Harmless Lorentz transformation question

Click For Summary
The discussion revolves around the application of Lorentz transformations in the context of a photon emitted from an inertial reference frame S. The main question is about determining the coordinates in a reference frame S' that is supposedly at rest with respect to the photon. Participants clarify that a reference frame where a photon is at rest cannot be an inertial frame, as it contradicts the principles of relativity where light always travels at speed c. The conversation also touches on the limitations of Galilean transformations and the necessity of Lorentz transformations to maintain the consistency of physical laws across different frames. Ultimately, the discussion emphasizes the foundational aspects of special relativity and the implications of Maxwell's equations in defining inertial frames.
  • #91
pervect said:
Originally Posted by Goldstein, pg 277
...Now, it is a postulate of physics, implicit since the time of Galielo and Newton, that all phenomena of physics should appear the same in all systems moving uniformly relatively to each other...

Right, I understand that.That is a statement of what Einstein called the Principle of Relativity. However, there is something about the buildup to SR that I have never understood. People other than Einstein were aware of the problem that the principle of relativity had become at odds with the law of the velocity propagation of light in vacuo. However, their favoured resolution to the problem completely flew in the face of the principle of relativity! Why is it that nobody else had a problem right from the start with the notion of an absolute rest frame, and differences in the perceived speed of light perpendicular and // to Earth's orbit? If the predictions of the Michaelson-Morely experiment were bourne out, would people have been willing to throw out the principle of relativity completely? This despite the fact that it was something so logically intuitive that nobody had even thought to question it since the time of Galileo and Newton! Everybody seemed willing to ignore this glaring problem and support a prediction that preserved the Galilean transformation, under which Maxwell's equations are not invariant.
 
Physics news on Phys.org
  • #92
cepheid said:
Right, I understand that.That is a statement of what Einstein called the Principle of Relativity. However, there is something about the buildup to SR that I have never understood. People other than Einstein were aware of the problem that the principle of relativity had become at odds with the law of the velocity propagation of light in vacuo. However, their favoured resolution to the problem completely flew in the face of the principle of relativity! Why is it that nobody else had a problem right from the start with the notion of an absolute rest frame, and differences in the perceived speed of light perpendicular and // to Earth's orbit? If the predictions of the Michaelson-Morely experiment were bourne out, would people have been willing to throw out the principle of relativity completely? This despite the fact that it was something so logically intuitive that nobody had even thought to question it since the time of Galileo and Newton! Everybody seemed willing to ignore this glaring problem and support a prediction that preserved the Galilean transformation, under which Maxwell's equations are not invariant.
No one is bothered by the fact that soundwaves in air don't look the same in every reference frame. As long as electromagnetic waves are vibrations in a physical medium, the ether, then the fact that the laws of electromagnetism don't look the same in every reference frame doesn't mean there are any fundamental laws of nature that disobey the principle of relativity, since the laws governing soundwaves in ether wouldn't be seen as any more fundamental than the laws governing soundwaves in air, it could be seen as just a matter of historical accident that the ether has the particular rest frame it does (you'd be under no obligation to believe the ether is at rest in absolute space, for example).

I don't know if 19th-century scientists thought of it this way though--maybe they just didn't consider the principle of relativity as fundamental as we do (after all, Newton himself believed in absolute space, since that seemed to be the only way to explain why acceleration is absolute).
 
Last edited:
  • #93
cepheid said:
If the predictions of the Michaelson-Morely experiment were bourne out, would people have been willing to throw out the principle of relativity completely? This despite the fact that it was something so logically intuitive that nobody had even thought to question it since the time of Galileo and Newton!

Why would they ??Would the Principle of Relativity ("all laws of mechanics are invariant under the Galieli group") be proven wrong,if the Michaelson-Morley experiment would have had another turn out...?

Daniel.
 
  • #94
cepheid said:
Why is it that nobody else had a problem right from the start with the notion of an absolute rest frame, and differences in the perceived speed of light perpendicular and // to Earth's orbit? If the predictions of the Michaelson-Morely experiment were bourne out, would people have been willing to throw out the principle of relativity completely?

While I'm not sure how accurate my insight into the mindset of those times is, my impression was that the usual practice was to compare ether waves with sound waves. So while I don't think the abandonment of the principle of relativity would be complete, I also don't think it would have the same emphasis as it does today.

However, I think a significant number of people even at the time were rather disturbed by the rather strange collection of properties that were being ascribed to the ether - that it was simultaneously present even in empty space, offering no resistance to the passage of uncharged bodies, but yet so rigid that it would transmit vibrations at the speed of light.

So people did what I think was a very sensible thing- they looked for some actual evidence that it (the ether) existed.

Ultimately, science is not about how long an idea has been around, but how well it works. If the ether theory had made testable predictions that were borne out by experiment, I think the theory would have stuck around. But it didn't.
 
  • #95
Physicsguru -- Read the literature. Physics has had a century to study and dissect relativity and E&M. Your objections have certainly been raised many times. How could you dispute the Lorentz invariance of Maxwell's Eq.? That invariance is central to much of modern physics, which, after all, is at least modestly successful. The debate was over certainly by the 1930s, and Einstein and Maxwell came through unscathed. Lasers, radar and masers, communication with space probes, radio and tv broadcasting, the Compton effect, photoelectric phenomena, atomic spectra and the classical theory of radiation all require the standard approach to realtivity and electrodynamics.

If you are serious about your objections then
1. Make sure you fully understand the electrodynamics and relativity that most of us treasure and love.(If you do not, few if any will pay attention to you)
2. Base your objections on experimental evidence. After all, physics is an empirical science.

There's no way a photon can be at rest in an inertial frame. If you can demonstrate that this is not true, buy your ticket to Stockholm.

Regards,
Reilly Atkinson
 
  • #96
reilly said:
Physicsguru -- Read the literature. Physics has had a century to study and dissect relativity and E&M. Your objections have certainly been raised many times. How could you dispute the Lorentz invariance of Maxwell's Eq.? That invariance is central to much of modern physics, which, after all, is at least modestly successful. The debate was over certainly by the 1930s, and Einstein and Maxwell came through unscathed. Lasers, radar and masers, communication with space probes, radio and tv broadcasting, the Compton effect, photoelectric phenomena, atomic spectra and the classical theory of radiation all require the standard approach to realtivity and electrodynamics.

If you are serious about your objections then
1. Make sure you fully understand the electrodynamics and relativity that most of us treasure and love.(If you do not, few if any will pay attention to you)
2. Base your objections on experimental evidence. After all, physics is an empirical science.

There's no way a photon can be at rest in an inertial frame. If you can demonstrate that this is not true, buy your ticket to Stockholm.

Regards,
Reilly Atkinson


Suppose that the charge density \rho which aappears in Coulomb's law satisfies the following postulate:

Postulate:

\nabla ^2 \rho = \frac{1}{c^2} \rho_t_t

From the postulate above, we have:

\nabla ^2 \rho = \nabla \bullet \nabla \rho = \frac{1}{c^2} \frac{\partial}{\partial t} \frac{\partial \rho}{\partial t}

Recall the continuity equation which holds if electric charge is conserved:

\nabla \bullet \vec J = - \frac{\partial \rho}{\partial t}

Therefore, if the postulate is true & electric charge is conserved then:

\nabla ^2 \rho = \nabla \bullet \nabla \rho = - \frac{1}{c^2} \frac{\partial}{\partial t} \nabla \bullet \vec J

From which it follows that:

\nabla \bullet \nabla \rho = \nabla \bullet - \frac{1}{c^2} \frac{\partial \vec J}{\partial t}

From which it follows that:

\nabla \rho = -\frac{1}{c^2} \frac{\partial \vec J }{\partial t}

Now, consider the formula for the electric field at (x,y,z) due to the presence of some non-zero charge density \rho which satisfies the postulate above. We have:

\vec E = \frac{1}{4\pi\epsilon_0} \int \rho d\tau \frac{\hat R}{R^2}

Now, take the curl of both sides of the expression above to obtain:


\nabla \times \vec E = \frac{1}{4\pi\epsilon_0} \int \nabla \times (\rho d\tau \frac{\hat R}{R^2} )

Recall the following mathematical theorem:

Theorem:

\nabla X (f\vec A) = \nabla f \times \vec A + f\nabla \times \vect A

For any scalar function f, and any vector function A.

Since the charge density rho is a scalar function, and the volume current density is a vector function, it follows that:

\nabla \times \vec E = <br /> <br /> \frac{1}{4\pi\epsilon_0} \int \nabla \rho \times \frac{\hat R}{R^2} d\tau <br /> <br /> +<br /> <br /> \frac{1}{4\pi\epsilon_0} \int \rho \nabla \times \nabla (\frac{-1}{R}) d\tau <br /> <br />

Since the curl of a gradient is zero, the second term above drops out, and we are left with:

\nabla \times \vec E = \frac{1}{4\pi\epsilon_0} \int \nabla \rho \times \frac{\hat R}{R^2} d\tau

Now, substitute the expression which was derived earlier from the postulate for the gradient of the charge density in the equation above to obtain:

\nabla \times \vec E = \frac{1}{4\pi\epsilon_0} \int <br /> <br /> <br /> -\frac{1}{c^2} \frac{\partial \vec J }{\partial t}<br /> <br /> \times \frac{\hat R}{R^2} d\tau

Suppose that the classical Maxwellian result for the speed of light is correct, so that:

\frac{1}{c^2} = \epsilon_0 \mu_0

We can now rewrite the curl of E as:

\nabla \times \vec E = - \frac {\partial}{\partial t}<br /> <br /> <br /> \frac{\mu_0}{4\pi} \int \vec J \times \frac{\hat R}{R^2} d\tau

Where we have arrived at the Biot-Savart expression for B:

Biot-Savart Law (definition of Magnetic field B)

\vec B = \frac{\mu_0}{4\pi} \int \vec J \times \frac{\hat R}{R^2} d\tau

Thus, we have arrived at one of Maxwell's equations, namely:

\nabla \times \vec E = - \frac{\partial \vec B}{\partial t}

The forumal above is true, provided that all the postulates used in the derivation are true. Namely:

Postulate 1: Charge density obeys a wave equation, namely:

\nabla ^2 \rho = \frac{1}{c^2} \rho_t_t

where the wavespeed is given by c, where:

c = \frac{1}{\sqrt{\epsilon_0 \mu_0}}

Postulate 2: The total electric charge of the universe cannot vary for any reason.

By experiment it is known that the speed of light through matter is less than the speed of light in vaccuum. The two speeds are related through the well known "index of refraction" n. Let v denote the speed of light through some substance with index of refraction n, we have:

v = \frac{c}{n}

So, if instead, the wave equation for a charge density wave is:

\nabla ^2 \rho = \frac{1}{v^2} \rho_t_t

where

v = \frac{c}{n}

Then instead we have:

\nabla ^2 \rho = \frac{n^2}{c^2} \rho_t_t

This would have altered the previous derivation above slightly. Instead, we would have had:

\nabla \times \vec E = \frac{1}{4\pi\epsilon_0} \int <br /> <br /> <br /> -\frac{n^2}{c^2} \frac{\partial \vec J }{\partial t}<br /> <br /> \times \frac{\hat R}{R^2} d\tau

Let c denote the speed of light relative to an emitter, and let the speed of light relative to the emitter satisfy:

\frac{1}{c^2} = \epsilon_0 \mu_0

Therefore, instead we have:

\nabla \times \vec E = - n^2 \frac{\partial \vec B}{\partial t}
 
Last edited:
  • #97
1.Your last two expressions coincide.So edit your post.EDIT:You did.
2.How did u get that formula giving \vec{E} as a function of \rho
3.What are your calculations trying to prove...?

Daniel.
 
  • #98
dextercioby said:
1.Your last two expressions coincide.So edit your post.EDIT:You did.
2.How did u get that formula giving \vec{E} as a function of \rho
3.What are your calculations trying to prove...?

Daniel.

Suppose there is a blob of matter at rest in some inertial reference frame, which has a nonzero electric charge, and pretend its inertia is infinite. Let all other matter in the universe be electrically neutral, except for one electron which happens to find itself in the electric field of the blob. The idea is that the electron, which otherwise would coast at a constant speed in a straight line experiences a force, and is accelerated, and the formula relating the inertia of the electron, to its change in speed, and the impressed force upon it is:

\vec F = \frac{d(m\vec v)}{dt}

And also that this force is related to the electric charge q of the electron, and the value of the electric field of the blob local to the electron, as:

\vec F = q \vec E

Couloumb's law for the electric force between two charged objects:

Let one object have electric charge Q1, and let the other object have charge Q2. By experiment, the electric force between them is:

\vec F = KQ1Q2 \frac{\hat R}{R^2}

Postulate: The total electric charge of anybody is quantized. A unit of electric charge is given by:

e = -1.6021773 x 10^{-19} C

So if there are N electric charges on one object, and n electric charges on another object, we have:

Q1 = ne
Q2 = Ne

Let the object with electric charge Q2 be the object with an infinite inertia, and let the other object have n=1, and call the other object an electron.

There is enough information above to solve for the electric field created by the object with electric charge Q2, and we find that:

\vec E = KQ2 \frac{\hat R}{R^2}

The formula above is obtained if

Q2 = \int \rho d\tau

Giving us the classical result that:

\vec E = K \int \rho d\tau \frac{\hat R}{R^2}

And letting K= \frac{1}{4\pi\epsilon_0}
we finally have the formula you asked about.

P.S. And yes I know, classically, electric charge is not quantized.

Regards,

Guru
 
Last edited:
  • #99
And how's that stuff related to the previous discussion ?And that \vec{E} in what reference frame is it considered...?

Daniel.
 
  • #100
dextercioby said:
And how's that stuff related to the previous discussion ?And that \vec{E} in what reference frame is it considered...?

Daniel.

If you read carefully, the speed of the test charge (a single electron in the example here) is being defined in the rest frame of the blob.

P.S.

The blob has an infinite mass, and therefore cannot be accelerated in this frame, regardless of whether or not it emits particles.

The electrons in the blob are the things that are responsible for a nonzero electric field local to the test charge, which is assumed far away from the center of mass of the blob, so that this force is a long range force.

If the electrons in the blob are at rest in this frame, then the electric field local to the electron way far away is irrotational, in otherwords, E would be an electrostatic field.

However, if the postulate about charge density waves is correct, then the blob could be supporting a charge density wave.

Regards,

Guru
 
Last edited:
  • #101
Physicsguru said:
My question is simple.

If, after time amount of time \Delta t has elapsed according to some clock which is permanently at rest in S, the photon has coordinates (L,0,0) in frame S, what is my x` coordinate in frame S`?

For the sake of definiteness, suppose that exactly one second has ticked according to a clock at rest in frame S. Therefore, the location of the photon in frame S is given by (299792458 meters, 0,0).

Let (M,0,0)` denote the location of the origin of inertial reference frame S, in reference frame S`, at the instant that the clock at rest in frame S strikes one. Solve for M.

Guru

I read all arguments, but then finally what is the answer to the first question raised by Guru?

What is M? and
(I am adding this) What is the speed of the photon in the frame S'?!

How I feel the question can be answered (possible ways) are:

a) Give exact values for M and Vs'
b) Give probability distribution of possible values for M and Vs'
c) Assert and prove that the Conceptualization of a reference frame attached to the photon, is meaningless and thus refrain from answering the question. (Here whether lorentz transformation holds good in that reference frame is besides the question as we do have some thing (photon) which moves at 'c' the lorentz constant and in our minds we can think of it as a reference frame. What I feel one needs to prove here is that thought is meaningless and doesn't make sense).

I didnt go thru the entire thing, but I suppose I haven't missed the answer to the initial question.

-cf
 
  • #102
Physicsguru said:
If, after time amount of time \Delta t has elapsed according to some clock which is permanently at rest in S, the photon has coordinates (L,0,0) in frame S, what is my x` coordinate in frame S`?
Guru

I think here the key is not the x' coordinate, but the t' coordinate. The light will move with the speed 'c' in the S' reference frame also, as the light speed is constant. But the time coordiante will stand still at 0. As no time will be elaspsed inspite of the fact the photon travels so fast, the photon will still be at the origin of S' frame as measured from S' frame.

The lorentz transformation becomes indefinite when when v/c goes to 1 if we do mere substitution,but I feel if we are able to take limits of the lorentz transformation as v/c goes to 1, through rules similar to L'Hospitals, it would be possible to find the t' and x' coordinates of the photon as limits and my guess is it will be equal to 0 and 0 resply always and in a way doesn't depend on t and x.

Also the location of the origin of S' reference frame measured in S reference frame will be moving at the speed of ct.

I haven't dwelled on any quantum reasoning here and I am sticking to classical physics.
 

Similar threads

  • · Replies 7 ·
Replies
7
Views
1K
  • · Replies 101 ·
4
Replies
101
Views
7K
  • · Replies 9 ·
Replies
9
Views
2K
Replies
12
Views
2K
Replies
15
Views
1K
  • · Replies 24 ·
Replies
24
Views
3K
  • · Replies 22 ·
Replies
22
Views
1K
  • · Replies 15 ·
Replies
15
Views
2K
  • · Replies 7 ·
Replies
7
Views
1K
Replies
32
Views
4K