# Tagged Questions

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

1answer
71 views

### Is it best to look at light as a particle when trying to understand special relativity?

So my course about special relativity explains time dilation using a moving train, where one sends up (i.e. perpendicular to the direction of movement) a light pulse which gets reflected etc. (a ...
2answers
648 views

### Definitions: 'locality' vs 'causality'

I'm having trouble unambiguously interpreting many answers here due to the fact that the terms locality and causality are sometimes used interchangeably, while other times seem to mean very different ...
1answer
327 views

### Two trains at the speed of light [closed]

Okay. Two trains travelling towards each other at the speed of light. So, from one train (let's call it train A), the other is moving towards it at the speed of light. The other train shines a torch. ...
1answer
109 views

### Galileo's dictum and how light cannot violate it

Okay. So I've been told that the speed of light is constant and cannot violate Galileo's dictum, but even if it weren't constant (in a vacuum), how would it violate it anyway? Say you are on a train ...
2answers
274 views

### Special Relativity: What differential equation describes an accelerated object from a non-inertial reference frame?

I am looking for a set of differential equations (to be solved numerically for an educational program) that would describe the position and apparent time of an accelerated clock relative to a ...
2answers
217 views

### A relative time dilation paradox.

Let us assume that there are two astronauts A and B who are floating in space. A sees B passing by and vice versa. A sends signals to B every minute. According to A since B is moving his clock will be ...
2answers
284 views

### relativistic acceleration equation

A Starship is going to accelerate from 0 to some final four-velocity, but it cannot accelerate faster than $g_M$, otherwise it will crush the astronauts. what is the appropiate equation to constraint ...
11answers
2k views

### Could the Heisenberg Uncertainty Principle turn out to be false?

While investigating the EPR Paradox, it seems like only two options are given, when there could be a third that is not mentioned - Heisenberg's Uncertainty Principle being given up. The setup is this ...
1answer
162 views

### Expression for the (relativistic) mass of the photon [closed]

I started learning a bit ahead from an old physics book, and they were discussing the photoelectric effect and after that Planck's hypotheses and energy quantas. The book said that the mass of a ...
5answers
420 views

### Special Relativity Second Postulate

That the speed of light is constant for all inertial frames is the second postulate of special relativity but this does not means that nothing can travel faster than light. so is it possible the ...
2answers
161 views

### Why does the (relativistic) mass change & why?

I studied that when an object moves with a velocity comparable to the velocity of light the (relativistic) mass changes...but I am really eager to know how does this alteration take place....If anyone ...
2answers
222 views

### Using Lorentz Invariance of Charge To Calculate Current Density

I'm attempting a problem from Zwiebach: A First Course in String Theory and am completely stuck. Could anyone give me a hint? The problem is as follows. Consider $S$, $S'$ two Lorentz frames with ...
4answers
333 views

### Why are objects at rest in motion through spacetime at the speed of light?

I read that an object at rest has such a stupendous amount of energy, $E=mc^2$ because it's effectively in motion through space-time at the speed of light and it's traveling through the time dimension ...
2answers
610 views

### Does the speed of light vary in noninertial frames?

The speed of light is the same in all inertial frames. Does it change from a non-inertial frame to another? Can it be zero? If it is not constant in non-inertial frames, is it still bounded from ...
3answers
191 views

### First Postulate of Special Relativity: What does it mean?

Wikipedia has this quote: Special principle of relativity: If a system of coordinates K is chosen so that, in relation to it, physical laws hold good in their simplest form, the same laws hold ...
2answers
116 views

### Relativity - time dilation

I'm learning about relativity and I'm having some issues with it and the twin paradox. I found many questions and answers on this subject but they did not answer my specific problem. In my thought ...
1answer
248 views

### Time dilation - why the observers see each other the slow one but then one of them is older or younger?

I'm in trouble with time dilation: Suppose that there's two people on the Earth (A,B), they are twins and each other has a clock. (So they are at the same reference frame). B travels in a spaceship ...
2answers
204 views

### Does the potential energy related to a particle determines its rest mass?

Would it be possible to determine the rest mass of a particle by computing the potential energy related to the presence (existence) of the particle, if this potential energy could be determined ...
2answers
178 views

### Inner product of four-vectors in special relativity

Reference) "Feynman lectures on Physics Vol.3 , p.7-4 ." With four vectors $x_{\mu} = (t,x,y,z)\ , \ p_{\mu} = (E,p_{x},p_{y},p_{z})$ the inner product of these two four vectors is scalar invariant ...
3answers
134 views

### Having trouble seeing the similarity between these two energy-momentum tensors

Leonard Suskind gives the following formulation of the energy-momentum tensor in his Stanford lectures on GR (#10, I believe): T_{\mu \nu}=\partial_{\mu}\phi \partial_{\nu}\phi-\frac{1}{2}g_{\mu ...
0answers
114 views

### How do I extend the Lorentz transformation metric to dimensions>4?

How do I extend the general Lorentz transformation matrix (not just a boost along an axis, but in directions where the dx1/dt, dx2/dt, dx3/dt, components are all not zero. For eg. as on the Wikipedia ...
2answers
111 views

### What should I call an n>4 dimensional Minkowski metric?

I am manipulating an $nxn$ metric where $n$ is often $> 4$, depending on the model. The $00$ component is always tau*constant, as in the Minkowski metric, but the signs on all components might be ...
1answer
119 views

### quantum curvature

If a state can be a superposition of energy states, and mass equals energy (special relativity), and mass curves space-time (general relativity), then could we say that space-time around a quantum ...
2answers
257 views

### Hamiltonian mechanics and special relativity?

Is there a relativistic version of Hamiltonian mechanics? If so, how is it formulated (what are the main equations and the form of Hamiltonian)? Is it a common framework, if not then why? It would be ...
0answers
86 views

### What is the proper time used in relativistic non equilibrium statistical physics?

In the literature one often finds covariant relativistic generalizations of classical non equilibrium statistical equations (Boltzmann, Vlasov, Landau, fokker-planck, etc...) but I wonder what is the ...
0answers
88 views

### maximum distance between accelerating objects started at different times [closed]

Let there be two objects that have zero relative velocity with respect to each other in an inertial frame. If they both undergo identical accelerations, but one starts the acceleration at t = T1 and ...
1answer
282 views

### Is 4-volume element a scalar or a pseudoscalar in special relativity?

In general relativity 4-volume element $\mathrm{d}^4 x = \mathrm{d} x^0\mathrm{d} x^1 \mathrm{d} x^2\mathrm{d} x^3$ is clearly a pseudoscalar (or scalar density) of weight 1 since it transforms as ...
1answer
207 views

### Relative Speed vs speed of light [duplicate]

Possible Duplicate: Travelling faster than the speed of light Someting almost faster than light traveling on something else almost faster than light I've got two questions which are ...
2answers
358 views

### Is the potential energy in a compressed spring a Lorentz invariant?

The total energy of an object comes from the time part of the four-momentum, and so isn't a Lorentz invariant. On the other hand, is the potential energy of a compressed spring a Lorentz invariant?
3answers
166 views

### Does the Lorentz transformation not apply to light?

Since you would know that light always travels at the constant velocity with respect to all frame of reference ....according to relativity whenever we are traveling at speed of light our time with ...
1answer
171 views

### Designing a plausible faster than light drive: the Space Skip Drive [duplicate]

Possible Duplicate: Is the Portal feasible in real life? I'm designing a plausible faster-than-light (FTL) drive for a SF universe. Here's what I have so far. I'm aware of existing ...
1answer
195 views

### Speed of light is not fixed?

In my research, I found that the speed of light is not fixed. IS it true? Namely, We know that light refracts when the medium it travels through changes. Actually, light travels in the same medium ...
3answers
668 views

### “Relativistic Baseball”

On Randall Munroe’s new blog “what if”, he answers the question: “What would happen if you tried to hit a baseball pitched at 90% the speed of light?” http://what-if.xkcd.com/1/ He concludes: ...
6answers
820 views

### Simple Experiment to Demonstrate Special Relativity

I am trying to think of a good experiment that can be done for under \$250 or so that would demonstrate some aspect of Special Relativity. Ideally this will be done in a few years with my kids when ...
1answer
541 views

### Phase space volume and relativity

Much of statistical mechanics is derived from Liouville's theorem, which can be stated as "the phase space volume occupied by an ensemble of isolated systems is conserved over time." (I'm mostly ...
2answers
283 views

### Why is ${\partial^i}{\partial_i\phi}$ = ${\partial^i {\phi}}{\partial_i{\phi}}$?

This notation can be found on page 254 of Victor Stenger's Comprehensible Cosmos and in David Tong's Lectures on QFT (Equation 2.4 http://www.damtp.cam.ac.uk/user/tong/qft/two.pdf), and in EDIT: on ...
5answers
1k views

### How do photons travel at a speed that should be impossible to attain?

If it requires infinite amount of energy to travel at the speed of light then how photon attains this speed? Its source is never infinitely sourced.
4answers
530 views

### Why did we need relativity to derive $E=mc^2$?

Okay, so the way I understand one of the "derivations" of $E=mc^2$ is roughly as follows: We observe a light bulb floating in space. It appears motionless. It gives off a brief flash of light. We ...
0answers
154 views

### Lorentz transformations of the polarization vector

Let $\bf{n}'$ be a unit vector in the direction of a wavevector in the plasma rest frame and $\bf{B'}$ be a unit vector along the magnetic field in the plasma rest frame. The electric field of a ...
1answer
62 views

### Tower redshift paradox

If photons are emitted at intervals a, from the top of a tower of height $h$, down to earth, is this formula correct for the intervals b in which they are received at earth? $b=a(1-gh/c^2)$ If so, how ...
2answers
477 views

### How to calculate speed difference between objects close to the speed of light?

If two different objects (for example two rockets) move in opposite direction at close to the speed of light (for example 0.8c and 0.9c), how do I calculate the difference in speed between the two ...
1answer
75 views

### In what subfields and how fare can the “naive limit” of special relativity be carried?

Even if many interesting similarities between the classical and the quantum mechanical framework have been worked out, e.g. in the subject of deformation quantization, in general, there are some ...
3answers
227 views

### How to connect Einstein's Special Relativity(SR) with General Relativity(GR)?

How Einstein's SR becomes GR? $$ds^2=dr^2-c^2dt^2,$$ $$ds^2=g_{\mu\nu}dx^{\mu}dx^{\nu}.$$ When the $s$ is constant $ds^2=0$, isn't it true? How to connect Einstein's SR with GR? What is the ...
1answer
104 views

### What kinds of inconsistencies would one get if one starts with Lorentz noninvariant Lagrangian of QFT?

What kinds of inconsistencies would one get if one starts with Lorentz noninvariant Lagrangian of QFT? The question is motivated by this preprint arXiv:1203.0609 by Murayama and Watanabe. Also, what ...
2answers
161 views

### Are there any known potentially useful nontrivial irreducible representations of the Lorentz Group $O(3,1)$ of dimension bigger than 4? Examples?

Are there any known potentially useful, nontrivial, irreducible representations of the Lorentz Group $O(3,1)$ of dimension more than $4$? Examples? A $5$-dimensional representation? EDIT: Is there ...
4answers
346 views

### Does $p=mc$ hold for photons?

Known that $E=hf$, $p=hf/c=h/\lambda$, then if $p=mc$, where $m$ is the (relativistic) mass, then $E=mc^2$ follows directly as an algebraic fact. Is this the case?
2answers
168 views

### Are the higher-order terms in the series for energy really negligible?

To show that energy in special relativity reduces to $E=m+mv^2/2$ for low velocities, if we make a Taylor expansion of $m\gamma$ around $v=0$ we get $$E=m+mv^2/2+3mv^4/8+\cdots$$ But why can we cutoff ...
1answer
204 views

### Here's a way to transmit data faster than the speed of light [duplicate]

Possible Duplicate: Is it possible for information to be transmitted faster than light by using a rigid pole? Assume there is a long rod or a string connecting two points separated by a ...
2answers
194 views

### why is there only one inertial frame that $ct$ and $x$ are orthogonal?

It is very long time ago that I took a physics lesson, so I want to refresh my memory. I think I learned that there is only one inertial frame in Minkowski spacetime (or special relativity time) that ...
1answer
194 views

### Why can't this speed be measured?

Superman and Supergirl were playing catch. When Superman is moving with a speed of 0.800c relative to Supergirl, he threw a ball to Supergirl with a speed of 0.600c relative to him. a. ...
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