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7

Light can obviously travel in any direction, but the magnitude of its velocity (in vacuum) is always $c$. The magnitude of the velocity is a scalar i.e. just a number, but the velocity is a vector. To specify the velocity we need to choose some axes. For example I might choose the Cartesian axes $x$, $y$ and $z$. In that case light approaching me from the ...


6

Nope, not an exception. A stick, like any solid object, is essentially a bunch of atoms held together at a particular equilibrium distance. Atoms of a solid are pretty well modeled by a lattice of spheres connected by springs (with damping). What actually happens when you hit one end of a stick, is that you compress the surface atoms inwards towards their ...


4

That the speed of light is a fixed constant in all inertial reference frames is a consequence of Maxwell's equations of electromagnetism (assuming that two other standard constants, $\mu_0$ and $\epsilon_0$ are, in fact, non-zero constants). The math goes like this: Consider $\nabla\times B = \mu_0 J + \mu_0\epsilon_0\frac{\partial E}{\partial t}$. In a ...


3

It is that the speed of light is the same for all inertial frames that causes Special Relativity, not the other way around. As to why it is the same, nobody knows. Or why it is a finite value. There is some speculation about quantum gravity and a magical æther called spacetime, but nothing has been proven yet.


3

Let's do a thought experiment. We'll prove by contradiction, that the speed of light must be the same in all reference frames. scenario: you're sitting in a train travelling at the speed of light. There's a mirror in front of you, inside the cabin of the train. question: Will you, or will you not see your reflection in the mirror? Answer: It's a logical ...


3

Speed of light is constant. But in some substances , still transparent , light is absorbed and retransmitted ( with the same properties ) , spending some time. With not well transparent material, things are more complex. Anyway, between 2 obstacles, it's the vacuum and the speed remains constant and maximum. How many are retransmitted and the specific ...


3

Light is described well by the classical electromagnetic theory and Maxwell's equations. In this framework, the classical one, the speed of light is constant in vacuum. When light impinges on transparent materials, its speed, classically changes, and this is measured with the index of refraction of the material: where c is the velocity of light in vacuum ...


3

You are asking us for the distance of the trip in the rest frame of the photon. The problem with asking that is that there is no rest frame of a photon. A photon can never be at rest, so it has no rest frame. This is like asking what a bowl of petunias thinks about its existence as it falls to the surface. A bowl of petunias doesn't think, therefore we can't ...


3

In a very real sense, the velocity of a light ray in a curved spacetime is constant, or at least as constant as it can be; this is because it follows a special path in spacetime called a geodesic. The problem with defining a "constant" vector on a curved surface (the surface of the Earth, say) is that you can't easily compare tangent vectors at two ...


3

There's no one-to-one relationship. With zero rest mass, a particle must always be observed to move at $c$. A particle with nonzero rest mass, on the other hand, can move at any speed in $[0,\,c)$ (note the closed-open interval). At the risk of putting words in your mouth, I think I can recall the exact same question in my mind and it went something like ...


2

Yes light does have different directions in different frames. Two observers with different velocities will see the same photon traveling in different directions. One observer standing still at noon sees light traveling vertically downward. Light that strikes the top of his head would also strike his toes. An observer running forward see light slanted ...


2

There is no such thing as "rotational speed". There is angular momentum, which has units of $\frac{\mathrm{kg} \cdot \mathrm{m}}{\mathrm{s}}$ (and so is not a speed), and angular velocity which has units of $\frac{1}{\mathrm s}$ (and so is not a speed either). There is no maximum angular velocity in special relativity. So long as $\omega r\lt c$ (the ...


2

If you punch one end, the atoms at that end are displaced. They push the neighbouring atoms, who push their neighbours etc. This "push" comes in the form of an increase electric field, because the atoms get closer. Such changes in electric field propatages with the speed of light. So if you could move an atom close to another atom instantaneously, then the ...


2

This is an interesting philosophical question(IMHO). In physics, we don't prove theories to be right, but we do prove theorems about the math used to hold together our theories. Which theories are right is determined by mere phenomena. We have to develop them together out of observation and mathematical logic. The key logic behind Special Relativity was ...


2

This answer here covers the general case where the aisle of the bus you run up, so your velocity, is not necessarily collinear (parallel) to the velocity of the bus relative to Earth. Let define the velocities : \begin{equation} \text{(1) Velocity of bus relative to Earth : } \qquad \mathbf{v}=\upsilon \;\mathbf{n} \tag{A-01a} \end{equation} ...


1

Actually we can't reach to a thing that is more speeder than light. Because as we read in the Einstein's theory, the speed of light is constant. When you reach the speed near the speed of light, the time will be stretched, and more you get close to the speed of light, more the time will be stretched and YOU WILL NEVER GET TO THE SPEED OF LIGHT. So, due to ...


1

If you ask why $c/v$ is equal to 299,792,458 (this is the right value!), the question obviously depends both on $c$ and $v$. We know that the speed of light is 299,792,458 m/s because that follows from the modern definition of one meter adopted 30+ years ago: one meter is exactly so long that the speed of light in the vacuum is 299,792,458 m/s. This random ...


1

And while it is philosophically acceptable to just "know" that the speed of light is constant but it not to just "know" that it is invariant. Fixed constant values such as the mass of an electron or the spin set of an electron are things one can accept as given. Kind of: be careful. We can fix some of the fundamental constants by choosing covarying unit ...


1

Comment on the answer of @Michael: The short answer is that you need antiparticles is false. In Quantum Field Theory you have perfectly working solutions also without antiparticles, i. e. for real fields. Even if you do want to consider antiparticles, always have in mind that despite the misleading name they are in fact different particles from the ...



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