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

learn more… | top users | synonyms (5)

0
votes
0answers
29 views

Relativistic Decay of Particles (Questions about Landau's derivations)

I'm currently reading Classical Theory of Fields by Lifshitz & Landau. In §11 "Decay of Particles" I can't really understand the solution to Problem 1: I have several questions: 1) How can one ...
0
votes
0answers
12 views

Derivation of the kinetic energy of decay product [on hold]

By conservation of momentum: $$M_{\alpha }V_{\alpha }+M_{Y}V_{Y}$$ By the conservation of energy: $$(\frac{1}{2})M_{\alpha }V_{\alpha }^{2} + (\frac{1}{2})M_{Y }V_{Y }^{2}$$ Here I begin the ...
0
votes
1answer
60 views

Question about scalar product of 2 four-vectors

I'm looking for help to prove that $\mathbf{a\cdot b}=-ab\ cosh \ \theta$, where: $a=\sqrt{\mathbf{-a\cdot a}}$, $b=\sqrt{\mathbf{-b\cdot b}}$, and $cosh \ \theta$ is $\gamma$, for a Lorentz ...
17
votes
1answer
842 views

Causality and how it fits in with relativity

I was talking to my teacher the other day about Einstein's spacetime and there's one thing he couldn't explain about the nature of Cause. I may be being stupid or just unable to comprehend, thanks for ...
-1
votes
1answer
28 views

Time dilation: reference frame [on hold]

So I'm calculating how long it will take for half of some muons to decay for in a stationary observers frame of reference. They have a half life of 2.2 * 10^-6 and are moving at a speed of .98c ...
1
vote
2answers
89 views

Speed of light paradox [duplicate]

If I send two rockets from the Earth in opposite directions, at, say, 60% of the speed of light relative to the Earth, then relative to each other they are travelling at 120% of the speed of light. ...
0
votes
1answer
35 views

Absoluteness of Simultaneity

I think this is a very important question because if simultaneity is absolute, then it shakes the foundation of relativity. But it was asked here a month ago, and no one answered it. Neither does ...
0
votes
0answers
26 views

Why wouldn't any Emission Theory work?

http://www.pitt.edu/~jdnorton/teaching/HPS_0410/chapters/origins_pathway/#Emission Here, at the Emission theories of light, I loved the discussed theory. There seems to be a contradiction right ...
0
votes
0answers
17 views

Klein paradox in graphene

When considering the Klein paradox in graphene Katsnelson, Novoselov and Geim introduce a potential barrier (see http://www.nature.com/nphys/journal/v2/n9/full/nphys384.html). But I cannot understand ...
1
vote
1answer
49 views

If time-like paths are geodesics, what physical principle applies to space-like intervals?

If I have a number of particles interacting with one another locally, then the center of mass of the system moves along a geodesic. Taking this further with the particles interacting via an EM field, ...
0
votes
1answer
58 views

Problem understanding Lorentz invariance [duplicate]

So they usually started with "...This is obviously Lorentz invariant, because of the 4-vector character of the quantity,..., (and after a two page long derivation) another quantity is also obviously ...
-5
votes
0answers
28 views

Posibility of time travel/multiple realities/time phase shifting [on hold]

Sounds odd and unlikely, but let me explain the situation. Going on the de ja vue ideal, which we probably have all experienced, I believe things of such a matter are possible. Just today when I ...
-1
votes
0answers
30 views

Thunder Clap Armageddon [migrated]

In a superhero setting a thunder clap is where someone claps with superhuman strength causing damage such as broken windows and burst eardrums. What I want to know is, taken to the extreme of the ...
12
votes
7answers
1k views

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 ...
1
vote
0answers
47 views

Why do we hyperbolas for distance? [on hold]

I'm confused about how distance is measured in spacetime. I've read a few texts that say that our normal distance equation doesn't apply because it violates causality and because it won't work for a ...
1
vote
2answers
188 views

Transverse doppler effect in light

In most books to explain transverse Doppler effect the following example is given: Consider a source that emits flashes at frequency f0 (in its own frame), while moving across your field of vision at ...
1
vote
0answers
14 views

what is the charge and current density in system K2? [on hold]

Consider there are two inertial reference of frames K1 and K2 in which K2 is moving with the boost velocity of V in direction of X2. If someone at the rest in system K1 finds the charge density $ \rho ...
3
votes
1answer
49 views

Special relativity kinematics problem [on hold]

I have this problem in SR kinematics. a spaceship travels near earth at c/2. it shoots off a light-ray at 45deg to its direction of travel (measured in its own ref frame). What is this angle in the ...
2
votes
1answer
89 views

Perfect fluid and Cauchy momentum equation

The stress-energy tensor of a perfect fluid is given by $$T^{\mu\nu}=\left(\rho+pc^{-2}\right)u^\mu u^\nu+pg^{\mu\nu}$$ The divergence of the stress-energy tensor is zero: $\nabla_\mu T^{\mu\nu}=0$. ...
0
votes
4answers
111 views

Is it possible to not move?

Whenever we consider velocity, it is always from a frame of reference. This means that if I am travelling .5c, I'm travelling at a speed between the speed of light and my current frame of references' ...
0
votes
0answers
56 views

Does EPR permit information to travel faster than light? [duplicate]

This question always throws me: Discuss if the EPR paradox violates special relativity? The information of the state of one particle is instantaneously transmitted to the other particle, but ...
1
vote
1answer
51 views

Representations of Lorentz algebra

It is well known that the Lorentz algebra can be written as two $SU(2)$ algebras. By defining $$N_i=\frac{1}{2}(J_i+iK_i), \qquad N^{\dagger}_i=\frac{1}{2}(J_i-iK_i)$$ we have ...
1
vote
1answer
132 views

Navigating a Time Machine

Notes: The background for this question is working out details of a sci-fi story. Answers to the effect of "time travel isn't possible" or "FTL isn't possible" are therefore not helpful. I'm looking ...
1
vote
1answer
38 views

Relativistic Lorentz Force with constant, perpendiuclar, and uniform E and B fields

I am trying to solve the following problem from the Fundamentals of Plasma Physics by Bittencourt (Problem 2.7): Analyze the motion of a relativistic charged particle in the presence of crossed ...
1
vote
1answer
53 views

4-momentum of photon

The 4-momentum is defined as $p=mU$ where m is the rest mass of the particle and $U$ is the 4-velocity. Now I am confused as to how this applies to a photon for which one can't define $U$ since there ...
3
votes
1answer
99 views

Is it possible to apply force to a light particle?

I know, the universal speed limit is the speed of light $c$. So, is it possible to apply more force to a light particle? What would happen to it if we could?
0
votes
1answer
105 views

No length contraction!

Suppose a person standing on a platform were to observe and measure the length of a train passing by. Instead of the usual approach involving a clock and knowing the speed of the train, the person ...
2
votes
3answers
95 views

What can change a photon's frequency?

I found this question: Is it possible to apply force to a light particle? As it states, gravity can change the frequency of light by changing its momentum. My question regards other phenomena that ...
0
votes
2answers
40 views

Lorentz force Law

I want to ask a fundamental question about Lorentz force equation. Why is it important to incorporate both electric and magnetic forces into one single expression? I know magnetism is electricity's ...
0
votes
0answers
34 views

One extra conservation equation or angle relation for a relativistic collision

Edit: It seems that the derivation for the center of momentum frame was incorrect, due to the fact that the center of momentum velocity changes after the collision. This is also true for the ...
2
votes
1answer
107 views

Physical interpretation of distances

Suppose we have a universe with only two distinguishable particles, neglecting gravitation. These particles start off next to each-other, ostensibly "touching" each-other and at rest. If it takes ...
0
votes
0answers
26 views

Where does the lorentz boost for a dirac spinor come from?

I have read, that if you have a dirac spinor \begin{equation} \psi = \begin{pmatrix} \phi_R\\ \phi_L \end{pmatrix} \end{equation} that you can apply a lorentz boost along the z-direction with ...
0
votes
1answer
48 views

Can someone explain what's the difference between all these terms in “Simple Words” with their “applications”? [on hold]

I'm very confused between all these terms. Can someone explain what's the difference between Classical Mechanics, Relativistic Mechanics, Quantum Mechanics, Quantum Field Theory, ...
1
vote
0answers
26 views

How to find the generators of a deformed boost?

I'm reading the paper arXiv:gr-qc/0012051 on doubly special relativity. In page 7, the author wants to find the generators of a deformed boost that preserves $$E^2 = p^2 + m^2 - l_p p^2 E$$ ($l_p$ is ...
4
votes
4answers
934 views

Time dilation derivation of special relativity

In almost all of the derivations using the postulates of special relativity (SR), we use experiments involving light signals. For example, we make a clock using a light signal or measure lengths using ...
6
votes
3answers
3k views

Deriving the Lorentz Transformation

I have been trying to understand a more or less geometric derivation of the Lorentz transformation, and I'm getting stuck at one spot. The wikipedia article for the Lorentz transformation for frames ...
3
votes
1answer
132 views

Where does this relation between gamma, energy and mass come from?

I am doing a course that has used this relation a few times now: \begin{equation} \gamma = \frac{E}{mc^2} \end{equation} It makes sense physically, when $E = mc^2$ the particle is at rest and ...
0
votes
1answer
49 views

Extreme temperatures, relativity and kinetic theory

According to kinetic theory, average kinetic energy is proportional to temperature. Supposing $k_BT/2$ per particle, can we use relativity and kinetic theory to calculate, e.g., the temperature and ...
2
votes
1answer
156 views

Graphene and relativistic dynamics

Is it possible to rewrite the Lorentz-transformations (for quantum particles) in terms of effective mass m* known from condensed matter physics? "From pencil lead to relativistic quantum physics" "A ...
1
vote
0answers
45 views

Travelling at a relative speed greater than the speed of light [duplicate]

I've wondered about this since I was a kid. I've been told the speed of light is a constant ($c$), which would mean that light always travels at the same velocity. Now, here's what interests me: if ...
0
votes
1answer
60 views

Twin paradox, with a meeting at a halfway point

Suppose body A, at rest in its reference frame, is passed by body B which is travelling at a constant 0.6 c, heading toward Z, a point 6 light years away by A's reckoning, and as bodies will, they ...
4
votes
4answers
191 views

How to derive $E=mc^{2}$?

Is there some way to derive $E=mc^{2}$? I can understand that energy in something is proportional to its mass, but the $c^{2}$ part. I have no idea. It seems like the way the units are going it would ...
3
votes
1answer
116 views

How do I transform onto a relativistic rotating frame of reference?

In classical mechanics, the usual formula to translate the evolution of a quantity as seen from an inertial frame of reference to a rotational frame is: $$\frac{d \textbf{A} }{dt} \vert_{Inertial} = ...
0
votes
1answer
28 views

Relativity and observable universe

Will cosmological horizon "expand" for an observer approaching the speed of light (at least in the direction of observer's motion) ?
29
votes
6answers
4k views

What speeds are “fast” enough for one to need the relativistic velocity addition formula?

In this question the accepted answer says: For objects moving at low speeds, your intuition is correct: say the bus move at speed $v$ relative to earth, and you run at speed $u$ on the bus, then ...
0
votes
1answer
52 views

Has the effect of magnetic fields on time ever been examined experimentally?

Relativity of time can be examined with satellites as gravitational or acceleration. Is there any similar experiment with intense magnetic fields?
0
votes
0answers
48 views

Speed of light, comparison of two light waves travelling side by side

okay so.here is setup of my question...say we have two observers A and A'..A is fixed to origin of coordinate axis x,y while A' is inside a hollow box (shown as rectangle MNRP) whose one side MN has ...
1
vote
3answers
83 views

Negative energy of free particle: classical and quantum picture

Classically, the energy of a free particle consists of only the kinetic energy given by $E=\frac{|\textbf{p}|^2}{2m}$ Since $|\textbf{p}| $is real and $m>0$, $E\geq 0$. However, since ...
1
vote
1answer
310 views

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 traveling would reach infinity. However in a medium, would there be some sort of theoretical ...
1
vote
1answer
30 views

Time for two relativistic objects to meet

How do you prove that the time for two objects traveling towards each other at relativistic velocities is still d/(v1+v2). Answer seems to have something to do with velocity addition but I'm ...