Do Photons Gain Infinite Mass at Light Speed?

[nakurusil]

what's there to check? you said two things that are demonstratively incorrect. check the NIST site to see how these get defined so that c = 299792458 m/s. and how \mu_0 is defined, not derived.

Look at "Overview" here:

http://en.wikipedia.org/wiki/Speed_of_light

$$\mu_0$$ value is chosen such that $$1/\sqrt(\epsilon_0 \mu_0)=299,...$$

 

[rbj]

c is constant by definition in relativity. In addition, there is ample experimental confirmation, so I don't think that c being a constant has much if anything to do with the meter is defined.
Actually, it is exactly the other way around, the definition of the meter is dependent on c:

http://en.wikipedia.org/wiki/Meter

and on the definition of the second.

a postulate is not a definition. a definition is something that is not a "truth" that is proposed. a postulate (such as the invariancy of c in SR) is something akin to a physical law.

we could define a meter to be the distance that sound in air at STP travels in 1/331.5 second and that would fix the speed of sound in air at STP to be 331.5 m/s. does that mean that the speed of sound is defined to be constant in the same way that c is believed to be constant?

c = 299792458 m/s because the meter was defined to make it so. now, if these VSL guys are correct (i don't think they are) then the distance between the two little scratch marks on the prototype meter will have changed, in terms of the present definition, if the speed of light actually did change sufficiently.

but i agree, that to speak of a changing speed of light, especially when the base units are defined as they are, is meaningless. the only numbers about the that we measure are ultimately dimensionless numbers. if \alpha changes, that is meaningful, but there is no meaning in saying that c, G, \hbar changes in and of themselves. if the number of Planck lengths in the Bohr radius changes, that is meaningful and, assuming the old prototype meter stick is a "good" meter stick (and it doesn't lose or gain atoms), then the number of Planck lengths between those little scratch marks will have changed.

but the fact that c=299792458 is a matter of how they defined the meter and SR could not have predicted or determined that.

 

[rbj]

Look at "Overview" here:

http://en.wikipedia.org/wiki/Speed_of_light

$$\mu_0$$ value is chosen such that $$1/\sqrt(\epsilon_0 \mu_0)=299,...$$

baloney. you need to read up on some of this.

\mu_0 = 4 \pi \cdot 10^{-7} in whatever units because of how they defined the Ampere. check it out yourself.

 

[nakurusil]

baloney. you need to read up on some of this.

\mu_0 = 4 \pi \cdot 10^{-7} in whatever units because of how they defined the Ampere. check it out yourself.

Yes, I know that. How do you explain the "Overview" then? It indicates that $$\epsilon_0, \mu_0$$ are arranged, chosen, fitted, however yo want to call them such that $$1/\sqrt\epsilon_0\mu_0=299,...$$ . And that is the value attributted to c

 

[nakurusil]

a postulate is not a definition. a definition is something that is not a "truth" that is proposed. a postulate (such as the invariancy of c in SR) is something akin to a physical law.

we could define a meter to be the distance that sound in air at STP travels in 1/331.5 second and that would fix the speed of sound in air at STP to be 331.5 m/s. does that mean that the speed of sound is defined to be constant in the same way that c is believed to be constant?

c = 299792458 m/s because the meter was defined to make it so. now, if these VSL guys are correct (i don't think they are) then the distance between the two little scratch marks on the prototype meter will have changed, in terms of the present definition, if the speed of light actually did change sufficiently.

but i agree, that to speak of a changing speed of light, especially when the base units are defined as they are, is meaningless. the only numbers about the that we measure are ultimately dimensionless numbers. if \alpha changes, that is meaningful, but there is no meaning in saying that c, G, \hbar changes in and of themselves. if the number of Planck lengths in the Bohr radius changes, that is meaningful and, assuming the old prototype meter stick is a "good" meter stick (and it doesn't lose or gain atoms), then the number of Planck lengths between those little scratch marks will have changed.

but the fact that c=299792458 is a matter of how they defined the meter and SR could not have predicted or determined that.

You missed the point , entirely. I reacted to MeJennifer's statement that said textually c is a constant because...

 

[rbj]

Yes, I know that.

no, you're saying something else.

\mu_0 has had a defined value long before c did. \mu_0 gets its defined value from how they defined the SI unit of current (the Ampere) which, along with the SI unit of time, defines the SI unit charge. don't know if it was Faraday or Boyle or Coulomb or who it was that first measured the inverse-square nature of the electrostatic force, but i imagine that they may have had a crude measured value for \epsilon_0. presumably with Maxwell, they got to relate \epsilon_0 and \mu_0 to c which was measured independently (i presume by the likes of Michaelson) and offered both experimental confirmation of the unified E&M theory of Maxwell, but also offered a better value for \epsilon_0 since they can relate it to the defined \mu_0 and a more accurately measured c. but c was still measured, in terms of the existing meter and second. until they redefined it in 1960 and 1983. they defined the meter in terms of c as they did so that the length of the new definition agreed, as well as they could determine at the time, with the length of the old definition. it's possible now that the distance between those two little scratch marks in the prototype meter bar is measured (using the present definition of the meter) to be something slightly different than a meter (present definition). i would chalk that one up to experimental error or advancement of metrology since 1983 rather than to a change in the speed of light since 1983.

but \mu_0 did not magically come out to be \mu_0 = 4 \pi \cdot 10^{-7} because of how they defined c (or the meter, however you wish to put it).

some of this might be semantic, but you are objectively mistaken to say that \mu_0 = 4 \pi \cdot 10^{-7} because of how they defined c. that's just incorrect.

 

[rbj]

You missed the point , entirely. I reacted to MeJennifer's statement that said textually c is a constant because...

lessee...

MeJennifer said:

Actually the reason c is a constant has to do with how the meter is defined.

you said:

$$\mu_0$$ value is chosen such that $$1/\sqrt(\epsilon_0 \mu_0)=299,...$$

i'm placing my bets with MeJennifer.

 

[rbj]

Interestingly C was defined by definition in 1983.
Permittivity (E)of free space is defined by C and the Maxwell equations.
Permeability (u) is measured.

oh, i missed this! this is where the evil misconception started. most certainly the Permeability of Free Space $$\mu_0$$ is not measured, but defined as a consequence of the definition of the unit of electrical current, the Ampere.

 

[ZapperZ]

Can't someone get ahold of the latest CODATA and figure out how these values were obtained?

Zz.

 

[rbj]

Can't someone get ahold of the latest CODATA and figure out how these values were obtained?

what do you mean, Z? the stuff at NIST: http://www.physics.nist.gov/cuu/ reflects the CODATA values but c, \epsilon_0, \mu_0, Z_0 are not CODATA measured values. they are defined in SI. I'm sure you know that, Z. but I'm not sure what you're asking for here.

 

[rbj]

If at some point the speed of light is shown not to be constant, then the definition of the meter will surely be changed to reflect this.

JT, how would we ever know if the speed of light is changing? what would be the standard against which we measure the speed of light? when we measure anything, ultimately what we have are dimensionless quantities that we conceptually attach dimensionful units to. but since these dimensionless quantities are ratios of like-dimensioned physical quantities, if this ratio changes, how would one know which of the like-dimensioned physical quantities to attribute this change to?

 

[nakurusil]

lessee...

MeJennifer said:




you said:



i'm placing my bets with MeJennifer.

You are still missing the point. I give up, not worth it.

 

[rbj]

You are still missing the point. I give up, not worth it.

what duordi said and you confirmed and reiterated is factually incorrect (about the permeability \mu_0).

what MeJennifer said is factually correct.

whatever your point is, i guess i did miss it.

 

[nakurusil]

what MeJennifer said is factually correct.

whatever your point is, i guess i did miss it.

Actually the reason c is a constant has to do with how the meter is defined.

NOT. Or maybe you think it is correct?

 

[MeJennifer]

Here is the http://www.physics.nist.gov/cuu/Units/meter.html"

The meter is the length of the path traveled by light in vacuum during a time interval of 1/299 792 458 of a second.

Furthemore from the same page:

Note that the effect of this definition is to fix the speed of light in vacuum at exactly 299 792 458 m·s-1.

 

[nakurusil]

Here is the http://www.physics.nist.gov/cuu/Units/meter.html"
Furthemore from the same page:

Yes, the sentence means that it makes c equal to the specific value of 299,... Do you understand the difference between that and making c a constant?
c is a constant for profound physical reasons, not because of the definition of the meter. You don't simply make c a constant

 

[rbj]

Here is the http://www.physics.nist.gov/cuu/Units/meter.html"

while we're at it, here it is for the Ampere:

The ampere is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross section, and placed 1 meter apart in vacuum, would produce between these conductors a force equal to 2 \times 10^{-7} Newton per meter of length.

The expression "MKS unit of force" which occurs in the original text has been replaced here by "Newton," the name adopted for this unit by the 9th CGPM (1948). Note that the effect of this definition is to fix the magnetic constant (permeability of vacuum) at exactly \mu_0 = 4 \pi \times 10^{-7} H \cdot m^{-1}.

 

[MeJennifer]

Yes, the sentence means that it makes c equal to the specific value of 299,... Do you understand the difference between that and making c a constant?
c is a constant for profound physical reasons, not because of the definition of the meter. You don't simply make c a constant

It seems that you rather deny the reality of certain things than admit you are wrong.

 

[rbj]

Yes, the sentence means that it makes c equal to the specific value of 299,... Do you understand the difference between that and making c a constant?
c is a constant for physical reasons, not because of the definition of the meter. You don't simply make c a constant

if it changed, how would we meaningfully measure it? you might want to read some stuff from Michael Duff http://www.arxiv.org/abs/hep-th/0208093 or http://xxx.lanl.gov/abs/physics/0110060 or just check out the Physical constants or Planck units articles at Wikipedia.

here's a good quote from John Barrow:

[An] important lesson we learn from the way that pure numbers like \alpha define the world is what it really means for worlds to be different. The pure number we call the fine structure constant and denote by \alpha is a combination of the electron charge, e, the speed of light, c, and Planck's constant, h. At first we might be tempted to think that a world in which the speed of light was slower would be a different world. But this would be a mistake. If c, h, and e were all changed so that the values they have in metric (or any other) units were different when we looked them up in our tables of physical constants, but the value of \alpha remained the same, this new world would be observationally indistinguishable from our world. The only thing that counts in the definition of worlds are the values of the dimensionless constants of Nature. If all masses were doubled in value [including the Planck mass m_P ] you cannot tell because all the pure numbers defined by the ratios of any pair of masses are unchanged.

you see, whether it is c, h, G, or e, it's not really meaningful to consider the changing of any of these dimensionful physical constants. it's only the dimensionless physical "constants" that mean anything (have "operational distiguishability") if they changed.

still, MeJennifer had it right and, doggedly you have been holding on to your guns, you did not.

 

[nakurusil]

if it changed, how would we meaningfully measure it?

Who's talking about changing? What exactly do you understand in the post that you just quoted?

you see, whether it is c, h, G, or e, it's not really meaningful to consider the changing of any of these dimensionful physical constants. it's only the dimensionless physical "constants" that mean anything (have "operational distiguishability") if they changed.

Where am I talking about changing? Where did you see that in ANY of my posts?

 

[nakurusil]

It seems that you rather deny the reality of certain things than admit you are wrong.

You didn't answer my question, can you try?
How does the meter definition make c a constant? (when c is constant to begin with due to multiple physical reasons). Please try to answer the question without a personal attack, ok?

 

[MeJennifer]

You didn't answer my question, can you try?

You are giving yourself a distinct impression of trolling.

 

[nakurusil]

You are giving yourself a distinct impression of trolling.

For the second time, please try answering without a personal attack. It is a very simple question. You made a statement, I think that it is incorrect, try to defend it.

 

[MeJennifer]

In the interest of our member's valuable time I rather suggest that a moderator close this topic, enough is enough.

 

[rbj]

Who's talking about changing? What exactly do you understand in the post that you just quoted?
...
Where am I talking about changing? Where did you see that in ANY of my posts?

what you said is

Yes, the sentence means that it makes c equal to the specific value of 299,... Do you understand the difference between that and making c a constant?
c is a constant for profound physical reasons, not because of the definition of the meter. You don't simply make c a constant

by "specific value of 299,..." i am presuming you meant "specific value of 299792458 m/s". the fact is, by the very definition of the meter (post 1983), that's what they did. given that means of defining what a meter is, the dimensionful value of 299792458 m/s can be nothing else. now, perhaps that was a bad definition for the meter. it sure as hell would have been bad if they defined the meter to be the distance sound travels in air at STP in 1/(331.5) second. but they're more confident about the stability and repeatability of the speed of propagation of E&M waves.

i think i have a feel for why the speed of E&M propagation should be the same for all inertial reference frames. don't know if it is "profound" or not, but the contrary leads to problems. don't know of this is how Einstein would put it but it's how i gleaned it from the some of the words of Einstein. it really just comes from Maxwell's Eqs. and the knowledge (verified by the "negative" outcome of the Michaelson-Morley experiment) that there is no ether medium that E&M is propagated in.

consider the propagation of sound, for instance. if the wind is steady and blowing across your face at some velocity v from left to right and you measure the speed of some sound coming from your left, you will measure it to be 2v faster than if it came from your right. that is because you are moving relative to the "ether" (air) medium that carries the sound wave. but there is no such medium for light or any other E&M wave.

so then, how do we tell the difference between a moving vacuum and a stationary vacuum? if we can't, if there really is no difference between a moving vacuum and a stationary vacuum, that such a concept is really meaningless, then whether the light that you are measuring originated from a flashlight mounted on a rocket moving past you at c/2 or from a stationary (relative to you) flashlight, how does that change the fact that this changing E field is causing a changing B field which is causing a changing E field which is causing a changing B field which is causing a changing E field, etc.? that propagation of an E field and B field disturbance, which has velocity $$1/ \sqrt{ \epsilon_0 \mu_0 }$$? how is it different for you or for the observer that is traveling along with the flashlight at c/2? whether you are holding the flashlight or moving past it at high velocity, Maxwell's Eqs. say the same thing regarding the nature of E&M in the vacuum and you will both measure the speed of that propagation to be $$1/ \sqrt{ \epsilon_0 \mu_0 }$$.

so that might be the "profound" reasons for choosing the definition of the meter in terms of c as they did, but given that definition of a meter, the speed of light cannot come out to be anything different. there is nothing more profound about it (or that of any dimensionful universal "constant") which is why we should all be dubious of either renowned researchers or of crackpots making claims of changing c or G or whatever. we got to just ask "how would we even measure or perceive such a change in the first place?"

 

[duordi]

Go RBJ

I would choose to defend RBJ.
The difference in your approaches have to do with definitions created by humans versus physical reality.

Yes C is defined technically by standards of time and length by humans.

Physical reality doesn't care spit about our standards.
If C is a constant then C is a constant regardless what our definitions say.

It is impossible to contemplate variation in the speed of light if we cling to the definitions commonly used because they consider the speed of light a constant.

Einstein and Maxwell cut their own path and RBJ should be allowed to do the same if he so chooses.

By the way if all matter in the universe doubled the universe may go from open to closed.
It would that is not undetectable.

 

[nakurusil]

what you said is
by "specific value of 299,..." i am presuming you meant "specific value of 299792458 m/s". the fact is, by the very definition of the meter (post 1983), that's what they did. given that means of defining what a meter is, the dimensionful value of 299792458 m/s can be nothing else. now, perhaps that was a bad definition for the meter. it sure as hell would have been bad if they defined the meter to be the distance sound travels in air at STP in 1/(331.5) second. but they're more confident about the stability and repeatability of the speed of propagation of E&M waves.

i think i have a feel for why the speed of E&M propagation should be the same for all inertial reference frames. don't know if it is "profound" or not, but the contrary leads to problems. don't know of this is how Einstein would put it but it's how i gleaned it from the some of the words of Einstein. it really just comes from Maxwell's Eqs. and the knowledge (verified by the "negative" outcome of the Michaelson-Morley experiment) that there is no ether medium that E&M is propagated in.

consider the propagation of sound, for instance. if the wind is steady and blowing across your face at some velocity v from left to right and you measure the speed of some sound coming from your left, you will measure it to be 2v faster than if it came from your right. that is because you are moving relative to the "ether" (air) medium that carries the sound wave. but there is no such medium for light or any other E&M wave.

so then, how do we tell the difference between a moving vacuum and a stationary vacuum? if we can't, if there really is no difference between a moving vacuum and a stationary vacuum, that such a concept is really meaningless, then whether the light that you are measuring originated from a flashlight mounted on a rocket moving past you at c/2 or from a stationary (relative to you) flashlight, how does that change the fact that this changing E field is causing a changing B field which is causing a changing E field which is causing a changing B field which is causing a changing E field, etc.? that propagation of an E field and B field disturbance, which has velocity $$1/ \sqrt{ \epsilon_0 \mu_0 }$$? how is it different for you or for the observer that is traveling along with the flashlight at c/2? whether you are holding the flashlight or moving past it at high velocity, Maxwell's Eqs. say the same thing regarding the nature of E&M in the vacuum and you will both measure the speed of that propagation to be $$1/ \sqrt{ \epsilon_0 \mu_0 }$$.

so that might be the "profound" reasons for choosing the definition of the meter in terms of c as they did, but given that definition of a meter, the speed of light cannot come out to be anything different. there is nothing more profound about it (or that of any dimensionful universal "constant") which is why we should all be dubious of either renowned researchers or of crackpots making claims of changing c or G or whatever. we got to just ask "how would we even measure or perceive such a change in the first place?"

I never imagined that such a straightforward issue can get so messed up.
Let me try one more time: my contention to MeJennifer post is that it is not the "definition of the meter that makes c a constant".

1. c was known to be a constant long before 1983 when the current definition of the meter was coined out. We all know why c is a constant, we all know all the experimental confirmations and the mess we would be in if it weren't (as a parens, I never brought in any isue about changing c, so I do not understand your persistance in bringing it in in your posts)

2. So , we all violently agree that c is a constant, ok?

3. The re-definition of the meter in 1983 simply assigned the value of precisely 299792458 m/s to the constant. Did not make c a constant, c was already known to be a constant for about 80 years.
An interesting question is how did the metrologists arrive to the 299792458 number. Do you know?

 

[pmb_phy]

Just a thought, according to Einsteins relativity mass changes with speed and tends to become infnite as it approaches 'c'. Since photons too have mass, why doesn't their mass become infinite since they travel at c?

I'm not sure whether your question was answered to your satisfaction so let me take a crack at it.

The (relativistic/inertial) mass of a particle is the m in mv where v is the velocity of the particle and the vector quantity p = mv is conserved. This quantity is called the momentum of the particle. So mass is defined so that momentum is conserved in all frames of reference. Since a photon has momentum it has inertial mass. The value of its intrinsic mass (aka "proper mass") is zero.

Pete

 

[bernhard.rothenstein]

photon

I'm not sure whether your question was answered to your satisfaction so let me take a crack at it.

The (relativistic/inertial) mass of a particle is the m in mv where v is the velocity of the particle and the vector quantity p = mv is conserved. This quantity is called the momentum of the particle. So mass is defined so that momentum is conserved in all frames of reference. Since a photon has momentum it has inertial mass. The value of its intrinsic mass (aka "proper mass") is zero.

Pete

Hi Pete. Nice to find you on the Forum.
Is it wrong to say that in the case of the tardyon, momentum and mass transform in such a way that p/m=u and p'/m'=u' ,u and u' being related by the addition law of relativistic velocities and so mass and momentum transform via different transformation factors. In the case of the photon
we should have for the same reasons p/m=p'/m'=c momentum and mass transforming via the same transformation factor which happens to be the Doppler factor. Has the conservation some relevance in that case?

 

[pmb_phy]

Hi Pete. Nice to find you on the Forum.
Is it wrong to say that in the case of the tardyon, momentum and mass transform in such a way that p/m=u and p'/m'=u' ,u and u' being related by the addition law of relativistic velocities and so mass and momentum transform via different transformation factors.

Why not? By the very definition of relativistiv mass, m, p = mv in every inertial frame. This, i.e. "m" is just the time component of the 4-momentum of the particle.

In the case of the photon we should have for the same reasons p/m=p'/m'=c ...

Which is exactly what you'd find if you calcluate it.

..momentum and mass transforming via the same transformation factor which happens to be the Doppler factor. Has the conservation some relevance in that case?

Not that I'm aware of.

Pete