# Tag Info

### At what speed does information move through the atoms of a rigid object?

A mechanical impulse will travel along an iron bar at the speed of sound (which will depend on the material and also possibly on the frequency and amplitude of the impulse). But in the video the ...
Accepted

### Relativistic Light Velocity

You identified the mistake in your own question. You correctly stated "both time and length should receive a Lorentz transformation", but the mathematical operations that you performed were ...

### At what speed does information move through the atoms of a rigid object?

The question in the title is formally contradictory. A model is always a simplification of the real world for the sake of understanding and making predicions (usually mathematically) A "rigid ...
Accepted

### Is the speed of causality slower in water?

The speed of causality in water is still $c$. Although light does not travel at $c$ in water, it is possible to have particles travel through water faster than light does. Such particles emit a ...

### Is the speed of causality slower in water?

The "speed of light" inside a medium refers to different things, so they can't all be the speed of causality. First, there are the group and phase velocities. In a medium where the index of ...

### Relativistic Light Velocity

The mistake is that you have assumed that in the stationary frame the light has travelled a distance equal to the contracted length of the train in that frame, which is not the case at all. The actual ...
Accepted

### The effects of light needing to reach observers from observed objects

Some of the teaching of relativity is awful, and a common problem in text books is that they are written in a way that blurs the meaning of the word observer. As you point out, a literal observer ...

### "Pure Energy" in special relativity and why does the rest energy follow from a derivation at the speed of light?

As a consequence of the postulates of special relativity, it can be shown that the total energy possessed by a moving object is: $$E=\frac{mc^2}{\sqrt{1-\left(\frac v c\right)^2}}=\gamma~ mc^2$$ If we ...
Accepted

### Which experimental setup replicating Michelson-Morley experiment had the longest path of light?

Experimental setup in Ligo uses replica of Michelson Morley interferometer with additional etalon used to increase the optical path length .The effective path length along each arm by adding etalon ...

### Relativistic Light Velocity

Your Lorentz transform of the time supposes $x = 0$. That means: Alice in one of the wagons (supposed the origin in the frame of the train) is comparing her clock with synchronized clocks of the ...

### Is the speed of causality slower in water?

The main thing to note is that when electromagnetic waves propagate in some medium other than vacuum, then the medium itself may be driven by the electromagnetic waves into some wave-like oscillation. ...

### Is the speed of causality slower in water?

Different wavelengths of visible light travel in water at different speeds. This is why we have rainbows (as well as other important things). Does it mean that causuality for red events happens at ...

### Is the speed of causality slower in water?

Although your question only concerns water and other such media, it can also be directly associated with Maxwell’s equations showing that c is inversely proportional to the permeability and ...
Accepted

### Question about the norm of the four-velocity being equal to $c$

One easy way of grasping it is to say that an observer at rest moves forward along the time direction at lightspeed, while the driver tilts their velocity vector so it has a spatial component. It ...

### Why can't we see past the observable universe?

The Hubble law is $v=Hd$. When you multiply H by the distance, you get a velocity (units distance/time). That is what the Hubble law is saying. Once you reach a distance (the cosmic horizon) in which ...

### Why can't we see past the observable universe?

The observable universe gets bigger with each new advance in telescope technology. In this sense the "observable universe" is that part of it which is sending just enough photons our way to ...
1 vote

### Thermodynamics and Special Relativity

What happens to the temperature of a body (not in the body frame) as it approaches the speed of light? As discussed in this article, no one can tell you the answer, at least unless you choose a ...
1 vote
Accepted

### Will light of 405nm wavelength behave in a single mode (SM) fiber , in multimode manner?

You are correct that shorter wavelenghts will travel through the fiber as a higher order mode. Here you can see the V-number Vs. b parameter. V-number is proportional to the wavelength as \$V= 2\pi a (...
1 vote

### Why does the different speed of light in different media cause refraction?

Is there a macro phenomenon, say a stream of bullets, that will also bend as it changes from one medium to another or is light somehow unique? If you drive a car off of a smooth hard road onto a soft ...
1 vote

### Does a photon in vacuum have a rest frame?

I am going to approach the matter otherwise. I do not dispute the arguments made in answers in  or answers in . However, based on my learning style, these arguments do not work for me. This is ...

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