# Tagged Questions

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

161 views

### Geometric definition of the Lorentz inner product

In Euclidean space one can define the dot product as projecting one vector to the other and multiply the length of the projected vector with the length of the other vector. This definition doesn't ...
4k views

### Maximum speed of a rocket with a potential of relativistic speeds

Ultimately, the factor limiting the maximum speed of a rocket is: the amount of fuel it carries the speed of ejection of the gases the mass of the rocket the length of the rocket ...
58 views

### How do I form the four-momentum quantum operator?

I am trying to form the four-momentum quantum operator. These are the steps I have taken so far: The 3-momentum operator is given by $\hat{p}_{i} = -i\partial_{i}$. This is covariant because it is ...
78 views

### Understanding the formula in an exercise

I'm stuck for quite some time now on this issue. I have the following question. Spaceship with velocity of $0.5c$ passes above two points $A$ and $B$ the distance between $A$ and $B$ respect to the ...
30 views

### Special Relativity and Composition Law for Velocities with respect to an observer watching two objects moving away from a central point

So I understand that special relativity is all about the frame of reference and there is a lot to do with time dilation and how space-time is warped at velocities near $c$. So my question is what ...
46 views

### Why does length contraction seem to conflict with invariance of intervals?

Suppose we have two simultaneous events, $A$ and $B$, separated by a distance $L$ (the simultaneity is in frame of reference $S$). Now suppose we have a second frame of reference $S'$ moving with ...
2k views

### Do tachyons move faster than light?

I am trying to understand whether or not tachyons travel faster than light. The linked Wikipedia page shows some seemingly contradictory statements, and they are confusing. For instance, the first ...
140 views

### Is distance always 0 relative to an object moving at speed of light $c$?

As I understand it, when an object is traveling at the speed of light, relative to itself all travel is instantaneous and the distance is zero. If a photon traveling from the sun was aligned with the ...
140 views

### Can the constancy of the speed of light in vacuum be derived from a deeper theory?

Einstein's second postulate of special relativity is: The speed of light in vacuum takes the same value ($c$) for any observer in an inertial frame of reference. I know there is a lot of ...
99 views

### Particle here at a given time, in another galaxy a second later… Really?

I read "The Quantum Universe (Cox & Forshaw)" that a particle can be measured at a given position at a given time, and in another galaxy one second later. The probability of such event may be ...
22 views

### Hidden character in EPR paradox [duplicate]

I am a beginner in Quantum Mechanics so i am pretty new to the EPR paradox although i have heard about it a long time ago but finally studying in detail. And came across a doubt: Why the hidden ...
49 views

### Minimum gravity required to suck light? [closed]

I was studying about gravitational forces of black holes when I came to a thought of what is the minimum gravity required to capture light in vacuum?BY capturing I mean the inability of light to ...
2k views

### What do physicists mean by “information”?

On the question why certain velocities (i.e. phase velocity) can be greater than the speed of light, people will say something like: since no matter or "information" is transferred, therefore the ...
33 views

### Is light send from the top of a ship to deck different when the ship is moving or not?

If you drop a ball from the top downwards to the deck it will follow a rather straigth line because the ball gets also a force horizontaly because the ship is moving. That is the cause in history why ...
111 views

### Does antimatter travel faster than light

I have read in the answers here that an electron traveling backwards in time would behave as a positron. I also read in another there that antimatter is matter traveling backwards in time As far as I ...
86 views

### When calc. time elapsed for an inertial reference frame during travel between two points, are length contraction and time dilation taken into account? [closed]

When calculating the amount of time elapsed for an inertial reference frame over the course of its travel at constant velocity between two points, are the effects of both length contraction and time ...
44 views

### Direction of the lightbeam in SRT thought experiments [duplicate]

Extending on the question about the time dilation on Wikipedia, namely this diagram in the accepted answer: Image credit: John D. Norton The thing that I don't understand is, why does the light ...
4k views

### What is the proper way to explain the twin paradox?

The paradox in the twin paradox is that the situation appears symmetrical so each twin should think the other has aged less, which is of course impossible. There are a thousand explanations out there ...
45 views

I start with 1st (non-radiative) term from Lienard-Wiechert fields: $$\vec{E} = q (1-v^2) \frac{\vec{R_{t'}} - \vec{v}R_{t'}}{(R_{t'} - \vec{v}\vec{R_{t'}})^3}$$ $$\vec{H} = - q (1-v^2) ... 1answer 161 views ### Does Inertial time dilation demonstrate that Time is not a dimension? [duplicate] If time is a dimension and 'now' simply an expression of your position with respect to that dimension, the progress of any object along that dimension should remain in step with all other objects. By ... 1answer 101 views ### Objects travelling relatively to each other faster than light? When we say that something is travelling a certain speed, it's really travelling that speed relative to the Earth. When saying the speed of anything, it is, for the most part, relative to something ... 2answers 96 views ### A twin-like paradox, with no acceleration and no 3rd reference point. What would happen? The following is a variation on the twin paradox, with no acceleration and no 3rd reference point (i.e. described purely in terms of relative motion). Imagine two observers initially located right ... 1answer 146 views ### Does the rest mass energy include the potential energy of the particle? The potential energy (as far as I have studied - that is, mainly classical physics) depends on the reference level, since its absolute value cannot be calculated. It can therefore be negative as well. ... 5answers 187 views ### Is this a fundamentally relativistic phenomenon? This question was inspired by some silliness in other threads but is independent of that silliness. Say that a train car sitting on a track is accelerated uniformly along its length if each point on ... 1answer 83 views ### How is the Lorentz group related to spin? [closed] I've been reading about the agebra of the Lorentz group. It is given by,$$G\equiv SO(1,3) ~\cong~ SU(2)\times SU^*(2)$$Now, representations of this group G as labelled by (j,j') where j is ... 1answer 48 views ### Ether (LET) vs electromagnetic field in the double slit experiment According to this answer, Lorentz Ether Theory (LET) is experimentally indistinguishable from SRT, because the Ether is immobile and practically redundant. However, on quantum scales, photons are ... 3answers 152 views ### A train on a track--relativity paradox While answering a question over on Worldbuilding.SE I found myself looking at a situation that I can't figure out: You have a train track of length L that makes a very large circle. You have a train ... 3answers 243 views ### When a body accelerates, it gains (relativistic) mass; after stopping, is the (relativistic) mass different from before it started accelerating? When a body accelerates, it gains (relativistic) mass m according to the relation$$m=\frac{m_0}{\sqrt{1-(v/c)^2}},$$where m_0 is the (rest) mass. But after it stops is the gained (relativistic) ... 2answers 2k views ### Does the existence of “gravitational waves” (assuming they exist) imply that time exists as a 4th dimension in the universe? [closed] I'm new to thinking about special and general relativity and I have no formal training as a physicist. However, I've been doing a bit of thinking about spacetime recently. I was wondering if ... 2answers 4k views ### Is kinetic energy a relative quantity? Will it make inconsistent equations when applying it to the conservation of energy equations? If the velocity is a relative quantity, will it make inconsistent equations when applying it to the conservation of energy equations? For example: In the train moving at V relative to ground, ... 3answers 63 views ### Can the passage of time be measured in the absence of motion? [duplicate] All of the ways I have heard of to measure the passage of time involve measuring some sort of motion (e.g. vibrations of a cesium atom, movement of the hands on a clock, etc.). Can the passage of ... 1answer 54 views ### Relativity of simultaneity: two light sources A. Suppose a moving train. The train has two wheels(front and back) and each wheel is connected to light source inside the train. The light source is triggered(light is emitted) when train passes ... 2answers 135 views ### Lorentz contraction explanation correct? [closed] I was having a heated SE chat debate with someone re relativity. One of us believes the explanation above is correct, while the other believes it is not. Who is correct, and, if the explanation is ... 1answer 74 views ### Would forcing a photon to travel under c even after it leaves a medium break Relativity? [duplicate] So, I stumbled onto this article which really blew my mind (http://www.bbc.com/news/uk-scotland-glasgow-west-30944584). To sum it up, these researchers set up some kind of material that slowed a ... 1answer 58 views ### Does travelling towards something at relativistic speed cause it to appear to speed up? If one were to travel towards a giant TV floating in space, at a fraction of the speed of light, would a video playing on the TV appear to play faster? I'm guessing the answer is no. From what I ... 5answers 314 views ### How is it possible for the wavelength of light to change in a medium? So my physics class has just finished a long unit on optics while at the same time I've been trying to teach myself relativity. I admit my understanding is probably rudimentary, but I figured all the ... 1answer 37 views ### In relativistic electrodynamics , why doesn't the Lorentz force not change its formulation ? And only the electric and magnetic fields transform ? My question is that when we want to find the Lorentz force acted on a particle moving in an electric and magnetic field , the equation is invariant in any two inertial relativistic frames. Why is that ... 5answers 3k views ### High speed and low speed photons Looking at the discovery of the neutron, and I came across this page: http://www-outreach.phy.cam.ac.uk/camphy/neutron/neutron3_1.htm The animation on the left, talks about low energy photons and ... 1answer 35 views ### Lorentz Invariants and Time dilation/Distance contraction? If distances contract in direction of motion and time dilates at high speeds, why are the rest mass m_0 and proper time t_0 called "invariant" under Lorentz transformations. Since depending on ... 3answers 85 views ### Mass has the same value in all inertial reference frames? Is mass the same in all inertial frames? If it is, why is that? If not, can you also explain? 1answer 133 views ### What are the expression for Energy and Momentum of tachyons in terms of their 'rest mass'? We examine the possibility of tachyons under the criteria that they form the energy momentum four vector that transforms according to Lorentz Transformations. Therefore, E$$'$... 2answers 71 views ### Is the speed limit as light speed just based on what we see? [duplicate] I know, the title makes no sense, generally. Let me explain. I am just asking a question for which I found an answer nowhere (not even here). i think similar questions has been asked before and, no ... 1answer 52 views ### Fermi's understanding of the Doppler effect I am now reading the classic paper by Dicke, The Effect of Collisions upon the Doppler Width of Spectral Lines At the very beginning of the paper, Dicke said ''Quantum mechanically, the Doppler ... 6answers 61k views ### How do moving charges produce magnetic fields? I'm tutoring high school students. I've always taught them that: A charged particle moving without acceleration produces an electric as well as a magnetic field. It produces an electric field ... 1answer 83 views ### Michelson Morley experiment? [closed] Its not that I question the conclusions reached concerning the Michelson–Morley experiment, however I would like to know how the following issue was addressed please? If I could pass bob through a ... 1answer 31 views ### Force required to produce a specific motion on a particle [closed] This exercise comes from the Exercises for the Feynman Lectures, Chapter 15. The full question: 15-6 A particle of rest mass$m_0$is caused to move along a line such that its position is: ... 1answer 50 views ### Using light clocks, can one derive the length contraction formula without the 'bouncing' of the photon? In the following link, the equation for time dilation is derived by allowing the photon to just go from the lower mirror to the top, without reflecting back to the bottom: ... 1answer 117 views ### How is it possible to change the direction of a spin by boosting? In Weinberg QFT section 2.5.5, he defines the states of momentum$p$by $$\Psi_{p,\sigma}=U\bigl(L(p)\bigr)\Psi_{k,\sigma}$$ up to some irrelevant normalisation, and$L(p)\$ is the Lorentz ...
The contravariant 4-displacement is: $${x}^{\alpha} = (ct,\mathbf{r})$$ And the contravariant 4-gradient is: $${\partial}^{\alpha} = (\frac{1}{c}\frac{\partial}{\partial{t}},-\nabla)$$ From what I ...