have mass but they have a momentum

If the answer to the above question is : yes a radiation is a form of matter then this guy is right
By the way many scientits disagree with this claim but it is not a bullshit

Now that is a really interesting topic! Kinda suprised that there hasn't been a topic on it.

Personally, I'm still skeptical of it. Relativity has passed so many tests by now that it seems unlikely to be wrong. But, of course, experiments always win -- if they reliable measured it, then we need to tweak the theory. Exciting!

But, again, the relative difference to the speed of light is very small, and the errors given in the paper can't explain them, it might just be that they've "forgotten" effects, or perhaps underestimated others.

What I'm saying is: To overturn a huge, established theory such as relativity, it takes more than a single experiment, I'm afraid.

Of course, neutrino-experiments are notoriously difficult anyhow...

What do you guys think about it?


~~



From what I've been told - and mind you, it hasn't been long since then, is that the modern interpreation is that an object, any object, does only have it's "intrinsic" mass, that is, it's mass is ALWAYS it's rest mass.
Of course, for a massive particle it is:

E=\gamma m c^2

causing people to "define" \gamma m as some kind of "relativistic mass". This, however, hasn't proved to be very helpful or enlightning, and thus, nowadays, most people do not use that terminology anymore. Contrary to the above, the formula

E = mc^2

is unfortunate, since E here is *NOT* the total energy, but the "rest energy". And that term is, for a massless particle, such as the photon, simply zero.

The history of physics, and relativity, is interesting, I give you that, but I feel that this is a different discussion. "Do photons have mass?" is a physical question, and there's no need to invoke history to answer it. The answer is, quite simply, no. (Real) Photons are completely massless.

This can be seen and shown in theory (such as in Quantum field theory, but we find the same in relativity -- in fact, the very fact THAT photons move with c means they cannot HAVE mass). It is the same in relativity (see my post above). It also does not apply to the Photon in the Higgs-mechanism, which, if true (News on Tuesday, everyone!), would be further proof of this.

Of course, you can say that any energy could technically be turned into mass (as you later in your post do). That is true, but unless we observe that happen, it does not seem likely. Photons, in particular, seem perfectly happy with just having a frequency

Let's nitpick a little.



Well, again, photons do not have mass. But they're not the only massless particles. Gluons don't have mass, either, at least not in the standard model. Of course, no one has ever seen a free gluon (since they have color, they instantly hadronize), but there's very strong evidence for their existance (3-jets).





Wait. What? I'm sorry, I don't think I understand what you're trying to say with this sentence

I simply replied with a silly sentence to your silly remark
" a photon at rest ? "
I dont know whether for a native english speaker the expressions "mass at rest " and "rest mass" sound completely diffrent
For me it is the same stuff
I suppose that nobody on the surface of the earth think that you can stop the light ,thus I found your comment a useless little provokation
Yet the term "rest mass" even for a photon is widely used in scientific literature ( ex wikipedia )
Why not simply : "a photon has no mass ", then ?
If some authors emphasized the term "rest" even for a photon there is a reason , I suppose

This is the point , I was expecting your comment while you are asking again the same question

Not to mention that I myself said that in modern quantum physics photon is considered a massless particle
I said "The scenario has changed again.."
Also I said that the mass in the Einstein's equation must be understood as the rest mass
Please read the other people post too,besides reminding the others to read yours

Anyway the discussion was about a possible interpretation of energy and matter

I supposed that it is more interesting to go through the evolution of these concepts in the hystory of physics rather than jumping straight away to the standard model
It is true that the "kinetic" mass in general , as well as the one of the photon is not considerd anymore a , so to speak , "true" mass but is not necessary to evoke the standard model
Yet, it is also out of discussion that photons share with material particles many "massive" properties
If the photon of light push away ( together with the solar wind) the tail of the comet which are made of thin fragment of ice, well I dont think it is an heresy to claim that a stream of photons is somehow similar to a stream of material particles, such as, in this example, the solar wind

Thus the difference between matter and energy is not that sharp as scientits assumed before the great revolution in physics at the beginning of the 20th century and as many people still assume nowadays

Listen, I don't feel like arguing about this. I stand by what I said. Perhaps this is all just a big misunderstanding.

I don't see why you're angry, given that my post is neither particulary offensive, nor intended to hurt anyone. I'll just assume that this is due to language issues, and move on, without replying to what could easily be called provocations in your post. Please understand that not everyone is out to get you.


Let me just say this:



I give you that - photons have momentum (E=p*c for them, as you know, but perhaps, non-physicists are listening). Sadly, we cannot invoke the formula p=m\gamma v, since for photons, \gamma goes to infinity.




~ ~ ~ ~

So, we haven't yet found the Higgs. But apparently, they have some unusual not-yet-statistically-significant patterns, and they say they were able to give a shorter possible range for it to be in.

Exciting!
But, to be honest, wouldn't it be even more exciting if we didn't find the Higgs? After all, that would imply our current understanding is wrong, and thus that'd mean lots of new stuff to discover. Not to say that things are over with the Higgs, but the other possibility just sounds more interesting.

What do you guys think? Alberto? I don't know if particle physics is your "favorite physics", so to speak, but I'm sure you've followed recent developments

Error014

First of all thanks for your answer
I was not angry, rather surprised

I just want to clear up one point

Even though the average scientific education of 3dgs members is probably high I dont think that most of them can really grasp the meaning of a claim :

Photon has no mass but it has a momentum

What ? momentum is mass by velocity isn't it ?

Hoewever this concept can be explained in simple ( but not that simple:) ) words
without evoking esoteric stuff such as standatd model or virtual particles
Actually they just make more confusion
It is much better to explain the Hystorical evolution of physical concepts
This is the only point where I dont agree with you

Getting back to the topic

In the 19th century is was already known that an electromagnetic wave can make a container explode thus there is a pressure inside
Given the electric and magnetic field you can calculate the momentum of an electromagnetic wave, which was supposed to be something different than a mechanical momentum

However this momentum can not explain the photoelectric and the Crompton effect

if a beam of light collide with a stream of electrons (or fragment of thin ice) , the electrons should oscillate about their centre of gravity
They should not be pushed away

also for this reason Einstein assumed that the radiation is atomized in order to
have a sort of particles / particle collision
Each photon carries its own momentum and quantum of energy

at this point it is evident that the , so to speak, mechanical and electromagnetic momentum, converge

Does it mean that a photon in motion has also a mass ?
Well actualy...no, but you get very close

Joey a time ago accused me to be ...a teacher

I dont mean that you a Joey dont know this things but for sure many people dont

I just try to be cooperative....that's all
Since I studied this stuff many years ago, maybe from time to time I tell also some bullshit