# 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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### How does a particle of light reach the max speed of light? [duplicate]

Possible Duplicate: How can a photon have no mass and still travel at the speed of light? First of all I am not a professional physicist. I was curious as to how a particle of light can reach the ...
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### Criticism of the Thomas precession literature

From an earlier version of the Wikipedia article on Thomas precession concerning TP and LP=Larmor precession, regarding the paper: G B Malykin, "Thomas precession: correct and incorrect solutions", ...
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### Is time travel possible? [duplicate]

Time travel -- often featured in movies, books, or facetiously in conversation. There are also theories treating time as simply another dimension, which to the layperson might imply forward and ...
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### Does change of coordinate system require acceleration?

This question came about from a side discussion that arose on this: Does GR provide a maximum electric field limit? Can we change our choice of coordinate system completely independent of physical ...
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### How do I develop an intuitive model of spacetime?

I am your nightmare poster - a layman trying to learn special relativity. (I'm also a refugee from stackoverflow.com: trust me, it's only going to get worse). Apologies in advance if my question is ...
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### Horizon and Unruh radiation for a finite period of acceleration

It's a well known fact that an observer that accelerates at a constant rate from $-c$ at past infinity to $+c$ at future infinity sees a horizon in flat Minkowski spacetime. This is easy to see from a ...
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### If I run along the aisle of a bus traveling at (almost) the speed of light, can I travel faster than the speed of light?

Let's say I fire a bus through space at (almost) the speed of light in vacuum. If I'm inside the bus (sitting on the back seat) and I run up the aisle of the bus toward the front, does that mean I'm ...
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### Quaternions and 4-vectors

I recently realised that quaternions could be used to write intervals or norms of vectors in special relativity: $$(t,ix,jy,kz)^2 = t^2 + (ix)^2 + (jy)^2 + (kz)^2 = t^2 - x^2 - y^2 - z^2$$ Is it ...
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### What is the effect of temperature on electrostatic-gravitational balance?

We have two identical massive metal spheres at the same temperature at rest in free space. Both have an identical charge and the Coulomb force [plus the black-body radiation pressure if the ...
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### Acceleration in special relativity

I am currently studying the motion of relativistic charged particles in electromagnetic fields. More exactly, we first derived the equation of motion in the 4-vector formalism. I was a bit confused ...
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### Relativistic charged particles in a constant uniform magnetic field

How can i derive the dynamic of a relativistic charged particle in a uniform magnetic field $B=(0,0,B)$?
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### How much energy can be extracted from hydrogen?

http://en.wikipedia.org/wiki/B-L says that the difference between baryon number and lepton number is conserved. Ordinary hydrogen has one of each, but turning it into helium releases only the binding ...
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### Is This The Answer To Artificial Gravity?

Einstein teaches that as an object gets faster, its relativistic mass increases... Newton teaches that as an object's mass increases, so does it's gravitational pull... So... if you a tethered some ...
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### Knotted token-ring network

Suppose we have a rigid token-ring network. An observer at any node can seemingly determine the angular momentum of the network by measuring the time it takes for a packet to travel around the ring ...
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### Why are four-vectors needed in the Dirac equation, when there are 4 linearly independent 2D matrices?

I was taught that for the Dirac-equation to "work", you need matrices of the following form: $Tr(\alpha^i) = 0$. Eigenvalues +1 or -1 2 previous points together: equal number of negative and positive ...
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### Calculate the electric field of a moving infinite magnet, without boosting

Consider a rectangular slab of permanently magnetized material. The slab's dimensions are $L_x$, $L_y$, and $L_z$, and the slab is uniformly magnetized in the $\hat{x}$-direction. The slab is not ...
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### Paradoxical interaction between a massive charged sphere and a point charge

Suppose we have a sphere of radius $r$ and mass m and a negatively charged test particle at distance d from its center, $d\gg r$. If the sphere is electrically neutral, the particle will fall toward ...
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### Time Dilation - How does it know which Frame of Reference to age slower?

Okay, I'm asking a question similar to this one here: Time Dilation - what happens when you bring the observers back together?. Specifically, I am curious about a specific angle on the second part of ...
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### Special Relativistic Time Dilation -- A computer in a very fast centrifuge

Ok, I've stumbled onto what I think is a bit of a paradox. First off, say you had some computer in a very fast(near light speed) centrifuge. You provide power to this computer via a metal plate on ...
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### Magnetic moment of relativistic rotating ring

Let's consider rotating charged ring. Theoretically mass of this ring has no limit as rotation speed increases. So what about magnetic moment of the ring? Is it limited by the value of speed of light?...
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### Does magnetic propagation follow the speed of light?

Does magnetic propagation follow the speed of light? E.g. if you had some magnet of incredible strength and attached an iron wire that is one light year long to it, would the other end of the iron ...
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### Wavefunction collapse in relativity

It is well accepted that quantum theory has well adapted itself to the requirements of special relativity. Quantum field theories are perfect examples of this peaceful coexistence. However I sometimes ...
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### What does Lijun Wang's experiment about supraluminal speed of light in a medium mean?

This is a summary from Physics World of the paper: L J Wang et al. 2000 Nature 406 277-- "Wang and colleagues begin by using a third continuous-wave laser to confirm that there are two peaks in the ...
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### Why doesn't the existence of proper-time $\tau$ imply a preferred reference frame?

A proper time interval $\Delta\tau$ for a given observer is a relativistic invariant. However, the calculation of $\Delta\tau$ requires reference to some arbitrary coordinate time t: \Delta\tau = \...
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### What is Lorentz contraction?

Can we describe our universe without it? Example; Explaining a muon able to hit Earth, by its time dilation only? Or a spinning disc contracting? Does it exist, and if it does, does it do so from ...
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### Is there a relativistic (quantum) thermodynamics?

Does a relativistic version of quantum thermodynamics exist? I.e. in a non-inertial frame of reference, can I, an external observer, calculate quantities like magnetisation within the non-inertial ...
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### Why can't massless particle exceed speed of light?

Why massless particle can't exceed speed of light?
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### Under what conditions do the Galilean transformatons become more accurate?

For S and S' in standard configuration, the Galilean transformations are: x' = x - vt, y' = y, z' = z, t' = t From the Lorentz transformations for v << c: x' = x - vt, y' = y, z' = z, t' = t -...
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### Do light and sound waves have mass

I have been reading Hawking's 'A Brief History of Time' and it has gotten me thinking about Einstein's theory of relativity, in that it assumes that an object must have infinite mass if it is to be ...
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### twistor-spacetime correspondence

Could someone explain the correspondence between lines in twistor space and minkowski space-time points? a basic derivation would suffice
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### Time Dilation - what happens when you bring the observers back together?

I'm having trouble getting my head around the time dilation paradox. Observer A and B are at the same "depth" in a gravity well. Observer B then descends into the well. A will observe B's time as ...
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### Is there an intuitive geometric view of the effects of Lorentz transformations?

Is there a time + two spatial dimension representation of a Minkowski-space surface which could be constructed within our own (assumed Euclidean) 3D space such that geometric movement within the ...
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### What kind of invariants are proper time and proper length?

Under the Lorentz transformations, quantities are classed as four-vectors, Lorentz scalars etc depending upon how their measurement in one coordinate system transforms as a measurement in another ...
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### Are Classical Field Theory and Quantum Mechanics of a single particle (nonrelativistic or "classical") limits of Quantum Field Theory?

Recently I talked about QFT with another physicist and mentioned that the Quantum Field Theory of a fermion is a quantisation of its one-particle quantum mechanical theory. He denied this and ...
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### Newton's Bucket [duplicate]

Newton's Bucket This thought experiment is originally due to Sir Isaac Newton. We have a sphere of water floating freely in an opaque box in intergalactic space, held together by surface tension and ...
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### Is causality a formalised concept in physics?

I have never seen a “causality operator” in physics. When people invoke the informal concept of causality aren’t they really talking about consistency (perhaps in a temporal context)? For example, if ...
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### Does the discreteness of spacetime in canonical approaches imply good bye to STR?

In all the canonical approaches to the problem of quantum gravity, (eg. loop variable) spacetime is thought to have a discrete structure. One question immediately comes naively to an outsider of this ...
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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 ...