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Are we meant to be discusing Protons or Photons here.

Peep Toom asked the Naked Scientists: Hello Naked scientists,On your recent show you said the photon carries energy but no mass. If E =mc^2, and the photon transfers energy, how can it have no mass even if the mass is miniscule?Thank you.Peep ToomBallarat, Victoria, Australia.What do you think?

I thought of using relativistic mass as a definition Pete, but what builds a relativistic mass?Accelerations?

And a photon has no accelerations as I understand it.

Ahh need to read up on how Euler thought there.

In Einstein's general relativity, GR, the old familiar parameters don't have the same meanings as those of classical physics. I believe this is a subtle result of the redefinition of space and time. Consequently, it is claimed that light has no mass, as mass is defined in GR. The concept of force is absent from GR. I am not comfortable applying GR, so I cannot dispute those claims. Instead, I shall demonstrate that, in classical physics, it is entirely consistent to attribute mass to a photon.

You can demonstrate nothing. A photon has no mass, period....sorry, you cannot view external links. To see them, please REGISTER or LOGIN

Hi lightarrow! How's if going?Regarding your comment A photon has no mass, period. Do you recall our discussions elsewhere?

If so then you would have seen me state that whether a photon has mass or zero-mass mass is a matter of taste. The PDF file you gave did not give zero proper mass for the photon but an upper limit of the photon's proper mass. It is experimentally impossible to prove that a quantity can be measured to be exactly zero.

Physicists learned their lesson when it was finally proven that neutrinos have a finite mass. What Phractality posted in this thead in the part you quoted was very unclear since he didn't speak of what he was talking about i.e. inertial mass or proper mass. Between yor_on and myself we have together touched on these points. i.e. proper mass and inertial mass.Good to see you again.

Should we say that physics was wrong when it stated, before of that discover, that neutrino's mass was zero?

...Section 47-5 Beta Decay - The symbol v represents a neutrino, a massless, neutral, particle .....Footnote - ... Finally, whether the mass of the neutrino is truly zero or not is subject under current investigation

...If you were to look under a different text you'd see the neutrino's mass stated as zero but with nothing mentioned about the mass being truly zero. E.g. see Sears, Zemansky and Young, (1985) page 894.

As for the non-zero proper mass of the photon seek out the Proca Lagrangian and see what it's used for. Hint; there are two sections in Jackson's Classical Electrodynhamics - 3rd Ed. text about it. The PDF file you linked to is about the photon's proper mass upper bound, the proper mass upper bound that the PDF file you linked to spoke of.A wise physicist would read those chapters of Jackson.

If you measure it as zero, how can you reasonably say that it's not?

But physics is not phylosophy, ....

Quote from: lightarrow on 08/05/2012 07:49:20If you measure it as zero, how can you reasonably say that it's not? Because nothing can ever be measured to have a given value with zero experimental error in the measurement.

It is easy to account for why a seemingly mass-less particle can have momentum but I am not allowed to answer it in this thread.Please seehttp://www.thenakedscientists.com/forum/index.php?topic=44008.0Original post 2nd. para. from end."A photon has no mass but does have momentum."

It is even easier: light has no mass but momentum even classically, it comes from Maxwell's equations. Momentum is not m*v, in general.

Proof that stress-energy tensor is symmetricCalculate in a specific Lorentz frame. Consider first the momentum density (components T^j0) and the energy flux (components T^0j). They must be equal because energy = mass ("E = Mc^2 = M")T^j0 = (energy flux)= (energy density) x (mean velocity of energy flow )^j = (mass density) x (mean velocity of mass flow )^j= (momentum density) = T^0j

A massive object has mass. An object traveling backward in time has negative mass.

Possibility of Faster-Than-Light Particles, G. Feinberg, Phys. Rev. 159(5), 25 July 1967 We consider the possibility of describing, within the special theory of relativity, particles with spacelike four-momentum, which therefore have velocities greater than that of light in vacuum. The usual objections to such particles are discussed, and they are found to be unconvincing within the framework of relativistic quantum theory. A quantum field theory of noninteracting, spinless, faster-than-light particles is described. The field theory is Lorentz invariant, but must be quantized with Fermi statistics. The associated particle theory has the property that the particle number is not Lorentz-invariant, and the no-particle state is not Lorentz-invariant either. Never the less, the principle of relativity is satisfied. The Lorentz invariance implies a relation between emission and absorption processes, in contradiction to the usual case. Some comments are made about the problem of introducing interactions into the field theory. The limiting velocity is c, but a limit has two sides.

You can't change lights speed therefore you cannot measure its resistance to change in speed. Therefore by definition light cannot have mass.

In Einstein's general relativity, GR, the old familiar parameters don't have the same meanings as those of classical physics. I believe this is a subtle result of the redefinition of space and time.

...old familiar parameters don't have the same meanings as those of classical physics.

In Einstein's general relativity, GR, the old familiar parameters don't have the same meanings as those of classical physics.

Consequently, it is claimed that light has no mass, as mass is defined in GR.

The concept of force is absent from GR.

For a particle with rest mass, at non-relativistic speeds, force is defined by the formula, f = ma. Turning that around to m = f/a serves as a definition of mass.

However, those formulas are not valid at relativistic speeds because it takes greater force to account for the increasing mass. A better definition of force is f = dp/dt, the rate of changing momentum, which is valid at all speeds. That formula is even valid for a photon at the speed of light.

A photon's momentum does change in response to gravity, ...

For particles with rest mass at relativistic speeds, ...

For a photon, f = dp/dt = d/dt(E/c) = d/dt(mc), where m is the mass equivalent of E, as in E = mc^2. So a photon has inertial mass.

Quote from: MikeS on 10/05/2012 05:22:56A massive object has mass. An object traveling backward in time has negative mass. I'm curious. Where did you get this idea that a particle traveling backward in time has negative mass?.

Quote from: MikeS on 10/05/2012 05:22:56A massive object has mass. An object traveling backward in time has negative mass. I'm curious. Where did you get this idea that a particle traveling backward in time has negative mass?Quote from: MikeS on 10/05/2012 05:22:56You can't change lights speed therefore you cannot measure its resistance to change in speed. Therefore by definition light cannot have mass.You have incorrectly used the flawed definition of mass wherein mas is supposed to be related to force and acceleration, i.e. mass is not defined as the m in F = ma. The m as defined here is known as the Euler definition of mass. The correct definition of mass is the m in p = mv. This is how Newton defined it in his Principia and how most SR texts have it right (e.g. p = mv and F = dp/dt). The correct definition of mass is that uised by Newton, of course, but also by Weyl. See ...sorry, you cannot view external links. To see them, please REGISTER or LOGINAs I keep saying , this is how mass is properly defined throughout the classical physics literature (classical physics by definition includes Newtonian physics as welll as relativity. Merely disagreeing with me by stating that I'm wrong cannot be considered a logical proof of any sort.

Since this is right on target for the OP's opening question it makes sense to respond to this post.Quote from: lightarrow on 08/05/2012 20:01:13It is even easier: light has no mass but momentum even classically, it comes from Maxwell's equations. Momentum is not m*v, in general.That's completely a matter of taste. I.e. actually p = mv is general.

p = mvIf you reverse time, then velocity becomes negative. If velocity is negative then mass is negative (from our perspective).

An electron has mass m = 9*10^{-31} kg; it moves at v = 299,792,457.99 m/s. How much is it its momentum?

Quote from: lightarrow on 11/05/2012 11:17:01An electron has mass m = 9*10^{-31} kg; it moves at v = 299,792,457.99 m/s. How much is it its momentum?The velocity you gave is faster than light (FTL).

Quote from: Pmb on 11/05/2012 12:30:28Quote from: lightarrow on 11/05/2012 11:17:01An electron has mass m = 9*10^{-31} kg; it moves at v = 299,792,457.99 m/s. How much is it its momentum?The velocity you gave is faster than light (FTL). What do you mean? c = 299,792,458 m/s.

Quote from: lightarrow on 11/05/2012 21:20:02Quote from: Pmb on 11/05/2012 12:30:28Quote from: lightarrow on 11/05/2012 11:17:01An electron has mass m = 9*10^{-31} kg; it moves at v = 299,792,457.99 m/s. How much is it its momentum?The velocity you gave is faster than light (FTL). What do you mean? c = 299,792,458 m/s.Correct. We no longer measure the speed of light because meters and seconds are defined by the speed of light, which is now exactly 299,792,458 m/s by definition. That number came about as a result of older definitions of meters and seconds, which are no longer valid. 299,792,457.99 m/s / 299,792,458 m/s = 0.9999999999666 cThat makes the relativistic gamma = 122432So the electron's inertial mass, m ≈ 1.22 * 10^{5}) * (9*10^{-31}) kg ≈ 1.1 * 10^{-25} kg, and its momentum is mv ≈ (1.1 * 10^{-25} kg) * (2.998 * 10^{8 }m/s) ≈ 3.3 * 10^{-17} kgm/s.

299,792,457.99 m/s / 299,792,458 m/s = 0.9999999999666 cThat makes the relativistic gamma = 122432

Quote from: Phractality on 11/05/2012 22:14:27299,792,457.99 m/s / 299,792,458 m/s = 0.9999999999666 cThat makes the relativistic gamma = 122432What method did you use to calculate gamma? My calculator does not have that many significant places. Do you have a better calculator? Used another method of calculation than the calculators basic functions?

You have to learn a few tricks, ....

Can you show me exactly what you did in this particular case? It would be of great help. I'm going to be doing a lot of relativity and will be dpoing a lot of gama function calculations and would like to know how to do it. It would be greately appreciated.

Lorentz factor γ: Input value: velocity | 2.9979245799×10^8 m/s (meters per second) Result: relativistic gamma 122432.116

Lightarrow. What was the purpose of asking the electrons's momentum since that's basic relativity?

I see lightarrow never answered my question.

Pete, I have a private life. You are not 24 hours in front of the computer, are you? []

Wrong. Electron's mass = 9.1*10^{-31} kg.Your mass is no longer used by most of the physicist community. When we say "mass" it means "invariant mass" (or "proper mass").

Lecture Notes 7Black-Body Radiation And the Early Universe...We are perhaps not used to thinking of electromagnetic radiation as having mass, but it is well-known that radiation has energy density. If the energy density is denoted u, then special-relativity implies that the electromagnetic radiation has a mass density rho given by(7.3) rho = u/c^2To my knowledge nobody has ever actually "weighed" electromagnetic radiation in any way, but the theoretial evidence in favor of (7.3) is overwhelming - light does have mass.

Pmb - In this forum it is unacceptable to say that a member "doesn't know what he's talking about". That is a personal attack.