Why is the Speed of Light Constant?

[Varnick]

Varying the frequency of light will alter its wavelength, not its speed. In the special case of electromagnetic waves moving through a vacuum, then v = c0 , where c0 is the speed of light in a vacuum, and this expression becomes:

f = \frac{c_o}{\lambda}

 

[navneet023]

So according to your logic, nothing can have zero electric charge either?

yes...electric charge can't be zero...how can u say its zero or not..we say smthng is not there just coz we r not able to detect it...its zero with respect to us...our calculation system...nd not even zero...so small that we can't get any instrument so as to measure it...

 

[Ich]

I have only found one serious attempt to compare the speed of light at different frequencies.

There's a sticky " FAQ: Experimental Basis of Special Relativity". And it's there for a reason.

 

[navneet023]

In relativity there are a number of different definitions of mass. In particular,

- rest mass = invariant mass = proper mass which excludes kinetic energy

- relativistic mass which includes kinetic energy

Most modern physicists mean invariant mass when they say "mass". But there are some people who mean relativistic mass when they say "mass".

A photon has zero invariant mass but non-zero relativistic mass.

The equation

m_{rel} = \frac {m_{inv}} {\sqrt{1 - v^2 / c^2}}​

is valid only for objects moving slower than light. For photons both the top and bottom of the fraction would be zero which makes it meaningless.

An equation that is true for all particles, including photons, is

m_{rel}^2 = m_{inv}^2 + p^2 / c^2​

where p is momentum.

even then...since the invariant mass of a photon is zero..wont it lead to smthng like m(rel)=p/c?

and how r we supposed to calculate this momentum, p?

 

[Norman Graves]

Why are these the only two options you allow?

Energy is (rest)massless, yet it travels through vacuum.

Michelson and Morely proved that the ether does not exist. What this shows is that space is empty - it contains nothing. If space contains nothing and photon exist in space then the photon must be something - here are no other values in the set.

 

[schroder]

yes...electric charge can't be zero...how can u say its zero or not..we say smthng is not there just coz we r not able to detect it...its zero with respect to us...our calculation system...nd not even zero...so small that we can't get any instrument so as to measure it...

I agree with this because I do not believe it is possible to conduct any experiment that can measure down to an absolute value of “zero”. Even the “absolute zero” of temperature is not zero! This is where the concept of limits in calculus assists us in evaluating measurements of extremely small quantities. All we can hope for is to establish some maximum limit to a quantity which cannot be measured down to zero. This has been done with the photon by the Charge Composition Explore Spacecraft and in various laboratory experiments. I believe the limit to be 7 x 10-17 eV. This does not prove that the photonic mass is not zero, nor does it establish the rest mass of a photon as any particular number. But it does leave open the possibility at least that the photon just might have rest mass. I don’t think we can discount this possibility. It may well be that some day we will establish that the basic unit of mass is one photon mass. It seems reasonable to me that such a minimum should exist as it would explain why the speed of light has a maximum value.

 

[Varnick]

There are (fundamental) particles, such as neutrinos, that are electrically neutral. They have NO electrical charge, no matter how accurately you attempted to measure it.

 

[navneet023]

There are (fundamental) particles, such as neutrinos, that are electrically neutral. They have NO electrical charge, no matter how accurately you attempted to measure it.

cant that imply these particles are not fundamental at all...may be some more basic particles of equal charges but opposite nature be inside them...it can make the whole charge as zero na??just an assumption...

 

[Varnick]

To my knowledge, there is no reason to consider neutrinos as non-fundamental, they are elementary particles in the standard model.

 

[FunkyDwarf]

No. Photons are not like normal particles. They can't be viewed as discrete particles that are vibrating. But they have a frequency/wavelength.

I don't think that's neccisarily (sp) correct. You can ascribe a freq/wavelength to any 'normal' particle and it would be an equivalent of the photons freq/wavelength. As for not being discrete well particle theory treats them as such with no real trouble. Of course you could bring up the whole wave particle duality thing but i think that's just semantics tbh =)

 

[malawi_glenn]

yes...electric charge can't be zero...how can u say its zero or not..we say smthng is not there just coz we r not able to detect it...its zero with respect to us...our calculation system...nd not even zero...so small that we can't get any instrument so as to measure it...

So then according to what reference is the electric charge of the say Photon non zero. And how can your theory be verified? Extra ordinary claims needs extra ordinary proofs. Ofcoruse we can never experimentally get exactly zero, but you also need a coherent and elegant theoretical framework to interprent the experimental data. There is no such things as 100% empirical data.

I agree with this because I do not believe it is possible to conduct any experiment that can measure down to an absolute value of “zero”. Even the “absolute zero” of temperature is not zero! ... I don’t think we can discount this possibility. It may well be that some day we will establish that the basic unit of mass is one photon mass. It seems reasonable to me that such a minimum should exist as it would explain why the speed of light has a maximum value.

The difference here is that you have a perfect theory for the zero point energy.

Why do we need a 'minimum mass' which is NONZERO in order to have a maximum value? It works just find as it is today. Please develop a coherent theory.

 

[DrGreg]

even then...since the invariant mass of a photon is zero..wont it lead to smthng like m(rel)=p/c?

Yes

and how r we supposed to calculate this momentum, p?

p = h / \lambda, where h is the Planck constant and \lambda is the photon's wavelength. Or equivalently p = h \nu / c where \nu is the photon's frequency.

See Photon on Wikipedia.

 

[navneet023]

Yes

p = h / \lambda, where h is the Planck constant and \lambda is the photon's wavelength. Or equivalently p = h \nu / c where \nu is the photon's frequency.

See Photon on Wikipedia.

what i wanted to imply...doesn't it sound a bit weird that we are generating mass out of nothing...I mean...There should be something around which to form the remaining mass...something which can act as a base... also...if the mass comes out of nothing...then how are we going to exactly predict its position or velocity...(since it won't be moving anymore with the velocity of light)so we are not able to predict either position or velocity of this newly generated mass...Please clear this doubt..

 

[jtbell]

I have only found one serious attempt to compare the speed of light at different frequencies. (Lovell B, et al (1964) Relative Velocity of Light and Radio Waves In Space Nature 202, 377 (25th April 1964)

The results were consistent with light traveling slower than radio waves, but the differences were within the range of experimental error, so the result was inconclusive.

It would make a very interesting experiment were someone to try it again over a wider frequency range.

From the FAQ that is stickied at the top of this forum:

http://www.edu-observatory.org/physics-faq/Relativity/SR/experiments.html#frequency

 

[DaveC426913]

Michelson and Morely proved that the ether does not exist. What this shows is that space is empty - it contains nothing.

Well, for starters, space is full of energy, even when there's nothing in it.

If space contains nothing and photon exist in space then the photon must be something - here are no other values in the set.

No one said photons were nothing, merely that they have no rest mass.

Really. Our knowledge has advanced quite a bit since Newtonian days and the world is full of subtleties. No need to see it so black & white as you are doing.

 

[malawi_glenn]

what i wanted to imply...doesn't it sound a bit weird that we are generating mass out of nothing...I mean...There should be something around which to form the remaining mass...something which can act as a base... also...if the mass comes out of nothing...then how are we going to exactly predict its position or velocity...(since it won't be moving anymore with the velocity of light)so we are not able to predict either position or velocity of this newly generated mass...Please clear this doubt..

Who has said that mass is genereted out of nothing?

 

[Varnick]

Malawi, I think navneet is using the definition of Newtonian momentum p=mv, and applying it to a photon, so if a photon has momentum, it must have mass.

V

 

[schroder]

There are (fundamental) particles, such as neutrinos, that are electrically neutral. They have NO electrical charge, no matter how accurately you attempted to measure it.

That is quite the bold statement! However, it is not factual. The fact is, the more accurate you make the scale, the less likely it is you will ever get it to balance! The electric charge of the neutrino, like the mass of the photon, can only be measured down to be within certain limits, because of the possible inaccuracies in measurement. It is simply Not Possible to say that either of these is zero! Would anyone care to disprove that statement? The latest method for measuring the charge of a neutrino is based on generating an alternative potential between a metallic hemisphere and ground by a time-variant neutrino beam. Transitions between hyperfine sublevels of polarized nuclei undergoing beta decay are then used to produce a time-oscillating neutrino beam. The resulting estimates suggest that the upper limit achievable on neutrino electric charge may reach 10-12e0, where e0 is an electron charge. As far as all the elementary particles go, the ones we can measure all exhibit a non-zero value. (This is a Min-Max Universe).There is no reason to believe that anything in our Universe has a value of zero. (There is no perfect vacuum). I am convinced that physicists, by convention assign that value only to hide the fact that we cannot yet measure it.

 

[malawi_glenn]

Malawi, I think navneet is using the definition of Newtonian momentum p=mv, and applying it to a photon, so if a photon has momentum, it must have mass.

V

Well I have no clue yet, but he thinks that there is an a priori reson for that no entity can have a zero value of a property.

According to this, the photon must have a very small amount of charge, strangeness, mass, spatial extension etc. But there are no a priori resons for why things can't have exactly zero as value for a certain property.

He must first prove his assumption, then create a coherent theoretical framework to interprent emperical data (as I argued in one of my last posts).

 

[malawi_glenn]

There is no reason to believe that anything in our Universe has a value of zero. (There is no perfect vacuum). I am convinced that physicists, by convention assign that value only to hide the fact that we cannot yet measure it.

The reason is due to the theoretical framework (once again).

Where is the 'proof' that there are no such thing as a perfect (generelized) vacuum?

Also, vacuum energy, is per definition the ground state energy. Just because the zero-point energy exists, you can not a priori extrapolate that result into the "charge vacuum".

 

[DaveC426913]

That is quite the bold statement! However, it is not factual. The fact is, the more accurate you make the scale, the less likely it is you will ever get it to balance! ... I am convinced that physicists, by convention assign that value only to hide the fact that we cannot yet measure it.

Empirical observation is only one method of scientific analysis. There are ways of showing something to be true that go beyond empirical observation. Thus is the nature of our universe as mathematically quantifiable.

For example, we can demonstrate - without the ability to observe or measure it - that an electron is a fundamental particle, with no substructure. The reasoning is that, as per HUP, any smaller particles will have a proportionately larger momentum of uncertainty. The upshot of this is that the substructural components of the electron would be larger and more massive than the electron itself.

Another famous one is the hypothesis of hidden variables affecting entangled particles at a distance. Read up on Bell's hidden variables theorem, which demonstrates, astonishingly, that there can't be hidden properties that we don't know about.

This is just a couple of simple examples. I'm simply trying to point out to you that empirical measurement is only one of many tools.

 

[Varnick]

That is quite the bold statement! However, it is not factual.

I disagree with both of these points. Firstly, I can't see how the statement is "Bold" when it has been a part of established standard model physics for over 40 years. As to your second point, Neutrinos are described as electrically neutral by the standard model, which is backed up by large amount of experimental data. If your principle of "Nothing can be 0" were true, then leptons would take part in strong interactions, which is not the case. Your idea that we only measure things to a certain level, and that they do have a charge but it's just really small is, simply, false. Please see

http://en.wikipedia.org/wiki/Standard_model
http://en.wikipedia.org/wiki/Neutrino

 

[DrGreg]

what i wanted to imply...doesn't it sound a bit weird that we are generating mass out of nothing...I mean...There should be something around which to form the remaining mass...something which can act as a base... also...if the mass comes out of nothing...then how are we going to exactly predict its position or velocity...(since it won't be moving anymore with the velocity of light)so we are not able to predict either position or velocity of this newly generated mass...Please clear this doubt..

Well, as I said before, most physicists nowadays don't use the concept of relativistic mass at all; they call it "energy" instead via the equation E = m_{rel}c^2, and so they write instead

E^2 = m_{inv}^2c^4 + p^2c^2​

and they say a photon has energy but no mass (meaning "invariant mass").

So, your comment above would translate as "we are generating energy out of nothing" -- except that it's not out of nothing, all the energy that goes into a photon (or any other particle) will have come from somewhere else. The total energy of all particles in a "closed" system (not interacting with anything outside) is always constant.

Note also that in quantum theory you cannot exactly predict the position or velocity of anything, you can only calculate a probability.

 

[schroder]

I disagree with both of these points. Firstly, I can't see how the statement is "Bold" when it has been a part of established standard model physics for over 40 years. As to your second point, Neutrinos are described as electrically neutral by the standard model, which is backed up by large amount of experimental data. If your principle of "Nothing can be 0" were true, then leptons would take part in strong interactions, which is not the case. Your idea that we only measure things to a certain level, and that they do have a charge but it's just really small is, simply, false. Please see

http://en.wikipedia.org/wiki/Standard_model
http://en.wikipedia.org/wiki/Neutrino

So your argument is based upon the “Authority” of Wikipedia and the Standard Model?
Great advances in science are not made by defending the status quo and not daring to rock the boat. If Newton and Einstein did science in this way, where would we be now? Still in the 17th century, I reckon. All I am proposing here is that we should not accept zero values because it makes our equations “come out right” and reinforce our existing theories. Good science is not done by people with closed minds. I have not claimed a non-zero value but I do accept the possibility of such for the mass of a photon and there is no empirical evidence that refutes that possibility. Instead of simply accepting zero mass, I would much rather see what minimum value of mass a photon can possesses without completely undermining physics as we know it and also what minimum value might account for the finite velocity of light. A zero value, in my estimation, should result in an infinite velocity of light, which we know is nonsense. What else can cause light to have a velocity less than infinity if not some amount of mass? And don’t you think the mathematics is ugly when we use one equation for objects that are traveling at less than c, and another for light? If you placed light in a box in such a way that it did not change the momentum of the box, only increased the energy of the box, what effect would that have on the mass of the light box? These are questions that even Wikipedia cannot answer!

 

[Fredrik]

A zero value, in my estimation, should result in an infinite velocity of light, which we know is nonsense.

Your estimation is wrong. When we take Minkowski space to be the mathematical model of spacetime and take the postulates of quantum mechanics seriously, a mass of zero implies that the speed is =1, not infinity. We could of course reject SR or QM, but then the question is, what would we replace them with? I assume you have an answer to that, since you were able to estimate the speed of massless particles to be infinite.

If you placed light in a box in such a way that it did not change the momentum of the box, only increased the energy of the box, what effect would that have on the mass of the light box? These are questions that even Wikipedia cannot answer!

The mass would increase by the amount E/c², where E is the energy. I would be surprised if that answer isn't somewhere in Wikipedia.

 

[Varnick]

So your argument is based upon the “Authority” of Wikipedia and the Standard Model?
Great advances in science are not made by defending the status quo and not daring to rock the boat. If Newton and Einstein did science in this way, where would we be now? Still in the 17th century, I reckon. All I am proposing here is that we should not accept zero values because it makes our equations “come out right” and reinforce our existing theories. Good science is not done by people with closed minds. I have not claimed a non-zero value but I do accept the possibility of such for the mass of a photon and there is no empirical evidence that refutes that possibility. Instead of simply accepting zero mass, I would much rather see what minimum value of mass a photon can possesses without completely undermining physics as we know it and also what minimum value might account for the finite velocity of light. A zero value, in my estimation, should result in an infinite velocity of light, which we know is nonsense. What else can cause light to have a velocity less than infinity if not some amount of mass? And don’t you think the mathematics is ugly when we use one equation for objects that are traveling at less than c, and another for light? If you placed light in a box in such a way that it did not change the momentum of the box, only increased the energy of the box, what effect would that have on the mass of the light box? These are questions that even Wikipedia cannot answer!

So first my statement is bold, now it is the status quo? How intriguing. Of course we should look to improve on theories, I know that there are flaws in the standard model, but unless you can propose a better theory with charged neutrinos, I think we must agree that neutrinos are, for now, electrically neutral.

V

 

[DaveC426913]

Great advances in science are not made by defending the status quo and not daring to rock the boat. If Newton and Einstein did science in this way, where would we be now?

Oh jeez. Here we go... The ol' "let's break all the rules" gambit.

If you were to "do science" the way Newton and Einstein did, you would have brushed up on the science and done your research rather than make wild, fanciful armchair speculations about things you've never studied.

Sounds like food for the Lock ness monster...

 

[Varnick]

Thank-you, DaveC426913, for some sense finally. This last page or so of discussion is because of my wild and spurious claim that a photon had no mass. If I knew such unsupported ideas weren't welcome here, I would not have posted. </frustrated sarcasm>

V

 

[George Jones]

I have not claimed a non-zero value but I do accept the possibility of such for the mass of a photon and there is no empirical evidence that refutes that possibility. Instead of simply accepting zero mass, I would much rather see what minimum value of mass a photon can possesses without completely undermining physics as we know it

Any non-zero rest mass for photon would destroy gauge invariance of electrodynamics, which, in turn, would probably mean that quantum electrodynamics would not be renormalizable. Renormalizable QED is, over a wide range of experiments, the most accurately verified theory of physics.

Also, while we do not have direct empirical that the rest mass of a photon is exactly zero, very small experimental upper bounds on the rest mass of the photon exist.

[EDIT]
http://www.ncbi.nlm.nih.gov/pubmed/12633416

puts an experimental upper bound of about 10^{-54} kg.
[/EDIT]

With this, I'm putting this thread to bed.