# 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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### What is the meaning of complex distances in relativity?

In flat Minkowski space with the metric signature (-,+,+,+) the spacetime interval $ds^2$ is defined as $ds^2 = -c^2 dt^2 + x^2 + y^2 + z^2$ This interval (or better the square of the distance) can ...
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### Can something be a black hole in one frame of reference but not in another? [duplicate]

So my understanding is because E=mc^2, and kinetic energy is included in E, something moving quickly relative to you gains mass in your reference frame, compared to that same object at rest. Black ...
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### Would the universe shrink to a point in light's frame? [closed]

Hypothetical: If "light-based life" somehow existed and the universe was in fact finite (choose your topology), would the universe appear 2-dimensional? Would length contract to a point in ...
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### Is this the correct equation for the four force on a system in special relativity?

Four velocity is the derivative of spacetime coordinates with respect to proper time, and four acceleration is the derivative of four velocity with respect to proper time. Also the four force is the ...
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### Does the 1/2 coefficient in kinetic energy stem from Newton, the De Broglie wavelength and length contraction?

This is my thinking. Everything has a De Broglie wavelength. From this the energy of an object can be calculated. Now due to Special Relativity anything with a velocity experiences length contraction. ...
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### What can I assume about Mandelstam variables in four-photon scattering?

In the case of four-photon scattering, the Mandelstam variables must satisfy $s+t+u=0$ Where: $s=-(p_1+p_2)^2, t=-(p_1+p_3)^2, u=-(p_1+p_4)^2$ And we know that $s>0$ is true because it is the ...
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### $k$-calculus in $3+1$ dimensions?

So I've recently been intrigued by $k$-calculus. Is there a formulation of $k$-calculus in $3+1$ dimensions? More specifically the relation between the $k$-factor and and a $4$-vector?
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### Interaction of particles with a spectrum of waves, relativistic equation of motion

I have to work with a poorly documented code, that simulates the interaction of particles with a spectrum of waves. The previous author has made the equations of motion dimensionless by using the ...
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### Acceleration and Reference Frames

If I'm in a car and hit the gas, an outside observer on the ground sees my car accelerate to some speed, and given the mass of my car and the horsepower of my engine, this obeys Newton's second law. ...
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### Are brains quantum or relativistic? [closed]

Are brains quantum or relativistic? They involve high speeds for the electricity.
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### If length in a frame moving with respect to a stationary frame contracts, how do we define the speed of the moving frame?

Under Galilean transformations, the speed of a moving system $k$ with respect to a stationary system $K$ is straightforward. Assuming the origins of $k$ and $K$ coincide at t=0, the spatial ...
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### Spinning a rod in relativistic speed

This is a random question that just comes into my mind. Say you have a rod with a length of 6 x $10^8$ m and I spin it at its mid point at 1 rad/s. By classical mechanics the end of the rod would have ...
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### Whats happening with the timelike component in non-relativistic approx.?

If we look at the non-relativistic approximation of an equation in Minkowski space, what happens to the timelike component in general? Like, my question is not the call for a specific Problem, but ...
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### What does the $O$ mean in "$O(v^4)$"?

I am currently studying General Relativity and I am using the book by Schutz. I encountered a problem when I was reading about the gravitational redshift experiment. Here is the website (it is not the ...
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### Some help in understanding energy conservation relating to special relativity

Consider the decay of particle $A$ into particles $B$ and $T$ where $T$ is the tachyon particle. The conservation of energy equation can thus be expressed as m_A=\sqrt{p^2+m^2_B}-\sqrt{p^2-m^2_T}\,.\$...
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Suppose that a man departs from Earth to reach a planet that is ninety-nine light-years away with 0.9802 light speed, c. According to special relativity, it takes him 40 years (measured by his own ...
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### Why does the derivation for the Michelson & Morley time difference assume Earth moves in only one direction relative to the Aether?

In the Michelson and Morley experiment, we predict with Galilean relativity and the assumption of the existence of a luminiferous aether that there should be a time difference between the two beams of ...
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### Does a relativistic rocket's combustion slow with time dilation?

If a relativistic rocket travels slower in time, does that mean the actual combustion in the thrusters happens slower and slower from an outside frame of reference? Is this part of what causes a ...
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### Would this equation for electromagnetic four force be valid when the energy of electromagnetic radiation is negligible?

The first piece of information I used in my derivation is that, with respect to electric charge, anti-matter is mathematically equivalent to ordinary matter traveling backwards in time, which means ...
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### Speed of photons in Cosmic Microwave Background Radiation relative to the earth (or solar system)

At https://living-universe.com/questions-and-answers/absolute-motion-of-photons-in-the-2-7cbr/, we are told that the Cosmic Microwave Background Radiation (CMBR) is propagating relative to the earth (...