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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Another Special Relativity Paradox inspired by Herbert Dingle's Paradox

Suppose spaceships A and B with clocks A and B move along the same straight line at uniform speeds differing by 161 miles per second. At the moment when B passes A, their clocks both read noon. ...
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74 views

How does the comparison of rates depend on mutual speed?

If I'm moving with a considerable fraction of speed of light, the time and any process in my system will be going slower. If B is an outstanding person that is watching me passing by, he will see ...
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682 views

Relativity of temperature paradox

The imagined scenario: Part A: From special relativity we know that velocity is a relative physical quantity, that is, it is dependent on the frame of reference of choice. This means that kinetic ...
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96 views

Why $F_\mu V^\mu=0$ in special relativity? [closed]

I'm trying to prove why $F_\mu V^\mu=0$. Where $F^\mu$ is the four force and $V^\mu$ is the four velocity. Can anyone help me?
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125 views

Information, entanglement and EPR

I'm currently trying to understand why superluminal information transport is not possible. Therefore, I would like to get some help concerning the definition of information or the general setting. ...
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61 views

Relativity on a universal scale

Imagine there was a clock on a planet the same size as earth, travelling at the same speed through space, and that this planet was at the most distant part of the universe from earth. If we had a ...
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1answer
66 views

Why does $k^z/E$ have dimensions of inverse velocity rather than velocity?

I'm studying quantum field theory and I want to prove the cross section. In Peskin's book, equation 4.77 says that: $$ \frac{1}{\left | \frac{k_{A}^{z}}{E_{A}}-\frac{k_{B}^{z}}{E_{B}}\right |}=\frac{...
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2answers
79 views

Special relativity: is this a known paradox, or one at all?

Two ships of the same proper length $L$ move towards each other, as in the diagram below (which shows it in the reference frame where the ship at the left is at rest). The fronts (noses) are pointing ...
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1answer
62 views

Relativity theory - lower end of falling to Earth object moves slower or faster than higher end?

According to relativity, time slows close to gravitational field (I prefer to say the processes with which we measure Time slows). (https://en.wikipedia.org/wiki/Hafele%E2%80%93Keating_experiment) - "...
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1answer
33 views

Lorentz contraction (or not) of an accelerated electron bunch

It's a well-known fact from special relativity that moving objects are subject to a length contraction: an object with a proper length $L_0$ moving at a high velocity $v$ will appear to a stationary ...
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3answers
150 views

Interpretation of terms in the Lorentz Transformation

What is the physical significance of the term "${vx}/{c^2}$" in $$ t' = \gamma \left(t- \dfrac{vx}{c^2}\right) $$ It has got something to do with clock synchronization but cant derive the 'correction ...
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1answer
31 views

Anti-commutator relation of supercharges

Reading mutiple references on SUSY (e.g. Baer and Tata's Weak Scale SUSY and A SUSY Primer by S.P. Martin, arXiv:hep-ph/9709356), there seems to be different anti-commutation relation conventions for ...
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206 views

Herbert Dingle's paradox modified

Suppose two spaceships A and B have on them clocks A and B move along the same straight line at uniform speeds differing by 161 miles per second. And suppose they are at positions A and B in space ...
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1answer
67 views

If we traveled at almost the speed of light would objects appear the same?

We see objects with our eyes as object emits/reflects light. Say if we travel at almost the speed of light can we see the objects normally?
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1answer
112 views

Is there a physical upper limit on how fast a physical object can rotate?

Objects can not travel faster than the speed of light. Angular velocity is $\omega = \frac{s \times v}{|s|^2}$ It seems to me that for infinitely small objects (like electrons) there is no limit on ...
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71 views

Four Vectors in SR and QFT

I'm covering both special relativity and quantum field theory in the summer. I'm currently using Spacetime Physics by Taylor and Wheeler to cover SR. Since I'm covering SR on the side with QFT, I'm ...
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2answers
68 views

Difference between KE “relativistic” mass and heat mass

I have been told in several answers that the term "relativistic mass" is no longer considered adequate wrt. the mass/energy increase in KE. Yet, I read that 1 Kg of gold's mass ingreases by 10^-14 Kg ...
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3answers
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What is meant by rest in rest-mass?

As far as I know only photons are considered to have no rest-mass. In common words when it doesn't move it 'disappears'. Electrons and quarks should have a rest-mass. But are they really at rest? ...
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43 views

Observer travelling near speed of light observing a laser pointer seeing photons at speed of light?

I'm a little confused by the following classic example about the speed of light being constant for all observers (paraphrasing): Jack and Jill are both travelling at a significant portion of the ...
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2answers
64 views

Time dilation: time shortest where events occur at same place?

In his book Simply Einstein Richard Wolfson describes time dilation this way: "The time between two events is shortest when measured in a reference frame where the two events occur at the same place." ...
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75 views

The criteria of light

My question is regarding if the "speed of light" 299,792,458 m/s should only apply to certain frequencies of light/colors/amplitude and most the medium in which it travels at 299,792,458 m/s . Shouldn'...
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2answers
111 views

Is the speed of light constant in all directions?

Consider the shape of the waveform emitted by a moving source: (with the horizontal axis "x" and the vertical axis "y"). According to the stationary observer, the wavefront of a short pulse ...
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165 views

Relevance of spinor in relativistic (classical) electrodynamics

I'm following a course about relativity and electrodynamics (not the "quantum" one), and the lecture notes introduces the concept of spinor by a map between an orthogonal basis in Minkowski spacetime ...
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How do I draw the Minkowski diagram of a event in two inertial systems when t=t'=0?

An Introduction to Mechanics" by David Kleppner & Robert Kolenkow, 1st Edition, 12.1 Assume that v= 0.6c . Find the coordinates in S' of the following events: a). x=4 [m], t= 0[s] b). x=4 [m], ...
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Special Relativity: Verifying a general boost matrix is in the Lorentz group

I'm attempting the problem shown below. Using the hint, I have so far found: \begin{align}B^T \eta B &= \begin{pmatrix}\gamma & -\gamma\beta^j \\ -\gamma \beta_k & \delta_k^j+\frac{(\...
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4answers
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Does $E=mc^2$ mean that an object with mass $m$ travels at the speed of light in empty space? [closed]

I cannot wrap my head around this (Im new to physics), and this concept is troubling me. I think no, but im not sure why?
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Speed of Dark experiment, does it *really* travel faster than light? [duplicate]

I have seen many folks doing this Moon shadow experiment concluding that shadow from point A on the Moon can travel to point B on the Moon faster than light. What I fail to understand here (and I am ...
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1answer
120 views

Action of a massive free point-particle in relativistic mechanics

I was reading about the formulation of mechanics in special relativity and found that the action for a massive free point-particle as $$ S = -mc\int_a^b ds $$ So, I did a few observations, ie. $$ S =...
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39 views

A photon lives life as a flat-lander? [duplicate]

Think length contraction... I was thinking about going close to the speed of light the other day, and I was thinking about the idea that, if you went close to the speed of light, time would slow down ...
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Is Young's Modulus a Lorentz Scalar?

If a spring is at rest and lies along $X$ axis in a frame $O$ with a spring constant $k_{0}$ then its spring constant in a frame $O'$ which is moving with a speed $v$ at an angle $\theta$ with the $X$ ...
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2answers
332 views

Why are usually 4x4 gamma matrices used? [duplicate]

As far as I understand gamma matrices are a representation of the Dirac algebra and there is a representation of the Lorentz group that can be expressed as $$S^{\mu \nu} = \frac{1}{4} \left[ \gamma^\...
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54 views

Why shift in fringe is expected in Michelson Morley Experiment?

When I (beginner) learnt Michelson Morley experiment, it was expected that there should be a shift in the fringe pattern when the setup was rotated by 90 deg if the presence of ether is true. But why ...
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38 views

Is an extended spring gain mass from potential energy? [duplicate]

Is it true that when a spring is extended, potential energy is stored in spring and result in gain of mass of the spring?
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575 views

Spacetime and quantum mechanics

In special relativity, the particle has a fixed world line in spacetime. So its whole trajectory is determined. But how can we represent the world line of the particle in spacetime when we take ...
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1answer
91 views

SR and a rotating observer [closed]

What if an observer took a particle placed it some distance away from himself and began to circle the particle approaching the speed of light. Would the observer himself observe that the circle is ...
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A question from “The meaning of the relativity, by A.Einstein” - Lorentz transformations [duplicate]

Let $K$ and $\bar K$ be two cartesian co-orditate systems in $\mathbb{R}^3$. The element: $$s^2=(\Delta x^1)^2+(\Delta x^2)^2+(\Delta x^3)^2$$ is an invariant in all co-ordinate system. I want prove ...
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2answers
96 views

Can't reach speed of light, but relative to who?

Imagine a stationary person (with respect to a spaceship) floating in space and looks at a clock on the spaceship going 0.5c and sees it's clock ticking slower than his own, and concludes that it is ...
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1answer
48 views

Lorentz transformation and symmetries of the Lagrangian [duplicate]

Since the Lagrangian of our quantum field theories is covariant under Lorentz transformations I'm asking myself if there is any link to some symmetries (like that we get from gauge transformations ...
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58 views

Electric field around a moving charge

A moving charge has magnetic field around it. Is it just increased electric field in relativistic frame? If yes, does a moving charge has more electric field around it?
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Relativity paradox with mirrors and light pulses

Consider two very short light pulses emitted from the centre (C) of two mirrors A and B (as shown in the diagram). From the point of view of the lab frame, the apparatus is all moving to the left at ...
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1answer
86 views

Wave operator applied to electromagnetic field tensor

I'm trying to understand an argument in "An introduction to general relativity" by Hughston and Todd (p37). Let $F_{ab}$ be the electromagnetic field tensor, I'm trying to show: $$\Box F_{ab} = -4 \...
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1answer
55 views

Poincare Group (Wald, Chapter 4 Page 59)

In Wald's text on general relativity, he mentions that in special relativity, many different global inertial coordinate systems are possible and can be put into one-to-one correspondence with elements ...
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Difference between space of reference and system of coordinates

In the book "The meaning of the relativity" by A. Einstein, it is referring to two different concepts: space of reference and system of coordinates. What it is the difference? It says: "we ...
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2answers
107 views

Relativistic Rocket Fuel Requirements

I am trying to reconcile two different calculations of fuel for a 1g constant acceleration/deceleration trip to Alpha Centauri. One found here (http://math.ucr.edu/home/baez/physics/Relativity/SR/...
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53 views

Relativistic mass of electron in a potential [closed]

An electron is accelerated through a 6 MV potential difference. what is the mass of the electron at the end of the path?
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18 views

Variation of spin in the rest frame of an electron in an external magnetic field

In Jackson's Classical Electrodynamics, the equation of motion for the angular momentum $\textbf{s}$ of an electron in an external magnetic field is given by (eq. 11.101) $$\left(\frac{d\textbf{s}}{dt}...
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1answer
46 views

Why would two inertial frames be distinguishable, if the coordinate of an event perpendicular to the motion would be different? [closed]

This question arises from the classical example: imagine a laboratory frame, and a space shuttle frame, the space shuttle moving in the laboratory frame with a constant velocity, let's say in the $x$ ...
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1answer
77 views

Will the speed of gravity and the speed of light always be the same?

I have been considering the following though experiment: If the entire sun were to disappear at once, would we feel it first due to the sudden drop in gravity or see it first? The answer I have come ...
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Does Heat Cause Time Dilation?

Since heat is defined as the movement of molecules, and because of relativity time slows for faster moving objects, would a hot object be in a slower time frame then a cooler object, because the hot ...
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Decrease in density due to relative velocity [closed]

A moving frame sees density of a spherical body decreased by 20% of its density in rest frame . What is the velocity of the moving frame ? I thought moving frame observes change in length of object ...