# All Questions

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2answers
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### Does a static electric field travel at the speed of light?

Suppose we have a large area where the net electric field is zero. We place a charge q somewhere in this area. How long will it it take for another positive charge q' standing at a distance d from q ...
4answers
132 views

### Do photons interact or not directly?

All I am asking about is photon (EM wavepacket) photon (EM wavepacket) interaction. I have read this question: If photon-photon interactions are impossible, how are higher harmonics generated? ...
2answers
37 views

### Changing Refractive Index

Is it possible that the refractive index of a material could change?if so,what are those factors and what kind of variation could they bring?
2answers
87 views

### How did Maxwell figure out the speed of light?

The Wiki article is about 2 graduate years of physics beyond my understanding. What is a good high-school rendition of his thought process: regarding his use of the "distributed capacitance and ...
1answer
158 views

### How to derive $c=1/\sqrt{\varepsilon_0\,\mu_0}$ from integral form of Maxwell equations? [closed]

I've read similar questions and answers given thereto but find them unsatisfactory. So please don't mark my question as "duplicate". The question may as well be a duplicate, but it's still waiting for ...
2answers
220 views

### How do we get a expression for speed of light $\frac{1}{\sqrt{\mu\epsilon}}$ using Maxwell's equation? [duplicate]

I am a $12^{th}$ class student in India. I am quite new to these interesting concepts. And, I think I've learnt about electrostatics, magnetism, Maxwell's equations very thoroughly. But, my books ...
1answer
45 views

### “flowing” speed of an electric field?

Consider a capacitor made of two infinite plates which are at a very large distance $d$ from each other. We place a positive charge on a plate and a negative one on the other one. Two electrical ...
2answers
57 views

### How was the value of vacuum permittivity originally found?

The vacuum permittivity appears originally in Maxwell's equations, used to describe electric fields. The permeability of vacuum was defined using Ampere's force law (itself derived from Biot-Savart ...
3answers
223 views

### Another objection to Feynman's moving infinite sheet of charge “radiator”

In The Feynman Lectures on Physics Vol II 18-4, A travelling field we are provided the thought experiment of two uniformly and oppositely electrically charged planar sheets of infinite extent lying in ...
0answers
54 views

### Purported non-constant speed of light

What is going on there to make it appear that light is slower than $c$ ? Here is the popular science news article: Speed of light not so constant after all; Pulse structure can slow photons, even ...
5answers
6k views

### Do Maxwell's equations predict the speed of light exactly?

I know that $\frac{1}{\sqrt{\mu_0\varepsilon_0}}$ is equal to the speed of light but is this prediction accurate? I mean is it 100 percent accurate?
2answers
75 views

### Paradox of the magnetic field's collapse [closed]

Imagine two strong electromagnets (X and Y) enough far from each other. Facts: 1.) We know the magnetic fields spreading with the speed of light. 2.) When the magnetic fields reach the other ...
1answer
51 views

### Has the velocity of propagation of a magnetic field been measured?

I understand the velocity of propagation of a magnetic field is supposed to be c, the velocity of light, but I'm not able to find any experimental tests that this is the case. I believe there have ...
3answers
103 views

### Will the electron affected by another electron via Coulomb's force affect this electron instantly? [duplicate]

Coulomb's law is strict: $F=k\dfrac{q_1*q_2}{r^2}$ that means that between two charges occurs force. I.e. occurs force on $q_1$ and on $q_2$. If there are two electrons in vacuum with 300 00 000 ...
2answers
100 views

### Speed of EM waves differ from GWs?

I understand that light travels at speed c in vacuum, when measured locally. This speed has an exact value, not an approximation, because it is defined as 299 792 458 m / s. It is an exact value, ...
2answers
351 views

### Do electromagnetic fields have inertia? Or, what sets the speed of light?

In all mechanical waves, there is a restoring force and an inertial influence. For example, a plucked string oscillates because the restoring force brings it back to being straight and then the string'...
1answer
53 views

### In how far can a source term influence the propagation speed of an EM wave?

Unfortunately, I can only find discussions of signal speed for solutions of the homogeneous EM wave equations (e.g. https://arxiv.org/pdf/1111.2402.pdf). Therefore, let me ask here: Can the form of ...
1answer
221 views

### Virtual photon exchange instantaneously

I have read these questions: Virtual Photon transmission speed of a Static Electric Field? where John Rennie's comments say: Virtual photons can travel faster than light Is there a "...
4answers
165 views

### Problem with relativistic mass

We know that relativistic mass of a object is given by: $$m=\frac{m_{0}}{\sqrt{1-\frac{v^2}{c^2}}}.$$ So the mass of an object will become very large if it travels with a speed near to the speed of ...
6answers
3k views

### Is force due to electric field instantaneous? If not then why?

Suppose a charge $q$ is experiencing a force due to charge $Q$. Suppose we move the charge $Q$ very slowly (no acceleration) what's the instantaneous impact on the charge $q$? How will the $q$ react?
1answer
48 views

### Shapiro test interstellar/intergalactic

I understand that the Shapiro time delay effect is one of the four classic solar system GR tests. I understand the EM waves have to pop back from a planet beyond the star and then pass next to the ...
6answers
504 views

### Give an interpretation of what $c=\frac{1}{\sqrt{\varepsilon_0\mu_0}}$ actually means

From my later school days on, the formula that impresses me the most is $$c=\frac{1}{\sqrt{\varepsilon_0\mu_0}}$$ I know how this can be derived, both from Maxwell Equations and more intuitively. I ...
3answers
100 views

### Electromagnet would emit light?

Light is an Electomagnetic wave. When I create an electromagnet by passing electricity wound around a core and keep changing the electric field, does it emit photons? Is the frequency of ...
0answers
179 views

### How fast does the electrostatic force travel?

Suppose an electron is created somehow in the universe -- through particle-pair production or what have you. Suppose it is stationary so that electrodynamics (and specifically propagating ...
1answer
79 views

### Speed of electromagnetic interactions

We know that electromagnetic waves fly with the speed of light, but my question not about waves. Consider a very strong electromagnet that creates a substantial field 3 meters away. Then we send a ...
4answers
1k views

### Is my argument deducing from Maxwells equations the exclusion of faster than light travel flawed?

When Maxwell's equations are solved, one of the solutions is electromagnetic waves that should move at a certain speed ($c = \frac{1}{\sqrt{\mu_0 \epsilon_0}}$). Now, one could argue that since ...
1answer
95 views

### Constancy of $c$ from Maxwell's Equations [duplicate]

How do you prove from Maxwell's equations that the speed of light is independent of the motions of the sources that produce the wave?
1answer
83 views

### Cause of Magnetic Fields

I read this on Quora in this question in an answer by Peter Webb. The magnetic forces are not real forces, they are virtual forces. They essentially derive from the time lag it takes an electric ...
1answer
56 views

### Oscillating Electromagnetic field

Hypothetically, if I were to move at the speed of light, would I see an oscillating electromagnetic wave or photons oscillating or what?
1answer
116 views

### Is there any physical significance to the fact that magnetic and electric field are proportional by $c$?

Being that this relation can be derived from Maxwell's equations, is there any physical significance to it outside the fact that the same equations make it the phase velocity of EM waves?
3answers
299 views

### Does light travel through a waveguide faster than electricity through a wire

So I have heard that using photons to compute is faster than electrons. I was wondering why exactly that is? Is it because light travels through a waveguide faster than electricity through a wire?
1answer
73 views

### Are these two definitions for propagation speed be the same?

We know for a fact that for any medium with permittivity and permeability $\epsilon$ and $\mu$, the propagation speed of electromagnetic waves is given by $$v = \frac{1}{\sqrt{\mu\epsilon}}$$ We ...
2answers
86 views

### Evidence for $\epsilon_{0}$ and $\mu_{0}$ being invariant between different inertial frames

Are $\epsilon_{0}$ (permittivity) and $\mu_{0}$ (permeability) invariant between different inertial frames? If yes, what hard evidence does it support that? If no, why would $c$ (velocity of light) ...
1answer
2k views

### How does $\epsilon \mu = 1/c_m^2$ change when $\mu$ or $\epsilon$ (permeability or permittivity) is a tensor?

I've read that in some materials, $\mu$ can be a tensor, not a mere scalar. (I haven't actually dealt with tensors before, but I'm assuming for my purposes here, it is synonymous with "matrix".) I'...
10answers
8k views

### Without the Michelson-Morley experiment, is there any other reason to think speed of light is the universal speed limit?

If the Michelson-Morley experiment hadn't been conducted, are there any other reasons to think, from the experimental evidence available at that time, that Einstein could think of the Special Theory ...
3answers
785 views