Constancy of the speed of light

[dayalanand roy]

The goal is to create laws of physics that are more general. Galilean relativity, when applied to light, doesn't work. Einsteinian relativity does. It works for both light and for what you are calling material objects. Galilean relativity works for neither of those. It seems to work at low speeds, so it seems to work for what you are calling material objects but only when they move at low speeds.
He did not propose them. He showed that they are consequences of the two postulates.

Thanks a lot. I am gaining.

 

[dayalanand roy]

I'm not sure this makes sense. I can imagine a universe where the principle of relativity applies, and I can imagine one where it doesn't apply. I have trouble with one where the principle applies to some things and not others. What about interactions between members of the two classes? Would they respect the principle of relativity and violate the rules for the no-relativity particles? Or vice versa? Either way, something ends up behaving in a way it can't behave.

The idea of the ether respected relativity. The laws of physics would be the same in all frames, but the physical situation picked out an interesting frame for electromagnetism - the rest frame of the ether. Once we'd found that, more sensitive experiments would be expected to show electromagnetic phenomena deviating from Maxwell in other frames, and we could fix his equations so that they respected Galilean relativity. Of course, it didn't work out that way.

Although you are correct that we had quite a lot of evidence of the invariance of light speed before 1905, I don't think anyone had recognised that this was what we had. We just had a lot of inexplicanle experimental results. (In a way, this is the exact opposite of what we have now, where we know we have theoretical problems but the experiments keep stubbornly matching the theories...)

Thanks and regards.

 

[dayalanand roy]

But that is exactly what the Einstein’s relativity says. The principle of relativity applies in both Galileo and Einstein, but Galileo’s spacetime laws were incompatable with Maxwell’s equations. Einstein fixed that problem, so that the principle of relativity didn’t hold for just mechanics (like Galileo’s did).

Thanks and regards.

 

[Sorcerer]

Thanks and regards.

Just for a quick little history nugget, in Einstein's first major paper on relativity he highlighted the issue. For example, it was believed that the following two situations were two different phenomena:

(1) If you move a magnet near a conductor at rest, it produces an electric current.
(2) If you move a conductor near a magnet at rest, it also produces an electric current.​

Einstein recognized that they were the same phenomenon, and in fact, were manifestations of the principle of relativity. The key question is, of course, which one is "really" moving and which one is "really" at rest? Here is a PDF of his paper. The very first paragraph discusses this:

http://hermes.ffn.ub.es/luisnavarro/nuevo_maletin/Einstein_1905_relativity.pdf

Here is the relevant quote:

"It is known that Maxwell’s electrodynamics—as usually understood at the
present time—when applied to moving bodies, leads to asymmetries which do
not appear to be inherent in the phenomena. Take, for example, the recipro-
cal electrodynamic action of a magnet and a conductor. The observable phe-
nomenon here depends only on the relative motion of the conductor and the
magnet, whereas the customary view draws a sharp distinction between the two
cases in which either the one or the other of these bodies is in motion. For if the
magnet is in motion and the conductor at rest, there arises in the neighbour-
hood of the magnet an electric field with a certain definite energy, producing
a current at the places where parts of the conductor are situated. But if the
magnet is stationary and the conductor in motion, no electric field arises in the
neighbourhood of the magnet. In the conductor, however, we find an electro-
motive force, to which in itself there is no corresponding energy, but which gives
rise—assuming equality of relative motion in the two cases discussed—to elec-
tric currents of the same path and intensity as those produced by the electric
forces in the former case."​

 

[dayalanand roy]

Just for a quick little history nugget, in Einstein's first major paper on relativity he highlighted the issue. For example, it was believed that the following two situations were two different phenomena:

(1) If you move a magnet near a conductor at rest, it produces an electric current.
(2) If you move a conductor near a magnet at rest, it also produces an electric current.​

Einstein recognized that they were the same phenomenon, and in fact, were manifestations of the principle of relativity. The key question is, of course, which one is "really" moving and which one is "really" at rest? Here is a PDF of his paper. The very first paragraph discusses this:

http://hermes.ffn.ub.es/luisnavarro/nuevo_maletin/Einstein_1905_relativity.pdf

Here is the relevant quote:

"It is known that Maxwell’s electrodynamics—as usually understood at the
present time—when applied to moving bodies, leads to asymmetries which do
not appear to be inherent in the phenomena. Take, for example, the recipro-
cal electrodynamic action of a magnet and a conductor. The observable phe-
nomenon here depends only on the relative motion of the conductor and the
magnet, whereas the customary view draws a sharp distinction between the two
cases in which either the one or the other of these bodies is in motion. For if the
magnet is in motion and the conductor at rest, there arises in the neighbour-
hood of the magnet an electric field with a certain definite energy, producing
a current at the places where parts of the conductor are situated. But if the
magnet is stationary and the conductor in motion, no electric field arises in the
neighbourhood of the magnet. In the conductor, however, we find an electro-
motive force, to which in itself there is no corresponding energy, but which gives
rise—assuming equality of relative motion in the two cases discussed—to elec-
tric currents of the same path and intensity as those produced by the electric
forces in the former case."​

Thank u so much sir.
I have read this paper. I do have a copy of the book that compiles his papers.
But as the papers are highly technical, his book, Relativity: special and general theory helps me better. And more helpful is the book, The universe and Mr Einstein by Lincoln Barnette, to which Einstein himself wrote a preface. Barnette has given some hint towards the physical meaning of time dilation, and as Einstein has wrote its preface, I shall presume that he might not be against Barnette's views.
But sir, my limited thinking capacity can appreciate the above case of relative motion only for limted contexts, for calculation of distance and relative speeds ect. I am unable to treat the photons and the stationary torch that is emitting them on equal footing. This needs a higher thinking capacity that I do not have.
My intention behind starting this thread was only to know one thing- why cannot we accept that Maxwell's theory of electrodynamics (about constancy of light speed) is itself a law and we do not need any other law to explain it?
And I have gained a lot from the discussions done here.
Thanks and regards.

 

[Sorcerer]

Thank u so much sir.
I have read this paper. I do have a copy of the book that compiles his papers.
But as the papers are highly technical, his book, Relativity: special and general theory helps me better. And more helpful is the book, The universe and Mr Einstein by Lincoln Barnette, to which Einstein himself wrote a preface. Barnette has given some hint towards the physical meaning of time dilation, and as Einstein has wrote its preface, I shall presume that he might not be against Barnette's views.
But sir, my limited thinking capacity can appreciate the above case of relative motion only for limted contexts, for calculation of distance and relative speeds ect. I am unable to treat the photons and the stationary torch that is emitting them on equal footing. This needs a higher thinking capacity that I do not have.
My intention behind starting this thread was only to know one thing- why cannot we accept that Maxwell's theory of electrodynamics (about constancy of light speed) is itself a law and we do not need any other law to explain it?
And I have gained a lot from the discussions done here.
Thanks and regards.

Based on my limited knowledge of Maxwell’s equations, they come with special relativity already built in. I would say special relativity isn’t any new laws, but is rather the logical consequence of Maxwell’s equations applying to both electromagnetism and mechanics.But if you are looking for something that appears a bit more fundamental, look up the fine structure constant, something I’ve learned a little bit about since being here.

 

[dayalanand roy]

Based on my limited knowledge of Maxwell’s equations, they come with special relativity already built in. I would say special relativity isn’t any new laws, but is rather the logical consequence of Maxwell’s equations applying to both electromagnetism and mechanics.But if you are looking for something that appears a bit more fundamental, look up the fine structure constant, something I’ve learned a little bit about since being here.

Thanks sir.
This is what i have thinking about- Special relativity is just a logical consequence of Maxwell's law and Lorentz transformation, not a new law.
But i am unable to follow what you mean by "fine structure constsnt". I shall be obliged to have it more clarified..
Regards.

 

[PeterDonis]

i am unable to follow what you mean by "fine structure constant"

Have you tried Google?

 

[Ibix]

Special relativity is just a logical consequence of Maxwell's law and Lorentz transformation, not a new law.

Special relativity is the Lorentz transforms. Everything follows from them.

But if you want to regard relativity as a consequence of electromagnetism then you should also argue that it is (independently) a consequence of the strong force, the weak force, and gravity, since all of those are also relativistic theories.

 

[dayalanand roy]

Have you tried Google?

I am trying.
Thanks

 

[dayalanand roy]

Special relativity is the Lorentz transforms. Everything follows from them.

But if you want to regard relativity as a consequence of electromagnetism then you should also argue that it is (independently) a consequence of the strong force, the weak force, and gravity, since all of those are also relativistic theories.

Thanks.
I agree.

 

[Ziang]

Time dilation and length contraction are actually logical consequences of the speed of light (really, electromagnetic waves) being independent of its source of motion.

They are logical or not depends on how light propagates in a vacuum.
If light travels like a particle then they are logical.
If light travels like waves then they might not be logical.

 

[PeterDonis]

If light travels like a particle then they are logical.
If light travels like waves then they might not be logical.

No, this makes no difference. Maxwell's Equations are Lorentz invariant, and they describe the propagation of electromagnetic waves.

 

[Sorcerer]

They are logical or not depends on how light propagates in a vacuum.
If light travels like a particle then they are logical.
If light travels like waves then they might not be logical.

Are you aware that Maxwell’s equations, which are built inherently with the rules of special relativity, describe electromagnetic propagation as a wave? This can be relatively easily derived from Maxwell’s equations with undergraduate math (“easily” if you’ve taken two semesters of differential equations and three or four of calculus). You get a second order partial differential equation - in particular, a WAVE equation - with a phase velocity that “just so happens” to be the speed of light.

As has already been stated, Maxwell’s equations are Lorentz invariant (that is, they are built with special relativity already in them), AND they describe light as a wave.

So basically, and sorry if this comes as rudeness, but your claim is simply false.Edited to add:

More information: https://en.m.wikipedia.org/wiki/Electromagnetic_wave_equation

 

[Ziang]

Let us see the following picture

Light is a transverse wave. In the spaceship the light path is vertical. So the oscillations (blue waves) are horizontal as shown in section a.
According to SR, the diagonal red line in section b is a light path with respect to a stationary observer (who is standing on the ground).

Argument
Physics laws are the same to all observers. Light must be a transverse wave to all observers.
When the spaceship is moving at a constant velocity v, the blue waves would appear as shown in section b, with respect to the stationary observer. These blue waves are not perpendicular to the diagonal red line. So the diagonal red line is not a light path with respect to the stationary observer.

 

[Ibix]

Let us see the following picture

View attachment 223357Light is a transverse wave. In the spaceship the light path is vertical. So the oscillations (blue waves) are horizontal as shown in section a.
According to SR, the diagonal red line in section b is a light path with respect to a stationary observer (who is standing on the ground).

Argument
Physics laws are the same to all observers. Light must be a transverse wave to all observers.
When the spaceship is moving at a constant velocity v, the blue waves would appear as shown in section b, with respect to the stationary observer. These blue waves are not perpendicular to the diagonal red line. So the diagonal red line is not a light path with respect to the stationary observer.

This argument is based on a misconception. Light is not a mechanical wave. There is nothing moving transversely in an EM wave. The electric and magnetic field vectors at a point change in a sinusoidal way, but nothing moves. Transforming the electromagnetic field tensor will give you a transverse electromagnetic wave in both frames, qualitatively because there's cross talk between the electric field as measured in one frame and the magnetic field in the other.

Furthermore your argument is obviously wrong, since you are arguing that light waves aren't light waves in another frame.

 

[PeterDonis]

Light is a transverse wave.

Not in the sense you mean. @Ibix's response is correct. Please do not post further on this without taking the time to learn the correct model of light.

 

[Dale]

Light is a transverse wave.

You clearly don’t know what this means. It does not mean that light wiggles back and forth as you show in your diagram, it means that light can be polarized.

Write the equation for a plane wave, propagating along z and polarized along x. Transform it and you have a polarized plane wave. Similarly for a plane wave propagating along z and polarized along y.

 

[Sorcerer]

I believe this link covers most of the bases here. It is extremely informative about why electromagnetic phenomena are waves AND why they are inherently Lorentz invariant. It also points out the similar relation ship the E and B fields have with the space and time interval. Very interesting read for those interested in learning.

http://www.mathpages.com/rr/s2-02/2-02.htm

 

[JulianM]

You clearly don’t know what this means. It does not mean that light wiggles back and forth as you show in your diagram, it means that light can be polarized.

Write the equation for a plane wave, propagating along z and polarized along x. Transform it and you have a polarized plane wave. Similarly for a plane wave propagating along z and polarized along y.

What then is the meaning of a polarized wave? It is created by placing a slit in the light beam so that only waves which are aligned with the slit pass through.

I don't see how the equations help one to visualize this.

 

[Nugatory]

What then is the meaning of a polarized wave? It is created by placing a slit in the light beam so that only waves which are aligned with the slit pass through.

Nothing is wiggling though. At every point in space there is an electrical field, and at every moment that field points in some direction. If the direction is always the same (so only the amplitude of the field is changing with time - written as a vector we have $\vec{E}=(A\sin{\omega}t)\hat{x}$ where $\hat{x}$ is a fixed unit vector in the direction of polarization), then we say that the wave is linearly polarized.

 

[JulianM]

Nothing is wiggling though. At every point in space there is an electrical field, and at every moment that field points in some direction. If the direction is always the same (so only the amplitude of the field is changing with time - written as a vector we have $\vec{E}=(A\sin{\omega}t)\hat{x}$ where $\hat{x}$ is a fixed unit vector in the direction of polarization), then we say that the wave is linearly polarized.

If that were true then surely a light beam could be polarized by a pinhole and a slit would not be necessary.

 

[Nugatory]

If that were true then surely a light beam could be polarized by a pinhole and a slit would not be necessary.

The direction of polarization (the direction that vector $\hat{x}$ points) is necessarily perpendicular to the direction of propagation of the light.

A linear polarizer doesn't really use a slit - that's just the way the illustrations are drawn. For an easy-to-understand example of how a polarizer might really work, you might try the Wikipedia description of a wire-grid polarizer.

 

[Ibix]

If that were true then surely a light beam could be polarized by a pinhole and a slit would not be necessary.

A polariser (or, at least, one kind of polariser) works by letting electrons flow in the x direction but not the y direction. So an incident EM wave with an electric field in the x direction starts electrons wiggling in the x direction, and one with its electric field in the y direction does not excite electrons. But nothing about the wave itself is moving in the x direction; the field is time varying, that's all.

A pinhole does not have a preferred direction for electron motion, so is not a polariser.

 

[JulianM]

A polariser (or, at least, one kind of polariser) works by letting electrons flow in the x direction but not the y direction. So an incident EM wave with an electric field in the x direction starts electrons wiggling in the x direction, and one with its electric field in the y direction does not excite electrons. But nothing about the wave itself is moving in the x direction; the field is time varying, that's all.

A pinhole does not have a preferred direction for electron motion, so is not a polariser.

Nugatory said nothing is wiggling. Your opinion is electrons are wigling.

The OP was talking about light so presumably photons. What are the opinions on light?

As far as I am aware a wire grid is just a matrix of slits and is quite similar to the lines in polarizing sun glasses.

 

[jbriggs444]

Nugatory said nothing is wiggling. Your opinion is electrons are wigling.

The light is not wiggling. Electrons in the polarizer are wiggling.

As far as I am aware a wire grid is just a matrix of slits and is quite similar to the lines in polarizing sun glasses.

A wire grid has effect not because of the slits, but because of its interaction with the time varying field which is light.

 

[Dale]

What then is the meaning of a polarized wave? It is created by placing a slit in the light beam so that only waves which are aligned with the slit pass through.

A polarizer usually does not have slits, at least not at optical wavelengths. Typically it is simply a material whose electrical properties are anisotropic.

I don't see how the equations help one to visualize this.

The equations are not about visualizing anything. They are simply about refuting his absurd claim.

 

[Nugatory]

Nugatory said nothing is wiggling. Your opinion is electrons are wigling.

@Ibix and I are not disagreeing - I was just careless in my wording when I said "nothing is wiggling" when I should have said that nothing in the wave is wiggling back and forth. When electromagnetic radiation encounters any conductive material, electrons in the material do respond by wiggling in response to changes in the electrical field at the position of the electron - but the wave itself is not anything wiggling, and there's nothing wiggling back and forth where the electromagnetic wave is propagating through the empty space on either side of the polarizing filter.

The OP was talking about light so presumably photons

OP was talking about light, so was talking about classical electromagnetic radiation, not photons. Polarization of light is a classical electromagnetic phenomenon understood by solving Maxwell's equations, has nothing to do with photons and quantum electrodynamics. (The relationship flows the other way - you have to understand the classical model before you can start leaning the quantum theory).

As far as I am aware a wire grid is just a matrix of slits and is quite similar to the lines in polarizing sun glasses.

That's not correct. Polarizing sun glasses use fine lines etched in the surface of an otherwise transparent material, and the physics of how that transmits light polarized in one direction and reflects light polarized in another direction is very complicated (How complicated? Well, first you have to explain how any ostensibly transparent material can ever reflect any light). The physics of the wire grid is completely different and much simpler - first-semester electrostatics is all it takes to show that the grid will only allow the electric field to point in one direction.

 

[Ibix]

Nugatory said nothing is wiggling. Your opinion is electrons are wigling.

The OP was talking about light so presumably photons. What are the opinions on light?

In a light wave in vacuum, nothing is wiggling. There is no piece of EM field bobbing up and down as the wave passes - it is just the field vectors at each point changing magnitude and direction.

That wave can start other stuff wiggling, however, such as electrons. And that's how the wire grid polariser works - the electrons absorb energy if the E field is parallel to the wires. All polarisers have a preferred direction, which is why a pinhole doesn't polarise - that's all I was pointing out.

 

[JulianM]

@Ibix and I are not disagreeing - I was just careless in my wording when I said "nothing is wiggling" when I should have said that nothing in the wave is wiggling back and forth. When electromagnetic radiation encounters any conductive material, electrons in the material do respond by wiggling in response to changes in the electrical field at the position of the electron - but the wave itself is not anything wiggling, and there's nothing wiggling back and forth where the electromagnetic wave is propagating through the empty space on either side of the polarizing filter.
OP was talking about light, so was talking about classical electromagnetic radiation, not photons. Polarization of light is a classical electromagnetic phenomenon understood by solving Maxwell's equations, has nothing to do with photons and quantum electrodynamics. (The relationship flows the other way - you have to understand the classical model before you can start leaning the quantum theory).
That's not correct. Polarizing sun glasses use fine lines etched in the surface of an otherwise transparent material, and the physics of how that transmits light polarized in one direction and reflects light polarized in another direction is very complicated (How complicated? Well, first you have to explain how any ostensibly transparent material can ever reflect any light). The physics of the wire grid is completely different and much simpler - first-semester electrostatics is all it takes to show that the grid will only allow the electric field to point in one direction.

That's not correct. The polarizing lens, developed and patented by Edwin Land, uses long strand molecules oriented I one direction.

 

[Nugatory]

That's not correct. The polarizing lens, developed and patented by Edwin Land, uses long strand molecules oriented I one direction.

Ah - right, I was thinking about a different type of polarizer.

The oriented long-strand molecules work similarly to a wire-grid polarizer(electrons move freely along the strands), so pass light that is polarized perpendicular to the direction of the strands.

 

[Dale]

That's not correct. The polarizing lens, developed and patented by Edwin Land, uses long strand molecules oriented I one direction.

On the scale of optical wavelengths there are no slits. The material is a continuous medium. Its material properties are simply anisotropic.

 

[Mister T]

If the direction is always the same (so only the amplitude of the field is changing with time - written as a vector we have $\vec{E}=(A\sin{\omega}t)\hat{x}$ where $\hat{x}$ is a fixed unit vector in the direction of polarization), then we say that the wave is linearly polarized.

If that were true then surely a light beam could be polarized by a pinhole and a slit would not be necessary.

Can you explain what you mean? How can a pinhole be used to produce an oscillating electric field described by $\vec{E}=(A\sin{\omega}t)\hat{x}$?

Or for that matter, how could a slit? Slits can polarize a mechanical wave, so that's an analogy often used to describe polarization.

By the way, phrases like "light is a wave" can be easily misunderstood. What it means in this case is that the behavior of light can sometimes be described by equations that describe wave behavior. In physics we're describing behavior, not assigning a status of "real" to any of the models that are a part of those descriptions. What I mean is that saying light's behavior can be described using a wave model may mean to some people quite a different thing than saying light is a wave. But in fact the former is just a more precise way of stating the latter.

 

[JulianM]

Can you explain what you mean? How can a pinhole be used to produce an oscillating electric field described by $\vec{E}=(A\sin{\omega}t)\hat{x}$?

Or for that matter, how could a slit? Slits can polarize a mechanical wave, so that's an analogy often used to describe polarization.

By the way, phrases like "light is a wave" can be easily misunderstood. What it means in this case is that the behavior of light can sometimes be described by equations that describe wave behavior. In physics we're describing behavior, not assigning a status of "real" to any of the models that are a part of those descriptions. What I mean is that saying light's behavior can be described using a wave model may mean to some people quite a different thing than saying light is a wave. But in fact the former is just a more precise way of stating the latter.

Please read carefully. I said - "if that were true"
Clearly it is not, therefore it is caused by something else.

Grids, slits and long chain molecules do polarize light, therefore it seems logical to infer that having a dimension greater than a pinhole is a requirement for polarization.

 

[Dale]

Grids, slits and long chain molecules do polarize light, therefore it seems logical to infer that having a dimension greater than a pinhole is a requirement for polarization.

Polarization also can happen just by reflection from a nonconductive surface, or by scattering off individual molecules in a gas. So a dimension greater than a pinhole is not necessary.

In any case, this discussion of polarization is very much a distraction from the topic of the thread. If you have questions about polarization then please start a new thread.