# Questions tagged [special-relativity]

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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### Does the mass variable $M$ in the Schwarzschild radius equation $R = 2GM/c^2$ refer to rest mass or relativistic mass?

Does the mass variable $M$ in the Schwarzschild radius equation $R = 2GM/c^2$ refer to rest mass or relativistic mass?
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### Is $\hat{H}^2 = c^2\left(\hat{p}^2+m^2c^2\right)$ valid quantum physics?

From Schrödinger equation we know that $\hat{H}=\hat{E}$ for non-relativistic systems. For relativistic scenarios we use the usual $\hat{E}^2 = c^2\left(\hat{p}^2+m^2c^2 \right)$. From this we can ...
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### Why cant I measure proper length on spacetime curvature with the following formula? [closed]

I'm struggling right now from the definition of proper length along spacetime curvature, it is said as I found online the length that object covered on his spacetime rest frame , so why cant I use the ...
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### Doesn't shooting a ray of light in the horizontal direction tell you whether you're moving or not?

The first postulate of the special theory of relativity states that the laws of physics are the same for all inertial frames of reference, and since things are the same whether you are stationary on ...
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### How do we find the spin an equation “represents”? [duplicate]

It is very well known how the Klein-Gordon equation describes 0-spin particles (fields), Dirac’s 1/2-spin, Proca’s 1-spin… But how do we know these functions describe particles with those spins? Given ...
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### Find mistakes in my deriving of Time Dilation and Lorentz Transformations [closed]

I am currently studying special relativity and have encountered some difficulties with derivations. I would appreciate it if someone could point out any errors in my personal derivation. Time ...
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### Mass-Energy relation

Einstein mass- energy relation states $E=mc^2$. It means if energy of a paricle increases then mass also increases or vice-versa. My question is that what is the actual meaning of the statement "...
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### Deriving the Minkowski Metric from homogeneity of space-time and the isotropy of space

In this wikipedia page, it says that one can derive the spacetime interval between 2 arbitrary events from the second postulate of special relativity, together with the homogeneity of spacetime and ...
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### Lorentz invariance of KG equation

I am confused with the equation 3.3 in Peskin & Schroeder: Given a Lorentz transformation $$x^\mu\rightarrow x'^\mu=\Lambda^\mu_\nu x^\nu,$$ the field transforms as\phi(x)\rightarrow\phi'(x)=\...
In French, Special Relativity, pg. 16-17, the 'Einstein's box' thought experiment is explained. It starts with a box of length $L$, mass $M$ and we emit photons from one end that have energy $E$. Then ...
Two photons, photon A and photon B, are traveling in the same direction at the speed of light, $c$. How fast does photon B appear to be traveling from the perspective of photon A?