# Tagged Questions

The special theory of relativity describes the motion and dynamics of objects moving at significant fractions of the speed of light.

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### Is spacetime absolute?

As I understand it Newton's Laws imply that space is relative, as the laws of physics are the same in all inertial frames and as such there is no way, even in principle, to distinguish a frame that is ...
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### Special Relativity and Gravity

As Einstein was seeking a relativistic theory of gravity, he thought that special relativity should be upgraded to general relativity thus promoting the Minkowski space to curved pseudo-Riemannian ...
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### What do physicists mean when they say “speed of light”?

Does it make sense to say, "The speed of light varies?" Some may say right off the bat "Yes, it changes as a wave passes through a different medium." However, I'd like to say no, because when I hear ...
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### Theoretically if you passed the speed of light in a medium, would there be a sonic boom equivalent?

I know that it is technically impossible to reach the speed of light in vacuum since the mass of the object travelling would reach infinity. However in a medium, would there be some sort of ...
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### Does quantum mechanics allow faster than light (FTL) travel?

Let's suppose I initially have a particle with a nice and narrow wave function[1] (I will leave these unnormed): $$e^{-\frac{x^2}{a}}$$ where $a$ is some small number (to make it narrow). Let's also ...
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### Physics behind this neutrino-related joke

In the comment section of a newspaper article reporting on the 2015 Nobel Prize for Physics, which was awarded for work on neutrino oscillation, I found the following joke: "I'm sorry, we do not ...
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### Does my interstellar pole thought experiment violate casualty? [duplicate]

I have heard that it is impossible to send a signal faster than light. However, what if I had a long (4.5 light years) pole and placed one end on Earth and the another end around Proxima Centauri. If ...
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### How can length contraction result in electron circular motion in a magnetic field?

If you ask around about magnetic fields, you will read seemingly-authoritative articles which say magnetism is a consequence of length contraction. This is widely taught and is repeated in answers ...
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### Why does gravity affect time?

So Special Relativity states that for all non-accelerating objects of matter the laws of physics are the same. I'm confused on why this law of physic applies to objects in acceleration and gravity ...
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### Could this be a valid reason for why the universe could be infinite?

So, special relativity says that every frame is as good as any other frame, and there is no absolute frame of reference. All good. Suppose there is a finite universe (let's just assume the universe ...
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### How to find kinetic energy given relativistic linear momentum?

The relativistic energy of a particle is given by the expression $$E^2 = m^2c^4 + p^2c^2$$ The rest energy is $E_{0}=mc^2$ and the momentum is $p=mc$. In the rest frame, ...
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### Minkowski spacetime: Is there a signature (+,+,+,+)?

In history there was an attempt to reach (+, +, +, +) by replacing "ct" with "ict", still employed today in form of the "Wick rotation". Wick rotation supposes that time is imaginary. I wonder if ...
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### Question/Doubt about Time Dilation Symmetry in Special Relativity

Apologies if this has been asked and answered before, as I am still having doubts reconciling the symmetrical effects of time dilation (i.e each frame sees clocks of other frame slowing down), and I ...
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### Observing light speed

Consider an observer B is moving with speed $0.8c$ relative to another observer A in standard-configuration (I think it is called that in english: that the system B is only moving along the $x$-axis ...
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### The origin of the value of speed of light in vacuum

Meaning, why is it the exact number that it is? Why not 2x10^8 m/s instead of 3? Does it have something to do with the mass, size or behavior of a photon? To be clear, I'm not asking "how we ...
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### How can relativistic effects be the cause of magnetic forces when the normal speeds of carrier charges are so low?

I had read here that magnetism arises from a current because of the special relativistic effect associated with the speed of the moving charges in that current. However that speed is only on the ...
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### What is the physical significance of Rapidity in Relativistic kinematics?

Recently I have came across a new term called rapidity in relativistic kinematics. We are using it in relativistic kinematics due to its additivity whereas velocity is not a additive quantity in ...
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### Can an object spinning near the speed of light be accelerated significantly in any direction?

This question made me curious. It refers to a ball which is rotating so that its equator is approaching the speed of light, then talks about what it would look like if the observer moved. What I'm ...
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### Reconciling Minkowski and 3+1 view of special relativity

I am having some trouble reconciling the Minkowski (4-dimensional) and the pre-Minkowski (3+1-dimensional) approach to special relativity. Let me describe (how I interpret) the Lorentz transformations ...
Considering, in flat spacetime, two participants, $A$ and $B$, who were "chronometrically rigid" with respect to each other (i.e. $A$ found constant ping duration with respect to $B$, and $B$ found ...